As shown on this Triumph,...
As shown on this Triumph, fitting EFI to a bike originally equipped with carburetors is a neat trick...
Speed-Density
Speed-Density systems determine engine load by measuring the manifold air pressure, that's the density part, and correlating it to the engine rpm, which is the speed portion of the equation. The ECU then uses that calculation to determine how much fuel the engine requires under the current conditions. Speed-Density systems came about because a system was needed that would lean out the mixture at idle and cruising speeds, enhancing fuel economy and lowering emissions under steady state throttle conditions yet still provide good response when the grip was twisted, especially from low speeds.
The answer was to monitor pressure changes in the intake manifold, via a Manifold Air Pressure or MAP sensor, and use that information to help determine fuel needs. Here's the deal; any changes in the engine's air requirements are reflected by near instantaneous changes in the intake manifold. If the ECU monitors the intake manifold pressure it can use the information to determine how much air is entering the combustion chamber and similarly how much fuel that air will need to combust properly. It sounds simple, and in principle, it is.
Although Speed-Density systems are more sophisticated than the Alpha-N types they do have their drawbacks. The most serious is that they don't adapt themselves very well to single and twin cylinder bikes. By nature, singles and twins suffer from constant manifold pressure variations especially at low speeds. Consider what happens at idle. When the intake valve is closed air pressure in the intake manifold is high because atmospheric pressure is pushing air through the air box, past the throttle and into the manifold where it piles up against the intake valve. As the intake valve opens, the air flows into the cylinder causing manifold pressure to drop. As the valve closes, and the incoming charge refills the manifold, pressure again raises.
These huge swings from low pressure to high and back again occur quickly, but because some time is involved, they complicate using a speed density system. However, there are ways to cope with the problem, and it's important to realize that at higher engine speeds the airflow smoothes out and the fluctuations diminish so it becomes much easier to take your readings. I should also mention that as you add cylinders, manifold pressures begin to settle down, simply because when one cylinder is at a low point another is high, so if you tie a pressure sensor into all your cylinders the overall readings will average out, which works just fine as far as an ECU is concerned.
The last thing worth mentioning is that because Speed-Density systems rely on manifold pressure they do their best work at low and moderate speeds where pressure changes are pronounced. At high speeds where manifold pressure changes have less impact, speed-density systems are less accurate.
Now that we've peeked at the philosophies behind the most common EFI systems let's look at how they actually function.