How Every Gasoline Engine Works — The Four-Stroke Cycle Explained

Why You Need to Know This

This is lesson number one for a reason. Every single diagnostic path you will ever follow on a gasoline engine traces back to the four-stroke cycle. No-start? One of the three combustion requirements is missing. Misfire? One of the four strokes is not happening correctly in that cylinder. Loss of power? Something is compromising the efficiency of the cycle. You cannot diagnose what you do not understand.

I have watched technicians with ten years of experience throw parts at a misfire because they never truly understood what was happening inside the cylinder. They swap coils, swap plugs, swap injectors — and when none of that fixes it, they are stuck. The tech who understands the four-stroke cycle asks better questions. Which stroke is failing? Is the cylinder sealing? Is the valve opening and closing at the right time? That is the difference between diagnosing and guessing.

The Three Requirements for Combustion

Before we get to the strokes, burn this into your brain: a gasoline engine needs exactly three things to produce a power stroke.

• Fuel — the correct amount of gasoline, mixed with air at roughly 14.7 parts air to 1 part fuel (stoichiometric ratio). Too much fuel (rich) or too little (lean) and combustion suffers.
• Air — drawn through the intake system, measured by the MAF or MAP sensor, and controlled by the throttle body. Restricted air = restricted power.
• Spark — a precisely timed electrical arc across the spark plug gap, delivered at exactly the right moment in the compression stroke. Too early = detonation. Too late = wasted energy.

On every no-start, your first job is to figure out which of these three is missing. Crank the engine — do you hear it spinning normally? Good, the starter is not your problem. Now check for spark. Check for fuel pressure. Check for injector pulse. If all three are present and the engine still will not fire, you are looking at a mechanical issue — the fourth player that does not make the list but matters just as much: compression. Without adequate compression, the air-fuel mixture will not ignite reliably even with perfect spark and fuel delivery.

The Four Strokes — Step by Step

Stroke 1 — Intake

The piston starts at top dead center (TDC) and moves down to bottom dead center (BDC). As it descends, it creates a vacuum in the cylinder. The intake valve opens, and the pressure difference pulls the air-fuel mixture into the cylinder. On a port-injected engine, the fuel is sprayed into the intake port and mixes with air before entering the cylinder. On a direct-injected engine, the air enters alone and fuel is sprayed directly into the cylinder during or after the intake stroke.

What can go wrong here: a restricted air filter chokes airflow. A stuck-closed intake valve blocks the cylinder entirely. Carbon buildup on the back of intake valves (common on GDI engines like the Ford EcoBoost and many VW/Audi TSI engines) restricts airflow into the cylinder. A vacuum leak downstream of the MAF sensor lets unmetered air in, throwing off the fuel calculation. A failed intake manifold runner control can reduce airflow to specific cylinders.

Stroke 2 — Compression

Both valves close. The piston moves from BDC back up to TDC, compressing the air-fuel mixture to about one-tenth of its original volume — a 10:1 compression ratio on a typical engine. Some modern engines run higher — the Mazda Skyactiv-G runs 13:1, and the 2024+ Toyota Dynamic Force engines run 14:1 with port and direct injection combined.

Compression is what makes the power stroke powerful. The more you compress the mixture, the more energy you release when it ignites. Think of it like a spring — the harder you squeeze it, the harder it pushes back. If you lose compression due to worn piston rings, a burned exhaust valve, or a blown head gasket, you lose power. If you lose enough compression, the cylinder will not fire at all.

What can go wrong here: worn piston rings let compression blow past into the crankcase. A burned exhaust valve cannot seal, so compression escapes into the exhaust port. A blown head gasket leaks compression between cylinders or into the cooling system. A jumped timing chain or belt means the valves open and close at the wrong time, killing compression entirely.

Stroke 3 — Power

This is the only stroke that produces work. Both valves are still closed. The piston is near TDC with the mixture fully compressed. The spark plug fires — typically a few degrees before TDC (called ignition timing advance) because it takes a fraction of a second for the flame front to travel across the combustion chamber. The expanding gases push the piston down with tremendous force, turning the crankshaft.

At 3000 RPM, each power stroke lasts about 10 milliseconds. In that time, the combustion chamber temperature hits around 4,500 degrees Fahrenheit and pressure spikes to roughly 600-1,000 PSI. That is the kind of environment your spark plug, piston, rings, and valves live in every single day. It is honestly impressive that engines last as long as they do.

What can go wrong here: a fouled or worn spark plug produces a weak spark or no spark. A failing ignition coil cannot deliver enough voltage. Carbon deposits cause hot spots that ignite the mixture before the spark plug fires (pre-ignition), which can destroy pistons — this is the LSPI problem that plagued early turbo GDI engines. Detonation (knock) occurs when the mixture ignites from pressure and heat instead of from the spark, causing the piston to get hammered from both directions.

Stroke 4 — Exhaust

The exhaust valve opens near the bottom of the power stroke. The piston travels from BDC back to TDC, pushing the burned gases out through the exhaust port, through the exhaust manifold, through the catalytic converter, and out the tailpipe. By the time the piston reaches TDC, the exhaust valve closes, the intake valve opens, and the whole cycle starts over.

What can go wrong here: a clogged catalytic converter creates back-pressure that prevents exhaust gases from leaving, which chokes the engine. The car might start and idle but dies under load, or it gradually loses power. A stuck-closed exhaust valve traps burned gas in the cylinder — the next intake stroke pulls in exhaust gas instead of fresh air-fuel, and the cylinder misfires. A cracked exhaust manifold leaks hot gas and can fool the upstream O2 sensor, causing fuel trim problems.

