Step 4 of 6
Four-stroke engine step 4 of 6: Stroke 3: power
Power, from 360 to 540 degrees: just before the top, the spark plug fires a spark across its gap. The flame front spreads through the squeezed mixture, the pressure leaps to about 50 bar and drives the piston down hard, turning the crankshaft. This is the only stroke that does work.
Tools: none, just your hands
Common mistake: a timing belt that has slipped four teeth. The toothed belt keeps the camshafts in step with the crankshaft, so the valves are shut whenever the piston is near the top. If a worn or loose belt jumps four teeth on its sprockets, the cams run 36 degrees ahead (72 of the crankshaft’s): the intake valve opens early, and on its way up the piston hits it. In an interference engine like this one that bends the valve and can break the piston. Change the belt when the car’s handbook says. The switch in the player shows what goes wrong.
All 6 steps
What is in it
- Piston · with two compression rings and an oil ring
- Connecting rod · and the piston’s pin
- Crankshaft · with its counterweights
- Intake and exhaust valves · each opened by its own camshaft
- Spark plug · in the middle of the head
- Timing belt · turning the camshafts at half speed
Questions
What are the four strokes of an engine?
Intake, compression, power and exhaust. The piston goes down to draw in air and fuel, up to squeeze them, down again when the burning mixture pushes it, and up to push the burnt gases out.
Why does the crankshaft turn twice in each cycle?
Each stroke is one trip of the piston, up or down, which is half a turn of the crankshaft; four strokes take two turns. That is why the camshafts turn at half the crankshaft’s speed: each valve opens once a cycle.
What is top dead centre?
The top of the piston’s travel, where it stops for an instant before going back down. Bottom dead centre is the bottom of its travel.
What is the compression ratio?
How many times smaller the space over the piston is at the top than at the bottom. A petrol engine’s is usually 8 to 12 to 1; this one’s is 10 to 1.
What happens if the timing belt slips?
The valves no longer open and shut in step with the piston. In an interference engine an open valve can meet the rising piston, bending the valve and damaging the piston, so belts are changed at the interval the car’s handbook gives.
Safety
This is a model to show how an engine works. Never reach into a running engine’s bay, and leave timing belt work to a garage: a belt fitted a tooth out can wreck an interference engine.
About this guide
This is one cylinder of a four-stroke petrol engine as a working model, sized like VW’s EA888 two-litre engine (bore 82.5 mm, stroke 92.8 mm, connecting rod 144 mm, from MAHLE’s data): a piston with two compression rings and an oil ring, a connecting rod, a crankshaft with counterweights, an intake and an exhaust valve opened by two camshafts, which a toothed belt turns at half the crankshaft’s speed, and a spark plug.
The valve timing is typical of a petrol engine, as MIT’s internal combustion engines course gives it: the intake opens about 10 degrees before the top and shuts 50 after the bottom, the exhaust opens 50 before the bottom and shuts 10 after the top. The 10:1 compression ratio is typical too (8 to 12:1). The pressures on the readings are illustrative, for an engine at part throttle.
Sources: MIT OpenCourseWare 2.61: Internal combustion engines (lecture notes), MAHLE: VW EA888 engine data, Wikipedia: Four-stroke engine.
The 3D models, drawings and text are our own.