fuel saving tips please

[stever750]

Steve, to drive a TSI at it's max efficiency point, you'll be doing probably 180km/h.

This is of course impractical and illegal. So you can pulse accelerate using points near that max efficiency and glide in between. This gives better fuel economy.

Petrols at part load are terribly inefficient. They strangle themselves with their own throttle plates.

But surely you are ignoring wind / tyre / oil resistance etc etc. OK so maybe speed limits get in the way, but in the real world, I get the best mpg by sticking cruise on on not smashing the throttle, ever. Same with the few diesels I've owned.

I'm not alone either: quick google and sofa book search:

http://www.theaa.com/motoring_advice/fuels-and-environment/drive-smart.html

http://www.telegraph.co.uk/motoring/2752696/Economy-driving-How-to-cut-your-fuel-costs.html

IAM handbook (P58 Driving Economically) "Keep your cruising speed constant. Use cruise control where possible (even in town)"

IIRC Top Gear and Myth Busters have both disproved this being effective

[Kiwibacon]

Steve, wind/tyre/oil resistance is not relevant to this argument.

You get the best economy cruising because you haven't been able to pulse and glide properly.

As I've said earlier in this thread, gains on diesels from pulse and glide are limited. It works best on petrol engines as they have a bigger difference between cruise and maximum efficiency.

My own results show pulse and glide is good for about 10% improvement over steady state open road cruising. With more dedication, you can do even better. But it is an exhausting way to drive. In urban driving you can do far better. Because there is no steady state for comparison.

I don't go to the AA or news sites for technical information. Those two links are just generic fuel saving comments.

Mythbusters and topgear are good entertainment, but they arent in any way proof of much.

[stever750]

Ok, so where is the scientific evidence that it works, links? I couldn't find any.

Sent from my GT-I9300 using Tapatalk 2

[Kiwibacon]

The science is sound, a BSFC map shows it all quite clearly. But just like gravity, it's best if you experience it yourself.

[dancingbear84]

In the Picasso when my wife drives she get 40-44 on the average mpg if i reset it at the beginning of the trip. On the return leg if i drive and reset the count with a cold engine having been in town for a few hours i can get 55 or higher by using a variety of techniques all mentioned above.

Don't rag the engine hard, don't brake hard, read the road ahead and predict what may happen. In a diesel keep the revs low don't labour the engine, I coast where possible come off the throttle where i can. It really does help. I could probably drive more economically but it would be a lesser saving than Mrs bear could achieve.

[john.errington]

.. You get the best economy cruising because you haven't been able to pulse and glide properly.

So properly is . . You ACCELERATE the car (using fuel) then you "glide".

The engine does MORE work when you are accelerating the mass of the car. Keep the speed constant you dont have that unnecessary work. (work = energy = used up fuel)

Also, the engine must work harder to overcome air resistance. the "drag" is proportional to the SQUARE of the velocity. Thats the main reason why you get better economy at lower speeds.

Lets say you want to average 50mph for an hour. 1 * 50^2 = 2500

Using pulse & glide you need to accelerate gently to 70 and glide to 70 repeatedly. Say each phase takes 5 min. Then in 1 hour you do 6 pg's

0.5 * 70^2 + 0.5 * 30^2 = 0.5 * 4900 + 0.5 * 900 = 2450 + 450 = 2900

The physics proves it doesnt work.

Tests "IIRC Top Gear and Myth Busters have both disproved this being effective" prove it doesnt work

Hopefully this silly myth is now put to bed.

[Kiwibacon]

John. No.

The physics does work and the practice does work. You've tried to simplify the model to the point where it has lost all relevance.

Your approach needs to be modified to include the following points.

1. A BSFC map with cruise, acceleration and coast points.

2. Road speed/load information with the power required to move the car at the target average speed as well as the highest and lowest speeds in the pulse/glide.

3. Integration of power and time with the instantaneous BSFC to get actual fuel energy spent.

4. Integration of the road loads to get the actual work done.

5. Compare with steady state cruise.

Once you've done all that, get back to us. I'll volunteer to check your working.

[john.errington]

..The physics does work and the practice does work. ..

Can you give some PROPER references for that?

