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

[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.

 

[xxChrisxx]

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).

I'd say that's just about right, even the numers are nearly bob on for my car :P. Going from steady part open throttle at cruise in my car in 5th to WOT causes the MPG to drop from 40-45 to about 12.

I average 37 if I drive like an undertaker (I do mostly long distance motorway) and 23-27 if the car is driven properly.

A diesel is sadly starting to look quite attractive now

 

[jack action]

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".

But the sentence that you emboldned is followed by:

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.

So I don't see how it is misleading or in contradiction with what you are saying. Acceleration kills your mpg, but if it takes you forever to accelerate inefficiently (i.e. the throttle plate closed), it will take more fuel too. It's a balancing act.

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.

Nobody argue with you that the pumping losses are going to be less at WOT than any other position. But at WOT, the AFR is usually on the rich side (to increase performance), which in turn means that WOT is never the most efficient spot. Because of that, it is usually somewhere around 75% WOT. And to get the same acceleration, meaning the same power output on a given vehicle, to be as close as possible to WOT, means lower rpm, which means running in top gear, as long as at around 75% WOT, at that rpm, you have the power required, of course.

 

[mender]

So I don't see how it is misleading or in contradiction with what you are saying. Acceleration kills your mpg, but if it takes you forever to accelerate inefficiently (i.e. the throttle plate closed), it will take more fuel too. It's a balancing act.

With the best balance being much closer to WOT than closed throttle. I say that the method of data presentation could be considered misleading, you don't. Fair enough.

Nobody argue with you that the pumping losses are going to be less at WOT than any other position. But at WOT, the AFR is usually on the rich side (to increase performance), which in turn means that WOT is never the most efficient spot. Because of that, it is usually somewhere around 75% WOT. And to get the same acceleration, meaning the same power output on a given vehicle, to be as close as possible to WOT, means lower rpm, which means running in top gear, as long as at around 75% WOT, at that rpm, you have the power required, of course.

And we agree on that:

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.

Just a note, 75% throttle doesn't always correspond to 75% load. Picking a nit.

But since getting to speed requires acceleration, saying that acceleration is bad is kind of silly, don't you think? Not quantifying what the best acceleration rate would be and why leaves a pretty big gap in the FE scenario in my mind. Maybe that's just me.

On to pumping losses: I haven't delved into this as much as I should have, but am curious about how that compares to the drop in thermal efficiency from the lower dynamic compression ratio caused by throttling. Any thoughts?

 

[xxChrisxx]

You can tell this is in the physics thread :P.

I'll take my engineering stance and say perform an experiment.

Buy 1 tank of fuel, drive in one way.
Buy another tank, drive another way.

most miles to a tank wins.

 

[mender]

Or just use the dyno test results from one of the many engines that I've tested and extrapolate.

I suspect a slush box will benefit from a more sedate driving style to limit slippage during acceleration.

 

[jack action]

But since getting to speed requires acceleration, saying that acceleration is bad is kind of silly, don't you think? Not quantifying what the best acceleration rate would be and why leaves a pretty big gap in the FE scenario in my mind. Maybe that's just me.

OK, you're right, I'm going a wrong way. More acceleration shouldn't require more fuel. What counts is the difference between initial and final energy energy state. The energy required is ½m(v_f²-v_i²). If you accelerate faster, it will take less time, so same energy required, hence same quantity of fuel (assuming same efficiency). It is accelerating and decelerating constantly that hurts your FE.

Answering what is the best acceleration to obtain the best FE brings us back to http://us1.webpublications.com.au/static/images/articles/i1102/110216_9lo.jpg" .

The most efficient power is the one right in the middle the red island. So say this point represents 50 hp. If you're driving at a speed that requires 20 hp, then the acceleration corresponding to a 30 hp (=50-20) increase will be the most effective acceleration. As the speed increases, the ideal acceleration will decrease as more power will be needed to fight the new acquired velocity.

I guess, when comparing engines, we should look at their most efficient power output.

 

[mender]

If you accelerate faster, it will take less time, so same energy required, hence same quantity of fuel (assuming same efficiency). It is accelerating and decelerating constantly that hurts your FE.

Especially the decel.

The most efficient power is the one right in the middle the red island. So say this point represents 50 hp. If you're driving at a speed that requires 20 hp, then the acceleration corresponding to a 30 hp (=50-20) increase will be the most effective acceleration. As the speed increases, the ideal acceleration will decrease as more power will be needed to fight the new acquired velocity.

The accel rate should taper off by itself if the throttle is held at the 50 hp until the desired speed is reached; good point, and one that I hadn't taken to your conclusion. Thanks!

I guess, when comparing engines, we should look at their most efficient power output.

http://ecomodder.com/wiki/index.php...umption_(BSFC)_Maps#Toyota_Prius_1.5L_1NZ-FXE

With a renewed emphasis on lean burn operation, the maps are slightly different for the hybrid car engines.

Just a note: when testing my daughter's car with a wide band O2 sensor and the wastegate wired open, I was a little surprised to see that the AFR was still 14.7 at full throttle but no boost. The mixture didn't fatten up until boost reached 4 psi.