So what's the consensus on wide open throttle for gas mileage?

[halik]

Originally posted by: fuzzybabybunny
I read here once that gently accelerating to cruising speed actually isn't the best for gas. For example, to get to 60mh you'd shift to 1, 2, 3, 4, and then cruise in 5th. With WOT you'd gun it throughout 1st, gun it in 2nd to 60, and then shift to 5th to cruise at 60. So which is best for better gas mileage?

The trip computer on my audi shows lower gas consumption when i take off slow as oppposed to gunning it.

 

[Zenmervolt]

Originally posted by: halik

Originally posted by: fuzzybabybunny
I read here once that gently accelerating to cruising speed actually isn't the best for gas. For example, to get to 60mh you'd shift to 1, 2, 3, 4, and then cruise in 5th. With WOT you'd gun it throughout 1st, gun it in 2nd to 60, and then shift to 5th to cruise at 60. So which is best for better gas mileage?

The trip computer on my audi shows lower gas consumption when i take off slow as oppposed to gunning it.

They all do. But you are accelerating for longer, which negates the fuel savings. For me, the difference between light acceleration and heavy acceleration is about 3 mpg difference, but with heavy acceleration, even though mpg is worse during acceleration, the overall average is better because I am accelerating for less total time.

For overall mileage, the best is to use low RPM and a large throttle opening.

ZV

 

[Assimilator1]

Originally posted by: Zenmervolt

Originally posted by: Assimilator1
Zenmervolt
The big efficiency gain of a Diesel engine is largely due to the fact that it has vastly lower pumping loss when compared to a gasoline engine.

Actually that's one of it's smaller gains ,the bigger gain is from their higher compression ratios.

Yes, Diesel fuel has a higher energy content, but not enough higher to account for the majority of the efficiency advantage that a diesel enjoys.

No it doesn't ,Petrol (gasoline) has a higher energy content hence part of the reason a petrol engine with the same aspiration setup & engine size etc will have a higher power output.

Nope, sorry. Wrong on both counts.

An increase in compression ratio will also allow an increase in efficiency, but the larger gain is from the lack of a throttle butterfly in terms of mpg efficiencies. A Diesel's increased thermal efficiency is indeed due largely to its higher static compression ratio, but the player in mileage efficiency is the absence of a throttle butterfly and the corresponding absence of any pumping losses. It takes a great deal of energy for a gasoline engine to maintain manifold vacuum.

Diesel has more energy per unit volume than gasoline. A gallon of Diesel fuel contains, on average, 147,000 BTU, while a gallon of gasoline has, on average, 125,000 BTU. Diesels have lower HP ratings because the engines are tuned to produce high torque at low RPM, a design consideration that necessarily restricts total HP output. The nature of Diesel engines requires heavier components than gasoline engines, which also limits RPM.

Thanks for playing though.
ZV

No ,not wrong on either counts ,greater thermal efficency means better fuel economy or power depending upon useage & it's effects are significant.
I'm not saying that the lack of a throttle plate doesn't help efficiency, it does, I'm just saying that the compression ratio has a bigger effect. Hence petrol engines without a throttle plate still can't match the economy of a Diesel, though it does help to close the gap.
If you've seen tests to show otherwise then link me.

Looking into the petrol vs diesel energy content it seems we are both right! lol ,if you look here, here, here, & here you'll see that Petrol has a higher energy (calorific) value as I thought, but this assumes that all the energy is extracted (Higher Heating Value) which it isn't, so the Lower Heating Value is also given which doesn't include the heat from the water vapour given off (though probably at least some is used in IC engines). The LHV of diesel is indeed higher than petrol by a narrow margine hence your right too :thumbsup:. LHV figures here (confirmed by non wiki sites too FYI). Seems that petrol loses more of its energy to water vapour than diesel.
Well I learnt something new their, didn't know about HHVs & LHVs.

