Cars · Lesson 2 of 15Beginner10 min read

The Engine

An engine is an air pump that powers itself. It pulls in air and fuel, squeezes them, burns them, and pushes out what’s left, over and over, many times every second. Watch it happen, then meet each part.

Press play, or drag the slider to turn the crank by hand. Tap any part to name it. The valve timing follows typical figures from MIT’s engine course: the intake valve opens a little before the top of the stroke and closes 30 to 50 degrees after the bottom.

The four strokes

Intake. The intake valve opens and the piston moves down, drawing air and fuel into the cylinder.
Compression. Both valves close and the piston moves up, squeezing the mixture into a small space at the top.
Power. The spark plug fires. The burning mixture gets very hot, its pressure soars, and it shoves the piston down. This is the only stroke that makes power.
Exhaust. The exhaust valve opens and the piston moves up, pushing the burned gases out.

Four strokes take two full turns of the crankshaft. Each valve opens once per cycle, so the camshaft that opens the valves turns at half the crankshaft’s speed. Its gear or sprocket is twice the size of the one on the crankshaft, and a timing belt or chain links them.

Real engines don’t open and close the valves exactly at the top and bottom of each stroke. The exhaust valve opens 30 to 50 degrees before the power stroke ends so pressure can blow the gases out, and the intake valve stays open 30 to 50 degrees past the bottom so moving air keeps ramming in. For a moment near the top of the exhaust stroke, both valves are open at once. That’s called valve overlap.

The parts and how they fit

The engine block is the frame. Bored into it are the cylinders, and in each one a piston slides up and down. Piston rings seal the gap between piston and cylinder wall. A connecting rod links each piston to the crankshaft, which runs along the bottom of the block. On top sits the cylinder head, with the valves, the spark plugs and usually the camshafts. The head gasket seals the joint between head and block, and the oil pan closes off the bottom.

Main engine parts
Part	What it does	Signs of trouble
Block	The engine’s frame; holds the cylinders and the coolant and oil passages.	Leaks; freezing can crack a block if the coolant lacks enough antifreeze.
Cylinders	Round bores where the pistons move.	Wear lets compression and oil slip past the rings.
Pistons and rings	The piston takes the push of the burning gas; rings seal compression and scrape oil off the walls.	Blue exhaust smoke and oil use can mean worn rings.
Connecting rods	Link each piston to the crankshaft.	A rhythmic heavy knock can mean worn rod bearings.
Crankshaft	Turns the pistons’ up-and-down push into rotation.	Heavy knocking from worn main bearings.
Flywheel	A heavy wheel on the crankshaft that stores energy from each power stroke and smooths the engine out.	Rarely a problem on its own; a technician checks it.
Cylinder head	Caps the cylinders and holds the valves, spark plugs and often the camshafts.	See head gasket; a technician checks it for warping.
Valves	Let air and fuel in and exhaust out, then seal for compression and power.	Low compression; a click that follows engine speed can be a stuck valve lifter or low oil.
Camshafts	Lobes on a shaft that push the valves open at the right moments.	Valve-train noise; a technician diagnoses it.
Timing belt or chain	Keeps the camshafts in step with the crankshaft at half its speed.	Belts must be replaced at the interval in your maintenance schedule.
Head gasket	Seals the head to the block, keeping gas, coolant and oil in their own passages.	Coolant loss with no outside leak, overheating, white exhaust that doesn’t clear.
Oil pan	Holds the engine’s oil at the bottom of the block.	Drips of brown or black oil under the engine.

Displacement and compression

Displacement is the volume all the pistons sweep through in one stroke: the area of the bore times the length of the stroke, times the number of cylinders. It’s a rough measure of how much air an engine can breathe, and it’s usually given in liters. One liter is about 61 cubic inches.

The compression ratio compares the cylinder’s volume with the piston at the bottom to its volume with the piston at the top. Squeezing the mixture harder gets more work out of each drop of fuel, but in a gasoline engine too much squeeze makes the fuel explode on its own, a problem called knock. Knock holds gasoline engines to about 12:1, or about 13:1 with direct injection. Diesels need high compression to light their fuel at all, and run about 14:1 to 22:1; the Navy’s mechanic course lists passenger-car diesels as high as 24:1.

