How does a four-stroke engine work?

A four-stroke engine completes one power cycle in four movements of the piston: intake, compression, combustion and exhaust. Only one of the four produces power. Each cylinder fires once every two crankshaft revolutions, and that single fact governs how ignition timing has to be referenced.

The four strokes, in order

• Intake. The inlet valve opens and the fuel and air mixture is drawn in as the piston travels down.
• Compression. The inlet valve closes and the piston travels back up the cylinder, compressing the mixture. Just before the piston reaches the top, the spark plug fires.
• Combustion. The piston is forced back down by the pressure wave from the burning mixture. This is the stroke the engine’s power comes from.
• Exhaust. The exhaust valve opens and the piston travels back up, expelling the burnt gases. At the top of the stroke the exhaust valve closes and the cycle repeats.

That is the cycle for one cylinder. Most engines have two or more cylinders working in concert, offset from each other so that the power strokes are spread across the cycle rather than arriving together.

Why one cycle takes two revolutions

Each stroke is half a turn of the crankshaft. Four strokes is therefore two complete crankshaft revolutions, and in that time each inlet valve, each exhaust valve and each spark plug operates exactly once.

This is the detail that catches people out. A crankshaft at 1,500 rpm is not firing each plug 1,500 times a minute. It is firing each one 750 times a minute.

Ignition advance and top dead centre

Top dead centre, usually shortened to TDC, is the point at which the piston reaches the top of its travel. The spark is not delivered at TDC. It is delivered a number of crankshaft degrees before it, and that lead is the ignition advance.

The reason is that combustion is not instantaneous. The mixture takes time to burn, so the spark has to be delivered early enough that peak cylinder pressure arrives just after the piston has passed TDC and started back down. Too little advance and peak pressure arrives late, so power and efficiency are lost. Too much and the pressure rise fights the piston while it is still travelling up.

The correct advance is not a single number. It changes with engine speed, load, fuel composition and charge temperature, which is why ignition is mapped across the operating range at commissioning rather than fixed at a single setting.

Why the timing reference comes from the camshaft

Because the cycle spans two crankshaft revolutions, a crankshaft signal on its own is ambiguous. The crankshaft passes the same angular position twice per cycle, and only one of those two passes is the one that should produce a spark.

The camshaft turns at half engine speed, so one camshaft revolution is one complete engine cycle. Taking the timing reference from the camshaft, or from a half engine speed signal derived from it, removes the ambiguity and tells the ignition system which revolution it is on.

What this means for gas engine control

On a gas engine, ignition control has two jobs: deciding when each cylinder sparks, and delivering enough spark energy for long enough to ignite the charge reliably. Lean gaseous mixtures are harder to ignite than a rich petrol charge, so continuous spark duration becomes a specification that matters rather than a detail.

Ignition modules are selected mainly on cylinder count and on the spark duration the engine needs.

Timing and energy are only half of engine control. The other half is the mixture itself. A closed-loop air and fuel ratio controller reads exhaust oxygen from a lambda sensor and drives a stepper or current-controlled bypass valve to hold the mixture on target, which keeps the charge inside the window the ignition system was mapped for.

The two are complementary, not alternatives. The ignition module owns when and how hard the plug fires. The ratio controller owns what it is firing into.