Rotax 912ULS CHT temperature Range

The experts must be on vacation.

I offer my opinion, based on limited knowledge;

Cylinder Head Temperature (CHT) in Rotax engines is now coolant temperature, taken at the front left and rear right heads (tractor view)

Coolant temperature should be close to crankcase oil temperature ie between 90-100C. There should not be a large temperature variation between major components.

Oils & coolants are designed to operate most efficiently within certain temperature ranges. Manufactures usually publish performance specifications that may give this information.

Oils should achieve over 100C, for a sustained period,  at least once (preferably on first flight), in an operating day, to drive off harmful volatile fractions. Coolant does not have this requirement.

All internal combustion engines have an optimum operating temperature range. Ideally (not always possible) engine operating temperatures should be the same all year round.

A range of strategies can be adopted to acheive near consistent engine temperatures no matter the climatic conditions. These include;

Pilot controlled draft modification systems eg Cowl Flap, blinds, etc

Fixed draft restrictors, of diffrent dimensions, for winter & summer flying.

Oil/coolant thermostats that will automatically restrict/enhance flow to heat exchangers, so as to maintain a preset minimum temperature (but not a maximum).

Reducing the efficacy of heat exchangers, by blanking off portions of the surface using heat resistant tape or aluminium sheet/foil. 

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CHT doesn't have a lower limit like the oil does. Care more about the oil temp and don't sweat normal CHT unless something very odd shows up on it like no temp or an off the chart high temp.

Hi Roger,

"CHT doesn't have a lower limit like the oil does."

I agree that coolant lower temperature is not critical however what of-

"...internal combustion engines have an optimum operating temperature range."

To this end -

"Coolant temperature should be close to crankcase oil temperature ie between 90-100C."

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