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Old 05-19-2008, 11:01 AM   #61
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Originally Posted by theholycow View Post
it was my guess (based on my ideas about cruising, which I described later) that P&G would still be more efficient if you had the same pumping losses during the pulse as you otherwise would at steady speed.
You're talking about a scenario where pumping losses during the pulse are as great as pumping losses at a steady speed. But that scenario doesn't exist, because, by definition, "pulse" means a larger throttle opening than when you're at a steady speed. And larger throttle opening means diminished pumping losses.

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I can give it a LOT of throttle and not have it downshift unless I really stab the pedal
That's the key to effective P&G in an automatic.

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in another thread there is someone reporting success with 4000rpm pulsing
He would do better if he shifted earlier.

Just above the comment you cited is a comment by PaleMelanesian (http://www.gassavers.org/showpost.ph...&postcount=13), where he says he shifts at 2200. He has described his driving habits many times. This is what he says: large throttle, low rpm.

There's a reason his vehicle appears on the home page of this forum (http://www.gassavers.org/), on the top ten list: he uses P&G very effectively. What's even more impressive is his EPA score; i.e., his percent above EPA rating (http://www.gassavers.org/garage/topten). He is more than 100% over EPA. Only two cars in the garage score higher than that. He ranks 3 out of 354 (http://www.gassavers.org/garage/viewall/epa).

You person you cited is doing about 19% over EPA, which makes his rank 178 out of 354. You have to decide who to believe, and whose habits are worth imitating.

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All that rev matching and clutch work (in addition to moving the shifter) gets tiring in my VW.
Good point. P&G with an automatic can still provide a lot of value, and for not a lot of extra work, compared with the work of doing P&G with a stick.
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Old 05-19-2008, 11:01 AM   #62
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Dosco, thanks for providing all that information. It's interesting and helpful.

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Originally Posted by dosco View Post
So you say, if you can use 100% load at 2,000 rpm, you'll use even less fuel. Indeed. However I can't do that in my automatic transmission without triggering a downshift.
Exactly. This is really the heart of the matter. You're noticing, correctly, that part-throttle at low rpm can be more efficient than WOT at a higher rpm. This is important to know when you're driving an automatic, because that's the choice the tranny is forcing you to make.

But if you have a stick, you have another choice (have big throttle and high gear at the same time). And it's always the case that assuming a constant rpm, big throttle is more efficient than little throttle (assuming that you haven't triggered open loop, which I mentioned earlier). The power of a stick, in P&G, is that it lets you increase throttle without increasing rpm (or without increasing rpm very much).

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Originally Posted by dosco View Post
What is the engine SFC at idle, especially compared to cruise?
I don't know what SFC is. Do you mean BSFC? Or do you mean FC?

Here's the answer if you mean BSFC. 100% of the fuel we use at idle is wasted, because it's not doing any useful work for us. In that sense BSFC at idle is infinite.

Here's the answer if you mean FC: a typical consumption figure is 0.3 gph.

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what if you could magically reduce the coefficient of drag on your car? If that happened, the engine load would decrease (particularly at highway speeds), which theoretically would reduce the fuel efficiency of your vehicle.
No, no, no. You're confusing the concept of load with the concept of drag, or braking force. They are separate concepts.

I think Gary has the same problem, and I think perhaps you are becoming confused by reading his posts.

When we use the term "load" in this context (e.g., in the article you cited), it means something very specific. It doesn't mean how heavy is the car. It doesn't mean how much power are we producing. It's simply a ratio that describes how much air is moving through the engine, as compared with the theoretical maximum volumetric capacity of the engine, at the current rpm, based on its static displacement.

100% load generally means WOT, because that's when the most air is being allowed to move through the engine. But 100% load doesn't mean the engine is producing the most power it's capable of producing. It just means the engine is producing the most power it is capable of producing at the current rpm.

And 100% load doesn't tell us if the car is speeding up, slowing down, or maintaining a steady speed. That's a separate issue, and it's a matter of how much power the engine is producing right now, as compared with the braking forces (aero drag, inertia, friction, whatever) that are being imposed on it.