Also here in the UK we AFAIK don't have petrol with a lower RON Octane rating of 95 (that I've seen in donkeys years anyway) ,so our lowest grade petrol is equivalent to your medium grade petrol according to this article, if you were quoting a lower grade petrol then that also explains some of the difference.

Btw you're only partially right about why Diesels have a lower output (when other factors are the same).Car Diesels (excluding commercial vehicles etc) aren't intentionally tuned for high torque at low revs as such, that's a consequence of the limitations of diesels.
1. As you said they have heavier components (crank,rods & pistons).
2. Diesel burns more slowly than petrol, so that without resorting to tiny & many pistons you are limited as to how fast you can spin the engine anyway as the Diesel can't be burnt fast enough. So even if the bottom end was strong enough to rev to say 6000RPM & you had a camshaft tuned to give power up there you still couldn't produce the power higher up anyway, unless you had more & smaller pistons like I mentioned. So because they are 'RPM limited' they thus (should) make best use of the RPM range they have which results in an engine giving more torque at lower revs than a typical petrol engine would which can rev higher. Also Diesels give more low speed torque largely because they usually use turbos now but also the higher compression ratio gives better torque.

And what are you talking about when you say 'playing'??
(man that lot took ages!).

 

[Elstupido]

Zenmervolt, I am assuming you are using a manual transmission. What do you recommend for an auto transmission? Is it the same for about 3/4 throttle for acceleration... such as an on ramp to the interstate? Maybe I have been a bit too light on the throttle, to save fuel.

 

[overst33r]

Originally posted by: Elstupido
Zenmervolt, I am assuming you are using a manual transmission. What do you recommend for an auto transmission? Is it the same for about 3/4 throttle for acceleration... such as an on ramp to the interstate? Maybe I have been a bit too light on the throttle, to save fuel.

When I drive my parents car I usually accelerate at 3/4 to about 2-3k rpms and then retract myfoot just enough for the computer to up shift to the next gear, do this until you get to cruising speed, just don't push the gas too hard or it will downshift.

 

[gunslash]

at WOT my big 6.0 liter opens it's gas guzzling maw even wider...this is the dumbest theory I've ever heard....
maybe moderate acceleration should be used, if anything, but definitely not WOT to save gas folks...

 

[overst33r]

Originally posted by: gunslash
at WOT my big 6.0 liter opens it's gas guzzling maw even wider...this is the dumbest theory I've ever heard....
maybe moderate acceleration should be used, if anything, but definitely not WOT to save gas folks...

Try it and report back. Don't go WOT, just 7/8 or 3/4. Short shifting is a must.

 

[gunslash]

if I got WOT, I'd be too busy watching the road as it gets up in speed quiet fast.

Also, my real time fuel meter isn't really up to speed it seems. When I had a GTP it seemed to be real time, in this car I won't get the true results.

 

[heymrdj]

Originally posted by: alkemyst
In basics the least rpms you turn the engine the better mileage you are going to get though. If you shift as soon as you can without bogging the engine you'll get great mileage. The trade off is a slow start.

That's not always true. Try pulling a 7,500lb camper with a 5.4L expedition. You get more mileage out of OD turning 3,200RPMs then you do in OD turning 1,900RPMs. The engine isn't lugging. It's healthier for the motor and drinks less gas. Confirmed test results via Edge monitors.

 

[Assimilator1]

OD= overdrive?
What's an edge monitor btw?

If your camper is doing better MPG at 3200 RPM than 1900 (same road speed right?) then your engine must be out of its main torque curve (where the baulk of the torque is produced), though I'd be suprised by that with a big 5.4Ltr engine, I thought your yank engines were good for low speed torque? .

I think the key to this practice (if it works, I don't know either way) is keeping the revs low but not so low that your engine drops off the main torque curve.
I remember reading about this near WOT low revs theory years ago in Autocar & I adopted it at the time but was never able to accuratley compare it.
I can't help wondering though even if you limit your revs to 2500-3000 & use 3/4 throttle won't the quicker accleration through that stage use more fuel even though the engine is operating more efficiently?

