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Development of noise reduction technologies for a small direct-injection diesel engine
JSAE Review 21 (2000) 327}333
Development of noise reduction technologies for a small direct-injection diesel engine Masahiko Kondo , Shuji Kimura , Izuho Hirano , Youichi Uraki , Ryoichi Maeda Powertrain Research Laboratory, Nissan Research Center, Nissan Motor Co., Ltd., 1, Natsushima-cho, Yokosuka-shi, Kanagawa 237-8523, Japan Powertrain Operations Division, Engine Engineering Department, Powertrain Performance Planning Group, Nissan Motor Co., Ltd., 6-1, Daikoku-cho, Yokohama-shi, Kanagawa 230-0053, Japan Received 5 November 1999; received in revised form 7 January 2000
Abstract The DI diesel engine has an advantage in terms of fuel economy, but disadvantages with respect to exhaust emissions and large combustion noise. To overcome these drawbacks, we have previously proposed the Modulated Kinetics (MK) concept [1,2] of low-temperature, premixed combustion. This paper presents the results of an investigation into the potential of a new combustion system to reduce combustion noise and improve emission performance simultaneously. As a result of applying heavy EGR and retarding the injection timing, combustion excitation forces are reduced without any increase in exhaust emissions, and with reduction of fuel injection system noise. 2000 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved.
1. Introduction E!orts are under way throughout the world today to prevent global warming. Reduction of carbon dioxide (CO ) emissions, representing a typical greenhouse gas, is being pursued with the aim of attaining concrete numerical targets. In this regard, what is required of future automotive engines is a substantial improvement in fuel economy. Against this backdrop, the demand for direct-injection (DI) diesel engines has been increasing in recent years. With their maximum thermal e$ciency exceeding 40%, DI diesels are clearly superior in fuel economy to gasoline engines and indirect-injection (IDI) diesels with an auxiliary swirl chamber. However, the high noise levels of DI diesel engines, due to their large combustion excitation forces, have prompted repeated demands for quieter operation. Another issue has been to achieve cleaner exhaust gas by reducing emissions of black smoke and nitrogen oxides (NO ). In general, the resolution of these issues requires V improvement of combustion, and vigorous e!orts have been directed toward that end over the years. However, delaying the fuel injection timing, which is e!ective for reducing NO , results in an increase in parV ticulate matter (PM). Similarly, exhaust gas recirculation (EGR) is also e!ective in reducing NO emissions as well V as the cylinder pressure level due to the shortening of the
ignition delay period. However, the level of EGR that can be applied is limited because it causes fuel economy and PM emissions to deteriorate. Increasing the pressure of the fuel injection system works to suppress smoke formation, but it also causes greater fuel injection system noise. Simultaneous improvement of these con#icting parameters is one technological challenge that engineers have been grappling with for many years. This paper presents the results of an investigation into the potential of a new combustion system, which has been described in detail elsewhere [1,2] to reduce combustion noise and improve emission performance simultaneously. It also touches on the reduction of fuel injection system noise.
2. Characteristics of combustion excitation forces and combustion noise The DI diesel engine investigated in this study is characterized by its large excitation forces in the low load region, the magnitude of which is shown in Fig. 1 in terms of the cylinder pressure level. The upper "gures indicate the change in the cylinder pressure as a function of the crank angle. The lower "gure shows the change in the 1-kHz component ( octave band) of the frequency characteristics of the cylinder pressure level in relation to the load, represented by the brake mean e!ective
0389-4304/00/$20.00 2000 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved. PII: S 0 3 8 9 - 4 3 0 4 ( 0 0 ) 0 0 0 5 2 - 7
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of the fuel injection system in particular is one example of such forces whose in#uence has become impossible to ignore in recent years. This is related to the e!ect of increasing the fuel injection pressure for the purpose of reducing exhaust emissions or achieving other improvements. For the DI diesel engine examined in this study, the excitation force of the fuel injection pump and that of the chain system for driving the pump were more dominant at full load than the contribution of the combustion excitation forces, as seen in Fig. 2 [3].
