The basie characteristics of direct injection (DI) combustion fuclled with compressed natural gas (CNC) and gasoline was studied using a rapid compression machine. The characteristics of stratified combustion and cmission of natural gas and gasoline direct injection at the optimum injection settings over a wide range of equivalence ratios were investigated. The results showed that. similar to premixed combustion. natural gas stratified combustion was of shorter duration that gasoline DI combustion. In contrast to this, the heat release pattern for gasoline DI combustion was similar to that of diesel combustion, which seems to have both a premixed phase and a diffusion phase. This phenomenon tends to be more obvious at a lower overall equivalence ratio. which suggests that fuel and charge stratification have a greal inflence on DI stratified charge combustion. Thus. this faster burn for natural gas promotes extremely lean combustion and a higher pressure rise. However. natural gas DI stratified combustion produces more hydrocarbons (HC) than gasoline DI stratified combustion at a low overall equivalence ratio. Combustion effiiency is at the same level for the two fuels. and natural gas DI combustion was shown to have a slightly leaner combustion capability than gasoline DI combustion, which suggests the better fasibility of natural gas stratified combustion.

The effect of fuel injection timing relative to ignition timing on natural gas direct-injection combustion was studied by using a rapid compression machine (RCM). The ignition timing was fixed at 80ms after the compression start. When the injection timing was relatively early (injection start at 60ms), the heat release pattern showed a slower burn in the initial stage and a faster burn in the late stage, which is similar to that of flame propagation of a premixed gas. In contrast to this, when the injection timing was relatively late (injection start at 75ms), the heat release rate showed a faster burn in the initial stage and a slower burn in the late stage, which is similar to that of diesel combustion. The shortest duration was realized at the injection end timing of 80ms (the same timing as the ignition timing) over a wide range of equivalence ratio. The degree of charge stratification and the intensity of turbulence generated by the fuel jet are considered to cause this behavior. Early injection leads to longer duration of the initial combustion, whereas late injection leads to a longer duration of the late combustion. Early injection showed relatively lower CO concentration in the combustion products while late injection gave relatively lower NOx . It was suggested that early injection leads to combustion with weaker stratification, and late injection leads to combustion with stronger stratification. Combustion efficiency was kept at a high value over a wide range of equivalence ratio.

The combustion characteristics and heat release of a direct injection (DI) compression ignition engine fuclled with diesel dimethyl carbonate blends were investigated on a compression ignition engine. The study showed that the premixed combustion is prolonged and the duration of the diffusive combustion is shortened with increase in the dimethyl carbonate (DMC) addition. For a specific brake mean effective pressure (b.m.e.p.), the maximum cylinder gas pressure, the maximum rate of pressure rise and the maximum rate of heat release increase with increase in the DMC addition at medium and high loads, while they exhibit less variation with the DMC addition at small load. Meanwhile, the maximum gas temperature decreases with increase in the DMC addition, The ignition delay increases while the rapid combustion. duration and the total combustion duration show lss variation with the DMC addition. The brake speeific fuel consumption (b.s.f.c.) inereases while the diesel equivalent b.s.f.e. deereases and the thermal efficiency increases with increase in the DMC addition. The CO and smoke decrease with increase in the DMC addition, and NOx does not inerease with increase in DMC.

Experimental test for premixed laminar combustion of liquefied petroleum gas-air mixtures is conducted in a constant volume combustion bomb. Spherically expanding flames have been employed to measure laminar flame speeds over wide equivalence ratios, at the initial pressures of 0.05, 0.1 and 0.15 MPa, and preheat temperatures from 300 to 400 K. To study the effects of stretch on burning velocity, various Markstein numbers for both strain and curvature have been measured and the effects of initial temperature and pressure on these parameters have been discussed. Following the linear relation between flame speeds and flame stretches, one has then obtained the corresponding unstretched laminar burning velocity after omitting the effect of stretches imposed on these flames. Over the ranges studied, laminar burning velocities are fit by a functional form ul=ul0(Tu/Tu0)aT(Pu/Pu0)BP; and the dependencies of aT and bP upon the equivalence ratio of mixture are also discussed.

A visualization study of natural gas direct injection combustion was carried out by using a high speed video camera. The results show that the distribution of the stratified mixture differs with the injection mode, with parallel and single injection tending to form a higher degree of mixture stratification than opposed injection. Flame propagates toward the downstream direction in the cases of parallel and single-injection combustion, and flame propagates outward from the centre of the combustion chamber in the case of opposed injection combustion. A characteristic of turbulent combustion with a wrinkled flame front is presented in natural gas direct injection combustion. Superlean combustion can be realized owing to the formation of an ignitable stratified mixture with the optimum setting of the fuel injection timing.

