Hypermiling videos to better fuel consumption

[puntmeister]

A few more points:

1) There is only 1 highly accurate method of measuring MPG - manually keeping track of the amount of fuel you put in the tank, relative to the miles your drive on that tank. Any measurement based on the fuel gauge is worthless.

2) It is very important for MPG to keep correct tire-pressure.

3) I read through this thread - there were a couple posters who were confused over the idea of 'gliding in neutral'. PLEASE, do NOT glide down the highway with your clutch pedal pressed in the whole time! To make it clear - the idea is to put the gear in neutral, then REMOVE YOUR FOOT FROM THE CLUTCH!

4) Although the P&G method - ie, accelerate to 55, glide in neutral to 45, accelerate to 55...repeat repeat repeat....may result in optimum MPG, I am curious: who is actually able to pull this off over any reasonable distance?

I drive in a relatively modest population area - not rural, but not typical inner-city either - lots of flat, long, straight roads with few/distant stop lights. Yet, there is no way I could safely practice this technique over any significant distance.

As it is, I drive 55 to 60 on the highway, and that alone causes a lot of honking/angry drivers who are forced to go around me. I'd end up with a bullet in my head if I consistently practiced the P&G technique.

 

[Maffken]

3) I read through this thread - there were a couple posters who were confused over the idea of 'gliding in neutral'. PLEASE, do NOT glide down the highway with your clutch pedal pressed in the whole time! To make it clear - the idea is to put the gear in neutral, then REMOVE YOUR FOOT FROM THE CLUTCH!

4) Although the P&G method - ie, accelerate to 55, glide in neutral to 45, accelerate to 55...repeat repeat repeat....may result in optimum MPG, I am curious: who is actually able to pull this off over any reasonable distance?

I drive in a relatively modest population area - not rural, but not typical inner-city either - lots of flat, long, straight roads with few/distant stop lights. Yet, there is no way I could safely practice this technique over any significant distance.

As it is, I drive 55 to 60 on the highway, and that alone causes a lot of honking/angry drivers who are forced to go around me. I'd end up with a bullet in my head if I consistently practiced the P&G technique.

I do it, but I don't drive on the interstate much and my area the speeds naturally vary from 60-55-45mph. It's great for mileage overall, and it still lets me get some good RPM's in for EGR health reasons.

People who clutch glide really don't know what they're doing. #clutchsmoke

I use that technique in my auto sometimes, and more so in dsg's because while it might be worse for it on occasion it's just so jerky to slow down with. But you do not "kill" the engine when is in N, while it normally shuts the ASV during 'glides'.

DSG's are weird space machines, yup, yup.

I get much better MPG when I spool up my turbo and then let it dump it at a higher end rev (2200-3200rpm) and then keep it about 1700rpms.

Worst part about flooring the turbo is after it kicks down and you keep the rev's high, you'll get 32mpg doing stuff like that (flat out to 40+mph; mileage dropping with the speed incrementally.).

Still not bad considering how many cars I passed that day at 65 mph + .

 

[puntmeister]

I guess I would summarize the real-world difficulty of implementing the various hypermiling techniques as, modifying a quote from Sartre, "Hell is other cars". Or, perhaps, to quote Mike Tyson, "everybody has a plan, till they punched in the face".

Over time, I've managed to get pretty good at timing lights - such that I arrive at red lights just when they turn green, for example, so I don't have to slow down/stop. BUT, that strategy is foiled at least half the time because of idiots who whip around me from behind, floor it all the way to the red, then slam the brakes. I arrive at the intersection in a perfect position to drive right through the green light - but can't, cause of the idiot who whipped around me, sitting like a dead duck at the green light, blocking my way.

For a hyper-miler, there is absolutely nothing more frustrating than being forced to come to a dead stop at a green light.

There are dozens of other related examples.

Entering the highway on my daily commute, there is a metered entrance - flashing green/red light which allows one car per green to enter the highway. Its a long, downhill stretch to the light - I can glide all the way down. However, if/when I do so, cars zip around me from behind, piling up in line in front of me - any fuel I saved by gliding, instead of flying at full speed to a dead stop, are negated by the increased wait time....

