Premixed diesel combustion offers the potential of high thermal efficiency and low emissions, however, because the rapid rate of pressure rise and short combustion durations are often associated with low temperature combustion processes, noise is also an issue. The reduction of combustion noise is a technical matter that needs separate attention. Engine noise research has been conducted experimentally with a premixed diesel engine and techniques for engine noise simulation have been developed. The engine employed in the research here is a supercharged, single cylinder DI diesel research engine with a high pressure common rail fuel injection system. In the experiments, the engine was operated at 1600 rpm and 2000 rpm, the engine noise was sampled by two microphones, and the sampled engine noise was averaged and analyzed by an FFT sound analyzer. The structural attenuation of the test engine was calculated from the power spectrum of the FFT analysis of the in-cylinder pressure wave data and the cross power spectrum of the sound pressure of the engine noise by the coherence method. With the heat release history approximated by the Wiebe function, the pressure history could be calculated from the fitted curves of the heat release, and the simulated combustion noise was calculated from the pressure history and structural attenuation.
Two simulations were conducted for this paper. Since combustion noise arises in a trade-off relation with thermal efficiency in general, the best heat release shape for low engine noise and high thermal efficiency was first investigated. The simulation parameters of the heat release shape are the crank angle at 50% burn (CA50), the maximum rate of heat release, the combustion duration, and the initial rise in the high temperature heat release. Based on the heat release data of the engine tests, initial conditions were set and the simulation was conducted with these initial conditions. The simulation results show that the CA50 and degree of constant volume are related to the thermal efficiency and the initial rise in the high temperature heat release is related to the engine noise. For the conditions studied in this research, the thermal efficiency is maximum when the CA50 of the heat release occurs at 4.0 °CA ATDC, and the optimum engine operation for lower engine noise with higher thermal efficiency can be achieved by a mild initial rise in the high temperature heat release (the parameter-M=2.0 in the Wiebe function) and a high degree of constant volume combustion.
The effects of the connecting rod/crank radius ratio (L/R ratio, Figure 10) on combustion noise and thermal efficiency were also investigated. The L/R ratio was changed from 2.5 to 8 in 0.5 steps, and the heat balance and combustion noise were calculated. The simulation results show that improvements in the thermal efficiency can be achieved with decreases in the L/R ratio, caused by the cooling loss reduction, but that L/R ratio changes do not result in a reduction in combustion noise. Further, the heat release shape for the optimum thermal efficiency and combustion noise was calculated from the points of the L/R ratio, maximum rate of heat release, and combustion duration.
The results of the pre-mixed diesel engine operation and the engine test data were compared with the simulation data. The parameters for this comparison were the intake oxygen content and the fuel injection timing. The engine test data shows that the parameter-M in the Wiebe function increases with delays in the CA50 position, and that there is a strong linear relation between the CA50 and the Wiebe parameter-M.