4. 9 & FR0105396

DEVELOPMENT OF MILLER CYCLE GAS

FOR COGENERATION

DEVELOPPEMENT D UN MOTEUR A GAZ A CYCLE DE MILLER

DESTINE A LA COGENERATION

N. Tsukida , A.Sakakura, Y.Murata, and K. Okamoto Tokyo Gas CO.,LTD , Japan

T.Abe and T. Takemoto Yanmar Diesel Engine CO.,LTD , Japan

ABSTRACT

We have developed a 300 kW gas engine cogeneration system for practical use that uses natural gas.Using a gas engine operated under conditions with an excess air ratio X =1 that is able to use a three way catalyst to purify the exhaust gases,we were able to achieve high efficiency through the application of the Miller Cycle, as well as a low NOx output. In terms of product specifications, we were able to achieve an electrical efficiency of 34.2% and a recovery efficiency of 49.3%, making an overall efficiency of 83.5% as a cogeneration system.

RESUME

Nous avons developpe un systeme de cogeneration de 300 kW avec moteur au gaz natural et tout a fait utilisable dans la pratique. Avec un moteur a gaz fonctionnant dans les conditions d ’un rapport d ’exces d ’air de X = 1 et capable d ’utiliser un catalyseur trois voies pour purifier les gaz d ’echappement, nous avons ete en mesure d ’obtenir un rendement eleve en appliquant le Cycle de Miller, et d ’obtenir egalement une faible emission d ’oxydes d ’azote. En termes de performances techniques du produit, nous avons obtenu un rendement electrique de 34,2 % et un rendement de recuperation thermique de 49,3 %, ce qui donne un rendement total de 83,5 % pour le systeme de cogeneration. DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. INTRODUCTION Environmental issues have been the focus of much attention for quite some time, but it is now becoming important that we not only make efforts on a regional level with issues such as air pollution, but on a global scale as well. It is crucial that we the people of this earth act quickly to reduce the level of C02 in order to prevent global warming. The use of gas cogeneration systems that supply both electric power and heat at the same time using natural gas as fuel are steadily growing as an energy source in metropolitan areas because they have high overall efficiency, are energy and cost efficient, and output clean exhaust gasses. Fig. 1 shows the actual growth of cogeneration systems in the areas supplied by Tokyo Gas Co. Moreover, their use is expected to rapidly expand worldwide because of their high overall efficiency and the fact they use natural gas - the main component of which is

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-86 88 90 92 94 96 98 year Fig. 1 Gowth of Cogeneration i n Tokyo Gis The power scale of the cogeneration system that uses a gas engine as the prime mover is quite large, with a single unit capacity ranging between 15 and 4,000 kW. The majority of the units installed though, are in the 300 to 1,000 kW class that is made up of gas having a bore diameter of 150 to 200 mm, and of which there are two types: the stoichiometric mixture combustion type that uses a three way catalyst, and the lean-burn type that does not require a catalyst. In recent years, use of the lean-burn type, which is has high electrical efficiency and with which there is no need to purify the exhaust gases, is growing. However, it is thought that NOx reduction through only the combustion of the gas engine has nearly reached its limit with the current regulation value levels, and there is concern about how to deal with the environmental regulations that can only be assumed will become stricter in the future. The stoichiometric mixture combustion type, on the other hand, is advantageous in that it has a high exhaust gas temperature and efficient exhaust heat recovery, making the high overall efficiency above 80%, so it is well suited for cogeneration systems. However, the electrical efficiency is below that of the lean-burn type, so it is hoped that this can be improved. Furthermore, according to an amendment to the laws within Japan, it is now possible to use those gas fuel generators that are for ordinary use as generators for emergency use as well if the gas supply conduit is deemed earthquake-proof. There is therefore a demand for a gas cogeneration system having high electrical efficiency and low NOx during ordinary use, as well as good characteristics for keeping up with the load during emergency use (bearable load characteristics: the load ratio where instantaneous output is possible with only a slight rpm drop from an idling state). We attempted to achieve high efficiency by applying the Miller Cycle ex(1) to the stoichiometric combustion gas engine that is able to realize a low level of NOx emissions relatively easily using a three way catalyst, in order to achieve high efficiency. As performance, durability and so forth demanded as a cogeneration system were able to be achieved, we will report on that process in this paper. 1. Results of the Basic Research (2>~(5) 1.1 Means to Realize the Miller Cycle The basic research was conducted with an altered commercial gas engine. Table 1 shows the specifications of the test engine. Table.1 Engine Specifications

