Problems with Pulse and Glide.

[JockoT]

Today I was out in the car, by myself, and traffic was extremely light. I tried Pulse and Glide, accelerating up to 60 mph, then knocking it out of gear until speed dropped to 35 mph before accelerating again. I found it very unsatisfactory because the speed bled off quicker than I was able to build it back up. To be fair, I accelerated in 5th gear (changing down to third would have put me in the 12 - 15 mpg category without a huge improvement in performance), but the real problem was the lack of inertia. I even thought I had a brake grabbing, but I jacked car up on my return and checked. The brakes were cold and the wheels spun freely. The problem seems to be a small, light, not particularly aerodynamic and under-powered car. It wasn't even windy and the road was dry and fairly flat. Don't know what to do.

[Daox]

P&G primarily helps when your vehicle's engine can provide much more power than is needed to push your car down the road. The pulse operates the engine at peak thermal efficiency, and the glide minimizes fuel usage when the power isn't needed. If you're already cruising at close to peak efficiency, then the results are much less. Also, at lower speeds you'll find P&G is more effective at increasing your fuel economy because the lower speed doesn't require as much power, thus your engine is oversized for that speed.

With your quite un-aerodynamic car and smaller engine, I bet you won't see huge gains from going to P&G. The only way to know for sure is to A-B-A test it. That should give you an idea of how much P&G is good for.

[cowmeat]

The problem seems to be a small, light, not particularly aerodynamic and under-powered car.

I was able to P&G my way to around 60 mpg in my old 1993 Ford Festiva, which had the basic aerodynamic shape of a brick glued to the front of a cinder block. There's a learning curve to P&G, but pumping the tires up and using double brake clips on the front may help a little.

Black Widow (Ford Festiva) | Fuelly

[Gasoline Fumes]

Originally Posted by JockoT

The problem seems to be a small, light, not particularly aerodynamic and under-powered car. It wasn't even windy and the road was dry and fairly flat. Don't know what to do.

My car is the same. On flat roads, I just keep a constant (and slowish) speed. I coast down hills and to stops. Gentle rolling hills are good for pulse & glide with cars like ours. Pulse up - glide down.

I tried a whole tank of constant P&G with my car a few years ago. I saw no improvement in MPG. And that was with engine-off coasting.


Edit: My 1,000th post on EcoModder!

[oldtamiyaphile]

Originally Posted by Gasoline Fumes

I tried a whole tank of constant P&G with my car a few years ago. I saw no improvement in MPG. And that was with engine-off coasting.

Most petrol cars will run open loop (rich) after a restart, which may be enough to counteract any gains from short P&G cycles.

[JRMichler]

Originally Posted by JockoT

The problem seems to be a small, light, not particularly aerodynamic and under-powered car.

This. Since you verified that you do not have brake drag.

You could watch intake manifold vacuum. If you consistently run at 5 to 10 or 12 inches vacuum, you have nothing to gain by P&G. Focus on DWG and DWL.

P&G benefits those who run at 15 inches or greater.

[JockoT]

Originally Posted by JRMichler

You could watch intake manifold vacuum. If you consistently run at 5 to 10 or 12 inches vacuum, you have nothing to gain by P&G. Focus on DWG and DWL.

I don't have a vacuum gauge, but my ScanGauge E is set to indicate MAP. It usually indicates 9 - 12 psi. Wide open throttle is 14+ psi and closed about
3.5 - 4 psi.

[JRMichler]

For those living less than about 1000 feet above sea level:

0 inches vacuum is about 14 PSI MAP
5 inches about 11.5 PSI
10 inches about 9 PSI
15 inches about 7 PSI
20 inches about 5 PSI

So you are right in the range of "forget P&G", and use DWL and DWB.

The exact conversion is 29.92 inches to 0 PSI at sea level, decreasing 1 inch per thousand feet. And 0 inches to 14.7 PSI. That's why I said "about" in the above table.

[Fat Charlie]

Nothing happens in a vacuum.

P&G can help a ton, but you've got to work the terrain, traffic and car. Use downhills and times when you'd be slowing down anyway. Use a 5-10 mph window instead of the 25 mph window you're talking about- your pulses will be a lot shorter, and by using circumstances better your glides will be longer. Make sure they're appropriate for the gearbox, because as you've spotted, pulsing from 35 in 5th just isn't right.

[JockoT]

I always glide down hills and to a stop. I also glide into roundabouts, common hereabouts (there are seven on the 1.8 miles I drive to take my wife to work). DWL and DWB are my primary methods of getting the numbers up. I am now averaging
60 mpg (UK)/50 mpg (US) for the last 10 tankfuls (calculated).