Pulse & glide with diesel?

[Brock]

Ok I have been doing some runs comparing pulse and glide to flat out driving. It is darn near a wash. Maybe I am doing something wrong? My normal trip is 11 miles all in a 35 mph zone. I often leave work late (after midnight) and have no traffic. So I have been alternating between pulse and glide and straight driving. My overall speed ends up being the same which surprised me.

Straight driving I set the cruise at 35 and go, when I get home on scangauge the trip overall mph will be 30 mph with right about 90 mpg. The next night I run pulse and glide, getting up to 45 and gliding to 30. Again overall trip speed on SG is 30 and mpg is the same at 90 mph.

I could try gliding to 25 mph, but that would surely slow down my overall speed, which would increase my mpg, but I am guessing slowing down 1 or 2 mph in straight driving would net the same gain.

Maybe this could be because diesels run more efficient at lower loads, but I wanted to make sure I am not doing this wrong. Thoughts?

I guess I could do the same test in the van and see what happens…

[MetroMPG]

Are you shutting the engine off for the glide?

[Brock]

No, I had done that in the past and the ECU (engine computer) really didn't like it. It must somhow see the car is moving. I suppose I could leave the iginition off? I know it shuts SG off, and remember reading about how to keep it on but have never tried it. Although the diesel only uses .2L idling.

[landspeed]

I have noticed that diesels seem to have quite good engine braking (at least the ones I have driven). I have found the best economy by accelerating slowly, and then staying in the highest gear possible (e.g. 5th gear at 30mph at 1100rpm), and using tiny amounts of fuel. The engine idles so efficiently that pulse and glide might not make much different. Engine-on coasting makes a huge difference though!.

This experience is from driving a Toyota Avensis D4-D, at average 50mph, and getting 92MPG (Imperial), from a cold start, with A/C on for 20% of the journey!

[omgwtfbyobbq]

Pumping losses dominate low load gas engine operation, so it makes a huge difference there, but for diesels, as long as the engine is above 2-4bar, or whatever it is, BSFC is pretty low, and P&G won't help much, if at all. Otoh, like landspeed said, engine on coasting can make a difference if the grade is steep enough to have the engine slow due to friction/rpm.

[Brock]

The one thing I have noticed is going just 1 mph slower for the trip nets me about 3-5 mpg. If I run the whole way at 30 SG will show about 110mpg. I should try to run a whole tank that way ;)

[James]

Because the diesel uses so little fuel to idle, and because it is more difficult to start, I would think that engine off coasting would be unnecessary. I would just coast in N, and if everything is cold, hold the clutch in as well.

Wouldn't you still average 30 if you speed up to 45 and slow down to 25 instead of 30?? I would think that this would help significantly the FE.

A very short glide does not help FE as much as a long glide.

I find that hilly terrain that you know well is the best for pulse and glide...like everyone says, a moderately steep uphill and a long gradual downhill where
you can coast at 35 for a long period of time yields astonishing FE.

If you try this and there is still no difference, dont bother with the pulse and glide, and do what the experts say for diesel: low RPM, low speed, low load. coasting might still be appropriate on hills. try and approach the top at a low speed and gather your speed by coasting. I am curious what happens, I have never driven a scanguage equipped diesel.

[omgwtfbyobbq]

The one thing I have noticed is going just 1 mph slower for the trip nets me about 3-5 mpg. If I run the whole way at 30 SG will show about 110mpg. I should try to run a whole tank that way ;)

Definitely! The slower you go in top gear the better your mileage should be... And as long as you stay above 2 bar at what? Nearly idle I suppose, which should be doable, you'll keep your engine in the 280-200g/kwhr area. I suppose you could drop it down into the 400-500g/kwh area at that speed, but you'd probably have to be doing 3000-4000rpm in first or second to actually see engine operation that's really inefficient.

http://home.earthlink.net/~rps1976/Images/Diesel/TDI%20Fuel%20Consumption%20Map.jpeg

[Lug_Nut]

Brock,
Add a kill switch to the fuel solenoid on the injector pump. You'll still generate a MIL fault, but the ECU won't become too grumpy as it remains switched on the whole time. You'll have to bump start the engine but that's no problem.
But I've found the P&G technique to be largely ineffective on the TDI diesel, not worth the trouble.
The TDI drive-by-wire idle speed of 903 rpm just so happens to produce about 35 mph in 5th gear. I think that 30 mph would require shifting down to 4th and isn't as efficient at turning fuel BTU into distance.

[The Toecutter]

omgwtfbyobbq, what specific engine is that map for? It that for the 90 HP TDi from Volkswagen?

That 90 hp TDi in a 1,300 pound Loremo-like car would have the power to weight ratio necessary for 0-60 mph in 7 seconds.

[omgwtfbyobbq]

Yup, it's supposedly from
B. Georgi, S. Hunkert, J. Liang and M. Willmann, Volkswagen AG, SAE972686 (1997) BSFC Map for 1.9L VW TDI engine.
Which can be bumped up to 130hp no sweat.

[Brock]

I was toying with the 5th gear upgrade, basically swapping to what would be a 6th gear in place of the current 5th gear. But since half of my driving is at that 35 mph I would be willing to be I couldn't go that slow, even at idle in the upgrades 5th gear. Lug, right now if I idle (903 rpm) in 5th I go about 28 mph on the speedo and about 25 in real life (GPS). Maybe I should do a run home at idle for kicks ;)

[niiloma]

But how does the engine hold out` Are they designed to withstand such low revs and still go for 300,000 miles? If an engine dies too soon it is a much bigger strain on resources than the few drops of fuel it saves..

[Lug_Nut]

I can't say from personal experience, I haven't had one last that long. My first TDI was a write off after an accident at 151k miles. I later sold the driveline to a friend that wasted his (don't use veg oil in a TDI). My second one I bought with 175k and sold at 295k. Still running as of last correspondence with the present owner. My third I bought with 150k and had that one written off as a total loss at 220k+ miles. That driveline is in my present car, now with 250+ on the engine.
I do know that the newer (1999+) TDI have variable inlet geometry on the turbocharger and aren't as forgiving on long duration, low rpm and low boost pressure operation as my first generation TDI with their fixed geometry turbos.

[Brock]

I also wouldn't think it would be an issue lowering the 5th gear as long as you aren't idling around in 5th all the time. Since you can't lug a diesel the only think I would worry about is the turbo not being exercised or maybe the turbo running all the time at a lower RPM? Either way at 55+ it is just like having a 6th gear.