www.tdiclub.com

Economy - Longevity - Performance
The #1 Source of TDI Information on the Web!
Forums Articles Links Meets
Orders TDI Club Cards TDIFest 2016 Gone, but not forgotten VAG-Com List Unit Conversions TDIClub Chat Thank You

Order your TDIClub merchandise and help support TDIClub


Go Back   TDIClub Forums > VW TDI Discussion Areas > TDI Fuel Economy

TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

Reply
 
Thread Tools
Old December 12th, 2013, 02:34   #166
VeeDubTDI
Good Ol' Boy
TDIClub Enthusiast
 
VeeDubTDI's Avatar
 
Join Date: Jul 2000
Location: Springfield, VA
Default

Yes, instant MPG will display lower when you're accelerating more briskly, but it will be lower for a shorter period of time. The average over a certain distance covered will be higher because the MPG when accelerating will be offset by the longer glide time.

This still applies even if you aren't using pulse and glide methods. Rather than gliding when you reach your speed of 50 MPH (or whatever), you can maintain that speed with very light engine load and keep your MPGs in the 50s or 60s.
__________________
TDIClub Chat: irc.freenode.net ##tdiclub
EV Chat: irc.freenode.net ##ev
VeeDubTDI is offline   Reply With Quote
Old December 12th, 2013, 05:15   #167
puntmeister
Veteran Member
 
Join Date: Feb 2013
Location: Arizona
Default

Ok, I think I am getting closer to the issue. A few comments:

1) Instant MPG is lower when driving 85mph, than when driving 55mph, but it is lower for a shorter period of time, because you cover the intended distance in less time - would you argue it is more fuel efficient, then, to drive 85 mph, than 55mph?

2) After taking a closer look, and googling around, I think I understand what the chart is showing.

The TDI engine is generally more efficient at higher levels of torque than low levels of torque.

But, when determining fuel consumption over a certain amount of distance, there are two fundamental calculations needed:

1) The amount of energy, in the form of engine-output, needed to move the car the distance in question.

2) The amount of energy, in the form of fuel, needed to generate the required engine-output from #1.


#1 is independent of #2. Assuming a (theoretically impossible) 100% efficient engine, the engine output needed will equal the fuel-energy input. A 50% efficient engine will require 2x the fuel-energy input to equal the needed engine-output.

The efficiency of a given diesel engine varies considerably depending upon conditions, which is what the chart is showing.

Since the TDI engine is most efficient at higher levels of torque, it might seem it is more fuel-efficient to always drive at high levels of torque. However, this fails to consider the other half of the equation - the varying level of engine-output needed, depending upon the varying drive profile.

Consider the question:

How much energy does it take to accelerate from 0 to 60 mph?

Answer: It depends on the rate of acceleration.

It will take MORE energy to go from 0 to 60mph in 10 seconds than it will to go from 0 to 60mph in 20 seconds.

This will be true EVEN THOUGH the engine is operating more efficiently in the 10 second acceleration profile. That's because, even though the engine itself is operating more efficiently, the required engine-output is much higher.

Last edited by puntmeister; December 12th, 2013 at 05:29.
puntmeister is offline   Reply With Quote
Old December 12th, 2013, 05:24   #168
puntmeister
Veteran Member
 
Join Date: Feb 2013
Location: Arizona
Default

Basically, it would be so complex as to be near impossible to accurately calculate what the best drive profile is to achieve maximum MPG while accelerating. To get to an accurate answer, the best way would really be real-world testing.

BUT, it would need to be fairly well controlled.

Anecdotes, like, "I drive sometimes with heavy acceleration, and I get better MPG than when I drive with modest acceleration" don't really fly, because there are simply too many other factors at play - and acceleration profile isn't likely the #1 determinate of average MPG.

Likewise, my anecdote of watching a real-time gauge doesn't fly either - because those gauges aren't likely terribly accurate and, it is true, the T factor (the amount of time) is a factor. In the above equation, accelerating from 0 to 60mph in 5 minutes would take the most energy - so, it is not as simple to just say the slowest rate of acceleration is the most efficient...(even then, not so simple, because you have to consider the distance travelled - which is what we are more concerned with - there is no T in MPG, but there is an M).

Until someone can really offer some credible real-world tests, I'm gonna have to go with the idea that it is more efficient to overcome inertia gently than rapidly.

Yes, the TDI engine will operate more efficiently - say, at 40% efficiency, while overcoming inertia rapidly, than it will gently, say, at 30% efficiency. But, if it takes 2x the engine-output to overcome inertia rapidly versus gently, well, the overall efficiency favors gentle acceleration.

