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The thermal efficiency of internal combustion engines can be improved dramatically with the right combination of engine technology and fuel technology. Super lean-burn technology is attracting attention as a means of boosting thermal efficiency. However, there is a limit to how lean a fuel-air mixture can be before combustion becomes unstable or misfire occurs. The authors evaluated the effects of various chemical compositions on the lean limit under super lean-burn conditions. By changing the composition of the fuel, it was possible to achieve excess air ratios of over 2.0, resulting in high thermal efficiency.

A simulation study was conducted to examine the transition from SI combustion to HCCI combustion in a two-stroke free piston engine fuelled with propane. Operation of the free piston engine was simulated based on the combination of three mathematical models including a dynamic model, a linear alternator model and a thermodynamic model. The dynamic model included an analysis of the piston motion, based on Newton's second law. The linear alternator model included an analysis of electromagnetic force, which was considered to be a resistance force for the piston motion. The thermodynamic model was used to analysis thermodynamic processes in the engine cycle, including scavenging, compression, combustion, and expansion processes. Therein, the scavenging process was assumed to be a perfect process. These mathematical models were combined and solved by a program written in Fortran.

In the HCCI (Homogeneous Charge Compression Ignition) engine, a mixture of fuel and air is supplied to the cylinder and auto-ignition occurs resulting from compression. This method can expand the lean flammability limit, realizing smokeless combustion and also having the potential for realizing low NOx and high efficiency. The optimal ignition timing is necessary in order to keep high thermal efficiency. The Ignition in the HCCI engine largely depends on the chemical reaction between the fuel and the oxidizer. Physical methods in conventional engines cannot control it, so a chemical method is demanded. Combustion duration is maintained properly to avoid knocking. In addition, the amount of HC and CO emissions must be reduced. The objective of this study is to clarify the following through calculations with detailed chemical reactions and through experiment with the 2-stroke HCCI engine: the chemical reaction mechanism, and HC and CO emission mechanisms.

Reduction of particulate emissions from diesel engine is an important theme from the view point of air pollution. Experiments were carried out using a four-stroke single cylinder direct-injection diesel engine. A new method to measure diesel particulates has been developed. Particulates were sampled with a freezing method just behind an exhaust valve and examined through a scanning electron microscope. Shape and structure of particulates and the size distributions are measured under wide operating conditions obtained with above method. The total mass of particulate emissions was measured using a dilution tunnel sampling system. The heat release processes were analyzed using indicator diagrams and the relation between burning condition and particulate emissions were discussed, after systematic experiments under constant revolution speed of 2000 r/min for several load and injection timing conditions.

An on-board measurement system that simultaneously measures road traffic, vehicle running conditions and exhaust emissions was installed in a diesel freight vehicle with two tons payload. Actual NOx mass emissions were compared with that measured in a typical test mode for urban cities on a chassis dynamometer. The frequency of vehicle accelerations in actual urban cities was found to exceed that of a typical test mode for urban cities on a chassis dynamometer, which resulted in increased NOx from actual running conditions compared with the typical test mode for urban cities. The dynamics of NOx emissions at an actual roadside was also analyzed. It was observed that NOx emission based on distance with an actual city route test was about two times higher than that of a free way route and a typical test mode for urban cities. The reason for high NOx with the city route was explained by the higher frequency of lower gears at which higher NOx is emitted.

A single action rapid compression machine was developed to observe the soot formation and oxidation processes in a diesel spray flame. Two color method was applied to analyze the flame temperature and KL factor from the flame image taken by high speed camera. Variation in gas oxygen concentration of the surrounding gas was achieved by adding different quantities of pure oxygen, nitrogen, carbon dioxide and argon gases to charged air within a range from 17 to 25 vol.% oxygen to examine the effects of the surrounding gas composition and the temperature, and of the flame temperature on soot formation and extinction. The initial gas temperature has much effect not only on the ignition but on soot formation speed. The higher oxygen concentration gives the higher flame temperature and the faster soot oxidation rate in the flame. Carbon dioxide has a soot reduction effect in spite of its lower flame temperature.

Super lean burn technology is conceived as one of methods for improving the thermal efficiency of SI engines[1][2]. For lean burn, reduction of heat loss and the due to decrease in flame temperature can be expected. However, as the premixed gas dilutes, the combustion speed decreases, so the combustion fluctuation between cycles increases. Also, to improve the thermal efficiency, the ignition timing is advanced to advance the combustion phase. However, when the combustion phase is excessively advanced, knocking occurs, which hinders the improvement of thermal efficiency. Knocking is a phenomenon in which unburned gas in a combustion chamber compressed by a piston and combustion gas suffer compression auto-ignition. It is necessary to avoid knocking because the amplitude of the large pressure wave may cause noise and damage to the engine. Also, knocking is not a steady phenomenon but a phenomenon that fluctuates from cycle to cycle.

CNG/diesel dual-fuel engine is using CNG as a main fuel, and injects diesel only a little as an ignition priming. In this study, remodeling an existing diesel engine into dual-fuel engine that can inject diesel with high pressure by CRDI (Common Rail Direct Injection), and injecting CNG at intake port for premixing. The results show that CNG/diesel dual-fuel engine satisfied coordinate torque and power with conventional diesel engine. And CNG alternation rate is over 89% in all operating ranges of CNG/diesel dual-fuel engine. PM emission is lower 94% than diesel engine, but NOx emission is higher than diesel engine. The output of dual fuel mode is 95% by the diesel mode. At this time, amount of CO₂ and PM are decreased while CO, NOx, and THC are increased. In NEDC mode, exhaust gases except NOx are decreased.

The HCCI engine is a next generation engine, with high efficiency and low emissions. However a rate of pressure rise is a major limitation for high load range. Recently, we are able to reduce the rate of pressure rise using thermal stratification. Nevertheless, this was insufficient to produce high power. Without the higher equivalent ratio, one way to improve the power is to increase the intake boost pressure. It is suggested that the rate of pressure rise is reduced by thermal stratification and the power is increased by boost pressure at the same time. The objective of this work is to understand the characteristics of combustion, knock and emissions for using both thermal stratification and the boost pressure. The calculations are performed by CHEMKIN and modified SENKIN. As a result of increasing the boost pressure, a higher IMEP was attained while the rate of pressure rise increased only slightly in the HCCI with thermal stratification.

The time of, and location where ignition first occurs in a diesel fuel spray were investigated in a rapid compression machine (RCM) using the two–dimensional techniques of silicone oil particle scattering imaging (SSI), and the planar laser induced fluorescence (LIF) of formaldehyde. Formaldehyde has been hypothesized to be one of the stable intermediate species marking the start of oxidation reactions in a transient spray under compression ignition conditions. In this study, the LIF images of the formaldehyde formed in a diesel fuel spray during ignition process have been successfully obtained for the first time by exciting formaldehyde with the 3rd harmonic of the Nd:YAG laser. SSI images of the vaporizing spray, and the LIF images of formaldehyde were obtained together with the corresponding time record of combustion chamber pressures at initial ambient temperatures ranging from 580 K to 790 K.