Piston speed and FE

[Ecky]

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.

[newbvetteguy]

Originally Posted by Ecky

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

[hat_man]

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.

[Ecky]

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.

[Piotrsko]

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.

[hat_man]

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.

[aerohead]

Originally Posted by hat_man

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.

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.

[Enki]

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.