According to previous papers on the combustion process in LHR diesel engines the combustion seems to deteriorate in LHR diesel engines. However it has been unclear whether this was caused by the high temperature gas or high temperature combustion chamber walls. This study was intended to investigate the effect of gas temperature on the rate of heat release through the heat release analysis and other measurements using a rapid compression-expansion machine. Experiments conducted at high gas temperatures which was achieved by the employment of oxygen-argon-helium mixture made it clear that the combustion at a high gas temperature condition deteriorated actually and this was probably due to the poorer mixing rate because of the increase in gas viscosity at a high gas temperature condition.THE LHR (Low Heat Rejection) DIESEL ENGINE promises improved engine fuel consumption by eliminating the traditional cooling system and converting part of increased exhaust gas energy into useful shaft work. The ability to burn lower-grade fuels because of the higher cylinder gas temperatures attainable without cooling system is also promising. Several simulation and feasibility studies have been made on the promises and challenges of the LHR engines(1, 2 and 3). The temperatures of the piston and fire deck made of monolithic ceramics are roughly 300 - 400 K higher than those of conventional metal engines. Accordingly the major processes which are involved in the LHR engines are heat transfer, combustion, and tribology.As for combustion, evaluation of the potential of the LHR engine to meet future EPA emission standards has been one of the current concerns to engine developers. The engine high wall temperature brings about a significant reduction in volumetric efficiency in the charge air amount due to heat transfer from walls to the charged air during the intake stroke. Since this essentially results in the deterioration of combustion, experiments of the combustion process in high temperature environment are usually performed at a condition of the same amount of air flow. Wade et al.(4) investigated experimentally fuel economy and emission opportunities using an uncooled light duty DI diesel engine with ceramic coated cylinder head and valves, a heat insulated steel topped piston and a short, partially stabilized zirconia cylinder liner in the area above the piston rings. The single-cylinder diesel engine used in this study has an 80 mm bore x 88 mm stroke and a compression ratio of 21. The combustion system consisted of a helical intake port, an oil cooled multi-hole injector and a re-entrant combustion bowl in the piston. A comparison of the measured indicated specific fuel consumption data for the uncooled and water-cooled baseline engines showed improvements in fuel consumption of the uncooled engine ranged from 4 % at the heavy load condition to 7 % at the light load condition. The test results on emissions showed that generally, HC, NOx and particulate emissions were reduced in the uncooled engine. They attributed the lower HC and NOx emissions to the reduction in the amount of premixed combustion resulting from the shortened ignition delay period. They also explained the trend of lower particulate emissions by the increase in the diffusion combustion rate.Henningsen(5) conducted engine tests similar to Wade's work and compared his results with those by Wade et al. He attempted to simulate LHR conditions in a research engine simply by eliminating cooling, and did not employ ceramic parts for the insulation of the combustion chamber. The start of combustion timing was set at top dead center in all tests as in the Wade's experiments. It is interesting to note that his results for the uncooled engine were contradictory to Wade's results, showing a slight improvement in fuel economy at light load and a slight worsening at high load, a significant increase in HC emission, a slight increase of NOx emission, and slight increase in total particulates at low load and a substantial increase at high load. In order to examine the reason for the discrepancy between the above two studies, he made a close comparison between the apparent rates of heat release in both experiments. He showed that the decrease in ignition delay led naturally to a decrease in premixed combustion and that most of the differences in apparent heat release may be attributed to the different leves of gas temperature which results from different degrees of insulation employed in these studies. It is suggested from this comparison that optimization of temperatures of combustion chamber walls is necessary for achieving better fuel economy and lower emission in an uncooled diesel engine.Recently Kawamura et al (6) investigated the rate of heat release at a high cylinder gas temperature condition using an uncooled light duty DI diesel engine. They found that even if the ignition delay was controlled by changing the cetane number of test fuel the combustion duration observed at a high gas temperature conditions was scarcely affected, and furthermore the combustion duration was always longer than that of a water-cooled baseline engine. Besides they observed the flame evolution on the high speed movies and revealed that the flame evolution was significantly degraded at a high gas temperature condition.The potential for LHR engines to meet current and future EPA heavy duty emission standards is a critical consideration. Dickey (7) conducted LHR enigne tests using a single cylinder test engine with a 137 mm bore, 165 mm stroke and a compression ratio of 14.5. The baseline metal engine was tested with 355 K coolant, while the insulated engine, which uses ceramic coated components, was operated by replacing the coolant with compressed air. He showed that the ceramic engine had significantly lower indicated thermal efficiency, with higher smoke and particulate emissions compared to the baseline metal engine, and that the NOx emissions for the ceramic engine were the same as the baseline metal engine at low load and were slightly reduced at the full load condition. He attributed the poor LHR engine performance to degraded combustion observed in the heat release rate curve.The conflicting results on fuel economy and emissions observed in LHR engine tests, are due to the large number of possible LHR engine configurations, test conditions, and analysis techniques used. As far as the heat release analysis is concerned, it seems difficult with engine experiments to separate the effects of cylinder gas temperature and combustion chamber wall temperatures on the rate of heat release. The present study is intended to investigate experimentally the effect of cylinder gas temperature on the rate of heat release using a rapid compression expansion machine. The cylinder gas temperature was raised to a value which is 300 - 400 K higher than that in conventional cooled engines, while the temperature of the combustion chamber walls was kept constant in all tests conducted.