Fuel Injection Information

Exotic as it may seem, this approach to fuel delivery solves many basic problems of good combustion efficiency. and it also applies to stock engines

Understand how the conditioning of air and fuel can affect overall engine performance. We’ve also touched on the importance of when air/fuel mixtures enter an engine’s cylinders, how they are best burned (oxidized), and some of the better ways of exiting exhaust gases. There has even been some mention of what happens when fuel particle size varies (from one droplet to the next) and the improvement in combustion efficiency when some measure of control is provided regarding atomization of fuel.

Carburetors, it has been suggested, are a form of compromise in comparing the best theoretical way to introduce fuel to an engine in the most cost effective way (just a fancy “accountant’s” way of saying does it cost more than it’s worth). But with the rising cost of today’s fuel, and the roads not getting any shorter, fuel economy is of increasing concern.
Fuel injection techniques offer some interesting solutions to the

classic problem of getting the correct amount and condition of fuel into an engine. And perhaps one of the first aspects to consider is the matter of fuel atomization (breakup into small particles).
Typically, fuel is delivered into a moving stream of air (sometimes heated air as in the case of diesel engines). As fuel leaves some sort of jet (or injector orifice), frictional contact between the fuel and air causes small particles of fuel to break away from the liquid stream (or “thread” of fuel) leaving the jet. Figure A shows the relationship between liquid and atomized fuel as both depart from a conventional injector nozzle. The amount or degree of fuel atomization can be seen by the smallness of particles or by how small the variation in size is among the atomized particles. At the risk of overemphasizing all this business of “how big the droplets are,” keep in mind that large droplets “burn” slowly while smaller ones burn more quickly (and efficiently). An injector nozzle that provides small and relatively equal droplet sizes of atomized fuel will allow an engine to produce more power and economy, assuming that something else doesn’t disrupt fuel particle size somewhere between nozzle and combustion chamber. Keep in mind also that the amount of pressure applied to a given injector will affect atomization efficiency: less pressure, larger droplets.
Also important to the effectiveness of injector nozzles is the direction in which the fuel is fed to the airstream. For example, even the most atomization-efficient nozzle can be rendered ineffective if it squirts fuel into a region of slow-moving air. This can result in “liquid sheathing” (reli-quefication of the atomized fuel) between nozzle and combustion chamber, effectively reducing the overall efficiency of the nozzle.
Timing of the injected fuel is also critical. Let’s suppose there is control over the point (relative to piston position) at which fuel is injected into the engine. Earlier Shop Series have indicated that at the time of intake valve opening, some amount of pressure is “blown back” into the induction system.

A. High-pressure fuel delivered through a typical injector nozzle comes into frictional contact with air, mechanically breaking up the liquid fuel into small particles that encourage good combustion efficiency. B. If timed injection fuel is delivered into the intake port at the time of intake valve opening, reversion pressure can cause fuel to be forced back into the induction system. Reductions in both power and fuel economy can result. Usually, such systems time fuel delivery after air flow is established toward the cylinder (35-45 degrees past TDC is common for such flow).

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