Zenmervolt
You said that even though that is the case it meant you were acclerating for less time & overall ended up with a higher MPG, by 3 ,was that calculated with the fuel computer?

I guess I might be able to test it myself seeing as the car I have now ('97 BMW 323i) has a fuel computer.
I'm guessing that it's most efficient range is between 1500-3000, going by the fact that it has very little power below 1500 (peak torque is at 4000 RPM), or should I have the minimium revs higher up? 2000?

 

[heymrdj]

Originally posted by: Assimilator1
OD= overdrive?
What's an edge monitor btw?

If your camper is doing better MPG at 3200 RPM than 1900 (same road speed right?) then your engine must be out of its main torque curve (where the baulk of the torque is produced), though I'd be suprised by that with a big 5.4Ltr engine, I thought your yank engines were good for low speed torque? .

I think the key to this practice (if it works, I don't know either way) is keeping the revs low but not so low that your engine drops off the main torque curve.
I remember reading about this near WOT low revs theory years ago in Autocar & I adopted it at the time but was never able to accuratley compare it.
I can't help wondering though even if you limit your revs to 2500-3000 & use 3/4 throttle won't the quicker accleration through that stage use more fuel even though the engine is operating more efficiently?

Zenmervolt
You said that even though that is the case it meant you were acclerating for less time & overall ended up with a higher MPG, by 3 ,was that calculated with the fuel computer?

I guess I might be able to test it myself seeing as the car I have now ('97 BMW 323i) has a fuel computer.
I'm guessing that it's most efficient range is between 1500-3000, going by the fact that it has very little power below 1500 (peak torque is at 4000 RPM), or should I have the minimium revs higher up? 2000?

OD is overdrive
Edge is a programmer/engine monitor. http://www.edgeproducts.com/product.php?pk=90&pvk=277

The powercurve of Ford modular engines is very high (4.6L and 5.4L V8's, and the 6.8L V10). All produce peak torque from 3,000-3,800RPM.

 

[Assimilator1]

Interesting, that'll be why then I suppose, depending how sharply torque drops off below 3000, got any graphs for that?

 

[heymrdj]

Sry, confused my numbers a bit, I was using the wrong model year of motor.

Peak torque for the 2 vavle model i'm using is achieved at roughly 2500RPM. Peak horses at 4,500RPM. Something like this I believe: 260HP @ 4500 and 350ft*lb @ 2500. The torque curve is flat at 4500RPM, only falling to approximately 305ft*lbs between 2,500-3,500RPM. It's HP @ torque is lackluster, at only 166HP@2,500RPM.

 

[Assimilator1]

2V engines usually do better at low speed torque/power than 4V, on average.
And 2500 RPM is more what I'd expect to see .

As far as seeing where the most fuel efficient RPM range is for an engine you want to look at the torque curve from peak going downwards, where it drops off sharply is where it's getting inefficient. I'd be surprised if 1900RPM is beneath the drop *shrug*.
Of course torque graphs like that aren't always readily available .

Interesting product the Edge, a tad pricey though!:Q

 

[Foxery]

Originally posted by: Zenmervolt
Unless you plan to be jettisoning ballast while driving though, there's no effective way to reduce the load on your engine. In essence, it's not a variable in the real world.

So yes, load does determine fuel use to a large extent. However, it's not something that is practical to consider to be a variable in the real world. The ways to reduce load are things like not towing a trailer everywhere or not keeping 500 pounds of concrete in your trunk. If you are towing the trailer, it's because you need to. If you have 500 pounds of concrete in your trunk, it's because you need to (in most cases).

What about dead bodies? Sometimes I need to jettison these in order to improve my car's top speed and handling for a brief period. Would a high-torque diesel (Cadillac, perhaps) be better suited to this job?

