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Old 04-04-2011, 04:35 PM   #41 (permalink)
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Unless you can tell me how to get 70+ mpg in my 29 mpg rated Paseo without P&G I just can't believe you. Until then, your engineer is wrong and P&G works amazingly well.

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Old 04-04-2011, 04:47 PM   #42 (permalink)
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Hackish: Leveraging the low BSFC of high-load, relatively low RPM pulses followed by engine-off gliding is the standard operating procedure for almost every fuel economy competition. (From the Shell one that's been going on for decades which was begun by the company's engineers, to the ones that members here parcipate in.)

Hang on a sec, and I'll go dig up another automotive engineer's account of why P&G works better than steady cruise at the same average speed.


edit: here it is...

Dr Andrew Frank is (was? still is?) a PHD engineering prof at UC. Davis. This is a quote from an early 90's Motor Trend article where Prof. Davis and some of his students demonstrate the "competition" technique, which they call "burn and coast", to the writer.

Spark-ignition engines are most efficient when run at either full throttle or turned off. What with aerodynamic drag so sensitive to speed, burn-and-coast is primarily useful at low speeds, and what do you know but the contest rules require a minimum average speed of 15 mph.

That's the bogey: Floor it up to, say, 25 mph, switch off and then coast -- seemingly forever -- down to a slow walking pace and then repeat the process. [...]

Burn-and-coast might seem a little impractical, but Dr. Frank and his students are adamant supporters.

"In Calcutta, all the taxis drive this way. It's economical, but total chaos," Dr. Frank laughs.
They conducted a controlled comparison using 2 vehicles:

Honda Civic VX (Cali emissions)
93 mpg (US) @ 20 mph cruise
104.8 mpg (US) pulse & glide @ 20 mpg avg speed

Geo Metro XFi
84.1 mpg (US) @ 20 mph cruise
116.6 mpg (US) pulse & glide @ 20 mpg avg speed

(And just a note: the writer got it wrong where he said "full throttle" in the above quote. You can bet that Dr. Frank and/or his students said "near full throttle".)
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Old 04-04-2011, 05:39 PM   #43 (permalink)
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Originally Posted by hackish View Post
The problem is that nearly all engines are designed to have the best BSFC at cruise, not accelration...
I don't know any engine like that. Steam locomotive perhaps.
Something isn't quite right with his explanation.
For instance look at this post.
I think it is pretty clear.

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Old 04-10-2011, 05:15 PM   #44 (permalink)
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The title of this thread is wrong. Should be pollute and glide, not pulse and pollute. I have yet to see a car that isn't even running pollute.
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Old 04-11-2011, 12:37 PM   #45 (permalink)
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When I did build Megasquirt EFI to one of my cars, I did learn that motor needs more fuel when starting, it is not very much of fuel, but it is more and it is quite unclean burn during startup, also it is needed to calculate that energy starter uses or if bump starting that startup energy must come from somewhere, so glide must be enough to overcome all that, pulse consumes more fuel than just accelerating from speed Y to speed X because of that.

Question is how much more it is? Making that energy back to battery with burning fuel is taking multiple times of energy because of poor efficiency of engine, belts etc.

Putting some solar cells to roof and charging battery with those, might be one solution, but they add weight and do they produce more energy to battery than what they add to consumption of fuel because of their weight?

When finding scientific reality of issue, it is needed to look every detail.

Surely P&G saves fuel, even there are these drawbacks, so that means less CO2 as CO2 is directly related to how much fuel is burned, so those interested in reducing that may continue without a worry, but those interested in air quality might need to consider upgrading engine management that can be programmed not to squirt so much extra fuel into engine during cranking and startup, even though amounts are still minimal, but if we would not be interested from minimal, then would we fill panel gaps?

That is at least how I see this subject.
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Old 03-17-2012, 01:59 PM   #46 (permalink)
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the answer!

Originally Posted by hackish View Post

The complicated engineering answer seems to say P&G cannot work. I've always questioned it myself. Can anyone describe how it possibly could work? Maybe it's a component of an overall driving style that as a whole provides gains?
I know this is an old thread, but I ran across it doing research, and I noticed that noone ever answered this question.

I had the same question myself, and it seems counter-intuitive, but there actually is a very good answer to it!

Here is why it works:

An ICE engine is, inherently, inefficient. Even in a prefect world there is a theoretical upper limit to any heat engine's efficiency.
In a typical gas engine, it is something like 25%.
In other words fully 75% of the energy in the fuel is used just to overcome internal resistance, to turn the engine itself.
On top of that, belt driven engine accessories and drivetrain losses absorb another 5-10% of the energy in the fuel.

That means all together, when the engine is running, more than 3/4 of the energy never even reaches the wheels.
And that is in optimal conditions, at the peak of the BSFC curve!

Any time the engine is off, you are not wasting that 75%.
However, the internal losses don't change (a significant amount) in acceleration vs stead state.

So, if you can P&G with a 1:3 ratio (10 sec pulse, 30 sec glide) the engine is off 3/4 of the time. Over that 40 sec span, the 75% of potentially wasted fuel energy is conserved.
In order for that not to save fuel compared to stead state, it would have to require 4x more fuel to accelerate - and of course that is before even taking into account pumping losses or the fact that acceleration puts the engine in a more efficient part of the BSFC curve.


Originally Posted by Piwoslaw View Post
A few months ago I returned home just as my neighbor pulled into his driveway. It was cold (around freezing) with some rain and sleet, and he yells to me: You rode your bike? In this weather?!?

So the other day we both returned home at the same time again, only now the weather is warm, sunny, with no wind. And I yell to him: You took the car? In this weather?!?

Last edited by JacobAziza; 03-18-2012 at 09:05 PM..
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