Does Revving Closer to Peak Power Improve Fuel Efficiency in Motorcycles?

[Spikeywan]

When I got my motorbike it had a non-standard rear sprocket with two extra teeth. I decided to swap it for a standard one. I expected to get a better overall mpg, but it didn't change noticeably.

So...

An engine has a red line at 14000 rpm. Peak power is delivered at 12000 rpm. Driving along in top gear at 70 mph, the revs are at 6500 rpm.

Because maxmimum power is delivered at 12k, surely this is the point where the engine is most efficient, and getting the most power from the fuel?

So, when cruising at a constant 70mph would it use less fuel if you changed down a gear or two, to get the engine revs as close to 12k as possible? Or is it better to drive in a high gear, keeping the revs as low as possible?

Why?

 

[Gordianus]

Even though the engine may deliver maximum power at 12000 rpm, best efficiency is roughly obtained when the engine delivers the maximum torque

 

[rcgldr]

The internal drag inside the engine, mostly due to moving air back and forth through the crankcase under the moving pistons is lowest at low rpms. However the engine is more efficient under some load and at higher rpms. As you've discovered the amount fuel rate consumption required to drive at 70mph is about the same within a range of rpms, but running in a much lower gear at 10,000+rpm would result in worse fuel milage. Motorcycles also accelerate to speed so quickly that unless you're in constant stop and go traffic, how fast you accelerate doesn't affect overall fuel milage that much because zero to 60 mph only takes about 4 to 5 seconds under moderate throttle, and zero to 45 mph only 3 to 4 seconds, then you're cruising at a somewhat fuel efficient speed.

 

[jack action]

Like Gordianus said, maximum efficiency is at the maximum torque. Because torque is a measure of the efficiency of the combustion and of the friction losses.

Power is a measure that depends on both torque and rpm. Torque is the result of the energy released by the fuel combustion times the efficiency of the thermodynamic cycle, i.e. how the engine was able to transform that heat into mechanical work. The bigger the efficiency, the lower the fuel needed to create the same torque. The rpm indicates how many of these cycles you do in row, hence the power you're going to get.

Depending on the engine design (valve event, intake & exhaust design, etc.), an engine often performs better at a certain rpm rather than an other (better efficiency), so it will also have a higher torque at a certain rpm rather than an other.

The friction losses are known to be more or less proportional to the mean piston speed of an engine, which is proportional to the rpm of the engine. This means that an engine @ 5000 rpm has twice the amount of friction losses than @ 2500 rpm. This is the number one reason for reducing the rpm as much as possible when looking for better mileage.

So when you perform a certain task (say cruisin' at 70 mph), you need a certain amount of power (say 20 hp), regardless of the engine's rpm. Whether you're at 5000 rpm or at 10 000 rpm, you're going to have to adjust your throttle such that your power output is 20 hp. So, before considering anything else, we already know that we need that 20 hp PLUS extra power to compensate friction losses, which are greater at 10 000 rpm than at 5000 rpm. Furthermore, let's say that your engine is more efficient at 5000 rpm. This means that you will need less fuel at that rpm to produce the already lower total power needed. If the opposite was true (better eff at 10 000 rpm), it might be possible that the better efficiency compensate for the friction losses and then you wouldn't see a difference in fuel consumption.

 

[madhatter106]

adding to jack actions great explanation, the piston speed is a function of the rpm and stroke. with a shorter stroke the rpm will be higher than one with a longer stroke for those frictional losses, and since torque is also a function of the stroke the longer stroke engine is going to have torque sooner than the short stroke engine.

of cousre this all can be engineered to an extreme where as in F1 the engines tolerances are so tight it requires pre-heating before turning over. oil pressure and viscosity also effect those numbers. idealy you want to have the engine operating in the peak torque band, hence the critical need to have the gear ratios correct and also when changing one component others also need to be looked at. the most common overlooked one is tire dia.

On the race cars I deal with changing the tire size relative to the front/back will cause problems with the braking systems and the transmissions gear selection due to integrated traction systems.

