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 hydrocarbon (Hc) emissions, better fuel economy which comes from a higher BSFC (Brake Specific Fuel Consumption is the ratio between the engine’s fuel mass consumption and the crankshaft power it is producing), and greater atomization of the liquid fuel for a more even burn. If injectors spray while the valve is closed, fuel will hit the back of the intake valve and the side walls along its way to the combustion chamber. These droplets atomize later, when the valve opens and air rushes through, but is impossible to measure because it has already been accounted for during the previous cycle. This approximate measuring leads to lower fuel economy and higher Hc emissions.

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.

Posted on September 29th, by admica in Mechanical, vehicles.

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