How Does A Motorcycle Fuel Injector Work?

Connecting the dots to deliver the correct fuel mixture for every condition

Automotive fuel injection in general measures the airflow going into an engine and then supplies the fuel that mass of air flowing every second requires. Motorcycle fuel injection, however, uses what is called an "N Alpha" system, which is mapped injection.

Manufacturers take an example of the engine, run it on a dyno at all different throttle levels and rpm, work out the correct fuel mixture for every condition, and then they make a map. They have all the N variables and the map connects those variables by supplying a particular amount of fuel and telling the injectors how long to stay open.

Air is controlled by a butterfly valve. If you have a steel throttle cable, when you turn the throttle the cable physically pulls the throttles open. If you have a throttle-by-wire system, like those found on more modern motorcycles, turning the grip tells the computer what you want and it opens the throttles a certain amount.

This Honda CBR600RR uses a more primitive system. The throttle-position sensor reports the throttle angle, called Alpha. The teeth on the wheel located on the end of the crankshaft whiz by another sensor, which determines engine rpm. With those two pieces of data, the ECU looks up in the map the correct amount of fuel for that condition.

When you go to the races and see a technician with a laptop plug into a bike and start doing what appear to be mysterious things, they are not mysterious at all. The rider has said, “Could you do something about that hiccup coming off turn 8?” And the technician fixes the problem with a few keystrokes. It’s a different way of doing things from the old carburetor days, but the end result is the same: trying to get to the correct fuel mixture.

Fuel injection compensates automatically for altitude, temperature, and barometric pressure because it has sensors that measure air pressure. If you ride up Pikes Peak, for example, you will have the same mixture at the top that you had at the bottom. You won’t have the same power because air density is lower. On a cold, high-pressure day, your engine will make more power but it also won’t go lean like a carbureted engine does because it has automatic mixture compensation.

Digital technology has a few things going for it.