Electronic Fuel Injection System - EFI Basics
Electronic Fuel Injection Is Not As Baffling Or Difficult As It May Seem, So Take A Quick Run Through ...
We also have to have a way to accurately introduce fuel into the engine. Most performance fuel-injection packages employ a multipoint system that specifies one injector per cylinder. These multiple injectors are then fed fuel at high pressure through a common fuel rail. Generally, most multipoint systems are designed to run at 3 bar (three times atmospheric pressure: 3 x 14.7 = 44 psi). One bar is actually 0.9869 of an atmosphere, which is why 43.5 is the 3-bar pressure reference number. With a set pressure, the volume of fuel the injector can deliver is determined by the amount of time the injector is open. This is called the injector pulse width. Properly sizing the injectors improves part-throttle performance because oversized injectors are difficult to control at very short pulse widths.
This is a typical multipoint fuel-injection package, specifically one of Edelbrock's Pro-F
Finally, all OEM and most aftermarket EFI systems also offer digital spark control. In the last few years, the OEs have elected to drop the distributor in favor of a distributorless ignition system (DIS). In this approach, the ECM needs a crank trigger to tell it when the No. 1 cylinder is at top dead center (TDC). Most engines also use a cam sensor (although it's not absolutely necessary) to tell the computer when the engine is at TDC on the firing stroke. Then the computer can trigger each individual spark-plug coil to light the fire in each cylinder much more accurately than a distributor with all its additional motion and clearances.
Systems Approach
EFI can be separated into three basic control configurations: alpha-N, speed density, and mass airflow systems. Alpha-N is the simplest, with the most complex name. This system requires input only from engine rpm and throttle position as input for the base fuel map and is mainly used for race engines operating primarily at wide-open throttle (WOT); no manifold pressure sensor is required. This is a very crude system and not recommended for street-engine operation.
Speed density is the most popular electronic fuel-control system and is also very simple, relying on engine speed and intake-manifold vacuum measured by a manifold absolute pressure (MAP) sensor. If you look at a speed-density base fuel map, it matches engine speed with a given amount of engine load (measured by the MAP sensor). Think of the MAP sensor as a digital vacuum gauge. High manifold vacuum at any given engine speed means there is very little throttle opening and therefore minimal load on the engine. A high engine speed with near zero manifold pressure means the throttle is at WOT and load is high. Given any combination of load and rpm, the base fuel map delivers a given amount of fuel. The trick here is building a base fuel map that accurately assigns the correct amount of fuel for each of these load situations for best efficiency and/or best power.
The limitation of speed density is that the engine does not measure the amount of air it uses, so the tuner must estimate how much fuel to use. Most current speed-density systems now use a wide-band oxygen sensor to aid in tuning the fuel curve. By actually measuring the amount of air that the engine breathes in, we can more accurately meter the proper amount of fuel. That's the job of a mass airflow (MAF) sensor. The limitation of this system is that most production MAF sensors are small, restricting the total volume of air they can flow. Plus, adding a MAF sensor increases both complexity and cost. This is why most aftermarket EFI systems choose the simpler speed-density design.
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