Driving for better mpg

[Wksg]

That is when that engine is making best use of the fuel, and creating the most energy.

Thanks for the description of your commute. I'm still trying to wrap my head around your driving technique.

I'm not sure about the "powering up the hill" part. It could be that the engine is operating in a more efficient mode during the climb. But the question is, when confronted with a hill, what will use the least fuel to get over it IN THE SAME TIME - i.e. with the same average speed.

The first approximation is that the slower the average speed, the better the fuel economy. That's just a function of friction. Since you slow down before the hill, you are reducing the average speed.

Anyway, your results suggests it works.

My drive, 120 miles RT, is flat. Not much to do besides cruise along.

 

[Bob_Fout]

from the second link:
How do any of these links support your outlandish claims of averaging 59mpg @ 75-80mph?

This is part of it:

 

[smck]

From my experience I must also disagree with the slow down before and then blast up the hill technique. That's glide/pulse not pulse/glide.

 

[UFO]

It makes sense to me to try to maximize engine efficiency by only applying power at times/locations where you can use maximum torque. It makes sense to use power climbing a hill, as long as you don't have to stop at the top, then you retain that kinetic energy to glide down the other side where you are less able to use the engine in its efficient range.

I seem to get better mileage driving in the mountains than on the flats, given the same average speed. And I do not "pulse and glide", just maintain speed up the hill, coast or engine brake down depending on the slope, and keep a constant speed on the flats.

 

[Wksg]

Well, I filled up tonight, 785 miles on 15.75 gallons, 49.8 mpg. That's driving about as conservatively as I can, typically 65 mph on highway. It's been cold lately, teens and 20's. Probably can't complain too much about 50 mpg in January. I still have to go through and replace air filter, clean MAF sensor, and probably there are a few other things that might help.

 

[tditom]

Unless you have reason to suspect that the MAF is acting up, I would refrain from cleaning it. You're more likely to do harm than good.

50mpg is pretty good on winter fuel.

 

[Wksg]

No real reason, just that the car is new to me, it's got 356K miles on it and I have no idea whether things like the MAF sensor are in good condition. OTOH Marty Bergel just put in a TB, probably he would have detected poor sensors or other major flaws.

 

[Bob_Fout]

No real reason, just that the car is new to me, it's got 356K miles on it and I have no idea whether things like the MAF sensor are in good condition. OTOH Marty Bergel just put in a TB, probably he would have detected poor sensors or other major flaws.

If you suspect the MAF isn't up to snuff, replacing it is relatively cheap. About $100.

You'd know if the MAF was bad, there'd be MPG or drivability issues. Possible check engine light too.

 

[Wksg]

If you suspect the MAF isn't up to snuff, replacing it is relatively cheap.

I think I recall reading a thread that said routine cleaning of the MAF was a good idea for optimum performance. I didn't have the impression it was tough or risky to do that.

No biggie.

What's the winter/summer mpg difference for these TDI's? On my other car, 6 cylinder Malibu, I could get 35 mpg in the summer but in the winter it fell to around 28 mpg, about 20% less. Similar ratio for the TDI's?

 

[tditom]

I think I recall reading a thread that said routine cleaning of the MAF was a good idea for optimum performance. I didn't have the impression it was tough or risky to do that.

No biggie.

What's the winter/summer mpg difference for these TDI's? On my other car, 6 cylinder Malibu, I could get 35 mpg in the summer but in the winter it fell to around 28 mpg, about 20% less. Similar ratio for the TDI's?

I disagree that cleaning the MAF is a routine maintenance thing. I would only clean it as a last resort if there are performance issues (while waiting for the replacement MAF to come in )

The biggest difference with operating any diesel in the winter time is winterized fuel. It is cut with what is essentially kerosene to prevent gelling issues. This has a lower energy content and will impact FE. I never saw a 20% hit, though. 10% maybe.

 

[TornadoRed]

What's the winter/summer mpg difference for these TDI's? On my other car, 6 cylinder Malibu, I could get 35 mpg in the summer but in the winter it fell to around 28 mpg, about 20% less. Similar ratio for the TDI's?