 

[puntmeister]

Oh - and don't forget: If you drive 55, you will get a fairly regular stream of troopers who tail you, running your plates into the system.

From their standpoint, anyone driving under the speed-limit clearly must have something to hide.

 

[2010nctaco]

^^^ if your in the left leave they probably just want you to get out of the way because they have somewhere to be. Everything isn't a conspiracy.

sent from the north pole

 

[VeeDubTDI]

I practice most of the techniques described. With one exception: I am not convinced rapid acceleration to speed is more efficient than gradual, light acceleration to speed.

With almost everything in the world, not just diesel autos - the faster you try to make something happen, the (exponentially) more energy that is required. ie - it is almost always more efficient to gradually do something (anything) than to do it rapidly.

The above is strictly in a theoretical sense - in the sense of having a perfectly clear straight-away, with no obstacles. Starting from a stand-still, and going a total of 5 miles, with top speed of 55mph, would use you less fuel if you accelerated to 55mph rapidly, or gently?

I vote gently. But I would LOVE to be proven wrong.

In a real world sense - in city driving - rapid acceleration will most often hurt you, as you will, on average, end up having to waste more energy to braking. The exception is timing lights - sometimes it will pay to accelerate rapidly to make it through a soon-to-change light.

The above nuanced argument aside, I don't think accelerating technique is the main factor in MPG. The bigger factors:

1) Top speed. MPG starts to go down after 55mph (true on all cars/trucks). MPG drops gradually between 55 to 65mph. Beyond 65mph, and it really starts to tank.

2) Braking. Less braking = better mpg. This is the hardest part, as it ultimately involves a lot of strategy, planning, and concentration - especially in urban driving.

Everything else is relatively minor.

The engine braking versus gliding in neutral arguments found in this thread are generally due to confusion over what everyone is intending to say. Clearly, if you are coming to a stop, slowing down in gear uses less fuel than gliding to a stop in neutral.

HOWEVER, that assumes the same start-point in the road. The argument that coasting in neutral is better is premised on the idea that you begin to coast at an earlier point, ie - you 'read' the situation better....you foresee you will need to stop, and let off the accelerator, and coast in neutral - ideally, perfectly timed such that you come to stop at the intended point of stop without ever having to touch the brakes, or touch the accelerator.

Again, that all plays into the 'minimizing braking' above, which takes a good deal of practice and, at times, a bit of luck.

 

[puntmeister]

^^^ if your in the left leave they probably just want you to get out of the way because they have somewhere to be. Everything isn't a conspiracy.

sent from the north pole

Ummm, tell it to the police officers who do, quite regularly, check my plates while I am driving 55 in the RIGHT lane.

Its not a conspiracy - they just run plates, somewhat randomly, when they drive on highways. This is nothing new, nothing strange. And nothing comes of it, as my car is not stolen...

Its not a major issue - and doesn't really bother me. I posted it as more of a funny (but very real) side-effect of driving 55, rather than as any kind of warning.

 

[puntmeister]

In regards to the graph - thanks. I had seen it, but could definitely stand to hear a clear explanation of how that graph proves fast acceleration is more fuel efficient than gentle accleration.

I do get that optimum efficiency is generally in the 1500 to 2500 rpm range. But even with gentle accleration, rpm is most often near, or above, 1500 rpm.

Of course, 'gentle' is subjective. I'm not talking so gentle that rpm bumps around 950.....

 

[VeeDubTDI]

In order to operate at your lowest (most efficient) BSFC, you need to accelerate moderately (approximatly two thirds of max torque) between 1,000 and 4,000 RPM, and then glide in neutral. You don't have to flog it, but leisurely acceleration will have you operating in the poor BSFC zone for a longer period of time.

It's also important to differentiate between engine speed and engine load.

 

[puntmeister]

Ahhh, yes, I do remember reading - its best to get up to speed, and 5th gear, because any gear lower than 5th, and any speed under 40, is less than optimal efficiency.

Best MPG is in the 40 to 55mph zone - faster than 55 and wind/drag reduces MPG. Under 40 mph, and the engine is running in an inefficent zone.

