How does a four-stroke engine work? Intake, compression, power and exhaust

Inside the engine you see when you open a car’s bonnet, a four-step cycle repeats dozens of times every second. In each cycle a little air and fuel is drawn into a cylinder, squeezed, burned and pushed out. The pressure of the burning gas pushes a piston, and the piston turns a shaft. In this post we open up that cycle, the four-stroke engine, step by step by looking inside a single cylinder.

Most petrol car and motorcycle engines, generators and lawnmowers work on this principle. The German engineer Nicolaus August Otto was the first to put the cycle to work in a running engine, in 1876, which is why its ideal form is called the Otto cycle. The French engineer Alphonse Beau de Rochas had described the same four steps in a patent in 1862.

The parts: from a turning shaft to a piston that moves up and down

Imagine cutting a single-cylinder engine down the middle and looking inside:

• Cylinder: The tube the piston slides in, with a very smoothly machined inner surface. Cooling water flows through the passages around it.
• Piston: A metal cup that moves up and down inside the cylinder, with a flat or slightly shaped top. The piston rings around its edge stop gas from leaking downwards.
• Connecting rod: The arm that links the piston to the crankshaft. Its top end fits on a pin in the piston, its bottom end on the crank of the crankshaft.
• Crankshaft: A shaft with a crank in it. It turns the straight-line motion of the piston into rotation; its counterweights balance the turning.
• Cylinder head: The part that closes the top of the cylinder. It holds the intake and exhaust ports, the valves that open and close them, and the spark plug.
• Camshaft: As it turns, the egg-shaped cams on it push the valves open at the right moment. Springs close the valves again.

Two positions have names: when the piston is at the very top it is at top dead centre (TDC), and at the very bottom it is at bottom dead centre (BDC). The distance between them is the stroke. They are called dead centres because the piston stops there for an instant and changes direction.

Four strokes, two turns

In the figure below you can drag the crankshaft, change the angle with the slider or let the engine turn slowly. We measure the crank angle from 0° to 720°: 0° is the top dead centre where the intake stroke begins. A full cycle takes two turns of the crank; each stroke lasts half a turn, or 180°.

1. Intake (0°–180°)

The piston moves down from top dead centre. The intake valve is open and the exhaust valve is closed. As the piston goes down, the volume in the cylinder grows; the pressure drops a little below the pressure outside, and a mixture of air and fuel flows in through the intake port. In today’s petrol engines injectors spray the fuel; whether it goes into the intake port or straight into the cylinder, the goal is a mixture in the cylinder that is ready to burn.

2. Compression (180°–360°)

The piston moves up from bottom dead centre. Both valves are closed. The mixture has nowhere to go; as its volume shrinks, its pressure and temperature rise. You feel the same thing when you block the end of a bicycle pump with your finger and push the handle: the pump warms up, because the work done to squeeze the gas is added to the gas’s internal energy.

3. Power, or combustion (360°–540°)

When the piston reaches top dead centre, the spark plug fires a spark. The mixture burns, and the temperature and pressure of the gas rise sharply in a very short time. This pressure pushes the piston down hard, and the connecting rod turns the crankshaft. Of the four strokes, only this one gives work to the shaft; the other three are preparation and cleaning up.

In a real engine combustion is not instant; it takes a few milliseconds. So the plug fires a little before top dead centre, so that the highest pressure comes just as the piston starts to move down. This head start is called ignition advance, and it grows as the engine speeds up.

4. Exhaust (540°–720°)

The piston moves up from bottom dead centre again. The exhaust valve is open; the piston pushes the burned gases out through the exhaust port. When the piston reaches top dead centre the cycle ends and a new intake stroke begins.

For simplicity, the valves in the figure open and close exactly at the dead centres. In real engines the valves open a little earlier and close a little later; using the momentum of the moving gas, the cylinder fills and empties better.

Why two turns?

There is only one power stroke in every two turns. What keeps the crankshaft turning for the other turn and a half is the flywheel, a heavy wheel on the shaft: it stores energy during the power stroke and gives it back during the others. In multi-cylinder engines the power strokes of the cylinders take turns; in a four-cylinder engine one cylinder fires every half turn, and the engine runs much more smoothly.

Why does the camshaft turn at half speed?

Each valve must open only once in two turns of the crank. So the camshaft turns at half the speed of the crankshaft: while the crank turns twice, the camshaft turns once. The two shafts are linked by a timing belt, a chain or gears; the gear or pulley on the camshaft has twice as many teeth as the one on the crankshaft.