3. Reduction of combustion noise with MK combustion
Fig. 1. Cylinder pressure characteristics of DI and IDI diesel engines.
Fig. 2. Contributions of noise components (2000 rpm) [3].
pressure. The cylinder pressure rises sharply from the vicinity of top dead center in a brake mean e!ective pressure range of 300}500 kPa. The 1-kHz component, which is representative of combustion excitation forces, also shows a peak in this load region. As a result, the combustion excitation force level in the low load region substantially exceeds that of a comparable IDI diesel engine. It is observed that the excitation force level decreases as the load increases. Since the engine used in this study was a DI turbocharged diesel, the ignition delay period was shortened as the boost pressure rose, resulting in a smooth waveform for the rise in combustion pressure in the indicator diagram, which would account for this characteristic. This cylinder pressure level at high load is approximately equal to that of a conventional IDI diesel engine. Excitation forces apart from those attributed to combustion are also present in an engine. The excitation force
Based on the foregoing results, an investigation was made of the possibility of reducing noise in the partialload region while at the same time improving emission performance. The engine used was a 2.5-l intercooled turbocharged diesel. A brief explanation is given here of the modulated kinetics (MK) combustion system. MK combustion is a low temperature, premixed combustion process. Heavy ERG is applied to reduce NO emissions while an inV crease in smoke is avoided by carrying out premixed combustion, which is accomplished by delaying the fuel injection timing so as to lengthen the ignition delay period and thereby prevent smoke formation. However, simply injecting fuel after top dead center would invite deterioration of fuel economy and higher hydrocarbon (HC) emissions. To avoid those undesirable e!ects, cooling looses are reduced and in-cylinder gas #ow is strengthened by optimizing the combustion chamber shape and intake port geometry. As the details of the MK combustion system were described in previous reports [1,2] we will next consider the e!ect of EGR and injection timing delay on reducing combustion excitation forces, as shown in Figs. 3 and 4. The results in Fig. 3 are for a condition close to a conventional combustion process without the application of EGR. It is seen that delaying the injection timing
Fig. 3. Reduction of CPL without EGR.
M. Kondo et al. / JSAE Review 21 (2000) 327}333
329
Fig. 4. Reduction of CPL with EGR. Fig. 7. E!ect of injection delay on CPL.
Fig. 5. Reduction of combustion noise without EGR.
Fig. 8. E!ect of injection delay on combustion noise.
Fig. 6. Reduction of combustion noise with EGR.
reduces excitation forces in the high-frequency region above approximately 1 kHz. Fig. 4 shows the results obtained when the injection timing was delayed under a condition of rather heavy EGR (44%). The e!ect on reducing excitation forces is evident from a lower frequency region than in Fig. 3, and the reduction of excitation forces shows greater sensitivity to the injection timing. The combustion noise is shown in Figs. 5 and 6. These noises were calculated by multiplying the struc-
ture-borne noise transmission characteristics, which were obtained by a multiple regression analysis [3], and the combustion excitation forces. With this regression analysis, the engine noise was treated as a criterion variable and cylinder pressure and engine load as explanatory variables. Consequently, not only are large e!ects seen in the frequency region from 500 Hz to 1 kHz under the application of heavy EGR (Fig. 6), but MK combustion also has a substantial e!ect on reducing combustion noise in the higher frequency region. The foregoing results are summarized in Figs. 7 and 8 as a function of the fuel injection timing shown along the horizontal axis. Combustion excitation forces (CPL) and combustion noise show changes in the 1 kHz component and overall changes of 50 Hz to 2.5 kHz. One distinct feature of these results is that excitation forces are markedly reduced under the application of EGR when the injection timing is delayed until after top dead center. The combustion characteristics obtained under these operating conditions are precisely those of MK combustion.
