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Old 10-01-2007, 09:33 AM   #1
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Interesting Hybrid - Nonhybrid comparison

I was on cars.com just now comparing the civic hybrid and insight gear ratios to the regular civic and corrolla. Given the stock tire sizes, here's what I found:

At 70 mph in od/5th the civic hybrid is running at 1,958 rpm; an '06 insight is at 2,396 rpm; the regular civic is at 2,947 rpm; and the corolla is at 2,767 rpm.

If you multiply the rpm by the length (inches) of the piston stroke, you get number that doesn't correspond to anything specific like actual piston speed, but it allows for a direct comparison among the four engines. The higher the number, the faster the pistons are moving in the cylinder walls.

civic hybrid: 6,167
'06 insight: 7,692
regular civic: 10,138
corolla: 9,961

So we can see that at 70 mph, the pistons in the regular civic are moving 164% faster than the pistons in the civic hybrid.

Now, according to my admittedly limited understanding of engines, as piston speed increases beyond the point at which it makes peak torque, the burn is actually chasing the piston down the cylinder. This requires greater and greater inputs of engery (fuel) inorder to make more power, which is why engines start consuming much more fuel as they rev higher and higher.

So the rpm x stroke numbers I just posted above should be a major factor in the improved fuel economy of the hybrids, which just goes to show that if the manufacturers would give us 6 speed trannies with really tall overdrives we could see huge FE gains in regular cars. But, no, evidently they want us to think we have to pay an $8,000 premium for fuel efficiency

Now, perhaps a straight comparison of the rpm x stroke numbers isn't a fair comparison, because the various engines might have very different cam timing and duration, as well as ignition timing. If someone knows more about this please chime in. However, I'm still struck by the huge differences in engine/piston speed between the hybrids and non-hybrids.
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Old 10-01-2007, 09:50 AM   #2
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Naw, I think that your comparison is pretty fair. I think they just don't want to have a car the market perceives as sluggish. With the Hybrid they can have the engine turn over at the slower rpm and still give users the feel of power, that they feel they need.
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Old 10-01-2007, 09:54 AM   #3
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Do you have the displacement numbers for each engine? You've basically made an offset comparison of linear distance traveled

I think it would be interesting to compare engine speed to surface area displacement.

I've normalized your figures above to show how close everything is and did a percent gain on all compared to the civic hybrid.
Code:
	normalized	% Gain
civic	0.181606691	0
insigt	0.226515107	24.7283931
rcivic	0.298545262	64.391114
corolla	0.293332941	61.5209989
sum	1
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Old 10-01-2007, 10:25 AM   #4
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How did you calulate piston speed? It is normally referred to in the total distance a piston travels in feet per minute. Your #'s seem too high to be in ft/min and too low to be in inches/min.

The aluminum limits the feet/min a racing engine can have its pistons survive and anything beyond about 5200 ft/min is considered unhealthy. I think you'll find that at 2900 rpm the standard civic is still well below this amount, but at redline is probably approaching it. Some racing engines go well beyond 5200 ft/min, but they also are replacing the pistons every couple hundred miles.

The ignition timing advance will make up for the speed the piston speed and is generally trying to keep the maximum cylinder pressure at the same crank rotation. For a far better explanation, see the article from Team-Integra.net below

http://www.team-integra.net/sections...?ArticleID=235
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Old 10-01-2007, 12:23 PM   #5
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How did you calulate piston speed? It is normally referred to in the total distance a piston travels in feet per minute. Your #'s seem too high to be in ft/min and too low to be in inches/min.
Like I said, I just multiplied rpm by stroke length. This results in the total distance (inches) a piston moves down the cylinder in one minute. Multiply it by two and you have the total distance a piston moves both up and down the cylinder in one minute. It is not a measure of maximum piston velocity (which, if my geometry is correct, would occur at some point partway down/up the cylinder when the connecting rod is pushing/moving tangentially to the rotation of the crank. I guess what my numbers actually correspond to is one half the average velocity of the pistons in inches per minute. It's half the average because the piston travels two stroke lengths -- down and then back up in one crank rotation -- and I simply multiplied one stroke length by rpm. Basically it's just a way to compare piston velocities relative to one another, since rpm alone doesn't account for the respective distances pistons travel in different engines, and so rpm doesn't give as precise an indication of piston speed, which is the more meaningful number.

