Is pulse & bleed a valid technique?

[MetroMPG]

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).

[omgwtfbyobbq]

What do you mean by bleed?

[MetroMPG]

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.

[SVOboy]

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.

[omgwtfbyobbq]

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.

[MetroMPG]

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?

[MetroMPG]

Originally Posted by omgwtfbyobbq

Probably depends on the rpm your car is at and whether bleeding involves any throttle.

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.

[omgwtfbyobbq]

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.

[MetroMPG]

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

Originally Posted by omgwtfbyobbq

An experiment seems to be in order.

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.

[omgwtfbyobbq]

Originally Posted by zpiloto

I played around with this not as an experiment bases but just to see if I need to go further with it. My car is an automatic. I was plusing 5 miles over the posted speed and gliding to 10 under. I did this over a 40 mile test loop with rolling terrain, mixed driving and multiple stops. Traffic is never a factor. Just one run plusing and the other normal driving style. Avg speeds for the loop were 36(plusing) and 40. The Plusing FE was 38.2 and the normal style driving(if you can call it that) was 38.6.
Take the results with a grain of salt. I'm sure there are better way to pulse than what I was doing and with time it would probably make a difference once you perfected the technique but for me with the auto it just not worth the effort.

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.