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that is subsequently followed by a second heat release peak. Because these two peaks are controlled in the MK combustion process, the maximum cylinder pressure and the cylinder pressure gradient are both reduced, thereby achieving a lower level of combustion noise.
4. Example of MK combustion system application 4.1. Reduction of steady-state combustion noise Fig. 9. Emission reduction with MK concept.
Fig. 11 shows an example of the use of the MK combustion system at a vehicle speed of 40 km/h and road load, driving conditions that represent a relatively low engine load. The results indicate the e!ect of a heavier EGR rate on the sound power level plotted along the vertical axis. The small inserted graph shows the cylinder pressure level (vertical axis) relative to the EGR rate (horizontal axis). The combustion excitation force level is shown for each EGR rate from 0 to 50%. It is observed that the excitation force level declines nearly uniformly until an EGR rate of 37%, but the rate of reduction increases sharply when the EGR rate is raised from 44% to 50%. However, the reduction of radiation noise saturates and reaches a region where fuel injection system noise and mechanical noise have a relatively larger e!ect than that of combustion excitation forces. Accordingly, further noise reductions must be pursued together with measures to reduce the mechanical system contributions. 4.2. Reduction of transient knock
Fig. 10. E!ect of MK concept on combustion rate.
The emission performance achieved under these operating conditions is shown in Fig. 9. Ordinarily, NO and V PM emissions require a trade-o!. In the range where EGR is not applied, delaying the injection timing has some e!ect on reducing NO emissions, but the PM level V increases. To overcome that trade-o!, the injection timing is delayed in the MK combustion process under the application of heavy EGR. Whereas the PM level initially increases, it changes to a reduction as a result of delaying fuel injection until ATDC. With a crank angle of 43 ATDC, PM emissions fall to the same level as when EGR is not applied. Accordingly, the MK combustion concept simultaneously reduces combustion noise and improves emission performance. Fig. 10 shows the change in cylinder pressure and heat release rate in relation to the crank angle. The heat release waveform with MK combustion nearly forms an isosceles triangle, indicating a single stage of combustion. In a conventional direct-injection diesel engine, the presence of both premixed combustion and di!usion combustion results in rapid heat release in the initial period
Transient knock is one index of noise performance that has a signi"cant e!ect on the customer appeal quality of a product. A major issue in previous combustion con-
Fig. 11. E!ect of EGR on sound power level.
M. Kondo et al. / JSAE Review 21 (2000) 327}333
cepts has been to reduce transient knock induced by the rapid rate of combustion that occurs due to a longer ignition delay period. That prolongation is attributed to the combustion chamber wall temperature being lower under transient operation than under steady-state operation. In this study, an investigation was made of the possibility of reducing knock in the low-temperature, premixed MK combustion process that requires a long ignition delay period. Using a stepper motor to control the accelerator wire, tests were conducted on an engine dynamometer in the same manner as acceleration tests carried out with an actual vehicle. Evaluations were made under three di!erent engine control modes. In control mode A, used as the baseline, the EGR valve was closed right after the throttle valve was opened. In control mode B, EGR was applied according to an engine map until nearly the point where smoke would form under transient operation. In control mode C, the EGR valve was kept open as in mode B and also the injection timing was delayed even though the acceleration response was dulled somewhat. As shown in Fig. 12, the results con"rmed that it was still possible to reduce engine noise by 2}3 dB even during transient operation. Large perceptible knock occurred when combustion proceeded rapidly, as indicated by the steep rise in the cylinder pressure waveform. In contrast, when knock was suppressed by the application of heavy EGR, the trough in the cylinder pressure waveform, formed by compressive pressure and combustion pressure, changed from a V-shape to a U-shape. It should be noted that transient emission performance was not evaluated in this investigation. However, it is thought that the second generation of the MK combustion system [4] which is designed to expand the MK combustion region, will be e!ective in reducing transient knock in a manner that also takes into account emission performance, because EGR can be applied until higher engine revolution and load without any increase in PM emissions.
Fig. 12. E!ect of EGR & IT on transient knock.
