Interesting Hybrid - Nonhybrid comparison

[2TonJellyBean]

Originally Posted by Bennet Pullen

The real lesson here is that taller gear ratios = better gas mileage. They also deliver worse acceleration characteristics, which is why you guys are right, they need more gears! I don't think that has anything to do with a sweet spot of piston speed though.

You make a strong case for the wide ratio CVT.

[rh77]

Originally Posted by 2TonJellyBean

You make a strong case for the wide ratio CVT.

Very true, but then the argument becomes, "Does the CVT use more energy to operate than a traditional automagic"...

One of the car mags did a mileage test with a Nissan model with a CVT and one with a traditional automatic -- the auto was slightly more efficient. Probably not a proper test. More data is needed, I s'pose...

Also, I have nightmares of hypermiling a CVT (probably no EOC, even Neutral coast with the engine on = not sure about longevity, no bump-start...)

RH77

[northboundtrain]

I just found this: http://www.isuzuengines.com/products...=5&model_id=13 It's some specs for the 7.8 liter Isuzu 6H I-6 diesel engine, made for medium duty trucks. The thing that jumps out right away is that the engine is capable of making 850 ft-lbs of torque while "only" having a top end hp of 300. This is a 2.8 : 1 ratio, compared to an '06 Jetta TDI which is rated for 177 ft-lbs and 100 hp, a 1.77 : 1 ratio. This is obviously because the engine is designed (cam timing and duration, injection timing, etc) for low-end pulling torque and fuel efficiency, as opposed to high reving power/acceleration. But it is interesting that the ratio of bore to stroke, 1 : 1.09, is even closer than in the jetta, which is 1 : 1.20. (longer stroke relative to bore creates a more torquey engine, but reduces top-end revs).

The isuzu could probably be "hopped up" if it were allowed to rev beyond 2,000 rpm (piston speed: 1,640 ft/min), but I'm sure it's governed to stay below 2,000 rpm for longevity purposes. The Jetta, otoh, can rev at 4,000 rpm (piston speed: 2,507 ft/sec). So, the relatively low hp rating of the Isuzu is somewhat misleading, but nevertheless, even with it's shorter stroke relative to bore, the Isuzu makes 17% more torque per liter than the TDI. Again, this suggests the top end of the Isuzu is designed for low-end torque.

Check out the fuel consumption graph on the bottom of the page: The 6H engine runs most efficiently at just under 1,600 rpm or 1,300 ft/sec piston speed. But the fuel consumption curve stays really flat (variation is less than 2%) from about 1,250 rpm all the way up to 1,875 rpm, or 1,025 ft/min up to 1,538 ft/min. Beyond 1,875 rpm, the fuel consumption curve starts to climb, and presumably would continue a steep upward path if the engine were allowed to rev higher.

Another interesting thing jumps out at me: The 6H engine consumes approx 1 gallon of fuel per 24 hp-hr. If I assume a 0.010 rolling resistance coefficient, a toyota prius with one or two occupants requires about 20 hp to cruise at 70 mph. So assuming 20% driveline and accessory losses, an engine opperating as efficiently as the 6H in a prius could get 70 mpg at 70 mph!

This makes me want to mess with the cam -- timing duration, etc. -- in my little jetta. I have an engine that I'm rebuilding so if I destroy this engine, it wouldn't be the end of the world for me. I should also do the 8v gasser tranny swap, which would give me 22% taller gearing.

[omgwtfbyobbq]

Here are a couple BSFC maps that show the difference between Turbo/NA and 4/2 valve engines, althought I don't have any info beyond that.

[omgwtfbyobbq]

If you can pull 50mpg@55mph in the Lambo, I think yer golden in terms of gear ratios. P&G is pretty much simulating taller gearing by loading up the engine for a shorter duration than at a cruise to improve overall efficiency, so I suppose it's a poor man's taller gearing...

[omgwtfbyobbq]

Meh... 50mpg is better than 25mpg. Why not grab another trans and crack it open to see if you can swap stuff around?

[omgwtfbyobbq]

22s playa1

[omgwtfbyobbq]

Sounds like a gud exkuse to get mas t00ls!

[omgwtfbyobbq]

The X axis is engine speed and Y-axis air/torque, with peak hp corresponding to peak torque at whatever rpm. Everything you wrote seems to be accurate. Regarding the gearing line for different ratios at a given power/speed output and efficiency, realistically, it can be anywhere from 1000-4000rpm depending on displacement and engine. An older small displacement SOHC engine designed to rev high and make power may have it relatively high, while a newer large displacement DOHC may have it around idle.

[northboundtrain]

Originally Posted by omgwtfbyobbq

. . . and Y-axis air/torque . . .

How do you convert from bmep [bar] to ft-lbs torque?