How a four-stroke engine turns fuel into motion

Most car engines are four-stroke engines, which means each cylinder completes four piston movements to produce one power stroke. The cycle is simple to describe but precise in execution. Air and fuel must enter at the right moment, be compressed, ignite at the right point and leave through the exhaust before the next cycle begins. Each cylinder repeats the cycle many times a minute, and the combined pulses from all the cylinders are what the crankshaft turns into rotation.

The explanation below starts with the strokes, then covers the valves and timing that keep them in step, how the pulses become rotation, and what can interrupt the cycle. Specific numbers vary between engines, so the owner's manual remains the reference for any particular car.

Four strokes and two turns of the crankshaft

The first stroke is intake. The piston moves down, the intake valve opens, and the cylinder fills with air mixed with fuel. In many modern engines, fuel is sprayed directly into the cylinder instead. The second stroke is compression. The intake valve closes, the piston moves up, and the mixture is squeezed into a small space, which makes it hotter and denser so that it burns with more force.

The third stroke is the power stroke. Just before the piston reaches the top of its travel, the spark plug fires and ignites the compressed mixture. The burning gas expands rapidly and pushes the piston down, and that push is the only stroke that produces work. The fourth stroke is exhaust. The exhaust valve opens, the piston rises again, and it pushes the burned gases out into the exhaust system. Together, the four strokes take two full turns of the crankshaft, so each cylinder fires once every two revolutions.

Diesel engines also run on a four-stroke cycle, but they have no spark plugs. Their air is compressed so strongly that it becomes hot enough to ignite the fuel as soon as it is injected. The strokes are the same, and the difference lies in how the mixture is lit. That distinction shows that the four-stroke pattern describes the sequence of piston movements and valve events, not one particular ignition method.

Valves and timing keep the cycle in step

The valves must open and close at the right moments relative to the piston's position. A camshaft controls them with lobes that push on the valves as it turns. The camshaft is driven by the crankshaft through a timing belt or timing chain, and it turns at half the crankshaft's speed, which matches the two-revolution cycle of each cylinder. If the camshaft and crankshaft drift out of step, the valves open at the wrong point, and the engine loses power, runs rough or stops.

Spark timing is just as important. Ignition occurs before the piston reaches the top of the compression stroke, and the exact advance is set in degrees of crankshaft rotation. That early start gives the mixture time to burn and build pressure as the piston moves down. The control computer adjusts the timing based on engine speed, load, temperature and other inputs. Small adjustments help the engine make power efficiently and resist knock, which is an uncontrolled rattling caused by the mixture igniting too early or unevenly.

How the pulses become rotation

The push on each piston travels through its connecting rod to an offset section of the crankshaft, which turns that push into torque. The torque is not constant. It rises sharply during each power stroke and falls off during the other three, so a single cylinder delivers its energy in short bursts. A heavy flywheel attached to the crankshaft stores some of that energy and carries the engine through the strokes that produce no power.

Adding cylinders smooths the result because their power strokes overlap. A four-cylinder engine has a power stroke in one cylinder roughly every half turn of the crankshaft, and those pulses blend into steadier rotation. Engine output depends on how much mixture each cylinder burns on every cycle, and on how many cycles happen each minute. Larger cylinders or more of them can hold more mixture, and higher speeds allow more cycles per minute, which is why power rises with engine speed up to a point.

What interrupts the cycle

The cycle fails when one of its requirements is missing. A misfire happens when a cylinder does not burn its mixture properly. Common causes include a weak or missing spark, a fuel delivery problem, air leaking past the intake, or low compression from worn piston rings or leaking valves. The result is a stutter or rough idle, and the engine can lose power in proportion to how many cylinders are affected. A brief misfire can happen under unusual conditions, but repeated misfires are a sign to have the car checked.

Severe misfires need prompt attention. A flashing check engine light is the usual warning, and it indicates that unburned fuel is entering the exhaust, where it can overheat and damage the catalytic converter. The usual advice is to reduce engine load, avoid hard acceleration and have the car inspected promptly. A technician can read stored trouble codes, check spark and fuel delivery, and perform a compression test to see whether the cylinders are sealing. Knowing the four strokes helps make that conversation clearer, because it shows which step in the cycle the symptoms point toward.