Spark Timing and the Pressure–Crank Angle Curve: Why Ignition Never Happens at Top Dead Centre

Why a spark-ignition engine fires the mixture before the piston reaches top dead centre, and how the resulting pressure-vs-crank-angle curve determines torque, efficiency, knock and fuel economy.

The four strokes are only the stage — the pressure curve is the play

Most explanations of the internal combustion engine stop at the four strokes: intake, compression, power, exhaust. That description is correct but incomplete, because it treats the piston's position as the whole story. The piston's motion — its up-and-down travel dictated by the rotating crankshaft — is just the stage on which the real physics happens. What actually determines how much useful work an engine produces, how efficient it is, and whether it destroys itself from detonation is the pressure inside the cylinder at every instant, plotted against the crank angle (the rotational position of the crankshaft, measured in degrees from top dead centre, or TDC). This pressure-versus-crank-angle curve, sometimes drawn as a pressure-volume (P–V) indicator diagram, is what engineers actually study when tuning an engine, and it reveals something that surprises most people: the spark plug never fires when the piston is at the top of its stroke.

Combustion is not instantaneous

A gasoline-air mixture does not explode the instant a spark ignites it. Instead, a flame front propagates outward from the spark plug's electrodes through the compressed mixture, consuming the fuel-air charge over a period of roughly 30 to 90 crankshaft degrees, depending on engine speed, mixture composition, cylinder geometry and turbulence. At idle this combustion event might take a couple of milliseconds; at high engine speed the crankshaft can rotate through the same number of degrees in a fraction of that time, so the flame simply has less real time to do its work per revolution. Because rotational speed changes how much physical time corresponds to a fixed number of crank degrees, the timing of ignition has to be adjusted with engine speed to keep the combustion event centred where it produces the most torque.

Why the spark fires before top dead centre

Peak cylinder pressure does the most useful work when it occurs shortly after TDC, while the piston still has significant leverage on the crankshaft through the connecting rod's angle, but has already started its descent so the expanding gas can push it down. If the spark fired exactly at TDC, the flame would still be spreading and building pressure while the piston was already several degrees into its power stroke, and a meaningful fraction of the combustion energy would be wasted heating gas that expands too late to contribute much torque. Engineers solve this by firing the spark plug several degrees of crank angle before TDC — commonly called spark advance or ignition advance — so that by the time the piston reaches TDC, combustion is already well underway, and cylinder pressure peaks at roughly 12 to 20 degrees after TDC on a typical automotive engine under load. This peak location is a deliberate compromise, not a fixed number: it is chosen empirically and by simulation for each operating condition.

Too much advance, too little advance

Spark timing is a balance between two failure modes. Advance the spark too far and peak pressure occurs too early relative to TDC — while the piston is still travelling upward on the compression stroke — which forces the piston to do extra work fighting against the rising pressure of its own compression plus combustion, wasting energy and increasing the risk of engine knock (uncontrolled auto-ignition of the remaining unburned mixture ahead of the flame front, which produces a sharp secondary pressure spike, an audible metallic knocking sound, and can physically damage pistons, rings and bearings over time). Retard the spark too far — firing it too close to or after TDC — and peak pressure occurs late in the expansion stroke, well after the piston has already started down; the leverage advantage is lost, torque falls, unburned fuel escapes into the exhaust unburned, and exhaust gas temperatures rise because combustion is still happening as the exhaust valve opens. Modern engine control units continuously adjust spark advance in real time based on engine speed, load, throttle position, coolant temperature, and knock-sensor feedback (a piezoelectric sensor bolted to the block that literally listens for the acoustic signature of detonation and retards timing on the cylinder where it hears it).

The rod-angle effect: why crank angle and piston position aren't proportional

A subtlety that a naive model misses: piston displacement is not a simple sine wave of crank angle, because the connecting rod is a finite length, not infinite. Near TDC, the piston's velocity changes more slowly with crank angle than a pure sinusoid would suggest, because the connecting rod's obliquity partially cancels the crank's rotation. Near bottom dead centre, the piston moves comparatively faster for the same change in crank angle. This asymmetry — captured by the standard slider-crank kinematic equation relating piston position to crank angle and the rod-to-crank-radius ratio — matters directly for combustion: it means the cylinder volume changes relatively slowly right around TDC, which is exactly the window in which spark timing has to work, giving the flame a brief, nearly constant-volume opportunity to build pressure before the piston starts pulling away and expanding the combustion chamber.

