How Electronic Fuel Injection (EFI) Works - How To

How EFI works
When a carbureted engine is running, airflow creates a low-pressure area in the carburetor venturi. Because the fuel in the float bowl is at atmospheric pressure while the pressure in the venturi is something lower, fuel is forced through the metering jets into the venturi, and from there it's carried by the moving airstream into the combustion chamber. In short, fuel metering is dependent on the size of the holes in the jets and the strength of the vacuum signal in the carburetor venturi. Because the vacuum signal is affected by everything from the engine's rpm and throttle position to the condition of the rings and the type of exhaust system you're using, it's a particularly crucial piece of the puzzle and one that can complicate the jetting process to no end. Granted that's a great oversimplification of how a carburetor works but it hits the high notes and hopefully, gives you some inkling of why it takes a very sophisticated carburetor to provide really accurate fuel metering.

In a sense, Electronic Fuel Injection is a lot simpler than a carburetor. Reduced to its essentials an EFI system is nothing more than a nozzle that sprays gasoline into the airstream whenever a computer tells it to. Of course, the devil is always in the details and those details, in particular the processes the computer uses to determine how much fuel to supply can get pretty complicated.

Mapping Strategies
All electronic fuel injection systems work in essentially the same way; information about the engines operating state is conveyed to the computer via sensors. The computer then compares that data to a stored map of fueling requirements. Once it finds what it likes, it tells the injector to remain open and spraying fuel for a given length of time.

Since all of the hardware is roughly the same, the real difference between EFI systems is in how that hardware is driven, or mapped.

Because we're primarily concerned with V-Twin cruisers here, we're going to limit the discussion to the mapping most commonly used on them, the Alpha-N system, which uses throttle position to compute engine load and the Speed-Density system, which determines load by measuring the intake manifold vacuum.

Alpha-N
Alpha-N systems are simple, a little crude by the latest standards and very effective. They don't require a whole lot of sensors or a very big ECU to get the job done, which makes them cost effective, and popular. While their simplicity, especially when it comes to remapping for tuning purposes makes them attractive to the go-fast crowd. As a bonus, they work very well when volumetric efficiency exceeds 100% as it does when forced induction is fitted, so if a blower or turbo is your thing, an Alpha-N system, with some upgrades to measure the increased airflow is just the ticket.

At its most basic, the system works by comparing the throttle angle or "alpha" to the engine rpm, or "N." Once it knows what rpm the engine is spinning and where you've got the throttle it makes a few quick, like microsecond quick, calculations and then selects the correct fueling points from the map. Other sensors are incorporated on an as needed basis to provide as much information as the systems designer deems necessary.

The downside to a pure Alpha-N system is that it's not the most precise form of EFI on the market and because of that, fuel economy and at times power output may fall short when compared to some of the more sophisticated systems. Alpha-N systems can also have some throttle response issues at low and cruising speeds, particularly if the throttle position sensor isn't precisely adjusted or needs cleaning and they are more sensitive to routine maintenance issues than other systems, something as small as a dirty air filter can throw one for a loop.

Lastly, because they are rather crude, straight Alpha-N systems are at the bottom of the food chain when it comes to emissions and mileage. Because of that, they are quickly falling by the wayside, at least as a stand-alone system.