—...... Baseline Tested Otto Cvcle Miller Cvcle Type Spark-Ignited 4 Stroke Cycle cylinders x Bore x Stroke (irm) 6 x f 170 x 170 Displacement (litre) 23.15 Natural Gas : IBV=41.6 MJ/Nrrt3 Fuel Z 88.5 % - CHi , 4.6 % - CaHs \ X 5.4 % - OB . 1.5 % - OHio ' Rated Power (kW / rpm) 220 / 1200 324 / 1500 BMEP (MPa) 0.95 1.12 Equivalence Ratio 1.0 with Three-way Catalyst Supercharging Turbo-charger / Inter-cooler Ratio of Turbo-charger 1.6 1.8 ~ 2.8 Expansion ratio (Geometric ) 10.3 10-18 Intake Valve IVO (deq.bTDC) 20 Timing IVC (deg. aEDC) 40 -60 , 40-140 Mechanism of (1) EIVC Low Compression Ratio (2) LIVC In realizing the Miller Cycle, numerical calculations and experiments were performed on the early intake-valve closing (EIVC) type and the late intake-valve closing (LIVC) type according to the changes in the cam profile, and were then examined. The experimental conditions were set so that the rated power was 324 kW / 1500 rpm, BMER 1.12 MPa, and the inter-cooler outlet air-fuel mixture temperature was set at 313 K. In addition, with the cogeneration system it was not necessary to take the engine rpm characteristics into consideration because the generator is driven by the engine shaft horsepower. Therefore, the experimental conditions were such that the engine revolutions were set at a general 1500 rpm when the generation frequency was 50 Hz. Fig. 2 shows a diagrammatic drawing of the valve lift of the EIVC type and the LIVC type, and their respective changes in air-fuel mixture mass flow rate at the rated power. Engine Output=324 kW

EIVC Base

120 deg.

60 deg. 40 deg.

Base 0.4 -

0.2 "

-0.2 "

-0.4 -270 Crank Angle deg.aEDC Fig.2 Intake Valve Lift and Mass Flow Rate With the EIVC type the maximum amount of lift decreases due to the fact that the shape of the cam profile is similar to that of the base. Furthermore, the mass flow rate peak is high because of the necessity to force air-fuel mixture in a short amount of time. This results in the need for a relatively large pressure boost. With the LIVC type, on the other hand, the maximum lift period is extended compared to the base cam profile and it is clear that the mass flow rate flows back past BDC. During development, it may be said that with the EIVC type, along with matching the it is highly likely that it will be necessary to reexamine the valve drive system (lift height, seating rate, fixed number of springs, etc.). With the LIVC type, however, that likelihood is quite low. Fig. 3 shows the effective compression ratio as the intake valve is closed with an expansion ratio of 14.2, and the air-fuel mixture temperature at the intake port. The effective compression ratio peaks around 40 aBDC because of the inertia force and an efficient compression ratio reduction effect is evident where there is left and right symmetry with the peak in the center with both types. In addition, with the LIVC type the air-fuel mixture temperature at the intake port rises due to the back flow of intake air, and it is feared that there will be a decline in knocking performance. Engine Output = 324 kW,Expansion Ratio - 14.2 Temperature at Exit of Inter Cooler = 313K -3 16 . oJ A EIVC 14 • □ LIVC Calculated X 12 / X / \ 10 7------X ii 8 / 8 / 6

320 -150 -100 -50 BDC 50 100 150 IVC Timing deg. aBDC Fig.3 Effect of IVC Timing on Effective Compression Ratio and Intake Temperature

Engine Output = 324 KW Tenperature at Exit of Inter Cboler = 313K

|l20/15|-

12 14 16 18 Expansion Ratio

Fig.4 Effect of Expansion Ratio on 1.2 Thermal Efficiency of the Late Closing Miller Cycle Fig. 4 shows the change in thermal efficiency as the expansion ratio is changed with a fixed rated engine load. The ignition timing was advanced up to the respective knocking limits, and the expansion ratio was increased in an attempt to drastically improve thermal efficiency. As a result, we were able to achieve almost equal thermal efficiency in both the EIVC and LIVC types. 2. Engine Developmentfor Practical Use 2.1 Course of Development As a result of the basic research, were able to confirm the large effects with both the EIVC and LIVC types regarding the improvement of thermal efficiency according to the Miller Cycle. In applying the Miller Cycle to the development of the gas engine for use in the cogeneration system, it will be extremely important to ensure not only thermal efficiency, but reliability as well. Also, the actual development period this time was limited to approximately one year so the main engine specifications had to be determined early on, and there was also the need to construct a prototype and conduct durability tests. For this, a gas engine which is well-proven in the market was used as a base. We narrowed it down to the LIVC type, which allows development to be done with few change items and at a low cost.

