Injected Mixture
Fallout from one more moon shot, as applied to the automotive industry, will probably require you to say “good morning” to your car just to get it started in the morning.
Two other aspects of fuel injection, at this point, are worth understanding. One is the point at which fuel is injected relative to piston position (“timing” of the injected mixture) and the other is how long this injection should last (“duration” of the injected charge).
Ideally, fuel should be introduced in the inlet air stream while air is moving toward the cylinder. In fuel injection systems where fuel is injected outside the combustion chamber, this is normally about 30-45 crankshaft degrees after top dead center of the intake stroke. By this time, any residual exhaust pressure (reversion) passing out of the cylinder and back into the intake system has subsided and fresh air is moving into the cylinder. It doesn’t mean that some amount of exhaust gas won’t still be in the cylinder; it just means the injected fuel has little chance of being blown back into the intake system once sprayed into the airstream.
For engines fitted with combustion chamber injection systems, the injection of fuel is normally timed so that the combustion process actually begins before a substantial amount of fuel is discharged into the chamber.
State-of-the-art electronic fuel injection systems utilize direct port injection as shown for this small-block Chevrolet V8. Temperature sensor (1) for engine coolant substitutes for conventional choke. Intake manifold vacuum taps—like (2) and others not visible—relate to net air flow, which is fed to a mini-computer for air/fuel mixture control. High-pressure fuel (35-40 psi) is maintained in “fuel rails” (3) that feed the nozzles. Inlet air is throttled by an air valve (4) that substitutes for a conventional carburetor.
E. This is a typical “continuous flow” injection system. Changes in engine air flow make the “air flow monitor” open or close, causing the “fuel quantity contol” module to provide more (or less) fuel to the engine. This system is common to Porsche engines in particular. F. As an example of how electronic fuel injection systems monitor a variety of engine variables for the ultimate control of high-efficiency air/fuel mixtures, here’s the crux of the Bendix system as designed for use on Cadillac Seville engines. (With the correct CB frequency, you can stop one of these critters right in the driveway—but that’s another story.) Note the data supplied for input and what the micro processor can supply to the engine as output. In the next few years, you’ll see a variety of “electronic mixture control” devices on new cars, many of which are offshoots of this system.