Compression ratio explained

On the compression stroke, the piston squeezes the air and fuel into a small space under the cylinder head before it burns. The compression ratio says how hard it squeezes: the space above the piston at the bottom of its stroke, divided by the space left at the top.

Take the 2.0-litre engine from the other demos. Each piston sweeps about 500 cc on its way up. At the top, there’s still 52.6 cc left in the combustion chamber, the thin gap of the head gasket and any dish in the piston crown. So at the bottom there’s about 552 cc above the piston, and at the top just 52.6 cc. 552 divided by 52.6 is 10.5, written 10.5:1 and said “ten and a half to one”.

Typical figures:

• Non-turbo petrol engines: about 10:1 to 13:1 today, and 8:1 or 9:1 on cars from the 1970s and 1980s.
• Turbo petrol engines: about 9:1 to 10.5:1.
• Diesel engines: about 15:1 to 18:1, and over 20:1 on older ones.

The ratio matters because the harder an engine squeezes, the more of its fuel’s energy it turns into push. But a petrol engine can only squeeze so hard before the fuel explodes on its own, so every engine is a balance between the two.

The demo shows one cylinder of that engine in section, drawn to scale, with a graph of its efficiency beside it: how much of the fuel’s energy it turns into push on the piston.

• Press Play or Next stroke and watch the compression stroke. The Charge-coloured mixture that filled the cylinder gets squeezed into the thin space under the head.
• Drag the compression ratio slider. The head moves down as the ratio rises, as if it had been machined thinner, and the space left at the top shrinks.
• At 10.5:1, squeezing alone takes the mixture to 24 bar (348 psi) and 406°C (763°F) before the spark has even fired. The engine turns 37% of the fuel’s energy into push.
• Follow the Mustard line on the graph. It climbs more and more slowly: going from 8:1 to 10.5:1 gets about 8% more power from the same fuel, but 10.5:1 to 12:1 adds only 3%.
• Slide past 12:1 and you reach the shaded Knock zone. The mixture is now so hot after the squeeze that once the spark lights it, the last of it explodes before the flame reaches it, shown in Coral.
• Press Diesel. There’s no spark plug: at 17:1 the squeeze heats the air to 531°C (988°F), and the diesel sprayed in near the top lights by itself. Slide it below 14:1 and the engine won’t start.

Why does squeezing harder help? The burning gas pushes the piston back down through the same ratio it was squeezed through. A high ratio means the gas expands further before the exhaust valve opens, so it gives more of its heat to the piston and less goes out of the exhaust. A tightly squeezed mixture also burns faster and more completely.

The model is simplified: it’s at full throttle, with the charge starting at 1 bar (15 psi) and 25°C (77°F), and the knock and no-start limits are for this engine on standard fuel. Real engines lose more at part throttle.

The fuel decides how high the ratio can go.

• petrol is squeezed along with the air, so it’s in the cylinder while it heats up. Squeeze it too hard and it knocks, which can crack pistons. Fuel’s resistance to knock is its octane rating, which Octane: what it actually means explains. Direct injection, spraying the fuel straight into the cylinder, cools the charge as the fuel evaporates, which lets modern engines squeeze a little harder. Mazda’s Skyactiv-G petrol engines run 13:1 or more.
• Turbo and supercharged engines squeeze the air before it even gets into the cylinder, so the piston starts with more to squeeze. Add a high ratio and the pressure and heat at the top would be too much, so they use a lower one and make their power from boost. Naturally aspirated vs turbocharged vs supercharged explains boost.
• Diesel engines squeeze only air and spray the fuel in at the top, so there’s nothing to knock. They need a high ratio to make the air hot enough to light the fuel, especially from a cold start, when the cold metal soaks up the heat. Glow plugs heat the cylinders to help. Diesels are also more efficient at part throttle because they have no throttle to suck against.

A few engines can change their ratio as they run. Nissan’s VC-Turbo moves the pistons with an extra linkage on the crank, from 8:1 under boost to 14:1 when cruising.

The ratio on a spec sheet is the static or geometric ratio: it assumes the squeeze starts at the very bottom of the stroke. In a running engine it doesn’t. As Camshafts and valve timing explains, the intake valve stays open well after the bottom, so the piston pushes some of the charge back out before the squeeze really starts. The ratio from the moment the valve shuts is the dynamic ratio, and it’s always lower.

That gives engineers another tool. Variable valve timing changes when the intake valve shuts, and so the dynamic ratio, while the engine runs. Many hybrid engines, including Toyota’s, use the Atkinson cycle: a high static ratio, often 13:1 or more, with an intake valve that shuts very late. The squeeze is gentle enough to avoid knock, but the burning gas still expands through the full high ratio, getting more out of each drop of fuel. The cost is less power, which the electric motor makes up for.

Compression can only do its job if the cylinder holds it. Anything that lets the squeezed charge leak out costs power.

• Low compression in one cylinder. The engine misfires or idles roughly and feels down on power, and a diesel may be hard to start. The usual causes are worn or stuck piston rings, a burnt or bent valve that can’t seal, or a failed head gasket.
• Compression test. A mechanic takes the plugs out, screws a gauge into each hole and cranks the engine. A healthy petrol engine reads about 11 bar (160 psi) to 14 bar (203 psi), and a diesel 25 bar (363 psi) to 35 bar (508 psi). That’s less than the demo shows, because the engine is only cranking slowly and the intake valve shuts late. How the cylinders compare matters more than the numbers: they should be within about 10% of each other.
• Finding the leak. Squirt a little oil into a low cylinder and test again. If the reading rises, the oil is sealing worn rings. If it doesn’t, suspect the valves or gasket. Two neighbouring cylinders low together usually means the head gasket has failed between them. A leak-down test, which pumps air into each cylinder and listens for where it escapes, pins it down.
• Carbon build-up. Deposits on the piston crowns and in the chambers take up space, raising the ratio slightly, and glowing lumps can light the mixture early. Both make knock more likely.
• Hydrolock. Liquid can’t be squeezed. If water, from driving through deep flooding, or coolant, from a failed gasket, gets into a cylinder, the piston can’t reach the top, and the force can bend a connecting rod. If the engine stops after driving through water, don’t try to restart it.

Changing the compression ratio means taking the engine apart, so it’s usually done as part of a bigger build.

• Raising it on a non-turbo engine gives a few percent more power and better economy. Skimming the head, machining its face flat, makes the chambers smaller: taking 0.5 mm (0.02 in) off this engine’s head would take it from 10.5:1 to about 11:1. A thinner head gasket does the same. High-compression pistons with a raised crown go further.
• The catch is knock. A higher ratio may need higher-octane fuel and less ignition advance, and the ECU will need retuning. It also brings the valves closer to the pistons, so the clearance must be checked.
• Lowering it for boost. Adding a turbo or supercharger to a non-turbo engine often means fitting pistons with a dished crown, or a thicker gasket, so it can take boost without knocking.
• Fuel. Fuels that resist knock better, such as E85 or race fuel, let tuners run more compression or more boost. Level 9 covers this in more detail.

As with any engine change, the gain should be checked on a rolling road, with the engine listened to for knock.