Mixture formation technology for a fuel injection system has been investigated. The effects of spray droplet diameter on engine performance were clarified. The combustion light Intensity was measured with a spark plug integrated combustion flame sensor. When sequential injection is used for better responsiveness in fuel injection systems, engine performance may be reduced through increased HC emissions. Reducing the diameter of the spray droplets and preventing fuel from adhering to the intake manifold walls promote vaporization, reduce fuel concentration on the cylinder wall, decrease HC emissions, improve cold start ability, and give good idling performance.