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

I'd like to add that mean piston speed, peak piston speed, and piston speed during the first few dozen degrees after TDC (probably most important to us), will all vary based on engine geometry. An engine with a larger rod ratio (reduces sideloading and friction) will increase the dwell time at TDC (possibly good for thermal efficiency?), after which the piston will accelerate downward at a greater rate, compared with another engine that has the same bore and stroke but shorter rods.

I've never really tried to wrap my head around all of the variables at once.

What's really fun is playing with Quench Velocity calculators and seeing how increases in quench velocity increase the turbulent burn speed and reduce the ignition timing advance required. -Quench really is "free octane booster in every tank" and the opportunity for free MPG gains, if planned for.

Right now I like the idea of being able to push the limits of cylinder pressure, both for power and fuel economy by using things like quench and tight control over air and coolant temps to eke out more power and fuel economy for more of the RPM band, anyway. (My engine has a strange aftermarket VERY long-runner version of a TPI intake and detonation around the torque peak becomes an issue, but lots of small tweaks and advanced electronic strategies can help push the peak torque and MPG a bit further.)

-I'm learning why the OEMs with DI have a huge advantage being able to use 100% of the cooling capacity of the fuel to cool the chamber- AND they can time it perfectly and stratify the charge to make for ultra lean AFRs... I'm jealous of the control they have, but I'm stuck trying to make my port injection behave as much like DI as I can.


Adam

I appreciate all the information, but I don't want to get so focused on the fine details that the bigger picture is lost. Things like metallic make up of blocks, pistons, and rings or the differences in pistons speed at TDC/BDC compared to mid stroke are great information and again I very much appreciate it, but for the average coroplast and duct tape Ecomodder it might be information overload. I would hate to try and figure the ROI from changing ones bore or stroke or replacing piston ring with something thinner. But using formulas like this one for piston speed as it relates to RPM (and by extension FE) and one to show RPM at any given speed/gear/tire size , someone could find out that driving in a different gear (manual transmission) might return better results than how they currently drive. Some SIMPLE math might help tighten up the nut behind the wheel, so to speak. Or, for the adventurous, finding a rear gear or tire size change that "dials in" their vehicle for better FE.

Again the information on the minute details will probably help someone, but keeping the bigger picture in focus might be for the greater good.

I worked up some numbers and have a couple of questions but I'll post those later tonight. If anyone want to jump in and help feel free. Thanks.

I don't think I've ever driven an inline engine that got better economy in a lower gear, at any speed. I'm of the opinion that the devil really is in the details with engine geometry though. The only hard and fast rule I can think of, is that you're best off getting the smallest engine and the tallest gearing, because the exceptions are exceedingly rare.

If you're building an engine, gains are probably more accessible through cam changes than bottom-end work.

I would think some of the big inline 8's might have better economy than the early V engines, but really have no experience or data there. Kinda apple orange comparison though.

induction change is the easy stuff, volumetric efficiency is tough and pricey.

I'm going to assume that the given range of 16.4-19.8 ft/sec is plotted as a bell curve and will have the midpoint (18.1 ft/sec) as ideal and giving diminishing returns down both sides. Please correct me if I'm wrong.

The stroke on my Ranger is 94mm (3.7 inches)
My RPM's at 55 MPH are just over 2K (I'll call it 2050)

Putting those numbers through the PS formula, I get right at 21.0 ft/sec. According to the linked thread this is too high. If 18.1 ft/sec is ideal, then I would need to drop my cruising RPM's down to the 1760ish range. 16.4 ft/sec is around 1600 rpm and 19.8 ft/sec is around 1940 rpm.

If all my assumptions and math are accurate, it looks like I would need to drop to a rear axle gear of 3.55 and keep my slightly oversized tires to run 55 MPH at 1790 rpm.

Any insight or advice would be greatly appreciated.

bell-curve
 

I don't believe that it's a purely Gaussian distribution. Some engine parameters vary linearly with rpm, while some geometrically.
So the curve would be 'skewed', just as in aerodynamics / hydrodynamics.
You need the BSFC map. That will tell you where the island of highest efficiency is, and the parameters associated with that island. If you can get the engine to that 'load', at that rpm, your 'ideal' surface-feet-per-minute with the piston rings will be achieved by default.
My grandad's 1961 Dodge D-100 got 11-mpg at 50-mph,and a top-speed of 50-mph, with bias-ply tires, three-on-tree 1:1 transmission, and 4.56:1 rear axle.
By arbitrarily swapping in a 1977 Dodge D-100, 4-speed overdrive, and 3.50 rear axle, plus all-season steel radials, she happened to go to 16-mpg, at 65-mph.
Then the aerodynamics pushed her to 21.5-mpg at 65-mph. Nearly a 'doubling' of her original mileage. And an indicated 100-mph, between White Sands, New Mexico and the missile range.
There was no 'science' involved. It happened to work out as HOT ROD Magazine implied it might. With a lot of $$$$$$$$$$$$$$ I could probably 'optimize' her performance, but that's outside the scale of the project.

Have some somewhat useful anecdotal evidence, despite current condition of my car.

The core engine mods:
Destroked from 2.26 L to 1.99 L
Long custom rods (ratio is 1.92:1 IIRC; I would reduce this to 1.8:1 or so)
High compression pistons (12.3:1)

On the head:
Custom cams, but I don't think they were done properly
Bowl work
Gasket matching and porting on the exhaust side

Others:
Modified stock intake manifold
Tubular exhaust manifold
Larger turbocharger
Larger turboback exhaust
Full E85 for fuel on stock injectors

More info:
The rings that came with the pistons aren't great, and the ringlands might also be damaged due to testing I shouldn't have done for the company that makes the tuning software I use.

Currently, I'm getting better than stock MPG (~25-28) on full E85, while the engine requires two quarts of oil per tank of fuel due to low compression (lots of smoke) so I'm sure it would be even better if I wasn't wasting cylinder pressure.

I think the compression ratio is what really gets the most bang for the buck as far as MPG goes, but if your only choice is to build the engine around the gearing for your transmission and the driving you do, I think you *can* optimize your rod ratio to hit the sweet spot of BMEP VS RPM that way (combined with other mods, of course). Kind of an expensive way to do it, but seems at least feasible.