liquid cooled engines verses air cooled engines

[dazza 38]

i was wondering what is the general consensus regarding liquid cooled engines eg-rotax 912, 582 etc against air cooled engines vw, lycoming jabirus etc, operating in hot conditions eg- northern australia.

 

 

[Guest mike_perth]

My answer only comes from car engines but they in theory both rely on air cooling one is direct and one indirect ie water in the radiator is cooled by air flow, and if the air flow is kept up excluding other differences they both work equally well, though in hotter conditions air cooled engines need every bit of air flow they can get....though in saying that so does water cooled!

 

Id be interested to hear about the differences in aviation and wonder if like cars the differences come down to the particular engine and not the fact that they are water or air cooled.

 

Mike

 

 

[stanzahero]

My experience is in ground based vehicles.

 

Most car engines that are air cooled use ducting and baffles to direct the air flow over the major 'hot spots' in an engine (top of the cylinder and cylinder heads mainly exhaust side). They are still prone to overheating which is why they run clearance (piston to bore) of about .008" to .012" compare to about .0005" to .0015" for the water cooled variety.

 

Once over about 30 kph, water cooled cars usually do not need their radiator fans because air flow is good enough to keep the radiator supplied with fresh air.

 

Engines are designed to handle a broad variety of conditions and the only thing that I can see would be extreme cold (providing all things being equal).

 

I'd personally like air cooled because if a water/glycol cooled engine fails due to heat at 4000 ft "she no go Mr Faulty" and you need to fix the issue before it starts again. Whereas, if you're at 4000 ft with an air cooled engine and it stops because of the heat, you maybe able to restart it before your glide altitude reaches zero.

 

This, like I said is with cars, not planes but I'm thinking it would be similar in some cases.

 

Also the clearances for the non motor people are in thousands of an inch (.0005 is five ten thousands of one inch). It doesn't take too much heat expansion to become an issue.

 

Stanzahero

 

 

[Guest ozzie]

remember oil plays a big part in cooling engines whether air or liquid cooled.

 

in hot climates larger oil capacity and larger coolers play a big part in keeping VW's and other air cooled engines runnning, not to mention life span. one thing that has always crossed my mind is the relatively small quanity that Jab engines carry. obviously more oil means longer warm up times and overcooling in cooler conditions but is usually overcome with shutters or by passing the cooler, it increases the complexity of the installation but there is always a tradeoff when doing things like this.

 

ozzie

 

 

[Guest Maj Millard]

If the Rotax 912 has a coolant failure it will still cool adequetly to get you back. The coolant basically cools, and keeps the heads warm. Without the coolant you still have airflow cooling the cylinders and heads, and the air still cooling the oil, via the oil cooler.

 

It is an air/oil cooled engine, and only uses around 2 lts of coolant anyway.

 

I have spoken to a gentleman who experienced a coolant failure, (failed hose) and frankly he said it didn't make a whole heap of difference to the temps. They were hotter of course, but still in range, and the engine didn't seem to mind.

 

 

[bilby54]

If the Rotax 912 has a coolant failure it will still cool adequetly to get you back. The coolant basically cools, and keeps the heads warm. Without the coolant you still have airflow cooling the cylinders and heads, and the air still cooling the oil, via the oil cooler.

That is something that I had never really considered and is probably one of the most important bits of information to come from these forums in a long while :thumb_up:

 

 

[eastmeg2]

That reminds me of something Brian Milton said in the video doco of his world circumnavigation in a Pegasus 912 trike in the late 90's.

 

Something along the lines of saying that Rotax had said that a 912 can run for up to an hour without coolant provided the oil temperature does not exceed 140c.

 

Not something I'd care to test myself and I've not been able to find anything supporting that claim in any Rotax literature. Has anyone else heard this, or even better documented anywhere?

 

 

[Yenn]

If a Rotax 912 has a coolant failure you are saying it will get you home. If an air cooled plane has a failure of it's cooling system, what fails? Does an air cooled system have anything to fail that a Rotax 912 doesn't have?

 

 

[facthunter]

Air cooling.

 

Air cooling has been used in the majority of aero engines, and it has the appeal of simplicity. Pushers are not the same they may need a fan in some cases. I believe that cowl gills should be used more than they are.

 

Regarding the Rotax keeping going (for a while) without coolant. I have heard that from a few sources, but I have seen instances where they overheat in normal circumstances, If the airflow is not correct through the radiator. Probably worth a try at reduced power as (as has been stated) they are cooled by air as well. (the cylinders) It's still a fairly complicated installation.

 

If you use a four valve per cylinder set-up or a supercharger, liquid cooling might be the best option. Nev

 

 

[Guest Crezzi]

That reminds me of something Brian Milton said in the video doco of his world circumnavigation in a Pegasus 912 trike in the late 90's.Something along the lines of saying that Rotax had said that a 912 can run for up to an hour without coolant provided the oil temperature does not exceed 140c.

Not something I'd care to test myself and I've not been able to find anything supporting that claim in any Rotax literature. Has anyone else heard this, or even better documented anywhere?

Yes I was told something similar as part of our preparation for a big trike expedition. It came from a reputable source (possibly the same one as Milton heard it from).

 

If I recall the conversation correctly - you should / could get up to an hour's use after coolant loss before the CHT exceeds the limit of 150c. It will run past that limit but you are looking at lifting the heads afterwards. So you are better off keep flying than ditching or force landing in jungle !

 

As an aside there may be other problems with coolant loss - I know of an accident that seriously injured a couple of guys when they lost power on take off due to the engine ingesting the coolant.

 

Cheers

 

John

 

 

[stanzahero]

I know of an accident that seriously injured a couple of guys when they lost power on take off due to the engine ingesting the coolant.

Hydrolic lock.

