Energy Fuels 2009, 23, 5460–5466 Published on Web 09/09/2009
: DOI:10.1021/ef900611t
Performance and Emission Characteristics of DME Engine with High Ratio of EGR Ruizhi Song, Ke Li, Yang Feng, and Shenghua Liu* School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, 710049, People’s Republic of China Received June 15, 2009. Revised Manuscript Received August 15, 2009
The combustion of dimethyl ether (DME) fuel, due to its simple chemical structure and high oxygen content, produces less exhaust gaseous products such as HC, CO and NOx. Moreover, soot-free combustion can be observed under all of the operating conditions, which allows a high ratio of exhaust gas recirculation (EGR) for NOx reduction without being restricted by the NOx-soot tradeoff. The effects of a high ratio of exhaust gas recirculation on performance and exhaust emissions from a DME engine were investigated. The results showed that EGR ratio has significant effects on power output and fuel consumption. NOx emission is considerably reduced with the decrease of oxygen concentration, and a higher level of EGR results in a greater reduction in NOx emission. Ultralow NOx emissions have been realized under a high ratio of EGR operations, however, accompanied with the increase of HC and CO emissions. High EGR ratios need to be applied under low loads, but low EGR ratios are sufficient for high loads. Considering all of the operating conditions, optimized EGR ratios were selected, and the tests were conducted under the European Steady-State Test Cycle. As expected, NOx emissions from a DME engine can easily meet the Euro IV limit with optimized EGR ratios.
apply it to the engine with a pump-line-nozzle fuel injection system. Alam et al.9 studied the performance of the catalysts designed for NOx reduction, such as Co-alumina and Snalumina catalysts. The results show that the NOx reduction rate strongly depends on the reaction temperature and DME content in exhaust. Exhaust gas recirculation (EGR) is a well known and established in-cylinder technology to reduce NOx emissions, particularly on modern direct injection compression ignition (DICI) engines.10,11 Many studies about the combustion and emission reduction of compression ignition engines combined with EGR have been carried out. Tsolakis et al.12 investigated the performance and emissions of a diesel engine operating on diesel-rapeseed methyl ester (RME) blends with a low ratio of EGR (not exceeding 20%). Saleh13 conducted the experimental investigations on the effects of EGR on diesel engine NOx reduction operating with jojoba methyl ester, and the maximum EGR ratio can reach about 45%. Zhang et al.14 used a common rail direct injection diesel engine equipped with an EGR system to study the impact of biodiesel on NOx emissions. All of the above researchers have proved that EGR is an effective method for NOx reduction, although it may result in an increase in fuel consumption and smoke. Thus far, most of the present research has focused on diesel and other alternative fuels. The knowledge accumulated as to the effects of EGR on DME engine was very limited. Wang et al.15 conducted the research on exhaust emissions from a multicylinder DME engine operating with EGR. Results indicate
1. Introduction Despite the fuel economy advantage, diesel engine faces great challenges when trying to comply with the increasingly stringent emission regulations, especially soot and NOx emissions. To meet this demand, very complicated technologies have been applied, such as an electronically controlled high pressure common rail injection system or a selective catalytic reduction (SCR) postcombustion treatment system. However, investigations have been carried out by using oxygenated alternative fuels and the new concept of engine combustion technology.1-4 Dimethyl ether (DME), as a kind of promising alternative fuel for use in diesel engines, has recently received considerable attention. Because of DME’s high cetane number, it can be easily compressed for autoignition. In addition, its simple chemical structure and high oxygen content (around 35% by mass) result in soot-free combustion in engines.5-7 However, DME engines still suffer from their high nitrogen oxide (NOx) emissions. For the successful use