ALT-1 Fuel Injection
The automotive term 'fuel injection' generally describes any system where pressurised fuel is injected into an engine in an atomized state. Atomization is merely a way of describing how finely the fuel particles are dispersed, but is an important factor for efficient combustion.
Electronic fuel injection (EFI) has now largely replaced older carburettor technology. The most propelling reason behind its success is the growing concern on vehicle emissions and fuel efficiency, where fuel injection delivers positive results. If any modern car manufacturer wants to succeed, it must take a serious attitude to the growing implications of carbon emissions, as well as considering inflating fuel prices for the consumer.
A typical fuel injection system will comprise of the following components:
Electronic fuel pump - Because of the speed in which an injector operates (sometimes opening and closing more than 7000 times per minute,) it is necessary for the fuel to be delivered at high pressure. The pump's purpose, then, is to increase the pressure of the fuel and provide a continuous flow to the injectors.
Pressure regulator - The regulator ensures a constant pressure is maintained at all times, only altering under instruction from the vehicle's electronic control unit (ECU.) Typical pressures for a petrol/gasoline car will be in the region of 2bar (29PSI) under normal conditions.
Sensors - There are a whole host of sensors on a modern engine, monitoring various conditions. The most important ones when considering fuel injection are the mass airflow (MAF,) manifold absolute pressure (MAP,) throttle position (TPS,) camshaft position, crankshaft speed, coolant temperature and exhaust oxygen level (O2) sensors.
Engine control unit - The ECU is the vehicle's brain. It receives inputs from each and every sensor and uses the relative information to calculate outputs in order to control various actuators, such as the fuel injectors. It's the ECU which decides how much fuel needs to be injected under any given circumstances, and therefore controls how long an injector's nozzle will stay open (this is called the pulse width, and is only a matter of milliseconds.)
Fuel injectors - Injectors are used to precisely control the amount of fuel which enters the engine's cylinders. Multi-point injection is widely used, where each cylinder has its own injector. In this system, the fuel is injected into the vehicle's inlet manifold, just before the intake valve for each combustion chamber. An injector works by use of electromagnetic induction. When the circuit for an injector is closed, a current is able to flow through the coil of a solenoid in the injector, causing an induced movement in the valve needle which runs through the center of the coil. Fuel is then able to flow through the open nozzle, and after a predefined time the circuit is broken by the ECU, causing the current to stop flowing and the magnet to switch off. The valve needle is then pushed closed again by a simple spring.
Fuel injection allows for a far more accurate amount of fuel to be combusted, compared to a carburettor, meaning there is less waste. Less waste, of course, leads to great fuel economy.
Not only does an injector give greater precision, but it also allows the air fuel ratio to be governed by a wider range of deciding factors. Pulse width (how long the injector's nozzle stays open,) is determined by 'lookup tables' in the vehicle's ECU. An example of this would be the relationship between the manifold absolute pressure reading (a measure of engine load) and engine rotation speed (RPM.) At 5000RPM and low engine load, a lookup table may tell the ECU that the pulse width should be 5ms (milliseconds.) Compare this to 5000RPM under high load, where the ECU may find that a pulse width of 25ms is required, to allow for more fuel to enter the combustion chamber. However, this is a simplified example, and in reality, an initial comparison between MAP and load would then be subjugated to further calculations based readings from other sensors.
All these sensors located around the engine also help with something called stoichiometry. This rather intimidating word simply refers to the balance of a chemical reaction. For total combustion of all the air and fuel which is drawn into the combustion chamber, they must be mixed at a certain ratio. This is generally accepted as 14.7:1 (air : fuel) and is known as the 'stoichiometric ratio,' or 'Lambda 1'. This ratio is desirable because it allows good fuel efficiency while maintaining low emissions. Fuel injection allows for much greater control over stoichiometry, compared to carburettors.