TDC is top dead center, the piston’s highest point; BDC is bottom dead center. The stroke is the distance between them.

displacement = (π/4 × bore²) × stroke × number of cylinders
compression ratio = (swept volume + clearance volume) ÷ clearance volume

WhyInside a running cylinder

At highway speed, a piston takes combustion pressures over 1,000 psi and temperatures well over 600°F, and it stops and reverses direction about 80 times a second. Engine parts are made with tight tolerances for that reason: a good cylinder is round to within about 0.0005 inch (0.012 mm).

Horsepower and torque

Torque is twisting force: how hard the crankshaft turns. Power is how fast that twisting does work. A rotating shaft’s power is its torque times its rotational speed, so an engine can make big power by twisting hard, by spinning fast, or both.

In U.S. units, one horsepower is 550 foot-pounds of work each second. Working that through gives a handy rule: horsepower equals torque in pound-feet times rpm, divided by 5,252. An engine making 200 pound-feet at 3,000 rpm is making about 114 horsepower. Torque is what you feel pulling away from a stop; power is what keeps the car moving fast. Gears in the transmission trade one for the other (Lesson 7).

power = torque × angular speed (P = τω)
horsepower = torque (lb·ft) × rpm ÷ 5,252
1 hp = 550 ft·lbf/s ≈ 745.7 watts

Clues an engine gives you

Oil level. The simplest engine check there is. See Check Your Engine Oil.
Ping. A high-pitched metallic tapping that rises with engine speed is usually fuel with a lower octane rating than the car needs.
Heavy knock. A rhythmic pounding can mean worn crankshaft or connecting rod bearings. Stop and have it checked.
Blue smoke. Oil is getting into the cylinders and burning, often from worn rings or valve guides.

Technicians measure an engine’s health with tests. A compression test, for example, reads the pressure each cylinder builds while cranking. The Navy’s training manual gives typical readings of about 125 to 175 psi for gasoline engines and 275 to 400 psi for diesels; your engine’s service information has the real numbers, and evenness between cylinders matters as much as the figure itself.

StopWhen to call a professional

Anything inside the engine, such as timing belts and chains, head gaskets, valves and bearings, takes special tools, exact specifications and the service information for your engine. Leave it to a trained technician, and stop driving if you hear a heavy knock or the oil pressure light comes on.

Check yourself

Tap an answer to check it, or open “Show the answer.”

How many times does the crankshaft turn in one complete four-stroke cycle?
Show the answer
Twice. Twice. Each stroke is half a turn, and there are four strokes.
Why does the camshaft turn at half the crankshaft’s speed?
Show the answer
Each valve opens once every two crank turns. A full cycle takes two crank turns, and each valve opens once per cycle, so the cam must turn once per cycle.
Which stroke pushes the piston down with burning gas?
Show the answer
Power. The power stroke. It’s the only one of the four that delivers work to the crankshaft.
An engine makes 200 pound-feet of torque at 3,000 rpm. About how much power is that?
Show the answer
About 114 hp. 200 × 3,000 ÷ 5,252 ≈ 114 horsepower.
What limits how far a gasoline engine can squeeze its mixture?
Show the answer
Knock. Squeezed too hard, the mixture ignites on its own before the spark. That uncontrolled burn is knock.

Go deeper

OpenStax College Physics 2e, 15.3 Heat Engines and Their Efficiency The four-stroke Otto cycle on a pressure–volume graph.
OpenStax University Physics Volume 1, 10.8 Work and Power for Rotational Motion Rotational work and power, with worked examples.
MIT OpenCourseWare 2.61 Internal Combustion Engines, Lecture 8: Gas Exchange Processes MIT lecture notes on how air gets in and out of an engine (advanced).
U.S. Navy, Construction Mechanic Basic (NAVEDTRA 14264A), Chapters 3–4: Construction of an Internal Combustion Engine; Gasoline Fuel Systems The Navy’s chapter on engine construction, testing and adjustment.

Sources

I finished this lessonSaved on this device only.

A free course from Teddy Rook Book. It explains how cars work in general and follows U.S. sources; your owner’s manual and your vehicle’s service information are the final word for your car, along with state and local rules.