It depends on where you are and how you drive. When I was in SoCal, December and January were my two best months for fuel economy. Cool but not cold weather, less traffic congestion because it was the off-season for tourism, no need to run the AC constantly.

In a broad band across the country, from the Pacific Northwest to Georgia, there is usually a drop in fuel economy in the winter, but not a big drop.

TDIs take a big hit in the super-cold northern and northeastern cities, especially for short trips, stop-and-go driving, etc... Chicago, Boston and Montreal, for instance. Not quite as bad for those with longer commutes and only one or two cold starts each day, even in Buffalo, Winnepeg or Minneapolis.

So if you hear from a TDI owner that he only got 25 mpg on his last tank, then you can draw certain conclusions about where he is and how he drives, without looking at his profile.

 

[Derrel H Green]

Not Cold Like WPG



Jerry,

Is it your position that we here in the Golden State are not getting winterized fuel
or if we are, and I believe we are, the fuel is not so winterized as some
other really cold locations such as Winnipeg, Manitoba?

Thanks in advance for any and all information. Just a kid asking a dumb question.



D

 

[TornadoRed]

Jerry,
Is it your position that we here in the Golden State are not getting winterized fuel
or if we are, and I believe we are, the fuel is not so winterized as some
other really cold locations such as Winnipeg, Manitoba?
Thanks in advance for any and all information. Just a kid asking a dumb question.

The fuel in each state is supposed to be winterized to a particular temperature, which varies by month. In many states, including California, the southern part of the state does not have the same requirement as the northern.

Note that this chart is from 1998:
http://www.tdiclub.com/TDIFAQ/TDiFAQ-10.html

 

[RalphVa]

A couple of the points are in conflict. One says to power up the hill. The other says to let the car slow some and not push it up the hill.

I think the best way is to power up hills and to maintain speed, ala like with cruise control or using it. This maintains momentum for a better coast down.

I also question that the engine runs at higher efficiency when used hard. Why did my 220D & 240D and my tractor put out a bit of black smoke when pushed up a hill or under an increase in load? Isn't the black smoke indicative of incomplete combustion? That would decrease efficiency rather than increase it. Yeah, the computer control of our newer TDIs will avoid this overfueling under load, but I seriously doubt the efficiency goes up, particularly as much as is mentioned.

 

[gdr703-2]

Thanks for the description of your commute. I'm still trying to wrap my head around your driving technique.

I'm not sure about the "powering up the hill" part. It could be that the engine is operating in a more efficient mode during the climb. But the question is, when confronted with a hill, what will use the least fuel to get over it IN THE SAME TIME - i.e. with the same average speed.

The first approximation is that the slower the average speed, the better the fuel economy. That's just a function of friction. Since you slow down before the hill, you are reducing the average speed.

Anyway, your results suggests it works.

My drive, 120 miles RT, is flat. Not much to do besides cruise along.

When operating the engine at the max power for the rpm you happen to be at, then you also happen to be operating at very close to the maximum efficiency, ie consuming say 210 gm of fuel for every KwHr produced.

If you happen to be tooling along on a flat road then the engine is using maybe twice (or more) that amount of fuel to generate the same amount of energy.

When confronted with a hill, you might take a run at it and cruise up the hill, or you could slow going into a power your way up.
Lets say both take the hill in the same time, which uses the least fuel? For a gasser the former, for a diesel the latter.

True the slower the speed the better the fuel economy, thats due to th aerodynamic resistance.
If you drive at a constant speed you will get a worse mpg than using pulse and glide, or glide and pulse, take you choice. The trick is to operate the engine in its most efficient envelope.

On a largely flat location, you'll have to improvise, but you can still practice the pulse and glide technique. Takes a bit more skill and determination, is all. So dont just cruise if you want some good mpg, try it out. If road conditions allow set yourself a range, lets say you normally cruise at 70. Ok the power the car from 60 to 80, and coast back down to 60, repeat ad nauseum, or whenever. then check you mpg's.

