piston engine thermal efficeincy.

[baobab72]

Hi
just a couple of questions about piston engine theory.
as far as i remember the cubic inch displacement determines together with the volume of the combustion chamber the compression ratio referred to the ratio of the volume in the cylinder when the piston is at the BDC to the volume in the cylinder when the piston is at the TDC.
The compression ratio, limited by the engine strength and by the number of octane in the fuel providing an indication of the fuel to resist detonation when ignited, determines both the thermal efficiency and the volumetric efficiency.
The thermal efficiency of an engine represents the ratio of the chemical energy stored in the fuel burnt to the work produced or in other words the ratio of the heat energy of the fuel to the work produced by the pressure acting upon the piston head.
Does the compression ratio affect the thermal efficiency by increasing the D variable in the Work formula W=F D and thus the work or there is something more to it? cos as far as i have understood the compression ratio provides merely an indication of how far the piston can travel, while i am having an hard time relating it to the pressure increase within the cylinder barrel - although i do understand that the higher up the TDC the more the fuel air mixture is squeezed.

Anybody can shed some light?

One more question: do manufacturers provide an indication of the thermal efficiency of each engine, or does it varies based on power setting? and if so what is the thermal efficeincy of the Lycoming IO360 vs the Lycoming 360?

Finally, does the Lycoming IO 360 mounted on the 172SP have a flywheel?

Many Thanks and Merry Xmas

baobab72

[Pilot DAR]

Baobab, you have a good idea already. I can help a bit...

The Lycoming has a "ring gear", which is used for starter motor engagement, and alternator drive belt (and maybe prop deice), but really has no significant flywheel affect. The metal propeller is the flywheel. This I know because I did testing of an IO-360 equipped with a small diameter Hoffman four blade propeller. The engine ran nicely, but was miserable to start. There was only a minor flywheel affect from the smaller, lighter wooden prop. During a start attempt, there was no flywheel affect to carry the engine through the compression stroke, so it acted as though it had a flat battery, even though the battery was fine. Similarly, the MT props can be a little difficult to start on the Lycomings. I did many shut down, feather and restart tests on the MT & IO-360 equipped DA-42-L360. The MT made the engine very hard to windmill start, and there were times in flight where I had to starter motor start the engine, or would not have go it running. Larger diameter heavier metal props seem to windmill start better - once you get them turning over, they stay turning, so the engine can start smoothly.

I'll have a look at my Lycoming data, to see if there is a reference to thermal efficiency, I do not recall having seen it before. If it interests you, compare the Lycomin IO-360, and HIO-360. Though the same displacement, they have different compression ratios. The HIO turns faster than the IO, though their powers are the same, or very close.

The compression ratio has a relationship to the torque of the engine. Though torque is not employed quite the same way in an aircraft engine, as it is in a car engine, there still is an effect. You can obviously pitch the propeller to be more course, and extract more power at the same RPM. The octane requirement of the engine is a direct relation to the compression ratio, which is a factor in combustion pressure, and the prevention of detonation. Detonation is also affected by engine speed. More speed = lesser chance of detonation, all other things being equal.

[NutLoose]

If the 172SP has air con fitted, it will also be belt driven (as the alternator is) off the rear of the ring gear... The diameter is really just for the correct gearing for the starter and to add a bit of weight to increase the flywheel effect of the prop, it also has the timing marks on it.

[jimjim1]

There is something more to it.

I don't mean to be rude at all but I think that you are quite far away from being able to understand this. I can recall studying some parts of this in University Physics but I have forgotten it all now so I cannot explain it to you.

A Google search for [thermodynamics engine efficiency] turns up some good looking links including:-

Thermal efficiency - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Thermal_efficiency)
"Surprisingly, even an ideal, frictionless engine can't convert anywhere near 100% of its input heat into work."

I think that you will find that there are NO shortcuts.

First year university maths and afterwards a thermodynamics course is what it will take.

If your reaction is No No No No - why is it not possible for an ordinary person to figure this out - then have a go at designing an atomic bomb, or a jet engine.

Of course a perfectly ordinary person CAN do these things given a decade of study.

Good luck.

[Pilot DAR]

I think that you are quite far away from being able to understand this.

To the depths of what could be known about thermodynamic, yes, I probably am.

I approve design changes to Lycoming and Continental engines, as well as certain aircraft, I don't pretend to be an expert in thermodynamics. As with anything, any aspect can be taken to much greater depths of understanding, in most cases at the expense of understanding another associated discipline to great depths.

If your reaction is No No No No - why is it not possible for an ordinary person to figure this out - then have a go at designing an atomic bomb, or a jet engine.

No No No No, I'm quite happy testing and approving small aircraft modifications. No need of atomic weapons. I am delighted to think that an ordinary person might want to expand their understanding, and am always happy to help a little. The question askers of today, could be the innovators of tomorrow - and we need to encourage them!

Of course a perfectly ordinary person CAN do these things given a decade of study

Yeah, that sounds about right, based upon the experience it took me to be granted Transport Canada Design Approval Representative delegation for piston powerplants

I don't mean to be rude

Oh, okay, I misunderstood..

do manufacturers provide an indication of the thermal efficiency of each engine, or does it varies based on power setting?

Baobab,

I have reviewed the Lycoming IO-390 Operator's Manual, which I'm sure would represent the "latest and greatest" from Lycoming. Though it presents lots of interesting written and charted information, it dd not mention thermal efficiency. There was lots of charted information about performance relative to power setting.

Don't let negative writers put you off... Your questions seem well considered to me.

[oggers]

Baobab:

There is a thread running in the Tech Log section that goes into this in some detail. However, I wouldn't recommend it for a clear answer.

Does the compression ratio affect the thermal efficiency by increasing the D variable in the Work formula W=F D and thus the work or there is something more to it? cos as far as i have understood the compression ratio provides merely an indication of how far the piston can travel, while i am having an hard time relating it to the pressure increase within the cylinder barrel - although i do understand that the higher up the TDC the more the fuel air mixture is squeezed.

You are on the right track but the 'D' in your formula is the 'stroke' length which is independent of compression ratio. Note that you increase the CR without increasing the stroke length by simply having a smaller combustion chamber. Thus you increase the compression during the stroke, but the stroke length itself remains the same (if you did change the stroke length you would also be changing the engine capacity).

The work formula you used can be restated as W = PΔV, which is more useful in this instance.