Interesting Hybrid - Nonhybrid comparison

[mrmad]

Originally Posted by northboundtrain

Yeah, fair enough, and I know that diesel fuel burns slower than gasoline, so diesel pistons should be slower. I guess my point is more that the truck seems to chug along nicely at 1,700 rpm on the highway, whereas a civic or corolla doing close to 3,000 rpm is very definitely well beyond it's FE "sweet spot". Of course, I base this assertion on the 1,000-1,200 ft/min figure I quoted and the fact that the hybrids are geared to run with these piston speeds. Maybe I'm wrong and for some reason like compression ratio or cam design almost 3,000 rpm is a fairly ideal engine speed for the civic and corolla. If so, I would love to hear why, so that I could feel better about possibly buying a new car and not think that I'm being manipulated by the manufacturers.

The regular Civic engine is designed more for performance then FE. They are setting the overal gearing and separate gear ratios so the engine can stay in the powerband between shifts. They could have designed it for better FE, which probably would have lowered the rom at 70mph, but I'm sure some marketing survey somewhere showed them they would not sell as many cars if they did.

[rh77]

2 inputs-

Interesting observations...

Not sure if anyone mentioned or factored the CVT in the Civic Hybrid, or if it was the older 5-speed used to compare (it probably doesn't change the final ratio at cruise, but holds another advantage up the speed range).

Finally, I think the only way we can get the cars we want, is to design them ourselves. To the mass public, Automakers don't have an incentive to make small, inexpensive, FE-oriented vehicles -- the profit margin is paper-thin compared to the truck-based SUVs.

It'll take a paradigm shift of customer education and demand through outlets such as these. First people have to care -- that generally translates to the wallet. Sigh, I'm preaching to the choir here...and in the wrong thread

RH77

EDIT: Based on the ideal piston travel speed, would there be interest in a custom calculator for ideal cruise (like a set of input questions in a web-based form or something?

[rh77]

OK -- so for 1000-1200 ft/min piston speed, with a 3.5 inch stroke in my auto,

Then (accounting for some slight tire wear) ...I'd have to drive in 3rd gear at 60, since that translates to:

1000 ft/min @ 3428 RPM = 55 mph in 3rd, 85 mph in 4th (wind resistance starts to hit).

only went up to 4000 RPM for engine longevity...

1283 ft/min @ 4000 RPM = 64 in 3rd, 98 mph in 4th

Should I be driving around in 3rd? What else is at play here?

RH77

[northboundtrain]

Originally Posted by rh77

OK -- so for 1000-1200 ft/min piston speed, with a 3.5 inch stroke in my auto,

Then (accounting for some slight tire wear) ...I'd have to drive in 3rd gear at 60, since that translates to:

1000 ft/min @ 3428 RPM = 55 mph in 3rd, 85 mph in 4th (wind resistance starts to hit).

only went up to 4000 RPM for engine longevity...

1283 ft/min @ 4000 RPM = 64 in 3rd, 98 mph in 4th

Should I be driving around in 3rd? What else is at play here?

RH77

I think you are multiplying the rpm by the stroke (inches) and then dividing by 12 to get ft/min. But the average piston speed must take into account two stroke lengths per revolution. Thus your piston speed is rpm x stroke x 2. Then divide by 12 to convert from inches per minute to feet per minute. Thus, to achieve 1,000 ft/min, your rpms should be half of what you figured -- 1,714. So according to your calcs, that means 27.5 mph in 3rd and 42.5 mph in 4th.

[northboundtrain]

Originally Posted by rh77

. . . I think the only way we can get the cars we want, is to design them ourselves . . .

Okay, I'd be game for doing this. Give me a good late model car to start with like a civic, corolla, echo/yaris, or whatever -- just something with air bags, a decent drag coefficient, clean emissions, etc. And now help me figure out how to modify the 5 speed transaxle (tranny and ring and pinion) to get the gear ratios I want. Hopefully this could be done without wacking out the car's computer (I don't know much about newer car electronics). Hopefully there'd be an easy way to recalibrate the speedometer. I'm not a machinist and I'm sure getting custom machined gears would be pretty expensive, but if only say the R & P gear needed to be remade, then perhaps it wouldn't be too unrealistic.

