The Basics Of The Car Petrol Engine

The basic principle behind the petrol engine and for that matter, the diesel engine, is one of internal combustion, although the two differ in their respective fuel systems and their method of initiating the combustion process.

The petrol engine used in many production cars, introduce an explosive mixture (petrol vapour and air) into a virtually sealed cylinder, compress it by moving one end of the cylinder toward the other then ignite the mixture with a spark. The resulting very rapid burn will force the moveable end of the cylinder (the piston) away from the fixed end (the cylinder head) and via a connecting rod and crank arrangement rotation will result.

The Petrol and Air Combustion Process

For the above scenario to take place effectively other conditions must be present both before and after the burn (combustion).

  1. The process starts with (1) a piston moving downward within a cylinder drawing in a charge of petrol vapour and air past an open inlet valve which then closes, at the bottom of this downward stroke the piston, via the connecting rod/crank arrangement, will change direction and start to move up.
  2. As the piston rises (2), the fuel/air mixture will be compressed and in doing so will increase in temperature, just before the top of the upward travel a spark will be introduced into this now highly volatile mixture and it will start to burn very rapidly,
  3. By this time the piston will have past the top of its travel and is now (3) being forced down by the expanding gases of combustion.
  4. The piston again reaches the bottom of the down stroke and (4) starts to rise pushing out the burnt gases through the now open exhaust valve which will close just before the top of the piston travel.

Just past the top of piston travel the inlet valve opens ready for the process to be repeated. This is the four stroke cycle explained and it is referred to in engineering circles as induction, compression, power, exhaust; it is also known by the shorter version; suck, squeeze, bang, blow!

Smoothing The Output

Obviously this process is now duplicated across all cylinders with each cylinder carrying out a different task in the cycle relative to its neighbour. This phasing is achieved by crank and valve position and will ensure an even power output.

As a general rule, the more cylinders an engine has the smoother its power output will be, this is mainly due to the shorter time between each power stroke. However, there will still be the situation involving Newtons third law which involves equal and opposite reactions to any action, to counteract, or at least minimise its effect, the crankshaft has strategically placed counter weights incorporated into its casting.

This has a very pronounced effect in evening out the severe direction changes experienced by the piston/connecting rod/crankshaft assemblies.

Engine Timing and Valve Arrangement

Mentioned above is the inlet/exhaust valve arrangement and its relevance to the correct operation of the engine. It is vital that these valves are opened and closed at exactly the right time for the engine to breath properly.

To achieve this a camshaft is mounted such that its rotation permits the valve operation relative to the piston position. To achieve this a drive arrangement is incorporated in the engine design so that as the crank turns drive is transmitted via a chain or toothed belt to the camshaft.

It is important that the drive between the crank and cam is correctly set at production and at replacement intervals during the life of the petrol engine, this is usually achieved by the use of specific marks on the engine allowing the correct positioning of the shafts relative to each other before the drive is fitted.

Should the timing of piston and valve come adrift there is a distinct possibility that the two will meet catastrophically resulting in a very large repair bill.

There are, of course, many other systems and components which enable the engine to run and these systems will be dealt with in later articles.