Electronic Fuel Injection

Electronic fuel injection (EFI) is ubiquitous in cars but still growing in aviation. Rotax is a major user of EFI in its iS line of engines, plus aftermarket EFI systems are commercially available for Experimental aircraft. Lycoming has sold a very limited number of EFI-equipped engines as well.

Note also that some of the Jet A (diesel) burning engines use their own form of EFI, which is a separate topic for a separate article.

As the name suggests, the big deal with EFI is the system is computer (digitally) controlled. This allows all sorts of neat tricks and efficiencies at the price of cost and complexity. Another fundamental of EFI is an electric fuel pump supplies abundant fuel pressure to enable a wide range of precisely metered fuel from idle to full power, along with more finely atomized fuel sprayed from the injector. Above all, it means less engine management work for the pilot. With EFI, there is no mixture knob, and starting means twisting the key or pushing a button.

Understanding electronic fuel injection beyond pushing the button begins by examining a typical system, if even at the basic level. And there is no better system to describe than Rotax’s, as the Austrian company dominates the fuel-injected aviation market.

Fuel is drawn from the aircraft’s tank by an electric fuel pump (there are two pumps for redundancy in the Rotax system). A water/sediment screen (similar to a gascolator) is placed before the fuel pump and a fine fuel filter after the pump. The fine filter uses a metal screen and bowl, again like a gascolator. The multiplicity of filters is your first clue that the close tolerances inside EFI systems are easily clogged by dirt, and water is never welcome in any fuel.

A vapor return loop starting just after the pumps returns any fuel vapors from the churning, pressurized fuel upstream of the pumps.

From the fine filter, the gasoline is plumbed to the engine, where it circulates through a fuel rail—metal piping connecting all the fuel injectors. There are two injectors per cylinder (more on this in a minute), and both fire into the cylinder’s intake port.

At the end of the fuel rail is a fuel regulator controlling the fuel pressure in the rail. Fuel not consumed by the injectors is returned to either a header tank or the fuel tank the fuel was originally drawn from. By restricting the return fuel flow to the tank, the regulator can raise the pressure in the fuel rail.

Rotax’s system maintains fuel pressure at a certain value above the air pressure in the inlet system’s airbox—manifold pressure, in other words. In practice, Rotax fuel pressure is nothing dramatic, roughly 40–100 psi. Obviously, the manifold pressure changes with throttle setting, rising altitude, ambient temperature, and so on, plus whether a turbocharger is boosting manifold pressure.

Unlike the simple, open nozzles in mechanical fuel injection, EFI fuel injectors are more complex electric switches. This allows the computer to open and close the injector for longer or shorter times as conditions demand. This controls how much fuel is delivered to the engine.

Fuel delivery can also be timed with EFI to squirt fuel sequentially—following the engine’s ignition firing order—or all the injectors can be fired simultaneously, called batch firing. It doesn’t make much difference in aviation engines because they run in a narrow, rarely varying rpm range. This is unlike auto engines, which use sequential injection exclusively these days.

Fuel pressure is much higher at the injectors with EFI than the barely there pressure found in mechanical, constant-flow aviation injection systems. This allows blowing the fuel into smaller, easier-to-burn droplets (atomization). It also means the passages in electronic fuel injectors are tiny and easy to clog, hence the emphasis on fuel filtering.

Another way to adjust how much fuel is delivered is to vary the fuel pressure. The higher the fuel pressure, the more fuel will spray from an injector in a given amount of time. Rotax uses both tactics, varying the duration the fuel injector is open along with raising or lowering the fuel pressure. All of this is decided by the engine control unit (ECU) many times per second.

For redundancy, Rotax uses two fuel injectors per cylinder; both fire into the intake port, and both injectors fire during normal engine operation.

There are two injectors because the Rotax fuel system is in many ways two separate fuel systems running in parallel. Rotax calls them Lane A and Lane B. This explains why there are two fuel pumps, two injectors per cylinder, two air pressure sensors in the airbox, and so on. During normal operation, Lane A could be called the primary Lane. Should something go wrong with Lane A—clogged injector, dead fuel pump, or blocked filter—Lane B automatically switches into primary mode. A cockpit display informs the pilot about the trouble, but the engine should run quite well so the aircraft can be safely landed and the system repaired.

