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Fuel properties are a very important factor to reduce particulate matter (PM) and other emissions with diesel engines. Especially the effect of aromatic contents has been discussed, though details of the influence differ in different reports. In this study the mechanism of PM formation was investigated in a small direct injection diesel engine. The fuels tested were paraffinic hydrocarbons (C7∼C12) with different boiling points (98∼216 °C), and the blending of aromatic hydrocarbons (1∼4 rings) with paraffinic and olefinic hydrocarbons. The effect of the structure of fuels with the same carbon numbers (dodecane and dodecene) was also investigated. The results showed that the amount of SOF decreases to about one tenth of that of diesel oil when using low boiling point paraffinic hydrocarbons like heptane. However, the total amount of unburned hydrocarbon increases due to over-leaning of the mixture due to the early evaporation.

This study proposes a new combustion chamber concept for small DI diesel engines. Reduction of fuel adhering to the cavity wall, improvements in mixture formation, and an optimum distribution of mixture inside and outside the cavity are the main characteristics of the combustion chamber. The spray formation and it's distribution inside and outside the combustion chamber was investigated photographically in a small DI diesel engine with transparent cylinder and piston. Optimization of the fuel spray distribution inside and outside the cavity was attempted by changing the shape of the cavity entrance and the location where spray impinges on the lip. In addition improvements in the mixture formation of the impinging spray and reductions in the fuel adhering to the cavity wall were attempted by introducing a small step on the cavity side wall. The results were confirmed by analyzing the combustion and emission in an actual DI diesel engine.

Blue smoke and HC emissions from a direct injection diesel engine increase gradually during long idling operation (for a few hours). The extent of this increase depends on the injection nozzle specification and engine operating conditions. The accumulation of carbon deposits on the nozzle tip and combustion chamber wall will depend on these factors. Since the carbon absorbs fuel well, low volatility components can not evaporate during the combustion period and the unburned fuel emissions increase over time. This tendency changes according to fluctuations of spray shape and cylinder to cylinder fuel quantity variations.

EGR reduces NO emission, but increases smoke and decreases BMEP in diesel engines. This paper describes the relationships between these behaviors and the effect of decreased oxygen with EGR in direct injection, pre-chamber, and turbocharged diesel engines. The results indicate that the reduction of NO depends on decreasing the rate of the incoming oxygen. The increase in smoke and the decrease in BMEP is due to a reduced rate of exhaust oxygen. Also the reduction of NO is due to increased ambient humidity which can be explained by the decreased oxygen in the incoming charge. With these results, it becomes possible to predict the ratio of the reduction of NO emission, the increase in smoke and the decrease in BMEP.

Diesel passenger car is superior to gasoline engine car in the fuel economy, but it has some defects to improve :noise, startability, particulate and transient performance, etc. Among these problems, this paper presents particularly transient performance in a diesel engine and clarifies the causes of its decline at lower temperature operation. As the results, it is found that the transient torque at the early stage of acceleration is only 50% at −20°C, and that when coolant temperature went up to 20°C, the transient torque approaches to that of the warmed up engine. The transient response becomes worse with retarding the injection timing and with decreasing the engine speed. On the other hand, since the normal response is not obtained despite of using high cetane number fuel, main cause of the inferior transient torque is not the poor combustion, but the increase of friction or cooling loss.

Direct injection diesel engines emit a far more disagreeable exhaust odor at idling than gasoline engines, and with increasing numbers of DI diesel engines in passenger cars, it is important to promote the odor reduction research. High pressure injection in DI diesel engines promotes combustion and decreases particulate matter (PM) emissions, but injection pressures at idling and warm up are limited to 30∼40 MPa considering engine noise and vibration. In this pressure range, a part of the fuel adheres on the relatively cool combustion chamber walls and causes incomplete combustion, producing higher concentration of unburned HC and intermediate combustion components (aldehydes, other oxygenated compounds, etc.) with objectionable exhaust odors. To reduce the exhaust odor, oxidation catalysts are effective, but catalyst activity is poor at idling, when the exhaust gas temperature is low (about 100°C).