Again, "gentle" is subjective..

Last edited by puntmeister; December 12th, 2013 at 05:32.
puntmeister is offline   Reply With Quote
Old December 12th, 2013, 05:28   #169
VeeDubTDI
Good Ol' Boy
TDIClub Enthusiast
 
VeeDubTDI's Avatar
 
Join Date: Jul 2000
Location: Springfield, VA
Default Hypermiling videos to better fuel consumtion

Accelerating is different than steady state cruising, just as overcoming inertia is different than overcoming drag. Just because it is more efficient to use more torque while accelerating does not mean that driving faster will give you better fuel economy.

While driving at 85 might have the engine operating at a more efficient point in the BSFC chart, that efficiency is negated by additional drag from the increased wind speed.

Vekke has very clearly demonstrated the effects of various practices in maximizing fuel efficiency.
__________________
TDIClub Chat: irc.freenode.net ##tdiclub
EV Chat: irc.freenode.net ##ev
VeeDubTDI is offline   Reply With Quote
Old December 12th, 2013, 06:09   #170
puntmeister
Veteran Member
 
Join Date: Feb 2013
Location: Arizona
Default

"Just because it is more efficient to use more torque while accelerating"

This is the point I contend with: The engine is operating more efficiently with more torque while accelerating, yes. But that doesn't necessarily equate to better over-all MPG, because the part about varying levels of required engine-output for varying rates of acceleration has been left out...

Vekke proposes gliding, with the engine idling, as a means of improving MPG. Out of curiosity, where is the operating point on the bsfc chart when gliding, with the engine idling?

Hint: Its not in the blue zone....

We are not ultimately concerned with operating at optimum engine efficiency, or optimum time efficiency - we are concerned with operating at optimum distance-per-fuel efficiency.

Forget about T. If you want to operate at optimum T, do like everyone else, and jack-rabbit start, fly at optimum torque till you get 25 feet from the next red light, then slam on the brakes.

Sure, braking is inefficient - but, by slamming on the brakes at the last second, you will have done so for a very minimal amount of T.

Operating at optimum energy efficiency does not equate to operating at optimum MPG. Someone who proposes gliding, while idling, should get this.
puntmeister is offline   Reply With Quote
Old December 12th, 2013, 06:35   #171
VeeDubTDI
Good Ol' Boy
TDIClub Enthusiast
 
VeeDubTDI's Avatar
 
Join Date: Jul 2000
Location: Springfield, VA
Default Hypermiling videos to better fuel consumtion

Good grief. You expressed skepticism of the recommendation to accelerate briskly and justify that skepticism by talking about driving 85 MPH and accelerating up to red lights and then slamming on the brakes. I'm not sure what else to say at this point, as all of the info and reasoning has been covered in this thread already.

Highlights: utilize low BSFC when accelerating, plan ahead, coast when possible, engine brake when you want to slow down. Use the mechanical brakes as little as possible. Preserve momentum.
__________________
TDIClub Chat: irc.freenode.net ##tdiclub
EV Chat: irc.freenode.net ##ev
VeeDubTDI is offline   Reply With Quote
Old December 12th, 2013, 06:41   #172
puntmeister
Veteran Member
 
Join Date: Feb 2013
Location: Arizona
Default

I was just trying to demonstrate a point by offering exaggerated scenarios.

Vekke gets Stellar MPG (or LPK, as they'd say in Europe). But his efficiency is due to many factors - I don't think his use of rapid acceleration is one of them. Its just that everything else he does masks the lower efficiency of rapid acceleration.

Of course, it also helps that he is in Sweden, where there are no mountains or red-lights, only prairies and lollipops.

Now, I hate to belabor the point - but the bsfc chart depicts engine efficiency - but it doesn't show energy required for different drive profiles.

It just isn't as simple as referring to a bsfc chart to determine optimal acceleration for optimal MPG.
puntmeister is offline   Reply With Quote
Old December 12th, 2013, 06:49   #173
VeeDubTDI
Good Ol' Boy
TDIClub Enthusiast
 
VeeDubTDI's Avatar
 
Join Date: Jul 2000
Location: Springfield, VA
Default

LOL @ prairies and lollipops.

I would disagree and say that the BSFC chart does indeed depict the most efficient operating ranges for acceleration, since factors like drag don't come into play until you're already at speed. Of course the entire drive cycle is important and you can't just focus on one small part of the equation.
__________________
TDIClub Chat: irc.freenode.net ##tdiclub
EV Chat: irc.freenode.net ##ev
VeeDubTDI is offline   Reply With Quote
Old December 12th, 2013, 08:39   #174
puntmeister
Veteran Member
 
Join Date: Feb 2013
Location: Arizona
Default

I agree drag isn't a meaningful issue at speeds below 50mph.

