An internal combustion engine using the combustion of fuel inside a combustion chamber in order to produce rotational force to move the wheels of the vehicle.
What happens when the engine starts?
When the car is switched on power is sent to the starter motor. This will turn the crankshaft and begin the combustion cycle (explained below). Air is drawn into the engine through a filter by a fan, removing any small pieces of debris which may damage the engine.
The clean air will be added to fuel as vapor and then is sent through the intake manifold and sent to the cylinders through the intake valves. Petrol engines also have a distributor which makes the spark plugs spark at the correct times.
A four-stroke engine has 4 stages in the combustion cycle. These are:
- Intake – During the intake stroke, the intake valve opens and the piston moves down. Fuel and air enter the combustion chamber.
- Compression – Piston moves up, fuel/air compressed. Both valves are closed so pressure increases.
- Combustion – Spark plug (only in petrol) sparks and fuel explodes. This pushes the piston back down.
- Exhaust – Exhaust valve opens and piston moves up and exhaust fumes leave the combustion chamber.
The combustion stroke is also known as the power stroke, as this is the part of the cycle where the force is produced and the engine is given its power. During one cycle, the crankshaft will rotate twice.
The crankshaft, which has the pistons on, is linked to the camshaft (containing rockers). A rotation in the crankshaft will cause the camshaft to move, along with the rockers. These rockers will open and shut the valves in time with the movement of the pistons. The camshaft will move one rotation in the time the crankshaft goes around twice.
Variable valve timing is now used in high performance engines or as a measure to improve fuel efficiency, the valve timings change depending on how high the RPMs are and the demands on the engine.
- Closing the intake valve late means air enters the combustion chamber at a higher pressure and has been shown to reduce NO2 emissions, with little reduction in power.
- Closing the intake valve early reduces the amount of fuel and therefore improves efficiency.
- Early intake opening allows some of the combusted vapour back in which helps with cooling and can help efficiency
- Changing exhaust valve opening times also usually improves emissions.
Petrol Vs Diesel
Petrol and diesel engines work in a very similar way, both converting chemical energy into kinetic energy by combustion. The main structure of the engine is the same but there are subtle differences in the way the combustion happens.
Spark plugs are required for a petrol engine in order to cause the combustion as compression alone isn’t enough. Diesel engines do not need a spark plug because the fuel will ignite with the compression. Unlike petrol engines, they require fuel injectors. Because diesel doesn’t mix as well with air this step occurs within the combustion chamber itself rather than before.
- Diesel engines burn less smoothly so are louder and often heavier
- Petrol engines therefore have more balanced power output
- Diesel engines have higher compression ratios which makes them more fuel efficient – in a petrol engine high compression leads to early ignition and something called knocking which damages the engine.