Mechanical Fuel Injection

Fuel injection is a catchall term covering nearly any method of delivering fuel into an engine outside of carburetion. Fuel injection systems can be entirely mechanical or nearly all-electronic and can differ wildly in detail. But the common thought behind fuel injection is the fuel is pressurized by a pump and sprayed into the engine. Carburetion, as we’ll recall, relies on the depression of air pressure created by a venturi to pull the fuel out of the float bowl.

Let’s begin with mechanical fuel injection by describing the most common small aircraft fuel injection, the robust Bendix RSA constant-flow, mechanical system commonly found on Lycoming engines. If your understanding of “fuel injection” is the electronic systems under automobile hoods, you’ll soon understand the Bendix system bears zero resemblance, and if you’re not familiar with automotive “EFI” systems, it’s hardly a deficit.

The Bendix system begins with fuel delivered in a single hose from the fuel system. Thus, fuel flows from the fuel tank, past an electric fuel pump, the gascolator (not commonly used in experimental installations), then the engine-driven fuel pump where the single fuel hose journeys to the fuel servo. Screens at the fuel servo inlet help keep dirt out.

The fuel servo is the heart and brain of the Bendix system and where the intake air and fuel systems begin to interact. The fuel servo is bolted to and is part of the intake system; it contains a large air passage with the throttle to control airflow. Off to the side are diaphragms in chambers; one senses fuel pressures and the other air pressures. Together these diaphragms move a valve to allow the correct amount of fuel to flow for the amount of air flowing past the throttle. Thus, the “fuel servo” meters fuel to the engine based on engine airflow consumption, holding a constant fuel-air ratio with changing engine power.

From the servo, another single fuel line carries the fuel to the flow divider, commonly called “the spider.” Looking like a downsized hockey puck, the flow divider is found atop the engine, where small, delicate-looking metal lines radiate from it to each cylinder. The flow divider evens fuel delivery among the cylinders, but is only in play at low fuel pressures (typically engine idle) and ensures a clean engine shutoff when the mixture control is pulled to the stop. It also evens the fuel pressure to each cylinder when the engine is inclined, such as sitting level (think of a taildragger on the ground).

At each engine cylinder the metal lines terminate at a fuel nozzle, also casually called an injector, although nozzle is far more descriptive of these simple orifices with no moving parts. Called an air-bleed nozzle, the nozzles direct the fuel into the intake port, an inch or so behind the intake valve (not directly into the cylinder). At idle, fuel is atomized at the nozzle by the differential pressure between manifold pressure and atmospheric pressure.

In operation, the Bendix system meters a constant flow of fuel according to how much air flows through the fuel servo. There is no pulsing of the fuel flow and no timing of fuel delivery to each cylinder. Fuel simply sprays constantly behind each intake valve, and if the intake valve is closed, fuel loiters in the intake port, then is rushed into the cylinder when the valve opens. This sounds crude but works just fine. At higher power levels, much energy is released when the intake valve opens and low pressure violently draws — you could say explodes — the fuel-air mixture into the cylinder. Much mixing and atomization of the fuel droplets occurs at this point.

The steady fuel flow also gives the Bendix system part of its general description as “mechanical, constant-flow fuel injection.” You can add “multi-point” to that description as the system delivers fuel individually to each cylinder and not one spot, as a carburetor does.

A few details are worth noting to the prospective Bendix-operating pilot or kitbuilt aircraft builder.

First, while the Bendix fuel servo is a mechanical symphony of small passages and serviced only by specialists, it is very reliable. At the latest by the 1970s, fluorosilicone diaphragms replaced early Buna rubber diaphragms, eliminating any issues with torn or stiff diaphragms.

While the Bendix RSA system is normally not equipped with altitude-sensing and compensating gear, the system inherently follows air density — sort of. The fuel-air ratio will not precisely lean with altitude, but it does lean sufficiently, and piston engines are tolerant of a relatively wide fuel-air ratio band so that the pilot need not adjust the mixture during climbs and descents, or if so, very little. A 5,000-foot change in altitude typically does not require adjustments of the mixture control by the pilot.

Expect to hear the electric pump called the “boost pump” and find it is used for priming, takeoff, landing, and emergencies, such as when the engine-driven pump fails (rare). Otherwise, the fuel is supplied by the engine-driven diaphragm pump.

Other than an electric boost pump, no electrons are required to run the Bendix fuel injection. It is powered by engine rotation running the diaphragm fuel pump. A total in-flight electrical failure in a Bendix-injected airplane poses little risk from a fuel delivery standpoint.

