Engines · Fundamentals · Demo
Light the fire early enough to push at the right moment — but too early, and the fuel lights itself.
Find MBT for this compression, then raise the compression and watch the shaded knock zone crawl toward it — until the efficient timing and the safe timing are no longer the same number.
Polytropic compression with a Wiebe-style burn: the spark starts a 55° flame, and torque follows how close the pressure peak lands to 14° ATDC. The knock limit tightens ~2.7° per point of compression — the exact squeeze that makes octane worth paying for.
In plain terms
The spark fires before the piston reaches the top — because flame takes time to cross the chamber, and the pressure peak it builds must land just after top dead centre, where it can shove the piston down its stroke. Fire too late and the push arrives when the piston is already leaving. Fire too early and pressure spikes against the still-rising piston — and the unburned mixture ahead of the flame gets squeezed and heated until it explodes on its own. That’s knock: the metallic ping of combustion happening all at once, and it eats pistons.
Best torque comes with peak cylinder pressure around 10–20° after TDC — early enough to catch the whole expansion stroke, late enough not to fight the compression. The spark advance that puts it there is called MBT (maximum brake torque) timing, and every degree away from it costs torque on a gentle, forgiving curve.
While the flame front crosses the chamber, the last unburned mixture — the end-gas — is being compressed and heated by the growing pressure. If it reaches auto-ignition before the flame arrives, it detonates: pressure spikes of hundreds of bar, ringing the block like a bell. More advance, more compression, more boost or hotter air all push the end-gas closer to the edge; octane rating is simply a fuel’s resistance to it.
High compression is efficiency, but compression invites knock — so modern engines run knock sensors and pull timing the instant they hear it, then creep back. Your engine is retuning itself every few seconds, riding as close to MBT as the day’s fuel, heat and load allow. This demo is that calibration table, with your hand on the spark.