Structure and operation of the combustion engine

The combustion engine converts chemical energy in fuel to mechanical work that drives the car forward. To find faults and perform service you must understand how the engine breathes ignites and delivers power. Most cars use a four-stroke engine where each cylinder goes through four strokes.

§The Four Strokes

• 01Intake — the piston moves down and draws air (and often fuel in petrol engines) through the open intake valve.
• 02Compression — the valves close and the piston compresses the mixture so it becomes easily flammable.
• 03Combustion — the fuel ignites (spark with petrol, compression heat with diesel), and the combustion pushes the piston down.
• 04Discharge—the piston goes up and pushes combustion gases out through the open exhaust valve.

The crankshaft converts the pistons' up and down movement into rotation, and the camshaft controls the valves so they open and close in time with the piston. The relationship between crankshaft and camshaft — ignition and valve timing — is crucial for the engine to run at all.

§Petrol and diesel — the most important difference

A petrol engine ignites a pre-mixed air-fuel mixture with a spark from the spark plug. A diesel engine sucks in pure air, compresses it forcefully so it becomes very hot, and then injects the fuel, which self-ignites from the heat. Therefore, the diesel engine has no spark plugs, but often glow plugs for cold starting, and it works at higher compression.

§Systems around the engine

The lubrication system delivers oil to bearings and sliding surfaces so metal doesn't wear against metal. The cooling system circulates coolant through the engine and radiator to keep operating temperature stable. The fuel and injection system meters the fuel, and the exhaust system with catalyst and possibly particle filter cleans the exhaust gas. When one of these fails, it shows quickly in the engine's performance and reliability.

§What it means for your work

Understanding the cycles and systems enables you to link a symptom to a probable cause: poor compression gives hard starting and power loss errors in lubrication can destroy bearings and a leaky intake disturbs the mixing ratio. The better you know how the engine works the faster you find the fault.