720 Degrees — Why Two Full Rotations

This confuses a lot of new techs. The crankshaft has to rotate 720 degrees — two complete revolutions — to complete one full four-stroke cycle. Each stroke takes 180 degrees of rotation. Four strokes times 180 degrees equals 720 degrees. That is why your cam turns at half crankshaft speed — one cam revolution for every two crank revolutions. It is also why a timing chain that jumps one tooth has such a dramatic effect. A single tooth can shift valve timing by several degrees of crank rotation, and the engine is designed to operate with precision measured in fractions of a degree.

At 3000 RPM, the crankshaft completes 3000 revolutions per minute. Since one cycle takes two revolutions, each cylinder completes 1,500 cycles per minute. A 4-cylinder engine at 3000 RPM produces 6,000 power strokes per minute. A V8 produces 12,000. That is why even an occasional misfire is noticeable — out of 6,000 events per minute, your body can feel just a handful of them missing.

Cylinder Numbering and Firing Order

Cylinder numbering is not universal, and if you get it wrong, you will diagnose the wrong cylinder. Here are the conventions that matter:

• GM V8: Cylinder 1 is the driver-side front. Odd cylinders (1, 3, 5, 7) are on the driver side. Even (2, 4, 6, 8) are on the passenger side. Firing order for LS engines: 1-8-7-2-6-5-4-3.
• Ford V8: Cylinder 1 is the passenger-side front. On older modular V8s (4.6L, 5.4L), the firing order is 1-3-7-2-6-5-4-8. On the Coyote 5.0L, it is 1-5-4-8-7-2-6-3.
• Inline 4-cylinder: Cylinder 1 is at the timing chain/belt end (front of engine). Numbering goes 1-2-3-4 front to back. Common firing order: 1-3-4-2.
• V6 engines: Varies wildly by manufacturer. Always look it up. A GM 3.6L V6 uses 1-2-3-4-5-6, while a Ford 3.5L EcoBoost uses 1-4-2-5-3-6.

Why does firing order matter? Because it tells you which cylinders share components and timing relationships. If cylinder 1 and cylinder 4 are companion cylinders (one is on the power stroke while the other is on the intake stroke), and both are misfiring, you might have a cam timing issue affecting an entire bank. If two cylinders that are physically adjacent both lose compression, suspect a head gasket breach between them.

What Happens When Each Stroke Fails

Here is the real diagnostic value of understanding each stroke. When you have a problem, you need to figure out which stroke is compromised:

Intake stroke failure symptoms: Low power, lean codes (P0171, P0174), rough idle, hesitation on acceleration. The cylinder is not getting enough air-fuel mixture. Check for vacuum leaks, restricted intake, failed intake valve, carbon buildup on GDI valves.

Compression stroke failure symptoms: Misfire codes (P0301-P0308), low or uneven compression readings, excessive crankcase pressure, oil consumption. The cylinder cannot seal. Run a compression test and a leakdown test to find where the leak is.

Power stroke failure symptoms: Misfire under load, knock/ping, poor fuel economy, P0300-series codes. Spark delivery or timing is off, or the combustion event is abnormal. Check ignition components, fuel injector flow, and knock sensor operation.

Exhaust stroke failure symptoms: Loss of power that worsens with RPM, sulfur smell, P0420/P0430 catalyst codes, excessive back-pressure. The burned gases cannot get out. Check for a plugged cat by measuring back-pressure at the upstream O2 sensor bung — anything over 1.5 PSI at idle or 3 PSI at 2500 RPM is restricted.

How This Applies to Real Diagnostics

Let me give you a real-world example. A 2018 Ford F-150 2.7L EcoBoost comes in with a P0302 — misfire on cylinder 2. The customer says it runs rough at idle and the check engine light is flashing under acceleration.

A parts swapper moves the coil from cylinder 2 to cylinder 4 and the injector from cylinder 2 to cylinder 4, clears codes, and test drives. If the misfire moves, they found it. If it does not, they are stuck.

A tech who understands the four-stroke cycle asks: which stroke is failing? They run a compression test. Cylinder 2 reads 95 PSI. Every other cylinder reads 180 PSI. That is a massive difference — this is a compression stroke failure. They follow up with a leakdown test. Air is hissing out of the tailpipe. That means the exhaust valve is not sealing. The fix is a valve job or a head, not a coil or injector.

The entire diagnostic took 30 minutes because the tech understood the fundamentals. The parts swapper could have spent hours swapping components that were never the problem.

Common Mistakes New Techs Make

• Assuming misfire = ignition. Misfires have three possible causes: ignition, fuel, and mechanical. Check all three before replacing anything.
• Not understanding TDC compression vs TDC exhaust. Each cylinder reaches TDC twice per cycle — once on compression (both valves closed) and once on exhaust (exhaust valve open). If you set a cylinder to TDC but the exhaust valve is open, you are on the wrong stroke. Your leakdown test will be meaningless.
• Ignoring the basics on modern engines. A 2024 engine still runs on the same four strokes as a 1965 engine. Variable valve timing, direct injection, and turbocharging are modifications to the cycle, not replacements for it. The fundamentals never change.
• Confusing cylinder numbers across manufacturers. Cylinder 1 on a GM V8 is driver-side front. Cylinder 1 on a Ford V8 is passenger-side front. Mix them up and you are diagnosing the wrong cylinder. Always verify in service information.

Frequently Asked Questions