We can be at 100% load and producing lots of power (if we're at high rpm), or we can be at 100% load and producing very little power (if we're at low rpm). We can be at 100% load and accelerating rapidly (if the car happens to be very light, or if we're going downhill, or if we're at high rpm, or all of the above). We can be at 100% load and decelerating rapidly (because we're suddenly struggling to go up a hill, or deal with a headwind, or we've picked a very high gear and are therefore at low rpm, or all of the above).

Or we can be at 100% load and be maintaining a steady cruise. Near my house is a hill, fairly long and steep. I usually climb it WOT, in top gear. Engine is at 1200 rpm, and vehicle speed is about 30. All those parameters are constant: the slope of the grade, the throttle opening, engine speed, and vehicle speed. All those parameters stay constant, for about half a mile, until I reach the crest of the hill.

Assuming I want to climb that hill, and assuming I want to do it at 30 mph, I believe I have optimized BSFC, i.e., I'm achieving the work that needs to be achieved, using the minimum amount of fuel. Any other throttle setting (i.e., short of WOT) would be less efficient.

These concepts are inherently confusing, and some people are indeed confused.
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Old 05-19-2008, 11:25 AM   #63
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Here is my thought.

How many states of energy transfer can we name, fuel to usable mechanical power (in your vehicle). I believe there are 5.

Idling=total waste of energy, you can easily carry a battery to run all accessories, indeed the acessories should be plug in modules, easily interchangeable. All accessories should be electric.

Accelerating=Here is where you can be efficient but you also use the most fuel. Your rate of acceleration determines your throttle position, its easy to feel when you accelerate. Put the car in gear and slowly increase the amount of gas pedal percentage. When you reach the point where more pedal really doesn't make much difference you are at your ideal. If you need to accelerate faster, you need to downshift, because flooring it will not make enough difference becasue you can not accelerate enough in the gear.

Acceleration is storing energy, for storage you use the mass of the vehicle, understanding that additional acceleration creates stored inertia in your vehicle. The reason you pulse is simple. When you pulse you are applying a load to the engine above the load it takes to maintain your speed. The secret is the additional energy stored in increased speed is much cheaper because your fuel use is only 50% more than just maintaining speed, for over twice the power. Your mpg shows adrop which is a false reading because you are measuring all the fuel consumed, including that to maintain speed and the 50% you are storing in additional speed.

Coasting=coasting is utilizing the stored energy in your vehicle as a means of propulsion while the engine can idle (if you dont shut it off). If the engine is off you use no fuel so that is as good as it can get, you are sacrificing your inertia for distance with no fuel. I dont use EOC except to coast the last .3 miles to my driveway.
Even with the engine idling your mileage is very high. My engine uses about .2 gal per hour idling. If I start my coast at 60mph at that instant I am getting 60 miles with only .2 gals of fuel per hour. Thats 300mpg.
You can calculate what you would get with eoc by simply counting the coast time. In one test for me it was 13:45 out of 18 minutes at about 32 mph.
almost 75% of my hour was spent using .2 gph, which worlks out to .15 of my hourly fuel was wasted idling.

MY last tank was 57.58mpg, probably about 1/2 of the time I was coasting.
Using my known idling consumption and assuming I used 1 gph to go 57.58. I can calculate my mileage if I used EOC. It would be 57.58 miles on .90 gal.
That works out to about 64 mpg give or take (used my old head instead of a calculator).
Now lets say my average speed was 40mph and my coasting was 75% (about what it works out to be at 40. My fuel consumption is 40/57 of a gallon while my idling is now .15 per hour. .7 gal-.15=.55 gal to go 40 mph. That's 40 miles on .55 gal of fuel or 72MPG. To me the difference between 57.58 and 72, is not enough to justify the exponentially greater effort using eoc in the traffic around here, but I bet I could do that under controlled situations averaging 40 mph.

Deceleration=Here is where I learned something new that works really well with my VX. Keep it in gear to decelerate employing fuel shut off by keeping the engine revs above the point where fuel is injected for it to idle. Now when I get caught by a light or traffic, I start decelerating, downshifting in the highest gear that will keep suel shutoff engaged. No fuel consumed. I try to use this to minimize idling. which wastes fuel, when I have the option of eliminating all fuel consumption.
The problem for me is if I slow my average speed down too much I start catching every light yellow, so I have to maintain may average speed which is slightly wasteful to avoid idling at almost all cost.

