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

[Christ]

You're right, you shouldn't follow my example - I screwed up the math horrendously there.

Accelerating the weight is definitely a factor, and no matter what anyone says, I believe it takes more energy to accelerate more mass, and more mass doesn't give an equivalent extension on your glide. IOW - if you add mass to the extent that it takes 30% more energy to do the same acceleration, you should see a 30% longer coast on flat ground, in theory. I don't believe this is true in practice, though.

I, too, would like to see "real" numbers, rather than just postulations on the apparent upside of P&G.

[MetroMPG]

FYI, Tasdrouille sometimes P&G's his diesel Jetta. He sees an improvement in fuel economy compared to steady state driving with load (DWL).

[tasdrouille]

Power units at high load cost less fuel than at low loads. That is shown by bsfc maps, and that's the reason why p&g works.

Lets just make the calculation from the TDI bsfc map.



We will compare steady state at 1800 rpm in 5th (~55 mph) to 1/3 pulse and 2/3 glide in neutral between 1500 and 2200 rpm in 5th for an hour.

Assumptions/approximations:

- The idling engine requires 2 hp
- Steady state at 1800 rpm requires 15 hp
- Pulsing 1/3 from 1500 rpm to 2200 rpm and gliding back 2/3 the distance back to 1500 rpm leads roughly the same speed as 1800 rpm constant.

So, steady state is easy, 15 hp (11.18549808 kW) at 250 g/kW-Hr is 2.79637452 kg of fuel used to drive an hour at 1800 rpm.

Pulsing and gliding for the idle part we have 2 hp (1.491399744 kW) for 2/3 Hr at 500 g/kW-Hr which means we'll use 0.497133248 kg of fuel for the idling parts of the P&G segment.

Now for the pulsing part between 1500 and 2200 rpm we'll average bsfc to 200 g/kW-Hr and hp to 45.

So we have 45 hp (33.55649424 kW) for 1/3 Hr at 200 g/kW-Hr which gives 2.23709961 kg of fuel

So in the end:

2.79637452 kg of fuel used for an hour cruising at 1800 rpm (~55 mph). That makes roughly 64.34 mpg
2.734232858 kg of fuel used for an hour pulsing and gliding at a speed average roughly the same, but probably just a tad higher than steady state 1800 rpm. That makes 65.81 mpg.

There are a lot of approximated assumptions which could have a relatively important impact on the results in this calculation, but at least you get the idea.

Personally I've been able to increase mileage by ~10% while P&G at highway speeds.

[roflwaffle]

Overall, P&G is just about how much energy out of the engine we can get compared to the amount of fuel we use. If we were fortunate enough to have ten speed transmissions like big trucks have, we proably wouldn't have to P&G much, but unfortunately we don't, and because of this we sometimes can't keep the engine operating at a speed/load where fuel consumption is minimized.

Transmission losses, at best, drop by a few percent at high torque, so there isn't a huge incentive to P&G from that end, and this can be offset by a richer mixture during acceleration, especially w/ OBDI and earlier vehicles. So really, it comes down to how far off of the minimum BSFC/maximum engine efficiency someone is during whatever driving situation. If they can move along at 35mph, or 55mph, or whatever, and the load is high enough, then P&G probably isn't worth it. If they're in an unfavorable portion of the BSFC map, then P&G may be able to help out w/ mileage.

[almightybmw]

ignoring efficiency talk, going back to wear and tear, take a gander at FedEx and UPS. How often do those trucks turn off and on in a given day? I watched a driver shut down, deliver, fire up, drive 50ft, shut down, deliver, fire up and drive away. Could he have walked that 50ft faster? who knows, but start-stops are very common on those rigs. I think you'd be alright with cutting the motor on glides, but if you're hauling 3.6T, I'd be VERY hesitant to cut the thing that will help stop me. I'd settle for idling.

[JacobAziza]

Originally Posted by almightybmw

I'd be VERY hesitant to cut the thing that will help stop me.

I replaced my mechanical vacuum pump with an electric, so the engine doesn't have any effect on the brakes at all.

When I want to stay slow on a downgrade, I hold the fuel cut-off switch on the gear shift while leaving it in gear - saves fuel, keeps the oil pump circulating, and helps stop more effectively all in one.

[MetroMPG]

Originally Posted by almightybmw

I'd be VERY hesitant to cut the thing that will help stop me. I'd settle for idling.

Do modern diesel passenger vehicles not have vacuum reserves for one or two full-assist stops with the engine off, like every gasoline car I've ever driven?

[JacobAziza]

Originally Posted by tasdrouille

Assumptions/approximations:

- The idling engine requires 2 hp
- Steady state at 1800 rpm requires 15 hp

(I'm assuming that is a diesel specific BSFC map?)

Why idle? EOC requires 0hp

What did you base the numbers 15hp for steady state and 45hp for accelerating from 45mph to 67mph on?

Steady state should depend on the Coefficient of drag,
Acceleration should depend on both the weight of the vehicle and the rate of acceleration.

In a motorcycle, very low weight and high CoD, it would take less hp to accelerate relative to steady state; in theory the benefit would be more pronounced. On the other hand, my truck is much more aerodynamic than my EX250 bike, and weighs 11 times as much, so lower power needed for steady state and higher to accelerate. If your numbers were for a car, considering how close they ended up, it still seeming like I shouldn't expect to see any benefit.

[JacobAziza]

Originally Posted by MetroMPG

Do modern diesel passenger vehicles not have vacuum reserves for full-assist stops with the engine off, like every gasoline car I've ever driven?

IDK, my truck isn't exactly modern, but it did not come with a vacuum reservoir. There is always residual vacuum in the brake booster though, enough for at least one stop.
I relied on that for a while, but an electric vacuum pump (and manual steering box) is really the way to go. Now EOC makes exactly zero difference to the handling and control of the truck in every way.

[tasdrouille]

Originally Posted by JacobAziza

(I'm assuming that is a diesel specific BSFC map?)

It is for the TDI engine found in my car.

Why idle? EOC requires 0hp

Good question. My turbo is oil cooled. I'm not sure repeatedly boosting at the maximum output and then cutting the oil supply would be a very good idea in the long run.

What did you base the numbers 15hp for steady state and 45hp for accelerating from 45mph to 67mph on?

The 15 hp is an approximation it takes my car to keep going down the highway at roughly 55 mph.

The 45 hp is the average available power I pulse at. You can see the hp lines on the bsfc map. Close to the maximum power output available in this rpm range.

Honestly the biggest guess here is not the power requirements, but whether the 1/3 pulse 2/3 glide ratio is right. This is what have the biggest impact on the outcome.