The Four-Stroke Engine

Fire Inside the Cylinder

A steam engine burns coal under a boiler and sends the steam to its cylinder. The boiler is big and heavy, and it takes a long time to raise steam. If the fuel burned inside the cylinder, the engine could be far smaller and start at once.

In 1860 Étienne Lenoir built the first gas engine to sell, but it fired the gas as it drew it in, without compressing it, and turned about 4% of the fuel’s energy into work. In 1876 Nicolaus Otto compressed the charge before lighting it, in a cycle of four strokes. In 1886 Karl Benz drove his three-wheeled motor car with a single-cylinder four-stroke.

1860 Lenoir: no compression, about 4% · 1876 Otto: compression, four strokes · 1886 Benz’s motor car

Four Strokes

Intake: the piston goes down with the inlet valve open and draws in the mixture. Compression: both valves shut, the piston comes up and squeezes the mixture to a ninth of its volume.

Power: just before the top the spark plug fires, the pressure leaps and drives the piston down. Exhaust: the exhaust valve opens and the piston comes up, pushing the burnt gas out. Four strokes, two turns of the crank.

The ideal Otto cycle: two adiabats, heat in at once at the top, out at once at the bottom · compression 9:1 · the real loop has rounded corners

Camshafts at Half Speed

Each cylinder’s inlet and exhaust valves open once in every two turns of the crank. So the camshafts, driven by the chain, turn at exactly half crank speed: their sprockets have twice the teeth of the crank’s.

The valves don’t open and shut exactly at the top and bottom. The inlet opens before the top and closes after the bottom, so the moving air keeps filling the cylinder; the exhaust opens before the bottom. Around the top, both are open for a little while: the valve overlap.

Inlet: opens 12° before the top, closes 48° after the bottom · exhaust: opens 48° before the bottom, closes 12° after the top · 24° of overlap

Crank and Connecting Rod

The connecting rod turns the piston’s to-and-fro into the crank’s rotation. The piston doesn’t follow a sine wave: at 90° of crank it is already past half its stroke, because the rod is leaning. It moves faster near the top than near the bottom.

Counterweights on the crank balance the throws and the rods’ big ends. The heavy flywheel at the back stores energy in the power stroke and gives it back through the other three, so the crank turns evenly.

Crank radius 44.25 mm · rod 152 mm · at 90° the piston is 50.8 mm down, more than half the stroke

Four Cylinders, 1-3-4-2

A single cylinder gives one push in two turns, and its torque swings from large to negative. Four cylinders half a turn apart, firing 1-3-4-2, give a power stroke in every half turn.

The throws of cylinders 1 and 4 point the same way; those of 2 and 3 point the other way. The torque of all four swings, relative to its mean, about a quarter as much as one cylinder’s.

Thin lines: each cylinder’s gas torque · thick line: the four together · piston and rod inertia left out

Compression and Efficiency

The ideal Otto cycle’s efficiency depends only on the compression ratio r: η = 1 − r^(1−γ), with γ about 1.4 for air. At 10:1 that is about 60%. A real petrol engine delivers 25% to 40% of its fuel’s energy at the crankshaft; most of the rest leaves in the exhaust and the cooling water.

The ratio can’t rise for ever. The harder the squeeze, the hotter the unburnt mixture, and it can light by itself before the flame reaches it, setting the pressure ringing: knock. The higher a fuel’s octane number, the better it resists knock.

Ideal efficiency rises with compression, more and more slowly · real efficiency 25%–40%

Taken Apart

The head on top holds the two camshafts, eight valves and their springs, and the spark plugs. The block in the middle is cast aluminium, with four cast-iron liners standing in the cooling water.

Below is the crank train: four pistons, four rods and a forged-steel crankshaft in five main bearings. The timing chain is at the front, the flywheel at the back, the sump at the bottom.

One Cylinder, Cut Open

Cut through the middle of cylinder 1: the inlet and exhaust ports on either side, the two valves facing each other at 80°, the hemispherical chamber with the spark plug at its top. The colour in the cylinder is the gas of the stroke under way.

Fresh mixture · compressed · burning · exhaust

What It Changed

The four-stroke made the car an everyday thing and took aircraft off the ground: the Wright brothers’ 1903 Flyer had a four-cylinder petrol engine of about 12 hp. Until the jet engine, aircraft flew on piston engines, and most small aircraft still do.

Now electric motors are taking over some of the engine’s work. In 2024 more than one in five new cars sold worldwide was electric, plug-in hybrids included. An electric motor turns its energy into motion far more efficiently; a kilogram of petrol stores far more energy than a kilogram of battery. How long the two share the road differs from place to place.

1876 Otto · 1886 Benz · 1903 the Wright Flyer · 2024 electric cars more than a fifth of new sales