Braking=Here is where you are really driving dumb, when you hit the brakes.
You are now throwing away the energy you spent soo much fuel to obtain. Even at your best your are spending 3 times the btus it cost you to get there. The mass of your car is your battery and braking is shorting that battery out (figuratively of course).
To put it bluntly your brakes should last forever. that means you have committed the least violations of the golden rule "thou shalt not brake".

I can look at the gas logs and notes of those who have contributed to a valuable knowledge base, and employ them with no instrumentation because I understand the dynamics of it and can feel when it is working right.

regards
gary
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Old 05-19-2008, 11:53 AM   #64
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Another stab eh Monroe LMAO.

Pulse and glide up the hill. You can produce more power at higher revs at the same efficiency probably even closer to the sweet spot in a BSFC MAP, using a lower gear.

I learned that driving up Afton Mountain in Virginia before they built the interstate highway in 1968.

Speed limit was 65, but my 63 Valiant wouldn't do 65 up the over 10%grade. In top gear it would not maintain speed, you could only get up to 55 in second gear (wound out- max revs). I used second to get to 55 and then third, because I didn't want to drive 5 miles up a mountain with it floored in second gear. In third gear it just kept slowing down as the revs dropped the power dropped and it would not maintain any speed.

Ah why bother, you probably don't understand that scenario either. That old valiant got 28.5 mpg. Was it before you were born?

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Old 05-19-2008, 12:07 PM   #65
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Quote:
Originally Posted by monroe74 View Post
Exactly. This is really the heart of the matter. You're noticing, correctly, that part-throttle at low rpm can be more efficient than WOT at a higher rpm. This is important to know when you're driving an automatic, because that's the choice the tranny is forcing you to make.
Yes.

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But if you have a stick, you have another choice (have big throttle and high gear at the same time).
Probably. If I still had my stick shift, I'd be trying it.

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I don't know what SFC is.
But you caught on to my question. Idling has low fuel flow, however it is inefficient. By maximizing my car's time in throttle, I'm using less fuel but I am operating my engine very inefficiently.

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No, no, no. You're confusing the concept of load with the concept of drag, or braking force. They are separate concepts.
Not my point. If I cut the drag of the car way down, and therefore reduced the amount of power needed to maintain highway speed, the engine would run at a lower rpm when compared to before. Which means in "traditional driving" (i.e. maintaining steady state throttle position) the operator would be commanding less rpm, and the engine would operate at lower thermodynamic efficicency. However one would expect the miles per gallon to improve.

Imagine taking this case to the extreme, where drag is reduced to the point where highway cruise could be maintained while only using idle rpm. The engine would be horribly inefficient, however the mpg would be astronomical due to the low FC.
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Old 05-19-2008, 12:08 PM   #66
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Originally Posted by monroe74 View Post
He would do better if he shifted earlier.
I also have a hard time believing that effective P&G could be had with revs up to 4,000 rpm.
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Old 05-19-2008, 12:26 PM   #67
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Originally Posted by monroe74 View Post
An effective FE strategy is not just about trying a achieve a lower flow rate, or something like that. It's not about simply using less fuel. It's about using less fuel per unit of work that's being produced. That's what the BSFC concept means.
Unfortunately that's not how we're measuring it, are we? We're trying to figure out ways to maximize miles while minimizing fuel used. Conceptually we agree, though, that this means maximizing work accomplished while simultaneously minimizing energy expenditure.

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But to really assess what's going on, you have to ask yourself the following question: was mpg high because I optimized thermodynamic efficiency, or was mpg high because I lowered my overall speed, for this trip?
Well, on that particular trip, I traveled to my destination in 7 hours. Which is nominal, accounting for traffic. I have made that trip before in 7 hours, and not achieved 40 mpg. Ever.

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And optimizing thermodynamic efficiency is almost always going to be a good idea, because it's essentially free (aside from some extra effort and skill on the part of the driver). In other words, when you optimize thermodynamic efficiency, you can save gas without making further sacrifices with regard to weight, distance, and time.
I agree conceptually, but again I wonder. As I mentioned in a post I wrote a moment ago, what if I could setup my car to run at highway speeds at idle rpm? Should I really believe that I would achieve horrible mpg, despite the fact that the engine's BSFC would be completely horrible?