 

[Assimilator1]

Actually weight (ignoring aerodynamic & rolling resistance effects) doesn't really affect top speed! , just how long it takes to get there, so don't worry about those bodies when you have miles of road to get upto speed .

 

[Howard]

I'm pretty sure a 10 000 lb car would have a lower top speed than a 1000 lb car given the same outer dimensions and power, even if for only higher bearing losses.

 

[91TTZ]

I'm surprised that nobody has mentioned that an engine is operating most efficiently at the RPM that it produces peak torque.

Also, due to laws of diminishing returns, you certainly aren't going to get the same gas mileage by flooring the cars as you would if you accelerated slower. I don't know if it would be a huge difference, but you'll definitely lose some efficiency by doing that.

 

[MrSpock53]

http://www.expha.com/exphabeta...ficfuelconsumption.pdf

Above is a performance map which shows an engine's BSFC over the entire load (BMEP) and speed (N) range. A dynamometer is used to calculate brake torque and fuel flow is entered into the computation to calculate brake specific fuel consumption. This map only incorporates the wide-open throttle condition, but adding a third independent axis of throttle position can create a 3D performance map; it is noted that throttling directly affects efficiency via an inverse proportion, and maximum efficiency and minimum BSFC is realized at 100% throttle.

 

[91TTZ]

Originally posted by: MrSpock53
http://www.expha.com/exphabeta...ficfuelconsumption.pdf

Above is a performance map which shows an engine's BSFC over the entire load (BMEP) and speed (N) range. A dynamometer is used to calculate brake torque and fuel flow is entered into the computation to calculate brake specific fuel consumption. This map only incorporates the wide-open throttle condition, but adding a third independent axis of throttle position can create a 3D performance map; it is noted that throttling directly affects efficiency via an inverse proportion, and maximum efficiency and minimum BSFC is realized at 100% throttle.

As ZV pointed out earlier, there's a difference in the type of efficiency we're talking about. While a 100% WOT race down the road may convert gasoline to power in the most efficient manner, that's completely different than trying to drive that same distance using the least gasoline possible. One is max BSFC and the other is max MPG.

 

[MrSpock53]

Originally posted by: 91TTZ

Originally posted by: MrSpock53
http://www.expha.com/exphabeta...ficfuelconsumption.pdf

Above is a performance map which shows an engine's BSFC over the entire load (BMEP) and speed (N) range. A dynamometer is used to calculate brake torque and fuel flow is entered into the computation to calculate brake specific fuel consumption. This map only incorporates the wide-open throttle condition, but adding a third independent axis of throttle position can create a 3D performance map; it is noted that throttling directly affects efficiency via an inverse proportion, and maximum efficiency and minimum BSFC is realized at 100% throttle.

As ZV pointed out earlier, there's a difference in the type of efficiency we're talking about. While a 100% WOT race down the road may convert gasoline to power in the most efficient manner, that's completely different than trying to drive that same distance using the least gasoline possible. One is max BSFC and the other is max MPG.

I still don't see how WOT acceleration would negatively affect the MPG though. The idea is to minimize the pumping losses to maximize the efficiency of acceleration. Once up to speed, you would shift into the lowest gear possible, again to minimize the pumping losses. I can't think of any real reason to run at partial throttle other than to maintain a given speed.

Besides, Zenmervolt seems to pretty much agree:

Originally posted by: Zenmervolt

They all do. But you are accelerating for longer, which negates the fuel savings. For me, the difference between light acceleration and heavy acceleration is about 3 mpg difference, but with heavy acceleration, even though mpg is worse during acceleration, the overall average is better because I am accelerating for less total time.

For overall mileage, the best is to use low RPM and a large throttle opening.

ZV

 

[Zenmervolt]

Originally posted by: Assimilator1

Originally posted by: Zenmervolt

Originally posted by: Assimilator1
Zenmervolt
The big efficiency gain of a Diesel engine is largely due to the fact that it has vastly lower pumping loss when compared to a gasoline engine.