of DME with respect to clean combustion, researchers have largely focused on NOx emissions. Teng et al.8 proposed a new fuel injection strategy on the basis of the physicochemical properties of DME to reduce NOx emissions from DME engines, but a very complex fuel injection system must be developed, and it is impossible to *To whom correspondence should be addressed. E-mail: shenghua@ mail.xjtu.edu.cn. (1) Knecht, W. Energy 2008, 33, 264–271. (2) Johnson, T. V. SAE Paper 2006-01-0030, 2006. (3) Lilik, G. K.; Herreros, J. M.; Boehman, A. L. Energy Fuels 2009, 23, 143–150. (4) Rakopoulos, C. D.; Hountalas, D. T.; Zannis, T. C. Levendis, Y. A. SAE Paper 2004-01-2924, 2004. (5) Arcoumanis, C.; Bae, C.; Crookes, R.; Kinoshita, E. Fuel 2008, 87 (7), 1014–1030. (6) Sorenson, S. C. ASME J. Eng. Gas Turbine Power 2001, 123, 652– 658. (7) Teng, H. McCandless, J. C. SAE Paper 2006-01-0053, 2006. (8) Teng, H. Regner, G. SAE Paper 2006-01-3324, 2006. r 2009 American Chemical Society
(9) Alam, M.; Fujita, O.; Ito, K.; Kajitani, S.; Oguma, M. Machida, H. SAE paper 1999-01-3599, 1999. (10) Abd-Alla, G. H. Energy Convers. Manage. 2002, 43, 1027–1042. (11) Zheng, M.; Reader, G. T.; Hawley, G. Energy Convers. Manage. 2004, 45, 883–900. (12) Tsolakis, A.; Megaritis, A.; Wyszynski, M. L.; Theinnoi, K. Energy 2007, 32, 2072–2080. (13) Saleh, H. E. Renewable Energy 2009, 34 (10), 2178–2186. (14) Zhang, Y.; Boehman, A. L. Energy Fuels 2007, 21, 2003–2012. (15) Wang, Y.; Zhou, L. Appl. Therm. Eng. 2008, 28, 1589–1595.
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: DOI:10.1021/ef900611t
Song et al. Table 1. Test Engine Specifications
combustion chamber bore stoke cylinder number displacement compression ratio rated power/speed rated torque/speed plunger diameter nozzle hole diameter number of nozzle holes nozzle opening pressure fuel delivery advance angle
diesel engine
DME engine
ω type 102 115 mm 2 1880 cm3 17.5 26.1 kW/2700 r 3 min-1 110.2 N 3 m/1400 r 3 min-1 8.5 mm 0.27 mm 4 19.1 MPa 25 °CA BTDC
r r r r r 26.8 kW/2800 r 3 min-1 108.9 N 3 m/2000 r 3 min-1 10.5 mm 0.35 mm 5 18.0 MPa 22 °CA BTDC
Table 2. Main Properties of DME and Diesel Fuel chemical structure cetane number lower heating value stoichiometric air/fuel mass ratio autoignition temperature carbon content hydrogen content oxygen content
DME
diesel fuel
CH3-O-CH3 55-66 27.6 MJ/kg 8.9 235 °C 52.2% (m) 13.0% (m) 34.8% (m)
40-50 42.5 MJ/kg 14.3 250 °C 86.0% (m) 14.0% (m)
Figure 1. Schematic diagram of the test engine and EGR experimental apparatus.
that NOx emissions can be reduced about 40% at a 20% EGR ratio without visible smoke and deterioration in thermal efficiency. DME engines allow a higher EGR ratio for NOx reduction without being restricted by the NOx-soot tradeoff. Thus, the DME engine has a great potential for further decreasing NOx emissions by means of increasing the EGR ratio. The objective of this study is to systematically investigate the effects of high ratio of exhaust gas recirculation on performance and exhaust emissions from a DME engine. Furthermore, the effects of EGR ratio on DME engine performance and emissions are analyzed and compared with the results obtained from engine operations under the European Steady-State Test Cycle (ESC).
Figure 2. Comparisons of the power output of a DME engine and a diesel engine at full load.
2. Experimental Section 2.1. Test Engine. The experiments were conducted using a direct-injection, four-stroke, water-cooled compression ignition DME engine. The major specifications of the engine are listed in Table 1, and the setup of the test bench is shown in Figure 1. The static injection timing of the DME engine and original diesel engine was kept constant at 22 °CA and 25 °CA BTDC, respectively. The main properties of DME fuel and diesel are shown in Table 2. The comparison of engine power output is shown in Figure 2, and emission measurement results are shown in Figure 3. Obviously, the DME engine has a significant advantage in exhaust emissions when equal power is produced. In the experiments, EGR ratio is defined to be intake CO2 concentration EGR ratio ¼ 100% ð1Þ exhaust CO2 concentration
Figure 3. Comparisons of emission measurement results of a DME engine and adiesel engine under the ESC 13-mode test.