In the figure, watch the small marks on the front of the two camshafts: while the crank turns 720°, each cam makes just one full turn. When the nose of the cam pushes down the bucket on top of the valve, the valve opens; when the nose has passed, the spring closes the valve.

Crank and connecting rod: from a straight line to a circle

The crank and connecting rod is one of the most common mechanisms for turning back-and-forth motion into rotation. Because the rod is not infinitely long compared with the crank radius, the piston does not move evenly: it slows down near the dead centres and stops for an instant, and it speeds up in between. When the crank has turned 90° from top dead centre, the piston has already covered a little more than half its stroke.

In the engine in the figure the crank radius is 40 mm (an 80 mm stroke) and the connecting rod is 140 mm. If you watch the volume reading, you will see it change slowly around the dead centres and quickly in between.

Compression ratio

The compression ratio is the volume of gas in the cylinder when the piston is at bottom dead centre divided by the volume left when it is at top dead centre. The small space left at top dead centre is the combustion chamber.

The engine in the figure has an 80 mm bore and an 80 mm stroke. The volume the piston sweeps (the swept volume) is about 402 cm³, and the combustion chamber is about 44.7 cm³. So:

• total volume with the piston down ≈ 402 + 44.7 = 446.8 cm³,
• volume with the piston up ≈ 44.7 cm³,
• compression ratio ≈ 446.8 / 44.7 = 10.

Why does the compression ratio matter? In the ideal Otto cycle the efficiency depends only on this ratio (r) and on one property of the gas (γ ≈ 1.4 for air):

ideal efficiency = 1 − 1 / r^(γ − 1)

For r = 10 this comes to about 60%. In real engines some heat escapes through the cylinder walls, there is friction, and combustion is not instant; petrol car engines usually turn about half that share of the fuel’s energy into work. Still, the trend holds: a higher compression ratio means better efficiency.

In petrol engines the ratio cannot be raised without limit. If the mixture is squeezed too much, it can heat up and ignite on its own without waiting for the spark. This uncontrolled burning is called knock; it strains the engine and can damage it. The octane number shows how well a petrol resists knock. That is why the compression ratio of petrol car engines is mostly between 9 and 13.

The P–V diagram: a picture of the work

The diagram under the figure plots the pressure in the cylinder against the volume. One cycle draws a closed curve, and the area inside it equals the net work the gas gives the piston in one cycle. During compression the piston does work on the gas (the lower curve); during the power stroke the gas gives more work back to the piston (the upper curve); the difference is what the engine gains. In the ideal model intake and exhaust run back and forth along the same horizontal line and leave no net area.

How is a diesel engine different?

A diesel engine is also a four-stroke engine, but it does not light the mixture with a spark plug:

• Only air is drawn in during intake.
• The compression ratio is very high; usually between 15 and 20 in car diesels. The air is squeezed so hard that its temperature rises far above the temperature at which the fuel ignites on its own. In an ideal calculation, air compressed 18 times heats up to about 680 °C.
• The fuel is injected straight into the cylinder at the end of compression and ignites by itself as soon as it meets the hot air. No spark plug is needed; the glow plugs that help with cold starts do not spark, they only heat.

Because a diesel gets its fuel only at the end of compression, the knock problem that limits a petrol engine does not arise in the same way, and the compression ratio can be kept high. This is one reason diesels are usually more efficient. The engine is named after the German engineer Rudolf Diesel, who developed and patented its principle in the 1890s.

Things to try with the figure

• Press Intake and watch: while the intake valve slowly opens and closes, the purple mixture fills the cylinder.
• Watch the pressure gauge during compression: while the volume shrinks tenfold, the pressure rises more than tenfold. Because the squeezed gas heats up, the pressure climbs faster than the volume alone would suggest.
• Drag the crank a little past 360°: the plug fires and the dot on the P–V diagram shoots straight up.
• Count the marks on the camshafts: while the crank turns twice, each cam turns only once.

Summary

• In a four-stroke engine one cycle has four steps: intake, compression, power (combustion) and exhaust. The crankshaft turns twice in one cycle.
• Only the power stroke gives energy to the shaft; the flywheel and having several cylinders smooth out the turning.
• The crank and connecting rod turn the up-and-down motion of the piston into rotation; the camshaft turns at half the crank’s speed and opens the valves at the right moment.
• The higher the compression ratio, the higher the ideal efficiency; in a petrol engine knock limits the ratio.
• A diesel engine squeezes only air at a very high ratio; the fuel ignites by itself in the hot air, with no spark plug.