Rotax 912 pulls through easy when cold

[facthunter]

 Hot oil becomes very thin and the engine can develop a stuck in a spot (hysteresis) feel. Oil film is not generated until the parts are moving in relation to each other .Piston rings  need some oil to aid  them sealing . Also a piston expanded closer to the bore will  have less "rock" and the rings seal better and wear less" barrell faced" like they do with worn pistons or run too cold. Nev

 

 

[onetrack]

I was always led to believe that cold engines were "tight" engines, with lowered component operating clearances, and therefore you kept engine RPM's down, until the engine warmed.

 

However, air-cooled aircraft engines suffer from lots of WOT operation and a fickle operating temperature environment, that encompasses a very wide range of temperatures.

 

As a result, air-cooled aircraft engines usually have bigger operating clearances, particularly in piston-to-bore clearances. If your aircraft engine is big-bore, the clearances are even greater.

 

However, I would have thought that the water-cooled Rotax 912 would have had fairly tight clearances when cold.

 

Rotax talk about being able to build the 912 with tighter clearances, simply due to water-cooling.

 

Here's a very well written and informative article about pistons, by a gent from Wiseco, U.S. forged piston manufacturer. It goes into detail as regards piston-to-bore clearances, for every type of engine.

 

http://blog.wiseco.com/piston-to-wall-clearance-myths-mysteries-and-misconceptions-explained

 

 

[skippydiesel]

Hi Onetrack - me thinks you have this around the wrong way.

 

Engines are designed to operate efficiently within quite a small temperature range ( air cooled usually  having a wider range than liquid).

 

For the most part  the cold clearances are designed to reduce (get tighter) as the engine warms, with minimum operating clearances being established at engine heat soak.

 

The clearances may differ but the principal is the same, no matter how your engine is cooled.

 

Liquid cooled are able to operate within a smaller temperature range, so in general clearances are smaller/tighter ( efficiency is higher) than an air cooled engine.

 

I am also lead to believe that ,  all petrol engines rely, to some extent, on fuel for cooling however this is less so in a liquid cooled engine. So again efficiency gains with this form of cooling over air.

 

 

[facthunter]

Heat always flows from hotter to colder so crankshafts and pistons will be hotter than what they run against. Many plain bearings have their clearances larger to allow  greater cooling oil flow.. Larger bores and bearing journals have bigger clearances but it's proportional. Usually say .001" plus .0025" per inch of bore  as a minimum for a "solid" type of piston.

 

     Liquid cooled engines do overall tend to run cooler as water boils a little too cool for best running temp so needs pressurising and anti-boil additives and it's still a little too cool for maximum efficiency.

 

  Another factor in clearances needed is the different rates of expansion of the metals as well as their temperature difference. An alloy piston  in an alloy bore with Nikasil on the surface can run a much smaller Cold clearance than if the bore is steel or iron. When BOTH are at the max temp allowed the running clearance for  both situations will be quite small. Just dictated by the need to not nip up. At no time should there be anything that is actually TIGHT as that will add  extra heat quickly and start  a path to destruction with more heat and MORE friction and more heat etc.

 

 Misalignment of components and metal picking up on parts (welding) where the oil film fails locally may produce a tight feel that needs dismantling and rectification. In these matters most engines do not "fix"them selves. Nev

 

 

[BirdDog]

Some really interesting discussions lads, but I think we are getting away from the original question.

 

More research is telling me this is all normal.  Remember, my engine is NOT tightening up - As in - more than normal.  It's the opposite - it gets loose cold.\

 

I will keep you posted.  Was going to try and get to the bird on the weekend, but ran out of time.

 

 

[spacesailor]

"For the most part  the cold clearances are designed to reduce (get tighter) as the engine warms, with minimum operating clearances being established at engine heat soak."

 

Didn't work that way for the 582 motor

 

That seize when taking off too early, & The cold radiator water hits the motor and "down we go" !.

 

spacesailor

 

 

[facthunter]

 Indeed that is so, but I didn't mention the word "soak" as you do in your QUOTATION,  is it introduces other issues.. My discussion is on heat flow from hot to colder and expansion with different temps and metals . Two strokes, which I have lots of experience with actually, are "special" beasts with distortion and marginal lubrication situation,s ring groove carbonning etc Carbon in the grooves makes the particular ring be forced against the cylinder when the cylinder has not yet expanded to it's hot dimension(s) and it usually breaks into little pieces and goes out the port or a least seizes and drags the bore, and you are suddenly without power.. Two strokes are now perhaps a bit of a "lost art" over the general network. In the earlier days they were well represented at Narromine and they all flew there for some large distances. Nev

 

 

[BirdDog]

OK... So here's an update.

 

Aircraft has been sitting for a little while now, and new leakdowns tested on a stone cold engine.  Results were 77/80 78/80 78/80 78/80

 

Leakdowns don't lie.  

 

So I am happy to continue on as normal.  The leakdowns, colour of the oil (still looks new), no oil use and colour of the plugs confirms that the motor is fine.

 

 

[facthunter]

Leakdowns check  piston and valve sealing and yours are good. Do you cut open your oil filters at each oil change?  Nev

 

 

[BirdDog]

Yes I do.  nothing on the magnet last time, and nothing in the filter either.

 

 

[facthunter]

Fine. It's done all the time in the "Industry". If you get metal look further. The magnet picks up ferrous metals only of course. Nev

 

 

[BirdDog]

Yep.  Thanks mate.  My LAME advised me to open the oil filter and make sure it's clean - so that's what I do.  ?

 

 

[antonts]

Some engines do not open valves until compensators are filled with oil, and after long pause oil drains out from there. It causes no compression at all, as no intake happens.

 

Mb this is the same for rotax. How are valve drives done there?

 

 

[Thruster88]

Some engines do not open valves until compensators are filled with oil, and after long pause oil drains out from there. It causes no compression at all, as no intake happens.

 

Mb this is the same for rotax. How are valve drives done there?

 

If you are talking hydraulic valve lifters then the valve still opens at least 75% even with the lifter collapsed (no oil). On a Lycoming the collapsed valve clearance is checked and adjusted with different length push rods. I presume the Rotax is similar. 

 

 

[facthunter]

 Yes . If they had that much slack they'd be popping pushrods off and ticking like clocks on steriods. . Most lifters hold their dimensions for long periods. I don't like them being used in Aero engines myself. They "mask" problems  and I've had an engine fail (Cont 0-300) because of one . Nev

 

 

[cscotthendry]

If you are talking hydraulic valve lifters then the valve still opens at least 75% even with the lifter collapsed (no oil). On a Lycoming the collapsed valve clearance is checked and adjusted with different length push rods. I presume the Rotax is similar. 

 

Don't know about the collapsed valve clearance on Rotax engines, but Rotax have a procedure for bleeding possible air out of the lifters. From what I've read, it only has to be done when (if) possible air has been introduced to the oil system as in, if the engine is turned backwards or air is allowed into the system during oil changes.

 

 

[facthunter]

 Dirt in the oil or a bit of metal swarf can cause the lifter to fail.( Not pump up). The geometry of the whole set up puts a limit on the total  extra play which is usually about a couple of MM max. As the valve seats wear this reduces. If it ends up being "nothing" the valve will not seat properly. IF you lose the 2 mm  of lift the motor loses power and may backfire and do more engine damage.  Investigate any unusual engine running. Don't hope it will come good or fix itself.  Nev.