Is there any point to P&G in a diesel?
 

I thought for sure someone would have asked this already, but nothing turned up in the search...

My understanding is that the whole reason pulse and glide improves mpgs is because during acceleration the throttle plate is open, reducing the force needed to suck air past it, otherwise known as "pumping losses".

A diesel regulates power by injecting less fuel, and always has an unrestricted air supply (which is why it has no vacuum and therefor needs a separate vacuum pump to run the brakes).

Therefor, not having any throttle plate, and therefor no pumping losses, it seems P&G should provide no benefit at all the diesel vehicles (aside from when coasting because of hills or red lights) - in fact, I imagine it would result in slightly worse fuel economy, due to the extra force required to accelerate.

Anything I am not factoring in? Any data or theory to support or counter that P&G might be effective in diesel vehicles?

Another thing to consider is that p&g effectively reduces engine and much drivetrain friction. If you pulse for 1/3 of the time then your engine (and valves and alternator and???) and most of your transmission gears are not turning for 2/3 of the time you are driving.

And of course if you glide up to stops and wait with the engine off, then less to zero fuel spent idling.

The throttle thing is only part of the P&G equation.

The biggie is portion of miles travelled with engine off vs engine on.

Say your glide is 3x the distance of your pulse. In 100 miles your engine is only on for 25 miles. A diesel should show improvement with that scenario too.

Actually ALL the trans gears are turning whenever the vehicle is moving, no matter what you do.

I'm not so sure, the metro actually makes gear noise and seems to decelerate faster in gear (clutch disengaged) vs in neutral (clutch in or out).

The engine isn't turning as often, however every time you reengage, it is starting with 0 oil pressure and has to build up again, so theres a second or two turning w/o oil which should cause a little wear, possibly more than steady state driving.

About being on only part time: if there is no penalty to steady driving (since there are no pumping losses) then accelerating should take more fuel than constant velocity (need power to change velocity, but to stay at a set speed only requires overcoming air resistance).
Outside of the engine losses, it should take a set amount of energy to go a certain distance at a certain speed, so if you have the engine off 50% of the time, presumably you should need to use twice the fuel when the engine is on, and they would cancel out (at best)

I can't help you quantify the wear concerns, suffice it to say some have P&Gd for hundreds of thousands of miles, and even in diesels.

Is there a point? Yes, it does help, people have demonstrated its advantages in diesels. It does not as much as it helps with a gasser, but rotating the engine and accessories 2/3 less helps in efficiency and perhaps in longevity.


There IS a significant "penalty" to steady state driving. keeping the pistons and valves and alternator and accessories and bits of the transmission all moving takes a significant amout of energy.

Hmm...

I suppose it would come down to the amount of internal engine friction losses being more energy than the additional energy needed to accelerate (alt and accessories don't factor for me, cause I don't have any anymore)
Mine is too old for a scan guage.

Anyone with a diesel and instrumentation that can quantify any loss/gain?

That's because when you leave it in gear, you're also spinning the clutch disc and input shaft, and your clutch is probably not adjusted properly, and thus dragging slightly on either the flywheel or the pressure plate, but not enough to cause significant trouble when shifting, or even noticeable trouble.

Even if you're not dragging your clutch, you're still wasting more of the momentum to spin the clutch disc while you're in gear.

The nature of a manual transmission leaves all the gears engaged on each other 100% of the time.

I don't follow your example.
During the glide you should be getting infinity mpg (instant).

If you get 8mpg during the pulse, over the 10 miles worth of pulse phase you use 1.25 gallons of fuel. Since the rest of the time the engine is off, you also used 1.25 gallons over the whole 40 miles, which works out to 32mpg.
So the question in this example would be, if you get 8mpg while accelerating, and can coast 3x for each pulse, do you get more or less than 32mpg from driving at a steady 35mph?

I have no idea how much more fuel it takes to accelerate the mass of the vehicle than it takes to maintain a given speed. That, and how far one can coast would determine the trade off. I can see how it might work either way, which is why hopefully someone with a diesel and a scan gauge can provide us some real numbers.

In my case, my vehicle is very heavy, so accelerating may be more a factor than for most of you?
I noticed when I had a 3.6 ton load recently that it would coast more than twice as far as I was used to, but none the less my mileage dropped from the 24-26 I've been getting the past few months to 22.