331
5. Reduction of fuel injection system noise The discussion so far has dealt with techniques for reducing noise in a relatively low-load range from idling to a medium-load region. However, as shown in Fig. 2, fuel injection system noise or mechanical noise originating in components that drive the injection system has a large e!ect on the overall sound pressure level under full-load operation, where the contribution of combustion excitation forces is small. Therefore, as the "rst step toward reducing such noise, an investigation was made of ways of lessening the excitation force of the distributor-type fuel injection pump that is the largest source of excitation in the full-load region. The vibration level of two pumps is compared in Fig. 13. A newly developed pump reduces the vibration level substantially by moderating the fuel delivery rate so as to suppress the peak drive torque. This works to reduce not only the direct noise of the pump, but also indirect radiation noise from the cylinder block and other parts due to the injection pump excitation. It should be noted that the same level of fuel injection pressure has been maintained by minimizing the volume of the fuel lines, among other measures. With this DI diesel engine, the valve train and the fuel injection pump are driven by a chain positioned at the front of the engine. Under full-load operation, the large torque applied to drive the pump became an excitation force that caused the chain cover to produce noise (Fig. 14). As an initial measure for resolving that problem, the structure of the chain cover was optimized with aim of suppressing the amplitude of the cover's panel vibration mode. However, because of the magnitude of the highfrequency excitation forces and layout restrictions, the e!ect on reducing chain cover noise was limited. Accordingly, an attempt was made to isolate the chain cover from chain and injection pump excitation forces. Fig. 15 shows the resulting e!ect on reducing noise. The soft
Fig. 13. Injection pump vibration reduction.
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Fig. 14. Sound intensity distribution at front of engine.
Fig. 16. Full load engine noise scatterbands. Fig. 15. E!ect of soft mounting and structural optimization.
mounting of the cover with molded seal rubber substantially reduced the noise caused by the panel vibration mode that occurs due to cylinder block vibration and chain drive excitation forces. The full-load engine noise level that was obtained as a result of applying these measures is shown in Fig. 16. The solid boldface line is for the newly developed DI diesel engine, which shows the lowest noise level among current diesel engines. Its noise level is so low that it enters the region generally seen for gasoline engines.
6. Conclusion This paper describes a new combustion system designed to reduce the combustion noise and exhaust emissions of a small direct-injection diesel engine while continuing its excellent fuel economy. The results of this study are summarized below: (1) Delaying fuel injection under the application of heavy EGR makes it possible to reduce combustion excitation forces in the frequency region above 500 Hz. The
M. Kondo et al. / JSAE Review 21 (2000) 327}333
reduction level in the 1 kHz component is approximately 10 dB. The resulting excitation force level is nearly comparable to that of IDI diesel engines. (2) Combustion noise at that point is reduced by 8 dB and the overall engine noise level, taking into account the contributions of the fuel injection system and other sources, is reduced by 2}3 dB. Additionally, perceptible transient knock is also reduced. (3) This combustion system reduces the partial-load combustion excitation forces characteristic of DI diesel engines, and is an e!ective improvement measure that has minimal repercussions on exhaust emission performance. Moreover, steps were taken to reduce the mechanical noise of the fuel injection system under full-load
333
operation, thereby achieving a noise level approaching that of gasoline engines. References [1] Y. Matsui et al., A new combustion concept for small DI diesel engines-1st report: introduction of the basic technology (in Japanese), Trans. JSAE, Vol. 28, No. 1 (1997). [2] S. Kimura et al., A new combustion concept for small DI diesel engines-2nd report: e!ects on engine performance (in Japanese), Trans. JSAE, Vol. 28, No. 2 (1997). [3] I. Hirano et al., Using multiple regression analysis to estimate the contributions of engine-radiated noise, JSAE Rev., Vol. 20, No. 3 (1999). [4] S. Kimura et al., Development of a new diesel engine concept based on second-generation MK combustion (in Japanese), Proceedings of Autumn Convention, No. 106 (1998).
