The Effect of a Longer Stroke on Improving Fuel Economy of a Multiple-Link VCR Engine

Some automakers have been studying variable compression ratio (VCR) technology as one possible way of improving fuel economy.

In previous studies, we have developed a VCR mechanism of a unique multiple-link configuration that achieves a piston stroke characterized by semi-sinusoidal oscillation and lower piston acceleration at top dead center than on conventional mechanisms. By controlling compression ratio with this multiple-link VCR mechanism so that it optimally matches any operating condition, the mechanism has demonstrated that both lower fuel consumption and higher output power are simultaneously possible. However, it has also been observed that fuel consumption does not reduce further once the compression ratio reached a certain level.

This study focused on the fact that the piston-stroke characteristic obtained with the multiple-link mechanism is suitable to a longer stroke. The effect of a longer stroke on improving fuel economy was investigated in the context of using this specification to suppress an increase in the surface-to-volume (S/V) ratio of the combustion chamber, which becomes a problem of a higher compression ratio. Study results showed that it is possible to increase the compression ratio further by applying a longer stroke and that a long-stroke multiple-link VCR mechanism is effective in reducing fuel consumption.