You've tried to simplify the model ..

yes I've ignored factors that have less effect than conservation of energy and increasing air resistance. For the purposes of calculation every real-world model in physics (other than simple stuff like the Schrodinger equation for an electron in a box) NEEDS to be simplified.

The effect of air resistance is easily shown to be significant. Drive your car at a steady 30mph on a flat road. Take the fuel consumption reading from the trip computer.

Now do the same at 40, 50 and 70. use your gears to try to keep the rpm near 2000. Plot the results and note that fuel consumption increases DRAMATICALLY

By contrast, the effect of changing load from 25% -50% - 75% is only a 6% increase away from the "sweet spot" see BSFC fig

http://pics.tdiclub.com/showphoto.php?photo=11194&title=fuel-consumption-map&cat=500

Thats why I ignored this element.

However I'll be interested to see your calculations and a summary of the factors your model leaves out.

Interesting discussion here

http://ecomodder.com/forum/showthread.php/got-2-25-l-100-km-104-36-a-22054.html

[Jockdooshbag]

..The physics does work and the practice does work. ..

Can you give some PROPER references for that?

You've tried to simplify the model ..

yes I've ignored factors that have less effect than conservation of energy and increasing air resistance. For the purposes of calculation every real-world model in physics (other than simple stuff like the Schrodinger equation for an electron in a box) NEEDS to be simplified.

The effect of air resistance is easily shown to be significant. Drive your car at a steady 30mph on a flat road. Take the fuel consumption reading from the trip computer.

Now do the same at 40, 50 and 70. use your gears to try to keep the rpm near 2000. Plot the results and note that fuel consumption increases DRAMATICALLY

By contrast, the effect of changing load from 25% -50% - 75% is only a 6% increase away from the "sweet spot" see BSFC fig

http://pics.tdiclub....ion-map&cat=500

Thats why I ignored this element.

However I'll be interested to see your calculations and a summary of the factors your model leaves out.

Interesting discussion here

http://ecomodder.com...36-a-22054.html

Interesting but complicated as hell. Effectively how much difference is there in the real world between cruising at constant speed and pulse and gliding? 5-10%??

I would have though bigger gains could be made in urban or rural road driving as opposed to motorways. Its quite anal but strangely enjoyable about thinking ahead in a town or on a rural road and therefore managing to avoid using the cars brakes. Very satisfying when you coast up to lights from a way off and they change to green just as you reach them or the same applies coating to a corner on a rural road and then managing to safely negotiate it without braking. The upside is you can still accelerate reasonably hard coming out the corner as long as you avoid using brakes at the next one. If you do a tedious journey in stop start trafic or on back roads i used to enjoy re-setting the trip computer each day and trying to beat my personal best in terms of MPG. It really makes you think far ahead and actually doesnt really slow you down overall. I would guess driving this way on these roads would see bigger savings compared to employing a pulse and glide technique. Obvioulsy this depends on the amount of time spent on a particular type of road but at least its a technique you can enjoy as pulse and gliding just seems boring.

Anyway, back to the physics and straight forward thermodynamics.

[john.errington]

Very true Jock. Its a bit academic in the real world, where bends, hills, other traffic, road works .. all prevent you from maintaining even a constant speed. And as for a measured "pulse and glide" well, I think its enough just to cope with driving responsibly. without having to watch the speedo and mpgometer all the time.

and thinking ahead and planning your driving I feel is not just about improving fuel economy but also about showing real skill as a driver, and delivering a smooth journey for yourself and passengers. Very satisfying when you coast up to lights from a way off and they change to green just as you reach them -- too right!

[Golf-Fiend]

Steve, to drive a TSI at it's max efficiency point, you'll be doing probably 180km/h.

This is of course impractical and illegal. So you can pulse accelerate using points near that max efficiency and glide in between. This gives better fuel economy.

Petrols at part load are terribly inefficient. They strangle themselves with their own throttle plates.

Kiwibacon,

I was doubting you but then came across this:

http://www.bath.ac.uk/mech-eng/news/news-items/news_0042.html

He knows what he's talking about. Very interesting, yet counter-intuitive theory. Yes, clearly if you drive a big engine at flat out speeds you will be going illegally fast and wind resistance will kill any efficiencies of optimum power, but run a small engine flat out and drive the car at sensible/legal speeds then the efficiency will be there.