However, energy required to overcome inertia is an issue - in fact, its the primary issue at low speeds, particularly when starting from zero mph.

In the real world, as you attempt to overcome inertia at a faster and faster rate, the energy required goes up exponentially. This is independent of the varying efficiency rates of the motor you use to overcome that inertia.

Given there are so many variables, it is difficult to calculate accurately on paper.
puntmeister is offline   Reply With Quote
Old December 13th, 2013, 15:47   #175
bvencil
Veteran Member
 
bvencil's Avatar
 
Join Date: May 2011
Location: Virginia
Fuel Economy: 43 combined average
Default

Quote:
Originally Posted by puntmeister View Post

In the real world, as you attempt to overcome inertia at a faster and faster rate, the energy required goes up exponentially.
Can you substantiate this claim? Maybe a site that has some data? Seems to me that the same amount of energy would be required to get the car to 60MPH in 10 seconds as opposed to 20 seconds. The energy required to overcome inertia is the same whether you do it quickly or slowly.
__________________

Bill's Music
bvencil is offline   Reply With Quote
Old December 13th, 2013, 20:01   #176
Jagerbecher
Member
 
Join Date: Oct 2012
Location: USA
Fuel Economy: 99% hw, 45 to 52mpg depending on speed
Default

A sidenote: accelerating does not necesarilly mean "overcoming inertia at a faster and faster rate". Simplest physical definition of accelerating is constant. Although the speed of an object increases it is caused by constant rate of acceleration. When costant amount of torque is applied it causes constant acceleration, for example every 1 seconds the speed increases 10Km/h so in 10 seconds final speed is 100Km/h. Not considering mechanical limitation of engine/transmission and other environmental factors that was steady acceleration caused by applying steady force(torque,energy). Same force (i.e. energy) used at second 1 all the way to second 10. This principle works perfectly in space, i.e. constant force cause constant acceleration and speed of object increases indefinitelly. Of course, there are cases of increasing acceleration, i.e. "overcoming inertia at a faster and faster rate" but this does not apply to automobile scenario since the available enery-torque is limited. But in that case the "the energy required goes up exponentially" will be true.
BSFC chart shows that you have to keep load on the engine high within a specific rpm range for most efficient converson of energy from every gram of fuel. How do you keep load on engine high? By brisk acceleration, i.e. applying right amount of throttle that gives a car constant acceleration by utilizing torque somewhere between 100-200 N/m. Of course acceleration will not continue endlessly and at some speed the engine enters the too low load-less efficient operating point. I even heard of engines that shut down a cylinder or two to bring the engine up the higher load and most efficient operating point. This is a good thread that explains BSFC very well http://forums.tdiclub.com/showthread.php?t=101422
Jagerbecher is offline   Reply With Quote
Old December 13th, 2013, 20:58   #177
puntmeister
Veteran Member
 
Join Date: Feb 2013
Location: Arizona
Default

Jager,

Your point about constant versus continuously increasing acceleration is perfectly understood - I didn't articulate my explanation well, as I never intended to suggest a drive-profile of continuously increasing rate of acceleration.

Instead, I was intending to compare rates of acceleration (all of them constant). Faster rates of acceleration, I would contend, would generally result in lower MPG.

I do understand that, per the bsfc, the highest efficiency of the engine is in a state of high load.

How to best achieve as close to a continuous state of high load as possible in a manual car is tricky. One of the keys is to shift early - ie, don't rev the engine up to 3,000 rpm. I'd say you'd want to upshift in the range of 1,800-2,000 rpm.

Upshifting in the 1800 to 2000 rpm range does not lead to rapid acceleration, of the kind being suggested by others here.

If you accelerate rapidly, and inevitably upshift in the 2,500 to 3,000 rpm range, you will likely be in a zone of lower load. You will also suffer greater losses during the actual shift - while the clutch is engaged, you lose some energy due to friction/momentum loss - the friction, hence losses, are going to be higher as the rpm increases.

Basically, shifting at lower rpms results in fairly constant high load.

All of this, however, is still just half of the equation - relating to engine output efficieny. But it doesn't address the varying rates of engine output required for varying drive profiles.

I have definitely gone beyond my true expertise - I don't really mean to present myself as a Physicist. So, to the poster questioning whether or not higher rates of overcoming inertia equate to exponentially higher energy requirements - I can't, personally, back this up....

