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Old 04-16-2012, 12:43 AM   #71 (permalink)
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Why rolling resistance increases with the speed?

edit:
Here is something that is in Wikipedia about diesel engines:
Quote:
For any given partial load the fuel efficiency (mass burned per energy produced) of a diesel engine remains nearly constant, as opposed to petrol and turbine engines which use proportionally more fuel with partial power outputs
https://en.wikipedia.org/wiki/Diesel_engine

If that is so, then there is no point of using high load with my old mechanical diesel.

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Old 04-16-2012, 04:47 AM   #72 (permalink)
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Quote:
Originally Posted by jtbo View Post
Why rolling resistance increases with the speed?
I imagine the tire sidewall has less time to deform when it is spinning faster, which means your car has to expend more energy to do so. mass X force = acceleration
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Old 04-16-2012, 07:38 AM   #73 (permalink)
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Quote:
Originally Posted by IamIan View Post
Because rolling resistance is the force needed to deform the tire ... the faster you go the more tire rotations per second ... the more tire rotations per second the more deformations per second ... even if each deformation requires the same force.

If takes 100 joules of energy to lift 1 rock a distance of 2 meters ... it will take 10x as much to lift 10x as many equal rocks an equal distance.

As far as vehicle energy per mile goes it cancels out ... because rolling resistance for a given Vehicle weight and tire Crr increases linearly at the same rate the speed does ... which means you cover a given distance as much faster as the rolling resistance increases.

- - - - - -

Although technically the Crr of the tire does not remain fixed ... and the vehicle weight also changes ... But if you want to get into that level of technicality then aerodynamics isn't as simple as CdA anymore either... and that is probably more than most people will ever care about.
Oh yes, that is indeed true, rolling resistance force is per meter from my memory in SI standard, so one would think it as a distance for given time, which gives you of course resistance force for given time, if that makes any sense for anyone else than me.

There is indeed lot happening with tires at speed, contact patch shape along changes a bit and so does pressure over contact patch, some part of rolling resistance comes from thread of contact patch and that also then changes quite a bit, sidewall deformation changes also as with speed tires 'grow' a bit, so there is bit helping force with deformation I believe.

From my memory sidewall deflection was larger cause of rolling resistance from those two.

Well, you could even start to think about heating of tire too and those effects if wanted to go further, but I think there is speed limit in human brain and certainly processing all that while driving will be rather taxing.

While attempting to create car to vehicle simulator of decent quality there is quite lot of physics stuff involved and some are bit related to this fuel saving hobby, I wonder if I could use simulator to do some testing of forces etc. Might try that at some point.
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Old 04-16-2012, 01:20 PM   #74 (permalink)
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Quote:
Originally Posted by jtbo View Post
Why rolling resistance increases with the speed?

edit:
Here is something that is in Wikipedia about diesel engines:

https://en.wikipedia.org/wiki/Diesel_engine

If that is so, then there is no point of using high load with my old mechanical diesel.
Not exactly, look at a BSFC map of a diesel engine, efficiency still goes down quite a bit at partial load. However I think the very high compression ratios and lack of throttle make the losses smaller. I guess a good thing to look at is idle consumption perhaps, you'll notice that a diesel uses less fuel than a gasoline engine of the same displacement when idling, but it probably doesn't even manage to cut idle consumption by 50% (I might be wrong, this is just some number I found once).
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Old 04-16-2012, 01:35 PM   #75 (permalink)
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Quote:
Originally Posted by serialk11r View Post
Not exactly, look at a BSFC map of a diesel engine, efficiency still goes down quite a bit at partial load. However I think the very high compression ratios and lack of throttle make the losses smaller. I guess a good thing to look at is idle consumption perhaps, you'll notice that a diesel uses less fuel than a gasoline engine of the same displacement when idling, but it probably doesn't even manage to cut idle consumption by 50% (I might be wrong, this is just some number I found once).
Problem is that I have not found BSFC map for anything that would really compare to my all mechanical small turbo diesel engine, I can see maps for more modern ones, also big ones, but nothing for this kind of engine. Modern ones have throttle butterfly, I think VW TDI engines has it too, at least some year model, but I can remember wrong too, does happen often.

I have been trying to look information about diesel and part throttle efficiency, but so far I have not found much of anything, except that Wikipedia quotation which as we all know can be false as anything in there.

I remember that diesel converted to run with ethanol and spark plugs got 40% efficiency with large range of loads, don't know if it does relate to diesel any way though.

It is huge guessing still, also when I have tested different styles I have not been able to make difference that would have had any statistical signification, so it makes guessing even more like guessing.
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Old 04-16-2012, 05:31 PM   #76 (permalink)
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Quote:
Originally Posted by jtbo View Post
Problem is that I have not found BSFC map for anything that would really compare to my all mechanical small turbo diesel engine
What is the BSFC for the engine you have now?

- - - - - - - -

Quote:
Originally Posted by jtbo View Post
There is indeed lot happening with tires at speed,
I agree.

Quote:
Originally Posted by jtbo View Post
I think there is speed limit in human brain and certainly processing all that while driving will be rather taxing.
Agree again

Don't make it a chore ... keep it fun.