 

[mender]

http://autospeed.com/cms/title_Brake-Specific-Fuel-Consumption/A_110216/article.html

Load makes much more difference in fuel economy than where the torque peak occurs. It would be more accurate to say that the engine should be at about 75% of peak load for best fuel economy at a specific power output.

A quick note: it isn't all about pumping losses like the author is saying; the increase in thermal efficiency caused by the increase in dynamic compression is also very important.

 

[Spikeywan]

Here's a torque graph for you...

 

[mender]

Which is of course at WOT only. That doesn't help very much when discussing what is happening at 25% or even 50% load.

Did you look at the article that I linked? It explains things quite well.

 

[Spikeywan]

Yes. Thanks.

Just processing it all now.

 

[jack action]

Load makes much more difference in fuel economy than where the torque peak occurs.

Changing the throttle angle is like changing the diameter of the intake port: you're changing the engine design and thus, restricting the flow. So the torque and power curves will also change.

No matter what, the lowest BSFC will always happen at maximum torque, FOR THOSE PARTICULAR CONDITIONS, throughout the rpm range. If BSFC happens at a lower rpm at part load, it is because maximum torque also happens at that same lower rpm.

Torque is proportional to BMEP and the volume of air per revolution that enters the engine:

$$T=BMEP\ V_{eng}$$

BSFC is inversely proportional to BMEP:

$$BSFC=\frac{\left(\rho_{air}/AFR\right)}{BMEP}$$

where $$\rho_{air}$$ is the density of the outside air and $$AFR$$ is the air-fuel ratio.

So when torque is at its maximum, BSFC is at its minimum. Although I must admit that maximum torque usually happens at a rpm slightly different than minimum BSFC, because the volume of air per rev varies also with rpm due to the variation of volumetric efficiency. But they're usually really close.

 

[mender]

When I got my motorbike it had a non-standard rear sprocket with two extra teeth. I decided to swap it for a standard one. I expected to get a better overall mpg, but it didn't change noticeably.

So...

An engine has a red line at 14000 rpm. Peak power is delivered at 12000 rpm. Driving along in top gear at 70 mph, the revs are at 6500 rpm.

Because maxmimum power is delivered at 12k, surely this is the point where the engine is most efficient, and getting the most power from the fuel?

So, when cruising at a constant 70mph would it use less fuel if you changed down a gear or two, to get the engine revs as close to 12k as possible? Or is it better to drive in a high gear, keeping the revs as low as possible?

Why?

So the answer is to select a gear that runs the engine at a high load, and not worry about the rpm that the torque peak at full throttle occurs at.

 

[Spikeywan]

Which is going to be the highest gear you have.

 

[mender]

That your engine will pull without lugging, yes!

 

[Spikeywan]

Hehe. So the obvious answer was correct after all. Thanks guys!

Still, it also explains why the MPG isn't much different between a standard rear sprocket and one with two extra teeth on it.

And for the extra Smiles Per Gallon given by those two extra teeth, they're well worth it.

I've decided that I'm going to fit a +2 rear sprocket again. I just need to wait until I wear the current chain and sprockets out so I can do the change.

 

[Lsos]

And when you throttle the engine, you're increasing the pumping losses by a whole lot, and therefore decreasing the efficiency. It doesn't do you much good to be at the torque peak (point of highest efficiency at WOT), when your throttle is almost closed...

 

[Spikeywan]

If you were cruising at 30 mph through town in top gear, then you come out of town and wanted to accelerate up to say 70, which is better if you want to use as little fuel as possible?

Originally, I thought it would be better to change down lots of gears, and use small throttle openings and all the gears to get up to your new cruising speed, but after reading this thread, it would seem that you're better off leaving it in top gear, and giving it WOT?

 

[mender]

With due consideration for the engine (again, no lugging), yes.

 

[jack action]

but after reading this thread, it would seem that you're better off leaving it in top gear, and giving it WOT?

