Review of Small Gas Turbine Engines and Their Adaptation for Automotive Waste Heat Recovery Systems

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International Journal of Turbomachinery Propulsion and Power

Review Review of Small Gas Turbine Engines and Their Adaptation for Automotive Waste Heat Recovery Systems

Dariusz Kozak * and Paweł Mazuro

Department of Aircraft Engines, Warsaw University of Technology, 00-665 Warsaw, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-509-679-182

Received: 28 October 2019; Accepted: 22 April 2020; Published: 30 April 2020

Abstract: Current commercial and heavy-duty powertrains are geared towards emissions reduction. Energy recovery from exhaust gases has great potential, considering the mechanical work to be transferred back to the engine. For this purpose, an additional turbine can be implemented behind a turbocharger; this solution is called turbocompounding (TC). This paper considers the adaptation of turbine wheels and gearboxes of small turboshaft and turbojet engines into a two-stage TC system for a six-cylinder opposed-piston engine that is currently under development. The initial conditions are presented in the ﬁrst section, while a comparison between small turboshaft and turbojet engines and their components for TC is presented in the second section. Based on the comparative study, a total number of 7 turbojet and 8 turboshaft engines were considered for the TC unit.

Keywords: auxiliary power unit; waste heat recovery; turbine engines; turbine wheel; opposed-piston engine; internal combustion engine; review

1. Introduction High-temperature exhaust gases from internal combustion engines (ICE) offer the potential for heat recovery systems. Mechanical turbocompounding (mech-TC) or electric turbocompounding (el-TC) systems have been used in heavy-duty diesel (HDD) engines. Recently, a few interesting studies were conducted on TC. Jääskeläinen and Majewski [1] carried out a review of TC systems for HDD engines; engine manufacturers such as Scania, Volvo, Daimler MB, Detroit, and Cummins have used serial TC systems in their engines. Axial turbines were used for the Volvo D13TC, MD13TC, and D12 500TC models, the Daimler OM 478 model, and the Detroit DD15 model. The implementation of TC resulted in a 3% efficiency improvement. Radial turbines were used in the Scania DT12, DTC11, and DTC12 models and the Cummins NTC-400 model; they resulted in a 4.5% efficiency improvement. The history of Scania engines has been presented in the literature [2]. The author mentions positive aspects of TC in Scania DT12 engines and that such an engine meets the Euro 5 emission regulations. In the literature [3], the available energy of the waste heat from a 6-cylinder HDD engine was analyzed. This engine was equipped with a high- and low-pressure loop exhaust gas recirculation (EGR) system. The authors compared three types of waste heat recovery systems, while 1-stage and 2-stage turbocharging systems were compared against a turbocharging system using the Rankine combined cycle (RCC). At full-load, the 1-stage turbocharging system provided 40.0 74.8 kW of available energy ÷ from the waste heat, whereas the 2-stage turbocharging system provided 43.0 76.3 kW of available ÷ energy. At low-load conditions, the available energy was small compared to high-load conditions. It was also found that turbocharging with the RCC improved the brake-specific fuel consumption (BSFC) by about 3%.

Int. J. Turbomach. Propuls. Power 2020, 5, 8; doi:10.3390/ijtpp5020008 www.mdpi.com/journal/ijtpp Int. J. Turbomach. Propuls. Power 2020, 5, 8 2 of 15

Ismail et al. [4] conducted a 0-D simulation on a 2 L diesel engine with a series TC unit. By locating the power turbine behind the turbocharger, the authors showed that the BSFC could be reduced; this was caused by the increase in the engine’s back-pressure. They also showed the potential of TC combined with an EGR valve; this combination resulted in a power gain of about 14 kW considering isentropic expansion. Furthermore, in their next work, Ismail et al. [5] investigated the effect of parallel TC on a 2 L diesel engine. It was shown that an engine model with parallel TC provided lower energy recovery potential than series TC. For low-load conditions, parallel TC had a negative effect on the global power gain, as most of the exhaust gasses passed through the turbocharger instead of the recovery turbine. At high load, a minor increase was observed for parallel TC; however, the authors showed the potential of parallel TC, as it produces lower back-pressure compared to series TC. In addition, in order to improve parallel TC, the authors suggested implementing a variable turbine geometry (VTG) in the turbocharger. With proper optimization of the VTG and reconfiguration of the turbocharger to run at higher efficiency, it is possible for the engine to benefit from parallel TC. Experimental tests of parallel TC were also performed on an IVECO 3 L engine with two exhaust configurations [6]—the first was with the original VTG turbocharger and the second was with a new fixed geometry turbocharger. The first configuration showed good results for the engine at low loads, however at high loads the advantages were minor compared to the fixed geometry turbine. However, both systems displayed potential for power recovery of about 2% 2.5%. ÷ Bin et al. [7] designed and tested a low-pressure turbine for TC applications in a 1.0 L gasoline engine. They conducted 1D simulations on the 1.0 L gasoline engine using three configurations of the low-pressure turbine—post-catalyst, pre-catalyst, and waste-gated configurations (in which the low-pressure turbine was located in a parallel configuration to the turbocharger). In low-speed, high-load conditions, the post-catalyst turbine position proved to be the best, with a reduction in the BSFC of 2.38% and an increase in the brake mean effective pressure (BMEP) of 2.4%. The pre-catalyst turbine position resulted in less of an improvement in both the BSFC and BMEP. This was caused by an increase in the back-pressure, thus causing higher pumping losses. The waste-gated, pre-catalyst turbine position had a negative effect on the engine’s performance at low speed. This was mainly due to the small bypass ratio of the main turbine used to feed the low-pressure turbine due to its parallel configuration. A comparison of an ICE diesel engine with TC against a turboshaft engine used as the propulsion system for a light rotorcraft was carried out in the literature [8]. The authors concluded that the ICE diesel engine offered a promising reduction in both fuel consumption and CO2 emissions, especially with medium and high loads and for long-range missions. In addition, implementing the TC system further increased the reduction in fuel consumption. The reduction in fuel consumption of 25% 50% ÷ compared to the turboshaft engine depended on the required power. The higher mass of the ICE was compensated by the lower fuel consumption compared to the turboshaft engine. He and Xie [9] investigated three modes of TC, namely electric TC, series TC, and parallel TC, on a 6-cylinder HDD engine. They performed simulations at a steady-state for different engine loads and speeds. In addition, three different driving cycles were investigated with these modes. The results showed that electric TC provided the best power increase at high-load and at a high-speed engine operating point. The best reduction in fuel consumption was achieved by electric TC at high-load and a high-speed operating point. The smallest power increase and reduction in BSFC were obtained from parallel TC. All the TC systems worked well for high-load and with a high-speed engine operating point. In different driving cycles, electric TC provided the best BSFC reduction of more than 7%. Bin et al. [10] developed and tested a low-pressure turbine for electric TC applications. A TC turbine was designed for a very low-pressure ratio in order to reduce the impact of the back-pressure on the engine’s performance. Tests with electric TC were performed on a 1.0 L EcoBoost gasoline engine in the series configuration. At 2500 rpm, a maximum reduction in the BSFC of 2.6% was achieved. Int. J. Turbomach. Propuls. Power 2020, 5, 8 3 of 15

Salehi et al. [11] studied the influence of electric TC on a 13 L HDD engine with an asymmetric twin-scroll turbocharger. The first approach of simply implementing electric TC downstream of the exhaust system resulted in a reduction in the BSFC and produced smoke emissions. This was a result of the increase in the back-pressure and higher pumping losses. After modification of the turbocharger’s volute and exhaust pipes to achieve a symmetrical volume, the BSFC was reduced by 1% at medium speed and load. Additionally, the transient operation of electric TC with a bypassed valve provided a 1.6% reduction in the BSFC. Although Yamaguchi et al. [3] revealed a greater potential for turbocharging when combined with the organic Rankine cycle (ORC), such a solution has not been considered in this paper due to its complexity and its greater increase in mass. The articles have shown the potential of series TC as a waste heat recovery system. Parallel TC offers small benefits for power recovery and BSFC reduction. Additionally, in order to control the back-pressure in the exhaust system, and thus emissions, a VTG turbine should be introduced into the exhaust system. A 2-stage mechanical TC system will be implemented in the 6-cylinder opposed-piston engine that is under development at Warsaw University of Technology. The recovered waste heat will be returned to the crankshaft by means of the gearbox.