2.2 Engine Specifications Table 2 shows the engine specifications and Fig. 5 shows a flow chart of the engine. We used a supercharging type that is a pre-mixing / supercharging type, which uses a venturi mixer and that allows operation even when gas is supplied under low pressure as well as has superior fuel gas mixture characteristics. For control of the air-fuel ratio, a feedback control method was used that allows precise control of the fuel gas bypass valve opening according to signals from oxygen sensors installed before and after the three way catalyst, allowing the excess air ratio to be constantly maintained in the vicinity of 1.0. For the three way catalyst, we employed a metal carrier using a stainless honeycomb material in an attempt to maintain high purification efficiency of the exhaust gases over a long period of time. To apply the Miller Cycle we manufactured trial camshafts of IVC =110, 120. Evaluation tests were then performed with an actual engine and the final specifications were determined. Table.2 New Engine Specifications Turbo-Charger Three-way Catalyst

" —— Depeloped Miller Cycle

[Yanmar Deisel 6NHLM-ST] -nfla Type Spark-Ignited 4 Stroke Cycle Cylinde x Bore x Stroke (rtm) 6 x (|>165 x 185 Displacement Volume (litre) 23.73 Bated Fewer (kW / rpm) 324 / 1500 BMEP (MPa) 1.09 Equivalence Ratio 1.0 with Three-way Catalyst Supercharging Turbo-charger / Inter-cooler Pressure Ratio of Turbo-charger 1.8 Intake Valve IVO (deq.MDC) 20 Timing IVC (deg.aBDC) -110 -120 Expansion ratio (Geometric Cbmpressicn Ratio) 13.3 14.2 Fig.5 Engine Flow

2.3 The Knocking Limit and Thermal Efficiency Fig. 6 shows the knocking limit and efficiency as the ignition timing is changed. The knocking limit expresses the traced knock read from the wave shape of the cylinder’s internal pressure as the overload ratio for the rated power of 324 kW. With the same ignition timing, the knocking limit of IVC=120 is lower. This is thought to be due to the effect of the temperature rise brought about by the degree of influence from the back flow of the air-fuel mixture. IVC=120 is thought to have a temperature rise somewhere between 5 and 10 K more than IVC=110, as can be seen in Fig.3. The thermal efficiency of IVC=120 is higher by approximately 1 point and is only slightly influenced by the ignition timing. Also, the standard for the cogeneration system is continuous operation under a full load, with peripheral maintenance of the combustion chamber also done at long intervals every 4,000 and 8,000 hours. Therefore, we set the knocking allowance taking into account mainly the accumulation of residual combustion elements from the lubricating oil for the pistons. With this Engine Qttput =324kWTenperature at Exit of Inter Cboler=318K IVG=120: Expansion Ratio 14. 2, IM3=110: Expansi on Ratio 13.3 130 'E ^ 3 2120

^110

100 >> 38 d

> 37 to 36

35 10 12 14 16 18 I gni t i on Ti rri ng deg. bTDC

Fi g. 6 Effect of Ignition Ti rri ng on Knocking and Therial Efficiency

development, we determined the ignition timing with a knocking limit at a load ratio of 115% with inter­ cooler cooling water at 318 K. The rough dotted line in Fig. 6 is equal to a load ratio of 115%. Thus performance that considers knocking allowance with IVC=120 is a thermal efficiency of 36.9% with an ignition timing of 13 deg. bTDC, and with IVC=110 that performance is a thermal efficiency of 36.1% with an ignition timing of 14.5 deg. bTDC. This is a thermal efficiency difference of 0.8 points between IVC=120 and IVC=110.

