Gen Shibata

Improvements in thermal efficiency of premixed diesel combustion with optimization of combustion-related parameters and fuel volatilities

The influence of combustion-related parameters and fuel volatility on the premixed diesel combustion was experimentally investigated in a modern direct injection diesel engine, and the evaporation process of the fuel spray in the combustion chamber was analyzed with computational fluid dynamics simulation. By optimizing the fuel injection timing and the intake oxygen content, ultralow NOx and smokeless premixed diesel combustion with high thermal efficiency and acceptable levels of CO and total hydrocarbon emissions is possible with both diesel fuel and normal heptane. The optimum fuel injection timing for the indicated thermal efficiency is obtained when the fuel spray does not enter the squish area, maintaining the 50% heat release crank angle at around top dead center. The indicated thermal efficiencies reach the maximum at around 12% intake oxygen concentration. The indicated thermal efficiency in the premixed diesel combustion with normal heptane is slightly higher than with diesel fuel and is very similar to the conventional diesel combustion in a wide indicated mean effective pressure range of below 0.8 MPa. The indicated thermal efficiency decreases when advancing injection timings mainly due to the deterioration in the combustion efficiency when the fuel is injected to the outside of the piston cavity. The degree of decrease in the indicated thermal efficiency with advancing injection timings is more significant with diesel fuel than with normal heptane due to the wall wetting.

Influence of fuel properties on operational range and combustion characteristics of premixed diesel combustion with high volatility fuel

The effects of fuel properties including ignitability, volatility, and compositions on operational range and combustion characteristics of premixed diesel combustion with various high volatility model fuels and an ordinary diesel fuel were examined in a direct injection diesel engine. The indicated mean effective pressure was limited by knocking with high-intake oxygen concentrations and by unstable combustion or significant increases in CO and total hydrocarbon emissions with low-intake oxygen concentrations regardless of fuels. The fuel volatility has little effect on the combustion characteristics and the stable operational range in premixed diesel combustion. With increasing octane number, the combustion phasing is retarded, and higher intake oxygen concentrations can be employed within the tolerance limits of rapid combustion, expanding the stable premixed diesel combustion indicated mean effective pressure range. The operational range of premixed diesel combustion with normal heptane and toluene blend fuels shifts to higher intake oxygen concentrations when compared with primary reference fuels with the same research octane numbers, showing lower ignition characteristics than primary reference fuel. The silent, low-NOx, and smokeless operation with high thermal efficiency was possible with both primary reference fuel and normal heptane and toluene blend fuel when the intake oxygen concentration is optimized corresponding to indicated mean effective pressure.

Optimization of multiple heat releases in pre-mixed diesel engine combustion for high thermal efficiency and low combustion noise by a genetic-based algorithm method

The reduction of diesel combustion noise by multiple fuel injections maintaining high indicated thermal efficiency is an object of the research reported in this article. There are two aspects of multiple fuel injection effects on combustion noise reduction. One is the reduction of the maximum rate of pressure rise in each combustion, and the other is the noise reduction effects by the noise canceling spike combustion. The engine employed in the simulations and experiments is a supercharged, single-cylinder direct-injection diesel engine, with a high pressure common rail fuel injection system. Simulations to calculate the combustion noise and indicated thermal efficiency from the approximated heat release by Wiebe functions were developed. In two-stage high temperature heat release combustion, the combustion noise can be reduced; however, the combustion noise in amplification frequencies must be reduced to achieve further combustion noise reduction, and an additional heat release was added ahead of the two-stage high temperature heat release combustion in Test 1. The simulations of the resulting three-stage high temperature heat release combustion were conducted by changing the heating value of the first heat release. In Test 2 where the optimum heat release shape for low combustion noise and high indicated thermal efficiency was investigated and the role of each of the heat releases in the three-stage high temperature heat release combustion was discussed. In Test 3, a genetic-based algorithm method was introduced to avoid the time-consuming loss and great care in preparing the calculations in Test 2, and the optimum heat release shape and frequency characteristics for combustion noise by the genetic-based algorithm method were speedily calculated. The heat release occurs after the top dead center, and the indicated thermal efficiency and overall combustion noise were 50.5% and 86.4 dBA, respectively. Furthermore, the optimum number of fuel injections and heat release shape of multiple fuel injections to achieve lower combustion noise while maintaining the higher indicated thermal efficiency were calculated in Test 4. The results suggest that the constant pressure combustion after the top dead center by multiple fuel injections is the better way to lower combustion noise; however, the excess fuel injected leads to a lower indicated thermal efficiency because the degree of constant volume becomes deteriorates.