 

Nasty because every thing stops dead, and usually they spit connecting rods out the side. Coolant is less "compressible" than air.

 

Although it would take a major catastrophe to cause that kind of coolant flow into the combustion chambers. Blown head gaskets and even cracked heads generally will not allow that kind of flow.

 

Stanzahero.

 

 

[facthunter]

Contaminant.

 

I don't know that you would want too much waterless (evans) coolant into a motor before you gunked it up proper. Nev

 

 

[Guest Crezzi]

Hydrolic lock. Nasty because every thing stops dead, and usually they spit connecting rods out the side. Coolant is less "compressible" than air.

Although it would take a major catastrophe to cause that kind of coolant flow into the combustion chambers. Blown head gaskets and even cracked heads generally will not allow that kind of flow.

 

Stanzahero.

In the case I'm thinking off it was simply a coolant pipe coming off - the power loss was due to the coolant being ingested through the air filters.

 

 

[facthunter]

More.

 

While a lot of water can go down the intake of a motor on high power, and it will cope. The salts used in a lot of the coolants would kill it quick. Nev.

 

 

[Guest Crezzi]

In this instance, it didn't actually stop the engine but the temporary power reduction during a critical phase of flight was the first link in the chain. The report can be found at http://www.aaib.gov.uk/cms_resources/dft_avsafety_pdf_500043.pdf

 

Cheers

 

John

 

PS lest this be used as another example of how "unreliable" 912 are, its worth pointing out that the cooling system had been extensively "modified" before the accident.

 

 

[facthunter]

Complexity.

 

Nontheless, there are a lot of hoses and connections with a "normal" 912 set up. The way the coolant flow occurs there is no guarantee that each head is cooling equally. Nev

 

 

[facthunter]

Cooling Efficiency.

 

Glycol does not transfer heat away as readily as water, but as you point out, the boiling point is higher with glycol added, Salts are also added with some coolants, ( Not common salt), which assist in removing the heat. Boiling point is also higher with a higher pressure cap. Nev

 

 

[Guest Crezzi]

I use 100% Evans NPG waterless coolant. No problems with cooling but, on a trike, the engine isn't generally working as hard.

 

 

[Guest check-in]

An air cooled engine in the heat of the NT will probably require more sympathetic handling than would liquid cooled. No long taxi times or prolonged run-ups, no slow, steep climbs, no rapid power off descents. Depending on the aircraft installation, air-cooled could be more critical, eg a real draggy airframe that staggers along at 80 knots may not give you the airflow you need.

 

On the other hand, if you crave simplicity, low weight, low purchase cost and your airframe is designed to take air-cooled, then Jabiru is cheaper to buy than Rotax and is direct drive which has its own appeal as one less thing to go wrong. But Jabiru engines generally do not seem to have the life of Rotax.

 

On the other hand, if properly installed, the smaller Lycomings and Continentals are well able to handle the heat even though they are air-cooled, and they seem to go well past overhaul time if used and maintained properly. But of course if something does go wrong they are expensive to overhaul. Also Lycoming and Continental are heavier than Jabiru for the same HP so not suited to some RA designs.

 

The aircraft I fly (Sonex) was specifically designed around either a Jabiru or VW powerplant and the designer was not prepared to option any liquid-cooled installation as he wanted to keep it as simple and light as possible. Even so and even with the optional oil cooler, oil temperatures in particular do need careful monitoring at all times. CHTs are rarely a problem as I never sit on the ground with the engine running for more than 5 minutes, and the airframe is obviously fast enough to give it the cooling it needs in the air. But if you were stuck on the ground somewhere like Darwin with ATC holding you up, I think the CHTs would run away within 10-15 minutes.

 

In summary, depending on the airframe and budget, for the tropics I would lean towards Rotax.

 

 

[facthunter]

Tropical operation.

 

The Merlins gave trouble in the desert and the tropics , Heated up too quickly on the ground and they had to get them airborne quickly. I have operated with largish radials frequently in the tropics and never had any trouble with head temps. Climb performance is bad at high weights, on hot days (as you would expect). Most tanks, even those used in the desert, had air cooled radials in them. Nev.

 

 

[Guest check-in]

Ah, but the Merlins were Pommy, the radials Yank. 'Nuff' said?

 

 

[Guest ozzie]

yanks made merlins under the packard badge

 

 

[Guest check-in]

Merlin was Pommy design, and in fact in one Packard version went quite well in boats (Fairmiles) where cooling was obviously not a problem.

 

But the poor old Poms never built anything that would fly in the tropics or anywhere else as well as Yank aircraft in general under the same conditions. Pom aeroplanes generally have very nice handling, so pilots often wax lyrical about them. But otherwise they are pretty ordinary. On the other hand the DC3 will cope with tropical heat and ice extremely well, when one considers how old the design. Compare that aeroplane with the crude offerings the Poms were producing at the time. Most of the Cessna, Piper and Beechcraft range cope with climatic extremes very well. Compare post-war Cessna or Beech with Auster of the same era, or Lycoming or Continental with Gypsy. No contest.

 

And anything Boeing builds is bloody brilliant.

 

 

[stanzahero]

Most tanks, even those used in the desert, had air cooled radials in them. Nev.

What tanks were these? What were the radials?

 

The British re-badged their Merlins as Meteors then used chassis cooling (chassis had water channels in them) to use in the tanks while I thought that the yanks had Ford and GM power plants (some were even petrol hence the nick names like Ronson and Zippo).

 

The Merlins that were built in America were modified and built as Allisons and (almost) powered the P51B & C's. It wasn't untill the Merlin went in that they (P51D) became useful as a fighter.

 

Stanzahero

 

 

[Guest ozzie]

when i was a kid there was a scrapyard that had a tank out front it had a radial in it. must have been torture for the crew.

 

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