How a four-stroke engine works.
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Power stroke
Crank 100° · 7.5 bar
173 Nm · 54 kW at 3,000 rpm
Shown 50× slower than real
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Cylinders
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Engine
Cylinders
Compression and combustion
Advanced: bore, stroke and rod length
The 3D model keeps its proportions; the numbers and the chart use your values.
Results for this engine
- Displacement
- 1,998 cc
- Efficiency
- 39.7%
- ideal Otto: 61.0%
- Mean effective pressure
- 12.4 bar
- Peak pressure
- 65.5 bar
- 13° after TDC
- Torque at the crank
- 173 Nm
- estimate, after friction
- Power
- 54 kW
- at 3,000 rpm, estimate
A six-step tour of the engine
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1 · The parts
A four-stroke engine is a block of cylinders with a head on top. Pistons slide in the cylinders, connecting rods join them to the crankshaft, and camshafts in the head open the valves. The engine is pulled apart here in the order a mechanic would take it down.
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2 · Intake: the charge comes in
The piston falls from the top of its stroke while the two intake valves stand open. The falling piston pulls the air–fuel charge in through the intake port. The cam lobe is pressing its bucket down to hold the valves open.
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3 · Compression: both valves shut
The intake valves have closed and the piston rises, squeezing the charge into the small space under the head. Pressure climbs from about 1 bar to around 20 before the spark.
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4 · Spark and power
The plug fires just before the piston reaches the top. The burning charge peaks at over 60 bar a few degrees after the top and drives the piston down, turning the crank through its connecting rod. This is the only stroke that makes work.
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5 · Exhaust: the burnt gas leaves
The exhaust valves open just before the bottom of the power stroke, and the rising piston pushes the burnt gas out into the exhaust manifold. Near the top both valves are open for a moment: that is valve overlap.
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6 · The work loop
The chart under the engine plots the pressure in cylinder 1 against its volume. Squeezing the charge costs work, the burn and expansion give back more, and the area inside the loop is what is left over: the work of one cycle. Change the compression ratio and watch the loop and the efficiency move.
Take it on the engine
The camera moves to each part, the engine stops at the right moment and the part glows. About three minutes.
Ten questions, answered from the engine
Which stroke is cylinder 1 on? Which valve opens next? Which cylinder fires next? Plus two quick sums: displacement and the ideal Otto efficiency. The engine freezes at each question's crank angle, and the readouts hide until you answer.
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Cylinder 1 through one cycle
The dot follows the engine above. The loop's area is the work of one cycle.
At the dot
Crank
100°
Stroke
Power
Volume
382 cc
Pressure
7.5 bar
Work per cycle, one cylinder
621 J
A live four-stroke petrol engine you can take apart. Every piston, valve and pressure reading comes from the engine's real geometry and a thermodynamic model of the burn: intake → compression → power → exhaust, two crankshaft turns per cycle.
How to use the engine simulator
- Press Start the engine, or pause and drag the crank scrubber under the engine to step through the four strokes by hand.
- Watch cylinder 1 through intake, compression, power and exhaust: the readout names the stroke and the pressure in the cylinder, and the dot on the chart below follows it.
- Pick one, two, three or four cylinders to compare firing orders, and use Take it apart to see the engine come down in the order a mechanic would strip it.
- Under Controls, change the compression ratio, speed or throttle and watch the pressure–volume loop, the efficiency and the power move. At 10.5 : 1 the ideal Otto efficiency is 60.9%; the modelled engine reaches about 40%.
- Take the six-step Learn tour, then try a ten-question round in Challenge.
How a four-stroke engine works
The four strokes (720° of crankshaft)
A four-stroke engine completes one power cycle every two crankshaft revolutions. Intake: the piston descends and the intake valves open, drawing in air and fuel. Compression: both valves close and the rising piston squeezes the mixture. Power: the spark plug ignites it and expanding gas drives the piston down. Exhaust: the piston rises again and the exhaust valves open to expel burnt gas.
TDC, BDC and what a “stroke” actually is
Top dead centre (TDC) is the highest point the piston reaches, where the space above it is smallest; bottom dead centre (BDC) is the lowest, where it is largest. A stroke is one trip between the two — half a crankshaft revolution, 180° — which is exactly why four strokes take 720°. Pause the engine and drag the crank scrubber under it to 0°, 180°, 360° and 540° to land on each changeover in turn.
Why the motion is exact
Piston height follows the slider-crank relation y = r·cos θ + √(L² − r²·sin²θ), where r is the crank radius (half the stroke) and L is the connecting-rod length. Every moving part, down to each valve and spring, is placed from a single crank angle θ, so the model is geometrically exact. The cam lobes are cut so their flat followers trace exactly the valve lift the pressure model assumes.
Bore, stroke and compression ratio
Bore is the cylinder’s diameter; stroke is how far the piston travels between TDC and BDC. Together they give the swept volume of one cylinder, V = π/4 · bore² · stroke, and multiplying by the cylinder count gives the engine’s displacement — the “2.0-litre” on a spec sheet: 86 mm × 86 mm × 4 cylinders is 1,998 cc. The compression ratio compares the volume above the piston at BDC with the much smaller volume left at TDC: around 10:1 for a typical petrol engine, closer to 18:1 for a diesel. Squeezing harder extracts more work from the same charge, right up to the point where the mixture ignites on its own.
