Cent. Eur. J. Eng. • 4(2) • 2014 • 110-118 DOI: 10.2478/s13531-013-0133-6

Central European Journal of Engineering

Internal combustion engine supercharging: turbocharger vs. pressure wave compressor. Performance comparison

Topical Issue: AMMA 2013

Atanasiu Catalin George1∗, Anghel Chiru1

1 ”Transilvania” University of Brasov, Brasov, Romania

Received 30 May 2013; accepted 24 September 2013

Abstract: This paper aims on comparison between a turbocharged engine and a pressure wave charged engine. The comparison was accomplished using the engine simulation software AVL Boost, version 2010. The grahps were extracted using AVL Impress, version 2010. The performance increase is limited by the mechanical side of the simulated engine. Keywords: Supercharging• Turbocharger• PWS, Engine• Comparison © Versita sp. z o.o.

1. Introduction

The will for the automotive engine builders to manufacture thermic engines with higher and higher efficiencies, but with lower emissions and lower fuel demanding, has led to research intensifications in terms of combustion improvement and geometrical optimization [1]. By comparing the internal combustion engines, in terms of efficiency, we can clearly see that the maximum efficiency has the diesel engine, or the compression-ignition engine. The spark ignition engine has an overall efficiency slightly Figure 1. Thermic engine efficiency comparison. lower. It is followed by the Stirling engine, gas turbine and gas machine (Figure 1). Although the electric engine has an efficiency of 85%, this does not compensate the losses of the electrical energy build-up [2, 3]. including transport, is 29.3%. The same calculation made for coal has an overall efficiency of 31.1%. In this case, The efficiency to convert the oil into electrical energy, the electric engine powered by oil-source energy is 24.9%, and powered by coal-source energy is 26.4% [2]. In ∗ E-mail: [email protected] order to improve the efficiencies of the internal combustion

110 Atanasiu Catalin George, Anghel Chiru

engines, more air is needed into the cylinder, to be able to burn more fuel. To do this, a supercharging or turbocharging equipment is needed [4].

2. Supercharging solutions

Superchargers and turbochargers are compressors mounted in the intake system and used to raise the pressure of the incoming air. This results in more air and fuel entering each cylinder during each cycle. This added air and fuel creates more power during combustion, and the net power output of the engine is increased [3, 5]. The internal combustion engines (ICE) can be supercharged in various ways, but he most common way to do this is to use a turbocharger [6]. A very Figure 2. 4 cylinder engine with turbocharger; 1 - compressor, 2 - intake manifold, 3 - exhaust manifold, 4 - turbine. rare type of supercharger, but pretty promising, is the pressure wave supercharger. Both of them are treated in this paper as a supercharging unit of the 1.8i gasoline engine. A gasoline unit has be chosen due to low torque and the fresh air. The PWS works based on the physical at low revs. The aim of this paper is to demonstrate principle that if two fluids having different pressures are that the pressure wave supercharger is way better than brought into contact, equalization of pressure occurs faster turbocharger, at low rev operation [3, 7]. than fluid mixing. The actual energy transfer occurs in a single cell joining area of different states. At the 2.1. Turbocharging beginning of a cycle, the fresh air fills in the rotor cell. When PWS works, the rotor keeps in continuous rotation At turbocharging, a compressor is powered with an and the hot exhaust gas flows from cylinder into the exhaust turbine using energy from the exhaust gases, cell, so that the fresh air meets the exhaust gas directly i.e. the engine is only fluidically connected to the during one phase of the cycle. As the rotor makes one turbocharger (Figure 2). The exhaust gas of the internal revolution, the ends of each cell are alternating either combustion engine flows through the exhaust manifold to nearly hermetically closed or widely open toward the the turbine and spins it. A compressor sitting on the passages of the casings. These alternative open and close shaft of the turbine, can convert the drive power of the of cells result in serial shock waves, compression waves turbine into compressing power (minus bearing friction), and expansion waves, which contribute to the energy itself compresses fresh air, raises its temperature, which transfer between the exhaust gas and the air. The shock must be cooled by an intercooler [8]. Via the manifold, the wave moves much faster than the gases. The compression compressed air enters the engine. In the case of highly waves build up the pressure of the charge air. The unsteady applications (e.g. vehicle operation), the power charge air with increased pressure flows out of the cell of the turbine should be controlled. This can be done in into the casing passage and then to the cylinder as the the schematic shown here with a wastegate [5, 7, 9]. end of cell opens. Simultaneously, the expansion waves cause the exhaust gas pressure to go down. With that, 2.2. Pressure wave supercharger the exhaust gas of low enthalpy finally goes toward the exhaust pipe [4, 7, 10]. The pressure-wave supercharger (PWS) utilizes the This direct contact action inside the PWS results in energy of the engine exhaust gas to build up the intake energy transfer in such short time that the PWS can air pressure [1, 2, 4, 10] like the more classic turbo respond quickly according to the different engine working charging. However, the operation principle of pressure conditions, hence there is no so-called ’turbo lag’ problem wave supercharger is some what different. As shown in like turbo charger [3, 5]. Furthermore, this unique direct Figure 3, The PWS is made of a set of tiny and narrow contact may also cause internal exhaust gas recirculation channels, called cells, placed on a rotor. The rotor spins (EGR) usefully for reducing the NOx emission [4, 5, 10]. between two casings, the exhaust gas housing and the Meanwhile, there are other attractive advantages, such as fresh air housing, with inlet and outlet for the exhaust gas large torque at small engine speeds and less soot during