2. Review of Potential TC Solutions After World War Two, increasing demand for fast travel forced engine manufacturers to increase the brake power of their engines. Hot exhaust gases from an ICE promised a great source of energy for increasing an engine’s power. Starting from the 1940’s, the first engines were equipped with an exhaust turbine coupled with the engine’s crankshaft via the gearbox. Despite the fact that the engine’s brake power was increased, a great deal of energy was wasted on driving the exhaust turbine. The use of hot exhaust gases as the energy source for the turbine improved the engine’s overall efficiency, and thus the engine’s brake power. This application was used in both aircraft and automotive engines. Further demand for increased aircraft speed and a low power-to-weight ratio for piston engines made turbine engines more promising as a propulsion system. Turbine engines were first used in aircraft, however there was consternation about the use of such engines in the automotive industry. Unfortunately, the difficulty of controlling such engines and the high fuel consumption made them inappropriate for automotive applications. However, rotary equipment is still widely used in piston engines for waste heat recovery. In this article, small turbojet and turboshaft engines that are available on the market, along with their rotary equipment, were considered as a 2-stage TC system for the 6-cylinder OP engine, PAMAR V, which is under development. This is one of the options, aside from designing a new radial turbine. The reason for this is that adapting turbine engines that are already on the market is still cheaper than designing a new impeller. Designing a new turbine is also an option, however it has not been discussed in this paper. Due to the highly isolated combustion chambers in the previous engines, namely PAMAR III and PAMAR IV, high exhaust temperature is available. Thus, extremely temperature-resistant turbine components need to be used, especially in the first stage of TC. A review and comparison of small turbojet and turboshaft engines was carried out, considering . the turbine inlet temperature (TIT) Tin, the maximal mass airflow mair, the maximal rotational velocity nmax or revolutions per minute (RPM), the bearing system, and the gearbox’s gear ratio.

2.1. Review of Small Turbojet Engines As stated in the literature [12], from the late 1960’s there was an increased interest in smaller turbojet engines. Engine manufacturers such as Teledyne Turbine Engines, Williams International, Turbomeca, Noel Penni, and Microturbo were leaders in the market. However, the main disadvantage of turbojet engines is the lack of an output shaft for transferring the mechanical power to other devices. The Teledyne J402 was a single-shaft turbojet engine, with a maximal TIT of 1116 K, a maximal RPM of 41,200 r/min, and a maximal thrust of 2.9 kN [13]. Int. J. Turbomach. Propuls. Power 2020, 5, 8 4 of 15

Bakaljev et al. [14] presented specification data for the known small turbojet engines that had been produced in the past. The Teledyne J408 was an example of a single-shaft turbojet engine with a TIT of 1039 K and a nominal RPM of 42,000 r/min; the nominal thrust of this engine was 4.45 kN and the engine was equipped with a single-stage axial turbine. Another model, the Teledyne 305-4, was a type of small single-shaft turbojet engine. However, this engine was much smaller than both the J408 and J402. Its maximal TIT was 1364 K and its maximal RPM was 81,000 r/min; due to its volume, it generated 0.18 kN of thrust. This engine was equipped with a single-stage axial turbine. The last engine from the Teledyne company was the model 312. Its TIT was 1100 K and maximal RPM was 71,000 r/min. This engine was also equipped with a single-stage axial turbine. However, no cross-section images could be found. The maximal thrust of this engine was 0.79 kN. Another interesting engine was the Williams International F107 series engine. Despite the fact that no cross-section images are available, it is known that it was a twin-shaft engine with high-pressure (HP) and low-pressure (LP) turbines. The HP turbine had a single stage with a maximal RPM of 64,000 r/min, while the LP turbine had two stages with a maximal RPM of 35,000 r/min; the maximal TIT was 1282 K and it produced 2.67 kN of thrust. Another type of Williams International engine was the YJ400 series engine. It was also a type of bypass engine with both HP and LP turbines; however, both the HP and LP turbines had two stages. This engine was a twin-shaft engine, with a maximal RPM of 60,000 r/min, a TIT of 1227 K, and produced thrust measuring 0.89 kN. Noel Penni was another manufacturer that produced well-known small turbojet engines. Their first model was the NPT 301. This engine had a single-stage axial turbine, with a maximal TIT of 1130 K and a maximal RPM of 40,500 r/min; it produced thrust measuring 1.37 kN. The model NPT 754 was similar to the NPT 301, but its maximal RPM was 34,100 r/min, with thrust measuring 5.09 kN. The Microturbo company also produced two models of turbojet engines. The first was a single-shaft model TRS 18-046, with a maximal TIT of 923 K and RPM measuring 47,000 r/min. The second one was the single-shaft model TRI 60-1, with a maximal TIT of 1198 K and a RPM of 28,500 r/min; the thrust of these engines measured 0.9 kN and 3.43 kN, respectively.

2.2. Review of Small Turboshaft Engines Small turboshaft engines are frequently used in aircraft to start the main engines and to produce electrical energy. Sometimes such engines are also used to provide auxiliary air for the aircraft; thus, they are equipped with a power output shaft. Nowadays there are many manufacturers of such engines around the world, most of which are located in the USA or Russia. Radikovic and Jevgjenievic [15] reviewed every Russian-made small turboshaft unit up to the year 2020. In their work, the AI-450, RU-19A-300, TA-6A, TA-8V, TA-12-60, AI-8, AI-9W, and VSU-10 units were mentioned. The AI-450 is a type of two-shaft auxiliary engine with a compressor turbine and a free turbine [16]. Its maximal TIT is over 1200 K, with a maximal RPM of 40,000 r/min and a 294 kW power rating (Nmax). Additionally, the maximal mass airﬂow rate for this engine is 1.127 kg/s and the reduction in the gearbox’s gear ratio is 6.5:1. A cross-section of this engine is shown in Figure1, where the reduction gearbox and the free turbine can be seen. Another type of engine that might be suitable for the ﬁrst stage of a TC unit is the RU-19A engine [17]. It is a single-shaft engine with a single-stage axial turbine and a 7-stage axial compressor. Little is known about its maximal TIT, however the maximal temperature after the turbine is 1173.15 K. Also, the maximal RPM is 16,000 r/min, with a Nmax of 720 kW. The turbine drives an AC generator via a set of bevel and helical gearboxes; the gearbox gear ratio is 0.447. The engine has its own fuel and lubrication system and the lubrication oil pressure varies from 1.2 bar at idle to 4.0 bar at 80% of the engine’s maximal speed. The cross-section of this engine is shown in Figure2. Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 5 of 16

Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 5 of 16

Figure 1. Cross-section of the AI-450 engine [16]. (Reproduced with permission from Ivchenko- Int. J. Turbomach. Propuls. Power 2020, 5, 8 5 of 15 ProgressInt. J. Turbomach., AI-450M Propuls. TurboshaftPower 2020, 5, x FORgas PEER turbine REVIEW engines; published by Ivchenko-Progress,5 of 2012. 16 ) Figure 1. Cross-section of the AI-450 engine [16]. (Reproduced with permission from Ivchenko- AnotherProgress type, AI of-450M engine Turboshaft that might gas beturbine suitable engines; for t hepublished first stage by Ivchenkoof a TC unit-Progress, is the 2012.RU-19A) engine [17]. It is a single-shaft engine with a single-stage axial turbine and a 7-stage axial compressor. Little is knownAnother about type its of maximalengine that TIT, might however be suitable the maximal for the fir temperaturest stage of a afterTC unit the is turbine the RU -is19A 1173.15 engine K. [17].Also, It theis a maximalsingle-shaft RPM engine is 16 ,with000 r/min a single, with-stage a N axialmax of turbine 720 kW. and The a 7turbine-stage axial drives compressor. an AC generator Little isvia known a set ofabout bevel its and maximal helical TIT, gearboxes; however the the gearbox maximal gear temperature ratio is 0.447. after The the engine turbine has is its1173.15 own fuelK. Also,and lubricationthe maximal system RPM andis 16 the,000 lubrication r/min, with oil a pressureNmax of 720 varies kW. from The 1.2turbine bar at drives idle to an 4.0 AC bar generator at 80% of viathe a engine set of ’bevels maximal and helical speed. gearboxes;The cross- sectionthe gearbox of this gear engine ratio is isshown 0.447. in The Figure engine 2. has its own fuel Figure 1. Cross-section of the AI-450 engine [16]. (Reproduced with permission from Ivchenko- and lubrication Figuresystem 1. Cross-section and the oflubrication the AI-450 engine oil [ 16pressure]. (Reproduced varies with permissionfrom 1.2 from bar Ivchenko-Progress, at idle to 4.0 bar at 80% of AI-450MProgress Turboshaft, AI-450M gas Turboshaft turbine engines; gas turbine published engines; by Ivchenko-Progress, published by Ivchenko 2012.)-Progress, 2012.) the engine’s maximal speed. The cross-section of this engine is shown in Figure 2. Another type of engine that might be suitable for the first stage of a TC unit is the RU-19A engine [17]. It is a single-shaft engine with a single-stage axial turbine and a 7-stage axial compressor. Little is known about its maximal TIT, however the maximal temperature after the turbine is 1173.15 K. Also, the maximal RPM is 16,000 r/min, with a Nmax of 720 kW. The turbine drives an AC generator via a set of bevel and helical gearboxes; the gearbox gear ratio is 0.447. The engine has its own fuel and lubrication system and the lubrication oil pressure varies from 1.2 bar at idle to 4.0 bar at 80% of the engine’s maximal speed. The cross-section of this engine is shown in Figure 2.

Figure 2. Cross-section of the RU-19A engine (Adapted from[17]).

The TA-6A is aFigure single 2.-Figureshaft Cross 2. engine-Cross-sectionsection with of ofthe thea RU3 RU-19A-stage-19A engine axialengine (Adapted turbine (Adapted from and [ 17from ]).a 3[17]-stage). compressor [18]. Its maximal TIT is 1110 K at a RPM value of 29,950 r/min, with a maximal Nmax of 45 kW; its maximal The TA-6A is a single-shaft engine with a 3-stage axial turbine and a 3-stage compressor [18]. massThe airflow TA-6A rate is ais single 1.35 kg/s.-shaft The engine turbine with drives a 3-stage accessory axial turbine devices and and a an 3- stageAC generator compressor via [18].a helical Its Its maximal TIT is 1110 K at a RPM value of 29,950 r/min, with a maximal N of 45 kW; its maximal Figure 2. Cross-section of the RU-19A engine (Adapted frommax[17]). maximalgearbox. TIT Themass is reduction 1110 airﬂow K rate at ratio isa 1.35RPM of kg the/ s.value The gearbox turbine of 29 drivesto,950 drive r/min accessory the, withgenerator devices a andmaximal anis 0.2505 AC generatorNmax for of the via45 aAC kW; helical generator. its maximal The massengine, airflow withgearbox. rate itsThe multi is TA The1.35-6A- reductionstage kg/s.is a single turbine, The ratio-shaft turbine of engineis the very gearbox driveswith promising a to3 -accessorystage drive axial the for generatorturbine devicesTC application and is aand 0.2505 3-stage an for scompressorAC. theIts generator ACcross generator. [18].-section Its via ais helical shown The engine, with its multi-stage turbine, is very promising for TC applications. Its cross-section is gearbox.in Figure The 3.maximal reduction TIT is ratio1110 K of at the a RPM gearbox value of to 29 drive,950 r/min the, withgenerator a maximal is N0.2505max of 45 for kW; the its ACmaximal generator. The shown in Figure3. engine, withmass its multiairflow- stagerate is 1.35 turbine, kg/s. The is turbinevery promising drives accessory for devices TC application and an AC generators. Its cross via a- sectionhelical is shown gearbox. The reduction ratio of the gearbox to drive the generator is 0.2505 for the AC generator. The in Figure 3. engine, with its multi-stage turbine, is very promising for TC applications. Its cross-section is shown in Figure 3.

Figure 3. Cross-section of the TA-6A engine (Adapted from [18]). Figure Figure3. Cross 3. Cross-section-section of of the the TA-6A-6A engine engine (Adapted (Adapted from [18] from). [18] ).

Figure 3. Cross-section of the TA-6A engine (Adapted from [18]).

Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 6 of 16

Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 6 of 16 AInt. single J. Turbomach.-shaft Propuls. TA-8V Power engine2020, 5, with 8 a radial inflow turbine wheel was another option for6 the of 15 first stage of TC [19]; its maximal TIT and RPM are 1100 K and 40,139 r/min, respectively. The Nmax of this A single-shaft TA-8V engine with a radial inflow turbine wheel was another option for the first engine is 30 kW and the maximal mass airflow rate is 0.85 kg/s. The turbine drives an AC generator stage of TCA single-shaft [19]; its maximal TA-8V engineTIT and with RPM a radial are 1100 inflow K and turbine 40,139 wheel r/min, was anotherrespectively. option The for theNmax first of this via a planetary gearbox. The gear ratio to drive the AC generator is 0.14952 and the engine has its enginestage is of30 TCkW [19 and]; its the maximal maximal TIT andmass RPM airflow are 1100 rate K is and 0.85 40,139 kg/s. r /Themin, turbine respectively. drives The an N maxAC ofgenerator this own fuel and lubrication system. The required oil pressure is 4.5 bar, while the minimal pressure via aengine planetary is 30 kWgearbox. and the The maximal gear ratio mass to airflow drive rate the is AC 0.85 generator kg/s. The turbineis 0.14952 drives and an the AC engine generator has its should not be lower than 1.2 bar. The cross-section of this engine is shown in Figure 4. ownvia fuel a planetaryand lubrication gearbox. system. The gear The ratio required to drive the oil AC pressure generator is is 4.5 0.14952 bar, andwhile the the engine minimal has its ownpressure shouldfuel not and be lubrication lower than system. 1.2 bar. The The required cross oil-section pressure of is this 4.5 bar,engine while is the shown minimal in Figure pressure 4. should not be lower than 1.2 bar. The cross-section of this engine is shown in Figure4.

