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).

C. Here are three common types of Bosch-designed fuel injector nozzles. The multiple-hole version (a) uses a tapered needle that is withdrawn from its seat for fuel delivery. The throttling nozzle (b) has a deflector cone just below the needle tip to improve fuel atomization. And the straight-tip pintle nozzle design (c) relies on sharp-edged discharge orifice openings and high fuel pressure to provide good atomized fuel. D. This for all you guys in 10th grade physics. Since air and fuel are of different weights (mass) for equal volumes, each will have different flowing energy at any given flow velocity. At point (1), both air and fuel are moving at the same velocity (speed). At points (2) and (3), a change in flow direction is required. The air particle is less affected by centrifugal forces and tends to slow down and make the turn more easily than the heavier fuel particle. By the time each gets to point (4), the fuel particle is traveling faster than the air particle (and is more influenced by centrifugal forces). And this is the basis for air/fuel mixture separation in engines for which mixtures must pass some distance from mixing valve (carburetor or upstream injector) to combustion chamber. Direct cylinder (or port) injection helps solve this fundamental problem of lost power and reduced fuel economy.

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.

By injecting fuel directly into the engine’s combustion chamber, less fuel will be left in the induction system. And since air can be passed more efficiently than fuel into an engine’s cylinders, late fuel entry is best achieved by fuel injection near the combustion chamber.
All this good theory is nice. But what we’re trying to achieve is the proper mixture of air and fuel (under favorable conditions of combustion pressure and heat) in the most-used range of engine rpm. If the engine is a wide-open-throttle race piece, little consideration should be given for light-throttle acceleration and/or idle characteristics. But if you’re faced with sub-zero cold-starts on your way to school or work, who cares how well an injection system works at 8400 rpm on the back straight at Riverside?
One thing’s for certain. With the present concern for fuel economy and the advantages possible from a correctly designed fuel injection (or electronically monitored) system, conventional carburetion may be in for some serious redesign. And for sure, we aren’t fueling.

REVIEW QUESTIONS: True or False
1. Carburetors tend to atomize liquid fuel into smaller particle sizes than conventional fuel injection systems.
2. Injector nozzle fuel pressure has little to do with the size of atomized fuel particles entering the airstream or combustion chamber.
3. Regardless of injector nozzle atomization efficiency, large fuel droplets burn more quickly than smaller ones.
4. Fuel injection systems that inject fuel directly into the combustion chamber usually provide less performance and economy than systems that inject fuel into the intake manifold or intake port.
5. Timed injection means fuel delivery based on precise matching of the ignition system’s rate of timing advance.
6. Solid fuel injection systems inject fuel into the engine’s cylinders at a time related to piston position, usually during the last half of the intake stroke.
7. One disadvantage of electronic fuel injection is that it cannot compensate for variations in altitude and incoming air temperature.
8. Fuel passing out of an injector nozzle is atomized primarily because of fuel particle friction with the air it is entering.
9. For optimum fuel delivery into an engine’s cylinders, it is best to time injector operation with the point at which the intake valve just begins to open.
10. Injector nozzle deflector cones (on the discharge ends of injector needles) are used to improve fuel atomization.
11. As air and fuel travel through an induction system, fuel has less kinetic energy than air, thus enabling it to turn corners more easily than air.
12. Solid fuel injection systems inject fuel with no particular regard to piston position.
13. Air injection systems rely on high-pressure fuel and low-pressure air to supply an engine with the proper air/fuel mixtures.
14. This month, answers to some of the True or False questions came from study of the graphics part of the story.