An interactive cutaway you can drive

How an engine breathes, burns and spins

Inside the block, a piston is pulled down, shoved up, launched by a controlled fire and pushed up again. At highway speed that happens twenty-five times a second, in every cylinder. Nearly every car on the road runs on that four-beat rhythm.

Drag the engine to orbit it; pinch or scroll to zoom. The strip under the engine is the crank angle: drag it to turn the engine by hand. Then keep scrolling. Each chapter takes the camera and slows the engine down to the stroke it is explaining. Corner readout: θ (theta) is the crank angle in degrees, and rev counts full crankshaft turns of 360°. TDC and BDC are top and bottom dead centre, the piston’s highest and lowest points. rpm is revolutions per minute, cyl is cylinder.

Anatomy

Twelve parts to know by name

The block and head are cut in half along the cylinder, so the moving parts are in plain view. Tap a part to light it up and bring the camera to it.

Tap a part to light it up.

Where is the oil? Not in the fire. Engine oil lives in the sump under the crankshaft, out of view here, and a pump pushes it through drillings to the bearings, camshafts and cylinder walls. The piston rings scrape it back down, so it never reaches the combustion chamber. That is the four-stroke rule: oil and fuel stay separate. A two-stroke has no sealed sump, because its crankcase is used to pump the mixture, so oil is mixed into the petrol and burns with it.

Stroke one, 0 to 180°

Intake: the engine inhales

The crankshaft drags the piston down from top dead centre (TDC) to bottom dead centre (BDC). The intake valve is open, the exhaust valve is shut. As the piston drops, the space above it grows and the pressure inside falls below atmospheric: about 0.9 bar with the throttle wide open, far less at idle when the throttle plate is nearly closed.

Nothing “sucks” the charge in. The pressure difference pushes it. Outside air at 1 bar flows toward the low-pressure cylinder through the intake port, picking up a mist of petrol on the way (port injection) or meeting it inside the cylinder (direct injection). Liquid petrol does not burn; its vapour does, so those droplets evaporate on the way in and arrive as vapour mixed with the air. The chemistry wants roughly 14.7 parts of air to 1 part of fuel by mass, the stoichiometric ratio.

Watch the strip under the engine: the intake valve opens a few degrees before TDC and stays open well after BDC. The column of air in the port has momentum, and it keeps ramming in even as the piston starts back up.

Stroke two, 180 to 360°

Compression: squeeze it small

Both valves close and the piston climbs. The gas that filled the cylinder at BDC is crushed into the small pocket under the head, the combustion chamber. The big volume divided by the small one is the compression ratio: around 10 : 1 in a modern petrol engine, 16 to 22 : 1 in a diesel.

Squeezing a gas quickly heats it. By TDC the charge is at roughly 15 to 20 bar and 350 to 500 °C (about 350 °C in this simplified model): hot, but not enough to light petrol on its own in the few milliseconds available. If it does light by itself, that is knock. That restraint is the whole design. In a petrol engine the spark decides when combustion starts. (A diesel compresses pure air so hard that it passes 700 °C, then simply injects fuel into it.)

Why compress at all? Thermodynamics: the harder you squeeze before you burn, the larger the share of the fuel’s heat you can turn into work. The Otto cycle chapter below has the formula.

Look closely just before TDC. The spark fires early, from about 10° before the top at idle to 30° or more at speed (the animation uses 10°), because the flame needs a couple of milliseconds to cross the chamber and the piston will not wait.

Stroke three, 360 to 540°

Power: the only stroke that pays

A spark of twenty-odd thousand volts jumps the plug gap and lights a kernel of mixture the size of a pea. The flame front races outward at 20 to 40 m/s, fanned by turbulence, and at 3,000 rpm within about two milliseconds the whole charge has burned. The gas does not explode; it burns extremely fast in a space that is barely changing size, so at full load the pressure rockets to 40 to 60 bar, peaking 10 to 15° after TDC in a well-timed engine (about 20° in this model).