Programming (software) in the Rotax ECU automatically provides the correct fuel (and ignition spark) for all engine conditions from starting to shutdown, so there is no mixture control knob. As with all EFI systems, the Rotax engines thus automatically provide either rich-of-peak or lean-of-peak exhaust gas temperature (EGT) operation depending on how much power the pilot demands—which he does by where he sets the throttle. And if Lane A has an issue, Lane B takes over using a rich-of-peak mixture to better avoid detonation and promote cooler cylinder temperatures. This is akin to the “limp home” mode in automotive engine management systems, but unlike the dramatic power loss of automotive limp home mode, the Rotax strategy delivers a likely imperceptible power loss.

Interestingly, Rotax uses EGT sensors as a sort of double-check on engine operation, but the system runs fine without them.

Other aviation EFI systems, not to mention their automotive big brothers, differ in detail. Two aftermarket systems—Fly EFii and Simple Digital System EFI—are popular in Experimental aircraft. They use a single fuel injector per cylinder and two separate ECUs (computers) for redundancy. Of course, their big advantage is they are available to retrofit Lycoming and Continental engines (among others), while the Rotax system comes only on Rotax engines.

For pilots, the big advantage of any EFI system is that the computer and its software eliminate the need for human control over fueling the engine. The intricacies of starting hot and cold engines, plus closely monitoring the air/fuel ratio or selecting rich- or lean-of-peak operation, are all computer controlled. The pilot’s fuel concerns are thus reduced to ensuring enough fuel is in the tanks and managing which tank he’s drawing from. Better fuel efficiency comes naturally as well because the computer does a better job of monitoring and delivering the correct amount of fuel than a distracted pilot fiddling with knobs. Plus, the EFI system can control the engine cylinder by cylinder, something a pilot can’t do. There are data acquisition possibilities with digital engine control as well; these help share fuel consumption information with flight management software, plus with trend monitoring for maintenance purposes.

If the EFI pilot no longer needs to twiddle with the now-missing mixture control knob, as an aircraft owner and maintainer he does need increased vigilance over the electrical system. That’s because any EFI quits instantly the moment the incorrect number of electrons are present. A very robust electrical system is required, it must be maintained scrupulously, and it must incorporate some form of backup alternator, battery, etc. This is even more important when EFI and electronic ignition are combined on the same engine, which is nearly always the case if EFI is fitted. Such an aircraft is electrically dependent and demands robust primary and secondary electrical systems.

Then there is the complexity of EFI. Carburetors may be old school, but they are eyewitnesses to the laws of physics. You don’t have to tell a carburetor much of anything, as it natively senses things such as density altitude. But EFI is inherently blind to the outside world and must be fitted with an array of sensors to provide the necessary information. Thus, there are crankshaft position sensors along with any number of temperature and pressure sensors, all connected by wires to the computer. It seems the computers are very robust and essentially never fail, but the wiring can get compromised and sensors sometimes go silent. EFI issues are rare—they’re as reliable as any other fuel system—but if issues arise, they’re likely going to be dead sensors or injured wiring.

Oxygen sensors would be nice, but can’t survive the lead in 100LL gasoline, so they are not regular aviation practice. Furthermore, the Rotax specialists at Motive Aero underscore the importance of running quality gasoline of the correct octane rating Rotax calls for. Low-octane gas is especially bad at altitude or high ambient temperatures. The system can compensate somewhat for poor gasoline, but can’t work miracles.

Finally, for no practical application other than the amusement of the tech heads in the audience, aviation EFI practice is to use speed density engine management systems, while automotive practice follows mass air principles. Rotax, Fly EFii, and SDS EFI are all speed density, so they have no mass air meter, meaning less weight, less cost, easier packaging, and lower air restriction. They use simple “look-up” fuel and spark tables so they are fast reacting, but must infer air density from throttle position, manifold pressure sensors, and the like. Automotive systems read air density directly with a mass air meter and require more processing power to run algorithms to arrive at fuel and spark values, but handle tricky rpm transitions and engine conditions a little better. All this doesn’t matter to the huge majority of pilots and A&P techs, but it’s a reminder to tinkering Experimental types that fine-tuning aftermarket EFI systems can be part of the plan when hot-rodding engines.