How does an engine work?
The mechanic says...
Actually that’s a very good question. The following is a much-simplified overview, but hopefully this will help you…
Essentially an ‘internal combustion’ engine (as still used in the vast majority of road vehicles around the word) burns a fuel (typically petrol or diesel) within the motor. The fuel burns within an enclosed cylinder (car engines are usually multi-cylinder units, typically employing three, four, five, six or even eight cylinders, for optimum smoothness and power output), and when the fuel, mixed with air, is ignited, rapid expansion of the gases takes place. This exerts a considerable force on the only moveable component within the cylinder – a tight-fitting piston.
So the piston is obliged to move away from the point of ignition. (Note: In a petrol engine, ‘spark plugs’ provide the spark required to ignite the fuel mixture. By contrast, in a diesel, or ‘compression ignition’ unit, the inherently higher level of fuel mixture compression in this type of motor heats up the fuel mixture sufficiently to ignite it without the need for a separate spark).
The moving piston is connected by means of a ‘connecting rod’ to the engine’s crankshaft, which is thus forced to rotate. In effect the linear motion of the piston is converted to circular motion of the crankshaft. The crankshaft in turn is linked to the transmission system – usually through a series of gears – and to the final drive (differential) assembly, then from there to the wheels, by means of driveshafts. The rotary motion of the crankshaft is thus eventually fed to the road wheels, which also have to turn, of course, to make the car move.
Most engines operate on what is known as the ‘four stroke’ Otto cycle. That is to say, through ‘induction’, ‘compression’, ‘ignition’, and ‘exhaust’stages.
The fuel/air mixture is admitted to each cylinder by means of an ‘inlet’ valve or valves, and the ‘spent’ exhaust gases are expelled by the piston moving back along the cylinder, through an ‘exhaust’ valve or valves, to the exhaust system. The inlet and exhaust valves are timed so that they open and close when required to make the engine operate. All valves in a cylinder are closed when the fuel/air mixture ignition takes place, so that the full force of the explosion can act on the piston, as described.
The fuel is delivered to the engine from a tank via a fuel pump and injection system, which accurately meters the amount required to make the engine work powerfully and efficiently. In virtually all vehicles produced within the last 10 to 15 years, an ‘Electronic Control Unit’ (‘E.C.U.’) computer controls the operation of the ignition and fuel systems. The E.C.U. receives input signals from a number of sensors (camshaft, crankshaft, oxygen, coolant temperature, and so on) and adjusts the ignition and fuel injection systems as required to ensure that, at all times, the engine runs efficiently and produces minimal emissions.
