Engine Horse Power

[The Hat]

When you buy a new piston engine it is rated at x horse power.

Lets say after several years use the compressions decrease to low 70's.

Is the horse power that the engine produces still the same or is it less... I assume that it has to be less. If I am correct how much less would it be 10-15%..

Thanks

[ferrydude]

In many cases it won't make rated horsepower out of the box when new. :eek:The rating is based on a standard day atmospheric condition and without any accessories installed.

[The Hat]

Its good to know that you dont even get what is advertised on the box...!

Okay let base the example on a standard day / temp etc etc ...and assuming that you dont have lots of additional extras running off the engine i.e. AC.. are the lower compressions making a significant effect on the HP.

Which then follows onto the performance figures in POH...How much of a margin should you then add to get somewhere near the actual figures for the plane.

[411A]

TCM operated several engines on the test stand with all piston rings removed, and the surprising results were...max 12% BHP reduction, as I recall.

Of course, the engines did use rather a large anount of oil...:uhoh:

[Brian Abraham]

did use rather a large anount of oil

Rolls Royce when developing the "R" 1,900HP engine for the Schneider races had troubles with the rings maintaining the gas seal. Result - an oil consumption of 112 gallons/hour.

[Keith.Williams.]

To keep it simple and stick to the basic question, let's assume that you actually got rated power in ISA conditions when the engine was new.

The short answer is that no you will not get the same power output if the compression has reduced with age.

Please note that I said "compression" rather than "compression ratio" because "compression ratio" is not a ratio of pressures, but is a ratio of volumes (Total volume / Clearance volume).

As engines get older their valves and piston rings become leakier, so the pressure at the end of the compression stroke decreases. This in turn reduces the pressure during the power stroke, so the power output decreases as engines get older.

We could say that the power output is equal to the power generated at the faces of the pistons minus the power that is lost transmitting this power to the end of the output shaft.

Power generated at the faces of the pistons is called Indicated Horsepower (IHP).

IHP = (Mean cylinder Pressure x Piston area x Stroke length x Number of pistons) 33000

Looking at the equation we can see that if cylinder pressure decreases then so does IHP.

It is a reasonable assumption (but by no means certain) that all of the other factors in the equation will remain constant as the engine gets older.

[ferrydude]

To complicate matters, there is not neccessarily a correlation between the compression reading and any loss of horsepower. You would need to know where the leakage was occuring. As 411 mentioned, TCM has loads of data that prove leakage past the dynamic seal, (rings), during a compression check does not represent what is occuring under engine operation, in fact they significantly lowered the compression test limits for leakage past the dynamic seal.

Leakage past the valves during the comp test is a better indicator of what will result in a loss of horsepower during operation.

[Keith.Williams.]

It is certainly true that not all leaks have the same effect on power output.

A leaky inlet valve will cause blowback of mixture (during the compression stroke) and burnt gasses (during the power stroke) into the inlet manifold.

Both of these effects will reduce the amount of fresh mixture that is available for combustion. This will clearly reduce the pressure in the cylinders during the power stroke, which will reduce the power output.


A leaky exhaust valve will cause unburnt mixture to leak away into the exhaust manifold during the compression stroke. It will also cause exhaust gas to flow into the cylinder during the inlet stroke. Both of these factors will reduce the amount of fuel that is available for combustion. This in turn will reduce pressure in the cylinders during the power stroke, which will reduce the power output.

Leaky piston rings will cause the crank case to become pressurised. This increased crank case pressure will tend to reduce the amount of gas that is able to leak past the leaky piston rings. Most crank cases are vented into the inlet manifold, so much of the lost gas will find its way back into the cylinders (albeit mixed with a good deal of oil). This in turn will tend to reduce the amount of power that is lost due to the leaky piston rings.

So although leaky piston riings will cause less of a pressure loss than will leaky valves, the overall effect of leaks, wherever they are, is likely to be reduced power output.

The magnitude of this power loss will be determined by the size of the leak and its location.

For most practical purposes, if you anyone tells you that their worn out old engine is producing the same power that it did when it was new, then they are probably trying to sell you a dodgy old car.

Don't buy it.