Actually that's one of it's smaller gains ,the bigger gain is from their higher compression ratios.

Yes, Diesel fuel has a higher energy content, but not enough higher to account for the majority of the efficiency advantage that a diesel enjoys.

No it doesn't ,Petrol (gasoline) has a higher energy content hence part of the reason a petrol engine with the same aspiration setup & engine size etc will have a higher power output.

Nope, sorry. Wrong on both counts.

An increase in compression ratio will also allow an increase in efficiency, but the larger gain is from the lack of a throttle butterfly in terms of mpg efficiencies. A Diesel's increased thermal efficiency is indeed due largely to its higher static compression ratio, but the player in mileage efficiency is the absence of a throttle butterfly and the corresponding absence of any pumping losses. It takes a great deal of energy for a gasoline engine to maintain manifold vacuum.

Diesel has more energy per unit volume than gasoline. A gallon of Diesel fuel contains, on average, 147,000 BTU, while a gallon of gasoline has, on average, 125,000 BTU. Diesels have lower HP ratings because the engines are tuned to produce high torque at low RPM, a design consideration that necessarily restricts total HP output. The nature of Diesel engines requires heavier components than gasoline engines, which also limits RPM.

Thanks for playing though.
ZV

No ,not wrong on either counts ,greater thermal efficency means better fuel economy or power depending upon useage & it's effects are significant.
I'm not saying that the lack of a throttle plate doesn't help efficiency, it does, I'm just saying that the compression ratio has a bigger effect. Hence petrol engines without a throttle plate still can't match the economy of a Diesel, though it does help to close the gap.
If you've seen tests to show otherwise then link me.

Looking into the petrol vs diesel energy content it seems we are both right! lol ,if you look here, here, here, & here you'll see that Petrol has a higher energy (calorific) value as I thought, but this assumes that all the energy is extracted (Higher Heating Value) which it isn't, so the Lower Heating Value is also given which doesn't include the heat from the water vapour given off (though probably at least some is used in IC engines). The LHV of diesel is indeed higher than petrol by a narrow margine hence your right too :thumbsup:. LHV figures here (confirmed by non wiki sites too FYI). Seems that petrol loses more of its energy to water vapour than diesel.
Well I learnt something new their, didn't know about HHVs & LHVs.

Also here in the UK we AFAIK don't have petrol with a lower RON Octane rating of 95 (that I've seen in donkeys years anyway) ,so our lowest grade petrol is equivalent to your medium grade petrol according to this article, if you were quoting a lower grade petrol then that also explains some of the difference.

Btw you're only partially right about why Diesels have a lower output (when other factors are the same).Car Diesels (excluding commercial vehicles etc) aren't intentionally tuned for high torque at low revs as such, that's a consequence of the limitations of diesels.
1. As you said they have heavier components (crank,rods & pistons).
2. Diesel burns more slowly than petrol, so that without resorting to tiny & many pistons you are limited as to how fast you can spin the engine anyway as the Diesel can't be burnt fast enough. So even if the bottom end was strong enough to rev to say 6000RPM & you had a camshaft tuned to give power up there you still couldn't produce the power higher up anyway, unless you had more & smaller pistons like I mentioned. So because they are 'RPM limited' they thus (should) make best use of the RPM range they have which results in an engine giving more torque at lower revs than a typical petrol engine would which can rev higher. Also Diesels give more low speed torque largely because they usually use turbos now but also the higher compression ratio gives better torque.

And what are you talking about when you say 'playing'??
(man that lot took ages!).

You are quoting energy per weight. That is the wrong way to measure it. Efficiency in automobiles is measured per unit of volume not per unit of weight. Diesel has a higher energy content per unit volume. The fact that it is also heavier per unit volume than gasoline is irrelevant since we don't measure efficiency for automobiles in terms of miles per pound of fuel, but rather miles per gallon. That's a simple, but crucial, distinction and it baffles me that you missed it.

The greater thermal efficiency is not as large a player as the lack of pumping losses.