To get total distance a piston travels in one minute, divide my numbers by six (divide by 12 to go from inches to feet, then multiply be two to account for both the up and down strokes).
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Old 10-01-2007, 12:43 PM   #6
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I'm looking at the corolla and the difference between 4th and 5th gears. 4th is 0.89 and 5th is 0.73. If we added a 6th gear with the same percent change, it would be 0.60. With this extra gear, the 70 mph rpms would drop from 2,767 to 2,271. rpm x stroke number would be 8,177 (or to get the average feet per minute number that mrmad mentioned 1,363 ft/min).

Why can't we get this in regular cars!!!???

By the way, just out of curiosity, does anyone know the ideal average piston velocity in feet per minute, since that's apparently the standard unit used, at which a gasoline engine makes maximum torque? That would help determine how far off the manufacturers are when they gear cars for typical highway cruising speeds
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Old 10-01-2007, 01:28 PM   #7
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I'm really surprised the regular civic is nearly at 3000rpm at 70mph.

The Civic hybrid has a 1.3 liter engine.
The Insight has a 1.0
The Corolla has at least a 1.8.
The regular Civic I believe has ballooned to 1.8 as well
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Old 10-01-2007, 01:42 PM   #8
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Originally Posted by northboundtrain View Post
By the way, just out of curiosity, does anyone know the ideal average piston velocity in feet per minute, since that's apparently the standard unit used, at which a gasoline engine makes maximum torque? That would help determine how far off the manufacturers are when they gear cars for typical highway cruising speeds
I may have mistated this question, the rpm at which max torque is made does not necessarily correspond to maximum fuel efficiency. I found this page that states that max fuel efficiency occurs with a piston speed of 1,000 - 1,200 ft/min. http://books.google.com/books?id=mX1...UCPU#PPA378,M1 The derivation of this number on subsequent pages is over my head, but assuming it's accurate and restating my numbers, let's look at how the four vehicles compare:

Civic hybrid: 1,028 ft/min
'06 insight: 1,282 ft/min
regular civic: 1,690 ft/min
corolla: 1,660 ft/min

So again, the hybrid engines are designed to opperate at the optimal rpm for fuel efficiency, while all other cars are designed with entirely different priorities.

By the way, I chose 70 mph as a good compromise between the more optimal highway speed for FE of 55-60 mph and the typical speeds seen on highways these days of 75-80 mph. It also happens to be the speed I'd like to be able to drive and not take a huge FE hit.

Interestingly, my Duramax diesel truck has a piston speed of 1,098 ft/min at 70 mph, so evidently the diesel trucks get more FE consideration than the so called economy cars
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Old 10-01-2007, 02:23 PM   #9
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Quote:
Originally Posted by northboundtrain View Post

Interestingly, my Duramax diesel truck has a piston speed of 1,098 ft/min at 70 mph, so evidently the diesel trucks get more FE consideration than the so called economy cars
Different engine cycle dynamics (makes direct comparison like this difficult) The ideal diesel power stroke is isentropic (constant entropy) while the Otto power stroke is adiabatic (constant heat).

Efficiency of an Otto engine is typically demonstrated in terms of compression ratio whereas the diesel cycle is in terms of the cut off ratio... It is very likely that it's not so much an FE consideration, and more of a general diesel design consideration...
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Old 10-01-2007, 02:53 PM   #10
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Different engine cycle dynamics (makes direct comparison like this difficult) The ideal diesel power stroke is isentropic (constant entropy) while the Otto power stroke is adiabatic (constant heat).

Efficiency of an Otto engine is typically demonstrated in terms of compression ratio whereas the diesel cycle is in terms of the cut off ratio... It is very likely that it's not so much an FE consideration, and more of a general diesel design consideration...
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.
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