Figure 4. Cross-section of the TA-8V engine (Adapted from [19] ).

Figure 4. Cross-section of the TA-8V engine (Adapted from [19]). Another option isFigure the single4. Cross-shaft-section TA12 of the-60 TA unit-8V, enginewhich (Adaptedhas a three from-stage [19]) . axial turbine with a maximal AnotherRPM of 24 option,470 r/min is the single-shaftand a TIT of TA12-60 1115 K unit, [20]. which The maximal has a three-stage mass airflow axial turbineis 2.1 kg/s with, with a a Another option is the single-shaft TA12-60 unit, which has a three-stage axial turbine with a Nmax maximalof 60 kW. RPM However, of 24,470 it rwas/min n andot possible a TIT of 1115to obtain K [20 ].a Thecross maximal-section mass image airflow of this is 2.1engine. kg/s, with a maximal RPM of 24,470 r/min and a TIT of 1115 K [20]. The maximal mass airflow is 2.1 kg/s, with a TheNmax AIof- 608 and kW. AI However,-9 are well it was-known not possible engines to obtain that are a cross-section used in helicopters. image of this The engine. first one is a two- Nmax of 60 kW. However, it was not possible to obtain a cross-section image of this engine. shaft engineThe with AI-8 andan axial AI-9 arecompressor well-known turbine engines and that an are axial used free in helicopters. turbine. The The TIT first of one this is aengine two-shaft is 1200 The AI-8 and AI-9 are well-known engines that are used in helicopters. The first one is a two- K andengine its maximal with an RPM axial is compressor 38,500 r/min. turbine The and reduction an axial gearbox free turbine. ratio The to drive TIT of the this AC engine generator is 1200 is K 0.25. shaftand engine its maximal with an RPM axial is compressor 38,500 r/min. turbine The reduction and an gearboxaxial free ratio turbine. to drive The the TIT AC of generator this engine is 0.25. is 1200 Unfortunately, no cross-section images are available for this engine. The output power (Nmax) of this K andUnfortunately, its maximal noRPM cross-section is 38,500 r/min. images The are availablereduction for gearbox this engine. ratio The to drive output the power AC (Ngenerator) of this is 0.25. engine is 14 kW. max Unfortunately,engine is 14 no kW. cross-section images are available for this engine. The output power (Nmax) of this As for the AI-9 engine, its single-shaft axial turbine is capable of withstanding 1100 K, its peak engine isAs 14 forkW. the AI-9 engine, its single-shaft axial turbine is capable of withstanding 1100 K, its peak RPM is at 39,150 r/min, and its Nmax is 3 kW; its maximal mass airflow rate is 1.1 kg/s. The turbine RPMAs for is atthe 39,150 AI-9 rengine,/min, and its its singl Nmaxe-shaftis 3 kW; axial its turbine maximal is mass capable airflow of ratewithstanding is 1.1 kg/s. 1100 The turbineK, its peak drivesdrives an AC an ACgenerator generator via via a gearbox a gearbox,, with with a a gear gear ratio ofof 0.2.0.2. TheThe cross-section cross-section of thisof this engine engine is is RPM is at 39,150 r/min, and its Nmax is 3 kW; its maximal mass airflow rate is 1.1 kg/s. The turbine presentedpresented in Figure in Figure 5. 5. drives an AC generator via a gearbox, with a gear ratio of 0.2. The cross-section of this engine is presented in Figure 5.

Figure 5. Cross-section of the AI-9W engine (Adapted from [20]). Figure 5. Cross-section of the AI-9W engine (Adapted from [20]).

The VSU-10 is a twoFigure-shaft 5. Cross engine-section with of a the2-stage AI-9W axial engine compressor (Adapted turbine from [20] and). a 2-stage axial free turbine [21]. The maximal TIT for the compressor turbine is 1050 K, with a maximal RPM of 31,975 The VSU-10 is a two-shaft engine with a 2-stage axial compressor turbine and a 2-stage axial free r/min. The free turbine has a maximal RPM of 19,625 r/min and it drives an auxiliary compressor via turbine [21]. The maximal TIT for the compressor turbine is 1050 K, with a maximal RPM of 31,975

r/min. The free turbine has a maximal RPM of 19,625 r/min and it drives an auxiliary compressor via

Int. J. Turbomach. Propuls. Power 2020, 5, 8 7 of 15

Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 7 of 16 The VSU-10 is a two-shaft engine with a 2-stage axial compressor turbine and a 2-stage axial free turbine [21]. The maximal TIT for the compressor turbine is 1050 K, with a maximal RPM of the gearbox31,975, with r/ min.a reduction The free turbine gear has ratio a maximal of 0.62. RPM The of 19,625 maximal r/min andmass it drives airflow an auxiliary for this compressor engine is 3.9 kg/s. via the gearbox, with a reduction gear ratio of 0.62. The maximal mass airﬂow for this engine is 3.9 kg/s. The cross-sectionInt. J. Turbomach. of this Propuls. engine Power 2020 is ,shown 5, x FOR PEER in FigureREVIEW 6. The compressor turbine has a roller7 of 16 bearing on either side, Thewhile cross-section the free of turbine this engine has is showna roller in Figureand ball6. The bearing compressor on turbineone side. has a roller bearing on theeither gearbox side, while, with thea reduction free turbine gear has ratio a roller of 0.62. and The ball maximal bearing mass on one airflow side. for this engine is 3.9 kg/s. The cross-section of this engine is shown in Figure 6. The compressor turbine has a roller bearing on either side, while the free turbine has a roller and ball bearing on one side.

Figure 6. Cross-section of the VSU-10 engine (Adapted from [21]).

Figure 6. Cross-section of the VSU-10 engine (Adapted from [21]). The lastFigure of the Russian 6. Cross small-section turboshaft of the engines VSU -is10 the engine GTDE -(Adapted117 [22]. This from unit is[21] used). to provide auxiliaryThe last power of the supply Russian for small the MIG turboshaft-29 aircraft engines and is the to provide GTDE-117 compressed [22]. This unit air tois used start to the provide main The lastenginesauxiliary of the of powerRussian the aircraft. supply small Its for cross the turboshaft- MIG-29section aircraftis shown engines and in Figure to provideis the 7. It GTDE compressedis a two--117shaft air [22]. engine to start This with the unita main compressor is engines used to provide turbine and a free turbine; the maximal Nmax for this engine is 16 kW. The compressor turbine’s auxiliary powerof the aircraft. supply Its cross-section for the MIG is shown-29 aircraft in Figure 7. and It is a to two-shaft provide engine compressed with a compressor air turbineto start the main maximaland a free TIT turbine; is 1123 the K maximal, with a maximal Nmax for RPM this engine of 43,000 is 16 r/min. kW. The The compressor maximal mass turbine’s airflow maximal rate for TIT this is engines of the1123engine aircraft. K, is with 1.0 akg/s. Its maximal cross The co RPM-mpressorsection of 43,000 isturbine shown r/min. has The ina maximalroller Figure bearing mass 7. It behind airﬂowis a two the rate- turbineshaft for this engine wheel engine and is with 1.0 a kgball a/s. compressor turbine andbearingThe a compressor free at turbine; the front turbine of the hasthe maximal a turbine roller bearing wheel Nmax behind, close for the to this theturbine engine place wheel where is and 16 the a ball kW. shaft bearing The couples at compressor the with front the of turbine’s maximal TITcompressor.the is turbine 1123 wheel,K The, with free close turbine a tomaximal the has place roller where RPM and the ballof shaft 43 bearings,000 couples r/min. on with one thesideThe compressor., andmaximal due to The its mass s freehort turbine airflowshaft, it has is rate for this easierroller andto balance. ball bearings on one side, and due to its short shaft, it is easier to balance. engine is 1.0 kg/s. The compressor turbine has a roller bearing behind the turbine wheel and a ball bearing at the front of the turbine wheel, close to the place where the shaft couples with the compressor. The free turbine has roller and ball bearings on one side, and due to its short shaft, it is easier to balance.