Development of noise reduction technologies for a small direct-injection diesel engine Masahiko Kondo , Shuji Kimura , Izuho Hirano , Youichi Uraki , Ryoichi Maeda Powertrain Research Laboratory, Nissan Research Center, Nissan Motor Co., Ltd., 1, Natsushima-cho, Yokosuka-shi, Kanagawa 237-8523, Japan Powertrain Operations Division, Engine Engineering Department, Powertrain Performance Planning Group, Nissan Motor Co., Ltd., 6-1, Daikoku-cho, Yokohama-shi, Kanagawa 230-0053, Japan Received 5 November 1999; received in revised form 7 January 2000
Abstract The DI diesel engine has an advantage in terms of fuel economy, but disadvantages with respect to exhaust emissions and large combustion noise. To overcome these drawbacks, we have previously proposed the Modulated Kinetics (MK) concept [1,2] of low-temperature, premixed combustion. This paper presents the results of an investigation into the potential of a new combustion system to reduce combustion noise and improve emission performance simultaneously. As a result of applying heavy EGR and retarding the injection timing, combustion excitation forces are reduced without any increase in exhaust emissions, and with reduction of fuel injection system noise. 2000 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved.
1. Introduction E!orts are under way throughout the world today to prevent global warming. Reduction of carbon dioxide (CO ) emissions, representing a typical greenhouse gas, is being pursued with the aim of attaining concrete numerical targets. In this regard, what is required of future automotive engines is a substantial improvement in fuel economy. Against this backdrop, the demand for direct-injection (DI) diesel engines has been increasing in recent years. With their maximum thermal e$ciency exceeding 40%, DI diesels are clearly superior in fuel economy to gasoline engines and indirect-injection (IDI) diesels with an auxiliary swirl chamber. However, the high noise levels of DI diesel engines, due to their large combustion excitation forces, have prompted repeated demands for quieter operation. Another issue has been to achieve cleaner exhaust gas by reducing emissions of black smoke and nitrogen oxides (NO ). In general, the resolution of these issues requires V improvement of combustion, and vigorous e!orts have been directed toward that end over the years. However, delaying the fuel injection timing, which is e!ective for reducing NO , results in an increase in parV ticulate matter (PM). Similarly, exhaust gas recirculation (EGR) is also e!ective in reducing NO emissions as well V as the cylinder pressure level due to the shortening of the
ignition delay period. However, the level of EGR that can be applied is limited because it causes fuel economy and PM emissions to deteriorate. Increasing the pressure of the fuel injection system works to suppress smoke formation, but it also causes greater fuel injection system noise. Simultaneous improvement of these con#icting parameters is one technological challenge that engineers have been grappling with for many years. This paper presents the results of an investigation into the potential of a new combustion system, which has been described in detail elsewhere [1,2] to reduce combustion noise and improve emission performance simultaneously. It also touches on the reduction of fuel injection system noise.
2. Characteristics of combustion excitation forces and combustion noise The DI diesel engine investigated in this study is characterized by its large excitation forces in the low load region, the magnitude of which is shown in Fig. 1 in terms of the cylinder pressure level. The upper "gures indicate the change in the cylinder pressure as a function of the crank angle. The lower "gure shows the change in the 1-kHz component ( octave band) of the frequency characteristics of the cylinder pressure level in relation to the load, represented by the brake mean e!ective
0389-4304/00/$20.00 2000 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved. PII: S 0 3 8 9 - 4 3 0 4 ( 0 0 ) 0 0 0 5 2 - 7
JSAE20004142
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M. Kondo et al. / JSAE Review 21 (2000) 327}333
of the fuel injection system in particular is one example of such forces whose in#uence has become impossible to ignore in recent years. This is related to the e!ect of increasing the fuel injection pressure for the purpose of reducing exhaust emissions or achieving other improvements. For the DI diesel engine examined in this study, the excitation force of the fuel injection pump and that of the chain system for driving the pump were more dominant at full load than the contribution of the combustion excitation forces, as seen in Fig. 2 [3].
