The Four-Stroke Engine Cycle Explained: What's Actually Happening Inside Your Aircraft Engine
- Nathan Hodell
- Aug 13, 2025
- 7 min read
Updated: Apr 20
Every GA pilot learns the four strokes early: intake, compression, power, exhaust. "Suck, squeeze, bang, blow" is the informal version. But knowing the names is different from understanding what's happening mechanically and chemically inside each cylinder — and that understanding is what allows you to make sense of mixture control, detonation, engine roughness, magneto checks, and why certain problems feel the way they do in flight.
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Setting the Stage: What the Engine Is Doing
A reciprocating aircraft engine is a controlled explosion machine. It takes a precisely metered fuel-air mixture, ignites it at exactly the right moment, and extracts the energy from the expanding combustion gases to push a piston and turn a crankshaft. This happens in every cylinder, in a staggered sequence, thousands of times per minute.
At 2,400 RPM in a four-cylinder engine, each cylinder completes 1,200 power strokes per minute — 20 per second. The timing tolerances for valve opening, ignition firing, and piston position are measured in degrees of crankshaft rotation, not seconds. When something is off by a few degrees, you feel it as roughness. When something is off significantly, the engine tells you with instruments.
The four-stroke cycle is how each cylinder produces power. Two full crankshaft revolutions complete one cycle.
Stroke 1: Intake
The piston starts at Top Dead Center (TDC) — the highest point of its travel — and moves downward toward Bottom Dead Center (BDC). As it does, it creates a low-pressure region in the cylinder. The intake valve is open, and atmospheric pressure pushes the fuel-air mixture from the induction system into the cylinder.
In a carbureted engine, the mixture arrives pre-blended from the carburetor. In a fuel-injected engine, air enters through the intake and fuel is injected separately — either at the intake port (port injection) or directly into the cylinder (direct injection). Both systems meter fuel in proportion to airflow, but the fuel injection system does so with greater precision and without the carburetor icing risk.
The pilot connection: The intake stroke is where mixture control becomes critical. At altitude, the air entering the cylinder is less dense — fewer oxygen molecules per unit of volume. If fuel flow stays constant while air density drops, the mixture becomes progressively rich. Rich of peak doesn't produce more power — it wastes fuel, lowers combustion temperature relative to stoichiometric, and can cause rough running and fouled plugs. Leaning the mixture compensates for density altitude, restoring the optimal ratio.
Carburetor icing affects this stroke directly — ice forming in the venturi restricts airflow into the cylinders, reducing the mass of mixture entering on each intake stroke. The result is a power reduction that can be subtle at first and dramatic if the icing progresses. The ice typically forms in clouds of moisture at temperatures between 20°F and 70°F — well above freezing — which surprises many pilots.
Stroke 2: Compression
With the intake valve closed and the exhaust valve closed, the piston travels back upward from BDC to TDC, compressing the fuel-air mixture into a fraction of its original volume. The ratio of original cylinder volume to compressed volume is the compression ratio — typically 7:1 to 9:1 in normally aspirated GA engines.
Why compression matters: a more compressed charge burns more completely and releases more energy per unit of fuel. Higher compression ratios extract more work from the same amount of fuel. This is why high-performance engines tend to have higher compression ratios — and why they require higher-octane fuel.
The pilot connection — octane and detonation: Octane rating is a measure of a fuel's resistance to autoignition under compression. A low-octane fuel in a high-compression engine may begin to ignite from the heat of compression before the spark plugs fire — or may spontaneously ignite in pockets separate from the advancing flame front after normal ignition. Both situations are forms of detonation or pre-ignition, which can destroy a piston in seconds.
Detonation occurs when the compressed air-fuel mixture spontaneously ignites in uncontrolled pockets before or during the normal combustion event, creating multiple pressure waves that collide violently. You may hear it as a knock or ping. It causes extremely high localized temperatures and pressures that can hole a piston, crack a cylinder head, or destroy the engine rapidly.
Pre-ignition occurs when the mixture ignites before the spark plug fires — triggered by a hot spot in the cylinder (carbon deposit, overheated spark plug electrode, or exhaust valve) rather than the controlled spark. Pre-ignition is typically more immediately destructive than detonation. The piston is still rising when combustion begins, meaning the expanding gases are fighting against an upward-moving piston, creating enormous stress.