Figure 7. CrossCross-section-section of of the the GTDE GTDE-117-117 engine (Adapted from [22] [22]).).

Woodhouse [23] carried out a revision and comparison of the different models of small turboshaft engines from the United States manufacturers, such as Garrett, Honeywell, Pratt and Whitney, and others. A great amount of information was provided for the Garrett GTCP331 model, with its single-shaft and a 3-stage axial turbine. Different variants of the GTCP331 were produced, starting with the GTCP331-250, GTCP331-9 (A, B, D), and GTCP331-500. The last version produced the highest amount of power. The TIT for the GTCP331-500 was 1050 K, with a maximal RPM of

Figure 7. Cross-section of the GTDE-117 engine (Adapted from [22]).

Int. J. Turbomach. Propuls. Power 2020, 5, 8 8 of 15

Woodhouse [23] carried out a revision and comparison of the different models of small turboshaft engines from the United States manufacturers, such as Garrett, Honeywell, Pratt and Whitney, and others. A great amount of information was provided for the Garrett GTCP331 model, with its single-shaft and a 3-stage axial turbine. Different variants of the GTCP331 were produced, starting with the GTCP331-250, GTCP331-9 (A, B, D), and GTCP331-500. The last version produced the highest Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 8 of 16 amount of power. The TIT for the GTCP331-500 was 1050 K, with a maximal RPM of 39,044 r/min and a Nmax of 169 kW. The highest value of the mass airflow rate was 3.53 kg/s. A cross-section of this 39,044 r/min and a Nmax of 169 kW. The highest value of the mass airflow rate was 3.53 kg/s. A cross- engine is shown in Figure8. section of this engine is shown in Figure 8.

Figure 8. The GTCP331-500GTCP331-500 engine:engine: ( a) cross cross-section;-section; (b) isometric view (Adapted from [[23]23]).). Another interesting engine is the Garrett AiResearch Model 85 [23]; however, very little is known Another interesting engine is the Garrett AiResearch Model 85 [23]; however, very little is known about this engine. It is a single-shaft engine equipped with a radial inflow turbine and its maximal about this engine. It is a single-shaft engine equipped with a radial inflow turbine and its maximal RPM is about 35,000 r/min. RPM is about 35,000 r/min. Another engine is the Garrett Model 700 [23], which is a two-shaft engine with HP and LP Another engine is the Garrett Model 700 [23], which is a two-shaft engine with HP and LP axial axial turbines. An interesting fact is that variable guide vanes are installed before the LP turbine. turbines. An interesting fact is that variable guide vanes are installed before the LP turbine. However, However, no information about the performance of this model is available. no information about the performance of this model is available. The next model is the Allied Signal RE220 [23]. This is a two-shaft unit with a 2-stage axial turbine. The next model is the Allied Signal RE220 [23]. This is a two-shaft unit with a 2-stage axial Its maximal TIT is 1055 K and its maximal RPM is 45,585 r/min. Its mass airflow rate is 1.22 kg/s and turbine. Its maximal TIT is 1055 K and its maximal RPM is 45,585 r/min. Its mass airflow rate is 1.22 the Nmax is 66.2 kW. However, no cross-section images were available. kg/s and the Nmax is 66.2 kW. However, no cross-section images were available. The model T-62T [23] is another engine that was widely used in military helicopters. It is a The model T-62T [23] is another engine that was widely used in military helicopters. It is a single- single-shaft engine with a maximal RPM of 61,565 r/min and a maximal TIT of 1000 K; the engine’s shaft engine with a maximal RPM of 61,565 r/min and a maximal TIT of 1000 K; the engine’s Nmax is N is rated at 44.1 kW. This engine is equipped with a radial inflow turbine, as shown in Figure9. ratedmax at 44.1 kW. This engine is equipped with a radial inflow turbine, as shown in Figure 9.

Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 9 of 16

Int. J. Turbomach. Propuls. Power 2020, 5, 8 9 of 15 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 9 of 16

Figure 9. Turbine wheel from the T-62T engine.

Figure 10 presents anFigure isometricFigure 9.9. TurbineTurbine view wheel wheelof an fromAPS3200 thethe T-62T T -engine’s62T engine. engine. cross -section [23]. It is a two-shaft engine with a 2-stage axial turbine. The engine’s maximal RPM is 49,300 r/min, with a maximal TIT Figure 10 presents an isometric view of an APS3200 engine’s cross-section [23]. It is a two-shaft Figure 10 presents an isometric view of an APS3200 engine’s cross-section [23]. It is a two-shaft ofengine 1173 K. with Additionally a 2-stage axial, the turbine. maximal The mass engine’s airflow maximal rate RPMis 1.8 is kg/s 49,300 and r/ min,the engine’s with a maximal Nmax is TIT91 kW. The engineturbine with drives a 2-stage an AC axial generator turbine. viaThe a engine’sgearbox ,maximal with a gear RPM ratio is 49of, 3000.4875. r/min , with a maximal TIT of 1173 K. Additionally, the maximal mass airﬂow rate is 1.8 kg/s and the engine’s Nmax is 91 kW. of 1173The K. turbine Additionally drives an, the AC maximal generator mass via agearbox, airflow withrate is a gear1.8 kg/s ratio and of 0.4875. the engine’s Nmax is 91 kW. The turbine drives an AC generator via a gearbox, with a gear ratio of 0.4875.

Figure 10. Cross-section of the APS3200 engine (Adapted from [23]). Figure 10. Cross-section of the APS3200 engine (Adapted from [23]). There is a smaller version, which is the APS2300 [23]. This has a TIT of 1120 K, with a maximal RPMThere of 45,225 is a rsmal/Figuremin,ler while 10. version Cross the engine’s-,section which of Nis maxthe the APS3200is APS2300 44 kW. engine No [23] cross-section. (AdaptedThis has froma images TIT [23] of were )1120. available K, with fora maximal RPMthis unit. of 45,225 r/min, while the engine’s Nmax is 44 kW. No cross-section images were available for thisThere unit.The is APS5000a smaller is version the biggest, which of all is the the APS APS2300 series [24 [23]]. Figure. This 11 has presents a TIT thisof 1120 engine, K, with which a hasmaximal a RPM2-stage of The45,225 axial APS5000 r/min turbine, iswhile drivingthe biggestthe a engine’s centrifugal of all theN compressormax APS is 44 series kW. and [24].No two crossFigure DC- generators.section 11 presents images However, this were engine no available data, which is for has a this2 unit.available-stage axial regarding turbine its driving performance. a centrifugal compressor and two DC generators. However, no data is availableThe ForAPS5000 thisregarding paper, is the aits biggest bigger performance. GTD-350 of all the turboshaft APS series engine [24]. wasFigure also 11 considered presents [this25]. engine It is a two-shaft, which has a 2-stageengine axial with turbine a compressor driving turbinea centrifugal and a freecompressor turbine. Forand the two compressor DC generators. turbine, However, the maximal no TIT data is availableis 1228.15 regarding K and its the performance. maximal RPM is 45,000 r/min. It has a maximal mass airﬂow rate of 2.19 kg/s, which makes its compressor turbine wheel very promising for use as the ﬁrst stage of TC; its maximal power (Nmax) is 294 kW. The cross-section of this axial compressor turbine is shown in Figure 12.

Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 10 of 16

Int. J. Turbomach. Propuls. Power 2020, 5, 8 10 of 15 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 10 of 16

Figure 11. Cross-section of the APS5000 engine [24]. (Reproduced with permission from Pamintuan, E., MESA/Hamilton Sundstrand Training Academy; published by Mathematics Engineering Science Achievement (MESA), 2012)

For this paper, a bigger GTD-350 turboshaft engine was also considered [25]. It is a two-shaft engine with a compressor turbine and a free turbine. For the compressor turbine, the maximal TIT is 1228.15 KFigureFigure and the 11.11. Cross-sectionCrossmaximal-section RPM of the is APS500045,000 r/min. engine [24]. [It24 has]. ( (ReproducedReproduced a maximal with with mass permission permission airflow from fromrate Pamintuan, Pamintuan,of 2.19 kg/s , which makes itsE.,E., compressor MESAMESA/Hamilton/Hamilton turbine Sundstrand wheel Trainingvery promising Academy;Academy; for published use as by the Mathematics first stage Engineering of TC; its maximalScience power (Nmax) is Achievement294 kW. The (MESA), cross- 2012)section of thisAchievement axial compressor (MESA), 2012 turbine) is shown in Figure 12.

For this paper, a bigger GTD-350 turboshaft engine was also considered [25]. It is a two-shaft engine with a compressor turbine and a free turbine. For the compressor turbine, the maximal TIT is 1228.15 K and the maximal RPM is 45,000 r/min. It has a maximal mass airflow rate of 2.19 kg/s, which makes its compressor turbine wheel very promising for use as the first stage of TC; its maximal power (Nmax) is 294 kW. The cross-section of this axial compressor turbine is shown in Figure 12.

FigureFigure 12. 12. TheThe GTD GTD-350-350 engine, engine, with with a a cross cross-section-section ofof thethe compressor compressor turbine turbine assembly assembly (Adapted (Adapted from [25]). from [25]). 3. Results and Discussion 3. Results and Discussion For the ﬁrst stage of TC, small turbojet and turboshaft units were both considered. Figure 13 Figure 12. The GTD-350 engine, with a cross-section of the compressor turbine assembly (Adapted showsFor the a comparisonfirst stage ofof theTC, TIT small and theturbojet Nmax ofand both turboshaft small turbojet units and were turboshaft both considered. engines. Figure 13 from [25]). shows a Fromcomparison Figure 13 of, it the can TIT be noted and the that N themax turbine of both wheels small from turbojet turbojet and engines turboshaft such as engines. the Noel Penni NPT754, the NPT301, the Teledyne J402, the 312, the 305-4, the Microturbo TRI60-1, the Williams YJ400, 3. Results and Discussion and the F107 are capable of withstanding temperatures above 1100 K, which is considered to be the temperatureFor the offirst the stage exhaust of TC, from small a developed turbojet opposed-pistonand turboshaft engine.units were Additionally, both considered. the maximal Figure RPM 13 ofshows such a engines comparison is relatively of the TIT high, and which the N ismax desirable of both small in exhaust turbojet turbines and turboshaft for automotive engines. applications. The low TIT of the Microturbo TRS 18-046 engine (923 K) makes it less suitable for TC applications. Apart from the Williams F107 and YJ400, all the engines have a single-shaft conﬁguration, which makes it easy to disassemble and assemble them. The main drawback of every turbojet engine is the lack of an output shaft and a reduction gearbox. In order to make TC operational, the power output shaft for a gearbox should be designed and manufactured to transfer the mechanical energy back to the crankshaft.

Int. J. Turbomach. Propuls. Power 2020, 5, 8 11 of 15 Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 11 of 16

1370 Teledyne 305-4 1320 Williams F107 1270 GTD-350

1220 Microturbo TRI60-1 Williams YJ400

RU-19A AI-8 AI-450 1170 Noel Penni NPT301 APS3200 T* [o K] GTDE-117 in Noel Penni NPT754 APS2300 1120 TA12-60 Teledyne J402 Teledyne 312 TA-8V TA-6A AI-9 GTCP331-500 1070 RE220 1020 VSU-10 Teledyne J408 T-62T

970 Microturbo TRS 18-046

920 10000 20000 30000 40000 50000 60000 70000 80000 nmax [r/min]

Figure 13. Comparison of the TIT *(T*in) and nmax of small turbojet and turboshaft engines. Figure 13. Comparison of the TIT (T in) and nmax of small turbojet and turboshaft engines.

Additionally,From Figure the 13 small, it can turboshaft be noted enginesthat the turbine TA12-60, wheels TA-6A, from TA-8V, turbojet AI-8, engines AI-9, AI-450, such as GTDE-117, the Noel APS2300,Penni NPT754, APS3200, the and NPT301, GTD-350 the have Teledyne a maximal J402, the TIT 312, equal the or305 greater-4, the Microturbo than 1100 K TRI60 and-1, a high the maximalWilliams RPM. YJ400 Due, and to the the F107 lower are TIT capable of the of wi turbinethstanding wheels temperatures from the above VSU-10, 1100 RE220, K, which T-62T, is andconsidered GTCP331-500 to be engines, the temperature they cannot of be the considered exhaust from for the a developed TC unit. Additionally, opposed-piston due engine. to the lowAdditionally maximal RPM, the value maximal of the RPM RU-19A of such engine engines (16,000 is relatively r/min), such high, an which engine is cannot desirable be in considered exhaust turbines for automotive applications. The low TIT of the Microturbo TRS 18-046 engine (923 K) makes for the TC unit. Small turboshaft engines are equipped with a power output shaft and a reduction it less suitable for TC applications. Apart from the Williams F107 and YJ400, all the engines have a gearbox, which makes it easier to adapt them for use in a TC unit. Figure 14 shows a comparison of the single-shaft configuration, which makes it easy to disassemble and assemble them. The main maximal TIT (T ) and the maximal m˙ of small turbojet and turboshaft engines. drawbackInt. J. Turbomach. ofin every Propuls. turbojet Power 2020 engine, 5, x FOR isair thePEER lack REVIEW of an output shaft and a reduction gearbox. In12 orderof 16 to make TC operational, the power output shaft for a gearbox should be designed and manufactured to transfer1370 the mechanical energy back to the crankshaft.

