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By using a four-stroke gasoline engine equipped with a fully variable valve operation system, combustion performance was investigated from the aspect of a gas exchanging difference at various internal exhaust gas recirculation conditions due to the negative valve overlap variations. The in-cylinder gas temperature throughout the cycle process was analyzed thermodynamically. The experimental data revealed that in-cylinder gas temperature at the end of compression stroke (TAI) dominates the onset of autoignition and ΔT, which is an index that represents the heat capacity of the working gas, dominates the heat release of auto-ignition. This paper intends to evolve the experimental knowledge to an engineering tool, which could predict possibilities and limits of auto-ignition. As a result, a controlling mechanism of auto-ignition is proposed. According to this mechanism, a possible maximum load of auto-ignition operation is estimated and also demonstrated in the engine experiments.

As an index to control the heat release of auto-ignition combustion, our previous paper introduced a concept of ΔT. It was the difference between the adiabatic flame temperature and the initial in-cylinder gas temperature before the heat release, i.e., ΔT physically represents the heat capacity of the in-cylinder gases relative to the calorific value supplied in a cycle. Firing tests of a four-stroke auto-ignition gasoline engine revealed that the heat release process could be successfully controlled when ΔT was maintained at a proper level. This paper evolved the ΔT theory into the every possible gas exchanging state in the four-stroke engines and found out a chain of the low-temperature combustion cycle (LTC), which continuously varied from the spark-ignition (SI) to auto-ignition (AI). By using a hydraulic-electromagnetic fully-free valve actuator system, the LTC was examined in our 650 cm₃ single-cylinder experimental-engine.

Auto-ignition may be considered as the ultimate combustion process for vehicle engines. Regarding combustion control, ignition timing and heat release are the chief concerns. The former is finding a solution, however the latter has, as yet, several approaches. As a means to control the heat release, this paper introduces a universal parameter; ΔT, which is the difference between the adiabatic flame temperature and the initial in-cylinder gas temperature before combustion. Firing tests of a four-stroke auto-ignition gasoline engine revealed that the heat release process could be successfully controlled when ΔT was maintained at a proper level. As a result, the auto-ignition zone was greatly enlarged in the tested engine with a moderate compression ratio.

Motorcycles have been classified by engine displacement from commercial and legal demands. If this restriction on displacement were removed, what could be done to improve fuel economy? “Down sizing” would be an obvious idea. However this paper proposes a quite inverse concept, i.e. a large displacement lower speed engine. Mechanical friction in four-stroke engines generally decreases in proportion to the square of engine speed. Thus mechanical efficiency should be better at lower operation speeds. According to this concept, developing a 200cm3 experimental engine was successful in improving fuel economy by 79% in the ECE40 mode and by 97% at 60km/h cruise condition, compared to the 100cm3 original engine.

For a better understanding of the auto-ignition phenomenon in internal combustion engines, consideration is given from the in-cylinder gas temperature aspect. Experimental results demonstrate that the in-cylinder gas temperature at the end of compression, namely, the “auto-ignition temperature” is deeply involved in the onset of auto-ignition. The relation between the gas exchange state and the auto-ignition temperature explains the mechanism of timing controlled auto-ignition, namely, Activated Radical (AR) Combustion. The auto-ignition temperature is maintained constant during the AR combustion state, thanks to the exhaust valve controlling the hot residual gas amount. Finally, the utilization of auto-ignition in gasoline engines is discussed from the methodology aspect.

A low pressure pneumatic direct injection engine by AR Combustion concept has been developed. In this engine, the scavenging process is performed by air only, prior to the fuel feed process, which minimizes the fuel short-circuiting accompanying the scavenging flow. The wide spray form generated by the rotary type injector facilitates homogeneous combustion even in the high speed high load range, which promises higher maximum power output. Meanwhile, in the light load range, AR Combustion effectively solves the irregular combustion. The bench test results show a remarkable reduction in the exhaust emissions and fuel consumption while successfully maintaining high power output of original two-stroke engine. Moreover, actual vehicle tests using a motorcycle reveal its good potential as a vehicle engine.

From the environmental conscious point of view, subjects of conventional two-stroke engines will be irregular combustion and new mixture shortcut. Concerning for the irregular combustion, our previous papers has however proven that timing controlled auto-ignition, namely Activated Radical (AR) Combustion, was an effective solution. Meanwhile for the new mixture shortcut, no solution is mentioned in those papers. This time, a low pressure pneumatic direct fuel injection engine has been experimented. This engine is however intended to perform homogeneous combustion, the A R Combustion solves irregular combustion effectively in the light load range. Fuel is pneumatically injected into the cylinder by using accumulated in-cylinder gas pressure, after the scavenging process performed by the air only. A mechanically driven rotary type injector is installed in the cylinder wall.

The improvement of the exhaust emission and fuel consumption in the conventional two-stroke engines would be urgent. Our previous papers have suggested that the timing controlled auto-ignition, namely Activated Radical Combustion(AR combustion) could be a solution for that. In this time, the AR combustion was applied to a 250 cm3 motorcycle for the intention of commercialization of the AR engine. The alternating phases between AR combustion and SI combustion were analyzed and successfully improved the typical pinking noise. The AR combustion finally decreased the HC emission by approximately 60% in the EC 40 emission evaluation mode. As the power units for the small motorcycles or outboards, two-stroke engines are yet majority. That is because they have advantages such as higher power output, simpleness and compactness of the structure, at the same time, their drawbacks in fuel consumption and exhaust emissions are also pointed out in the issues of preserving the environment.

Variable-valve-actuation mechanism is considered to be one of the most suitable solutions to realize the compatibility between higher power output and performances in the practical speed range. A new variable-valve-actuation mechanism named “Shuttle Cam” was designed and studied. In this mechanism which was applied to a conventional motorcycle engine with rocker arms and gear-train-driven valve system, the cam gears move along the idler gear. And cam shafts simultaneously slide along the rocker-arm slipper surfaces which are concentric with the idler gear. Consequently valve lift varies continuously in accordance with the alteration in the rocker-arm lever ratio and the cam phasing changes simultaneously in accordance with the cam gear rotation. Result of the experiments has confirmed that the mechanism functions accurately even at high speeds up to 10,000 rpm and some improvements were achieved in power output, fuel consumption, idling quality, and exhaust-noise level.

The one-way clutch mechanism seen on bicycles, etc. greatly ease the load of riders at times of coasting or descending downhill. Their use on motor vehicles are restricted to some automatic drive four-wheeled vehicles with torque converters. There are neither any example of mass-produced motorcycle with one-way clutch between the engine and the drive system nor any reports of quantitative study on its impact on the performance of the vehicle including fuel economy. The present paper reports the results of experimental research on one-way clutch employed in the drive system, obtained for motorcycles having five kinds of two-cycle engines of different displacements. The test on fuel economy showed an improvement of 4 - 9%.

Although two stroke cycle engines generally have less mechanical parts and higher specific power output, the fuel consumption is considered inferior to that of four stroke cycle ones. Nevertheless, for motorcycles with a 100 cc engine which are used as an practical means of transportation 1) Two stroke cycle engines have revolution region which provides specific fuel consumption (SFC) as favorable as that of four stroke cycle engines, at a part throttle adjustment. 2) Applying centrifugal governor type V-belt CVT (continuous variable transmission) allows to always utilize a certain revolution region of the engine. Through the experimental approach and results based on the above concept, this paper reports that the optimum combination of two stroke cycle engine and CVT enabled to achieve fuel consumption as favorable as that of four stroke cycle engines.