compression ratios of typical single piston engines
 

Very simple and quick question. Sorry for two posts in one day.

I just noted that the O-320-D2J engine I flew today has an 8.5:1 compression ratio. Question: why does a mere 8.5:1 compression ratio engine require 100 octane fuel? Surely it could run on 87, right? (I know it can't, but if I didn't know anything about aviation I would surely think it could.

I would think that one would need to be at 12:1 or even above to require "high test".
If you don't know what high test is it means you're even younger than me, or perhaps from Europe. :8

I think the fuel is leaded to prevent damage to the valve components in engines of old design, rather than to prevent detonation.

I fully understand that the more octane (or lead to an extent) a fuel has, the more resistant to pre-det it has. That's pretty common household knowledge.

I guess what I need to understand now is why aero engines need so much spark advance (?)

edit:

ahhh...forgot about that. Good point!

OK, starting to understand. Because we are firing the initiator (spark plug) so early, we need a fuel that can resist simply exploding immediately....we need to have a fuel that has a controlled burn and a (relatively) gradual pressure rise in the cylinder. If we used 87, the burn would be so fast that the piston would want to travel back down the up-stroke...to try to run backwards so to speak.

The detonation won't push the piston back down the cylinder but is will put a high load on the side of the piston furthest from the spark plug, this is likely to result in mechanical dammage to the piston.

which also explains the inherent superior efficiency of diesel engines with their much higher compression.

(And good volumetric efficiency).

The critical thing with respect to detonation in a spark ignition engine is peak pressure in the cylinders.

With a large cylinder at low rpm (compared to a small cylinder at high rpm), there is a relatively longer period of burning taking place. This leads to higher peak cylinder pressures, which are more prone to causing detonation in the remaining mixture, ahead of the flame front.

This same phenomenon can occur in car engines, albeit less often these days due to better engine design and the use of ECUs with knock sensors. If it happens, it is more likely to be in a high gear at low rpm.

GAMI have reported that it is actually very difficult to get a normal engine to detonate. The CHT has to be very high - around 500F - but a less than careful pilot of something with no engine instruments could well achieve that in a mismanaged climb, or slow flight.

I wonder whether an engine with say 9.5:1 CR and no turbo can be as efficient as the same engine with a 8.5:1 CR (achieved purely by different pistons) and a turbo.

I ask this because this has been claimed. I don't see it can be true on a turbonormalised engine but it could be true if the output pressure of the turbo is above normal atmospheric.

The TB20 does about 10% more MPG than the TB21, in reality, but there are also different operating practices.

Is this where a turbo gets its rotational drive ratio from...from non-fully-expanded exhaust gas? In other words, the turbo is not driven by the exhaust stroke piston "pushing" exhaust through it bu rather from said exhaust expanding further/fully and flowing through it (the turbo)?

Silvaire1
 

I think the word you are looking for is turbocompound it was a great idea that was not as reliable or as cheap as the turbo charger.