Pulse & Glide or In 5th Gear

[actwithclarity]

P&G wins for fuel efficiency by far. Now it's only a matter of pulsing and gliding every 20 seconds for your 2 hour trip. But driving at constant throttle can get boring too. Usually I just do a mash up of whatever I feel interested in, and traffic conditions factor in as well..

[CigaR007]

I find that P&G requires too much work.

I try to do my best in 5th gear.

[hackish]

I think P&G is the biggest farce out there. Physics does not support it. People who claim it gives them better fuel economy are seeing gains because of a lower average speed. They pulse up to 70mph and coast down to 50mph then compare it with a constant drive at 60mph while in actual fact their average speed is more like 56mph.

If it were valid then why would the scientists who work on eco - competitions not do it? Why wouldn't hybrids do it too? I mean they could P&G with electric and fuel.

An accelerating engine consumes more fuel per unit of energy produced. Since transporting your vehicle from point A to point B requires that x amount of fuel be turned into energy the most efficient way is to output the energy required in such a way that it consumes the least fuel, not deliver the energy in inefficient spurts.

Finally, P&G pollutes significantly more. Most vehicle emissions are tied to the cat temps. When you shut the engine off, the cat cools below it's effective range, restart it and for the next 20-30 seconds you will be sending somewhere around 5-10x the pollutants out the tailpipe.

[dennyt]

I'm with Arragonis here - I cruise at the posted speed limit on long flat stretches with traffic, but I also P&G with EOC when hills, stops, and traffic allow it.

Originally Posted by hackish

If it were valid then why would the scientists who work on eco - competitions not do it?

Because they build cars with small enough engines that they are operating near full throttle at steady-state cruise. Compare this to our daily drivers with engines sized for acceleration and "sportiness".

Also, it's not just "scientists" doing it - NASCAR of all places has started using pulse & glide.

Originally Posted by hackish

Why wouldn't hybrids do it too? I mean they could P&G with electric and fuel.

I've had this thought before as well. It would be fun to test!

Just because something isn't available on the market, doesn't mean it's not a good idea.

Originally Posted by hackish

An accelerating engine consumes more fuel per unit of energy produced.

Most engines have the lowest BSFC near 80% throttle at low RPM. Of course acceleration enrichment varies by fuel system, but in a high gear the RPM rate of change is pretty slow.

Originally Posted by hackish

Finally, P&G pollutes significantly more. Most vehicle emissions are tied to the cat temps. When you shut the engine off, the cat cools below it's effective range, restart it and for the next 20-30 seconds you will be sending somewhere around 5-10x the pollutants out the tailpipe.

This is an interesting consideration that isn't discussed much around here, perhaps because we don't have emissions instrumentation on our dashboards. Does anybody have experience driving around with emissions instrumentation?

[Daox]

Originally Posted by hackish

I think P&G is the biggest farce out there. Physics does not support it.

I don't know where you get your info, but physics totally supports it. Look at any BSFC map for any engine out there and you can see they're FAR more efficient at higher loads.

People who claim it gives them better fuel economy are seeing gains because of a lower average speed. They pulse up to 70mph and coast down to 50mph then compare it with a constant drive at 60mph while in actual fact their average speed is more like 56mph.

It is true to some extent that average speed speed is lower than just cruising at a set speed. However, you're talking a couple MPH at most. My personal experience shows I can increase my mileage at least 50% with P&G vs constant speed cruising.

If it were valid then why would the scientists who work on eco - competitions not do it?

Um, all the MPG challenge built cars I've seen do use P&G. Just the other weekend at drive green expo I talked to a team from Illinois and they were using P&G with their vehicle.

Why wouldn't hybrids do it too? I mean they could P&G with electric and fuel.

You don't get the electricity for free. Its more efficient to just run an engine than have an engine run a generator to spin an electric motor. This is one of the reasons the Volt is why it is.

An accelerating engine consumes more fuel per unit of energy produced. Since transporting your vehicle from point A to point B requires that x amount of fuel be turned into energy the most efficient way is to output the energy required in such a way that it consumes the least fuel, not deliver the energy in inefficient spurts.

Again, BSFC maps show this not to be the case.

Finally, P&G pollutes significantly more. Most vehicle emissions are tied to the cat temps. When you shut the engine off, the cat cools below it's effective range, restart it and for the next 20-30 seconds you will be sending somewhere around 5-10x the pollutants out the tailpipe.

This probably has some basis, and it would be very interesting to see it tested. But to say its completely true is rediculous unless you have some testing to back it up.



The short of it is P&G is the only way to get outstanding mileage without lean burn or some sort of displacement reducing technology. You NEED your engine to run at high loads or it is running inefficiently, even if its a diesel. This is why smaller (not bigger) engines get better mileage in the same vehicle. Its also why taller gearing returns better mileage.

[Fat Charlie]

Originally Posted by hackish

I think P&G is the biggest farce out there. Physics does not support it. People who claim it gives them better fuel economy are seeing gains because of a lower average speed.

Yep. There's no rational way to account for my jumping from a long line of 22 mpg tanks to a 28 mpg tank right after reading about P&G. It must be all because of the 5 psi I added to my tires.

[PaleMelanesian]

There is no way my car could ever do 80+ mpg at any speed without P&G.

Hybrids (some at least) do p&g, just in a different way. The Prius, for example. It runs the engine at it's efficiency peak. If that's more power than needed right now, the extra goes to charging the battery pack. If it's not enough power, it draws some off the pack to the motor for the extra needed. It's seamless and smooth and brilliant.

[California98Civic]

Originally Posted by hackish

I think P&G is the biggest farce out there. Physics does not support it. People who claim it gives them better fuel economy are seeing gains because of a lower average speed. They pulse up to 70mph and coast down to 50mph then compare it with a constant drive at 60mph while in actual fact their average speed is more like 56mph.

If it were valid then why would the scientists who work on eco - competitions not do it? Why wouldn't hybrids do it too? I mean they could P&G with electric and fuel.

An accelerating engine consumes more fuel per unit of energy produced. Since transporting your vehicle from point A to point B requires that x amount of fuel be turned into energy the most efficient way is to output the energy required in such a way that it consumes the least fuel, not deliver the energy in inefficient spurts.

Finally, P&G pollutes significantly more. Most vehicle emissions are tied to the cat temps. When you shut the engine off, the cat cools below it's effective range, restart it and for the next 20-30 seconds you will be sending somewhere around 5-10x the pollutants out the tailpipe.

Every one of these assertions is incorrect or unsubstantiated, for all the reasons other have cited and more. But you should also take a look at the test I recently did on P&G technique. Note the average speeds.

[kc7ekk]

I have been practicing to Pulse and Glide on a 12 miles stretch of highway. What have you found to be the best ratio for pulse:glide?
1: Pulsing in 5th gear and glide ratio is a 10 second pulse vs a10 second glide.
2: Pulse in 4th gear with higher RPM, fuel consumption and power band is a 4 second pulse vs a 10 second glide.
As my 1992 Honda Accord is pre-computer, I have to wait until my next fill-up to see if this technique is working.
I was getting 76% above EPA before I started my P&G experiments. My goal is to get 100%.

John

Drive your ICE like a MOSFET switch. All on or all off.

[Vekke]

With my 3L lupo I found first test that push to 90 km/h and glide to 75 km/h gived me 2.5 l/100km. But if I did the push and glided with gentle load (tried to keep instant fuel consumtpion at 2 l/100km mark) that gived me better result which was 2.3-2.4 l/km. So with my current knowledge on the lupo push and glide with gentle load works best. On my old seat the full P&G was the clear winner.