Engine Fundamentals

Auto and Shop rewards knowing what each part does and where it sits in a sequence. The four-stroke cycle is the sequence everything else hangs from.

The four-stroke cycle

Enlarge

Zoom in

The valve pattern is the part worth learning, because it is what questions ask. Both valves are closed for compression and power - the cylinder has to be sealed for the mixture to be squeezed and for the burn to push rather than escape.

Only the power stroke produces power. The other three are driven by the crankshaft's momentum, helped by the flywheel and by the other cylinders.

That is why an engine has a flywheel and why a single-cylinder engine runs less smoothly than a multi-cylinder one: with four cylinders, one is on its power stroke at all times.

Two crankshaft revolutions complete one four-stroke cycle, because four strokes at half a turn each is two full turns.

The parts

The crankshaft is the part that changes the kind of motion, which is the point a question about it is usually making: reciprocating in, rotating out.

The camshaft is driven by the crankshaft, through a timing belt, chain or gears, so the valves stay synchronized with the pistons. A broken timing belt stops the engine immediately and can let valves and pistons collide.

Displacement is the total volume the pistons sweep, and it is the number an engine is described by.

Compression ratio is the cylinder's volume at the bottom of the stroke compared with at the top. Higher ratios extract more energy from the same fuel.

Two-stroke engines

A two-stroke completes the cycle in two strokes and one crankshaft revolution instead of four and two.

A two-stroke fires twice as often, which is why it makes more power for its size and weight - and why it is used in chainsaws, outboards and small equipment.

Its oil is mixed with the fuel and burnt, which is why a two-stroke smokes and why it is cleaner-burning engines that have displaced it.

Diesel engines

A diesel has no spark plug.

It compresses air alone to a very high ratio, which heats it, and then injects fuel into that hot air, where it ignites on contact. That is compression ignition.

"No spark plug" is the answer to more diesel questions than anything else.

Glow plugs are not spark plugs. They warm the combustion chamber to help a cold diesel start, and they do not ignite the fuel.

Know it by sight

Compression ratio compares the cylinder's volume with the piston at the bottom of its stroke to the volume with it at the top; a higher ratio squeezes the mixture harder.

What you can skip

Across the 59 questions on this topic in our bank:

• Engine types beyond the four-stroke. Two-stroke engines come up once here and diesel twice; read those sections once. Rotary engines never appear.
• Displacement arithmetic. Displacement comes up twice and firing orders twice; neither is a calculation.
• Variable valve timing and turbocharger details do not appear here; the turbocharger is asked once, in the fuel and air lesson.

Where people lose points

Getting the stroke order wrong.

Saying a valve is open during compression or power. Both are closed.

Thinking every stroke produces power. One in four does.

Saying a diesel uses a spark plug, or that a glow plug is one.

Confusing the camshaft and the crankshaft. The crank converts motion; the cam times the valves.

Saying a two-stroke is more efficient. It is more powerful for its size and less efficient.

Work one in under a minute

During which strokes are both valves closed, and why does it matter?

Compression and power.

Compression: the cylinder must be sealed or the mixture would escape instead of being squeezed.

Power: it must stay sealed or the expanding gases would escape instead of pushing the piston down.

Both strokes depend on the cylinder being closed, and that shared requirement is why the two strokes in the middle of the cycle are the ones with both valves shut.

Where this leads

Every other Auto lesson attaches to this cycle: fuel and air feed the intake stroke, ignition triggers the power stroke, and the exhaust system takes what the exhaust stroke pushes out.