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Cyl #3 and #4 show hottest CHT, too hot for my taste during climb

In terms of cooling drag, the specific heat of the cooling medium is the same for all types of engines – because the cooling medium for all of them is air. However, the air cooled engine can be cooled with less air flow because each mass unit of air can be heated to a greater temperature above ambient before it runs out of temperature differential – the air cooled cylinder is 150 F hotter than a radiator.

Water cooling is an advantage for design of an engine in isolation, as opposed to an engine to power an aircraft. It provides higher heat transfer coefficients at the cylinder surface, which allows higher rpm, higher power per cylinder and more waste heat per cylinder. That’s OK as long as you don’t mind adding an extra cooling system, and a gearbox, as well as having a little higher cooling drag,

Water cooling did not become established as better in the 40’s – it was actually at that time when aircraft engines went firmly away from water cooling, for civilian aircraft of up to many times the power of our GA engines. You can make an argument that developments since then have shifted the technical balance back again, but I think it’s more of an expedience based on an interest in adapting car engines to create a product relatively inexpensively. However the resulting products are still more expensive than most GA buyers are ever going to pay based on comparison with other options they already have.

I’ve fixed quite a number of leaky water pumps and radiator hoses, actually more than I could count, but never on an aircraft. I’ve also never adjusted the valves on an aircraft engine, or replaced a gearbox, clutch or timing belt as I’ve done on many other engines – for instance I run five other engines with rubber timing belts like a Mercedes-based aircraft engine. For aircraft service, I intentionally selected the two most robust aircraft engines ever built, the A65 and O-320. They don’t overheat, they don’t break, they don’t require much service and they’re fine for me. Others can spend their money on whatever they’d like.

Last Edited by Silvaire at 02 Nov 00:00

The Extra 400 has a Continental TSIOL-550 which is liquid cooled. It was expected to be available in the 400hp range but never made it to more than 350/325hp which is why the Extra 400 eventually failed. However, from what I hear, the engine is very reliable and easy to keep within its operating temperature range and generally a good engine. Even the primitive avgas engines can benefit from liquid cooling.

The OM668 aero engines have very favorable reliability figures. Initially there were a bunch of issues, mostly connected to situations where the ECU could lose power which is obviously fatal for those engines. That was addressed by extra redundancy.

Obviously the very crude setup in the DA42-NG is not an argument pro liquid cooled engines. That was a huge step back in engineering quality from the Thielert engines. With the latest DA42 VI, it looks a lot better.

Last Edited by achimha at 01 Nov 16:20

I’ll gladly expand Silvaire. OM668s can run as high as 2 bars and 130C cooling.

Further to that, as I am sure you well know, “effective cooling” is a function not only of temperature delta and surface but also of specific heat. Liquid cooling has been operationally proved since the 40es to be vastly more effective at channelling heat away from cylinders in a more aerodynamic package. The only nag people could find about it was “it doesn’t deal with bullet holes very well” – which while true doesn’t apply much to GA operation.

So as an engineer you have to ask yourself whether you build a “simple” engine that is dependent on external factors like weather or airport infrastructure in order to reach design clearance parameters (truncated cone cylinders… great idea) – or a marginally more complex design that is self-sustainable in operation and does not impose performance penalties.

Well – you ask yourself these questions if you actually want to build a product that actually sells.

How many instances of water pump failure or heat stress induced internal failures on OM668 derivatives vs heat stress induced failures on air-cooled designs?

Timothy – have you checked your EDM is not out of adjustment?

Oh yes. This diagnosis has been going on for nearly 20 years!

EGKB Biggin Hill

thanks all for your comments, and the added info regarding why certain rules apply to lycomings.

I am going to tackle replacing the baffles in the morn!

Will take pics and post them if there is any interest.

Perhaps a little information and data in your posts might be helpful Shorrick Making endless, bitter one liners does make one look silly.

That aside, I said “limited to 100 C / 212 F or a little higher”. Late model automotive engines operate in the range of 200-250 F, consistent with what I actually said, but you’re right in your implication that they’ve increased coolant temperature by about 30 F over the last few years to move it into that range. One of the motivations to do so was to reduce radiator size. An air cooled engine does the same thing only more so by eliminating the radiator and raising the cooling surface temperature to perhaps 390 degrees F. Obviously that creates issues with material strength if taken too far, but it provides a much higher temperature differential relative to ambient air, and more effective cooling for the same surface area.

What temperature and pressure do the Mercedes car-based aero engines run their coolant?

Last Edited by Silvaire at 31 Oct 02:22

Pressurised liquid cooling isn’t limited to 100C.

Welcome to the 1990es.

The CHT and cooling issues are an airframe design compromise. Minimizing cooling air flow to that required for adequate cooling at cruise power/altitude maximizes cruise speed. Cowl flaps when fitted help reduce the compromise at the expense of another system. Also, the higher the CHT, the lower the required cooling flow & drag. The higher CHT of an air cooled engine is a cooling drag advantage over liquid cooling which is limited to 100 C / 212 F or a little higher. Increasing efficiency requires stressing material more – which is often the case in any design situation.

Last Edited by Silvaire at 30 Oct 18:25

Yes it could be the valves. Or any other area where differential expansion is an issue.

I think this “de-rated” is not very meaningful if you are comparing two engines with a different CR. There is a corresponding difference in stress. For example I have it on good authority from an Exp engine builder that while my engine will normally make 2000hrs easily (8.5:1), if you put in 9.5:1 pistons but keep all else the same it struggles to make 1000hrs. The 9.0:1 pistons are apparently OK and the engine doesn’t need 100LL (it can run on 91UL).

There is an IO540 which does 350HP, too, AFAIK. Probably at 3000rpm+

I am sure there is no problem with running at 75% in cruise all the time, so long as the CHTs are OK. I am not sure they would be (on a TB20) and when running in an engine with new or re-honed cylinders (when you have to fly at 75% for quite a number of hours) you have to fly ROP. I did that in 2002 and again in 2008. But I would have to test it to be sure.

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Shoreham EGKA, United Kingdom

@Peter: I understood that the reason Lycoming advises a max temperature change of 50 C per minute is due to the risk of sticking valves. Lycoming was able to reproduce valves sticking when shock cooling in a test cell. Can’t find the source at the moment.

Since the TB20 engine is de-rated from 300 HP to 250HP, running it at 75% HP should be equivalent to running it at less than 65% of its engineered maximum power, since it is really the same engine as the K series, say IO-540-K1A5, which achieves its 300HP through higher RPM of 2700, and slightly higher compression ratio, while still having the same TBO of 2000 hours (and same metallurgy and wear profile). In other words, in the case of the TB20 IO-540 you cruise never at more than 65% max power of what the engine is engineered for. At 16000 feet or above where you appear to cruise most of the time the effective power is less than 50% of max engineered power, or closer to where a car engine runs. I would therefore conclude, that practically speaking, the TBO of the 250HP IO-540-C should be 2300 hours, if the 300HP version has a TBO of 2000 hours. Right or wrong?

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