However, I would not agree that because optimum efficiency is above 40 mph/5th gear, that it is necessarily most efficient to get to 40mph/5th gear by any means possible.

Yes, by accelerating all-out, you'll spend less TIME getting to 40mph/5th gear - but how much FUEL will you burn, cranking the engine?

I could still be wrong - but, I am still not convinced.

Stating optimal efficiency is achieved by getting to 40mph/5th gear as fast as possible results in the best fuel economy is akin to arguments that "sure, I burn more fuel at high speeds, but I get to the location faster, so its more fuel efficient, because I drove for less time".

There is no T in MPG.

 

[VeeDubTDI]

When I say "coasting/gliding in neutral," I mean with the engine running.

Nobody is arguing that the most efficient speed is 40 - 55 MPH, and again, we aren't saying that you should accelerate all out, but instead accelerate based on the most efficient BSFC ranges (as illustrated by the chart).

 

[puntmeister]

What is missing from the discussion is the differentiation between efficiency at a steady speed/steady rpm, and efficiency during acceleration.

When it is stated that a particular TDI gets 60mpg at 55mph, it is getting that efficiency at a steady speed of 55mph.

If the same car is accelerating from 40mph to 60mph, the moment it passes through 55mph, the car is NOT getting anything even close to 60mpg. Depending on the rate of acceleration, the mpg would be closer to 25 to 35mpg.

As the rate of acceleration increases linearly, the consumption of fuel increases exponentially. Driving a car which has an instantaneous mpg gauge best demonstrates the effect of rapid acceleration. A car which may get 35mpg on average, under heavy acceleration, can be getting as little as 5mpg. Yes, 5 - as in five - mpg.

So, I revert back to my claim - all things considered, it is likelier more fuel efficient to accelerate gently than to accelerate quickly. That's a hard pill to swallow, as we all like the idea that we can get optimum mpg by flooring it! Trust me, I'd love that to be true...

Understand it takes a lot of energy to overcome inertia. The faster you want to overcome inertia, the more energy you are going to spend. Exponentially more.

 

[VeeDubTDI]

Accelerating using a higher amount of torque is more efficient than using a low amount of torque. The BSFC chart clearly demonstrates that.

I'm not sure how to explain it any better. Perhaps Vekke will chime in.

 

[puntmeister]

For someone who can make complete sense of the chart, and see clearly what each point/area demonstrates, relative to real-world characterstics, perhaps. I admittedly don't know enough about it to make enough sense of it.

I have seen from cars with instantaneous mpg gauges - when you increase your rate of acceleration, the mpg tanks - dramatically.

It just doesn't flow with common logic, that accelerating heavily would result in increased efficiency.

Yes, I do get that efficiency suffers at too low of rpms/too low of a speed - that I can pick up from the chart (and I did know this already) - but the notion that heavy acceleration is more efficient than moderate/gentle acceleration? It just flies in the face of everything else in the physical world.

Again, I could very well be wrong. The reason why I keep chiming in is not to prove anyone wrong, but because I am interested in getting to the bottom of this.

Ultimately, about the only thing that could definitely prove which methodology is more efficient, would be real-world testing (driving over a given distance, with varying rates of acceleration, calculating how much fuel was used at the varying rates).

 

[rotarykid]

Depending on the terrain you drive over you will have to decide what techniques are the most efficient. Over rolling terrain the hills can be used as fuel to keep momentum saving diesel. But over flat terrain there really isn't that much to be gained with pulse & glide...

condition anticipation saves fuel in what ever conditions you drive in.

Killing the engine while coasting then using the momentum to restart the engine "in a manual trans car" saves a lot of fuel if you doing this over rolling terrain. But over flat terrain there really isn't much to be gained...

 

[VeeDubTDI]

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.

 

[puntmeister]

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.

 

[puntmeister]

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..

 

[VeeDubTDI]

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.

 

[puntmeister]

"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.

 

[VeeDubTDI]

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.

 

[puntmeister]

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.

 

[VeeDubTDI]

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.

 

[puntmeister]

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.

 

[bvencil]

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.

 

[Jagerbecher]

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

 

[puntmeister]

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]

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]

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]

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...