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-   -   Is there any point to P&G in a diesel? (https://ecomodder.com/forum/showthread.php/there-any-point-p-g-diesel-9664.html)

JacobAziza 08-14-2009 08:19 PM

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?

dcb 08-14-2009 08:30 PM

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.

Frank Lee 08-14-2009 08:31 PM

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.

Frank Lee 08-14-2009 08:34 PM

Quote:

Originally Posted by dcb (Post 121366)
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.

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

dcb 08-14-2009 09:06 PM

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).

JacobAziza 08-14-2009 09:09 PM

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)

dcb 08-14-2009 09:57 PM

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.

JacobAziza 08-14-2009 10:05 PM

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?

Christ 08-14-2009 10:30 PM

Quote:

Originally Posted by dcb (Post 121370)
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).

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.

JacobAziza 08-14-2009 11:20 PM

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.

Christ 08-14-2009 11:30 PM

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 08-20-2009 09:49 AM

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 08-20-2009 12:28 PM

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.

http://cr4.globalspec.com/PostImages...C650E40A3F.bmp

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 08-21-2009 12:48 AM

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 08-21-2009 07:38 AM

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 08-21-2009 11:21 AM

Quote:

Originally Posted by almightybmw (Post 122636)
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 08-21-2009 11:30 AM

Quote:

Originally Posted by almightybmw (Post 122636)
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 08-21-2009 11:35 AM

Quote:

Originally Posted by tasdrouille (Post 122490)

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 08-21-2009 11:38 AM

Quote:

Originally Posted by MetroMPG (Post 122659)
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 08-21-2009 12:11 PM

Quote:

Originally Posted by JacobAziza (Post 122660)
(I'm assuming that is a diesel specific BSFC map?)

It is for the TDI engine found in my car.

Quote:

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.

Quote:

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.

bluetwo 08-21-2009 12:44 PM

I say any time you can coast regardless of the grade it's worth it to pulse a bit or get up a little extra momentum so you can get the most out of the coast or glide. I hate the term pulse and glide because it just sound deceptive, as if I'm not just accelerating then coasting. But whatever, glide whenever you can is my advise but don't get too carried away with trying to pulse and glade all the time.

JacobAziza 08-21-2009 01:39 PM

Ah, yes, I do always coast when possible with just the slightest down grade. It changes everything when you have gravity on your side.

When I first posted this, I was thinking about perfectly level ground.

bluetwo 08-21-2009 01:41 PM

Right.... and I'm not sure if downhill was meant but the main thing I think people should take from it is unless you're in a car with Hybrid Synergy Drive you shouldn't try to pulse and glide all the time like it's going to give you much better mileage. It works for those cars because the engine can autostop while the car is still at decent speed.

dcb 08-21-2009 02:08 PM

I don't follow. One can stop the engine anytime without owning a synergy whatsit.

MetroMPG 08-21-2009 02:28 PM

bluetwo: pulse and glide was invented decades before hybrids hit the market.
FYI, see: http://metrompg.com/posts/xfi-pulse-and-glide.htm

As dcb points out, it just takes a bit more fussing in a non-hybrid.

In a hybrid, lifting off the throttle kills the engine. Then you finesse throttle position to ensure you're truly "gliding" (no regen, no battery assist). In a non-hybrid, you kill the engine / shift to neutral.

bluetwo 08-21-2009 04:46 PM

I just hope people will be safe when they're navigating a 2,000 lbs. plus vehicle and realize that most of the laws and guidelines are put into place with the safety of everyone in mind. For the sake of the one driving and for all everyone else young and old think about what you're doing before you take an unnecassary risk while trying to set a hypermiling record or whatever by killing the engine at speed in a car that was not meant to be driven that way

dcb 08-21-2009 05:04 PM

I think you may find that the folks who take it seriously are paying a LOT more attention to the road and especially to potential obstacles than your average driver. But always practice new techniques in a safe place, and always THINK.

Personally I wish people cared more about folks who text and engage in other distractions while driving.

tasdrouille 08-21-2009 05:28 PM

I have to agree with dcb. When you are hypermiling, your results mostly depend on your ability to be constantly aware of the environment your evolving in and anticipating what is going to happen all around you. I don't see any real danger going down the road with the engine off when you know exactly what's happening (limited vacuum reserve, heavier steering at slow speeds (if it's not electric)), and see what's coming.

JacobAziza 08-21-2009 05:33 PM

Not only that, but excessive speed is the single largest contributor to injury and fatality accidents, and you won't see many hypermilers guilty of that

Neapolitan: Slow down. My philosophy for life also applies to the road.

tasdrouille 08-21-2009 05:54 PM

Thanks for reminding us Jacob. I wrote a piece on my blog (in French) some time ago about speed limits. During my research I came across a very interesting synthesis of safety research related to speed. From this source, we can see that for every variation of 1 mph in the average traffic speed of travel, there is an associated variation of roughly 4% of injury and fatal crashes.

MetroMPG 08-21-2009 06:03 PM

Quote:

Originally Posted by bluetwo (Post 122739)
I just hope people will be safe when they're navigating a 2,000 lbs. plus vehicle

So do I. I don't think anyone will argue against that.