AdditionallyTeledyne, the 305 -4small turboshaft engines TA12-60, TA-6A, TA-8V, AI-8, AI-9, AI-450, GTDE- 117, APS2300,1320 APS3200, and GTD-350 have a maximal TIT equal or greater than 1100 K and a high maximal RPM. Due Williams to the F107 lower TIT of the turbine wheels from the VSU-10, RE220, T-62T, and GTCP3311270-500 engines, they cannot be considered for the TC unit. Additionally, due to the low Williams YJ400 maximal RPM value of theGTD RU-350-19A engine (16,000 r/min), such an engine cannot be considered for 1220 the TC unit. Small turboshaft engines are equippedMicroturbo with TRI60 -1 a power output shaft and a reduction AI-450 gearbox, which makes it easierAPS3200 to adapt them for use inRU a-19A TC unit. Figure 14 shows a comparison of 1170 GTDE-117 the *maximalo TIT (Tin) andNoel Pennithe NPT301maximal ṁair of small turbojet and turboshaft engines.Noel Penni NPT754 T in [ K] APS2300 Teledyne J402

1120 AI-9 TA12-60 TA-8V TA-6A GTCP331-500 1070 Teledyne 312 RE220 VSU-10 1020 Teledyne J408

T-62T 970 Microturbo TRS 18-046

920 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 ṁ air [kg/s]

Figure 14. Comparison of the TIT (T* *in) and ṁair of small turbojet and turboshaft engines. Figure 14. Comparison of the TIT (T in) and m˙ air of small turbojet and turboshaft engines. From Figure 14, it can be seen that the TA-8V, Teledyne 305-4, and T-62T engines operate at very low values of mass airflow rate. Such values are not acceptable for the TC unit, since the expected exhaust mass flow rate will be greater than 1 kg/s. With such a low mass flow rate, the exhaust gases would be choked during the exhaust phase.

4. Adaptation for OP Engine The concept of an exhaust system with an adaptation of the turbine engines for the OP engine is presented in Figure 15. Such a system consists of a first-stage turbine, a turbine housing, a second- stage turbine, a hydraulic coupling, and a gearbox. The turbine housing has six inlets to prevent leakage of exhaust gases during the exhaust valve overlap. The turbine housing will be designed from the beginning and its configuration will depend on the first-stage turbine. The type of turbine impeller (radial or axial) for both first and second turbines depends on the chosen turbine engine. The first-stage turbine needs to withstand extremely high temperature from exhaust gases, which is why turbine engines with high TIT should be considered. Such a turbine should also withstand mass flow rates of at least 1.0 kg/s without choking. The second-stage turbine should be coupled with the gearbox and the hydraulic coupling in order to transfer torque back to the crankshaft.

Figure 15. Conceptual adaptation of turbine wheels for an OP engine.

Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 12 of 16

1370

Teledyne 305-4 1320 Williams F107 1270 Williams YJ400 GTD-350 1220 Microturbo TRI60-1

AI-450 APS3200 RU-19A 1170 GTDE-117 * o Noel Penni NPT301 Noel Penni NPT754 T in [ K] APS2300 Teledyne J402

1120 AI-9 TA12-60 TA-8V TA-6A GTCP331-500 1070 Teledyne 312 RE220 VSU-10 1020 Teledyne J408

T-62T 970 Microturbo TRS 18-046

920 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 ṁ air [kg/s] Int. J. Turbomach. Propuls. Power 2020, 5, 8 12 of 15 Figure 14. Comparison of the TIT (T*in) and ṁair of small turbojet and turboshaft engines.

FromFrom FigureFigure 14 14,, itit cancan bebe seenseen that the TA-8V,TA-8V, TeledyneTeledyne 305-4,305-4, and and T-62TT-62T enginesengines operateoperate atat veryvery lowlow valuesvalues ofof massmass airflowairflow rate.rate. SuchSuch valuesvalues areare notnot acceptableacceptable forfor thethe TCTC unit,unit, sincesince thethe expectedexpected exhaustexhaust massmass flowflow raterate willwill bebe greatergreater thanthan 11 kgkg/s./s. WithWith suchsuch aa lowlow massmass flowflow rate,rate, thethe exhaustexhaust gasesgases wouldwould bebe chokedchoked duringduring thethe exhaustexhaust phase.phase.

4.4. AdaptationAdaptation forfor OPOP EngineEngine TheThe conceptconcept ofof anan exhaustexhaust systemsystem withwith anan adaptationadaptation ofof thethe turbineturbine enginesengines forfor thethe OPOP engineengine isis presentedpresented in in Figure Figure 15 15.. Such Such a systema system consists consists of aof first-stage a first-stage turbine, turbine, a turbine a turbine housing, housing, a second-stage a second- turbine,stage turbine, a hydraulic a hydraulic coupling, coupling and a, gearbox. and a gearbox. The turbine The t housingurbine housing has six inletshas six to inlets prevent to leakageprevent ofleakage exhaust of gasesexhaust during gases the during exhaust the valveexhaust overlap. valve Theoverlap. turbine The housing turbine willhousing be designed will be fromdesigned the beginningfrom the beginning and its configuration and its configuration will depend will on depend the first-stage on the fir turbine.st-stage The turbine. type ofThe turbine type of impeller turbine (radialimpeller or axial)(radial for or both axial) first for and both second first turbinesand seco dependsnd turbines on the depends chosen on turbine the chosen engine. turbine The first-stage engine. turbineThe first needs-stage to turbine withstand needs extremely to withstand high extremely temperature high from temperature exhaust gases,from exhaust which isgases, why which turbine is engineswhy turbine with highengines TIT with should high be TIT considered. should be Such considered. a turbine Such should a turbine also withstand should also mass withstand flow rates mass of atflow least rate 1.0s kgof /ats withoutleast 1.0 choking.kg/s with Theout choking. second-stage The turbinesecond-stage should turbine be coupled should with be coupled the gearbox with and the thegearbox hydraulic and the coupling hydraulic in order coupling to transfer in order torque to transfer back to torque the crankshaft. back to the crankshaft.

FigureFigure 15.15. ConceptualConceptual adaptationadaptation ofof turbineturbine wheelswheels forfor anan OPOP engine.engine.

AsInt. can J. Turbomach. be seen fromPropuls. Figure Power 2020 15, ,5 the, x FOR first PEER and REVIEW second turbine stages will not be connected13 mechanically. of 16 Engines such as the APS500, APS 2300, APS3200, GTCP331-500, AI-9W, TA-6A, RU-19, J402, J408, As can be seen from Figure 15, the first and second turbine stages will not be connected TD 312,mechanically. NPT 301, NPT Engines 754, andsuch TRIas the 60-1 APS500, units APS are 2300, single-shaft APS3200, engines GTCP331 equipped-500, AI-9W, with TA one-6A, HPRU- turbine. Such configurations19, J402, J408, TD can 312, only NPT be 301, addressed NPT 754 by, and the TRI first-stage 60-1 units turbine. are single Additionally,-shaft engines equipped for the second-stage with turbine,one an HP LP tu turbinerbine. Such from configuration such enginess can as only the be GTD-350, addressed GTDE-117,by the first-stage AI-450, turbine. AI-8, Additionally YJ400, and, F107 units shouldfor the be seco considered.nd-stage turbine, Great an care LP turbine has to from be taken such engines considering as the GTD the- gearbox350, GTDE and-117, the AI- reduction450, AI- gear ratio to8, ensure YJ400, theand turbineF107 units matches should be with considered. the engine. Great care Figure has to16 b eshows taken considering the first-stage the gearbox turbine and from the the reduction gear ratio to ensure the turbine matches with the engine. Figure 16 shows the first-stage GTD-350turbine engine from that the was GTD obtained-350 engine for that the was project. obtained It is for an the axial-type project. It turbine, is an axial the-type operating turbine, parameters the of whichoperating are shown parame in Figureters of which 12. are shown in Figure 12.

