Engine Theory: Fuel Injection

Fuel injection is a catch-all term for any number of mechanical or electronic fuel delivery systems. Detail differences abound, so a bit of precision helps when addressing the subject. For example, when we hear “fuel injection” today, we mentally default to “multi-point sequential-fire electronic-port fuel injection,” or simply “EFI,” because that’s what cars have used for the last quarter century. But that’s not what we have in aviation (except for newer aftermarket systems).

The Bendix Baseline

At the start of WW-II the Germans were ahead of everybody with Bosch direct-cylinder mechanical fuel injection (a result of diesel engine development). Attempts at multi-port fuel injecting Allied airplane engines were mostly unsuccessful or not developed in time (your first clue fuel injection is not your average technical accomplishment). Post-war, Bendix developed their wartime single-point pressure carburetor system into the RS multi-point fuel injection, and by the late 1950s that was detail improved into the RSA system that’s still with us, both in original form and updated by several aftermarket sources, notably Airflow Performance and Precision Airmotive.

Bendix’s RSA is constant-flow, mechanical fuel injection. An engine-driven diaphragm pump supplies fuel to the fuel servo; this is a throttle body and fuel metering assembly that typically mounts in the same place as a carburetor. The servo senses air pressure and employs a series of diaphragms to meter the fuel flow for the mass of air passing through the throttle body portion of the servo. But unlike carburetion, the fuel is not administered to the air stream at the fuel servo; instead it is routed to the flow divider. Like a railroad roundhouse the flow divider parses the fuel to small lines running to each cylinder’s intake port. There fuel passes through a precision nozzle, spraying in a constant stream into the intake port, just upstream of the intake valve.

Note there is no pulsing of the fuel; it flows in a steady stream. Fuel pressure as delivered to the fuel servo varies with demand, and is often around 20 psi, but can rise to approximately 45 psi. Fuel pressure is the energy operating what could be called an analog fuel computer (the fuel servo), and so fuel pressure is, by design, consumed operating the various diaphragm springs, overcoming line losses, and pushing fuel through the main jet. Therefore fuel pressure is much lower at the fuel nozzles than at the fuel servo. Nozzle pressure can well be under 1 psi at idle and around 7 psi at full throttle.

Clearly the big advantage is the fuel is administered individually to each cylinder rather than a single point as with carburetion. Mixture variations are limited to intake manifold design, something the engine manufacturer can easily get close, plus you can fine-tune mixture variations by substituting different size nozzles. Each cylinder can be more closely maximized for power, economy, and aggressive lean-of-peak operation; greater maximum engine power is thus possible compared to rudimentary carbureted systems, and more economy is possible when leaned, too. The RSA system features a standard fuel mixture control knob in the cockpit, plus an automatic altitude compensation circuit so the pilot need not readjust the mixture because of subsequent climbs or descents.

Unlike a carburetor no fuel is administered at the venturi inside the fuel servo (there’s still a venturi to generate an airflow signal), so icing is eliminated. Instead, an alternate air source is provided in case the main engine air inlet gets clogged from toilet paper when you’re cutting up that roll you flung overboard—it only takes one square…

Disadvantages are cost, complexity, and therefore an increased number of failure points. That said, the simple Bendix system is tough to fail. The diaphragms have proven bulletproof, a backup boost pump saves the day should the engine-driven diaphragm pump fail (rare), leaving debris the only real-world worry. Even then, grit clogging the fuel servo causes the system to run dripping rich. Merely pulling the mixture knob to nearly idle cutoff typically restores a workable mixture and thus power.

More annoyingly, the small injection nozzles are easy to plug by minute bits. Typically this causes rough running until the nozzles are removed and the trash back-flushed. Obviously, fuel filtration and system cleanliness are required.

With no float bowl, a fuel injection system needs a non-engine-driven pump for priming. In practice an electric pump serves as the priming pump and as emergency backup to the engine driven pump. Otherwise the Bendix system is purely mechanical, needs no electrical system, thereby segregating the electrical system as a failure point to the fuel system in flight.

A rarely encountered limitation of the standard Bendix system is its fuel- metering window can be slightly narrower than needed, so fuel metering on a large displacement, hot-rodded engine can grow increasingly inaccurate when heavily leaned. This isn’t a normal issue on mainstream engines, but with high-power Experimental engines, the system fuels precisely at WOT and rich-of-peak high-power cruise settings, but cylinder-to-cylinder variations show up when lean-of-peak at low power (manifold pressure) settings. Think of an RV-10 leaned to near strangulation at 12,000 feet. Careful matching of nozzle diameters, fuel pressure, and diaphragm spring pressures in the flow divider can address this issue.

Electronic Fuel Injection

Sharing little more than the label “fuel injection,” EFI as we know it from automotives is completely different from aviation’s constant-flow, mechanical fuel injection standard bearer. But automotive EFI is where Experimental aviation seems headed, so it begs description here.

In now traditional automotive EFI, the action begins with an electric fuel pump supplying fuel at a metered pressure—typically around 40 psi—to the fuel rails. These are simple galleries fitted atop and linking the individual fuel injectors. The injectors are electrically powered, computer-controlled solenoid valves; when open they spray fuel into the intake port.

