What is EFI?Good question. No, it's not the latest boy/girl band (as if there weren't enough), but it is a vital part of a modern engine. An engine's fuel injection system controls three things: how much air an engine has, how much fuel is needed to mix with the air (dependent on conditions), and what the proper timing for the ignition of the mixture will be. All the ABCs of power (remember: stoichiometry) that dictate how well an engine can perform are controlled by an EFI system. For example, let's say your car has a turbocharged engine and at 4,000 rpm with full boost it will require 18-20 degrees of timing. All of that extra air and fuel, produced by the turbocharger, is pushed into the cylinder chamber resulting in a faster burn of the mixture. But with no boost and light throttle at the same rpm, 40 degrees of timing would be needed for the engine to operate properly. It's a big amount of timing that the EFI has to manage.
Let's move onto air. The ECU must know how much air is entering the engine so it can tell the injectors how much fuel is needed. There are a couple different ways for an engine to measure the amount of incoming air: mass air sensor or speed density system. Mass-flow systems use a mass air sensor to measure the mass of air entering the engine. This sensor measures the amount of air by directing it past a heated wire that is part of an electronic circuit. Air flowing across the wire draws away some of its heat and an increase in electrical current is required for it to maintain its fixed temperature. The current necessary to heat the wire is proportional to the mass of air flowing across the wire. Most mass-flow systems measure the air directly, so there is no need for the ECU to correct for air density. Once the ECU knows how much air is entering the engine, it looks at other sensors to determine the current state of operation. It then refers to an electronic table or map to find the appropriate air/fuel ratio and selects the correct fuel injector pulse width. Pulse width is the time in milliseconds that the injector is open, while duty cycle is the injector's overall percentage of open time. The only downside to having a mass flow system is the price and overall design, which can be restrictive in high-horsepower engines.
With a speed density system, the speed of the engine is used along with a measurement of manifold vacuum and the density of air to calculate airflow. This is accomplished by using a MAP (manifold absolute pressure) sensor and a predetermined table of how efficient an engine is at flowing air in all conditions. However, there is one problem with the table: it's created at the factory and is based on a new, bone-stock engine. It doesn't take into account wear-and-tear of the engine or if something mechanical is modified. To compensate for this, an oxygen sensor is used to measure the air/fuel ratio. If any errors are detected, then the computer will correct fuel delivery of the injectors. Honda has done a very good job with MAP sensors, especially with emissions, because it's harder to get precise emissions with a MAP sensor. From a performance standpoint, an engine with a MAP sensor usually makes more horsepower than one with an airflow meter because there's less restriction.
An injector allows fuel to be applied into the combustion chamber. Most import applications have one fuel injector for each cylinder. Really quick, here's how an injector works. There's an internal plunger that is activated by the application of the voltage. When the plunger is activated, an opening is created, allowing pressurized fuel to flow past it. Fuel flow is controlled by varying the pulse width or duty cycle of the injector. Injector Pulse width is the time in milliseconds that the injector is open, while duty cycle is the injector's overall percentage of time open to time closed.