Piston-engine cycles, power and combustion

Place the four strokes on two crankshaft revolutions

Follow intake, compression, power and exhaust through a four-stroke cycle and calculate compression ratio from cylinder volumes.

Mechanism and relationships

A four-stroke engine completes intake, compression, power and exhaust over two crankshaft revolutions. The piston draws in a charge, compresses it, receives work from expanding hot gas and expels the exhaust. Real valve opening and closing extend around the simple stroke boundaries to improve gas exchange. A connecting rod and crank convert piston force into shaft torque. A multi-cylinder arrangement spaces power events to reduce torque fluctuation and vibration.

Reason through the evidence

Compression ratio compares cylinder volume at bottom dead centre with volume at top dead centre; it is a geometric ratio, not the ratio of atmospheric to manifold pressure. Manifold pressure reflects induction conditions and is not itself a direct measurement of shaft power. Engine power depends on torque and rotational speed, with power = torque × angular speed. Brake power is the useful measured shaft output after internal losses; indicated power describes work calculated from cylinder pressure behaviour.

Apply the explanation to a different case

At 2,400 rpm a crankshaft makes 40 revolutions each second, but each four-stroke cylinder completes only 20 full cycles per second. A power stroke therefore does not occur on every revolution of each cylinder. Multiple cylinders are phased to provide more frequent torque events at the common shaft. The geometric compression ratio remains fixed by the engine dimensions unless a special design changes them, whereas manifold pressure changes with induction conditions and throttle setting. This distinction prevents a frequent error: describing a throttle reduction as though it physically changed piston clearance volume.

Keep this distinction clear

Volumetric efficiency concerns the actual inducted charge relative to a defined displacement-based reference. It is distinct from thermal efficiency and mechanical efficiency. An improvement in one part of the engine’s air handling does not imply that all friction or exhaust-energy losses have disappeared from shaft output.

Distinguish normal combustion, detonation and pre-ignition

Distinguish controlled spark-ignition combustion, detonation and pre-ignition by where and when combustion begins.

Mechanism and relationships

Normal spark-ignition combustion begins at the intended spark and propagates as a controlled flame through the charge. Detonation is abnormal auto-ignition of unburned end gas that produces damaging pressure oscillations. Pre-ignition begins before the intended spark because a hot spot ignites the charge. They are different mechanisms and can reinforce each other. High cylinder temperature, unsuitable fuel and excessive operating stress can reduce the margin against abnormal combustion.

Reason through the evidence

Dual ignition provides two independent spark paths in many conventional engines and improves combustion reliability. Conventional magnetos generate their own ignition energy when rotating; disconnecting the aircraft battery does not necessarily make the engine incapable of firing. A grounding lead disables a magneto by its designed path, so an open lead can leave ignition live. Modern electronic engine systems may have different power dependencies, requiring the actual engine documentation rather than an assumption that every piston engine uses magnetos.

Apply the explanation to a different case

Imagine two abnormal pressure events. In one, a glowing deposit starts combustion before the intended spark; in the other, the unburned end gas auto-ignites after the normal flame has begun. Their timing and mechanism differ even if both create damaging temperatures and rough operation. A single cockpit temperature indication may not uniquely distinguish them. Approved fuel grade and operating limits reduce exposure, while subsequent inspection depends on the event and engine data. The learning aim is to explain the mechanism, not to improvise an in-flight diagnosis or authorise continued use from a normal-looking gauge.

Keep this distinction clear

The spark must occur at the intended crank position so pressure development produces useful work without excessive opposing load. Ignition timing is therefore a relationship to piston motion, not simply the existence of a spark. A spark plug firing does not alone prove correct timing or healthy combustion.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.