valve. By measuring CO2 concentration, the EGR ratio can be calculated. The system design enables the EGR ratio to be adjusted in the range of 0%-70%. 2.2. Test Apparatus and Conditions. The cylinder pressure was sensed by a water-cooled piezoelectric pressure transducer (Kistler 7061B). The charge output from this transducer was converted to an amplified voltage using a charge amplifier (Kistler 5011). The crankshaft position and TDC signals were obtained using an optical rotary encoder (Kistler 2013B), which was attached directly to the crankshaft to determine the cylinder pressure as a function of the crank angle. Two Siments SITRANS FC300 massflow meters were used to measure engine
The EGR system consists of an EGR pump, a throttle valve, a cooler and pipes, as shown in Figure 1. Exhaust gas is pumped to the EGR mixer and mixed with fresh air in it homogeneously. EGR ratio is controlled by the opening of the EGR throttle 5461
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: DOI:10.1021/ef900611t
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Table 3. Accuracies of the Instruments and the Uncertainties of the Experimentation measurements load speed AFR fuel consumption HC CO CO2 NOx
measurement range
accuracy of the instruments
0-600 N 3 m 0-8000 r 3 min-1 0-20 0-80 kg/h 0-50000 ppm 0-12% 0-20% 0-1000 ppm
(2.4 N 3 m (1 r 3 min-1 (0.3 (0.1% (1 ppm (0.001% (0.01% (1 ppm
measurements
uncertainties of the measurements (2.0% (6.7% (4.6% (0.8% (2.9%
fuel consumption HC CO CO2 NOx
Figure 4. ESC 13-mode test cycle of a DME engine.
Figure 5. Effect of EGR ratio on 100% load power at speeds A, B, and C of the ESC test.
DME consumption. One was set prior to and the other after the injection pump. The difference of the mass flow rates was the DME consumption rate. Engine emissions were measured online by the exhaust analyzer (Horiba MEXA 7100DEGR), in which HC was analyzed with a flame ionization detector (FID), CO and CO2 were analyzed with a nondispersive infrared analyzer (NDIR), and NOx was measured with a chemiluminescent detector (CLD). Smoke was detected with a smoke meter (AVL Dismoke4000) during the test process. The air-fuel ratio (AFR) was measured with a heating type universal exhaust gas oxygen (UEGO) AFR analyzer (Horiba Mexa-700λ). The analyzers were calibrated before experiments. The accuracies of the instruments and the uncertainties of the measurements are given in Table 3. On the basis of the ESC 13-mode test cycle, the DME engine test speeds A, B, and C are 1600 r 3 min-1, 2000 r 3 min-1, and 2400 r 3 min-1, respectively, and the corresponding bmep values under 100% loads are 0.73 MPa, 0.73 MPa, and 0.62 MPa, respectively, at speeds A, B, and C. The 13 operating modes are shown in Figure 4. The effects of exhaust gas recirculation on performance and exhaust emissions (NOx, HC, and CO) from the DME engine were studied and particularly under 50% load (bmep = 0.36 MPa) and 100% load (bmep = 0.73 MPa) at 2000 r 3 min-1.
Figure 6. Effect of EGR ratio on 100% load AFR at speeds A, B, and C of the ESC test.
intake charge to a large extent. Air-fuel ratio as a function of EGR ratio is shown in Figure 6. The general trend of the three curves is quite similar. AFR decreases with an increase of the EGR ratio. The reason is that more exhaust gas was introduced into the inlet with the increase of the EGR ratio. 3.1.2. Fuel Economy Performance. The equivalent brake specific fuel consumption (beq) is used to compare the economy of a DME engine. For convenience, DME is converted to be equivalent to diesel according to its lower heating value. It is evaluated by the following formula: HU, DME ð2Þ beq ¼ be, DME HU, diesel
3. Results and Discussion 3.1. Performance. 3.1.1. Engine Power Output. The effect of EGR ratio on the power output from the DME engine at speeds A, B, and C is shown in Figure 5. The decrease in power output was as high as 25.5%, 28.8%, and 35.4% when the EGR ratio was, respectively, 59.4%, 50%, and 48.8%, which gave the maximum decrease in the power output for the speed range studied. It indicates that the EGR ratio has a significant effect on the power output from the engine. This decrease is mainly due to the reduction of air-fuel ratio in the DME engine resulting from the recirculation of exhaust gas, which dilutes the oxygen concentration of the fresh
where be, DME, HU, DME, and HU, diesel are the brake specific fuel consumption of the DME engine, lower heating value of DME, and lower heating value of diesel, respectively. Figure 7 gives the effect of the EGR ratio on equivalent brake specific fuel consumption of a DME engine at 2000 r 3 min-1. 5462
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Figure 7. Effect of EGR ratio on fuel consumption at speed B of the ESC test (50% and 100% loads).