Both detonation and pre-ignition are aggravated by: excessive leaning at high power settings, use of lower-octane fuel than required, sustained high power at low airspeed (reducing cooling), and ignition timing that is too advanced.
Compression check: During maintenance, a differential compression check measures how well each cylinder seals during the compression stroke by pressurizing the cylinder with air and measuring leakage. Low compression in a cylinder indicates leakage past piston rings (heard in the crankcase breather), intake valve (heard in the intake), or exhaust valve (heard in the exhaust stack). This test is one of the primary engine health indicators and is performed at every annual inspection.
Stroke 3: Power
This is the only stroke that produces work. The other three are necessary prerequisites.
As the piston approaches TDC, the spark plugs fire — typically 20–30 degrees of crankshaft rotation before TDC (called the ignition advance or timing). This advance allows the flame front to develop fully so that peak combustion pressure occurs slightly after TDC, pushing the piston downward at its most mechanically efficient angle. The high-pressure combustion gases expand against the piston, driving it from TDC to BDC and turning the crankshaft.
Why two spark plugs per cylinder? Most aircraft engines use two spark plugs per cylinder — one from each magneto — firing simultaneously (or nearly so). This dual ignition serves two purposes: redundancy (the engine runs on either magneto alone if the other fails) and efficiency (two flame fronts developing from opposite sides of the cylinder burn the mixture more completely and quickly, producing more consistent combustion and slightly more power than a single plug).
During the runup magneto check, you test each magneto individually. A normal drop when switching to a single magneto (typically 50–125 RPM depending on the engine) is expected — you're going from two flame fronts to one. An excessive drop indicates a problem with that magneto. A rough drop indicates fouled plugs — the affected cylinders are not firing efficiently on the magneto side with the compromised plugs.
CHT and the power stroke: Cylinder head temperature reflects the cumulative heat load of the combustion cycle. Very lean mixtures at high power cause CHTs to rise — the flame burns hotter and slower with less fuel to act as a coolant. High CHTs over sustained periods damage cylinder head metal. The target range for most GA engines is 300–380°F in cruise, with 400°F as a generally accepted upper limit.
Stroke 4: Exhaust
With the exhaust valve now open and the intake valve closed, the piston travels back up from BDC to TDC, pushing the spent combustion gases out of the cylinder and into the exhaust manifold. The exhaust gases travel through the exhaust system, where they give up heat (used to warm cabin air in aircraft with heat mufflers), and exit through the exhaust stacks.
As the piston reaches TDC at the end of the exhaust stroke, the exhaust valve closes and the intake valve opens — and the cycle begins again.
The pilot connection — back pressure and exhaust system health: Any restriction in the exhaust system — accumulated carbon deposits, a cracked muffler, a damaged baffle inside the heat exchanger — increases back pressure on the exhaust stroke. The piston has to work harder to push gases out, reducing net power output. More critically, a cracked or failed exhaust component inside the heat muffler creates the CO poisoning pathway discussed in the carbon monoxide article.
Exhaust valve health deteriorates over engine life. Exhaust valves operate in an extremely hostile environment — extreme heat, repetitive mechanical loading, and chemical attack from combustion products. Burned exhaust valves don't seal properly on the exhaust stroke, allowing hot gases to leak back and further damage the valve. A differential compression check identifies exhaust valve leakage (you hear the air escaping in the exhaust stacks during the check).
Valve Overlap: The Detail Most Pilots Don't Know
One aspect of the four-stroke cycle that isn't in most basic discussions: valve overlap. At the transition between the exhaust and intake strokes, there's a brief period where both the exhaust valve and intake valve are open simultaneously. This is intentional.
The outgoing exhaust gases create a scavenging effect — their momentum and slight pressure differential help draw the incoming fresh charge into the cylinder. Engineers design valve overlap to maximize this effect at the RPM ranges where the engine is expected to operate most efficiently. Valve timing (when exactly valves open and close, measured in degrees of crankshaft rotation) is optimized for specific power curves.
For pilots, valve overlap matters during engine run-up and at idle — at very low RPM, the scavenging effect isn't present and the overlap can cause some exhaust gas to remain in the cylinder, contaminating the fresh charge. This is part of why aircraft engines can run rough at very low idle settings and why enriching the mixture slightly at idle helps.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.