Another aspect is that aerodynamic drag. That can be reduced by adjusting your weight distribution. Firstly dont carry excess weight in the car, and secondly any weiight that you do carry, move it as far forward as possible. You want to achieve a nose down, tail up attitude, That will reduce the aerodynamic drag effect on your car that becomes very important at speeds > 60mph.
The reduction comes from the pattern of air flow that begins under the car, and as the car rides over that air how that air disperses rearwards, and how it spills out the sides, or not, and affects the fill in of the vpoid at the rear.

hth and cheers, Richard

 

[gdr703-2]

A couple of the points are in conflict. One says to power up the hill. The other says to let the car slow some and not push it up the hill.

I think the best way is to power up hills and to maintain speed, ala like with cruise control or using it. This maintains momentum for a better coast down.

I also question that the engine runs at higher efficiency when used hard. Why did my 220D & 240D and my tractor put out a bit of black smoke when pushed up a hill or under an increase in load? Isn't the black smoke indicative of incomplete combustion? That would decrease efficiency rather than increase it. Yeah, the computer control of our newer TDIs will avoid this overfueling under load, but I seriously doubt the efficiency goes up, particularly as much as is mentioned.

There is a chart posted on this site that clearly shows the fuel efficiencies of this engine, at least of the 1.9 tdi.
The best it does is to consume 198gm of fuel to produce a KwHr, and that happens at 1750 rpm, and almost full power.
Around that point there is a plateaui described, that is elogated between 1200 rom, and 2200rpm and sya in the range of 90% to full power situations where its better than 210.
At the other end of the spectrum, lest say when you are on cruise at 60mph on a flat road, the car needs about 8hp to keep on rolling, and its buring the fuel at a rate of about 450gm. thats twice as much. and if its a gentle down hill and you keep the cruise on it gets as bad as 600. of course you dont need much power at all, but the small amount of power required is consuming fuel at a poor rate.
Thats what makes it fun. you can drive it hard and get good mpgs.
The cruise control is only used for convenience, not economy, you can do better than that gadget,
Anyways, That is how the tdi engine is designed.
Most diesels are designed that way. The Yanmar in my boat, has its max fuel efiiency at 3,000 rpm, when its thumping out its max contimuous.

Dont believe me, try it out for yourself.

cheers.

 

[UFO]

If the engine is smoking, it's not in it's efficient range as you are pumping unburnt fuel out the exhaust. High EGT is good for clearing out accumulated soot, but for efficiency a low EGT indicates you are getting more thermal work out of the fuel.

"Italian tuneups" are only to be performed occasionally if you are interested in conserving fuel.

 

[Wksg]

There is a chart posted on this site that clearly shows the fuel efficiencies of this engine, at least of the 1.9 tdi.

Where's that chart? I'd like to see it.

I've done pulse & glide before on my commute, last fall. I was not sure how to do it, exactly, and without a scan gauge, it was hard to tell if I was accomplishing anything.

I experimented yesterday too. With your advice in mind, I accelerated moderately from 60 mph to 70 mph, in about 6 seconds. Then I'd depress the clutch and cost back to 60 mph; that took 10 seconds. (leaving it in gear you don't use any fuel but you get back to 60 mph much quicker, probably about 6 seconds).

So 6 seconds with moderate pedal, 10 seconds idling. Not sure if that's better than constant speed.

I don't have much trouble doing this kind of stuff on the highway. There's always a truck cruising along at a convenient speed, I can just hang out behind him and P&G while everyone else zooms past, oblivious to the fact that I'm speeding up and slowing down.

 

[tditom]

When you're coasting in neutral, simply multiply the speedo reading by 10 and you are real close to the instantaneous mpg.

(off-topic- where are you getting your fuel in the Ann Arbor area?)

 

[Wksg]

I threw together a spreadsheet and calculated the average speed for:

Pulse: 60mph to 70 mph in 6 seconds
Glide: 70mph to 60 mph in 10 seconds

Average speed (for comparing to constant speed scenario) is 64.4 mph. Turns out that's not really useful. Oh well.

Accelerating like that would take about 29 kW, 38 hp, if I'm calculating correctly. If fuel is being consumed at a rate of 300 gm/kWh to do this (not as good as the max efficiency you mentioned of ~200 gm/kWh), it took about 14 gm of fuel for that 6 sec acceleration.

If the 10 second coast down at idle uses fuel at 0.4 liters/hr, then adding that to the 14 gm and doing all the conversions, give me about 61 mpg for the pulse & glide.

