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Re: What creates engine rpm?
[Re: mopar_man]
#898370
01/13/11 02:30 PM
01/13/11 02:30 PM
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Joined: Dec 2006
Posts: 3,533
Indiana
Fury Fan
master
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master
Joined: Dec 2006
Posts: 3,533
Indiana
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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.
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Re: What creates engine rpm?
[Re: Fury Fan]
#898371
01/13/11 02:57 PM
01/13/11 02:57 PM
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Joined: Oct 2005
Posts: 8,879
Ontario, Canada
Stanton
Don't question me!
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Don't question me!
Joined: Oct 2005
Posts: 8,879
Ontario, Canada
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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.
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Re: What creates engine rpm?
[Re: Stanton]
#898372
01/13/11 04:26 PM
01/13/11 04:26 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Re: What creates engine rpm?
[Re: pressureangle]
#898374
01/13/11 05:30 PM
01/13/11 05:30 PM
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Joined: Oct 2005
Posts: 8,879
Ontario, Canada
Stanton
Don't question me!
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Don't question me!
Joined: Oct 2005
Posts: 8,879
Ontario, Canada
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Quote:
Sorry, it's a fact. (I'm an ASE Master Heavy Truck tech, and worked the past 3 years for Navistar Defense) It's true, modern diesels (and likely some modern gas engines) can detect misfire through time change in the crank sensor.
And yes, if you remove enough counterweight from a single cylinder engine, it stalls more easily and requires a higher idle RPM. Have a look at model airplane engines, the old COX .049 has no flywheel or counterweight at all.
Nobody's arguing that a misfire will slow down a motor - put enough misfires in sequence and its called "killing the ignition" and yes, the motor WILL slow down and (I'm going out on a limb here !!) eventually come to a complete stop.
And I won't argue that if you remove the flywheel and any counterweights you will also see the crank slow down. But let's be practical and talk about common, everyday engines in normal operating modes. Any properly running engine will have absolutely no detectable loss of speed between firing stokes.
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Re: What creates engine rpm?
[Re: Fury Fan]
#898376
01/14/11 09:44 AM
01/14/11 09:44 AM
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Joined: Jan 2006
Posts: 8,162
USA
360view
Moparts resident spammer
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Moparts resident spammer
Joined: Jan 2006
Posts: 8,162
USA
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Quote:
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.
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.
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.....just shows how fat these motors really are.................
.....pretty simple question but not so simple to answer 
