2000 rpm

[Big Dave]

For me, 2000 RPM in top gear would have me going about 85 MPH. I cruise 1700 RPM = 70 MPH. 1450 RPM = 60 MPH.

2000 RPM is considered the magic MPG engine speed for these school bus motors.

[almightybmw]

Dave made a point I was just thinking about. Diesels. The newish QSB3.3T cummins makes 300+tq at 1400rpm.

Question: peak torque /= best FE?
So does that mean lower the rpms to get better or raise them? Raising just injects more fuel, so that can't be right. From what I see, a 600rpm --> 2800rpm range with peak tq from 1400rpm on up seems to me that lowest rpm for highest given tq would produce ideal results. All of this of course depends on the body the engine propels.

No mean to get crazy, but this topic seems geared towards gas motors and their efficiencies. Pretty sure diesels work differently despite the ICE similarity.
Thoughts? Comments?
Someday I'd like a 3.3 in my Sonoma. I know it can be done, but the engine is the hard part; expensive and not easy to find.

[Daox]

Check the BSFC charts is all I got to say. They really say it all.

[PaleMelanesian]

+1. BSFC charts showed me the way and turned on the "Aha!" lightbulb.

[some_other_dave]

The plain BSFC charts are great for showing you what RPMs you use to accelerate, when the aim is to add energy to the car (in the form of velocity) in as efficient a manner as possible. I believe that the peak BSFC island for gasoline-powered cars is at a lower RPM than the peak torque is, just from my experiment with accelerating to 2000 RPM versus accelerating to 2500 RPM.

Cruise is a different matter. You would need to figure out how much power is required to run at a given cruise speed (and assume a flat road, no wind, etc.), then turn that into torque at a given RPM. Then you could plot that on the chart, and repeat for different speeds. That may give you an idea of what speed is most efficient at cruise for that given car/engine/gear combination.

If you want to change the overall gearing, then you need to re-figure the torque/RPM curves for each change of gearing. And, of course, the moment you encounter any wind, or go up hill or down hill, the whole "power required to maintain a given speed" has to be re-figured...

All that said, I still think that the most efficient cruising RPM for any gasoline-powered car out there is "as low as you are comfortable with".

Diesels have different torque characteristics, and different BSFC maps, but I wouldn't be surprised if their results were similar--that peak efficiency is almost always at lower RPMs than peak torque.

-soD

[PaleMelanesian]

Originally Posted by some_other_dave

The plain BSFC charts are great for showing you what RPMs you use to accelerate, when the aim is to add energy to the car (in the form of velocity) in as efficient a manner as possible.

Exactly. And this is why Pulse & Glide works. When you are using the engine, it's at its peak efficiency. The rest of the time it's off / idling, and using minimal fuel.

I believe that the peak BSFC island for gasoline-powered cars is at a lower RPM than the peak torque is, just from my experiment with accelerating to 2000 RPM versus accelerating to 2500 RPM.

That has been my experience as well - lower than peak torque rpm.

Cruise is a different matter. You would need to figure out how much power is required to run at a given cruise speed (and assume a flat road, no wind, etc.), then turn that into torque at a given RPM. Then you could plot that on the chart, and repeat for different speeds. That may give you an idea of what speed is most efficient at cruise for that given car/engine/gear combination.

I rarely / never cruise at steady speed. P&G all the time. My car's most efficient steady cruise speed is somewhere around 35 mph. I hate that it's so crippled with gears that run the rpm so high.

All that said, I still think that the most efficient cruising RPM for any gasoline-powered car out there is "as low as you are comfortable with".

Amen to this one!

[superchow]

Rearanging that formula to output RPM...

(720 * Piston speed) / 2 * Stroke in inches = RPM

Honda Civic 1.8L stroke = 87.3mm = 3.437
Ideal piston speed (for most engines) = 16.4 to 19.8 ft/s

Solving...

@ 16.4 ft/s, RPM = 1718 = 39.3 mph
@ 19.8 ft/s, RPM = 2074 = 47.4 mph

These numbers coincide with my high (successful) FE attempts. Now I just need to find that darn BSFC chart for the R18A... *sigh*

p.s.: Someone with a diesel engine shout out their speeds at a given RPM - then we can figure out why diesels can run so slow and still get their awesome FE. Logic would dictate a longer stroke than gasoline engines to stay within the "best" piston speed range.

[PaleMelanesian]

Huh. My 1.6L civic has a 90mm stroke (according to one reference... ). So the 1.8 is bigger in bore but not in stroke.

[superchow]

Just checked VW 2009 Jetta 2.0L TDI engine's stroke. 95.5mm

Solving for RPM @ ideal piston speed = 1570-1896 rpm.
Max. torque is listed at 1750 rpm.

Therefore, the engine feels gutsy/torquey in its ideal piston speed range! Don't know what that would translate into on-the-road speed...

[superchow]

Originally Posted by Big Dave

For me, 2000 RPM in top gear would have me going about 85 MPH. I cruise 1700 RPM = 70 MPH. 1450 RPM = 60 MPH.

2000 RPM is considered the magic MPG engine speed for these school bus motors.

Big Dave - if you have the 5.4L V-8, then your ideal range should be 1419-1713 rpm. If you are already at ideal engine speed going 60, then going slower would technically hurt your mileage, right? 60 creates a significant amount of aero drag. So unless you shorten your tallest gear to go 45-55 you are damned either way.

Either engine/gearing inefficiencies or aero drag.