This study investigated the effect of high pressure injection and an oxidation catalyst on the exhaust odor of DI diesel engines. At idling an injection pressure of 60∼80 MPa resulted in the minimum exhaust odor, with the least aldehyde and minimum total hydrocarbon (THC). This is because of decreases in fuel adhering to the combustion chamber walls due to the shortest ignition delay at this pressure range. However, above 60 MPa there is no further shortening of the ignition delay and overleaning of the local mixture dominates at injection pressures above 100 MPa, where the exhaust odor increases again. The odor reduction at the optimum injection pressure and injection timing is not significant, and further experiments with an oxidation catalyst were performed. The oxidation catalyst was found less effective to reduce exhaust odor at long idling where the maximum catalyst temperature is only about 120°C.

Modern small DI diesel engines are operated at high loads and high speeds. In these engines the spray spreading on the cavity walls during the main combustion is kept approximately constant at all engine speeds to optimize the air utilization. However, spray spreading on the wall during the early and late part of combustion changes with engine speed due to the changes in air motion. At the end of impingement much of the spray moves outside the cavity due to a strong reverse squish when the injection timing is set near TDC. This causes incomplete combustion of fuel and increase emissions of HC and soot. Therefore, the study of the behavior of spray affected by the reverse squish is very important. In this study the fuel spray development under high injection pressure and high gas charging pressure was investigated photographically in a small direct injection diesel engine with a common rail injection system.

Exhaust odor of DI diesel engines is worse than that of gasoline engines, especially at low temperatures and at idling. As the number of passenger cars with DI diesel engines is increasing worldwide because of their low CO2 emissions, odor reduction research of DI diesel engines is important. Incomplete combustion is a major cause of exhaust odor. Generally, odor worsens due to overleaning of the mixture in the cylinder and due to fuel adhering on the combustion chamber walls. To confirm this, the influences of different engine running conditions and fuel properties were investigated. The reason for the changes in exhaust odor with injection timing is evaluated by considerations of optimum positions of the maximum heat release. With n-heptane, a low boiling point fuel, odorous emissions increase because of overleaning of the mixture.

Early stage combustion systems, with lean homogeneous charge compression ignition (HCCI), have been studied, with the intent to decrease the pollutant emission characteristics of DI diesel engines. Early stage combustion enables drastic reductions in both nitrogen oxides (NOx) and smoke emission, but the operating load range is restricted, due to combustion phenomena, such as unsteady combustion and knocking. In this study, we explored the possibility of broadening the operating load range in HCCI and reducing pollutant emissions using Dimethyl Ether (DME) fumigated through the intake pipe. However, the improvements in load range were found to be less than 0.1 MPa in brake mean effective pressure (BMEP), even when compression ratios were reduced and Methane with high octane number was mixed. Therefore, a DME premixed charge could be used only at light loads. At heavier loads a hybrid fuel system with a DME premixed charge and diesel fuel injection is necessary.

The blue and white smoke (cold smoke) emitted from diesel engines during warm up at low temperatures and idling conditions contains pollutant gases which irritate eyes and nose, and reductions in this irritating, odorous gas have become important with the increasing numbers of DI diesel engine vehicles. To assess the blue and white smoke a qualitative assessment method is necessary, though, there are no simple and exact measuring methods. In this study a new assessment method using a digital camera and photo analysis system with a computer is introduced. With this method the luminance of the cold smoke is displayed as 8 bit data, and a quantitative evaluation is simple, when the influence of sunshine is corrected for the smoke luminance. This paper describes the correction method for the sunshine illumination and the technique for taking the photographs.

This study investigated the influence of aldehyde and hydrocarbon components (HC components) on exhaust odor in DI diesel engines. Aldehyde is an important odorous group in exhaust, and it correlates well with exhaust odor at any engine condition. Formaldehyde (HCHO) in the exhaust has been identified as an important component causing irritating odor. Water-washing of exhaust gases does not trap HC components, while most of the odorous components are trapped with remarkable odor reductions. This indicates that the HC components in the exhaust have no direct effect on exhaust odor. However, the exhaust odor increases with increases in the concentration of the low boiling point HC components. This maybe due to the increase in intermediate odorous compounds like aldehydes, organic acids, or other oxygenated compounds in the combustion condition where the low boiling point HC components increase.