Where the gases flow
The intake manifold feeds the air–fuel charge down its runners and past the two intake valves into each cylinder. After combustion, the rising piston pushes burnt gas past the exhaust valves into the exhaust manifold on the other side, which collects every cylinder into one outlet. In the cutaway the exhaust manifold is drawn as a ghost so it does not hide the cut. The overhead camshafts spin at half crank speed, their lobes pressing each valve open against its spring at the right moment.
Valve timing and valve overlap
The camshafts turn at exactly half crankshaft speed, so each valve opens once per 720° cycle rather than once per revolution. Switch the chart to Pressure–crank angle and the lift curves along its foot are a valve-timing diagram: intake in blue, exhaust in red. Near the TDC between exhaust and intake both curves are briefly off the seat at once. That is valve overlap, and it is deliberate: exhaust leaving at speed helps drag the fresh charge in behind it.
Firing orders
The four fires 1-3-4-2, one power stroke every 180°: pistons 1 and 4 rise and fall together, opposite to 2 and 3. The three has a 120° crank and fires 1-2-3, every 240°. The twin is a 360° parallel twin: both pistons move together and fire one turn apart. The single fires once every two turns, which is why it leans on a heavy flywheel.
The Otto cycle and the P–V diagram
The four-stroke petrol cycle is the Otto cycle: intake and exhaust move gas at near-atmospheric pressure, while compression and expansion do the thermodynamic work. The chart plots cylinder pressure against volume, a P–V diagram, where the area inside the loop is the work of one cycle. It is modelled, not measured: a single-zone model of the charge with a Wiebe burn curve, separate heat-capacity ratios for compression and expansion, and heat lost to the walls. The ideal air-standard efficiency at 10.5 : 1 is 61.0%; the modelled engine turns 39.7% of its fuel's energy into work on the piston, and friction takes a little more before the crank.
Explore it your way
Take it apart runs a mechanic's teardown in order, from the belts to the crank, with each part labelled. Cutaway opens the block and head down the middle. Pause and drag the scrubber, or step a stroke at a time, to freeze any instant of the 720° cycle, and use Fullscreen for presentations.
Frequently Asked Questions
What are the four strokes of an IC engine?
A four-stroke engine repeats a cycle of intake, compression, power and exhaust. Intake draws air and fuel in as the piston descends; compression squeezes the mixture as the piston rises with both valves shut; the spark ignites it for the power stroke that pushes the piston down; and exhaust expels the burnt gas as the piston rises again. The whole cycle takes two crankshaft revolutions (720°).
What is the firing order of an inline-four engine?
The most common firing order for an inline-four is 1-3-4-2, giving one power stroke every 180° of crankshaft rotation. Cylinders 1 and 4 move up and down together, opposite to cylinders 2 and 3, which balances the engine and spreads the power pulses evenly. Some engines use 1-2-4-3 instead.
How does a piston and crankshaft turn up-and-down motion into rotation?
The connecting rod links the piston to an offset journal on the crankshaft. As the piston moves up and down, the rod pushes the crank pin around a circle, converting linear motion into rotation. Piston height follows the slider-crank relation y = r·cos θ + √(L² − r²·sin²θ), where r is the crank radius and L the rod length — which is exactly how this 3D model is animated.
What is the difference between a two-stroke and a four-stroke engine?
A four-stroke engine fires once every two crankshaft revolutions and uses separate intake and exhaust strokes with poppet valves — efficient and clean, used in cars and motorcycles. A two-stroke engine fires every revolution by combining intake/exhaust with compression/power, making it lighter and more powerful for its size but less efficient. This simulator models the four-stroke cycle.
What is TDC and BDC in an engine?
TDC (top dead centre) is the point where the piston sits highest in the cylinder and the space above it is smallest; BDC (bottom dead centre) is where it sits lowest and that space is largest. One stroke is a single trip between the two — 180° of crankshaft rotation — which is why four strokes add up to 720°. In this simulator, pause and drag the crank scrubber under the engine to 0°, 180°, 360° and 540° to sit exactly on each changeover.
What is valve overlap?
Valve overlap is the brief window near TDC, between the exhaust and intake strokes, when both valves are off their seats at the same time. It is deliberate: exhaust leaving the cylinder at speed helps drag the fresh air–fuel charge in behind it, which fills the cylinder better at high RPM. The cost is a rougher idle and some unburnt mixture slipping through at low speed, so road engines use modest overlap and race engines use far more. You can see it in the valve-lift curves on the cycle diagram above.
Does this engine simulation run on my device?
Yes — it renders entirely in your browser using WebGPU where available (Chrome, Edge and recent Safari), automatically falling back to WebGL2 on other browsers. The camera-views menu on the model says which one it is using. The 3D model downloads once (about 1.4 MB) and your browser keeps it; after that, the motion and the pressure model are computed on your device and nothing is sent to a server.
Read next
Sources · Reviewed
- John B. Heywood, Internal Combustion Engine Fundamentals, 2nd edition (2018): the single-zone cycle model, slider-crank kinematics and typical engine data
- I. I. Vibe (Wiebe), Brennverlauf und Kreisprozess von Verbrennungsmotoren (1970): the burn-rate curve
- S. K. Chen and P. F. Flynn, SAE Technical Paper 650733 (1965): the friction mean effective pressure estimate
- Air-standard Otto cycle efficiency, 1 − 1/r^(γ−1)