- - - - - - - -

The only aspect about that I will give a few more of my 2 bits ... is ... I see all of that ... just fine tuning the results , but not changing the relationship of the forces involved enough to alter the rolls or the net results of the effects of energy consumption per mile at different speeds ... I still expect even with lots and lots of these fine tuning of the results ... faster speeds will still consume more energy per mile than slower speeds ... how you get that mechanical energy is a separate issue from the amount of energy needed.

- - - - - - - - -

Not to be forgotten ... it is entirely possible that the separate mechanism of how one gets the mechanical energy might be a larger factor in the Net MPG than the mechanical energy needed per mile factors.

For example if I go from ~26% ICE efficiency to ~38% ICE efficiency that is about a ~46% more mechanical energy output for the same amount of fuel energy input ... if my energy per mile needs did not change than I would expect to see a ~46% increase in MPG ... if my energy per mile needs increased by ~46% than I would expect to see no change in MPG as it equals the energy output increase.

Similarly ... if my energy per mile needs drop by ~46% and my ICE efficiency stays the same ... than I also would expect to see a ~46% increase in MPG ... even if the ICE efficiency had stayed at ~26%.
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Old 04-16-2012, 08:55 PM   #77 (permalink)
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Quote:
Originally Posted by jtbo View Post
Modern ones have throttle butterfly, I think VW TDI engines has it too, at least some year model, but I can remember wrong too, does happen often.

I have been trying to look information about diesel and part throttle efficiency, but so far I have not found much of anything, except that Wikipedia quotation which as we all know can be false as anything in there.
As I understand it, the Throttle plate in diesels is only used for shutdown, so when running it remains fully open, but when you shutdown, the throttle closes, which reduces the compression kick and gives a smoother shutdown.
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Old 04-17-2012, 05:01 AM   #78 (permalink)
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Quote:
Originally Posted by IamIan View Post
What is the BSFC for the engine you have now?
I start with simple one first

My engine is with code XUD9TE, but XUD7 map should be relatively similar. Only VW TDI map is what I have found to be enough readable for diesel, but that is whole lot different engine.

To get know engine one can read this article:
https://en.wikipedia.org/wiki/PSA_XUD

I find it odd that despite hours and hours of searching, I have found nothing for this engine, even this is very common and not so terribly bad motor. I did try to find contact to car maker, but they seem not want to be contacted, competitor which also used same motor had contact details so I did post question there, but don't know if I ever get reply, so at the moment there is no knowing what kind BSFC map there is for this engine.

Boost starts building up from 1900rpm anyway, below that there is lot less power but maybe also lot less consumption too, who knows.

Tesla, some modern diesels have throttle plate to cut emissions of some sorts, causing higher fuel consumption in progress.
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Old 04-17-2012, 08:19 PM   #79 (permalink)
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Quote:
Originally Posted by jtbo View Post
so at the moment there is no knowing what kind BSFC map there is for this engine.
With no BSFC or points to compare ... there is no knowing if the engine is even average ... it could be great ... or it could be crap.

If you don't know the BSFC do you know any information that can be worked backwards to get you at least some individual points for comparison?

Things like ... In Vehilce X that has __ CdA and ___ Weight , it achieved __ MPG at a steady state of __ MPH ... in __ Slope , ___ Wind, etc.... if you know enough about a given set of conditions and the performance it got under those conditions , some points on the BSFC could be estimated fairly well.

For example ... I've seen some automotive turbo Diesels run at ~39% Efficient and I've seen others run at ~15% Efficient ... ~15% is crap ... ~39% is very good ... so , what criteria do you have to be able to narrow down the range more for this specific engine you are interested in?

Or said another way ... you say you haven't found anything to compare to it ... if you don't know how efficient it is ... what are you comparing?
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Old 04-18-2012, 12:36 AM   #80 (permalink)
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Only thing I can compare is driving tank or two with gearing down at hills and driving tank or two without shifting down at hill, then try to find difference, which I can't find, weather seem to cause more differences.

I did mail to Peugeot France and asked from them to get BSFC map, not very high hopes, Citroen seemed to have no contacts at all, so at the moment I wait Peugeot's reply. From Peugeot Finland I got reply that they had data, but it was lost at move.

There are of course factory consumption figures to compare to also, problem is that I have found those only from interwebs and not knowing how reliable those are and are they for wagon mode is uncertain.

Best is history of consumption logs to compare to of course, that should show changes if enough is driven with relatively similar conditions, but so far that method has failed to show changes, it can of course be that climbing hills it does not matter which gear I'm using at speeds what I use (around 60kph for longer hills).

Nowdays it is just that I use car very little, so can't do really meaningful testing and theoretical calculation should overcome some of that limitation.

When I use aerodynamic drag calculator here in EM, it shows silly low consumption figures with values I put in, efficiency of motor is again big unknown, but at steady speed 26% efficiency put into calculator is giving correct results. Then again 65mpg(uk) was advertised by car magazines when car was new one touted as best and one of the fastest diesels in a world. But that figure is probably not for wagon, cd 0.32 vs 0.36 in wagon, coast down tests show that with current mods cd 0.299 would be likely, no consumption data with current mods.

At least there is now time to wait reply, I might drive next time at autumn at earliest.

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