Piston engine and the relationship with its prop

bomber
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Joined: Mon Nov 30, 2015 3:40 pm

Piston engine and the relationship with its prop

Postby bomber » Sat Nov 12, 2016 5:02 pm

Lets first understand the inherent characteristics of a piston engine...

PISTON ENGINES SUCK.

This is not an opinion but instead a description of just what they do... they suck fuel and air, know as the 'mixture'.
Take the air filter off the carb, put your palm on the opening to the throttle chamber and you can feel this suction.

Why then do they have a fuel pump if they suck ?

The answer is, what you don't want is a direct line from the fuel tank to the engine, so they interrupt the suction with a float chamber..

The second thing to understand is that there's no top RPM for a piston engine, its inherent behaviour is to 'run away'... more mixture it sucks the faster its rpm, the faster it's rpm the more mixture it sucks and the faster its rmp etc... until it runs out of fuel or breaks.

Now the Horsepower of an engine is also linked to it's rpm, so there's actually no limit to the horsepower of the engine if it never broke due to mechanical tolerances.

Yet I've got a 100HP engine here so how come ?

Yes you have... it's rated at 100HP @ 2750rpm...... if you rev it to 3000rpm it's a greater horsepower say 110HP

So why have I another engine over here that looks the same but is 120HP @ 2750rpm ?


Well the cylinder sizes are slightly bigger so it sucks in more mixture. more fuel = bigger bang = more energy released = more horsepower for same rpm.

So. we now understand that an engine will gobble up an ever increasing amount of fuel/air mixture until it explodes and the throttle is there to stop this, not to feed it.

Ok that's the first time you've mentioned throttle, where'd you get that from ?


Well the throttle allows the engine to suck fuel not from the tank but from the float chamber and this float chamber is supplied by a pump with a 'relatively' fixed flow rate.

When are you gonna get to brake horsepower ?

So I'm running an engine at 2750rpm and the throttle is hardly open because the engine will do a lot more rpms than that if I open up the throttle and let it suck what it wants to suck...
There's no load on the engine, just a spindle with a mechanical brake on it like a bicycle brake... and rather than use your hand we'll attach a horse and get it to pull (no horses were hurt in this exercise). The RPM's stay the same so we attach 10 and the rpm drops, no worries we open the throttle a bit more until the rpm's are back at 2750rpm. So we continue adding more horses and opening the throttle until the throttle is fully open at which point we know that the engine is sucking all the fuel out of the float chamber at the same rate as the fuel pump is filling the float chamber. Now the number of horses that are used in braking this engine is it's brake horse power rating.

who picked the 2750rpm ?

Oh that's the engineers who designed the engine... they're just scared that if you run the engine continuously at any faster rpm that it'd spring an oil leak or explode and brake their little baby. These guys will continue to improve their machining skills such that the engine can be run at a higher rpm... thus more horsepower.

But that doesn't explain when I'm flying why I'm seeing 170hp on an engine that's rated at 100hp at full throttle and doing 2750rpm.

I'll be back later to answer that...
"If anyone ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me - it's all balls" - R J Mitchell

bomber
Posts: 834
Joined: Mon Nov 30, 2015 3:40 pm

Re: Piston engine and the relationship with its prop

Postby bomber » Mon Nov 14, 2016 4:40 pm

updated...
"If anyone ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me - it's all balls" - R J Mitchell

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IAHM-COL
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Re: Piston engine and the relationship with its prop

Postby IAHM-COL » Mon Nov 14, 2016 4:43 pm

my brain is turning....
R.M.S.
If we gave everybody in the World free software today, but we failed to teach them about the four freedoms, five years from now, would they still have it? Probably not, because if they don’t recognise their freedoms, they’ll let their freedoms fall


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