Ideal cruise CHT for Jabiru

I've been trying to find the ideal cruise temperatures to aim for, and so far the most detailed info comes from an Oshkosh seminar on Lycoming engines.

cruise CHT.... If below 110 C, you get "poor lead scavenging"

Never above 204 C

Ideal between 120 C and 193 C

Well I reckon that for a Jabiru engine, the 193 is way too high, and I would have said that ideal is between 100 and 120 C, but this is just a guess, based on the fact that liquid-cooled heads run cooler than 120.

Does anybody know more? For example, do liquid cooled engines run on avgas get poor lead scavenging? Has anybody had a problem traced back to cruise CHTs being too low?

cheers, Bruce

Head temps.

Rotax engines that I know of, have had valve seating problems with 100LL put down to running too cool. These problems do not occurr with unleaded. Jabiru's recommended head temps are low for an air-cooled motor compared with P&W, Wright etc. I think I can recall a max of 230 with a P&W R-2000. (memory only) Nev

180 is max in cruise, up to 200 for 5 min I think

Id have thought 120-150 would be ideal

A BIG problem here is how and where the CHT is measured. Some probes could be a long way out. Also the spread of CHT is an issue on many Jabiru engines. mine in cruise are from 125 to 169 spread (old thick finned heads)

Jab CHT from manal

Hopes this helps.

Yours in flying

Mick

Run in Period

Oil 80 100 120

Outside Air Temp -17oC to 25oC 15oC to 35oC Above 35oC

(1o to 77oF) (59o to 95oF) (Above 95oF)

Normal Operations

Oil W80 W100 W120

Outside Air Temp -17oC to 25oC 15oC to 35oC Above 35oC

(1o to 77of) (59o to 95oF) (Above 95oF)

2.6 Cooling System

Free air cooled. Ensure that baffles are correctly fitted & located.

The required pressure drop across the cylinders at 1.3 Vs in take off configuration is 4.3 cm (1.7") water gauge, minimum.

2.7 Operating Speeds and Limits

Maximum continuous speed 2750 RPM

Maximum 3300 RPM ISO STD Conditions

Idle Speed: 650 RPM

Oil Pressure: Normal Operations Min 220 kPa (31 psi)

Max 525 kPa (76 psi)

Idle Min 80 kPa (11 psi)

Starting & Warm up Max 525 kPa (76 psi)

Oil Temperature: Min. 15 oC (59oF)

Max. 118 oC (244of)

Continuous Temperature: 80 - 100 oC (176o - 212oF)

Max. Cylinder Head Temperature: 175 oC (348oF)

(reading on the sensor spot

of the hottest cylinder)

Continuous 150oC (302o F) Max.

Im finding some contratictory information from Jab specs. They say one thing in one document then other data elsewhere.

Max cont rpm here says 2750, yet advice is to run aything up to 3000 continuous and stated cruise is 2850

If my idle was set at 650 it would stop on landing, set at more like 800

Theres also no mention of 15W40 oil here which seems to help a few issues and is std I thought on hydraulic lifter engines, and does help on solids too

15W40? misstype? 15W50 I think

Andy

cht temps

Yes the MAX rated cruise temp is 150, but the ideal would be less. But how much less?

Now the advantage of liquid cooling is low and stable cht, giving more reliability with respect to heat-related problems. Its the entire reason why most engines are liquid-cooled.Of course, the additional complexity has reliability costs, for example with a coolant leak.

So, if you could, why not operate your air-cooled engine at 120 or even lower? (if your system could achieve this). Well, as Nev has confirmed, you can have problems with 100ll avgas with rotax liquid-cooled heads. I take this to mean that the quoted figure of 110 as a minimum temperature to avoid lead scavenging problems is correct.

My current guess is that a cruise CHT of 120 may be ideal for avgas, and maybe 100 to 110 for unleaded.

Bruce

Head Temps.

It is not good to run heads too cool. You lose efficiency. With all carnot cycle engines the efficiency formula is directly related to max temp achieved in the combustion process.. When you use water as a coolant the boiling point limits the temps available. Cars run a pressurised system to achieve higher temps, but this causes unreliability if pressures above say, 1 atmos approx. is used..(pump seals and hoses etc) so often engines run cooler than optimum.. I think Jabiru have sought to get the head temps down for metalurgical reasons. Most other similar style engines run much hotter. Nev