2.4 Bearable Load Characteristics We disconnected the gas engine cogeneration system from the commercial electric power system’s grid. Importance was placed on the bearable load characteristics that express how much load it can take all at once from an idling state and still be able to output power, when operated singly. Even with the gas engine, pressure boost is generally high, and the higher the thermal efficiency, the worse the bearable load characteristics become, similar to turbo lag in a vehicle. Fig. 7 shows the bearable load characteristics of IVC=110, 120. An identical supercharger was used and the engine was warmed up. With IVC=120, the differential pressure was slight between the boost pressure and the intake manifold pressure. With the IVC=110, on the other hand, this differential pressure was quite large. For thermal efficiency it is more advantageous to have less differential pressure because it means less loss, but for load control by the throttle valve a large differential pressure is preferable. Also, pressure boost is low in an idle state so it relies solely on the force from atmospheric pressure. From experience, if the engine output has an intake manifold pressure of approximately -10 kPa, then a sudden output is possible without any problem, even when the engine is cold. With the engine that we developed this time, we achieved approximately 120 kW (37% load ratio) with IVC=120, and approximately 160 kW (50% load ratio) with IVC=110. —Ck—Boost Pressure IM3=110 —0 — Mnifold Pressure IVG=110 - - Boost Pressure IVG= 120 - -Ar - Mni fol d Pressure IVC=120

Engi ne Qit put kW

Fi g. 7 Bearable Load Qiar act er i s t i cs

2.5 Determining Engine Specifications After considering the balance of thermal efficiency according to the Miller Cycle and the bearable load characteristics, as well as the marketability of cogeneration in the future, we finally decided to use the IVC=110 with its superior bearable load characteristics despite the fact that is 0.8 points worse in thermal efficiency.

2.6 Durability Test It was feared that the lubricating oil would stick to, accumulate on, or obstruct the intake port due to the back flow of intake air, and that this would affect the reliability from the Miller Cycle that used the LIVC type. However there were no problems with the durability test where an actual engine was operated under a full load for 2,000 hours.

3. Development of a Cogeneration Package ■ 3.1 Package Outline The cogeneration system consists of a prime mover, generator, exhaust heat recovery equipment, and other various types of accessories. This time we developed a cogeneration package with a power output of 300 kW using the newly developed gas engine as the prime mover. It employs an all-in-one (AiO) package method that allows the on-site installation process to be drastically reduced thanks to both the fact that it is equipped with everything necessary for operation, as well as its functional assembly that is aimed at conserving spaced A picture of the package is shown in Fig. 8. The package is broken into two units: an enclosure containing a gas engine, generator, control panel, three way catalyst, heat exchanger for heat recovery of the engine cooling water and so forth, and an integrated sub-unit containing an exhaust gas boiler, exhaust silencer, and cooling tower. Compared to conventional systems, it is extremely compact and achieves a power output per installation area of 0.07m2 / kW. Fig.8 300kW Cogeneration System "All in One Package"

3.2 Performance of the Cogeneration System The heat balance of the cogeneration system are shown in Table 3. The high efficiency is able to realize as well as low NOx emission.

Table.3 Cogeneration Specifications

^——- Depelqped Miller Cycle Gas Engine Cogeneration System

Input / Fuel Consumption 877 kW / 76.0 mWh (Natural Gas Supplied by Tokyo Gas)

300 kW(50Hz) Generated Power / 34.2% Output / 252 kW(358K Hot Water) Heat Recovery Efficiency / 28.7% (Engine Jacket & Exhaut Gas) 181 kW(0.78MPa Saturated Steam) / 20.6%

NOx Emission Under 40ppm @C^=0%,Dry

Bearable Load Ratio 50% - 150 kW (from Idling) CONCLUSION (1 )We applied Miller Cycle technology to a three way catalyst specification gas engine for use in a cogeneration system and put it into practical use. An engine thermal efficiency of 36.1% was achieved. (2)We developed and commercialized a cogeneration package with an overall efficiency of 83.5% using a Miller Cycle gas engine as the prime mover.

REFERENCES (1) Fu-Rong Zhang et al., "Methods of Increasing the BMEP for Natural Gas SI Engines", SAE, No.981385, 1998 (2) K.Okamoto et al., "Development ofA High Performance Gas Engine Operating at A Stoichiometric Condition", CIMAC, No. 13.02, 1998 (3) K.Okamoto et al., "Study on Miller Cycle Gas Engine for Co-generation Systems", SAE, No.960949, 1996 (4) S.Shimogata et al., "Study on Miller Cycle Gas Engine for Co-generation Systems - Numerical Analysis for Improvement of Efficiency and Power SAE, No.971709, 1997 (5) K.Okamoto et al., "Development of A LIVC Miller Cycle for Stationary Natural Gas Engines", SAE, No.972948, 1997 (6) M.Takayanagi., "The Development of Genemaru AiO", The Cogeneration Japan, Vol. 13, No. 1, 1998.