You are welcome to continue to dispute either, but you will continue to be wrong.

ZV

 

[Zenmervolt]

Originally posted by: gunslash
if I got WOT, I'd be too busy watching the road as it gets up in speed quiet fast.

Also, my real time fuel meter isn't really up to speed it seems. When I had a GTP it seemed to be real time, in this car I won't get the true results.

Why does everyone confuse WOT with running up to redline? WOT can be combined with short-shifting and does not need to accompany flat-out runs. As has been said, WOT in EFI engines triggers an enrichment mode, which negates the gains from the reduction of pumping losses. This generally means that the best combination for modern cars is ~75% throttle opening and short-shifting.

ZV

 

[Zenmervolt]

Originally posted by: Assimilator1
Zenmervolt
You said that even though that is the case it meant you were acclerating for less time & overall ended up with a higher MPG, by 3 ,was that calculated with the fuel computer?

No, I said that when accelerating slowly, the mpg during acceleration is approximately 3 mpg higher than the mpg during fast acceleration. I made no claim as to the amount of improvement in overall mpg caused by fast acceleration taking less time.

For example, WOT acceleration from a stop is ~5 mpg. Accelerating slowly is ~8 mpg. However, it takes 3-4 times as long to reach cruise speed by accelerating slowly. So instead of 6 seconds at 5 mpg followed by 30 seconds at 35 mpg by accelerating at WOT, a person would instead have 18-24 seconds at 8 mpg followed by 18 or 12 seconds at 35 mpg. Overall, even though the first instance has "worse" mileage during acceleration, the overall mileage for the entire 36 second cycle is superior.

Note that this is a hypothetical example to show how this would work.

ZV

 

[91TTZ]

Originally posted by: MrSpock53

Originally posted by: 91TTZ

Originally posted by: MrSpock53
http://www.expha.com/exphabeta...ficfuelconsumption.pdf

Above is a performance map which shows an engine's BSFC over the entire load (BMEP) and speed (N) range. A dynamometer is used to calculate brake torque and fuel flow is entered into the computation to calculate brake specific fuel consumption. This map only incorporates the wide-open throttle condition, but adding a third independent axis of throttle position can create a 3D performance map; it is noted that throttling directly affects efficiency via an inverse proportion, and maximum efficiency and minimum BSFC is realized at 100% throttle.

As ZV pointed out earlier, there's a difference in the type of efficiency we're talking about. While a 100% WOT race down the road may convert gasoline to power in the most efficient manner, that's completely different than trying to drive that same distance using the least gasoline possible. One is max BSFC and the other is max MPG.

I still don't see how WOT acceleration would negatively affect the MPG though. The idea is to minimize the pumping losses to maximize the efficiency of acceleration. Once up to speed, you would shift into the lowest gear possible, again to minimize the pumping losses. I can't think of any real reason to run at partial throttle other than to maintain a given speed.

Besides, Zenmervolt seems to pretty much agree:

Originally posted by: Zenmervolt

They all do. But you are accelerating for longer, which negates the fuel savings. For me, the difference between light acceleration and heavy acceleration is about 3 mpg difference, but with heavy acceleration, even though mpg is worse during acceleration, the overall average is better because I am accelerating for less total time.

For overall mileage, the best is to use low RPM and a large throttle opening.

ZV

Even though the engine is more efficiently converting gasoline into power during that time, you get diminishing returns in acceleration the more power you produce. For instance, if you have a car that runs 15's with 200 hp, that same car will not runs 7.5's with 400 hp. Even though you burned twice as much gas you didn't get twice the acceleration.

If you calculated the time it takes to get to 60 mph, the car with 400hp at WOT will get to that point faster than the car with 200 hp, but it won't do it twice as fast. But it will burn about twice the gas during that acceleration.

Keep in mind that the time you spend accelerating is very small compared to the time spent cruising, so the overall affect in mpg will be minimal unless you're driving in the city.