[ferrydude]

all true, but again, leakage past rings detected during a compression check does not mean that leakage is occuring during engine operation. So assuming that there will be a reduction in rated horsepower based only on a trend of lower compression readings is faulty logic

[Keith.Williams.]

Nothing in either of my previous posts referred to compression checks.

You are of course correct in saying that a simple compression check does not provide an accurate indication of power output.

If compression checks are carried out using the starter motor to turn the engine, then clearly the rpm will be very low. Under these circumstances the compensating effects of the pressurised crank case will be minimal, so leakage past the rings will probably be much greater than when the engine is actually running.

But if the leakage past the valves or the piston rings is sufficient to reduce the compression when the engine is running, then the IHP that is being generated at the faces of the pistons will be reduced. And unless the wearing out of the sliding surfaces has reduced internal friction sufficiently to fully compensate for this loss of IHP, then the power output will be reduced.

[ferrydude]

I refer you to the orginal poster's question. Compression checks on aviation engines utilising the starter motor are very rare. The TCM data does not support your position. Leakage past rings during a static compression test is usually higher than when the engine is operating.

[411A]

Ferrydude is quite correct, and TCM has loads of data on the subject for confirmation.
Likewise Pratt&Whitney with larger radial engines.
UAL was authorized to operate their R2800CB16 engines (installed on DC-6B's) to 3,300 hours until overhaul, and when one of these engines was sent to P&W for analysis, it was put on a dyno and checked for power output, the results surprised no one...yes, specific BHP was reduced, but only by three percent, and this was in the dry takeoff mode, wet takeoff, two percent.

[hawk37]

Gentlemen, an interesting discussion. I gather that Keith Williams contends that leakage, if any, by piston rings will cause a decrease in IHP. 411A and Ferrydude seem to contend that TCM data does not support this.

Is there an academic type of explanation of why you may have some blow by of piston rings yet not decrease IHP? I find Keith's explanations quite convincing....

Separately Keith, have you ever written a book or published papers/articles on aerodynamics/performance/engines etc?

[ferrydude]

Again, the original poster is asking what sort of penalty he should assess to performance charts based on compression testing results indicating in the low 70's. Plain and simple answer is none. A determination of actual ring leakage during engine operation cannot be made from static testing. I refer you to the TCM testing wherein rated horsepower was obtained during engine operation that failed static compression testing!

[Keith.Williams.]

Ferrydude

Even the original post in this thread does not refer to compression checks. It simply asks if wear-induced loss of compression causes a reduction in power output and if so by how many %.

Your statement that

"The TCM data does not support your position. Leakage past rings during a static compression test is usually higher than when the engine is operating."

Seems to agree entirely with my statement that

"Under these circumstances the compensating effects of the pressurised crank case will be minimal, so leakage past the rings will probably be much greater than when the engine is actually running."

We appear to be agreeing that leakage when the engine is running at normal operating rpm will be less than when it is static or at low rpm.

But we also appear to be engaged in two separate arguments. You appear to be arguing that compression checks do not give an accurate indication of power output. I agree that this is probably true.

But I am arguing that if wear-induced leaks cause cylinder pressure to be reduced in a running engine, then this will cause a reduction in power output.

The question of whether or not we can actually measure the leakage or cylinder pressures when the engine is running is irrelevant.


411A

Your statement that

"UAL was authorized to operate their R2800CB16 engines (installed on DC-6B's) to 3,300 hours until overhaul, and when one of these engines was sent to P&W for analysis, it was put on a dyno and checked for power output, the results surprised no one...yes, specific BHP was reduced, but only by three percent, and this was in the dry takeoff mode, wet takeoff, two percent. "

Simply proves that a reduction in power output had occurred, but that this reduction was not very great. It does not mean that power output was not reduced.

These results do not prove me wrong, but actually illustrate exactly what I have been arguing. Wear-induced leakage causes a reduction in power output.

Nothing in any of my posts suggests that the rate of power loss in any particular type of engine is very great. Even for a specific engine type, the wear rate and power reduction rate will vary from one engine to another.
My argument is quite simply that if wear causes a reduction in cylinder pressure, then this in turn with result in a reduction in both IHP and BHP.

If anyone would like to use the IHP equation to demonstrate that a reduction in mean cylinder pressure will not cause a reduction in IHP, then I will be delighted to be proved wrong.

