Spark-Ignition vs. Compression-Ignition Combustion
The flame that chases a spark versus the flame that chases fuel — the two combustion strategies that split engines into petrol and diesel.
A petrol engine mixes fuel and air first, then lights it; a diesel squirts fuel into already-hot air and lets it ignite. That ordering difference cascades into efficiency, emissions, noise, and the whole character of the engine.
Two ways to light the charge
In a spark-ignition (SI) engine — the petrol/gasoline engine — fuel and air are mixed before they reach the cylinder (or, in direct injection, mixed early in the stroke), forming a roughly uniform, stoichiometric charge. A spark then ignites it at a precisely timed moment, and a premixed flame propagates outward through the charge. Because the whole charge is premixed and near-stoichiometric, SI combustion is fast, complete, and clean at the chemical level — but the premixed charge is also why SI engines are limited in compression ratio (knock, next lesson) and must throttle the intake to control load, which wastes pumping work.
In a compression-ignition (CI) engine — the diesel — only air is compressed (to a high ratio, ~14:1 to 22:1), raising its temperature above the fuel's auto-ignition point. Fuel is then injected directly into the hot air, and each droplet ignites almost as it arrives — a diffusion flame that burns at the fuel–air boundary wherever mixing allows. There is no spark and no fixed flame start; ignition is by temperature alone. Load is controlled purely by fuel quantity (no throttle), so the intake is always wide open and pumping losses are low. The high compression ratio and unthrottled operation give diesels their efficiency edge; the always-lean, locally-rich diffusion flame gives them their emissions challenge (particulates, NOx).
SI engines control power by throttling the intake — partly closing a butterfly valve to reduce air, and with it fuel. That throttling creates a partial vacuum the engine must pump against, a pumping loss that hurts part-load efficiency (you can feel it as engine braking). CI diesels have no throttle: the intake is always wide open, and power is set purely by how much fuel is injected. So a diesel has almost no pumping loss — one reason it beats a petrol engine's efficiency at part load, which is most of real driving. (Modern SI engines claw some of this back with variable valve timing, Atkinson cycle, and direct injection, but the fundamental throttle disadvantage remains.)
- phi = (F/A)_actual / (F/A)_stoich = (1/13.2) / (1/14.7) = 14.7 / 13.2 = 1.114
- phi > 1 ⇒ rich (a slightly rich mixture for more power or to protect components).
- phi = (1/30) / (1/14.7) = 14.7 / 30 = 0.49
- phi < 1 ⇒ lean (typical of diesel, which always runs with excess air).
Check your understanding
- SI premixes a stoichiometric charge and sparks a propagating flame; throttled to control load
- CI compresses air hot and injects fuel that self-ignites as a diffusion flame; load set by fuel quantity, no throttle
- Diesels are more efficient (high compression, no pumping loss) but lean/diffusion combustion makes NOx and particulates harder
- Equivalence ratio phi: SI ~1 (for the catalyst), CI always lean (phi < 1)