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In an SI engine, increasing the compression ratio could be one mean of achieving higher thermal efficiency. However, when the compression ratio is increased, knock occurs and it prevents higher thermal efficiency. It is generally known that if the burning velocity is increased and the combustion period is shortened, the occurrence of knock may be suppressed. Here, hydrogen was added to the gasoline engine as a mean of increasing the burning velocity. As a result, it has been confirmed that the occurrence of knock could be controlled to some extent, and knock could be completely avoided depending on the conditions for the distribution of hydrogen. Furthermore, it became clear that this result might have originated not only by the increase in the burning velocity but also by the hindrance of radical production by the hydrogen.

Squish flow control is well known as a key technology for improving knock limit in spark ignition engines. However, to acquire a sufficient squish area in a four-valve engine is difficult. In order to achieve a maximum effect of knock suppression with a minimum squish area, we have developed, what we call, a Slant Squish Combustion Chamber for new engines. A slant squish compared with a conventional squish produces an effective reverse squish flow in the early expansion stroke, resulting in higher flow velocity and turbulence. Furthermore, flame propagation to squish area and end gas is accelerated. These improvements are considered to suppress the knock phenomenon. Consequently, with a slant squish, a high compression ratio, to achieve low fuel consumption and high engine performance is realized.

A helical port with a swirl control valve (SCV) has been developed to satisfy two inconsistent requirements of achieving sufficient swirl generation to improve the combustion and still maintaining high volumetric efficiency. Their effects on combustion were confirmed in a single cylinder engine using high speed flame photography and cylinder pressure diagram analysis which has demonstrated faster combustion. As a result of a hot wire anemometer study, the differences in gas motion were clarified between two helical ports, one with and one without a SCV. A more active movement of the center of swirl was measured in the case of helical port with SCV which suggests the generation of higher turbulence in the cylinder.