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That's what's known as a false choice. You don't have to choose one or the other. They go hand-in-hand. Reducing rpm tends to be a key part of optimizing BSFC.
In my car, it might be the only choice. In your car, maybe not.

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Don't be misled by the fact that 3000 rpm at WOT is more efficient than 1500 rpm at WOT. You need to remember that the former setting produces probably about twice as much power. Yes, it produces that power very efficiently (in other words, let's say it provides double the power without using twice as much fuel), but you're wasting fuel unless you're in a position to put that power to good use.
Therein lies part of the rub. In my car, there are times when I can pulse up to 3,000 rpm. It's very easy. It doesn't get me a glide that is appreciably better than when I creep up to speed with low rpms and slow accelerations.

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P&G is so effective because it lets us temporarily run the engine in a very efficient way, where it is producing a lot of extra power for not a lot of extra fuel. And we store that extra power in the form of momentum, which we then use to coast.
I'm not convinced that P&G is solely to maximize engine thermodynamic efficiency. It seems like it is more about *both* creating power as efficiently as possible (when you need it) while also minimizing fuel consumption during the glide (hence EOC). Perhaps I'm restating the obvious. If so, sorry.
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Old 05-19-2008, 12:30 PM   #68
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My P&G occurs primarily at 1500-2000 unless I amg going over 50, when P&G starts to reach the point where the pulse and glide phases approach equal times for both.

For the VX at speeds higher than 50 mph I start looking for drafting opportunities, and I might still pulse and glide but the glide phase would be more tailored to downhill portions of the highway.

I live in an area with hills. When drafting the pulse and glide can be maintained up to higher speeds, but you have to do it almost unconsciously because when drafting you need to focus on whats going on in front of your car.


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Old 05-19-2008, 12:31 PM   #69
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Quote:
Originally Posted by monroe74 View Post
You're talking about a scenario where pumping losses during the pulse are as great as pumping losses at a steady speed. But that scenario doesn't exist, because, by definition, "pulse" means a larger throttle opening than when you're at a steady speed. And larger throttle opening means diminished pumping losses.
It was hypothetical, and my point was that in an automatic where you don't have the ability to go wide on throttle and tall on gear at the same time, it's still going to be worth it even though you sacrifice some of the pumping savings. It's still better than nothing.

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He would do better if he shifted earlier.
I think, like me, he doesn't have enough gears to have low RPM at highway speed. I'd need at least another 5 gears at the close ratios that are in mine. By the time I hit 40 mph I'm pretty strongly itching to shift but there's no 6th gear. I'm used to GM-style big displacement engines and tall gears. Don't get me wrong, I'm overall very happy with my car choice, I just have some complaints like that one (as I would with any car).

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Just above the comment you cited is a comment by PaleMelanesian where he says he shifts at 2200. He has described his driving habits many times. This is what he says: large throttle, low rpm.

There's a reason his vehicle appears on the home page of this forum (http://www.gassavers.org/), on the top ten list [...]

You person you cited is doing about 19% over EPA, which makes his rank 178 out of 354. You have to decide who to believe, and whose habits are worth imitating.
You're preaching to the converted. My point wasn't that it's better to rev high in the pulse, just that it's better to pulse revving high than to not P&G at all. I rarely reach the 2200 rpm that PaleMelanesian reports until I run out of gears. Usually I only reach that high if I've jumped out in fast traffic or am accelerating uphill from a stop. Otherwise I shift between 1200 and 1600 rpm, or even 2000 rpm if I'm not feeling patient.
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Old 05-19-2008, 12:58 PM   #70
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I rarely reach the 2200 rpm that PaleMelanesian reports until I run out of gears. Usually I only reach that high if I've jumped out in fast traffic or am accelerating uphill from a stop. Otherwise I shift between 1200 and 1600 rpm, or even 2000 rpm if I'm not feeling patient.
Interesting. 2200 is right about the rpm for my Camry cruising at 60mph on level ground in top (4th) gear.
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