A stabilized diesel-methanol blend was realized and a study on the performance and emissions of the diesel-methanol blend was carried out in a compression ignition engine. The study showed that the engine thermal efficiency increases and the diesel equivalent b.s.f.c. decreases with increase in the oxygen mass fraction (or methanol mass fraction) of the diesel-methanol blends due to an increased fraction of premixed combustion phase, oxygen enrichment and improvement in the diffusive combustion phase. Further increase in the fuel delivery advance angle will achieve a better engine thermal efficiency when the diesel engine is operated using the diesel-methanol fuel blends. A marked reduction in the exhaust CO and smoke can be achieved when operating with the diesel-methanol blend. There is not a large variation in the exhaust hydrocarbon with the addition of methanol in diesel fuel. NOx increases with increase in the mass of methanol added; methanol addition to diesel fuel was found to have a strong influence on the NOx concentration at high engine loads rather than at low engine loads, and a flat NOx-smoke trade-off curve exists when operating with the diesel-methanol fuel blends.

Spherically expanding flames of Chinese natural gas-air mixtures have been used to measure the laminar flame speeds, at equivalence ratios of ф=0.6-1.4, initial pressures of p=0.05, 0.1, and 0.15 MPa, and preheat temperatures of T=300-400 K. Following Markstein theory, one then obtains the corresponding unstretched laminar burning velocity after omitting the effect of stretch imposed on these flames. To study the effects of stretch on burning velocity, various Markstein numbers for both strain and curvature have been derived and the effects of initial temperature and pressure on these parameters have been discussed. Over the ranges studied, the laminar burning velocities are comparable with those previously reported for pure methaneair mixtures and fit by a functional form ul= ul0(Tu/Tu0)RT(pu/pu0)βp, where the dependencies of RT and βp on the ф value of the mixture are also deduced. Furthermore, it is presented that the extrapolation results are still in good agreement with the previous work at relatively high pressure. The effects of the dilution gases on the burning velocity have been studied at ф=0.7-1.2, and the variations in burning velocities are plotted as functions of the dilution ratio and the equivalence ratio of the mixture.

A stabilized diesel/methanol blend was developed and the combustion characteristics and heat release analysis of this blend was carried out in a compression ignition engine. The study showed that the increase in the methanol mass fraction will result in an increase in the heat release rate in the premixed burning phase and shorten the combustion duration of the diffusive burning phase. Ignition delay increases with the increase in the methanol mass fraction and the behaviour is more obvious at low engine load and high engine speed. The rapid-burn duration varies little with the methanol mass fraction and the total combustion duration decreases with the increase in the methanol mass fraction. At a low engine speed, the centre of heat release curve tends to be close to the top dead centre (TDC), with an increase in the methanol mass fraction at all engine loads. Fuel delivery advance angles, the maximum rate of pressure rise and the maximum rate of heat release increase with the increase in the methanol mass fraction. At a high engine speed, the centre of the heat release curve closes to TDC at high engine load and will depart from TDC at low engine load. The maximum rate of pressure rise and heat release gives an increasing trend with the increase of methanol mass fraction at high engine loads. The maximum cylinder pressure increases with the increase of the methanol mass fraction. The presence of oxygen reduces the peak pressure, but the reduction was found to be insensitive to the proportion of oxygen within the 6-11 per cent range of testing.

. results show that an optimum ignition position exists for a specified overall equivalence ratio where it has short combustion duration, high value of pressure rise, high combustion efficiency, rapid heat-release rate, low concentration of CO, and unburned hydrocarbon. The optimum ignition position will be close to the injector nozzle tip with decreasing overall equivalence ratio. Lean burn limit can be extended to even smaller overall equivalence ratios as the spark electrode gap approaches the injector tip, whereas the range of overall equivalence ratios for the flammability becomes narrow when closing the spark electrode gap to the injector tip. High combustion efficiency can be maintained at the overall equivalence ratio ф≥0.1 and decreases remarkably at ф<0.1 due to the increasing ratio of unburned fuel to injected fuel.

A stabilized diesel/methanol blend was described and the basic combustion behaviors based on the cylinder pressure analysis was conducted in a compression-ignition engine. The study showed that increasing methanol mass fraction of the diesel/methanol blends would increase the heat release rate in the premixed burning phase and shorten the combustion duration of the diffusive burning phase. The ignition delay increased with the advancing of the fuel delivery advance angle for both the diesel fuel and the diesel/methanol blends. For a specific fuel delivery advance angle, the ignition delay increased with the increase of the methanol mass fraction (oxygen mass fraction) in the fuel blends and the behaviors were more obvious at low engine load and/or high engine speed. The rapid burn duration and the total combustion duration increased with the advancing of the fuel delivery advance angle. The centre of the heat release curve was close to the top-dead-centre with the advancing of the fuel delivery advance angle. Maximum cylinder gas pressure increased with the advancing of the fuel delivery advance angle, and the maximum cylinder gas pressure of the diesel/methanol blends gave a higher value than that of the diesel fuel. The maximum mean gas temperature remained almost unchanged or had a slight increase with the advancing of the fuel delivery advance angle, and it only slightly increased for the diesel/methanol blends compared to that of the diesel fuel. The maximum rate of pressure rise and the maximum rate of heat release increased with the advancing of the fuel delivery advance angle of the diesel/methanol blends and the value was highest for the diesel/methanol blends.