Just keeping it honest. I am basically speculating, based on my layman's understanding of the natural world...
puntmeister is offline   Reply With Quote
Old December 13th, 2013, 21:13   #178
puntmeister
Veteran Member
 
Join Date: Feb 2013
Location: Arizona
Default

I would give two extreme examples:

When driving up a steep hill, the load on the engine is about as high as its going to get, and the engine will thus be operating in a very efficient zone - but, even with the engine operating in its highest efficiency zone, what kind of MPG are you getting?

When gliding down a steep hill in neutral, the load on the engine is roughly nil - the engine is in its worst efficiency zone - but, even with the engine in its lowest efficiency zone, what kind of MPG are you getting?

I do understand comparing going uphill to downhill is comparing very different drive profiles - but that is my point: MPG isn't only determined by engine efficiency. It is determined by drive-profile (uphill, downhill, speed, acceleration rates, etc).

It takes more energy for a car to go uphill than downhill - this is independent of engine efficiency. It is true for gas, diesel, hybrid, CNG, electric, windmill, solar, and tidal-wave driven cars alike.

I contend (although, I'll admit, I am not truly an expert on this) - it takes more energy for a car to accelerate at higher rates of acceleration than at slower rates of acceleration. Again, this is independent of engine efficiency, and, as above, would be true for all manner of vehicles.

Putting physics aside, think of it this way: Nothing in life is easy, and nothing in nature works the way you'd want it to. We'd all want it to be the case that flooring it results in optimum MPG. That would be fun - and it would be easy. So easy, in fact, that the majority of Tinheads (remember, T is for Tinhead), with no thought or effort, are actually getting the best MPG possible during their acceleration phase.

From my many years on this planet, the above is just highly unlikely.
puntmeister is offline   Reply With Quote
Old December 13th, 2013, 21:24   #179
puntmeister
Veteran Member
 
Join Date: Feb 2013
Location: Arizona
Default

PS - thank you for the link!

The second post on the link is most helpful. I will quote:

"The blue lines show where you would be operating the engine at a given steady speed (level terrain) given an infinitely variable transmission, and the lavender lines show the points accessible at a given fixed gear ratio. At 60 MPH, you could be in 5th, 4th, or 3rd gear. The lowest bsfc occurs for 5th gear. You can ask what point on this chart is the engine operating most efficiently (100MPH in 5th), which is a different question than how do you get from point A to point B with the least amount of fuel. "

Notice his point - the most efficient point on the bsfc is in 5th gear, at 100MPH.

Yet, as well all know, even though the bsfc chart shows 5th gear, 100 mph as optimum efficiency, driving 100 mph is NOT going to get you optimum MPG.

Optimum engine efficiency does NOT equate to optimum MPG.

There are a lot of other variables. My quack physics aside, I go back to one of my points - the bsfc chart alone is not sufficient to prove what rate of acceleration will result in optimum MPG.

For this, real-world testing - with tight controls -would be needed.

That is, unless we could get a physicist to chime in....
puntmeister is offline   Reply With Quote
Old December 13th, 2013, 21:28   #180
puntmeister
Veteran Member
 
Join Date: Feb 2013
Location: Arizona
Default

What we can glean from the chart - higher gears are uniformly more efficient. So, the sooner you can upshift, the higher the engine efficiency, per the bsfc, you will obtain.

The soonest a TDI can be upshifted, without lugging the engine, is generally about 1,700 rpms. A bit lower, actually, but I'll be conservative and run with 1,700.

I don't think the guys suggesting rapid acceleration are upshifting at 1,700 rpm...
puntmeister is offline   Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
fuel consumption lbpred TDI Fuel Economy 2 January 30th, 2012 12:53
Fuel consumption roypaci TDI Fuel Economy 0 June 16th, 2010 00:30
Fuel consumption roypaci TDI 101 0 June 15th, 2010 13:07
Fuel Consumption tdi-boy TDI Fuel Economy 2 July 2nd, 2008 16:39
fuel consumption TDI PD 130 TDI Fuel Economy 3 March 12th, 2006 17:07


All times are GMT -7. The time now is 03:35.


Powered by vBulletin® Version 3.8.5
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Copyright - TDIClub Online LTD - 2017
Contact Us | Privacy Statement | Forum Rules | Disclaimer
TDIClub Online Ltd (TDIClub.com) is not affiliated with the VWoA or VWAG and is supported by contributions from viewers like you.
1996 - 2017, All Rights Reserved
Page generated in 0.16436 seconds with 11 queries
[Output: 136.33 Kb. compressed to 115.42 Kb. by saving 20.91 Kb. (15.34%)]