Ignition delays in diesel combustion and intake gas conditions

Ignition delays in diesel combustion under several intake gas conditions, including different oxygen concentrations changed with exhaust gas recirculation quantities and different intake gas temperatures, were measured for four cetane numbers and three compression ratios in a single-cylinder, naturally aspirated, direct injection diesel engine (bore: 110 mm, stroke: 106 mm, and stroke volume: 1007 cm3). The engine has a common rail fuel injection system which can be set to optional injection timings and has an injector with a needle lift sensor to accurately estimate the injection timing. The intake oxygen concentrations were set by the quantity of exhaust gas recirculation gas, and the intake gas temperatures were changed with a water-cooled exhaust gas recirculation cooler and an electric heater in the intake pipe. Three compression ratios, 16.7, 18.0, and 21.3, were established with three pistons of different cavity volumes. Four fuels with different cetane numbers, 32 (CN32), 45 (CN45), 57 (CN57), and 78 (CN78), consisting of normal and isoparaffins, were examined for the three compression ratios, and the influence of exhaust gas recirculation and intake gas temperature is discussed for 12 combinations of compression ratios and cetane numbers. The results showed that the ignition delay increases linearly with the 1.67 power of the decrease in the intake oxygen concentration changed with cooled exhaust gas recirculation at the same cetane number and the same compression ratio. The ignition delay increases linearly with lowering intake gas temperatures, and the degree of increase in the ignition delay is more significant with lower cetane number fuels and lower compression ratios. Under practical conditions with the intake oxygen concentration between 21% and 11% and the intake gas temperature between 40°C and 100°C, the changes in ignition delays with the intake oxygen concentration are more significant than the changes with intake gas temperature. The ignition delay increases linearly with lowering compression ratios, and the degree of increase in the ignition delay with reductions in the compression ratio is larger in the cases with lower intake oxygen concentrations and lower cetane number fuels. The ignition delays at the higher compression ratios are significantly shorter than with the lower compression ratios in the case of the same in-cylinder gas temperature at top dead center due to higher in-cylinder gas pressures. The degree of increase in the ignition delay with lower cetane numbers is more significant at lower intake oxygen concentrations and lower compression ratios, and the ignition delay decreases linearly with the 0.25 power of the increase in cetane numbers.

Combustion Noise Reduction with High Thermal Efficiency by the Control of Multiple Fuel Injections in Premixed Diesel Engines

Premixed diesel combustion is effective for high thermal efficiency and reductions of NOx and PM emissions, but a reduction of combustion noise is necessary for medium-high load engine operation. The control of the fuel injection has become more accurate because of the technical progress of the common rail fuel injection system, and the target heat release shape, calculated by computation, can be achieved by control of EGR, boosting, fuel injection timing, and injection quantity of multiple fuel injections. In this paper, the reduction of premixed diesel combustion noise maintaining high thermal efficiency has been investigated by the control of injection timings and heating values of multiple fuel injections. There are two aspects of the combustion noise reduction by multiple fuel injections. One is the reduction of the maximum rate of pressure rise in each combustion cycle, and the other is noise reduction effects by the noise cancelling spike (NCS) combustion. The research was conducted with both engine simulations and experiments. In combustion noise simulations, the heat release history of multiple injections was approximated by Wiebe functions and the simulated combustion noise was calculated from the fitted curve of the heat release and the coherence transfer function. The structural attenuation (SA) 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, then the combustion noise (CNL) can be calculated from the structural attenuation and cylinder pressure level (CPL) in the simulation, as shown in equation 4. The simulation results were confirmed by the engine tests. First, the combustion noise reduction by two stage fuel injection was investigated. The maximum rate of pressure rise changes depending on the combustion occurring separately in the compression and expansion strokes. One heat release was set at TDC and the second before or after the TDC. In the late two stage combustion as shown in Figure 12 (b), the combustion noise reduction was most effectively achieved when the heating value of Q2nd is higher than that of Q1st, however in the early two stage combustion in Figure 12 (a), the Q1st heat release occurs during the compression stroke and the combustion noise reduction by the early two stage NCS combustion is more effective than the combustion noise reduction by the late two stage NCS combustion. Three stage combustion simulations were also investigated at 0.6MPa IMEP and 2000 rpm. The optimum heat release shape for low combustion noise and high indicated thermal efficiency was calculated and the role of each part of the heat release in the three stage combustion is discussed. The simulation predicted 87.1 dBA of combustion noise and 50.3 % of indicated thermal efficiency. Finally, the effects of multiple fuel injections on the degree of constant volume and combustion noise are analyzed and discussed.

Predicted diesel ignitability index based on the molecular structures of hydrocarbons

A predicted ignitability index for diesel combustion (the predicted diesel ignitability index) has been established with multiple regression analysis of parameters related to the bond structures in hydrocarbons as explanatory variables and the cetane numbers as a response variable. There were 116 hydrocarbons with known cetane numbers and molecular structures used for the calculations. The numbers of carbon atoms for the seven categories—CM (carbon in a main-chain), CSL (carbon in a side-chain longer than five atoms), C1A (carbon in a single-benzene ring), C2A (carbon in a double-benzene ring), CNA (carbon in a naphtheno-benzene ring), C1N (carbon in a single-saturated six-membered ring), and C2N (carbon in a double-saturated six-membered ring)—were included. The predicted diesel ignitability index was expressed with these seven parameters in the following equation PDI index = 3 . 95 C M + 0 . 99 C SL − 3 . 99 C 1 A − 3 . 31 C 2 A − 2 . 56 C NA − 2 . 15 C 1 N − 0 . 96 C 2 N + 25 . 7 The equation of the predicted diesel ignitability index comprises one-dimensional mono-nominal formulas for each molecular structure variable, suggesting the quantitative influence of the variable on the ignitability. There is good correlation between the predicted diesel ignitability index and the cetane number, showing the coefficient of determination with an R2 of 0.82. The simulation was validated with the ignition delays of 31 blends of hydrocarbons and gas to liquid in a diesel engine. The predicted diesel ignitability index showed better correlations with the measured ignition delays than the cetane number at all three intake oxygen concentrations examined here. Especially, at a low intake oxygen concentration, there was significant scattering between the cetane number and the ignition delay for low cetane number fuels, where the predicted diesel ignitability index showed much smaller scattering.