Nobody should attempt any newdriving technique without a full understanding of the way it will affect their vehicle (as Tasdrouille mentions), and certainly shouldn't use it around other road users until they're competent. Sometimes conditions dictate it's not appropriate at all.

JacobAziza 08-21-2009 06:05 PM

My source said for every 1% increase in speed there is a 4% increase in fatality rate, but yeah, either way the conclusion is the same.

Saving the environment, saving money, and saving your life are all compatible

bluetwo 08-21-2009 06:11 PM

I have nothing against pulse and glide or any other legal technique but I'm looking at it from a law enforcement perspective as well as from a safety conscience mind. I agree about people texting and driving too and I'm glad a lot more states have outlawed it. Hell most of the military bases I frequent have rules against operating a cell phone at all while on the installation so that's nothing new to me at all. I'm a military policeman if anyone cares and sometimes I actually want to pull someone over for stupid driving practices when I'm in my own car or truck but of course I have to just sit there and watch them put others in danger at those times.

I'm not saying coasting with the engine off is like drunk driving but if it kills one little kid or something that's one too many. I have a heightened awareness of the dangers on the road because I've seen too many vehicle related deaths and so it's for those reasons that I speak out against and oppose unsafe practices and definitely don't encourage people to talk about breaking the law on the internet. That's just not smart.

tasdrouille 08-21-2009 06:28 PM

Bluetwo, I do not condone breaking the law either. Around here, driving with the engine off is only potentially illegal, simply because there's a law stating that you can't do anything that could put others at risk on the road. AFAIK it's never been trialled though.

I honestly cannot see a single risk when done by someone knowing what he's doing.

It's kind of funny when people bring up the risks associated with hypermiling. Maybe they are not speaking up, but I've never heard of someone being responsible of an accident while hypermiling.

MetroMPG 08-21-2009 06:36 PM

I don't think anyone's advocating that safety not be the top priority.

From a strictly legal point of view... laws vary by region - I was unable to find anything on the books where I live that even mentions neutral coasting. I realize in some US states it's on the books.

From the intro to the 100+ ecodriving/hypermiling tips:

Quote:

Note that the list on this page describes some techniques that may be illegal in some areas, and potentially unsafe (or inconsiderate) depending on the traffic situation where they're used.


So don't be stupid! Make safety your first priority. Use good judgment and be considerate toward drivers around you. Take care to learn new techniques in an isolated environment, and incorporate them gradually into regular use.

bluetwo 08-21-2009 06:54 PM

Quote:

Originally Posted by tasdrouille (Post 122768)
Bluetwo, I do not condone breaking the law either......

Quote:

Originally Posted by MetroMPG (Post 122770)
I don't think anyone's advocating that safety not be the top priority.

From a strictly legal point of view... laws vary by region - I was unable to find a law on the books where I live that even mentions neutral coasting. I realize in some US states it's on the books.

Thanks guys. I'm just massively disappointed because I honestly never thought I would have this conversation with any of you and I don't mean to be a sore thumb or discourage anyone at all.

Like I said, I'm in law enforcement myself but I'm not a hypocrite at all, nor do I claim to be a perfect driver all the time, yet I rarely ever speed excessively, I stop at every stop sign and don't put other people at risk certainly not for some selfish obsession of mine. So the thing to take away from it really: is I don't want hypermilers, ecomodders or even just the slightly concerned mileage watchers to get a bad name because I can guarantee that there are people out there who would love to smear us at the first opportunity. Please everybody continue to be safe and aware drivers so none of our cars ever end up in pictures anywhere.

Frank Lee 08-21-2009 08:34 PM

I can't think of a circumstance where trying to power out of it would have helped.

Well, OK, if you're on or almost on r.r. tracks and a train is coming you might want to goose it.

99.99% of the time evasive maneuvering is 1. steering 2. braking. I guess I never encounter what I'd consider a potentially dangerous situation while gliding.

rmay635703 08-22-2009 12:43 AM

Pulse and gliding a NATURALLY APIRATED diesel is almost pointless.

My 1982 diesel Suburban I drove for years with a 3speed auto and 4.10 gears got the best MPGs accellerating very very slowly and holding constant speed and coasting to stop. I could get around 22mpg in town and approx 21.2 on the highway if I was careful.

I do recommend letting the vehicle slow uphill and speed up going down however, thats all I did to get decent mileage from such a poorly configured rig. heck I was in 3rd at 15mph or so.

Frank Lee 08-22-2009 01:10 AM

Quote:

Originally Posted by rmay635703 (Post 122855)
Pulse and gliding a NATURALLY APIRATED diesel is almost pointless.

I'm thinking no.

I'm thinking the biggest reason P&G works is simply due to the percentage of time the engine is off.

Maybe someone more ambitious than I could math it out. But I feel this is the case.

rmay635703 08-22-2009 01:26 AM

Quote:

Originally Posted by Frank Lee (Post 122866)
I'm thinking no.

I'm thinking the biggest reason P&G works is simply due to the percentage of time the engine is off.

Maybe someone more ambitious than I could math it out. But I feel this is the case.

I think it depends on the engine to weight ratio, my engine had little power and there was a STEEP penalty for accelerating more than lightly.

To pulse and glide effectively I would have had to enter a very inefficient area, though it might be worth a try if the thing was a manual, mine was not.


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