Is pulse & bleed a valid technique? - Page 2 - Fuelly Forums

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Old 11-28-2006, 08:55 AM   #11
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I don't think it's likely to do any good vs. cruising at a reasonable speed, preferably while DWL to keep engine loads down. True P&G works because it allows the engine to impart kinetic energy on the vehicle for a short period of time and then shut down completely. The combustion process for a typical engine is horribly inefficient, with lots of energy going out the tailpipe as well as to internal resistance and pumping losses. Accruing mileage without those losses brings in the gains. The problem with "pulse and bleed" is that you never cancel the internal resistance, and if anything the pumping losses are worse due to higher vacuum during the "bleed" portion. I would be interested to see a controlled experiment but I think it would be a wash at best.
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Old 11-28-2006, 09:17 AM   #12
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It would only help if the normal cruizing speed was a bad operating point of the engine efficiency.

Case in point - the Scion tC apparently gets over 30mpg at speeds over 75-80mph but 26-28mpg at 50-60mph speeds.
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Old 11-28-2006, 09:24 AM   #13
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I've read that kind of claim a few times for different cars, and I'm skeptical. I wouldn't believe it without seeing controlled data (speed vs. FE), eg:





I get the sense that people saying that about their cars are deceiving themselves to justify driving faster.
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Old 11-28-2006, 09:37 AM   #14
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The tC is a really aero sports car with 160hp engine and it may not get up to a good temperature until it is pushed a bit. Engine is probably not running efficiently at all at low speeds. I think it is smaller than my Geo was and more aero for sure. Guys were running Scangauges and took readings. I think they were drafting at the higher speeds without realizing it. The wake at 75mph is pretty far back and if there are enough big vehicles pushing the air ahead of you it can cover the entire highway and extend backwards for hundreds of feet. I watch the leaves on the road and look for which way they are moving.

MAN your graphs really tell the story! Wish I could find a long level road to take some readings at different speeds. Friend has a Honda Element and was drafting a bus at about 10 feet and getting 40mpg normally he gets 25mpg.
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Old 11-28-2006, 10:50 AM   #15
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MetroMPG : How about engine-on coasting?. Pulse, then just coast with the engine on. This is halfway between engine-off-coasting, and pulse-and-glide!. And that would show savings over remaining in gear. I do this when there isn't time to engine-off-coast; just remember to double-declutch to save the synchro.
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Old 11-28-2006, 11:16 AM   #16
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landspeed -

Quote:
Originally Posted by landspeed
MetroMPG : How about engine-on coasting?. Pulse, then just coast with the engine on. This is halfway between engine-off-coasting, and pulse-and-glide!. And that would show savings over remaining in gear. I do this when there isn't time to engine-off-coast; just remember to double-declutch to save the synchro.
I do the engine-on coasting all the time (one reasons is urban high density traffic conditions in LA). Can you explain double-declutch to me?

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Old 11-28-2006, 11:21 AM   #17
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Engine on coasting would be too much effort still, you lose speed so quickly, and after 8 hours, you just don't want to mess with shifting around and all that every 30 seconds, the idea is to have a very prolonged, and easy to accomplish, bleed.
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Old 11-28-2006, 11:35 AM   #18
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Throttle lock is the only way that would be easy and a bit steady - You would automatically slow for hills going up and speed up going down them. I don't see any way of improving MPG at highway speeds due to the air drag levels. Of course any reduction of the revolutions of the engine total will / should save some fuel.
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Old 11-28-2006, 11:43 AM   #19
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I was thinking about this very subject over the past few days. Pulse and Bleed will be on my variable list for the upcoming Prius testing. One case in which there might be a benefit would be with a car with lean burn (VX, HCH I, Insight). If you want to cruise at a speed that is too fast to maintain lean burn, instead of constant throttle in non-lean burn mode, you could accellerate in non-lean burn then bleed down in lean burn. Your overall speed would be faster and your FE should be higher. Just my opionion though. No data to back it up.
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Old 11-28-2006, 12:02 PM   #20
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Metro

I do this alot! I look at it as load and bleed. It is a basic standard tool for me. I use the lay of the land to make it happen. There always has to be I very slight up hill for the load or say pulse if you will. and then a slight bleed on the ever so slightly downhill. I never see so called flat ground to work with,,, Thank GOD!!!
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