Yes, if you want maximum fuel economy. But if you want performance (and not take forever to get to your new speed), you have to downshift such that you get more power from the upper rpm to give you a good acceleration. And performance has a price called fuel consumption.

 

[rcgldr]

If you were cruising at 30 mph through town in top gear, then you come out of town and wanted to accelerate up to say 70, which is better if you want to use as little fuel as possible?

Unless you're driving a very low powered vehicle, then maximum miles per amount of fuel used occurs around 55 mph. At any speed below 55mph, you milage is a bit worse. So there's a trade off in how fast to accelerate to 55 mph versus how much time is spent at a lower and less fuel efficient speed.

It's also unlikely that you'd be able to drive at 30 mph in top gear on most sport motorcycles (600cc and up) without lugging the engine. Most motorcycles are relatively overpowered and normally operate well out of peak milage mode at any reasonable speed, so driving technique isn't going to have much effect on milage, unless it's constant stop and go. The highest powered bikes, like the Suzuki Hayabusa and Kawasaki ZX14 get about 35 to 37 mpg (miles per gallon) regardless of driving method. 600cc sport bikes and cruisers will get between 40 to 50 mph, depending on the bike. Some mopeds can get over 50 mpg.

 

[madhatter106]

Unless you're driving a very low powered vehicle, then maximum miles per amount of fuel used occurs around 55 mph. At any speed below 55mph, you milage is a bit worse. So there's a trade off in how fast to accelerate to 55 mph versus how much time is spent at a lower and less fuel efficient speed.

Sorry, velocity is not the defining factor for BSFC, it's based upon the engine displacement, design and rpm with a given load under steady state conditions and acceleration. the final MPH will be determined by gearing and the optimum RPM for that engine.

The ol' double nickle was a political issue.

The newer DI EFI has better MPG than conventional EFI. Due to the DI cooling the cylinder along with better atomization and mass measuring, it has a higher CR and the fuel trim can be adjusted to maintain the needed Tq, then at idle trimmed back far enough to just run while not increasing the NOx levels due to running cooler, less fuel wash also helps with wear and lessens oil degradation decreasing frictional losses. However I'm off topic now.

I'll see about clipping in some of the data from a recent development engine that shows BSFC and BMEP per RPM, it'll be edited for sensitive reasons.

 

[Spikeywan]

Hang on...

If the vehicle is more fuel efficient when under more load, why don't we drive along with the brakes on, to increase the load, and improve fuel efficiency?

Something's not quite right here. (Probably my understanding)

 

[rcgldr]

Unless you're driving a very low powered vehicle, then maximum miles per amount of fuel used occurs around 55 mph.

Velocity is not the defining factor for BSFC.

The government version of this is close to reality for most cars. It would be different for a hybrid, but for a standard small sedan, the graph the second image on this web page is close. From 30 mph to 55mph there isn't much gain, but from 0 to 25 mph there's a lot.

http://www.fueleconomy.gov/feg/drivehabits.shtml

In the case of some low drag, high powered cars, like a Corvette Z06, best milage occurs around 70mph, with the engine running at 1500rpm (very tall 6th gear, red line is 7000 rpm, so 1500 rpm is relatively slow).

 

[xxChrisxx]

I think people are getting themselves confused about efficieny and fuel economy.

Basically if you want the best MPG you want to be traveling using the minimum power possible for a given speed (tis is overly basic as sometimes its better to be in a lower gear to get more fuel economy becuase of gearing). This is usually top gear. For the engine itsself converts fuel burnt to work best at max BSFC.

The reason you don't drive around with the brakes on is becuase althoug the engine is converting fuel to work as efficiently as it possibly can, you would be using more power than is strictly necessary to travel at that speed.

 

[russ_watters]

Hang on...

If the vehicle is more fuel efficient when under more load, why don't we drive along with the brakes on, to increase the load, and improve fuel efficiency?

Something's not quite right here. (Probably my understanding)

Because, for example, 30% efficiency at 10 kW output requires 33 kW of input while 35% at 20 kw output reqires 57 kw input.

