[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.