An investigation on the causes of cycle variation in direct injection hydrogen fueled engines
Introduction
The hydrogen-fueled engine is being developed into a hydrogen-fueled engine with manifold injection, direct injection in cylinder, or dual injection according to the fuel supply method [1], [2], [3], [4]. Of these, the hydrogen-fueled engine with direct injection can fundamentally keep backfires from occurring, so it can be utilized as a high-powered hydrogen power system if the reliability of high pressure direct injection valve is secured [5], [6]. Hydrogen gas is characterized by a rapid combustion speed, wide combustible limit and low minimum ignition energy. Such characteristics play a role to decrease engine cycle variation for the safety of combustion. However, it is frequently observed that the values of cycle variation for hydrogen-fueled engines with direct injection are higher than those of hydrogen-fueled engines with manifold injection or those of gasoline engines, due to a decrease in the mixing period by direct injection in the process of compressing hydrogen gas [7], [8], [9]. Since the stability of engine operation should be secured for the practical use of a hydrogen-fueled power system, it is necessary to investigate the causes of cycle variation in direct injection hydrogen-fueled engines.
Cycle variation in a spark ignition engine is produced by the change in the mean effective pressure caused by combustion variation, which is brought about by the flow of fuel–air mixture at the time of ignition, the heterogeneous fuel–air mixture, and the difference in the mixing condition of fresh mixture and residual gas near the spark plug. Theories on the cause of cycle variation are divided largely into two: one holds that it is caused by the variation in early combustion, while the other says that it is caused by the variation in rapid combustion. The former theory posits that the factors mentioned above vary ignition delay and this causes the difference in combustion on the whole. The latter theory says that variations in the period of rapid combustion are caused significantly by turbulence variation in gas flow in the cylinder rather than affected by ignition delay, etc.
Such theories on the cause of cycle variation are usually presented for gasoline engines, so it is not easy to apply them to the cause of cycle variation in hydrogen-fueled engines, which are significantly different from gasoline engines in terms of the burning velocity, ignition energy and combustible limit of hydrogen gas.
This study attempts to clarify by which theory the cause of cycle variation is governed in direct injection hydrogen-fueled engines. For this purpose, the cycle variation was investigated with respect to the engine operation variables such as engine speed, injection timing of hydrogen gas, air–fuel ratio and ignition time, and evaluated the coefficient of variation in the early combustion period and the coefficient of variation in the rapid combustion period by defining the coefficient of variation in the combustion period.
Section snippets
Experimental apparatus and method
Fig. 1 is a schematic diagram showing the overall configuration of the experimental apparatus. It is composed of a hydrogen-fueled engine, a high-pressure hydrogen supply system, a intake and exhaust system, an injection controller, and a data acquisition system. The hydrogen-fueled single cylinder engine of L-head type is shown in Fig. 2. High-pressure hydrogen gas is injected directly into the cylinder in the early period of compression stroke. Engine specifications are compression ratio of
Variation in engine speed
Fig. 5 shows the coefficient of variation in mean effective pressure, COVimep with respect to changes in engine speed. In this case, the air–fuel equivalence ratio was fixed at λ=1 and ignition timing at MBT. The COVimep of the direct injection hydrogen-fueled engine shows less than 3% on the whole at overall engine speeds so there is not a big problem in regard to the stability of engine operation, but as the engine speed increases, it tends to increase more or less. Normally, as the engine
Coefficient of variation in combustion period
The theories on the causes of cycle variation in spark ignition engines are largely divided into two: that it is caused by variation in the early period of combustion, and that it is caused by variation in the rapid combustion period. In the case of the hydrogen engine, we defined and evaluated the coefficient of variation in the combustion period to clarify whether cycle variation is caused by variations in the early period of combustion or by variations in rapid combustion. The coefficient of
Conclusions
In order to clarify by which theory of cause the cycle variation is governed in direct injection hydrogen-fueled engines, that is, either the theory that it is caused by variations in the early combustion period or the theory that it is caused by variations in the rapid combustion period, the cycle variation with respect to major operation variables was studied and defined the coefficients of variation in the combustion period. By analyzing the coefficients of variation, conclusions are
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