Mileage versus travelling Speed ?? Is there truely exists something like fuel economy Speed? 1

[ninjaz]

I am just curious to know the theory behind vehicle speed versus mileage.
My mechanic advised that vehicle should be driven at optimum speed to get the maximum mileage.Low speed or High speed may result in over fuel consumption.

Is it true?
How low speed consume more fuel.
How high speed consume more fuel.

 

[IRstuff]

Fuel consumption vs. speed

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[ninjaz]

Is that link provided is an advertisement or malware? Something strange happens when i clicked that link.

 

[hydroman247]

All IC engines will have an optimum efficiency RPM. If you kept the engine at this speed, you will get the most power for the smallest amount of fuel. Depending on the gearbox between the engine and the wheels, the speed can be derived. Most manual transmission cars in the EU will operate most efficiently (fuel/distance) at 40-60mph. Any less and your % of energy wasted as heat goes up and any faster and you start to see the effects of drag a lot more significantly.

 

[IRstuff]

That was an exhortation to try a websearch

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[Komodo86]

Speed requires power to overcome drag.

An engine consumes fuel according to it's BSCF and the power generated.

BSFC is lower at higher loads, so effectively as the speeds increase, the BSFC decreases.

Obviously, there is a sweet spot, and either side will see an increase in consumption, but every vehicle is different.

 

[GregLocock]

hydroman's answer is true in part but has the incorrect conclusion. The most efficient speed for a car is not necessarily at the speed at which the engine is most efficient.

Cheers

Greg Locock


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[MikeHalloran]

On Interstate 59, which goes diagonally in a nearly straight and nearly flat line across Mississippi and Alabama, a 2001 Z28 Camaro can touch 32 mpg, short-shifted into sixth gear and then held at a thoroughly boring 55mph.
No data was taken at higher speeds for fear of governmental intervention.
No data was taken at lower speeds for fear of being run over by locals.





Mike Halloran
Pembroke Pines, FL, USA

 

[140Airpower]

"...BSFC is lower at higher loads, so effectively as the speeds increase, the BSFC decreases...".
This is true for a spark ignition engine because lower loads are handled by throttling the engine (almost universally with a choke type throttle) and introducing pumping losses in the process. Therefore, for any given load it is best if the engine can meet that load at wide open throttle. This means that for cruising at 55mph with a drag load of 15HP, the best efficiency is achieved with an engine that makes 15 HP wide open while running at its torque peak.
A more powerful engine will be suffering from pumping losses at that speed. Going slower will require more throttling with even more pumping losses. Going faster will reduce the pumping losses, but will encounter more drag losses mainly from air drag.
For any given car with its own drag characteristic and with its own particular engine there will be a particular speed at which it gets the optimum balance and its best mileage.

 

[Tmoose]

"Pumping losses" are usually mentioned as "the" cause of part throttle BSFC inefficiency.

Cruising at 12 to 18 inches of mercury vacuum operating thruout the intake stroke would seem to seem to be worth about 7 psi of BMEP if totally unrecoverable.
And that would seem to be worth 10% (+/- a lot) at various points around various cruising rpm on a chart like this.

http://www.techno-fandom.org/~hobbit/cars/bmep-Final_29.gif

Then I start to think about the effective compression ratio being about half as a result of the 0.5 atmosphere pressure in the intake.
So a 10-1 compression ratio might be more like 5-1 running under part throttle/high vacuum. That looks like about a potential 20% reduction in theoretical thermal efficiency. 60% down to 47% as shown here.

http://www.fao.org/docrep/t0512e/T0512e0v.gif

The fact that an additional 10-15 degrees or so ignition advance when cruising on the level is called for seems to confirm combustion is mighty slow, whether just from rarified (not leaner) fuel mixture or even additionally from exhaust gas dilution.

Going the "other" way with a 70-80% overdrive has been worth a minimum of 10% better fuel economy a couple of times for me. When commuting over not-too-hilly terrain with 70s and 80s era Volvos.

 

[Lou Scannon]

Then I start to think about the effective compression ratio being about half as a result of the 0.5 atmosphere pressure in the intake.
So a 10-1 compression ratio might be more like 5-1 running under part throttle/high vacuum. That looks like about a potential 20% reduction in theoretical thermal efficiency. 60% down to 47% as shown here.

True, throttling affects the net compression ratio (i.e. from atmosphere to TDC). But the effect on BSFC is only felt via the pumping loss. It is the effective expansion ratio that directly affects thermal efficiency. Throttling does not affect the expansion ratio.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[Komodo86]

Therefore, for any given load it is best if the engine can meet that load at wide open throttle. This means that for cruising at 55mph with a drag load of 15HP, the best efficiency is achieved with an engine that makes 15 HP wide open while running at its torque peak.
A more powerful engine will be suffering from pumping losses at that speed. Going slower will require more throttling with even more pumping losses. Going faster will reduce the pumping losses, but will encounter more drag losses mainly from air drag.

Ah, but to offset this, a large engine typically runs much longer gearing, allowing the engine to turn slower giving lower friction losses and requiring more throttle to be used. Low RPM, wider throttle position usually means that they can return reasonable economy vs a much smaller engine with shorter gearing and higher RPM, which gives more fricion losses and requires more throttling due to reduced torque requirement.

