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Old 10-02-2007, 07:51 PM   #21
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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.
Can you further explain what you're getting at, because I have no idea what you're talking about and I'd really like to understand this better. I know from my own experience with my car (and it's a diesel, so things may be a little different in a gasser) that there is this "sweet spot" where I'm barely touching the accelerater and the car just chugs along effortlessly. It happens to be about 40 mph in 5th gear which I've calculated to be approx 2,000 rpm or 1,200 ft/min average piston velocity. And when I've been able to maintain this speed for any length of time (unusual here in Colorado where I'm either in town or on the open highway), my fuel economy skyrockets. increasing speed beyond this point requires a lot more right foot. So it makes sense to me that there is an ideal piston speed for maximum engine efficiency, but like I said I want to understand this better, so can you elaborate?
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Old 10-02-2007, 08:47 PM   #22
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There are more parameters that affect FE then just the piston speed. I think the best rpm for FE has as much or more to do with the cam, intake manifold timing, and port sizes then the piston speed. For instance, compare my Integra GSR and my CRX HF. The Integra has larger port sizes, longer duration cams, and longer intake runner lengths. Because of this, at lower rpm, the Integra's intake velocity is slower and the cam duration is too long such that the CRX HF is much more efficient in filling its cylinders at 2000-2500 rpm then the Integra is. At 1000-1200 ft/min piston speed (somewhere between 2000 and 2500 rpm), the Integra is anemic in power and is barely waking up. Because the Integra engine is designed for HP and not FE, it probably would give better gas mileage to operate it above 3000 rpm then lug it around at 2000 rpm.

I think you are probably right, the best FE is around the 1000-1200 piston speed, if the rest of the engine is designed for this.
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Old 10-03-2007, 12:48 AM   #23
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I think you guys are simplifying things way to much, and looking at the wrong variables.

First of all, the sweat spot (if there is one for piston speed) would be created because of a number of factors lining up. The problem is that way to many of those factors (such as atmospheric pressure, ambient air temperature, the stroke/bore ratio, octane, spark timing, etc) are variable. The only one that is really constant is the burn-rate of gasoline. I'm sure there is some sweat spot where the speed that gasoline burns matches up with the speed of the piston travel, but I think it's effect probably less than all of the other factors going on here.

The biggest factor is just internal friction. Every time crank turns one rotation there is a given amount of energy lost to friction. So if two cars are going the same speed, and one is turning less RPM, it is loosing less of the energy it is creating to internal friction, and is therefore more efficient. That is true no matter what the RPM in question is, or what the piston speed is.

If you did your test at say 45 miles per hour the hybrids would now have a piston speed lower than your "ideal" while the regular Civic or Corolla would be right in the sweet spot. I bet the hybrids would still be more efficient.

The idea that less RPMs is better is always true unless the airflow and other factors in the engine have been tuned in such a way that the engines inefficiency at the lower RPM can overcome the natural efficiency benefit of taller gears. There is a point where that equation where that doesn't work anymore because of that constant gas burn-rate property, but I think it is significantly lower than a piston speed of 1,200 ft/min.

If the engine were being run with no car/gearbox attached to it then maybe piston speed would play a significant role. However once you add a bunch of weight, wind resistance and gear reduction into the equation overall ratio makes a larger impact I think.

northboundtrain, the reason you hit that sweat spot has to do with two curves interacting. Those curves are the engine efficiency, and the aerodynamic efficiency. As speed goes up, aerodynamic efficiency goes down. On the other hand going faster gets you into higher gears (more efficient as per my above statements), and gets you closer to your engines peak efficiency. Where those two curves meet is that "sweet spot" that you describe. The question is, if you had a 6th gear and were in that, turning say 1600 RPM at 40 mph, would you be using less gas? At that point you would be at 960 ft/min piston speed, and out of the piston speed "sweet spot" but I bet you would find you would lose even less gas. Also, the sweet spot might move to a slightly different speed.

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.
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Old 10-03-2007, 07:06 AM   #24
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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.
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Old 10-03-2007, 07:57 AM   #25
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True, but then...