3. Reduction of combustion noise with MK combustion
Fig. 1. Cylinder pressure characteristics of DI and IDI diesel engines.
Fig. 2. Contributions of noise components (2000 rpm) [3].
pressure. The cylinder pressure rises sharply from the vicinity of top dead center in a brake mean e!ective pressure range of 300}500 kPa. The 1-kHz component, which is representative of combustion excitation forces, also shows a peak in this load region. As a result, the combustion excitation force level in the low load region substantially exceeds that of a comparable IDI diesel engine. It is observed that the excitation force level decreases as the load increases. Since the engine used in this study was a DI turbocharged diesel, the ignition delay period was shortened as the boost pressure rose, resulting in a smooth waveform for the rise in combustion pressure in the indicator diagram, which would account for this characteristic. This cylinder pressure level at high load is approximately equal to that of a conventional IDI diesel engine. Excitation forces apart from those attributed to combustion are also present in an engine. The excitation force
Based on the foregoing results, an investigation was made of the possibility of reducing noise in the partialload region while at the same time improving emission performance. The engine used was a 2.5-l intercooled turbocharged diesel. A brief explanation is given here of the modulated kinetics (MK) combustion system. MK combustion is a low temperature, premixed combustion process. Heavy ERG is applied to reduce NO emissions while an inV crease in smoke is avoided by carrying out premixed combustion, which is accomplished by delaying the fuel injection timing so as to lengthen the ignition delay period and thereby prevent smoke formation. However, simply injecting fuel after top dead center would invite deterioration of fuel economy and higher hydrocarbon (HC) emissions. To avoid those undesirable e!ects, cooling looses are reduced and in-cylinder gas #ow is strengthened by optimizing the combustion chamber shape and intake port geometry. As the details of the MK combustion system were described in previous reports [1,2] we will next consider the e!ect of EGR and injection timing delay on reducing combustion excitation forces, as shown in Figs. 3 and 4. The results in Fig. 3 are for a condition close to a conventional combustion process without the application of EGR. It is seen that delaying the injection timing
Fig. 3. Reduction of CPL without EGR.
M. Kondo et al. / JSAE Review 21 (2000) 327}333
329
Fig. 4. Reduction of CPL with EGR. Fig. 7. E!ect of injection delay on CPL.
Fig. 5. Reduction of combustion noise without EGR.
Fig. 8. E!ect of injection delay on combustion noise.
Fig. 6. Reduction of combustion noise with EGR.
reduces excitation forces in the high-frequency region above approximately 1 kHz. Fig. 4 shows the results obtained when the injection timing was delayed under a condition of rather heavy EGR (44%). The e!ect on reducing excitation forces is evident from a lower frequency region than in Fig. 3, and the reduction of excitation forces shows greater sensitivity to the injection timing. The combustion noise is shown in Figs. 5 and 6. These noises were calculated by multiplying the struc-
ture-borne noise transmission characteristics, which were obtained by a multiple regression analysis [3], and the combustion excitation forces. With this regression analysis, the engine noise was treated as a criterion variable and cylinder pressure and engine load as explanatory variables. Consequently, not only are large e!ects seen in the frequency region from 500 Hz to 1 kHz under the application of heavy EGR (Fig. 6), but MK combustion also has a substantial e!ect on reducing combustion noise in the higher frequency region. The foregoing results are summarized in Figs. 7 and 8 as a function of the fuel injection timing shown along the horizontal axis. Combustion excitation forces (CPL) and combustion noise show changes in the 1 kHz component and overall changes of 50 Hz to 2.5 kHz. One distinct feature of these results is that excitation forces are markedly reduced under the application of EGR when the injection timing is delayed until after top dead center. The combustion characteristics obtained under these operating conditions are precisely those of MK combustion.
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that is subsequently followed by a second heat release peak. Because these two peaks are controlled in the MK combustion process, the maximum cylinder pressure and the cylinder pressure gradient are both reduced, thereby achieving a lower level of combustion noise.
