What creates engine rpm?

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I suspect, even though it may be impossible to measure that the crank may not be travelling at a constant speed in one revolution as the piston speed changes four times every revolution. everybody agree?

Yep, it's true the speed changes throughout each revolution. Engines with greater # of cylinders get more 'bumps' per revolution and that's what makes them feel smoother with less NVH (all else equal, of course).

No, it's not impossible to measure. Truck diesel engines have increasingly-strict emissions control, and the crankshaft sensor can detect a void in acceleration of the crank due to a misfire, and then determine which cylinder it was.

I agree with Fury Fan.

With the right oscilloscope,
torque sensors,
rpm sensors and
pressure gauges inside the cylinder
you can see all the slight changes in rotation,
even with heavy counterweight
and flywheels.... even with Mazda Wankel design engines.

The NVH (noise vibration and harshness) engineering teams
at all the major automakers
do this routinely
even out on the
'convoy style' road trips you see a group of new vehicles making,
such as out at Death Valley National Park.

People think of crankshafts
as massive and so stiff they resist twisting completely,
but it is more accurate to think of them like big 'Chrysler Torsion Bar Springs'
because they not only twist
but they are constantly oscillating back and forth in very complex ways.

Posted By: DrCharles

Re: What creates engine rpm? - 01/08/11 02:17 AM

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Or are we asking what causes the rise and fall of rpm.

That is how I interpreted the question... open the throttle, more air comes in, which causes more fuel to go in, etc.

but is the engine mounted on a treadmill

Posted By: 69CoronetRT

Re: What creates engine rpm? - 01/08/11 02:26 AM

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the starter creates rpms by turning the engine over and starting it,as it has no rpms till the starter turns it first

I agree. Nothing 'creates' the RPM, which was the original question, within the engine except the starter. Everything after that just regulates it.

Posted By: Keith Black®

Re: What creates engine rpm? - 01/08/11 02:39 AM

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It's too simple, the revolution of the crank "creates" rpm.

I've changed my mine I reckon he is right.

Posted By: bboogieart

Re: What creates engine rpm? - 01/08/11 03:01 AM

What creates engine rpm?
This is what I read. The rest pertains to internal combustion, except the starter response.
But does an electric motor have rpm, how about a steam turbine, wind mill? Even what increases or slows down rpm is not just combustion, but force against a shaft to cause or slow rotation.
RPM is still just a measurement, kinda like time not a physical thing.
Guess what kids, time travel is not possible.
How ever when I get behind the wheel..... Hmmmmmmmm.

Posted By: JoesMopar

Re: What creates engine rpm? - 01/08/11 03:06 AM

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Guess what kids, time travel is not possible.

Then I should show you whats covered up in my basement...hint...it's sitting beside my anti-gravity machine and my hydrostatic precipitator..

Posted By: Stanton

Re: What creates engine rpm? - 01/08/11 04:04 AM

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The engine is an air pump.
The pistons create the suction that draws the air into the pump. More air= more R.P.M's.

I know what you mean but the way you're stating it is totally wrong. Yes, the engine is an air pump. However, the pistons don't create suction and they don't "draw" air in. The pistons merely create a "void" - atmospheric pressure fills that void.

Posted By: 474218

Re: What creates engine rpm? - 01/08/11 04:27 AM

"You should not argue with you father"!

Posted By: Charger69RT8

Re: What creates engine rpm? - 01/08/11 10:25 AM

Sorry Stanton, but you need to rethink this. If your statement were true a vacuum would never exist in the intake. Which does also exist in a cylinder when the piston is in the downward stroke and intake valve open. The piston rings seal against the cylinder wall and as the piston moves down air (air/fuel) is "drawn" in. This is why manifold vacuum is higest when the butterflies of the carb or throttle body are near closed, restricting that draw of air in, and falls to very low levels when they are wide open.

I agree with the earlier post about combustion. While it is true that the starter begins rotation, combustion is the driving force that maintains that. I think the heart of what the OP was asking is what causes an increase in RPM's. Therefore, cumbustion as regulated by an allowed increase in magnitude through increased flow via the throttle plates and fuel metering is what causes increased RPM's.

An internal comustion engine will naturally increase rpm's infinitely (to its mechanical limits) as long as it has an ever increasing supply of air fuel mixture.

Posted By: mopar_man

Re: What creates engine rpm? - 01/08/11 01:46 PM

Not to be a smart azz, and I know what you mean in the o/p but in reality nothing creates RPM , rpm is a measurement of rotations of the crankshaft.

here's another little tid bit of info:
I suspect, even though it may be impossible to measure that the crank may not be travelling at a constant speed in one revolution as the piston speed changes four times every revolution. everybody agree?
as to your o/p , fuel and air or lack thereof will change the rpm in an engine .

