Fuel Injector Spray Timing for Reduced Hydrocarbon Emissions and Higher BSFC
The amount of overlap depends on the engine, but most modern fuel injected high performance internal combusion engines have some valve overlap. While the intake valve is starting to open, the exhaust valve is starting to close. During this time there can be a back flow of air/exhaust in the direction of the fuel injector and intake manifold. This effect is stronger under high vacuum conditions (low absolute pressure). It is important to wait for this back flow to reverse before firing the injector. Once the exhaust valve is open, the velocity and flow of air will have increased as the exhaust gases have completely turned around and begun to escape through the exhaust port and out of the exhaust manifold. The optimal time to fire the injector is during this period, but as engine speed increases the injection must begin earlier to fully complete to avoid flowing fuel straight through the combustion chamber and right out of the exhaust port.
The advantages to injecting fuel during the phase of combustion where air is flowing into the cylinder is a reduction in
The window of opportunity to fire the injector will decrease as engine speeds increase. At 3000 RPM there is a 20 millisecond window for the piston to complete a full rotation. Since it spins twice per cycle, the maximum amount of time that the injector can fire is 20 milliseconds before it would go static at a 100% duty cycle. At this point control of fuel quantity delivery is lost and very little of the injection happens during optimal conditions. Much of the fuel injected during one cycle will be left in a puddle on the backside of the valve, waiting to enter the combustion chamber until the next cycle. Hydrocarbon emissions skyrocket and BSFC goes kaput along with fuel economy.