4. Example of MK combustion system application 4.1. Reduction of steady-state combustion noise Fig. 9. Emission reduction with MK concept.
Fig. 11 shows an example of the use of the MK combustion system at a vehicle speed of 40 km/h and road load, driving conditions that represent a relatively low engine load. The results indicate the e!ect of a heavier EGR rate on the sound power level plotted along the vertical axis. The small inserted graph shows the cylinder pressure level (vertical axis) relative to the EGR rate (horizontal axis). The combustion excitation force level is shown for each EGR rate from 0 to 50%. It is observed that the excitation force level declines nearly uniformly until an EGR rate of 37%, but the rate of reduction increases sharply when the EGR rate is raised from 44% to 50%. However, the reduction of radiation noise saturates and reaches a region where fuel injection system noise and mechanical noise have a relatively larger e!ect than that of combustion excitation forces. Accordingly, further noise reductions must be pursued together with measures to reduce the mechanical system contributions. 4.2. Reduction of transient knock
Fig. 10. E!ect of MK concept on combustion rate.
The emission performance achieved under these operating conditions is shown in Fig. 9. Ordinarily, NO and V PM emissions require a trade-o!. In the range where EGR is not applied, delaying the injection timing has some e!ect on reducing NO emissions, but the PM level V increases. To overcome that trade-o!, the injection timing is delayed in the MK combustion process under the application of heavy EGR. Whereas the PM level initially increases, it changes to a reduction as a result of delaying fuel injection until ATDC. With a crank angle of 43 ATDC, PM emissions fall to the same level as when EGR is not applied. Accordingly, the MK combustion concept simultaneously reduces combustion noise and improves emission performance. Fig. 10 shows the change in cylinder pressure and heat release rate in relation to the crank angle. The heat release waveform with MK combustion nearly forms an isosceles triangle, indicating a single stage of combustion. In a conventional direct-injection diesel engine, the presence of both premixed combustion and di!usion combustion results in rapid heat release in the initial period
Transient knock is one index of noise performance that has a signi"cant e!ect on the customer appeal quality of a product. A major issue in previous combustion con-
Fig. 11. E!ect of EGR on sound power level.
M. Kondo et al. / JSAE Review 21 (2000) 327}333
cepts has been to reduce transient knock induced by the rapid rate of combustion that occurs due to a longer ignition delay period. That prolongation is attributed to the combustion chamber wall temperature being lower under transient operation than under steady-state operation. In this study, an investigation was made of the possibility of reducing knock in the low-temperature, premixed MK combustion process that requires a long ignition delay period. Using a stepper motor to control the accelerator wire, tests were conducted on an engine dynamometer in the same manner as acceleration tests carried out with an actual vehicle. Evaluations were made under three di!erent engine control modes. In control mode A, used as the baseline, the EGR valve was closed right after the throttle valve was opened. In control mode B, EGR was applied according to an engine map until nearly the point where smoke would form under transient operation. In control mode C, the EGR valve was kept open as in mode B and also the injection timing was delayed even though the acceleration response was dulled somewhat. As shown in Fig. 12, the results con"rmed that it was still possible to reduce engine noise by 2}3 dB even during transient operation. Large perceptible knock occurred when combustion proceeded rapidly, as indicated by the steep rise in the cylinder pressure waveform. In contrast, when knock was suppressed by the application of heavy EGR, the trough in the cylinder pressure waveform, formed by compressive pressure and combustion pressure, changed from a V-shape to a U-shape. It should be noted that transient emission performance was not evaluated in this investigation. However, it is thought that the second generation of the MK combustion system [4] which is designed to expand the MK combustion region, will be e!ective in reducing transient knock in a manner that also takes into account emission performance, because EGR can be applied until higher engine revolution and load without any increase in PM emissions.
Fig. 12. E!ect of EGR & IT on transient knock.
