Piston speed and FE
 

Hello everyone,

I know I have seen this term used here before, but I'm not sure how it relates to FE, how to calculate it, or how to use that calculation to help me improve. Can anyone give me a tutorial? If you need any specific information about what vehicle, it would be Oh Deer in my signature. I cruise at just a hair over 2k RPM at 55 mph.

Thank you for any help.

Nor I.

I can say that bore and stroke determine displacement, and stroke and RPM determine piston speed.

Destroking an engine lowers the compression ratio all else equal. Stroking an engine improves low end torque.

It's complicated by things like wrist pin offset.

Back when I was street racing (oh the horror!) There was a semi physical limit on how fast a piston should travel and not self destruct. IIRC, it was mostly about skirt wear and rod or wrist pin strength. Not much related to efficiency. If you know displacement and rpm you can calculate volume and perhaps mileage but that would be above my pay grade

The line between racing and ecomodding gets blurry, especially above 200MPH. :)

Just found this......

https://ecomodder.com/forum/showthre...peed-1477.html

Now to read it all. Any other insight would be appreciated.

surface feet per minute
 

During transient loads between idle and WOT, the percentage of heat energy supplied, and the net power extracted varies with rpm.
Exhaust, Oil, & Miscellaneous heat rejects in lock step with rpm.
Cooling system follows it's own path.
Hydrodynamic shearing losses in the oil for sliding and rotating components increase with rpm.
Pumping losses fall with rpm, at minimum at WOT.
An old metric was to operate an engine at an 80% load factor to achieve the highest operating efficiency ( 40% thermal efficiency ) which tended to occur where torque peaks.
Piston speed has to do with hydrodynamic losses created as the reciprocating piston rings ride the cylinders. The 'speed' has to do with their averaged surface feet per minute travelled. Drag varies with the square of the velocity. Power to overcome the drag varies with the cube of the velocity. Just like in air with the car's body.
As an air pump, if you double the engine's rpm, you essentially double the gross horsepower, but at the cost of geometric increases in internal losses, so your 'net' horsepower falls off geometrically as it's trying to increase.
The rpm for the engine's rated torque is still probably a decent metric for determining a 'cruise' rpm.
Turbocharging artificially increases the apparent displacement of an engine, allowing really 'flat' torque curves, and at very low rpm. A 'two-fer.'
Electric motors are a 'three-fer.' All available torque is available at all rpms, from zero on up.

Just did the calcs for my CRV 1.5L turbo. At 2000 rpm I get 1173 ft/min piston speed (2 x 3.52 x 2000 x 60/720 = 1173). The original formula was for ft/sec so multiply by 60 sec/min to get final result. This seems to show that 2000 rpm for my engine is "ideal".

I'm not very good at using the Quote function, so this is a cut,copy,paste from the thread I linked above.

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.

If anyone has read the book cited in that thread I have a question......

It gives a range of of 16.4-19.8 ft/sec for best FE. Is this linear, with 19.8 ft/sec being "best of the best" and 16.4 ft/sec being "lowest of the best".....OR......is it more like a Bell Curve, with the middle (18.1 ft/sec) being best and tapering down both sides to both 16.4 and 19.8 being "lowest of the best" ?

Also, I'm assuming that the formula is 2 x ( stroke in inches) x RPM and then that whole quantity being divided by 720 rather than 2 x (stroke in inches) x (RPM/720).

feet/second
 

Check my numbers please:
Stroke = 3.52"
TDC to BDC to TDC ( or vice versa/revolution )= 2X 3.52"/ rev= 7.04"/rev
7.04" X 2,000 rpm = 14,080"/minute
14,080" / 12"/foot = 1173.33 feet/minute
---------------------------------------------------------------------------------------
If you're looking for feet/second, and there's 60-seconds/minute, wouldn't we divide by 60?
1173.33/60= 19.55 ft/sec?
I couldn't find the material attributed to 'Taylor'.

The Wallace Racing site has a simple online calculator that let's you submit the engine stroke and RPM and it will spit out the mean piston speed here: ww.wallaceracing.com/calc-pistonspeed.php

-I can't post links yet, so add one more W to the beginning of that and copy/ paste it.


These simple calculations only look at the MEAN /AVERAGE piston speed, though the piston velocity at each crank angle changes, though and depends upon the rod length / rod ratio, too.

A lot of the old rules-of-thumb of safe max piston speed are just that and based upon old assumptions on materials and things like piston weight, and modern materials are moving the actual limits faster and faster.


When it comes to fuel economy and balancing fuel economy and performance things get murkier; you can go with a larger cylinder bore and keep piston speed the same and you'll make more power from having more cubic inches, but the larger bore makes for a wider combustion chamber that requires more ignition advance / a burn that takes longer to accomplish and you'll have more negative work and reduced efficiency from that wider chamber. You could instead choose to go with a longer stroke and the same initial engine bore and you'd have more efficient combustion because of the more compact chamber -this would increase the piston speed / FPS at the same RPM as the original unstroked engine.

Increased stroke AND increased bore size increase the friction between the rings and the cylinder walls and oil ring friction is the largest source of friction in an internal combustion engine.

IMO, Increases in stroke or bore should be paired with thinner piston rings to reduce friction.

With wedge-type heads that have a decent quench area and quench distance, the increase in piston speeds increases the max quench velocity which can increase the detonation resistance of an engine and allow higher "dynamic" compression / cylinder pressures and reduced friction + the ability to support higher max cylinder pressures without detonation can together help increase fuel economy. -Low RPM eco-focused SBCs have particularly poor quench velocity at their low RPM torque peaks where knock / detonation risk is highest and rebuilding with a tight quench and more piston speed can help them to run with more cylinder pressure / compression on a given octane fuel.


Adam