Posted By: 360view

Re: What creates engine rpm? - 01/08/11 01:49 PM

Geeky.... but here goes:

At any given second,
the force on the piston top area
multiplied by the crank arm length out from the center of the crankshaft,
equals the friction force of the engine's parts
multiplied by that parts distance out from the center of the crankshaft,
plus
the torque load on the output shaft.

If any of the above 3 things changes
{1. piston area force, 2. friction, 3. torque load}
... then the RPM changes

The force on the piston top area
changes with the amount of air and fuel combusting above it,
and the time the combustion takes {Fast Burn}. The time is important because the crank arm length away from the crankshaft centerline is constantly changing,
going from zero at top and bottom,
to maximum at 90 degrees.

The friction changes with RPM.

It changes more than is commonly thought.

If you double the piston up and down speed, the friction of the piston rings and skirt goes up almost eight times.
If you double the crankshaft speed, the friction in the bearings almost goes up four times.
There is also a friction if a vacuum is drawn by the piston descending on the intake stroke,
and another friction if the exhaust system has any back pressure above zero that opposed the piston as it rises on the exhaust stroke.
As a combination of the above things,
it can be seen that friction in an engine is complicated,
but a 'rule of thumb'
is that if the rpm doubles
the internal friction will increase between 5 and 7 times.

The torque load on the output shaft is uncertain unless we are told exactly what the shaft drives,
but generally,
the torque load will be doing two things:
1. overcoming friction
2. accelerating some mass... either in a straight line or in a circle
Since friction is involved again,
we can guess that
this friction in the load will also change with RPM.

A steady RPM will happen
when the combustion force on the piston multiplied by the crank arm distance at that instant,
is exactly equaled by
the internal friction forces in the engine each multiplied by their little crank arm distances,
plus
the torque load on the output shaft.

If the force on the piston top times the instant crank arm,
exceeds the friction torque
plus the instant torque on the output shaft,
then the output shaft will
accelerate any mass connected to it
by increasing the output shaft RPM.

At idle RPM
torque on the output shaft is zero,
and average force on piston top times crank arm equals all the combined friction torques.

If you let an engine with nothing attached to its output shaft go to full throttle,
the RPM will continue to increase
until the friction torques
equal the piston top forces times the crank arm,
... with the exception that at some high RPM the valves will 'float' and less air & fuel will make it into and out of the combustion chambers above the pistons,
which will decrease the piston top forces
or,
something will break.

All of this is easier to understand,
for any size engine,
if the idea of
"Brake Mean Effective Pressure"
or
BMEP
is studied and used.

http://www.epi-eng.com/piston_engine_technology/bmep_performance_yardstick.htm

http://www.bmepfuelandtuning.com/html/what_is_bmep_.html

http://en.wikipedia.org/wiki/Mean_effective_pressure

Posted By: fox

Re: What creates engine rpm? - 01/08/11 03:06 PM

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Wouldn't the load through the transmission regulate RPM's also?

WOT RPM @ the end of first gear isn't the same as the beginning of 4th, even if the plates are completely open.

Some guy named OTTO!!!!

Posted By: 451 D*O*D*G*E

Re: What creates engine rpm? - 01/08/11 03:52 PM

Very nice descriptions and without naming names i am sure the answers above are correct.Yes my initial post was what makes the rapid increase in the rpm's..some may think dumbest thread this winter but i enjoed it..thanks for the replies and i wasnt arguing with him he brought this to me because he likes to ask guys this when there talkin cars at work and he walks off leaving them in deep discussion and it gets talked about for a couple days.

Posted By: 6PakBee

Re: What creates engine rpm? - 01/08/11 09:11 PM

What creates engine rpm?

You guys have waaaay too much free time on your hands.

Posted By: bboogieart

Re: What creates engine rpm? - 01/08/11 09:56 PM

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Very nice descriptions and without naming names i am sure the answers above are correct.Yes my initial post was what makes the rapid increase in the rpm's..some may think dumbest thread this winter but i enjoyd it..thanks for the replies and i wasnt arguing with him he brought this to me because he likes to ask guys this when there talkin cars at work and he walks off leaving them in deep discussion and it gets talked about for a couple days.

you are a chip off the old block. This one created a lot of comments.
I kinda figured what you meant, and was just playing with semantics.

So what we want to know now is what increases rpm.
combustion is the answer, how you get it to be more efficient is another question. Anyone else?