331
5. Reduction of fuel injection system noise The discussion so far has dealt with techniques for reducing noise in a relatively low-load range from idling to a medium-load region. However, as shown in Fig. 2, fuel injection system noise or mechanical noise originating in components that drive the injection system has a large e!ect on the overall sound pressure level under full-load operation, where the contribution of combustion excitation forces is small. Therefore, as the "rst step toward reducing such noise, an investigation was made of ways of lessening the excitation force of the distributor-type fuel injection pump that is the largest source of excitation in the full-load region. The vibration level of two pumps is compared in Fig. 13. A newly developed pump reduces the vibration level substantially by moderating the fuel delivery rate so as to suppress the peak drive torque. This works to reduce not only the direct noise of the pump, but also indirect radiation noise from the cylinder block and other parts due to the injection pump excitation. It should be noted that the same level of fuel injection pressure has been maintained by minimizing the volume of the fuel lines, among other measures. With this DI diesel engine, the valve train and the fuel injection pump are driven by a chain positioned at the front of the engine. Under full-load operation, the large torque applied to drive the pump became an excitation force that caused the chain cover to produce noise (Fig. 14). As an initial measure for resolving that problem, the structure of the chain cover was optimized with aim of suppressing the amplitude of the cover's panel vibration mode. However, because of the magnitude of the highfrequency excitation forces and layout restrictions, the e!ect on reducing chain cover noise was limited. Accordingly, an attempt was made to isolate the chain cover from chain and injection pump excitation forces. Fig. 15 shows the resulting e!ect on reducing noise. The soft
Fig. 13. Injection pump vibration reduction.
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Fig. 14. Sound intensity distribution at front of engine.
Fig. 16. Full load engine noise scatterbands. Fig. 15. E!ect of soft mounting and structural optimization.
mounting of the cover with molded seal rubber substantially reduced the noise caused by the panel vibration mode that occurs due to cylinder block vibration and chain drive excitation forces. The full-load engine noise level that was obtained as a result of applying these measures is shown in Fig. 16. The solid boldface line is for the newly developed DI diesel engine, which shows the lowest noise level among current diesel engines. Its noise level is so low that it enters the region generally seen for gasoline engines.
6. Conclusion This paper describes a new combustion system designed to reduce the combustion noise and exhaust emissions of a small direct-injection diesel engine while continuing its excellent fuel economy. The results of this study are summarized below: (1) Delaying fuel injection under the application of heavy EGR makes it possible to reduce combustion excitation forces in the frequency region above 500 Hz. The
M. Kondo et al. / JSAE Review 21 (2000) 327}333
reduction level in the 1 kHz component is approximately 10 dB. The resulting excitation force level is nearly comparable to that of IDI diesel engines. (2) Combustion noise at that point is reduced by 8 dB and the overall engine noise level, taking into account the contributions of the fuel injection system and other sources, is reduced by 2}3 dB. Additionally, perceptible transient knock is also reduced. (3) This combustion system reduces the partial-load combustion excitation forces characteristic of DI diesel engines, and is an e!ective improvement measure that has minimal repercussions on exhaust emission performance. Moreover, steps were taken to reduce the mechanical noise of the fuel injection system under full-load
333
operation, thereby achieving a noise level approaching that of gasoline engines. References [1] Y. Matsui et al., A new combustion concept for small DI diesel engines-1st report: introduction of the basic technology (in Japanese), Trans. JSAE, Vol. 28, No. 1 (1997). [2] S. Kimura et al., A new combustion concept for small DI diesel engines-2nd report: e!ects on engine performance (in Japanese), Trans. JSAE, Vol. 28, No. 2 (1997). [3] I. Hirano et al., Using multiple regression analysis to estimate the contributions of engine-radiated noise, JSAE Rev., Vol. 20, No. 3 (1999). [4] S. Kimura et al., Development of a new diesel engine concept based on second-generation MK combustion (in Japanese), Proceedings of Autumn Convention, No. 106 (1998).