Chapter 1 — The four-stroke engine, in the order it matters

Almost every petrol and diesel car sold in the UK uses a four-stroke engine. The four strokes are the four phases the piston completes in one cycle: intake, compression, combustion (sometimes called power), and exhaust. Every other engine system — the valvetrain, the fuel system, the ignition system, the cooling system, the emissions system — exists to make one of those four strokes happen at the right moment.

Understanding the cycle in this order is what lets a service receipt make sense.

01Intake

On the intake stroke, the piston moves down the cylinder. The intake valve opens. Air (in a petrol engine) or air-and-fuel (in a diesel) is drawn into the cylinder.

The systems that hang off this stroke are the air filter, the mass airflow sensor (which measures incoming air so the ECU can calculate the right amount of fuel), the throttle body on a petrol engine, the intake manifold, and the variable-valve-timing system that decides exactly when the intake valve opens and closes.

On modern downsized turbo engines, the turbocharger sits in front of the intake — it uses exhaust gas to spin a compressor wheel that forces extra air into the cylinder, which lets a smaller engine make the power of a larger one.

02Compression

Both valves close. The piston moves back up the cylinder, compressing the air (petrol) or air-and-fuel (diesel) to a fraction of its original volume. Compression ratios are typically 9:1 to 12:1 on petrol, 15:1 to 22:1 on diesel.

Compression is what makes the next stroke produce useful work. A failing head gasket, a burnt valve or worn piston rings reduce compression in one or more cylinders; a compression test (measured in bar or psi at each cylinder) is one of the standard diagnostic checks when an engine misfires.

On a direct-injection petrol engine, fuel is injected near the top of the compression stroke. On a port-injection petrol engine, fuel has already been added to the air during intake. On a diesel, fuel is always injected at the top of compression — the heat of compression itself ignites it, which is why diesels have no spark plugs.

03Combustion (power)

On a petrol engine, the spark plug fires shortly before the piston reaches the top of its stroke. The flame front travels across the combustion chamber, expanding gases push the piston down, and this is the stroke that produces the work the rest of the cycle exists to enable.

On a diesel, ignition is by compression alone. Multiple injection events per cycle are normal on modern common-rail diesel — a small pilot injection to soften the combustion noise, the main injection for power, and sometimes a post-injection to manage the diesel particulate filter.

The ignition system on a petrol engine is the coil pack (or coil-on-plug), the spark plugs, and the engine control unit timing them. A misfire warning on the dashboard usually points to one of these three.

04Exhaust

The exhaust valve opens. The piston travels back up, pushing burnt gas out into the exhaust manifold. The cycle then restarts at intake.

Hanging off this stroke: the exhaust manifold, the turbocharger turbine wheel (which extracts energy from exhaust gas to drive the intake compressor), the catalytic converter (which converts CO, unburnt hydrocarbons and NOx into less harmful compounds), and — on a diesel — the diesel particulate filter (DPF) and the AdBlue/SCR system that further reduces NOx.

The oxygen sensors (lambda sensors) sit in the exhaust before and after the catalytic converter. They tell the ECU whether the air-fuel mixture is right; an aged or contaminated lambda sensor is one of the most common causes of poor fuel economy on a car that drives normally otherwise.

05The systems that span all four strokes

• Cooling. The water pump circulates coolant through the engine block and the radiator. The thermostat opens at a set temperature (usually around 87–92°C) to let coolant flow to the radiator. An engine that runs too cool wears prematurely; an engine that overheats can warp a cylinder head in minutes.
• Lubrication. The oil pump draws oil from the sump and pushes it through galleries to the bearings, the cam journals and the turbocharger. Oil pressure is monitored by a sensor; loss of pressure for more than a few seconds at speed will destroy the engine.
• Engine management. The ECU reads dozens of sensors — crank position, cam position, mass airflow, manifold pressure, coolant temperature, oxygen sensors, knock sensors — and adjusts fuelling, ignition timing and valve timing many times per second. The OBD-II port under the dashboard gives a diagnostic tool access to fault codes from the ECU.

06Why this matters at the garage counter

A bill that says "replaced coil pack on cylinder 3" is fixing the ignition system feeding combustion on one of (usually four) cylinders. A bill that says "new DPF and EGR cooler" is two parts of the emissions system, both downstream of the exhaust stroke. A bill that says "timing chain and tensioner replaced" is the mechanism that keeps the valvetrain in sync with the crank — if that fails, the engine usually fails with it.

The four strokes are the map. Every other term in motoring is an address on that map.