Engines · Fundamentals9 min read

Why engines waste most of their fuel

Fill a car's tank and roughly seventy of every hundred euros of fuel never move the car — they heat the street. That is not sloppy engineering: it is a law of physics with a number attached, and the whole history of engine design is the fight to claw a few percent back from it.

The fuel bill

Petrol is astonishing stuff. One kilogram holds about 44 MJ of chemical energy — enough to lift a family car a kilometre straight up. A typical car engine turning that fuel into motion manages roughly 30% on a good day: of every litre you buy, less than a third becomes distance travelled. A big ship's diesel does better at around 50%, the best Formula 1 power units touch a similar figure with hybrid help, and your legs, for comparison, run at about 25%.

The obvious question is why. Not "why is my particular car lazy" — why does every heat engine, from a lawnmower to a power station, throw most of its energy away? The answer comes in two parts: where the energy physically goes, and the ceiling that says a lot of it had to go there.

Where the energy goes

Burning fuel in a cylinder makes gas at well over 2000 °C. The piston extracts work as that gas expands and cools — but the expansion has to stop when the piston reaches the bottom of its stroke, and at that moment the exhaust gas is still at several hundred degrees. All the heat still in it leaves through the exhaust pipe, unconverted. That alone is about a third of the fuel.

Another third leaks out sideways: cylinder walls, head and piston crown would melt if they were not cooled, so the coolant and oil carry heat to the radiator and dump it into the air. And a last slice — under a tenth — is eaten by the engine rubbing against itself and pumping air in and out past a half-closed throttle.

Where 100 units of petrol energy end up in a typical car engine at cruise. The green slice is the only part that moves the car.

Why 100% is impossible

Here is the deep part. A heat engine only produces work while heat is flowing from something hot (burning gas) to something cold (the outside air). Work is extracted from the flow — and a flow needs somewhere to go. If the exhaust left the engine at air temperature, carrying no heat away, the flow would have stopped and so would the engine. Some rejected heat is not a defect; it is the price of admission. This is the second law of thermodynamics wearing overalls.

The size of the price is set by temperatures: the hotter the burn and the colder the exhaust, the larger the fraction of the flow you can capture. That is why efficiency talk is always temperature talk — and why the compression ratio, which controls how much the gas expands and cools before the exhaust valve opens, is the single most important number in the engine.

The actual ceiling: Carnot and Ottofor engineers

The theoretical maximum for any heat engine working between a hot source at $T_h$ and a cold sink at $T_c$ (in kelvin) is the Carnot efficiency:

\eta_{\text{max}} = 1 - \frac{T_c}{T_h}

Peak combustion at ~2500 K against ambient at ~300 K gives a ceiling of 88% — so the second law alone is not what limits a petrol engine to 30%. The tighter bound is the cycle shape. For the idealised Otto cycle the efficiency depends only on compression ratio $r$ and the gas's heat-capacity ratio $\gamma \approx 1.35$ :

\eta_{\text{Otto}} = 1 - r^{\,1-\gamma}

That gives 48% at $r = 8$ and 56% at $r = 12$ — before subtracting real-world losses (finite burn time, heat loss to walls, friction, pumping), which take a production engine down to ~30–38%. The equation says: raise $r$ . The fuel says: raise it too far and the mixture detonates before the spark — knock. Petrol engines live pinned against that limit, which is exactly what the ignition & knock demo lets you feel.

How engineers chase points

Once you see the losses, every modern engine technology reads as an attack on one of them. Turbochargers recycle exhaust heat that was leaving anyway, using it to cram in more air. Lean-burn and diesel engines run with excess air, which burns cooler but expands more usefully — efficiency peaks lean of the "perfect" mixture, as the air–fuel demo shows. Hybrids attack the problem from outside the cylinder, catching braking energy that friction would have turned to heat. And ship engines cheat on scale: enormous cylinders lose proportionally less heat through their walls, and at 100 rpm there is time to expand the gas almost completely — which is how they reach 50%.

Electric motors, for contrast, convert stored electricity to motion at 90%+ — they are not heat engines, so the second-law price never applies. The heat-engine question just moves up the wire to whatever power station charged the battery.