Reducing compression ratio

[Old Koreelah]

I fitted a jab 2.2 to my little special, mostly because of its light weight. If I cruise at 75% power I expect to be well over VNE, so a cruise at c.2500 is more likely.

 

All advice suggests at those revs I could glaze the bores. To prevent this I guess all I can do is vary the speed regularly, (This will certainly happen during the months of testing).

 

What is likely to result from shimming up the barrels to reduce compression ratio? The power loss wouldn't bother me, but would this reduce the chance of glazing, extend engine life and allow use of lower grade fuel?

 

 

[Relfy]

Have you thought a different prop to bring the revs up and keep the speed down?

 

 

[Yenn]

When running in Jab engines it is recommended to use full power and then slightly reduced power to bed in the rings, what you should not do is run on reduced power I am told. So what would be the advantage of reducing compression? Would it not be the same as running at reduced power by throttling back.

 

 

[Guest watto]

I think relfy has the medicine with a prop change, gives you the best of both worlds then.

 

 

[Old Koreelah]

Thanks fellas for the replies. Having spent years streamlining the airframe to improve efficiency, I'm a little reluctant to fit a prop that wastes engine power.

 

The beast only needs about 60hp but all available engines were heavier than the Jabiru. The ideal would be a Jab 1600 if it were as reliable as the 2200. It occurs to me that the next best thing might be to detune the 2.2 so it delivers the 60 hp in a safe rev band. Perhaps it would also be under less stress and therefore last longer.

 

 

[Vev]

It’s not so much the engine speed (RPM) that causes bore glazing, it's the load …. The engine needs to be loaded up and run at the correct engine temps. Light loads and low temps will cause glazing, high oil consumption and short engine life. Full power at take-off and 75% + at cruise is the way to go.... not sure how you do this as it sounds like you have more grunt than you need.

 

 

Regards

 

Jack

 

 

[Old Koreelah]

Exactly, so if a detuned 2.2 Jab was working harder to pull me thru the air at the same speed, it should be loaded up and running hot enough to avoid glazing...

 

If that were true, would the lower compression put less train on the engine and improve longevity?

 

 

[Guest Maj Millard]

I know the Jab uses standard aviation grade oil. On a normal aviation engine after a rebuild or new cylinders/rings, the engines are run on standard mineral oil for 50 hrs. This is then taken out and the standard oil put in.This is to assist in the seating of the rings. Don't know if the jab uses this process. To seat the rings the engine has to be working hard, and getting a bit hot. Worst thing you can do is baby it, and take it easy. Once you glaze the bore, it's glazed, and you have to pull the cylinders off again and re hone. Even tieing it up to a tree and running the guts out of it for an hour or two (providing you can keep it cooling ok) would help a lot towards seating the rings. If you don't seat the rings you will have excessive oil consumption, lower power, and probabily less than TBO life.

 

Seating the rings is necessary to achieve the best fit/seal between piston ring and cylinder wall. The actual 'seating of the rings' is the new rings wearing down the little hills and valleys put there by the honing and crosshatching process, on the cylinder walls. The honing and crosshatching is put there in the first place to retain additional oil, whilst the 'seating in' process occurs. Hope this assists you. I did notice a Corby Starlet in a hangar recently with a new 2.2 Jab on it also. Because you are going to have too much power for your aircraft, it may be a good idea to overprop it (more pitch than you are going to fly with) and run it in on the ground, tied up securely.

 

 

[BigPete]

I know the Jab uses standard aviation grade oil. On a normal aviation engine after a rebuild or new cylinders/rings, the engines are run on standard mineral oil for 50 hrs. This is then taken out and the standard oil put in.This is to assist in the seating of the rings. Don't know if the jab uses this process.

Yep, that's right :thumb_up: - and Jabiru does do the same. :thumb_up:

 

regards

 

:big_grin::big_grin:

 

 

[geoffreywh]

the proper way to reduce power would be to fit a smaller choke carburettor....