Now I throw in a few simple aero mods -- grill block, rear wheel skirts, etc. and maybe I've got my 50 mpg at 70 mph.

Any ideas?

[trebuchet03]

EDIT: Based on the ideal piston travel speed, would there be interest in a custom calculator for ideal cruise (like a set of input questions in a web-based form or something?

Something tells me there isn't 1 ideal piston speed.... I'll bet there's a relation with the torque curve, piston dimensions, intake tract length etc....

[rh77]

Originally Posted by northboundtrain

I think you are multiplying the rpm by the stroke (inches) and then dividing by 12 to get ft/min. But the average piston speed must take into account two stroke lengths per revolution. Thus your piston speed is rpm x stroke x 2. Then divide by 12 to convert from inches per minute to feet per minute. Thus, to achieve 1,000 ft/min, your rpms should be half of what you figured -- 1,714. So according to your calcs, that means 27.5 mph in 3rd and 42.5 mph in 4th.

OK this makes sense: had to calc the complete travel of the piston...

Even more sense, around 45 is the perfect speed for FE in this car. My best segments are on a road with slight hills while using P&G/DWL between 40 and 50 mph.

OK, designing cars. Take something like the Pruis and ditch the drivetrain/batts. Now you have 4-doors + the use of a hatch + a slick Cd. Throw-in a 1.3L like the Yaris' x-US version: 2NZ-FE, VVT-i 10.5:1 Compression. 88 hp @ 6000, 89 ft-lbs @ 4400. There's also a 1.0L, but with a larger platform, it may not be efficient on the highway.

Gearing -- dunno. 6-speeds. You'll have some torque with the VVT and compression. To sell it to the U.S., it would have to be manual only in this version, then have a bigger engine and automatic for everyone else (or sport version with the big engine and 6-speed). One could argue all day about the auto being CVT or traditional, I suppose...

My ideal new car is a CNG 4-door Civic, using the older 1.6L variety with the unavailable standard shift -- maybe a 6-speed -- only one I can think of is from the Si, but the top gear might not be efficient, so a swap of sorts is appropriate, or throw in a 5-speed. Same for the Toyota example.

A CNG-Hybrid needs more research, but sounds pretty cool.

RH77

[northboundtrain]

According to specs I got from metrompg's site and the team swift site, a 1.0L metro engine coupled to the 1.3L suzuki tranny (metrompg did this swap, and it's documented on his site) with 13" wheels would yield piston speeds of 1,192, 1,291, and 1,391 ft/min at 60, 65, and 70 mph respectively. This is the only non-hybrid gearing that I've so far found that could put you in the ideal-piston-speed ballpark at highway cruising speeds (still need to check out the CRX HF gearing).

By the way, the stock XFI trannies give piston speeds of 1,282, 1,389, and 1,496 while the regular non-XFI trannies from '89-'94 give piston speeds of 1,389, 1,505, and 1,621. The '95 and up trannies have a shorter R & P gear, so the piston speeds are even higher.

[omgwtfbyobbq]

I think you're making a bit too much out of it. Just think in terms of load and yer set. This holds true for small and large, diesel and gas, AFAIK. The only notable difference comes from friction reduction in hybrid engines, which allows 'em to rev higher w/o the usual drop in efficiency. Every other BSFC map is pretty consistent in terms of efficiency. Something less than peak load at ~2-3k rpm.

[rh77]

Originally Posted by omgwtfbyobbq

I think you're making a bit too much out of it. Just think in terms of load and yer set. This holds true for small and large, diesel and gas, AFAIK. The only notable difference comes from friction reduction in hybrid engines, which allows 'em to rev higher w/o the usual drop in efficiency. Every other BSFC map is pretty consistent in terms of efficiency. Something less than peak load at ~2-3k rpm.

Yah, but it's still fun to design your own car, eh?