Figure 8. Effect of EGR ratio on air-fuel ratio at speed B of the ESC test (50% and 100% loads).
As expected, the equivalent brake specific fuel consumption increases with an increase of the EGR ratio under high load conditions, which also corresponds to the maximum power output as shown in Figure 5. However, fuel consumption can be improved by operating the engine below a certain EGR ratio (around 30%) under low load conditions. In general, EGR influences the DME engine fuel consumption by means of the following two aspects: (1) reduction in air-fuel ratio by EGR may lead to incomplete combustion, which results in increased fuel consumption. (2) Diluted mixture by EGR can lower the combustion temperature and thus the heat losses. Otherwise, pumping losses are also reduced with this EGR system. Figure 8 shows the relationship of EGR ratio and air-fuel ratio at 2000 r 3 min-1. When the EGR ratio is below a certain EGR ratio (around 30%) under low load conditions, although the increase of the EGR ratio dilutes the fresh charge to some extent, the overall air-fuel mixture still retains a relatively lean level. As observed, when the EGR ratio was 42.7%, the air-fuel ratio is still up to 13.9, much higher than the stoichiometric air-fuel mass ratio of DME fuel, and therefore has little impact on fuel consumption. At this time, factor 2 dominates, resulting in the decrease of the fuel consumption rate. However, when the engine runs under high load conditions, the mixture is relatively rich compared to that under low loads. Therefore, factor 1 plays a major role, and hence, the fuel consumption increases with the increase of the EGR ratio under high loads. 3.1.3. Pressure in-Cylinder and Rate of Pressure Rise. Hideyuki Ogawa et al.16 reported that there is a relationship between combustion noise and the maximum rate of pressure rise from diesel engine combined with ultra-high EGR. The
Figure 9. Comparisons of cylinder gas pressures under various conditions. (A) 2000 r 3 min-1 50% load (bmep = 0.36 MPa); (B) 2000 r 3 min-1 100% load (bmep = 0.73 MPa).
maximum pressure rise rate during combustion is presented as a measure of combustion noise. By cylinder pressure sampling and data analyzing, the effects of EGR ratio on DME engine pressure in-cylinder and the rate of pressure rise can be studied. Figures 9 and 10 show the comparisons of cylinder gas pressure and rate of pressure rise under various conditions, respectively. From the figures, it can be seen that the peak pressure and maximum rate of pressure rise decrease with the increase of EGR ratio, which indicates a reduction of combustion noise.
(16) Ogawa, H.; Miyamoto, N.; Shimizu, H. Kido, S. SAE paper 2006-01-1147, 2006.
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Figure 11. Comparisons of COVimep under various conditions. (A) 2000 r 3 min-1 50% load (bmep = 0.36 MPa); (B) 2000 r 3 min-1 100% load (bmep = 0.73 MPa).
Figure 10. Comparisons of rate of pressure rise under various conditions. (A) 2000 r 3 min-1 50% load (bmep = 0.36 MPa); (B) 2000 r 3 min-1 100% load (bmep = 0.73 MPa).
