Car Parts and Accessories

Most modern turbocharged cars and many naturally aspirated cars have petrol injection instead of carburettors.

With a carburettor system, the petrol is mixed with the air as it passes through the carburettor and is then distributed to the cylinders; but with an injection system, the petrol is squirted under pressure through small injector nozzles where it is atomised and vaporised.

Generally injectors are located in the intake passages near the inlet valves. The quantity of fuel injected and the correct timing of the injection varies with the type of system used and the conditions prevailing in the engine at the time.

The venturi is replaced by an air metering device in a petrol injection system so the flow of air is less restricted than with a carburettor. The inlet manifold is generally much longer than with a carburettor as fuel injection systems usually take advantage of the standing pulses of air which are generated by the inlet valve opening and closing. The momentum of air moving in the inlet manifold adds to its flow and gives a better charge into the cylinder than would be the case if a shorter untuned manifold was used.

Compared with carburation, fuel injection can be tuned to give improved fuel consumption, better power and improved performance since mixture distribution and cylinder charging are more efficient.

The engine usually responds much more rapidly to throttle changes because of the very small time lag between throttle movements and the change in the amount of fuel injected.

The modern injection system also permits far more accurate control over the emission levels of an engine under a wide range of operating conditions.

Electronic system

Electric pulses operate a solenoid on an electronic fuel injector permitting a very accurate flow of atomised fuel into the intake manifold. An electronic control unit determines the period of time the injector remains open—permitting fuel to flow into the intake manifold—by taking information from sensors.

The computer has been programmed to provide a mixture based on driver requirements for performance and preordained emission levels.

Sensors monitor engine temperature, throttle position, air flow and ambient air temperature on a basic system. The computer will often take crankshaft position readings from the distributor. It may take crankshaft position readings from a sensor adjacent to the flywheel. Once the position of the crankshaft is accurately determined, the computer knows when each piston is moving down the bore to take in a charge of fuel and air.

It can also trigger the spark on those combined systems of electronic fuel injection and electronic ignition.

Fuel is taken from the tank by a pump which raises its pressure to a level sufficient for it to atomise when it is permitted to exit the injector nozzle. The fuel is then filtered and flows to a distributor pipe. From the distributor pipe, fuel is channelled to each injector.

Once the computer has analysed all the information sent by the various sensors, it sends an electric signal to the injector’s solenoid. This lifts the valve about 0,1 mm and permits the pressurised fuel to spray out of an orifice and over a shaped pipe where it atomises.

The injector will remain open for typically, between 1,0 and 1,5 milliseconds (one to one-and-a-half thousandths of a second). The variation in time is the difference between the amount of fuel needed for heavy load or low load.

On some fuel injection systems the injection pulses are cut off under closed throttle deceleration— usually in the region of about 1800 to 2000 rpm. This improves fuel efficiency as the engine coasts without using fuel when the car is on a trailing throttle.

Engines which use fuel injection and turbocharging will often have an additional sensor which detects the onset of engine `knocking’—a trait of turbocharged engines. With the engine under load and knocking about to start, the knock sensor retards the spark or reduces the turbocharger pressure.

Engine management

For strict exhaust emission control, the fuel injection system’s central processor unit (CPU) may be used to control all aspects of the combustion process, fuel and ignition other than the mechanical processes of moving the piston in the bore and the valves in the cylinder head (although electronically controlled variably timed camshafts with multiple valves have passed through the research stage).

A further refinement is to have an oxygen sensor in the exhaust manifold. This detects emission levels by assessing the amount of oxygen remaining in the burnt gases. It can then adjust the injection quantity of fuel to keep the engine operating at optimum economy and control exhaust pollutants.

An electronic engine management system greatly reduces the stress placed on frequently unreliable mechanical devices when controlling performance and emissions. It also improves reliability and frequently has a self-testing facility in the CPU’s memory for diagnosis during servicing. It may also contain a limp-home’ facility if any part of the sensory chain is damaged or inoperative. In this case a warning light on the instrument panel will warn the driver and expert assistance should be immediately sought.

Electromechanical injection

A simpler system of fuel injection uses many of the components of a purely electronic system but has a mechanical distributor to allocate fuel to the injectors. These are not triggered electronically by solenoids but by fuel pressure. A large flap valve in the air flow sensor rises as air flows over it to determine the quantity of air being drawn into the engine. This determines the quantity of fuel supplied to the injectors. Correction of the hydraulic load placed on the flap valve varies the quantity of fuel according to cold start or heavy load conditions.

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