 

[Lsos]

If the vehicle is more fuel efficient when under more load, why don't we drive along with the brakes on, to increase the load, and improve fuel efficiency?

The vehicle is NOT more efficient under more load, the engine is.

Brakes take a car's mechanical energy, and throw it away. What good is even a 100% efficient engine...if you're going to throw away 90% of it's energy?

 

[jack action]

I was just reading my Automotive Engineering International april issue, where there is an article called Focused on fuel economy about driver behavior effects on fuel economy (FE). Here are some extracts from this article:

Speed kills (efficiency)

(...)

Many factors contribute to the difference in FE from one driver to the next. The speed at which one drives is the biggest factor, according to Roger Clark, Senior Manager of GM's Energy Center.

Aerodynamic drag and tire rolling resistance increase with speed. So do engine friction, transmission pumping losses, bearing and brake drag, final-drive losses, and electrical accessory losses, Clark said. These losses kick in after the vehicle reaches its most efficient speed, which for an average car is about 30 mph, he noted.

"Wind resistance is proportional to vehicle speed, but the engine and transmission get more efficient as the load on them increases," said Clark. At about 30 mph, that is a good balancing act. But for every 10-mph increase in speed above that, a conventional vehicle suffers an FE degradation of about 4 mpg, he noted. The penalty curve begins to flatten out at about 80 mph. (...)

David Woudstra, a Technical Specialist in FE at Ford, said the aerodynamics penalty "ramps up fairly quickly." Roughly speaking, he said, the FE penalty at 30 mph is about 10%, from 30-50 mph "maybe 20%," then after that about 40%.

(...)

Easy on the g force

(...)

Clark pointed out that at a steady cruise of about 40 mph, one's FE could be 40 to 50 mpg. But during a moderate acceleration event - about 0.1 g - FE drops to about 10 mpg, he said. At idle, a car burns fuel at a rate of roughly 0.2 g/s whereas at the acceleration of 0.1 g it burns perhaps 2 or 3 g/s.

At WOT, fuel intake could be as much as 10 g/s. An average driver launches at about 0.2 g, Clark said, although most vehicles are capable of 0.5 g.

"Any little bit of acceleration, you're adding a huge amount of extra fuel," Clark said. "That's why trying to anticipate stops and trying to coast as much as you possibly can really make a big difference in your FE, It's all about acceleration and the conservation of kinetic energy. It takes a lot of fuel to get that mass to accelerate, even in a small car."

Ford's Woundstra noted that, at idle, the 2010 Ford Fusion with a six-speed automatic transmission and 2.5-L four-cylinder engine consumes fuel at a rate of 0.28 gal/h compared to a rate of 16.4 gal/h at WOT. He said the Fusion's FE is best between 40 and 45 mph, the main reason being that at that speed under light cruise the transmission will be in sixth gear. "Lowest speed in top gear is your best fuel point," he said.

Steve Payne, a Senior Engineer at Ford, said shifting schedules for Ford automatics are designed with drivability at the forefront. Someone opting for a manual transmission on the same Ford model can get better FE by upshifting sooner, but at the expense of drivability. Payne noted that too slow an acceleration rate is bad for FE also "because, especially on a spark-ignition engine, you're keeping the throttle plate closed, which is making the engine run inefficiently. But that would only be for extreme cases, and people would be honking their horns if you drove that slowly."

(...)

There is an FE penalty for use of every electrical and electronic feature and function. Clark said electrical losses for an average vehicle are about 4% when the vehicle's comfort and convenience features are turned off and only the engine control unit, brake lights, and similar functions are used.

On a winter night with the defroster, heated seats, audio system, and other functions at maximum, electrical losses can total 15%. A basic radio alone can amount a loss of about 2%, a high-end one "with a big subwoofer" can be 4%, said Clark.

(...)

 

[rcgldr]

vehicle's most efficient speed, which for an average car is about 30 mph, he noted.