However, if I assume a constant speed, requiring 8 hp (6 kW) and consuming fuel at 450 gm/kWh to produce that 6 kW, I calculate 77 mpg average.

So, this first attempt at figuring it out doesn't quite work out. I have to check my math, but now my brain hurts

 

[gdr703-2]

I experimented yesterday . . . , I accelerated moderately from 60 mph to 70 mph, in about 6 seconds. Then I'd depress the clutch and coast back to 60 mph; that took 10 seconds.

I'll see if I can locate the chart.

Pulse, means hard, (not moderate) you need to be in the top 90% to 100% power, thats where the fuel efficiency is. ie < 210 Diesels like to work hard.

Coast with clutch depressed, or better still out of gear. (disclaimer: maybe illegal) or even better engine off. (disclaimer: not only illegal but possibly dangerooos)

cheers, Richard

 

[gdr703-2]

Where's that chart? I'd like to see it.

Look up the thread:
Can someone help me read a BSFC chart.

cheers, Richard

 

[shizzler]

I dont seem to have the oficial VW data on my photobucket acct, so you'll have to find that BSFC thread.

However I re-created it fairly accurately in excel. Then overlaid vehicle speed in 5th gear (manual trans, Mk4 tire size, ALH engine). Followed by a curve for the estimated road load (= aerodynamic drag + rolling resistance).

Wksg, it takes a lot more than 8 BHP to cruise at 64 mph.

I've always meant to calculate some acceleration rates and see how pulse and glide fares when analyzed on the BSFC chart, but I've been lazy to do the math. However steady state conditions come out about exactly right for my personal driving results (on avg) from this chart.

 

[Wksg]

(off-topic- where are you getting your fuel in the Ann Arbor area?)

All over the place. I work in Perrysburg OH so I often fill up at a Mobil down there. Sometimes Speedway up here. Two days ago at a Marathon in Saline area.

However I re-created it fairly accurately in excel. Then overlaid vehicle speed in 5th gear (manual trans, Mk4 tire size, ALH engine). Followed by a curve for the estimated road load (= aerodynamic drag + rolling resistance).

Very nice graph, thanks, I'll read up on it. And hey, you're in AA! I've only had my TDI for a few months, got to learn more. I'm on the highway a lot so some of your aero mods could be good for me. Where do you get your bio? I've thought of making my own, but I've got too many projects already.

 

[shizzler]

Hey, cool, didn't see your location there. In the summer, Wacker fuel and oil out in the sleepy farm town on Manchester, about 17 miles from me on the SW side of A2. In the winter I buy B20 from the Miejer on Ann Arbor-Saline rd. I have heard some sketchy things about their fuel quality before but in general it's never let me down (for years now). I also thought of making my own, but yeah, the financial investment and time required are just too much.

If you ever need TDI / ALH (your engine code) advice, let me know! I'm fairly knowledgeable after trolling this forum for years and wrenching most of my own mods.

 

[tditom]

Hey, cool, didn't see your location there. In the summer, Wacker fuel and oil out in the sleepy farm town on Manchester, about 17 miles from me on the SW side of A2. In the winter I buy B20 from the Miejer on Ann Arbor-Saline rd. I have heard some sketchy things about their fuel quality before but in general it's never let me down (for years now)...

Wacker ftw!! I drove out of my way every week to buy their fuel. Top quality fuel and good people.

I would not trust Meijer fuel after some of the issues that were reported.

 

[Wksg]

If you ever need TDI / ALH (your engine code) advice, let me know! I'm fairly knowledgeable after trolling this forum for years and wrenching most of my own mods.

Thanks, I'll probably take you up on that. I've met a few other TDI locals too, notably over at compu_85's place in Ypsi last week. He drove my car before I bought it from Howard Cooper.

I took a chance on it, at 350K miles, but I think I got lucky. I took it up to Marty Bergel and he did a TB change, and found otherwise that the cylinder head and cam looked good, and the turbo still was fine, except it needed a new actuator. Nozzles were shot, so I ended up getting the T4's from Frank06. Some work on the hydraulic clutch too.

I'd like to get a tune this summer. Maybe a new turbo to pick up some more power, probably need a new clutch since it seems the car is still on the original.