111 Internal combustion engine supercharging: turbocharger vs. pressure wave compressor. Performance comparison

Figure 3. Engine fitted with pressure wave supercharger.

acceleration. All these good characteristics make the • s - distance between crank axis and wrist pin PWS suitable for automobile engines whose loads vary A a lot [3, 11, 12]. • m valve area

Air flow rate: 3. Theoretical approach of the simulation   1   γ−1  2     gamma CD · AR · p pT  · γ pT  m 0 · 2 · −  The theoretical approach of the simulation is supplied ˙ = 1 p γ − 1 p R · T 2 0  1 0  by [7, 9, 13, 14]. ( 0) (3) Volumetric efficiency: • CD - discharge coefficient · m 2 ˙ a p ηv = (1) • 0 - upstream stagnation pressure ρa · Vh · N T • m˙ a - mass flow inducted into the engine • 0 - upstream stagnation temperature p • ρa - air density • T - pressure at the restriction

• N - engine speed • Am - reference area of the valve

• Vh - engine displacement • γ - adiabatic coefficient

Flow velocity: Equivalence ratio:

1 dV π · B2 ds A
vps = · = · (2) φ F actual Am dθ 4 · Am dθ = A
(4) F theoretic • V - cylinder volume A • - Air-fuel ratio • B - cylinder bore Z

112 Atanasiu Catalin George, Anghel Chiru

Intake mass speed: Burnt fuel quantity:

 2 −a·y· m X X Ap ( +1) 2 x = 1 − e (8) patm − pc = ∆pj = ρa · S¯p ξj · (5) Aj

• patm - atmospheric pressure 4. Practical approach of the

• pc - cylinder pressure simulation

S¯p • - mean piston speed The AVL code is a full software bundle used to simulate the processes within an internal combustion engine and • Ap - piston area includes: Boost, Fire, Cruise and Excite. • Aj - component minimum flow area In this paper, the AVL Boost had been used to simulate a complete engine and its internal processes. After the • ∆pj - total quasy steady pressure loss simulation was completed, AVL Impress was used to read and easily generate the graphs. ξj • resistance coefficient for that component which AVL Boost can simulate a very wide range of parameters: depends on its geometric details performance evaluation for different operating points in accordance with geometrical and functional optimization, Mass flow rate: comparison of different concepts of internal combustion s engines, etc. dm · ψ A · p · 2 The construction of the model in AVL Boost follows a = eff 0I (6) dt R0 · T0I verification and also an analyze of the real-world engine, the Audi 1.8 TFSI. In Figure 4 and Figure 5, the engine dm model is presented, used for turbocharged (Figure 4) and • - mass flow rate dt pressure wave supercharged (Figure 5) engine operation. • Aeff - effective flow area The model is built with the help of the AVL Boost internal library and is formed by: C1, C2, C3, C4 - engine p • 0I - port upstream static pressure cylinders; PL1, PL2 - plenums; TH1 - engine throttle; CO1 - air intercooler; CL1 - air filter; TC1 - turbocharger; T • 0I - port upstream static temperature PWSC1 - pressure wave supercharger; CAT1 - engine R catalyst; MP1 to MP5 - key measuring points; R1, R2 • 0 - gas constant - flow restrictions (used to simulate EGR opening and Combustion model: turbocharger bypass); SB1, SB2 - system boundaries; J1 to J14 - joints; 1 to 42 - connecting pipes. The fluid flow within pipes is simulated using dx a m −a·y· m = · (m + 1) · y · e ( +1) unidimensional pipes, with the friction coefficient input da ∆αc dQ required. The pipe bending losses are simulated also dx = Q (7) with a friction coefficient. The software describes the fluid flow through the pipes using 3 basic equations: the α − αc y = continuity equation, the impulse conservation equation ∆αc and the energy conservation equation. • Q - total heat amount received For each element of the model, the input data is required. For example, in Figure 6, Figure 7 and Figure 8 are α • - rank angle degree presented some of these required inputs. α The Vibe function is a very convenient method for • 0 - corresponding angle for beginning of describing the heat release characteristics. It is defined by combustion the start and duration of combustion, a shape parameter ’m’ and the parameter ’a’. These values can be specified • ∆αc - combustion duration either as constant values or dependent on engine speed • m - form coefficient (in rpm) and engine load (expressed as BMEP in bar). In this case, fixed values were chosen [14]. • a = 6:9, Viebe coefficient The heat release characteristic of gasoline engines,

113 Internal combustion engine supercharging: turbocharger vs. pressure wave compressor. Performance comparison

Figure 4. AVL Boost turbocharged engine layout.