That conflicts with the data I've seen from magazines and web sites, a couple of links:

http://www.fueleconomy.gov/feg/drivehabits.shtml

for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph.

http://auto.howstuffworks.com/fuel-efficiency/fuel-economy/question477.htm

In the case of small econoboxes, or hybrids like the Prius, lower speeds are better. A 1.0 liter Geo Metro proably peaks below 30mph, scroll down to see the Metro milage versus speed graph on this web page:

http://www.metrompg.com/posts/speed-vs-mpg.htm

On the other extreme, something like a Corvette Z06, probably gets it's best gas milage around 70mph (1500 rpm), a combination of low aerodynamic drag and a high powered (505 hp) engine that's inefficient at making a low amount of power.

acceleration

Again somewhat car dependent. You use more fuel during acceleration, but the time to speed is short enough that it doesn't matter unless you're in stop and go traffic. Full throttle for 4 or 5 second intervals isn't going to have much effect on milage if those intervals are separated by 60 seconds or more of cruising.

Also if you're commuting on a city freeway in the USA, you can constantly cycle between 50 mph to 65mph in the slow lane, dealing with trucks and cars merging onto the slow lane, or on the other extreme, you can cruise in the fast lane at 75mph, occasionally pulling over to let faster cars by.

 

[russ_watters]

for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph.

Basically, it's the lowest speed the car can comfortably maintain speed while in its top gear. For my current car, that's about 50mph, though there isn't a major difference between 4th and 5th gear.

On the other extreme, something like a Corvette Z06, probably gets it's best gas milage around 70mph (1500 rpm), a combination of low aerodynamic drag and a high powered (505 hp) engine that's inefficient at making a low amount of power.

And, I would think, a high gear ratio.

 

[jack action]

That conflicts with the data I've seen from magazines and web sites, a couple of links:

http://www.fueleconomy.gov/feg/drivehabits.shtml

for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph.

http://auto.howstuffworks.com/fuel-efficiency/fuel-economy/question477.htm

In the case of small econoboxes, or hybrids like the Prius, lower speeds are better. A 1.0 liter Geo Metro proably peaks below 30mph, scroll down to see the Metro milage versus speed graph on this web page:

http://www.metrompg.com/posts/speed-vs-mpg.htm

It doesn't really conflict. If you read carefully the link from metrompg.com:

Finally, what's the message these charts all send? For the best fuel economy, drive as slow as practical in the tallest gear.

Which is what my article says:

He said the Fusion's FE is best between 40 and 45 mph, the main reason being that at that speed under light cruise the transmission will be in sixth gear. "Lowest speed in top gear is your best fuel point," he said.

Except that, for the average car, there is a lower limit of about 30 mph because the engine becomes more inefficient when the load gets too light:

"Wind resistance is proportional to vehicle speed, but the engine and transmission get more efficient as the load on them increases," said Clark. At about 30 mph, that is a good balancing act.

 

[rcgldr]

Except that, for the average car, there is a lower limit of about 30 mph because the engine becomes more inefficient when the load gets too light:

Clark pointed out that at a steady cruise of about 40 mph, one's FE could be 40 to 50 mpg.

Based on these statements, Clark is talking about a hybrid or a econobox with a tiny engine, since he's quoting 40 to 50 mpg. An "average car" in the USA doesn't get 40+ mpg, and it's "sweet spot" will be in the 40mph to 60mph range mentioned in the how stuff works article. However, over time, what constitutes an "average car" in the USA is changing, towards more fuel efficient cars. Still with the EPA quoting highway milage, there's going to be an effort to make the cars reasonably efficient at freeway speeds.

 

[mender]

Clark pointed out that at a steady cruise of about 40 mph, one's FE could be 40 to 50 mpg. But during a moderate acceleration event - about 0.1 g - FE drops to about 10 mpg, he said. At idle, a car burns fuel at a rate of roughly 0.2 g/s whereas at the acceleration of 0.1 g it burns perhaps 2 or 3 g/s.