Posted By: Stanton

Re: What creates engine rpm? - 01/12/11 03:12 AM

Quote:

If your statement were true a vacuum would never exist in the intake. Which does also exist in a cylinder when the piston is in the downward stroke and intake valve open. The piston rings seal against the cylinder wall and as the piston moves down air (air/fuel) is "drawn" in. This is why manifold vacuum is higest when the butterflies of the carb or throttle body are near closed, restricting that draw of air in, and falls to very low levels when they are wide open

Vaccuum is merely the absence of pressure - atmoshperic pressure! The piston merely creates an absence of pressure and when the valve opens and exposes that area atmospheric pressure fills it - starting with whatever is in the manifold. Therefore you see manifold negative pressure - commonly referred to as "vaccuum". Regardless of how you look at it, there is no suction, merely the absence of pressure being repressurized by atmospheric pressure.

Posted By: charger440sixpak

Re: What creates engine rpm? - 01/12/11 09:53 PM

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What creates engine rpm?

Assuming a system predisposed to rotation (ie. designed to rotate), I would think the simplest general answer is 'to add energy to the system'. Once sufficient energy of the required type is applied in the proper manner to overcome the resistance in the system, rotation occurs.

Regulation of said rotation is controlled by some combination of only two things, the amount of energy supplied and/or the resistance within the system.

Posted By: scratchnfotraction

Re: What creates engine rpm? - 01/12/11 10:08 PM

I still say the starter,final answer

I will take door #3

Posted By: Mr.Yuck

Re: What creates engine rpm? - 01/12/11 11:08 PM

What creates engine rpm?

A heavy right foot....

thank you...I'll be here all night

Posted By: pressureangle

Re: What creates engine rpm? - 01/13/11 02:17 AM

No no no, you all have it all wrong.

a Clock, Duh.

Posted By: 360view

Re: What creates engine rpm? - 01/13/11 01:54 PM

Quote:

Vacuum is merely the absence of pressure - atmoshperic pressure! The piston merely creates an absence of pressure and when the valve opens and exposes that area atmospheric pressure fills it - starting with whatever is in the manifold. Therefore you see manifold negative pressure - commonly referred to as "vacuum". Regardless of how you look at it,
there is no suction,
merely the absence of pressure being repressurized by atmospheric pressure.

=======

This is so hard to get across on automotive forums!

The idea that differences in pressures creates inefficiencies on both the intake and exhaust of an engine is perhaps the most common misunderstanding,
perhaps because hot rod magazines spend almost no time writing about part-throttle operation of an engine.

In engineering classes,
when the
"loop-de-loops"
of the
pressure versus volume
inside the cylinder
graph
is explained,
and how to spot
usable power output
versus
wasted energy from friction
by noticing whether
the direction of the loop is either
clockwise or counter-clockwise
... it is of such simple beauty,
that students are usually kinda awe struck,
with a
" Wow! That's really neat! "
reaction from many.

They don't call it a Carnot engine for nothing:

http://en.wikipedia.org/wiki/Carnot_cycle

Posted By: 602heavy

Re: What creates engine rpm? - 01/13/11 05:54 PM

Shoot some nitrous (no fuel) into the the motor when idleing & see how it picks up.

.....just shows how fat these motors really are.................

Posted By: Fury Fan

Re: What creates engine rpm? - 01/13/11 06:30 PM

Quote:

I suspect, even though it may be impossible to measure that the crank may not be travelling at a constant speed in one revolution as the piston speed changes four times every revolution. everybody agree?

Yep, it's true the speed changes throughout each revolution. Engines with greater # of cylinders get more 'bumps' per revolution and that's what makes them feel smoother with less NVH (all else equal, of course).

No, it's not impossible to measure. Truck diesel engines have increasingly-strict emissions control, and the crankshaft sensor can detect a void in acceleration of the crank due to a misfire, and then determine which cylinder it was.

Posted By: Stanton

Re: What creates engine rpm? - 01/13/11 06:57 PM

Quote:

I suspect, even though it may be impossible to measure that the crank may not be travelling at a constant speed in one revolution as the piston speed changes four times every revolution. everybody agree?

You have the counterweights on the crank doing their part to maintain inertia. The piston may be changing speed as it changes direction but the crank is still rotating in the same direction. With the additional weight of a flywheel on the end of a crank I'd have to disagree and say the speed would be constant.

If your theory were true then it would be a miracle that single cylinder 4 stroke motors run at all.

Posted By: Anonymous

Re: What creates engine rpm? - 01/13/11 08:26 PM

Spark.