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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
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Old 10-03-2007, 11:31 AM   #26
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Instructive diagram

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.
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Old 10-03-2007, 01:27 PM   #27
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Can you further explain what you're getting at, because I have no idea what you're talking about and I'd really like to understand this better. I know from my own experience with my car (and it's a diesel, so things may be a little different in a gasser) that there is this "sweet spot" where I'm barely touching the accelerater and the car just chugs along effortlessly. It happens to be about 40 mph in 5th gear which I've calculated to be approx 2,000 rpm or 1,200 ft/min average piston velocity. And when I've been able to maintain this speed for any length of time (unusual here in Colorado where I'm either in town or on the open highway), my fuel economy skyrockets. increasing speed beyond this point requires a lot more right foot. So it makes sense to me that there is an ideal piston speed for maximum engine efficiency, but like I said I want to understand this better, so can you elaborate?
Engine efficiency, for everything from semi engines to small gasoline engines is a function of load and engine speed. High load and low engine speed are typically where efficiency is the best. Although, low load hurts efficiency more than high engine speed does IMLE. Efficiency is given in terms of grams of fuel used per kWh generated, so lower is better. Here are some examples

-Mercedes I4 Circa 2000
-1.9L TDI circa the lates ninties
-Heavy duty diesel engine circa (?)
-Toyota V6 engines circa the early nineties
-Unkown, likely gasoline
-The Prius' engine

Due do friction reducing design, newer hybrid engines are unique in that they can rev relatively high w/o hurting efficiency as much as most engines do. Diesel engine efficiency tends to peak at about half of their designed operating speed, and gasoline engine efficiency tends to peak at about a half to a quarter of their designed operating range. This is just a byproduct of the different air utilization rates, which limits the speed of diesels compared to gassers. All engines operate better at around full load, usually a little less than full load, so all things being equal more pedal is better.

Given a certain car that needs X kW at N mph, in order to get best mileage we want to optimize load such that the engine is working as efficiently as possible. Low output (compared to displacement) TDI diesels are nice in this respect since engine efficiency doesn't vary compared to load nearly as much as it does in other engine types, so gearing isn't as crucial for good economy, although it can help. For instance, the 1.9L TDI has fairly good efficiency of ~300-200g/kWh from ~10-90hp. Unlike the mercedes I4 gasser which has efficiency of ~500-250g/kWh from ~10-150hp.

Here's the thing, most cars today have decent highway efficiency given gearing and speeds of ~70-80mph, so there really isn't much to improve from the factory in this respect. However, if we don't mind driving 55, then with an appropriate OD ratio we can see the same engine efficiency w/ much less kWh needed by making sure the engine is at similar load. Here's a crappy edit of the Toyota V6 I linked earlier. My engine is the one w/ the dotted lines, and each square corresponds to a different OD ratio, w/ admittedly high numbers for the power needed to go down the freeway at 55mph. The bottom one is where the gearing is at now, with BSFC of ~350g/kWh(?) which corresponds to ~35mpg@55mph. If I drop the engine speed to ~1500rpm by a taller gear, BSFC drops to ~260g/kWh since engine load increased, and my mileage would increase to ~45mpg@55mph. Going to 1000rpm doesn't increase efficiency, but it does reduce available power in that gear, so it's probably no worthwhile unless I can drop the energy needed to move the car enough, likely cut it in half ala Basjoos.

Here's some more on it, and like I said, the road load line is how efficiently the engine operates in a certain gear over that gear's range of speeds. If you put a taller gear in that engine, the line would go through BSFC ovals with smaller values, and you would get better mileage. That's the VW 1.5L IDI diesel, so more or less the same as our diesel engines. At the end of my diesel swap post I calculated approximate mileage via that BSFC map assuming certain conditions.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 10-03-2007, 01:32 PM   #28
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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.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 10-03-2007, 01:43 PM   #29
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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...
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 10-03-2007, 01:57 PM   #30
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Meh... 50mpg is better than 25mpg. Why not grab another trans and crack it open to see if you can swap stuff around?
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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