Four strokes of the piston, two turns of the crankshaft and a single spark. Stop time, turn the crankshaft with your finger and see how a drop of fuel becomes the motion that has powered cars, motorcycles and lawn mowers for almost 150 years.
Vs = 443 cm³ · Vc = 49 cm³
Less open = less mixture = less power
Shortcuts: Space start/pause · ←→ ±15° (Shift ±1°) · +− speed · L labels · D Diesel/Otto · G gases · O outline
Every point on the diagram is a state of the gas in the cylinder: its volume V and pressure p. In one cycle (two turns of the crankshaft) the point travels around a closed loop — and the area inside it is the work the gas has done on the piston.
When the gas expands at high pressure, it pushes the piston harder than the piston had to squeeze it before. The difference is the gain — the shaded area of the loop:
The efficiency of the ideal Otto cycle depends only on the compression ratio ε = (Vs + Vc) / Vc and the adiabatic index κ ≈ 1.4:
In a diesel, heat is added at constant pressure, so the cutoff ratio φ = V3/Vc comes in as well:
Gasoline can't be compressed without limit — at too high an ε it ignites by itself before the spark (so-called engine knock). A diesel relies on exactly this self-ignition, so its ε can be 14–22.
The dashed line is the textbook ideal cycle: compression and expansion with no losses, combustion in the blink of an eye.
The solid line is an approximation of a real engine: ignition before TDC, combustion lasting tens of degrees, valves opening early.
The small lower loop is the pumping work — the loss from moving gases in and out. Close the throttle a bit and switch on the log scale to see it. Real engines reach about 30–45% efficiency.
Four such cylinders on a common crankshaft. The crank throws of cylinders 1 and 4 point the same way, while 2 and 3 are offset by 180°. Cylinder 1 moves exactly like the engine above.
Firing order
In a single-cylinder engine only one stroke in four drives the crankshaft — the rest of the time it keeps turning on the flywheel's momentum, and the engine shakes and jerks.
With four cylinders a power stroke comes every 180°, so some piston is always pushing the crankshaft. The pulses are four times as frequent and smaller relative to the average — the engine runs more smoothly and the flywheel can be lighter.
Stop the engine exactly at the moment of ignition — about 12° before TDC, within ±3°. In a diesel, the start of injection counts (10° before TDC). Shift+←/→ will help.
Not done yet
Find and freeze the moment when both valves are open at the same time. Hint: look on the cycle dial for where the blue and brown arcs overlap.
Not done yet
Drag the crankshaft by hand through two full turns (720°). How many turns does the camshaft make in that time?
Turned: 0° of 720°
A former traveling salesman and self-taught engineer builds the first practical four-stroke engine with mixture compression at the Deutz works near Cologne. Gottlieb Daimler and Wilhelm Maybach — later pioneers of the automobile — also worked there at the time. That is why the intake–compression–power–exhaust cycle is called the Otto cycle.
Diesel receives a patent for a compression-ignition engine and builds the first prototype in Augsburg. It needs no spark: air compressed more than ten times over gets so hot that the injected fuel ignites by itself. In 1897 his engine reaches about 26% efficiency — far more than the steam engines of the day.