Is pulse & bleed a valid technique?
 

Anyone have any info or experience on whether this would offer a net gain over just cruising at the same average speed?

I tend not to P&G on long highway trips to minimize wear on the car (and the driver).

But I've often wondered if this "mild" form of P&G would be productive: Pulse up, then bleed off the speed holding a relatively high instantaneous MPG readout for the duration of the bleed.

You can sustain a high-mpg bleed much longer than an ICE-off coast.

But is it enough to offset the FE hit from the pulse?

Anyone looked at this methodically yet? (I'm just being lazy about running the numbers).

What do you mean by bleed?

Bleeding: just really slow deceleration (in gear, engine on). ("Bleeding off" speed.)

It's not difficult to see 80-90+ mpg on the instant readout if I back off the throttle starting around 80 km/h, bleeding speed at a rate of roughly -1 km/h every 2 or 3 seconds.

I do this just to avoid the difficulty of keeping my pedal pressure constant, I feel like it probably helps, like the steep hill/slow downgrade is the best type for hill driving, if you get my bad analogy.

Probably depends on the rpm your car is at and whether bleeding involves any throttle. If your engine speed drops below some set range, according to coolant temp usually, then the engine will essentially start idling in gear, which is as in efficient as it gets. Otoh, if you're bouncing around above the idle fuel cut, then I'm pretty sure you could see an increase in mileage. I could see it being useful for high speed driving, but most FI cars end up idling in gear below 45-55mph. You could put an injector switch to get around that, although I'm not sure if the ECU would like it.

I get what you're saying. And based on your FE on the trip to school, it didn't seem to hurt!

But I have a suspicion if it does work, it's going to be fairly close to the FE you'd get just cruising at the same average speed. Or maybe a wash?

My definition of bleeding definitely involves throttle pressure. It's like: achieve peak speed, then back off the throttle nearly the smallest amount possible. Instant FE spikes up as speed s l o w l y falls. Fuel cut-off wouldn't be active for that kind of decel rate.

If there's throttle pressure then you shouldn't see any (significant) gains. It seems like a net sum deal to me, even though you're pulsing to decrease the BSFC, the proportionally lower load, higher BSFC bleed will wipe out that gain, with the whole thing being equivalent to cruising at the average speed. An experiment seems to be in order. :D

Yeah, I'm kind of thinking what you said.

I was just hoping maybe someone had already looked at this methodically. I don't want to add another one to the to-do list. It's already overflowing. :)

It seems that it has no positive benefit. And may actually hurt economy, since, for some cars anyhow, constantly accelerating/decelerating may prevent EGR from functioning as well as it could. Assuming of course that the idling below the fuel cut corresponds to light throttle.

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.

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.

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:

https://www.metrompg.com/posts/photos/gcc-autobild1.gif

https://www.metrompg.com/posts/photos...ed-chart-z.gif

I get the sense that people saying that about their cars are deceiving themselves to justify driving faster.

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.

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.

landspeed -

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?

CarloSW2

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.

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.

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.

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!!!

cfg83 : I am assuming the car is a manual. Double-declutching is just a way of reducing the stress on the synchromesh when changing gear.

Normally, if you are coasting, then input shaft will be spinning the speed of the engine (700RPM), whereas the output shaft will be spinning at a different speed.

When you go to put the car in gear (with the clutch down), the two shafts must be spinning at exactly the right speed, or the gear cogs will crunch badly. Synchromesh makes the input shaft spin at the correct speed as the output shaft, so that they can engage properly. If the synchro is worn, then the gears crunch (even with the clutch pressed down).

When changing from 1st-2nd, 2nd-3rd, 3rd-4th, 4th-5th, you can quickly change gear (synchro will help you, gearstick will have some resistance to
going in gear), or gently do it (gently resistance, when speeds match, gear will go in).

If changing down from 4th - 3rd... Or, from neutral - 5th, the synchromesh has to do a lot of work. Double-declutching can reduce the amount of work it has to do.

examples of double-declutching

(1) Car is in neutral, coasting along at 40mph
(2) If 5th gear would be 1500RPM, then rev engine to 1700RPM, *then*
press clutch in, change gear, and it will change a lot more easily.

(1) Car is in 5th, and you want to change to 4th
(2) Press clutch down, put it in Neutral
(3) Rev engine to a little bit above the RPM you would want to be in 4th
(4) Press clutch down, and change gear.

Double-declutching can also be useful for cars with worn synchromesh, particularly on 1st gear (like mine) :)

SVOBoy : I do engine-on coasting, but, the motorways here tend to
have lots of rolling hills, and the lorries are speed limited to 56MPH by
law - gives lots of opportunity for getting good fuel economy!

Well put. Pulse and glide no. Pluse and bleed yes. I also do this quite a bit using the terrain as your friend. No conclusive data except that the 90 day MPG figure kept going up once I got it down.

lol, what kind of car makes the EEEEEEoooo noise?

Also, isn't pulse and bleed exactly what we are tryign to avoid when we use cruise control? I thought that it's more efficient since we're not constantly speeding up and slowing down?

hahaaha i thought you were talking about bleeding the brake lines. or maybe the clutch line.

I've tried 'pulse and bleed' with the piece of **** Suzuki I drive. Didn't work well in my case. Better kept at a constant speed.

While of no intention of my own, that is pretty much what the Triumph sounded like. Only louder and even more obnoxious. Then when you step on the gas...
"R-I-I-I-I-I-I-I-I-I-NNNNNNNN". Like a Harley getting *** raped by a Formula race car... Had no problem setting off alarms on parked cars.

omgz, i hated that my beater diesel sounded like rice. but a glasspack was $20 while a new exhaust would've been at least $100, and running around with an unmuffled diesel is just crazy. sounded like a tug boat or something, i could idle through a parking garage and set off alarms.
https://www.fuelly.com/attachments/fo...dd07625374.jpg

landspeed -

Yes, a manual.

Ok, I *almost* do something like this already, because I hit the accelerator right before I press the clutch to go from Neutral to 5th, but I am not consciously trying to match RPMS, it's just a smoother way to engage the gear. I am lucky in the sense that with my car, in 5th gear, the MPH and RPM needles "line up" at the same angle, so it is easy to "match the needles" to engage 5th.

CarloSW2

I never thought reading a series of posts would be so loud! My eyes are ringing!!! :eek: ROFLMAO !!!

And lets not forget the rice burners honnnNNNNNNNNnnddaaaa yaaaAAAAAAAAMMMMMMMMAAAAAAhhhhaaaaaaaaaaa and KaaaaaAAAAAAWAsssssaaaaakkkkkkkkkiiiiiiii