I suppose starting a 912, Thielert, or Austro engine by -25 does no more harm than when starting your Mercedes class A or B engine in the same environment. Plenty of those North of 60° North.
Good point. I wouldn’t give a second thought to starting my car engine in cold temperatures. Why is it so different starting an aircraft engine? The air cooled vs water cooled can’t be relevant. So is it just the oil?
One issue is that not all operating clearances in an engine scale with the size of the engine, but thermal contraction does scale directly with the size of the engine. So a physically larger engine with larger cylinder (aluminum inside steel) and larger crankshaft (steel inside aluminum) would see a greater change in cylinder or bearing clearance at a given low temperature.
I suppose starting a 912, Thielert, or Austro engine by -25 does no more harm than when starting your Mercedes class A or B engine in the same environment.
If that were true beyond doubt, I’d be quite relieved. But I understand aviation engines – and the Rotax in especial – are made to a set of compromises quite different from those applied to car engines. Tolerances must be tighter, margins on operating temperatures also; if only because the cooling systems will be less overdimensioned. Ok, that applies to the upper end of the temperature scale and we are now discussing the other side, still I feel there must be less room for variance.
Then again, perhaps the mere existence of this thread has made me over anxious? Actually, it makes me thinking twice about precautions before starting my van’s 3,0 Iveco in -10C that we may soon expect here.
the bigger the bearing, the more clearance
eggssactly. The Rotax is amazingly small with its 1211 cc, the bearings must be rather tight.
A couple more points:
The cylinders on modern car engines are typically made from an alloy that matches the thermal contraction of the pistons. A while ago there was a thread about aluminum cylinders being sold for Lycoming engines, and this was one reason I found it interesting. Less variation in cylinder clearance is a good thing.
One interesting issue is when you have rolling element main bearings, which thankfully is not the case for most small aircraft engines. When the aluminum crankcase changes size due to temperature changes, it is possible for the crankshaft to be pinched lengthwise, or even come loose lengthwise between the main bearings. For this reason when you assemble one of these engines you may shim the crankshaft to a negative thrust clearance, i.e. pinched a little at room temperature.
Prior to seeing the aluminum Lycoming cylinders, I did some research into (diesel style) steel pistons that would match the thermal expansion of the steel cylinders. On a boxer engine it seemed like an interesting idea because of the 100% primary balance, but the con rod inertial forces would be high. Higher revving engines use aluminum pistons, in Nikasil or similar, like the air cooled BMW motorcycle engines Vic mentions.
A lot of rolling element main bearing engines are temperature critical in terms of thrust clearance regardless of it being ideal design practice (or not!). I was surprised when I first saw it on a Ducati V-twin.
Vic, I’m impressed that you guessed bevel gear Ducatis… Those are the ones that specify a slightly negative clearance at room temperature for crankshaft end float. The later non-bevel one-piece crankshaft engines engines use a similar main bearing setup but without the negative clearance. I got about 60,000 km out of my first bevel Ducati prior to overhaul, in the mid-90s… But I did learn a lot doing the built up crankshaft myself.
Oil film bearing design for thermally varying clearnace sounds like an interesting problem to me. There’s a lot to this machinery design stuff, eh?