Mechanical Fuel Injection Testing

Friends,

One of the things that we have been testing lately is a mechanical fuel injection system from Precision, makers of certified aircraft fuel systems.

Precision has developed a single point fuel injection system that is entirely mechanical for engines in the Corvair’s power category. We have had one of these for several months and conducted a number of tests. Below is a quick outline of some of the data we’re collecting.

Above on the left is the Precision fuel injector, on the right for size comparison is an MA3-SPA Marvel Schebler carburetor, the most popular Corvair carburetor. The Precision injector is designed to fit in exactly the same space with the same bolt pattern as an MA3.

Just for starters, let me say that many people do not understand the function of mechanical fuel injectors on aircraft. The above unit is closely related to the operation of an RSA fuel injection system, the gold standard of mechanical fuel injectors. Part of what confuses experimental aircraft builders is the fact that there are a number of carburetors that include in their name the misnomer “throttle body injector” or “Aero injector.” Both the Ellison and the Aerocarb are useful carburetors but in no way shape or form are they fuel injectors. They are simply flat slide carburetors that do not have float bowls.

The defining characteristic differentiating a mechanical fuel injection system and any other type of carburetor is simply that the mechanical fuel injector meters off density, not off volume. Anything that meters fuel off the volume of air flow that comes through the throttle is a carburetor. Such systems will always change their air/fuel ratio as the density of air changes. Conversely, mechanical fuel injectors, such as the Precision unit, meter off density. When they are set to a specific air fuel ratio they hold it, no matter what altitude you climb to, nor how the conditions change. If you look closely, there are four chambers on the diaphragm of the Precision unit. These four chambers allow this to function as a very precise pressure regulator and metering device based on the mass flow of the air passing through the assembly.

To give you some idea of the quality of this unit, and its adaptability to different airframes, the directions actually spell out that it can be run on any fuel pressure unregulated between 20 and 80 PSI, and it will handle momentary over pressures to 180 PSI without damage. Because of the diaphragm assembly, the pressure can actually fluctuate between any of these pressures and it will not change the air/fuel ratio.

The primary difference between the Precision system, and typical certified aircraft systems, is that this is a single point injector that does not have injector nozzles in the intake ports. It has one nozzle that is in the body of the unit after the throttle plate. This unit is immune to carburetor icing. Yet in operation the fact that the fuel is vaporizing 18 inches upstream of the intake ports allows a very significant evaporative cooling effect. Unique to this unit is the fact that it is equipped with a very potent accelerator pump that gives it instantaneous throttle response that one associates with port fuel injection.

Above, the Precision injector mounted on a 3,000 cc Corvair on our engine test stand. Bolting on the injector in the place of our typical MA3 only took 30 minutes. Even the throttle arms are in the same location. As far as I know, this is the first mechanical fuel injection system that has ever been used on a Corvair engine turning a propeller.

The system is not cheap. Its suggested retail price is more than $2500. If you are building a Pietenpol and were planning on using a Stromberg, you’re probably not going to change plans and pick up one of these injectors. However let’s look at this from a different perspective. People who are spending $18,000-$20,000 to buy a Rotax 912 or a Jabaru 3300 will find that their engines are equipped with one or two Bing motorcycle carburetors. Although these carburetors are allegedly altitude compensating, in practice they are far from it. You can ask any operator of a Jabiru engine and they will tell you that at high power settings and high altitudes, their engines are very thirsty, and they have no way to compensate for this.  These expensive buy-it-a-box engines come with Bing carburetors because they are cheap. 

Now let’s look at the 3 L Corvair engine with the injector above. It is the most expensive Corvair powerplant I have built in the past couple of years; the price of the engine complete without the fuel injection on it is $11,500.  At $14,000, it is $4000-$6000 less expensive than imported engines fed by motorcycle carburetors. Combine this with the Corvair’s made in America pedigree, and its reputation as tough as nails, and many people find that it’s a choice they’re interested in. Corvairs are not for everybody. Most people don’t actually care where their products are made, or if they will be serviceable in five years. It’s a free world and those people can find engines that suit their needs. For people with different value systems, I am glad to conduct R&D to find out what is a real value in high-end engines.

Above, the engine runs on the stand in front of our hangar. The fuel line leads down to a rack of instrumentation, a high-pressure pump, and an external pressure regulator used for testing purposes. The engine ran very smoothly, and passed our early tests. I will have the unit on display and more information in our NO34 booth outside Building C at Sun N Fun all week.