At WOT, fuel intake could be as much as 10 g/s. An average driver launches at about 0.2 g, Clark said, although most vehicles are capable of 0.5 g.

"Any little bit of acceleration, you're adding a huge amount of extra fuel," Clark said. "That's why trying to anticipate stops and trying to coast as much as you possibly can really make a big difference in your FE, It's all about acceleration and the conservation of kinetic energy. It takes a lot of fuel to get that mass to accelerate, even in a small car."

Ford's Woundstra noted that, at idle, the 2010 Ford Fusion with a six-speed automatic transmission and 2.5-L four-cylinder engine consumes fuel at a rate of 0.28 gal/h compared to a rate of 16.4 gal/h at WOT. He said the Fusion's FE is best between 40 and 45 mph, the main reason being that at that speed under light cruise the transmission will be in sixth gear. "Lowest speed in top gear is your best fuel point," he said.

Steve Payne, a Senior Engineer at Ford, said shifting schedules for Ford automatics are designed with drivability at the forefront. Someone opting for a manual transmission on the same Ford model can get better FE by upshifting sooner, but at the expense of drivability. Payne noted that too slow an acceleration rate is bad for FE also "because, especially on a spark-ignition engine, you're keeping the throttle plate closed, which is making the engine run inefficiently. But that would only be for extreme cases, and people would be honking their horns if you drove that slowly."

They should also add that the lowest gear with high engine load for the desired acceleration rate is also the best for FE. The consumption data given for the idle and WOT conditions could be considered misleading as there are no conclusions drawn from that data but is presented in a way that implies that WOT is not an efficient way to accelerate. The sentence that I emboldened is correct but appears to discourage the technique at the same time; could just be corporate speak for "we did it (shift schedules for the automatic) our way even though we knew it wasn't the most efficient".

That is one of the problems when quoting certain sources, the bias has to be accounted for when considering the validity of the arguments presented. There are other things that I could pick at in that article but 'nuff said.

A better way to consider the effect of acceleration on FE would be to consider how much fuel is consumed when changing speed in 10 mph segments at various throttle openings and gear selections.

 

[mender]

Based on these statements, Clark is talking about a hybrid or a econobox with a tiny engine, since he's quoting 40 to 50 mpg. An "average car" in the USA doesn't get 40+ mpg, and it's "sweet spot" will be in the 40mph to 60mph range mentioned in the how stuff works article. However, over time, what constitutes an "average car" in the USA is changing, towards more fuel efficient cars. Still with the EPA quoting highway milage, there's going to be an effort to make the cars reasonably efficient at freeway speeds.

My daughter's old 2.5 turbo auto registers just over 40 mpg on the trip computer at 30 mph steady state but can hardly be called a hybrid unless you're referring to the heritage of some of the parts.

 

[mender]

This is usually top gear. For the engine itself converts fuel burnt to work best at max BSFC.

That would be min BSFC.

 

[xxChrisxx]

That would be min BSFC.

Indeed it is :D

 

[jack action]

Based on these statements, Clark is talking about a hybrid or a econobox with a tiny engine, since he's quoting 40 to 50 mpg. An "average car" in the USA doesn't get 40+ mpg, and it's "sweet spot" will be in the 40mph to 60mph range mentioned in the how stuff works article.

Man, take the time to read before getting all defensive:

one's FE could be 40 to 50 mpg. But during a moderate acceleration event - about 0.1 g - FE drops to about 10 mpg, he said.

An average driver launches at about 0.2 g, Clark said,

With no acceleration, i.e. at constant speed, you get 40-50 mpg, but if you drive with a 0.1 g you drop to 10 mpg. Your normal driving behavior is a mix of both (and even greater accelerations for the average driver).

The reason why EPA highway mileage is better than the EPA city mileage is because there is a lot more acceleration in the city than on the highway, and even small acceleration takes a lot more fuel than "higher" speed.

However, over time, what constitutes an "average car" in the USA is changing, towards more fuel efficient cars.

This article is about 2010 vehicles.