 

[IRstuff]

Another reason for not using WOT at max vehicle efficiency is that there is no performance headroom to do anything else, like accelerate for passing, etc.

In the Honda Pilot, the engine management system supposedly shuts off cylinders to get fuel efficiency.

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[pickler]

I had created a thread regarding BSFC and best acceleration efficiency last year. Might want to look that up. But generally best speed vs mpg is achieved at lowest possible speed in highest gear. In automatic it is the slowest speed when torque converter locks and highest gear is usually used (with exception of some modern transmission That have massive number of gears some of which are reserved for highway speeds only ie. BMW 8 speed auto). Not gonna go into technicals but higher speed means higher aero drag no matter how much more efficient your engine gets it won't make up for aero drag.

 

[pickler]

Oh and as for bsfc or optimum engine efficiency its usually ~1000rpm below peak torque on your average 4 cylinder engine with minimal manifold vacuum (vacuum induces pumping losses). It doesn't take much throttle to eliminate vacuum at low RPMs so you don't have to generally go full throttle to achieve this efficiency. Specially with modern drive by wire systems now days engines are always at peak efficiency during acceleration with minimal vacuum. This doesn't mean you should go wide open throttle though. Also modern ecus account for tip in enrichment. That is rate of change in accelerator pedal position. So be gentle and avoid lots of vacuum. Get a vacuum gauge or something like scangauge or ultra gauge to monitor your numbers.

Here is a 1.8l corolla BSFC map

 

[140Airpower]

"...Ah, but to offset this, a large engine typically runs much longer gearing, allowing the engine to turn slower giving lower friction losses and requiring more throttle to be used. Low RPM, wider throttle position usually means that they can return reasonable economy vs a much smaller engine with shorter gearing and higher RPM, which gives more fricion losses and requires more throttling due to reduced torque requirement..."

There are indeed methods to offset the inefficiencies of running a powerful engine far removed from its most efficient mode, ie WOT at torque peak. But, current engines being operated like that do not match the economy of a much smaller engine operating IN its most efficient mode.
Among methods mentioned was cylinder deactivation which virtually eliminates air pumping losses in the deactivated cylinders, leaving the active cylinders to perform closer to the efficient mode of a much smaller engine and suffering mainly only from the unproductive frictional losses from the deactivated cylinders.
Low rpms also help especially if the cams and tuned elements can be varied to match the low engine speed, effectively retuning the engine to be efficient at ultra low rpms.
Modern methods may eliminate the economy disadvantages of large, powerful engines.

 

[IRstuff]

Interesting curves. The Corvette curve is what I expected to see, but the Firefly curve seems to be more common today than before, even though the Firefly is the same vintage as these:

Most of the curve(s) used for arguing going 55 mph, show a hump that peaks at ~55mph, but there seems to be a raft of curves that show at even slower speeds result in even better mileage, like:

What makes the difference?

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[GregLocock]

"What makes the difference? "

Among other things, the losses in the diff.

These have two main parts, a relatively large constant friction torque, and then a load dependent part. The upshot of this is that low speeds (hence low loads) see much greater %age driveline losses than high speeds.

Then in manual cars you have the mismatch between the possible gear ratios and the desired engine speed. In autos you have the same, slurred by the torque converter, which adds another set of losses, until you hit the lock up clutch.

The easiest case is things like the Prius where the gear ratio can be correctly adjusted to the road speed, so for a given demand power the engine can be run at its most efficient rpm.

With a typical bsfc map and typical gearing it is very hard to get near the optimum bsfc point on the map, and the Prius suffers from the same problem, the optimum bsfc is at a huge power output compared with road load. So you have to settle for trying to get near the best sfc for a given road load power.



Cheers

Greg Locock


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[Lou Scannon]

With a typical bsfc map and typical gearing it is very hard to get near the optimum bsfc point on the map, and the Prius suffers from the same problem, the optimum bsfc is at a huge power output compared with road load. So you have to settle for trying to get near the best sfc for a given road load power.

Agreed, unless you have a largish battery to charge to and draw from, there is little opportunity to operate at optimum BSFC and also realize the benefit, in terms of actual miles per gallon, in a typical hybrid passenger car. Largish batteries exact their own penalty, in terms of mass. Ay, there's the rub.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[GregLocock]

Charging the battery and using the stored power later is very inefficient, say 60% efficicnt at best, so you'd have to be a long way from the ideal to make that pay. FWIW a prius operates above 30% engine efficiency whenever it is ins't at idle, so it would never be worth running harder just to get nearer to its best, 37%.

The Volt on the other hand probably has a bigger margin between road load efficiency and its best, so it might make sense to run the engine a bit hard and then turn it off.

The other thing that gets in the way is that you want to leave enough headroom in the battery for any regen stops.

With current battery technology a self powered (ie not plug in) hybrid's fuel economy is affected to a surprisingly small extent as the battery capacity drops (typically due to old age), apparently, which may be another way of saying that the optimum battery size might actually be quite small, if you aren't going to plug the car in. Here's a study on this, note that increasing the battery size (CD range in their termininology) REDUCES mpg once you have a certain minimum CD range, 15 miles in this example which annoyingly lacks resolution, because all you are doing is lugging extra mass around.

https://mospace.umsystem.edu/xmlui/bitstream/handle/10355/32077/Ferdowsi.pdf?sequence=1

Cheers

Greg Locock


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