Simple engine question

[omcaree]

Hey all,

I have what I'm sure will be a simple question about engines, it stems from my thermodynamics coursework but its just as relevant to aviation.

I'll admit that I don't fully understand how manifold pressure and RPM can be used to find engine power output (despite recently completing a VP checkout), thats not really the problem in this case. with the throttle set in a constant position, a reading will be obtained for manifold pressure, if nothing else changes appart from the engine RPM being increased then what should happen to the manifold pressure reading? my logic says it should decrease because the engine is trying to suck more air through the same gap, however from the experiment which i conducted i found that the manifold pressure remained constant from 1800 to 4700 RPM (rover k series engine) which is just making me confused.

Any help would be greatly appreciated

Cheers

Owen

PS. this wont help with my coursework as that concerns thermal efficiency, this was just an observation which confused me!

[Skylark4]

If the only thing you were changing was the RPM I assume you were doing it by varying the load on the engine, Please confirm.

I suspect that the situation you do not understand involves a Constant Speed Prop. Is this the case?

Mike W

[omcaree]

Thats correct, the engine was kept under load by a dynamometer and the load was varied to change the RPM. Am i right in saying this is the same situation at with a constant speed prop? coarser pitch puts a higher load on the engine, thus reducing RPM ?

if that assumption is correct then yes i'm generalising what i saw in the labs to what i'd expect to see in the air. I must admit i've never observed changed in MP with changes in RPM simply because I haven't been looking for it (and i wont be flying for a few weeks so i thought i'd ask for help here)

[homeguard]

omcaree

read the following;

http://www.avweb.com/news/columns/182081-1.html[/URL]

[omcaree]

cheers for the link but i found that on google earlier, and it supports what I said.

keeping throttle position constant while increasing RPM should cause a fall in MP because the pistons are trying to suck more air through the engine (i believe the linked article has the opposite example, fixed throttle, lower RPM, MP goes up).

Are there any situations where this wouldn't be the case? surely car engines can't differ from aircraft engines in that respect (since they both work the same, although the RPM/torque range my vary). So far everything supports what i thought, but not the data i saw.

If no one else has any ideas then i'll just ignore the data and accept what i thought, but more ideas are welcome

[IO540]

I'll admit that I don't fully understand how manifold pressure and RPM can be used to find engine power output

That's because it can't :O

That sort of statement, stated in isolation, is simply wrong. Engine power output comes from the chemical energy in the fuel. Not from the air!

The first step is to establish what the fuel flow is. If you run rich of peak then not all the fuel is getting burnt, so that makes it harder.

If you run at peak EGT or lean of peak then all of it is getting burnt, and the engine power will be proportional to the fuel flow.

(The fuel flow v. power output line won't pass through zero,zero on the graph because the engine needs a certain amount of power to suck the air in, and to overcome friction, etc. Have a look at some engine performance graphs from e.g. Lycoming)

An engine running as above, at a given fuel flow rate, will develop much the same power at any reasonable RPM; all that happens is that at a higher RPM the torque will be lower (because HP = revs * torque) and at a higher RPM it will try to suck in more air (because the whole bit of the engine before the inlet valves is just a fancy air pump; the more you rev it the harder it will suck) so the throttle needs to be closed more in order to get back to the desired power output.

MP is a crude indicator of torque and this is why it is relevant to engine management but it is nothing to do with determining what the power output is.

There are tables showing that a particular engine, at a given altitude, with a given MP and at a given RPM, will develop X HP. That is probably what is confusing you. These tables "work" because that particular engine model will have a particular carb or a fuel injection unit bolted onto it, and these accessories will be set up at the factory to deliver a certain fuel flow rate at a given airflow, and the airflow obviously depends on the altitude, the temperature, the engine suction (RPM) and how much that suction is constricted by the throttle valve.

Also the vast majority of light planes don't have a fuel flow rate indicator, which is probably why we have these tables.

If you have a plane with a decent fuel flowmeter then (when in cruise) you just set the engine up at an RPM which gives a nice engine life, say 2300, set the MP for a value which is within limits for that RPM (because MP is a crude indicator of torque i.e. crankshaft stress, there is often a limit on the MP at a given RPM), say 23", and then lean the engine so the fuel flow gives you the desired power setting.

[omcaree]

IO540 thanks for the reply, it seems to confirm my belief. If MP is a method (all be it not very precise) of measuring torque then at a fixed RPM and fixed MP you'll be developing a particular power output, if the RPM is decreased then for the power output to remain constant the torque (therefor MP) has to go up (because Power (HP) = Torque (MP) * Angular speed (RPM)).

In that case this has been an informative, but slightly pointless thread as it has just confirmed what I thought, but with a better explanation than i could have given.

Cheers All :ok:

Owen

[IO540]

You will be developing a given power at a given fuel flow rate.

You can run an engine at a given RPM and a given MP, and vary the fuel flow rate (by the mixture control) and the power output will still vary - because you are varying the fuel flow rate.

You are right in that if the RPM is decreased (by increasing the load) then the torque will go up, and the power remains about the same. That's why if one is flying behind a VP prop, and fiddles with the RPM lever, the speed (which is the best measure of power) doesn't vary much at all.