[Kiwibacon]

John, lets start with the major error in your wind resistance calculations.

You are comparing pulse and glide with a much higher average speed to steady speed cruising at lower.

In your original calculation set compares 50mph cruise with 50-70mph P&G. Which gives a 60mph average speed.

If you were to pulse and glide from 45-55mph the average wind resistance and average speed is the same. But the fuel consumption is lower.

Do you agree or should I move onto the next error?

The next error is using a diesel BSFC plot to discuss petrol fuel saving methods.

[john.errington]

You are comparing pulse and glide with a much higher average speed to steady speed cruising at lower.

In your original calculation set compares 50mph cruise with 50-70mph P&G. Which gives a 60mph average speed.

Sorry its a typo; but the calc is for pulse to 70 and glide to 30. OK is extreme but the calc is only for drag. however it is the same average speed.

The next error is using a diesel BSFC plot to discuss petrol fuel saving methods. Well, my car IS a diesel.

OK we'll agree to differ.

[Wardy-]

My 2.0T vRS TFSI was maxing out at 32mpg, which I thought was a bit low. Reset the trip computer completely and long and behold it went up to 32-36mpg! That's for a 40-50minute commute with light traffic and A-roads only.

The way I get good MPG and did also on my passat, was that I found cruising to a stop saves so much fuel and also shifting in the gears as opposed to using your brakes to slow you down. These are the 2 best tips I can recommend. If you're really struggling then avoid accelerating hard in low gears and avoid topping the rev counter... but then that's no fun is it

edit: also worth noting that this is using normal Unleaded. Will be filling up with the premium for the next few fills, so this hopefully will increase it further!

Edited 13 February, 2013 by Wardy-

[Gazman]

Pulse and glide, don't you just hate those people, there you are doing a steady speed and the same car passes you over and over again.....aaaaahhhhhhh

[whitep]

or they just coast to the next corner even if they nearly stop!!!

[TDIum]

Pulse and glide - I don't think I've ever heard of anything as stupid. Oh hang on ..

[Kiwibacon]

Sorry its a typo; but the calc is for pulse to 70 and glide to 30. OK is extreme but the calc is only for drag. however it is the same average speed.

Well' date=' my car IS a diesel.

OK we'll agree to differ.[/quote']

No-one would pulse and glide between 30 and 70mph. As soon as you've dropped below the speed you can run smoothly in top gear you've lost all fuel economy.

Doing P&G either side of your target speed, the average wind resistance will be within very little of a steady cruise wind resistance. To see exactly how close it is would require knowing the exact time spent at each speed. But it's a wash, the wind resistance isn't significantly different as long as the average speed isn't significantly different.

[Jockdooshbag]

Fuel saving tips - sit right behind a vehicle travelling at the speed you wish to travel i.e slipstream. In other words buy an Audi. Its what radar cruise control was invented for.

[TDIum]

Sit behind an old person - travel at the speed they want to travel at because its more economical than the speed you want to do. Enjoy the lavender and **** aroma :think:

[Gazman]

Sit behind an old person - travel at the speed they want to travel at because its more economical than the speed you want to do. Enjoy the lavender and **** aroma :think:

'Old people' used to be easy to spot them from the back in there Austin metro's, driver with check trilbee,blue rinser in the passenger seat, one indicator blinking, i loved the 80's..

[john.errington]

now they are affluent so you see them driving fast cars - and because the car can do 180mph they think they sit in the overtaking lane doing 30.

"the wind resistance isn't significantly different as long as the average speed isn't significantly different." and by that same reasoning we can eliminate all variables. The arrow can nevr reach the tortiose!

[Kiwibacon]

"the wind resistance isn't significantly different as long as the average speed isn't significantly different." and by that same reasoning we can eliminate all variables. The arrow can nevr reach the tortiose!

Straw-man much?

Tortiose vs arrow is in no way comparable to 45-55mph pulse vs 50mph steady.

Feel free to do the maths on it and prove yourself wrong.