Electronic Fuel Injection how it Works
Electronic Fuel Injection Explained.
You may think it intimidating, but when you understand the philosophy of electronic fuel injection, it all becomes clear.
Introduction
For efficient fuel consumption and optimum performance from an internal combustion engine, the air-fuel ratio must be as near to the ideal as possible all the time the engine is running. When all the the free oxygen in an engine cylinder, on an inlet stroke, combines with all fuel delivered into that cylinder’s combustion chamber, the air/fuel mixture is chemically balanced. Keeping the air/fuel ratio as ideal as possible is an important measure to ensure low-pollution exhaust emissions, and also for performance tuning reasons.
For gasoline the ideal air/fuel mixture is approximately 14.7 parts air to 1 part gasoline. Less than 14.7 parts of air to 1 part gasoline is considered to be a rich mixture, more than 14.7 to 1 is a lean mixture. The actual air/fuel mixture being used by an engine at any one time, divided by the ideal air/fuel mixture is known in automotive technology as lambda. The math show us a lambda value of 1 is ideal, a rich mixture will be less than one, and a lean mixture will be greater than 1.
The Modern Electronic Fuel Injection System
At the core of nearly all modern fuel injection systems is the fuel rail. This carries fuel supplied from an electric pump mounted near the fuel tank, and keeps the system pressurized and free from air-locks. When the engine is running, the pressure of the fuel in the fuel rail varies continually, in line with the fuel requirements of the engine. A pressure valve in the system is used to regulate the pressure of the fuel in the fuel rail and allows excess fuel to be returned to the tank.
The fuel is introduced into the engine via a set of fuel injectors. There may be just a single injector, or there may be one injector for each cylinder of the engine. The injectors are fed by the fuel rail, and spray atomised fuel into the inlet manifold ports. The amount of fuel delivered into the engine from the injector(s) is constantly adjusted by the ECU (the Electronic Control unit) by varying the amount of time that the injector(s) is/are open. For this to work efficiently and precisely, the difference in pressure between the fuel in the injector(s) supplied by the fuel rail, and the air pressure in the inlet manifold must stay constant. This is achieved by using the pressure valve that controls the pressure of fuel in the fuel rail to also monitor manifold depression (vacuum), and adjust the actual pressure in the rail, to follow the varying degrees of depression in the manifold.
The amount of fuel and when it is delivered into the engine depend on a number of variables, including throttle position, air temperature and how completely the air/fuel mixture burns. Modern fuel injection systems include sensors to measure the air temperature, throttle position, and the amount of oxygen in the engine’s exhaust emissions. This last sensor is also known as the lambda sensor. As mentioned in the introduction, lambda is the figure derived at by dividing the engine’s actual air/fuel mixture ratio by the ideal air/fuel ratio. All the time the engine is running, its fundamental fuel requirements are ascertained by one of two methods.
An air-mass meter bodily measures the volume of air entering the engine over a specified period. The higher the engine speed the greater the volume of air drawn in A Manifold Absolute Pressure (MAP) sensor is used to monitor the manifold depression, and a sensor on the engine crankshaft monitors the engine speed.
This data is passed to ECU for final calculation as to the delivery of the fuel into the engine. For the majority of systems these are:
Town Driving.
This is signified by a constantly varying engine speed and throttle position. The ECU keeps the air/fuel mixture as close to the ideal as possible.
Cruising
The sensors detect a relatively constant engine speed and throttle position, and the air/fuel mixture is lowered to about ten percent below lamda for maximum economy.
Rapid Acceleration
This is signified by wide throttle openings or suddenly increased throttle movement. The fuel mixture is enriched to about ten per cent higher than lambda.
Starting
A signal from the starter motor circuit to the ECU triggers an increase in the quantity of fuel delivered.
Cold Running
The temperature sensor sends a signal to the ECU, and the fuel mixture is kept richer than normal until the engine has warmed up.
Idling
The ECU receives information about the engine speed from either the crankshaft sensor or air-mass meter (depending on the system) and the throttle disc is closed. The engine draws in air through a valve called a stepper motor. If the engine is warm, the oxygen sensor sets the air/fuel mixture to below lamda, if the engine is cold, a richer mixture is used.