Hawk37

No, I have not published any books or papers on this subject.

[ferrydude]

Erm, fromthe original post;
"Lets say after several years use the compressions decrease to low 70's."

This does not refer to compression checks? What would you suppose he meant by "low 70's"??????

"But I am arguing that if wear-induced leaks cause cylinder pressure to be reduced in a running engine, then this will cause a reduction in power output."

And while you may be technically correct, it is a moot point.
In most cases the reduction in power output is insignificant. No reduction in horsepower. The TCM test data support this. Where did your data come from?

[hawk37]

No, I have not published any books or papers on this subject

Well, you should, it may be your second calling if you're looking for one. I was once told that for nearly every complex problem, there is a solution that is simple, neat, logical and wrong. Fortunately, your responses seem to fall only into the first 3.

[ferrydude]

Also fortunate is the fact that this is not a complex problem. Quite simple really;)

[Keith.Williams.]

Ferrydude

The fact that the first post specified "low 70s" as an example of the reduced cylinder pressure does not make any reference to compression checkes and does not restrict the question to the subject of compression checks.

I have already agreed that compression checks are unlikely to provide an accurate indication of power output.

Now if you would like to look beyond the validity of compression checks, to consider the effects of reduced cylinder pressures in a running engine, we can move the debate forward.

If you would like to use the IHP equation to demonstrate that reducing cylinder pressure will not reduce IHP and hence BHP, I would be delighted to learn something new.

But further argument about compression checks would be quite futile.

[ferrydude]

Futile is correct. So then, what is your answer to the orginal poster other than theory ?

[Brian Abraham]

Top Hat - Not a direct answer to your question, but http://autospeed.drive.com.au/cms/A_108913/article.html has a discussion about rings and compression. An extract (deals with motor vehicles but it reads across to aero engines in basic principles)

During operation of an engine, blowby passing through the ring gap is under very high pressure and it reaches what is termed "sonic velocity". At this point any increase will not result in any increase in gas velocity (hence the rate of escape of the gases). From this point of view, leak-down tests are of little or no value in determining engine condition, as they measure leakage under static and low pressure conditions.

The gas which does escape the piston rings, and which is returned to the intake charge, is termed blowby. The control of blowby to an acceptable level is important because excessive blowby means loss of engine power, leads to ring sticking and an increase in emissions. That is why manufacturers tightly control piston ring shape and flatness, two factors important for good gas sealing. Blowby is measured under laboratory conditions by sealing off the engine's crankcase and attaching an accurate gas meter by means to the breather on the oil filler cap. A typical 2-litre engine can expected to have a blowby figure of 20-30 litres per minute under full load conditions.

[barit1]

A reasonably good measure of trending any HP degradation is available to the operator of a fixed-pitch prop aircraft, and that is to keep a log of static runup RPM at home base. Record the OAT, wind, and QFE along with RPM and you'll get a good picture of what's going on. Every 50 hours might be a good interval for these checks, at least until you have a good database.

Unfortunately a variable (constant-speed) prop complicates the matter, unless there's a torquemeter like the R-2800 has.

[411A]

Unfortunately a variable (constant-speed) prop complicates the matter, unless there's a torquemeter like the R-2800 has.

Indeed, BMEP tells all...mostly.

[Keith.Williams.]

Brian Abraham,

You post is interesting but we need to take care in interpreting it.

The words

"At this point any increase will not result in any increase in gas velocity (hence the rate of escape of the gases)."

Mean that increasing cylinder pressure will not increase the leak rate. This is just basic high speed duct flow theory.

But it does not mean that increasing the size of gaps between the rings and the cylinder wall will not increase leak rates.

Larger gaps will result in larger leakage rates, lower cylinder pressures and ultimately lower power output.

[The Hat]

Many thanks for all of the replies.

The original question was raised because you see a lot of aircraft for sale with the seller stating that the compression is "x" per cylinder.

This got me thinking abt the actual engine performance and other directly related issues and whether the performance figures in the POH are correct for a new and an engine that is obviously mid life and has lower compressions.

[ferrydude]

"The original question was raised because you see a lot of aircraft for sale with the seller stating that the compression is "x" per cylinder."

Which is a reference to the static differential compression test performed during the required inspection(s). :)