Finding the best balance between fuel pressure and nozzle diameter is a challenge for Bendix engineers thanks to the relatively low energy (low fuel pressure) inherent in the RSA system. Typically, these systems use an engine-driven diaphragm fuel pump delivering a relatively low 20–35 psi fuel pressure. Therefore, at maximum engine power, a large-diameter fuel nozzle may be required to supply enough fuel volume given the system’s low fuel pressure, but at idle, only a tiny 0.1 psi of fuel pressure is needed to avoid delivering too much fuel through the big nozzles. Fuel flow is thus quite slow at idle, such that once fully warmed, engine heat can boil fuel in the nozzle lines, leading to a lumpy idle and hot-start issues. In some cases, optimizing fuel distribution by tuning nozzle sizes and pump pressures (on experimental aircraft) can mitigate some of the issues but never completely solve them. Specialist help is best when nozzle tuning (see Airflow Performance).

Speaking of engine start-up, fuel pressure must be present, and if the engine isn’t running, the diaphragm pump does no good. A hand-operated wobble pump was the WW-II era answer; for many decades now, electric pumps do this job. The electric pump is briefly energized by the pilot to prime the engine with fuel before cranking the engine when starting. How long to prime is one of those things learned by experience and varies with engine and ambient temperature. It is exactly what computers now do on car engines and thus seems a black art to a new pilot in the 21st century. Not the least because it is indecipherable unless you have a clue how the Bendix system works and how much fuel the engine might require, plus you’ve made priming a side hobby in your aviation journey. We can address engine starting in a future article.

It’s worth noting the correct diaphragm-style, engine-driven fuel pump is required; a pump designed for a carburetor application can be visually identical but will not supply enough pressure to run the Bendix system properly. It’s imperative to install the correct engine-driven pump at overhaul or repair time.

The small metal fuel lines running from the flow divider (spider) to the cylinders are best treated respectfully while servicing the engine. Setting heavy tools or leaning on them is poor practice; these lines can crack at their ends where a half-sphere is soldered on to form a sealing surface. After thousands of hours in operation, vibration can eventually lead to cracks; rough handling the lines doesn’t help any.

Like any system delivering fuel cylinder by cylinder, the Bendix RSA system allows more effective leaning and inherently supports lean-of-peak operation than carburetion, something to discuss in future articles.

The Bendix fuel injection has been duplicated and improved by others and offered as Silver Hawk EX injection for experimental aircraft and the FM series by Airflow Performance. In fact, Airflow Performance is absolutely the last word in Bendix RSA fuel injection; Don Rivera at Airflow Performance was employed at Bendix and worked directly with the developer of the RSA system there. Thus, Airflow Performance services and overhauls existing systems, offers its upgraded FM series, and serves as the expert consultant on the Bendix system. Today, AVStar and Airflow Performance supply Lycoming Engines with the majority of certified and experimental engine fuel injection systems.

There is an earlier fuel injection system offered by Bendix in the post-WW-II timeframe. Confusingly, it is known as the “PS5 carburetor” even though no carburetor is present. The confusion dates from big WW-II engines that ran “pressure carburetors,” which weren’t really carburetors, but the term “fuel injection” wasn’t in wide use at the time. They were Bendix systems similar to the multi-point Bendix RSA system detailed here with the same pump arrangement and a somewhat similar fuel servo (misnamed the carburetor), but minus the flow divider and fuel lines to each cylinder. Instead, the fuel was injected by one very large fuel nozzle ejecting into the intake manifold, or very typically into the entry of a supercharger. “PS5 carburetors” are still with us on older engines and some homebuilts. They’d best be called “single-point, constant-flow, mechanical fuel injection,” but we’re stuck with “PS5 carburetor.” It is also a reliable, accurate way of delivering fuel and requires very little adjustment by the pilot.

Continental engines have their own mechanical fuel injection system, which is ubiquitous on big-bore Continentals but rarely seen elsewhere. A simplified description shows it, too, uses an electric boost pump for priming and emergencies, but the main pressure actor is an engine-driven vane-type injection pump. This injection pump can easily supply more fuel than is needed, so a fuel-air control unit returns fuel via a bypass loop to the injector pump according to where the mixture and throttle are set. Downstream of the fuel-air control unit is the manifold valve, which looks similar to the flow divider in the Bendix system. It has two spring-loaded circuits, one for idle and the other for all higher power settings, and like the Bendix “spider,” it distributes the fuel to simple nozzles at each cylinder.

Like the Bendix system, the Continental injection is mechanical, constant-flow, and multiport. It, too, is very reliable and has the advantage of being able to purge fuel through the injection pump and air-fuel control with the electric boost pump. This ensures liquid fuel is present to avoid hot-start drama.

In the next article, we’ll peek behind the electronic fuel injection curtain.