 

 

[Yenn]

The above ways of reducing power won't make any difference, reducing carby size and reducing compression ratio both reduce the compression pressures, which is just the same as running at reduced throttle. I am running a 2.2 Jab in a Corby Starlet and cannot pull high revs in cruise because I would exceed Vne. I made sure that when I ran the engine in I used full throttle and a steep climb attitude to prevent glazing the bores and since then I don't cruise at above 2800rpm. No problems with glazing, the max amount of oil used in 25 hours would be 500ml and usually only about 250 to 300ml suffices. Of course I don't know what would happen with Old Koreelah's plane and if it is a Jodel D9 it would probably have to run at lower rpm than mine, but while there may be some glazing of the bores, the power required may be so little that it doesn't matter and fuel consumption would probably be 10 l/hr or less. I seem to be using 10 to 11 l/hr and cruising at above 100kts.

 

You could of course talk to Rod Stiff at Jabiru, but I asked him the same question when I bought my engine and he recommended against it.

 

 

[Vev]

Hi Yenn,

 

 

I agree with you … I really can’t see any of these engine mods doing much to reduce bore glazing. In my old motorcycle days there were inlet or exhaust restrictors fitted to engines to detune them for children to ride them with less power. Be it a 4 stroke or 2 stroke engines they all had deposits problems along a high propensity to suffer with bore glazing. Apart from your suggestions to use full throttle and steep climb out, I think a chat with Rod Stiff a good idea as well.

 

 

Regards

 

Jack

 

 

[Old Koreelah]

Avoiding glazing a J2.2

 

Thanks lads for the replies. All food for thought. (I started out with a VW motor which I liked because of its long-life/low-stressed power delivery. Perhaps I'm subconsciously trying to turn Rod Stiffs' little wonder into a lightweight VW.)

 

I sure plan to carefully run-in the engine, including using full power in climb.

 

Yenn, you're right about my D9 needing less revs than the Corby, but I need to to explore the following idea:

 

"... while there may be some glazing of the bores, the power required may be so little that it doesn't matter and fuel consumption would probably be 10 l/hr or less. I seem to be using 10 to 11 l/hr and cruising at above 100kts."

 

My original 1500 VW used about 6 l/hr doing circuits and short trips at c. 65kt, so I hope the Jab will use about 10 at 80-90kt.

 

 

[Guest Qwerty]

A thought from the lateral bin....

 

How about getting in touch with a prop maker and get him to make a really "bad" prop for you.

 

You will burn fuel unneccessarily but then you bought an unneccessarily high powered engine and there is no was around the use of additional fuel if you want to load the engine.

 

You may get some torque reaction/roll effects but these shouldn't be too much of a problem.

 

But it would work. and if the prop maker is worth his salt he should be able to make a suitably inefficient prop for you. :thumb_up:

 

 

[Guest Qwerty]

And again....

 

Dont tidy the fairings, no spats, HUGE BALLOON WHEELS. You could attach all sorts of external accessories aerials and stuff....just make it really aerodynamically inefficient.

 

Need any more help :thumb_up::thumb_up:

 

 

[Old Koreelah]

Now you're just being silly!

 

 

[Guest Qwerty]

Hahhh, I'm not, these are actual solutions to your problem. You are in a situation where you need to produce 60 hp to stop the bores from glaseing and these suggestions will help you to achieve this without cruising beyond Vne.

 

What else are you going to do?????? Now that I have posed the question backwards........there is nothing else you can do.

 

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Maybe a 25 KW generator??? OK NOW I'm being silly.

 

 

[slartibartfast]

Sounds like you need a half jab.

 

At Wagga about 3 years ago John Corby was ground running a half 2200 engine (2 cyclinder) in a Corby fuselage. I guess that didn't take off (pun intended).

 

Got a hacksaw?

 

 

[Guest drizzt1978]

CAMSHAFT people...

 

Camshaft?

 

???

 

Not to mention a restrictor on the inlet side. (this are used on race car with hp restrictions) they restrict up to 100hp sometimes?

 

Remember and engine is a big air pump, want more put more air in (and fuel) want less dont put as much air in.

 

Any takers on that debate!

 

 

[jetboy]

Just get a finer pitch prop. It will keep the cruise revs out of the forbidden zone, and the carb will keep the mixture and fuel flow corrected for the higher rpm. There is no change to fuel consumption, you get a better climb and keep off vne. It will make more noise and wear out faster, and you have to watch for over rev, but thats not an often observed problem on any Jabiru i've seen.

 

I did the prop change on my CH701, but with all my drag vne is never an issue, staying aloft using only 75% is!

 

Ralph