TECH UPDATE: Cold Seizures

Engine pre-heating helps prevent cold seizures (and pistons from “clattering about”).

KTM RC8 pistons

Formula 1 engines and the first 800cc Suzuki pneumatic-valve MotoGP engines have had to be externally heated before they can be started. A mechanic wheels up a heater whose hoses connect to the cooling system of the engine to be heated. A pump in the heater circulates electrically heated engine coolant through the engine in the car or bike. Once suitable instrumentation reveals that the engine's cylinder block is at operating temperature, the starter can be engaged to start the engine normally.

I suspect this relates to the old argument in hot-rod circles over whether ‘tis nobler in the mind to run low-expansion pistons made of high-silicon alloy 4032, or to run more fatigue-resistant pistons made of the other popular and much talked about alloy 2618, which expands more with heat.

If the F1 engine were built with low-expansion pistons, they could be fitted at a small clearance and the engine could be started and warmed up normally—no pre-heating needed because the warming-up of the low-expansion piston material would not outrun the expansion of the aluminum cylinder block.

But with the higher-expansion 2618 material (or with possible later refinements of that alloy), you have two choices:

1) Fit the pistons loose enough that rapid piston expansion after start-up would not result in one or more seized pistons (but the loose fit, combined with an F1 engine’s extreme rpm, would rattle such loose pistons to an early death).

2) Fit the pistons much more closely (possibly even to an interference fit), such that starting a cold engine would result in seizure as the pistons, being small, light, and made of higher-expansion material, would expand much faster than the massive cylinder block.

If this sounds unfamiliar, I call upon the once-very-familiar phenomenon of the cold seizure in two-stroke racing engines of the 1980s and 1990s. Fitted tightly enough for best thermal contact once warmed-up, pistons in TZ250s could easily seize if a bike was taken onto the track before being thoroughly warmed-up. Engine pre-heating would have been an excellent way to avoid this.

This reveals that engine pre-heating is a way to avoid seizure while fitting pistons made of high-expansion material closely enough to their cylinders that they are not damaged at high revs by “clattering about,” yet do not cold-seize at start-up.