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A study of the mixing mechanism of fuel with surrounding air is necessary in order to clarify the combustion process. In this study, the flow field near non-evaporating diesel spray as well as spray surface were observed and analyzed using a Nd-YAG laser light sheet. A single shot fuel spray was injected into a high pressure vessel and photographed under double-pulse laser illumination. The images of dispersed particles in the vessel were processed and velocity vectors were obtained by the auto-correlation method. Measured results showed temporal variation in the air movement around the spray. Just after the start of injection, air near the nozzle was pushed outward by the spray tip, after which the flow direction reversed. The air velocity ahead of spray tip was very low compared to fuel spray tip velocity. At a stable injection condition, air near the nozzle tip was pulled by the spray movement and flowed uniformly, and the spray-air boundary was smooth.

The biomass fuel is expected to solve the global warming due to a carbon neutral. A rapeseed oil methyl ester (RME) as biomass fuel was selected, and also a low sulfur diesel fuel is tested as reference fuel in this study. The experiments were carried out to improve diesel emissions and engine performance using high boost and high rate EGR system and a common rail injection system in a single cylinder engine. The diesel emissions and engine performance have been measured under the experimental conditions such as charging boost pressure from atmospheric pressure to 401.3kPa maximum and changing EGR rate from 0% to 40% maximum. RME contain about 10 mass % oxygen in the fuel molecule. Furthermore, RME does not contain aromatic hydrocarbons in the fuel. Due to these chemical properties, RME can be used at 40% high EGR condition.

Premixed Lean Diesel Combustion (PREDIC) has very low NOx combustion because of early injection timing, for example, at -120 degrees ATDC; however, it has some problems. One problem is that so much fuel spray reaches the cylinder wall, which causes high HC emission and high fuel consumption. The other problem is that compression ignition timing control is difficult due to the dependence on the in-cylinder temperature. To solve these problems, an impingement spray system with two nozzles is attempted to obtain the spray increasing at the center of the combustion chamber instantaneously. This impingement spray system has two nozzles, which are located diagonally. Two sprays, one injected from each side injector, impinge each other at the center of the cylinder to create an air-fuel mixture.That is,this impingement spray system creates the air-fuel mixture by using the penetration of both sides of the sprays instead of early timing injection.

A conventional single cylinder direct injection diesel engine was fitted with three fuel injectors: one mounted vertically on the center, and the others mounted diagonally from the side direction. With this system, it was possible to control the fuel injection timing and injection quantity of each injector independently. It was also possible to independently control the fuel injection pressure of the center and side injectors. Using this system, it was possible to control the spatial and temporal distributions of the fuel injected into the combustion chamber, which are impossible to obtain with conventional injection equipment. In this study, an improvement in particulates and specific fuel consumption was obtained, while maintaining low NOx, by injecting a small amount of fuel from the two side injectors after the main fuel injection from the center injector.

The effects of fuel injection velocity and ambient gas pressure on the spray formation and atomization process for a non-evaporating diesel spray were observed and analyzed with greatly magnified photographs illuminated by a pulsed ruby laser light sheet. Individual fuel droplets were distinguishable at the peripheral regions of the spray in these photographs. The spray width became narrower with an increase in injection velocity, and the spray spread out further with increase in ambient gas pressure. The branch-like structure in the spray originated from local high and low fuel particle number density regions and the difference in number density between these two regions increased with higher injection velocity. The ruby laser was double-pulsed to enable fuel particle velocity vectors to be characterized at the peripheral regions of the fuel spray. The vorticity scale was smaller and vorticity magnitude grew higher with increase of injection velocity.

Typical DI diesel engines operate with fuel injection taking place within a range of about 30 crank angle degrees before top dead center, at the end of the compression stroke. When injection takes place far earlier, at the beginning of the compression stroke, another form of combustion occurs, which we termed PREmixed lean Diesel Combustion, or PREDIC. With PREDIC operation, self-ignition occurs near top dead center and NOx emissions are drastically lower. When ignition occurs, the fuel-air mixture is thought to be nearly homogeneous, with only slight heterogeneity. Appropriate fuel spray formation is very important for successful PREDIC operation. Using a single-zone NOx formation model, calculations showed that the mean excess air ratio in the PREDIC combustion zone was 1.87, which resulted in very low (20 ppm) NOx emissions. Conventional combustion at the same conditions resulted in a mean combustion zone excess air ratio of 0.88.

The mixture formation process in a premixed compression ignition engine was numerically analyzed. This study aimed to find out effective injection conditions for lean mixture formation with high homogeneity, since the NOx and soot emissions in the engine are closely related to the mixture homogeneity. To calculate fuel spray behavior, a practical computer code GTT (Generalized Tank and Tube) was employed. In a model for the premixed compression ignition engine, the effects of injection parameters, such as injection timing, initial droplet size, spray angle, injection velocity, nozzle type (pintle and hole) and injection position / direction, on the mixture homogeneity near ignition timing (or TDC) were investigated. To evaluate the homogeneity of the mixture, an index was defined based on the spatial distribution of fuel mass fraction. The fuel vapor mass fractions as well as the homogeneity indices, obtained as a function of time, were compared under various boundary conditions.

To find more effective lean mixture preparation methods for smokeless and low NOx combustion, a numerical study of the effects of in-cylinder flow field before injection on mixture formation in a premixed compression ignition engine was conducted. Premixed compression ignition combustion is a very attractive method to reduce both NOx and soot emissions, but it still has some problems, such as high HC and CO emissions. In case of early direct injection, it is important to avoid wall wetting by spray impingement, which can cause higher HC and CO emissions. Since it is not easy to examine the effects of initial flow and injection parameters on mixture formation over the wide range by practical engine tests, a computer program named “GTT (Generalized Tank and Tube)” code was used to simulate the in-cylinder phenomena before autoignition.

Previous research in our laboratory has shown that NOx emissions can be sharply reduced by PREDIC (PRE-mixed lean DIesel Combustion), in which fuel is injected very early in the compression process. However some points of concern remained unsolved, such as a large increase in THC and CO, higher fuel consumption, and an operating region narrowly limited to partial loads, compared to conventional diesel operation. In this paper, the causes of PREDIC's problem areas were analyzed through engine performance tests and combustion observation with a single cylinder engine, through fuel spray observation with a high-pressure vessel, and through numerical modeling. Subsequently, measurable improvements were achieved on the basis of these analyses. As a result, the ignition and combustion processes were clarified in terms of PREDIC fuel-air mixture formation. Thus, THC and CO emissions could be decreased by adopting a pintle type injection nozzle, or a reduced top-land-crevice piston.

For further increase in thermal efficiency of heavy-duty diesel engines, flexible regulation of the heat release rate (HRR) profile combined with higher compression ratio could have more rooms to improve indicated thermal efficiency by overcoming various drawbacks relevant to higher compression ratio. A new ideal HRR profile, which starts as a kind of delta shape to fulfil the isobaric cycle from top-dead-center (TDC) and is followed by the significant increase in HRR to reach the maximum cylinder pressure in the retarded timing, was proposed. We call it as ‘High-heels’ HRR profile from its two-step-increase delta shape. To confirm the potential of the ideal HRR profile by utilizing a single- cylinder heavy-duty diesel engine, a variable fuel injection rate equipment, novel combustion chamber designs, and an offset orifices nozzle were investigated as the technologies for modifying HRR profile.