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Old 03-19-2008, 08:52 PM   #1 (permalink)
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Optimum piston speed?

I don't recall where, but someone once provided a link to an engineering text that mentioned 1000-1200 ft/mn as optimal piston speed for economy.

My question is, is that true and why is it?

Why wouldn't, say, 700-840 ft/mn be even better?

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Old 03-19-2008, 09:55 PM   #2 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
I don't recall where, but someone once provided a link to an engineering text that mentioned 1000-1200 ft/mn as optimal piston speed for economy.

My question is, is that true and why is it?

Why wouldn't, say, 700-840 ft/mn be even better?
I recall the same. At the time, I did the calcs for my engine and came up with the same RPM at which my best, constant FE is supposed to occur, which it, indeed, does:

1714 RPM @ 42.5 MPH (4th gear, Full TC lockup)

Here is a link to an article on it.

The text may hold some advice on the slower speed question.

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Old 03-19-2008, 10:45 PM   #3 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
Why wouldn't, say, 700-840 ft/mn be even better?
Did you suggest that specific range for a reason?
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Old 03-19-2008, 11:30 PM   #4 (permalink)
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is there a calculator to determine the best speed for fuel economy? if so let me at it!
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Old 03-20-2008, 12:09 AM   #5 (permalink)
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Here you go;

As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s). Piston speed (ft/s)= 2*stroke(inches)*rpm/720.
OR:
Piston speed = 2 x Stroke in inches x rpm / 720


Please keep in mind this is only a rule of thumb and specific situations will alter the "rule".
That said it is a good starting point.

The source by the way is Taylor as linked above.
A good book too since it has all the necessary info in one place and is more up to date than many others including Judd and the now classic Ricardo publications.

Cheers , Pete.
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Old 03-20-2008, 12:30 AM   #6 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Did you suggest that specific range for a reason?
I picked 700 ft/mn out of thin air, and went with a range of 20% as in the 1000-1200.
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Old 03-20-2008, 01:05 AM   #7 (permalink)
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It's a figure relating displacement to engine speed such that for relatively (+/-10%) square engines efficiency is pretty good.

Last edited by roflwaffle; 03-20-2008 at 01:11 AM..
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Old 03-20-2008, 03:56 AM   #8 (permalink)
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The text linked also has the volume 1 (under 'other editions') with nearly the whole discussion on efficiency. It centers on the efficiency graphs of piston speed vs mean effective pressure. They seem to orbit around the highest efficiency point, but mainly on MEP, not piston speed. So MEP is the key more than speed. At low MEP, lower speeds seem to hurt efficiency.

The Sulzer RTA96C that showed up as the 'most powerful diesel engine' is also quite efficient. It's rated at 92 to 102 rpm, with a 2.5 m stroke, listed 8.5 m/s (1670 ft/min) piston speed but it must be average speed. The highest efficiency is the high speed, lowest horsepower rating. Cross heads don't have much in the way of side pressure problems, but surely they don't have bore size or weight limits either. They must have chosen high piston speeds for efficiency.
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Old 03-21-2008, 01:27 AM   #9 (permalink)
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I would think that as slow as possible piston speed would yield the highest efficiency. After pondering it for a while maybe this ideal piston speed is best used to determine the bore to stroke ratio. After all if your piston speed is below what they consider optimum, you coulda designed you motor with more stroke and less bore for the same displacement.
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Old 03-21-2008, 04:44 PM   #10 (permalink)
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I wonder why an optimum rpm exists? The only things I can think of are:

Increased Inertial Loads at High Rpm?
Increased Friction at High Rpm?
Varying Disparity Between Piston Speed and Flame Front?
*(I know this set a limit on piston size during WWII @ ~5.5" Bore + Stroke)
Piston Position @ Peak Pressure?
Vibration?
Combustion Gas Dwell Time in Piston
Little Magic Elves Need Time to Do Their Work

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