3.1.4. Coefficient of Variation in Indicated Mean Effective Pressure. Exhaust gas is recirculated to the engine and takes part in the combustion, which may make stable combustion more difficult to achieve. Because the engine was fueled with DME, the EGR ratio was not restricted by the NOx-soot tradeoff but by combustion instability, which was encountered in the tests. Figure 11 gives the comparisons of the coefficient of variation of indicated mean effective pressure (COVimep) under various conditions. From Figure 11, it can be found that when a high EGR ratio (around 50%) is adopted, COVimep is maintained at an acceptable level (less than 10%) at both test load conditions. However, when the EGR ratio further increases, the combustion becomes unstable, the COVimep increases, and the engine cannot keep its speed, even sudden flameout occurs. In this study, the ultrahigh EGR is conducted to investigate the utmost emission level from the DME engine. However, when the DME engine runs with optimized EGR ratios, COVimep keeps at an acceptable level. 3.2. Emissions. Two load conditions at speed B of the ESC 13-mode test were selected to study the effects of EGR ratio on DME engine emissions in detail. They were 50% load (bmep = 0.36 MPa) and 100% load (bmep = 0.73 MPa), respectively. HC, CO, and NOx were measured and analyzed, and no smoke was detected during the whole test process. 3.2.1. Effects of EGR Ratio on NOx Emissions. NOx emissions under two loads (50% and 100%) at the characteristic speed B (2000 r 3 min-1, close to the peak torque speed) are presented in Figure 12. As widely recognized, the formation of NOx is favored by high oxygen concentration, high combustion temperature, and longer time.17,18 As shown in Figure 12, NOx emission is considerably reduced with the decrease of
Figure 12. Effect of reduction in oxygen concentration by EGR on NOx emissions in a DME engine for speed B (2000 r 3 min-1).
oxygen concentration, and a higher ratio of EGR results in a greater reduction in NOx emission. The reason is that NOx formation can be reduced by diluting charge and lowering the charge temperature during combustion. In other words, combustion under lean oxygen and low temperature will suppress NOx formation. In addition, the effectiveness of NOx reduction by EGR also varies with the load. When the engine runs without the EGR system, NOx emission under high load is higher to some extent compared to that under low load conditions, and it has a more rapid reducing tendency with the increase of the EGR ratio. The main reason for the higher NOx reduction under high load is the lower oxygen concentration compared to that operating under low load conditions (for the same EGR level). 3.2.2. Effects of EGR Ratio on HC and CO Emissions. Figures 13 and 14 give the curves of HC and CO emissions versus excess air ratio, respectively. As might be expected, while EGR reduces NOx production, it simultaneously increases HC and CO emissions. HC emission increases with the reduction in excess air ratio by EGR under the same load operating conditions, which can be explained by the dilution effect of exhaust gas. Otherwise, EGR reduces the oxygen
(17) Heywood, J. B. Internal Combustion Engine Fundamentals, 1st ed.; McGraw-Hill: New York, 1988. (18) Hill, S. C.; Smoot, L. D. Prog. Energy Combust. Sci. 2000, 26 (4-6), 417–458.
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Figure 15. Optimized EGR ratios under ESC 13 test conditions. Figure 13. Effect of reduction in excess air ratio by EGR on HC emissions in a DME engine for speed B (2000 r 3 min-1).
Figure 14. Effect of reduction in excess air ratio by EGR on CO emissions in a DME engine for speed B (2000 r 3 min-1).
concentration in the charge and, consequently, the combustion pressure and temperature, which causes only partial oxidation of unburned HC to occur. Thus, unburned HC emissions relatively increase. Moreover, it is also observed that HC emission is greatly influenced by the load. Under high load conditions, HC emission from a DME engine is maintained at a higher level compared to that operating under low load conditions at the same EGR level. Emission of CO is mainly a function of excess air ratio. CO emission remarkably increases with the reduction in excess air ratio, as shown in Figure 14. The oxygen concentration lowers, and the oxidation of CO is suppressed with the increase of the EGR ratio; thus, an increased emission level of CO is detected. Particularly, CO emission is kept at a high level and has a sharp increase trend with the increase of the EGR ratio under full load conditions. Otherwise, the increasing HC emission also leads to a rapid rise of CO emission. 3.2.3. DME Engine Exhaust Emissions under the ESC 13-Mode Test Cycle. Test results indicate that high EGR ratios need to be applied under low loads but that low EGR ratios are sufficient for high loads. When operating at lower loads, a DME engine generally tolerates a higher EGR ratio because the engine runs with the higher oxygen concentration compared to that operating under high load conditions (for the same EGR level), as shown in Figure 8. However, the exhaust oxygen becomes scarce, and the maximum EGR ratio is limited under high load conditions, such as modes 2, 8, and 10 of the ESC test. Thus, considering all operating conditions in this study, the optimized EGR ratios are selected and illustrated in Figure 15. When the DME engine runs with the optimized EGR ratios, it should comply with
Figure 16. Comparisons of NOx emissions at speeds A, B, and C of the ESC test. (a) Without EGR; (b) with optimized EGR ratios.