FigureFigure 16. 16.Compressor Compressor turbine turbine from from the the GTD GTD-350-350 engine engine..

This turbine wheel benefits from high mass flow rate and high efficiency; however, such efficiency is achievable within a narrow range of linear speed. It was possible to obtain a K44 automotive turbocharger turbine. This turbine is shown in Figure 17, along with the scanning process and turbine model generated in the Unigraphics NX software.

Figure 17. (a) Geometry of the K44 turbine, (b) turbine wheel scanned with the 3D scanner, and (c) the turbine wheel model.

5. Conclusions To conclude, small turbojet and turboshaft engines were considered for automotive adaptation in an opposed-piston engine that is under development at Warsaw University of Technology. The adaptation aimed to recover waste heat energy from exhaust gases using a two-stage TC unit. The ORC system was considered in this paper. • The literature review showed that parallel TC offered smaller BSFC reduction compared to series TC. This is attributed to the fact that in a parallel configuration, a greater amount of exhaust gases flow through a compressor turbine rather than a power turbine. • Parallel TC produced smaller back-pressure compared to series TC, as the power turbine is not placed directly behind turbocharger. This position of the power turbine does not limit the pressure ratio of the turbocharger. • In total, 25 turbine engines were compared, considering their maximal TIT, RPM, and ṁair.

Int. J. Turbomach. Propuls. Power 2020, 5, x FOR PEER REVIEW 13 of 16

As can be seen from Figure 15, the first and second turbine stages will not be connected mechanically. Engines such as the APS500, APS 2300, APS3200, GTCP331-500, AI-9W, TA-6A, RU- 19, J402, J408, TD 312, NPT 301, NPT 754, and TRI 60-1 units are single-shaft engines equipped with one HP turbine. Such configurations can only be addressed by the first-stage turbine. Additionally, for the second-stage turbine, an LP turbine from such engines as the GTD-350, GTDE-117, AI-450, AI- 8, YJ400, and F107 units should be considered. Great care has to be taken considering the gearbox and the reduction gear ratio to ensure the turbine matches with the engine. Figure 16 shows the first-stage turbine from the GTD-350 engine that was obtained for the project. It is an axial-type turbine, the operating parameters of which are shown in Figure 12.

Figure 16. Compressor turbine from the GTD-350 engine.

Int. J. Turbomach. Propuls. Power 2020, 5, 8 13 of 15 This turbine wheel benefits from high mass flow rate and high efficiency; however, such efficiency is achievableThis turbinewithin wheel a benefitsnarrow from range high mass of flow linear rate and speed. high effi ciency; however, such efficiency It was possibleis achievable to obtain within a a K44 narrow automotive range of linear speed. turbocharger turbine. This turbine is shown in Figure It was possible to obtain a K44 automotive turbocharger turbine. This turbine is shown in Figure 17, 17, along with thealong scanning with the scanning process process and and turbine turbine model model generated generated in the Unigraphics in the NX Unigraphics software. NX software.

Figure 17. (a) GeometryFigure 17. of(a )the Geometry K44 turbine, of the K44 ( turbine,b) turbine (b) turbine wheel wheel scanned scanned withwith the the 3D 3D scanner, scanner, and (c) the and (c) the turbine wheel model. turbine wheel model. 5. Conclusions To conclude, small turbojet and turboshaft engines were considered for automotive adaptation in an 5. Conclusions opposed-piston engine that is under development at Warsaw University of Technology. The adaptation aimed to recover waste heat energy from exhaust gases using a two-stage TC unit. The ORC system To conclude,was small considered turbojet in this paper. and turboshaft engines were considered for automotive adaptation in an opposed-pistonThe e literaturengine review that showedis under that parallel development TC offered smaller at BSFCWarsaw reduction University compared to series of Technology. The • adaptation aimed toTC. recover This is attributed waste to theheat fact thatenergy in a parallel from configuration, exhaust a greatergases amount using of exhausta two gases-stage TC unit. The flow through a compressor turbine rather than a power turbine. ORC system was consideredParallel TC produced in this smaller paper. back-pressure compared to series TC, as the power turbine is not • • The literature reviewplaced directly showed behind that turbocharger. parallel This TC position offered of the power smaller turbine BSFC does not reduction limit the pressure compared to series ratio of the turbocharger. TC. This is attributedIn total, 25 turbineto the engines fact werethat compared, in a parallel considering configuration, their maximal TIT, RPM, a greater and m˙ . amount of exhaust • air gases flow throughThe Noel a compressor Penni NPT754, NPT301, turbine Teledyne rather J402, than 312, Microturboa power TRI60-1,turbine. Williams YJ400, • • Parallel TC producedand F107 turbojet smaller engines back were-pressure considered appropriate compared for the to TC series unit. However, TC, as such the engines power turbine is not are not equipped with a power output shaft, which has to be manufactured. placed directlyThe behind TA12-60, TA-6A, turbocharger. AI-8, AI-9, AI-450, This APS2300, position APS3200, of and the GTD-350 power turboshaft turbine engines does not limit the • pressure ratio ofwere the considered. turbocharger. These engines meet the requirements regarding the TIT, RPM, and m˙ air. Additionally, these engines are equipped with a power output shaft and a gearbox. The output • In total, 25 turbineshaft andengines the gearbox were can becompared used to transmit, considering torque onto the crankshaft.their maximal TIT, RPM, and ṁair. The adaptation of the turbine engine into an ICE engine was presented, including the turbine • wheels of the GTD-350 engine and the K44 radial inflow turbine. 6. Plans for Further Research As mentioned in the previous sections, a two-stage TC unit will be used for the PAMAR V engine, which is a six-cylinder, opposed-piston engine with the cylinder axis positioned parallel to the main shaft. The engine is being developed at Warsaw University of Technology. The TC unit will be used as a waste heat recovery system, placed between the cylinders and the silencer. Additionally, the possibility of using automotive radial inflow turbine wheels for the two-stage TC unit was considered. Int. J. Turbomach. Propuls. Power 2020, 5, 8 14 of 15

In the near future, an initial calculation of the ﬁrst stage of the TC unit will be conducted, including numerical computational ﬂuid dynamics (CFD) calculations.

Author Contributions: writing—original draft preparation, D.K.; writing—review and editing, P.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to acknowledge Military Aviation Works No. 4 in Warsaw for sharing the GTD-350 compressor turbine wheel. Additionally, the authors would like to acknowledge BorgWarner company Poland for sharing the K44 turbine wheels. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature The following nomenclature is used in the manuscript: Symbols Tin Turbine inlet temperature [◦K] . mair Air mass flow rate [kg/s] r nmax Maximal turbine revolutions per minute [ min ] Nmax Maximal engine power [kW] Abbreviations TC Turbocompounding ICE Internal Combustion Engine mech-TC Mechanical Turbocompounding el-TC Electric Turbocompounding HDD Heavy-Duty Diesel EGR Exhaust Gas Recirculation RCC Rankine Combined Cycle BSFC Brake Specific Fuel Consumption VTG Variable Turbine Geometry BMEP Brake Mean Effective Pressure ORC Organic Rankine Cycle TIT Turbine Inlet Temperature RPM Revolutions Per Minute HP High-Pressure Turbine LP Low Single-Pressure Turbine AC Alternating Current DC Direct Current

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