Figure 5. AVL Boost pressure wave supercharged engine layout.

114 Atanasiu Catalin George, Anghel Chiru

Figure 6. Engine burning model - Vibe function was used. Parameters ”m” and ”a” are shape parameters.

with essentially homogeneous mixture distribution in the increase at low revolution. Test setup: cylinder, is mainly determined by the flame propagation • Engine: 1.8i, gasoline speed and the shape of the combustion chamber. A high flame propagation speed can be achieved with • Operating point: 1500 rpm, full load, high compression ratio and high turbulence levels in the cylinder. In diesel engines on the other hand, • Turbocharging: standard Audi turbocharger the combustion characteristic depends strongly on the (unknown manufacturer), capabilities of the fuel injection system, compression ratio • Supercharging: Comprex CX-93 solution, and the charge air temperature [14]. For accurate engine simulations the actual heat release • Run setup: turbocharged engine, turbocharged characteristic of the engine, (which can be obtained by engine with VVT, supercharged engine using an analysis of the measured cylinder pressure history), pressure wave supercharger. should be matched as accurately as possible [14]. Measuring point 2 location: compressed air intake, before In order to simulate the burning process within the engine plenum PL1, right before entering the engine. It is good cylinder, the uni-zonal model is adapted witch assumes to know the intake pressure after all pipe and connection that the fuel and the air within the cylinder is always in losses. thermodynamic balance, with no temperature gradients, no pressure waves and no unequilibrate composition. 6. Conclusions

5. Results The pressure wave supercharger has great influence on the performance of the gasoline engine, as showed in Following results represent the benefits of the pressure this paper, and also of diesel engine. The 1.8i gasoline wave supercharging process. The engine was simulated engine performance increased dramatically at low revs in at only one operating point, to indicate the performance term of power and torque and also develops perfectly fast

115 Internal combustion engine supercharging: turbocharger vs. pressure wave compressor. Performance comparison

Figure 7. Intercooler configuration.

Figure 8. Pressure wave supercharger setup.

116 Atanasiu Catalin George, Anghel Chiru

Figure 11. Cylinder temperature for the 3 setups.

Figure 9. P-V diagram of the 3 supercharging solutions.

Figure 12. Mechanical engine work for the 3 setups.

Figure 10. Cylinder pressure for the 3 setups.

instant response. The turbocharged gasoline engine fitted with VVT witch comes closer, in terms of performance, to the pressure wave supercharged engine also it is more expensive due to integration of VVT. The pressure wave supercharged engine is a little bit heavier (about 10kg+) than the turbocharged one, but overall has a better weight/power ratio. Figure 13. Air pressure at measuring point 2 in the simulation for the 3 setups. Some factors can affect the pressure wave supercharged engine. The simulation shows that the intake air mass

117 Internal combustion engine supercharging: turbocharger vs. pressure wave compressor. Performance comparison

flow and the exhaust gas temperature are the most [4] Weber F., Guzzella L., Control oriented modeling important influence factors of the 1.8i PWS gasoline of a pressure-wave supercharger (PWS) to gasoline engine. Therefore, larger intake manifold volume and engine [C], SAE paper 2000-01-0567, 2000 higher exhaust temperature are recommended for better [5] Heisler H., Advanced engine technology, 1995, ISBN: power performance. 978-1560917342 [6] Haider, G.: Die mechanische Aufladung, 2nd edn. Published by the author, Wien, 2000 Acknowledgements [7] Heywood J. B., Internal Combustion Engine Fundamentals [M]. New York, McGraw Hill, 1988 This paper is supported by the Sectoral Operational [8] Spinnler, G: Ecodyno®: a new supercharger for Programme Human Resources Development (SOP HRD), passenger car engines. abb Techn. Beschreibung, financed from the European Social Fund and by 1991 the Romanian Government under the contract number [9] Pulkrabek W. W., Engineering Fundamentals of POSDRU ID76945. the Internal Combustion Engine [M]. New Jersey: Prentice Hall, 2003 [10] Gyarmathy G., How does the Comprex pressure-wave References supercharger work [C], SAE paper 830234, 1983, 91- 105 [1] Despreiries P., L’avenir du petrole. Ingineurs de [11] Hermann H., Peter P., Charging the internal l’automobile 10, 1979, 545-550 combustion engine, Springer Wien New York, 2003 [2] Chiru A., Posibilitati de imbunatatire a [12] Spring P., Onder C. H., Guzzella L., EGR control performantelor unor agregate energetice pentru of pressure-wave supercharged IC engines. Control autovehicule. Buletinul sesiunii stiinitifice, I.I.S. Engineering Practice [J] 2007, 15, 1520-32 Pitesti, 4-5 Dec. 1981, III, 169-174 [13] LMS Amesim 10, user guide pdf book [3] Stone R., Introduction to Inernal Combustion Engines, [14] AVL Boost 2010, user guide pdf book Third Edition, SAE 1999, ISBN: 0-7680-0495-0

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