Compression ratio sets the ceiling, timing decides how close you get to it

The theoretical efficiency ceiling of a spark-ignition engine, described by the idealised Otto cycle, rises with compression ratio: a higher ratio compresses the mixture into a smaller volume before combustion, extracting more work from the same expansion. But a real engine's compression ratio can't be raised arbitrarily, because higher compression raises the mixture's temperature and pressure before the spark ever fires, making it easier for the unburned end-gas to auto-ignite — knock — before the flame front reaches it. This is why premium fuels with higher octane ratings resist auto-ignition at higher pressures and temperatures, letting an engine run higher compression ratios or more aggressive spark advance for the same knock margin. Spark timing and compression ratio are therefore two knobs on the same underlying constraint: the pressure-crank-angle curve must peak at the right location without the end-gas ever crossing its auto-ignition threshold.

Diesel engines skip the spark plug entirely

Diesel (compression-ignition) engines take a fundamentally different route to the same pressure-crank-angle problem. Instead of compressing a premixed fuel-air charge and igniting it with a spark, a diesel engine compresses air alone to a much higher ratio (commonly 14:1 to 23:1) until it is hot enough — often above 500 degrees Celsius — to auto-ignite fuel the instant it is injected near TDC. Because there's no premixed charge waiting to be triggered, there's no risk of the pre-ignition knock that limits spark-ignition compression ratios; the entire combustion event is instead controlled by fuel injection timing and rate rather than spark advance. This is why diesel engines can run much higher compression ratios and achieve higher thermal efficiency, at the cost of a combustion process that's inherently noisier and produces more particulate matter and nitrogen oxides unless carefully managed.

Frequently Asked Questions

At exactly what crank angle does the spark fire in a typical car engine?

It varies continuously with engine speed and load, but a common cruising figure is roughly 10 to 30 degrees before top dead centre (BTDC). At idle it may be closer to 10 degrees BTDC; under light-load cruising at higher RPM it can advance to 30 degrees or more, because the flame has less real time available per crank degree at higher speed.

Why doesn't the engine just fire the spark at top dead centre for simplicity?

Because combustion takes tens of degrees of crank rotation to complete, not zero. If the spark fired exactly at TDC, peak pressure would arrive too late in the power stroke to give the piston good mechanical leverage through the connecting rod, and torque output would drop significantly compared to properly timed ignition.

Is engine knock the same thing as the normal combustion 'bang'?

No. Normal combustion is a controlled flame front spreading smoothly from the spark plug outward over tens of crank degrees. Knock is uncontrolled auto-ignition of the remaining unburned end-gas ahead of that flame front, which burns almost instantly and creates a sharp pressure spike and shockwave — the metallic pinging sound — that can crack piston crowns or damage bearings if it happens repeatedly.

Why do modern cars automatically retard timing instead of a mechanic setting it once?

Because the ideal spark advance shifts constantly with engine speed, load, intake air temperature, humidity, altitude and even fuel quality. A fixed mechanical setting (as older engines used, via centrifugal and vacuum advance mechanisms) could only approximate the ideal curve. Modern engine control units use real-time sensor data, including knock sensors, to adjust timing every combustion cycle, cylinder by cylinder in some designs.

Why can diesel engines use a much higher compression ratio than gasoline engines?

Because diesel engines compress air alone, without fuel present, so there is nothing to auto-ignite prematurely during compression. Fuel is injected only near TDC, and combustion is deliberately triggered by the heat of compression itself. Gasoline engines compress an already-mixed fuel-air charge, so raising compression ratio too far risks that mixture auto-igniting before the spark fires, which is exactly the knock phenomenon that limits how high gasoline engine compression ratios can go.