Of course there are filters and fuel regulators, and the fuel can either run in a constant loop from the fuel tank, through the fuel rails, and back to the fuel tank (old school, less fuel heating at the injector during hot starts), or be a one-way returnless design (newer emissions-driven design with less fuel heating and vapor-inducing agitation of the in-tank fuel).

The EFI advantage is computer control. A small army of sensors measures many things including engine speed, crankshaft, camshaft and throttle positions, plus intake air mass is measured directly by a hot-wire style mass air sensor. About ten times per second the computer uses all this information to compute when and how long to fire the injectors, thus controlling the air/fuel ratio by the amount of fuel delivered.

Automotive computer strategies vary greatly among manufacturers, and the computations are more complex than sensing rpm and airflow, then looking up spark and fuel values in a table. And yes, the computer also controls the ignition timing and camshaft timing (sometimes that’s four camshafts all moving independently of each other) and is programmed to trim the fuel (and spark and cam) computations as required by possibly 30 different parameters. These include engine coolant temperature, rate of engine acceleration, knock sensor input, what gear the transmission is in, emission requirements such as EGR function and carbon canister purging, WOT enrichment, accessory loads from the air conditioning and possibly the alternator, engine de-tuning during automatic transmission shifting, emergency engine air cooling (via cylinder deactivation) in the case of coolant loss, and seemingly if the dome light is on. These systems even adapt mildly to the driver’s historical driving style and are sometimes also adjusted for traction conditions (snow, rain, mud, dry) as the driver selects from a dial. Adjusting the software code to a particular engine and car application, called mapping, is a long, laborious process for the manufacturer; four technicians with access to every tool, climate control lab, and a host of worldwide proving grounds (Abu Dhabi in summer and Fairbanks in winter) can take three years to fully map a new engine’s management software. Things such as setting the cold start strategy can take weeks to map simply because you only get one cold start per overnight heat soak. You get the idea.

In the 1980s such systems fired all the fuel injectors at once (batch fire), or one cylinder bank in a vee engine at a time (bank fire). But with advances in computing, sequential firing has long been the norm, where the injector triggering is synchronized with the cylinder’s firing order. The efficiencies in batch vs. bank vs. sequential firing are small and mainly driven by emission and transient response (changes in engine rpm) concerns.

EFI on the Fly

Today companies such as EFII—there are several others besides the EFII system detailed here—are offering aftermarket electronic port fuel injection systems for Lycomings. Like the auto systems just described, these are actually engine management systems incorporating the ignition along with the fuel. Unlike auto systems, the aviation systems (including Continental and Lycoming efforts that haven’t reached the market) are much simpler in that they concern themselves strictly with the engine and don’t bother with interacting with the rest of the airplane (responding to propeller pitch or flap position, lets say). Also, airplane engines run a far narrower rpm range and change rpm much less often and more slowly than auto engines, no knock sensors are used because our loose-tolerance air-cooled engines are mechanically too noisy, and 100LL is universal. EFII’s system is also batch fire, eliminating the need for a camshaft sensor.

Furthermore, unlike mass air auto systems, aviation EFI systems are speed density. They don’t directly measure air mass, but infer it from the air temperature, barometric pressure and engine rpm. This is notably less expensive, but requires mapping the software to each engine, and if something meaningful is changed (cam timing), it has to be remapped. Thankfully the mapping requirements for our aviation applications are hugely simplified from automotive needs. Heck, your lawn tractor might require more mapping if it were EFI.

Such aftermarket aviation systems are a big step forward and provide experimenters new opportunities. Ultimately outfits such as EFII, SDS, Precision Airmotive, and others are showing the way to reduced pilot workload and more easily-gained fuel economy among other things. But they are aftermarket items from tiny development budgets and also require modern thinking and are absolutely electrically dependent. If that electric fuel pump quits, it’s going to get very quiet, so an airplane running EFI must be electrically robust. Professional wiring standards, dual alternators, batteries, buses or some combination of these are mandatory. In short, EFI needs integration into the entire airframe and the builder’s thinking.

Hot and Cold Manifolds

One last thing: hot intake manifolds. In the flat-engine beginning (1940s), carb icing was a big fear, and an easy answer was to preheat the intake air. An easy solution on a horizontally-opposed engine is to package the intake runners through the oil pan. This reduces intake icing, but also air density and thus power.

In response, the aviation aftermarket offers cold air intakes for use with fuel injection, and they are a must if maximum power or fuel efficiency is the goal. While these cold air intakes absolutely make power, recent tests suggest the majority of their gains are from something besides cooler intake air. Optimized runner length and shape, plus plenum volume and other tuning are likely their largest benefits.

Unfortunately, these systems are too costly at aftermarket economies of scale to pencil out in fuel budget savings, so they remain a hot rod trick for aerobatic and racing types. But they are available if you’re experimenting for maximum efficiency or have a need for speed.

The Future

Going forward, electronic engine management (fuel injection and ignition commanded by the same computer) seem obvious as new aircraft become electronically intensive and robust. Reduced pilot workload (no mixture knob), easier starting, smoother operation, better fuel economy, more power at altitude (fewer misfires and adjustable ignition timing), no-hassle lean-of-peak cruising, and reduced spark plug fouling (lean ground operation) are all benefits. Still, such systems are more expensive and relatively untested in aircraft. In the short term, financial reality shows there’s plenty of life left in legacy aviation intake systems when it comes to aspirating our simple, steady-state rpm engines. In the long term, the march of progress will continue.