That pressure on the piston crown is enormous. At the top of that range, sixty bar on an 86 mm bore is about 35 kN, the weight of a loaded van, delivered down the connecting rod to the crankpin, which turns the push into torque. The expanding gas drives the piston all the way to BDC, cooling and dropping to a few bar as it goes.

This is the only stroke that produces work. The other three consume it. The flywheel stores enough momentum to carry the engine through them, which is why a single-cylinder engine thumps and a multi-cylinder one hums.

If the last pocket of unburned mixture gets hot enough to ignite before the flame reaches it, you get knock: the metallic ping that octane ratings exist to prevent.

Stroke four, 540 to 720°

Exhaust: clear the room

The exhaust valve cracks open about 40° before BDC, while the gas is still at 3 to 5 bar. It rushes out on its own in a burst called blowdown. That pulse, repeated thousands of times a minute, is the sound an exhaust pipe shapes. Opening early gives up a little work at the end of the power stroke but saves more by not making the piston fight the pressure on the way up.

The piston then rises and sweeps the remaining burnt gas, at 700 to 900 °C, out through the port. Near TDC the intake valve starts to open before the exhaust valve has fully closed. For about twenty degrees both are open, the valve overlap, and the departing exhaust helps pull in fresh charge.

The piston is back at TDC. Two full turns of the crank, one cycle. That is why the camshafts that open the valves must turn at exactly half crankshaft speed.

The whole cycle

720 degrees, in order

Everything in the engine is choreographed against crank angle. The bars show when each valve is open; the dashed line is the spark. Drag the cursor to move the engine by hand.

IN and EX are the intake and exhaust valve bars. IVO and IVC are intake valve opening and closing, EVO and EVC the same for the exhaust. Getting the valves open wide enough, early enough, and slammed shut again by a spring 25 times a second at 3,000 rpm, 50 times at 6,000, is most of what separates a strong engine from a wheezy one. Variable valve timing shifts these events as speed changes.

Theory

The Otto cycle, drawn live

Plot pressure against volume for one cycle and you get the loop below. The dot is the engine above, right now. The area inside the big loop is the work delivered per cycle. The thin loop at the bottom, intake running below exhaust, is pumping work the engine spends just to breathe.

Idealise the cycle, with instant combustion at TDC and no heat lost to the walls, and its efficiency depends on one number:

where r is the compression ratio and γ is about 1.4 for air. Try it. The engine above changes too: watch the combustion chamber shrink and the pressure peak climb. This simplified model peaks lower than a real engine at full load, about 35 bar at 9 : 1.

Real engines land between 25 and 40 %. Heat leaks into the water jacket, the burn takes time, the gas is not ideal, valves and bearings cost friction. Higher compression helps right up to the point where the charge starts to knock, which is why premium fuel, direct injection and turbocharging are all compression-ratio stories in disguise.

Scaling up

Four cylinders, one crankshaft

A single cylinder fires once every two revolutions and coasts the rest of the time. Put four on one crankshaft, offset so that one is always on its power stroke, and the torque evens out. The crankpins of cylinders 1 and 4 point one way, 2 and 3 the other, and the cylinders fire in the order 1-3-4-2: a power stroke every 180°.

Cylinder 1 is on its power stroke.

At any instant each cylinder is on a different stroke. While cylinder 1 burns, cylinder 3 is compressing, cylinder 4 is on intake and cylinder 2 is pushing exhaust out. The valves give it away: look along the row of cylinders and you will see all four states at once.

By the numbers

What 3,000 rpm actually means

By default the engine above runs at a scaled 300 rpm, one tenth of the speed in the panel, and even that is too fast to follow by eye. The slider only recalculates the figures; it does not drive the engine. A real engine at 3,000 rpm, a typical motorway speed, runs ten times faster than the one on screen. Every valve in it opens and shuts in the time it takes you to blink.

Keep going

Things to try

Pause the engine and drag the strip very slowly across 350°, then watch the flame front leave the spark plug. Push the compression ratio to 14 : 1 and compare the pressure peak. Switch to four cylinders, turn the labels on and find the one cylinder whose intake valve is open.

Then go and listen to a real engine. Every sound it makes is one of these four strokes.