Optimum piston speed?

[RH77]

Originally Posted by aerohead

Some guys at Bonneville are running series transmissions to get all kinds of gear splits,as their cam grinds and induction provide very narrow power bands,requiring very small rpm drops between gears.

It sounds like a CVT may be a good candidate for such a project. Barring any mechanical inefficiencies of this arrangement, programming the infinitely-variable band-set to a vehicle's exact aero vs. power output could get the job done efficiently.

The problem is finding a "spare" CVT lying around -- so cost would likely be a limiting factor. Also, I haven't heard of re-programming one, but that doesn't mean it isn't possible

RH77

[aerohead]

All I would know to do,is look into the Subaru Justy.I think Nissan launched a CVT this year,it's part of their "wave" marketing promotion.Don't know if thats a FWD or RWD setup.Theoretically,the CVT should be the "hot-ticket" for mpg.I don't understand the disappointing numbers for the Subie.On a side note,I believe Bombadier uses a CVT in their snowmobiles.Other manufacturers may also.I older variable air volume HVAC systems,variable sheaves were used to very the rpm of air-handlers.Rubber v-belts eat alot of power,but the CVT and it's steel belt is supposed to cancel this effect out.Let us know what you find,and good hunting!

[metromizer]

I recently read an article on advanced automotive transmissions, that essentially said CVT are high maintainance, unforgiving, soak up power and are noisey, and that several companies were dropping or considering dropping them as an option... at the same time, the author said they were great at keeping the engine operating in it's sweet spot rpm.

you guys are right about snowmobiles and HVAC equipment, don't leave out the famous Bridgeport milling machine with vari-drive. Most of the automotive units use constantly lubricated steel chain instead of rubber belts, according to that article.

My Metro could easily make use of a 6 or 7 speed gearbox and taller final drive. I've played with gearing a fair amount, it seems a relatively low powered engine or high weight vehicle 9for the power it has) has the same 'feel' as narrowing the torque band of the engine.

[Frank Lee]

Because my head is the thickest part of me, I still haven't fully wrapped it around this piston speed thing. So I did a lil lookin' at some numbers:

Tempo seems to be geared spot-on for the 1000-1200 ft/mn piston speed theory, as that calc's out to 50-60 mph. It was made in the era of the national 55 mph speed limit, so that makes perfect sense.

Gold Wing 1100 has 1297 piston speed at 55 mph- winding out a little fast? It always did give me the impression of "wanting" a slightly taller gear. 1000-1200 ft/mn gives a calc'd 42-51 mph cruise speed. Makes sense too, I can see the bike getting way better fe at those speeds vs 55. It's got horrible aero and a very lossy drivetrain too. I wonder how much it would benefit with a higher final drive unit that pulls the ft/mn to, say, 1100 at 55?

Honda Super Cub 50: Interestingly enough, the stats I found for max fe (343 mpg) said it was tested at 19 mph which calc's out to 1,168 ft/mn piston speed- nicely within the 1000-1200 parameters even for such a small one-lunger! And this engine is capable of a 9500 rpm redline (?) which is 2580 ft/mn.

1994 F150: OK now I'm baffled. The above three seemingly agree with the piston speed theory. But this truck has a calc'd engine rpm of 1500 @55 mph; with the 3.0" stroke that's 825 ft/mn piston speed- way too slow according to theory. Calc'd speed for my F150 at 1000-1200 ft/mn is 73-88 mph! I know for a fact the fe on that truck plummets like a lead balloon above 60 mph and in fact usually I cruise it at 52-55. Would it benefit from MORE rpms via a gear DOWN?

[gone-ot]

...see Fig. 10-7, on page 247, of "The INTERNAL-COMBUSTION ENGINE, 2ed., 1961, International Textbook Company, by Professors C.F.Taylor and E.S.Taylor of MIT:

Fig. 10-7. Volumetric efficiency vs. mean piston speed and Z of the MIT geometrically similar engines under similar operating conditions.

...it's a graph showing that Vol. Eff. "peaks" at piston speeds between 1000 and 1500 fpm, which corresponds to Mach index air-flow Z numbers of 0.3-0.4-0.5.