the following principles. First, NOx emissions from the engine do not exceed 3.5 g/(kW 3 h) at any mode of the ESC test cycle, which is the limit of the Euro IV NOx emission standard. Second, if the first condition is met, it should make sure that HC and CO emissions are kept at the lowest level with the optimized EGR ratio. In this case, a DME engine runs with low EGR ratios under high load operating conditions and with high EGR ratios under low load operating conditions. As a result, an improved thermal efficiency as well as reduced NOx emissions can be reached, and tests prove that the DME engine operates smoothly with the optimized EGR ratios under the ESC test cycle. To compare the emissions of a DME engine with and without EGR, an electronic control unit (ECU) of the EGR system was employed for the study. A section control program of optimized EGR ratios was pre-stored in the microprocessor of the ECU to control the stepper motor that was directly connected with the EGR valve. Figure 16 shows NOx emissions from the DME engine with and without the EGR system under the ESC 13-mode test. With an optimized EGR ratio, NOx emissions are lower than the Euro IV limit 5465
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: DOI:10.1021/ef900611t
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low load conditions at 2000 r 3 min , which is mainly due to the reduced pumping work and heat loss to the cylinder walls. Otherwise, increasing the EGR ratio can result in a decrease of the maximum rate of pressure rise, which is considered as an indication of the reduction of the combustion noise level. (2) NOx emissions are considerably reduced with the decrease of oxygen concentration, and a higher ratio of EGR results in a greater reduction in the NOx emissions. Ultralow NOx emissions have been realized with a high ratio of EGR. However, CO and HC emissions increase when the EGR ratio is added. (3) It is better for the DME engine to run with low EGR ratio under high load operating conditions and with high EGR ratio under low load operating conditions. In this case, an improved thermal efficiency as well as reduced NOx emissions can be reached. Considering overall operating conditions, optimized EGR ratios were selected, and tests were conducted under the ESC 13-mode test cycle. As expected, NOx emissions from a DME engine can easily meet the Euro IV limit with optimized EGR ratios. (4) HC and CO emissions from the DME engine with optimized EGR ratios are higher than those of the DME engine without EGR. Thus, further experiments are needed to reduce HC and CO emissions by means of an oxidation catalytic converter. Moreover, no smoke emission from a DME engine was observed through the entire engine operational region.
Figure 17. Comparisons of NOx emissions (ESC 13).
Figure 18. Comparisons of exhaust gaseous emissions (ESC 13).
under all of the 13 modes; thus, we can ensure that NOx emissions from the engine would be lower than 3.5 g/(kW 3 h). Figure 17 gives the comparisons of NOx emissions under the ESC 13-mode test. As expected, NOx emissions from a DME engine can easily meet the Euro IV limit with optimized EGR ratios. Comparisons of exhaust gaseous emissions are shown in Figure 18. Observe the figure, accompanied with the significant reduction of NOx emissions and increase in HC and CO emissions. HC and CO emissions from DME combustion have been discussed in a previous paper.15 These two gaseous incomplete combustion products can be treated relatively easily with the oxidation catalyst in the after treatment. Therefore, when an oxidation catalyst was combined with the DME engine equipped with an EGR system, HC and CO emissions can be reduced considerably, and the hazardous emissions from a DME engine are promising to meet the Euro IV limit.
Nomenclature AFR = air-fuel ratio bmep = brake mean effective pressure (MPa) beq = diesel equivalent brake specific fuel consumption (g/(kW 3 h)) be, DME = brake specific fuel consumption of DME engine (g/(kW 3 h)) CA = crank angle (°) CI = compression ignition CO = carbon monoxide COVimep = coefficient of variation of indicated mean effective pressure DME = dimethyl ether DICI = direct injection compression ignition EGR = exhaust gas recirculation ESC = European steady-state test cycle HU, DME = lower heating value of DME (MJ/kg) HU, diesel = lower heating value of diesel (MJ/kg) HC = hydrocarbon NOx = nitrogen oxides ppm = parts per million Pe = brake power of the engine (kW) TDC = top dead center
4. Conclusions The performance and emissions of a compression ignition DME engine were investigated, and the main results can be concluded as follows. (1) The EGR ratio has a significant effect on the power output from the engine. The power output decreases with the increase of the EGR ratio. This decrease is mainly due to the reduction of AFR in a DME engine that results from the recirculation of exhaust gas. Fuel consumption can be improved by operating the engine below a certain EGR ratio (about 30%) under
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