I like driving a car with so many miles. People are amazed. And I don't have to worry about piling up a lot more miles on it, what's another 100K?

 

[shizzler]

Man, that's great. These things will run forever with proper maintenance. I feel the same way about my 03 with only 116k on it - whats stopping it from going another 116k without issue?

Yeah tditom, I was nervous for a while with the miejer fuel. But being thats its so close to my house, offers somewhat competitive pricing, and B20 straight from the pump, it was enough to quash my fears. No problems after years now.

 

[gdr703-2]

So, this first attempt at figuring it out doesn't quite work out. I have to check my math, but now my brain hurts

Try this:
At 70 mph the engine consumes 250gm/KwHr
You can Pulse, and burn fuel at 210,
and then you'll need to Glide. the engine burns more fuel at 2,500 rpm than it does at 900, say 1.1 vs 0.4
So, if it you have 50 litres of fuel to burn, at the steady cruise rate of consumption (250 nad 0.85 sg) thats 170KwHr
running at the 210 level saves you 8 litres fuel, but some of that is used to keep the engine turning over whilst gliding. The gliding time is likely to be say 4 hours? thats 1.6 out of the 8, leaving you with 6.4 litres still in the tank to go someplace with, and to add to your mpg's.

Did that add up? - cheers

 

[Wksg]

Try this:
At 70 mph the engine consumes 250gm/KwHr
You can Pulse, and burn fuel at 210,
and then you'll need to Glide. the engine burns more fuel at 2,500 rpm than it does at 900, say 1.1 vs 0.4
So, if it you have 50 litres of fuel to burn, at the steady cruise rate of consumption (250 nad 0.85 sg) thats 170KwHr
running at the 210 level saves you 8 litres fuel, but some of that is used to keep the engine turning over whilst gliding. The gliding time is likely to be say 4 hours? thats 1.6 out of the 8, leaving you with 6.4 litres still in the tank to go someplace with, and to add to your mpg's.

Did that add up? - cheers

Thanks again! Well, it still didn't quite add up.

What I miss is that when you pulse at 210 g/kWh, you are using 16% less fuel per kWh, but your using more kW during the pulse (maybe twice as many?), so you still are using more fuel during the pulse. I don't think you can directly apply that 16% to a 50 liter tank and save 8 liters.

How about this, using shizzler's graph above:

Assume that at cruise, 2500 RPM, 73 mph, I use 265 g/kWh, and generate about 65 nm torque (interpolating these values from the graph). Doing some conversions, 65 nm = 48 lb-ft which at 2500 RPM gives 22.8 hp or 17.0 kW.

So I'm using 17 kW to cruise, and every hour I use 17*265 = 4.5 kg/hr = 5.3 l/hr = 1.4 gallons/hr. Since I'm going 73 mph, I'm getting 73/1.4 = 52.1 mpg.

This seems like a reasonable result. I can go 886 miles on a 17 gallon tank, and it takes 12.1 hours to use up the tank.

Now, imagine that I'm using P & G for the whole tank. Here are some assumptions:

- same average speed, 73 mph
- pulse and glide time evenly split: 1/2 of the time spend pulsing, 1/2 gliding (don't know how accurate this is).
- when pulsing, I use 210 g/kWh
- when pulsing, I generate 150 nm torque, or 40 kW.
- when gliding, I use fuel at 0.4 l/hr.

With these assumptions, I use fuel at these rates:

- during glide: half the time is 12.1 hr/2 * 0.4 l/hr = 2.4 liters
- during pulse: half the time is 12.1 hr/2 * 40 kW * 210 g/kWh/1000 * 0.85 = 59.8 liters

So I've used 2.4 + 59.8 = 62.2 liters = 16.4 gallons to go the same 886 miles (since I averaged the same speed).

I did get better mileage, 54.0 mpg instead of 52.1 mpg.

It seems likely that the pulse time would be shorter than the glide time. Let's say the time is split 40/60 pulse/glide, instead of the 50/50 above. Working through the same gyrations, I'd calculate now 65.6 mpg.

A scangauge would probably help to nail down those fuel consumption rates to make a more accurate calculation, but in any case, your actual results seem to say that this really can be done.

Fantastic!