...Z number is tied into both piston speed and valve size (ie: trying to suck/push air past/though changing valve areas), which gets into Mach index (Z) and maximum air speeds...but, piston speed 'starts' every thing.




addendum: see posting #53 below

[Phantom]

I just wanted to add a bit on CVTs and clarify they are NOT infinitely-variable-transmissions as the marketing would like you to believe, they are continuously-variable-transmissions. Whats the difference? An IVT would have to be able to expand and shrink the spools that the drive chain runs on and be able to do this infinitely. A CVT actually is only infinitely variable on a continuous scale between the max and min ratios so if the range is from 3.5:1 to .6:1 the CVT can only run ratios between those points.

Also CVT is not always best for the job look at the 2005 Ford 500 the CVT was marketed as being more efficient and having better acceleration than the same car with a automatic 5-speed transmission and thats why the other transmission option was a auto 6-speed that had better MPG and acceleration not by much.

[TheEnemy]

Originally Posted by Old Tele man

...see Fig. 10-7, on page 247, of "The INTERNAL-COMBUSTION ENGINE, 2ed., 1961, International Textbook Company, by Professors C.F.Taylor and E.S.Taylor of MIT:

Fig. 10-7. Volumetric efficiency vs. mean piston speed and Z of the MIT geometrically similar engines under similar operating conditions.

...it's a graph showing that Vol. Eff. "peaks" at piston speeds between 1000 and 1500 fpm, which corresponds to Mach index air-flow Z numbers of 0.3-0.4-0.5.

...Z number is tied into both piston speed and valve size (ie: trying to suck/push air past/though changing valve areas), which gets into Mach index (Z) and maximum air speeds...but, piston speed 'starts' every thing.

Wouldn't that also depend on factors such as your intake runners, cam and such and what those parts flow best at?

[gone-ot]

...fundamentally, it's all about the "air" and it's maximum speed Mach index Z-number, so anything affecting "air" flow IN and OUT is a determinant.

...cam, timing, lift, valve dia, rate of lift, etc., intake & exhaust gas velocities through orifices (valves).

[TheEnemy]

Thats MACH index as in speed of sound?

[gone-ot]

Originally Posted by TheEnemy

Thats MACH index as in speed of sound?

...yep.

...quoting from page 245 of Taylor & Taylor (Compressible Flow Through a Fixed Passage, in Chapter 10):

"Inlet-Valve Mach Index. Some years ago (10.41) experiments were made under the author's direction to determine if some easily evaluated velocity could be used for µ in Eq. 10-17. The velocity finally chosen was the following:

µ = (Ap·s)/(Ci·Ai)

where Ap is piston area, Ai is nominal inlet-valve opening area, and Ci is the inlet-valve flow coefficient based on the nominal area Ai. It will be noted that the resultant value of µ is the velocity of an incompressible fluid through an opening of area Ci·Ai into a cylinder of piston area Ap with piston velocity s. Using this nominal velocity, the ratio µ/a in Eq. 10-17 can be written:

µ/a = (b/Di)^2 × [s/(a·Ci)] = Z

...where:
b = cylinder bore;
Di = inlet-valve diameter, the nominal inlet-valve area being taken as (PI·Di^2)/4;
s =mean piston speed;
a = inlet sonic velocity;
Ci = inlet-valve average flow coefficient."

"...the variable, µ/a, refers to the Mach index [Z] through the controlling flow section, which is usually the inlet-valve opening."

"...Mach index (Z): Z = (b/Di)^2 × [s/(a·Ci)]"

...and, lastly, from pages 248 and 249:

Page 248: "Effect of Piston Speed. Where piston speed is the only variable, the effect on volumetric efficiency is through Z, which is directly proportional to piston speed."

Page 249: "One important relation is observable...namely, that volumetric efficiency starts to fall off rapidly as Z exceeds 0.5 to 0.6."

"...from the aforementioned considerations...the inlet-valve size and design should always be such that Z will not exceed 0.6 within the operating regime of any engine."

...and, similar, but higher order limitations (due to higher pressure and gas temperature) occur getting exhaust gas through/past the exhaust valve.


addendum: see posting #53 below