Revs and fuel efficiency - Sportbike Forum: Sportbike Motorcycle Forums
Reply
 
LinkBack Thread Tools Display Modes
post #1 of 20 (permalink) Old 10-09-2007, 11:07 PM Thread Starter
 
Join Date: Nov 2006
Posts: 307
Revs and fuel efficiency

Considering ideal conditions such as cruising on a flat highway, is there a certain rev range that is more fuel efficient than another? Or is the difference negligible?
OSUrid3r is offline  
Sponsored Links
Advertisement
 
post #2 of 20 (permalink) Old 10-09-2007, 11:24 PM Thread Starter
 
Join Date: Nov 2006
Posts: 307
I think I've just answered my own question. Maintaining a constant speed requires a certain amount of fuel energy. If the engine was spinning slower more fuel would be used in one cycle to maintain that speed. If the engine was spinning faster, less fuel would be used in a cycle (but more cycles per second) so the amount of fuel consumed over a period time would more or less stay the same. However, consumption at high revs may be a little higher to compensate for the various forms of friction throughout the engine and transmission such as mechanical, fluid, etc, which would probably be apparent at higher revs thus lowering efficiency slightly. I guess the difference is negligible afterall... but it should still be on Mythbusters.
OSUrid3r is offline  
post #3 of 20 (permalink) Old 10-10-2007, 03:37 AM
Strength and Honor
 
kanwisch's Avatar
 
Join Date: Oct 2002
Location: Central IN
Posts: 6,144
I had always had the belief (pulled from thin air) that the lower the RPMs the better the mileage. This definitely holds true for our Grand Prix and I've noted over the years that the top gear of most of our cars has close to its low RPM point at about 50-60mph, which coicides with the average speed on highways (for most people anyway ).

My theory goes like this: if the engine spins slower, less energy is needed to achieve and maintain that spin. However, I've ridden my bike hard for a tank or two and then ridden it like a puss and found the mileage to be roughly the same. That seems to conclude what you have: it doesn't matter.

SportbikeWorld Supermoderator

Dragging knee is for the track, and dragging tail is for the lot. --Kane Friesen

When you're in a car, you're watching a movie; when you're on a bike, you're in the movie. --Robert Pirsig

Identity theft is not Fun
kanwisch is offline  
 
post #4 of 20 (permalink) Old 10-10-2007, 05:44 AM
 
Join Date: Mar 2004
Posts: 8,798
you have answered your own question

Not only does friction increase with speed, but so does momentum (mass*speed squared) since each piston has to stop and change direction, the less momentum you are fighting with the better. But considering how light the engine internals are the difference is pretty negligable.
The key factor is going to end up being the valve overlap (the time in the cycle when both exhaust and intake valves are open). Over lap depends on certain gas (intake and exhaust) velocity to work right, so that fuel enriched air doesnt end up traveling directly to the exhaust or vice versa,



Vash is offline  
post #5 of 20 (permalink) Old 10-10-2007, 06:02 AM
 
Join Date: Apr 2007
Posts: 29
I believe that it is due in large part to the fact that when our bikes are at higher revs it doesn't necessarily mean the engine is working a lot harder. Considering that most liter sportbikes weigh around 400lbs, our engines don't carry the same kind of load cars/trucks do. I think that is mainly why running the bike harder doesn't take much affect on mileage.

As in running a car harder the engine has a lot more strain on it when the car wieghs like 3,000lbs or more. And getting the best mileage doesn't necessarily mean keeping the lowest rpms while driving. If your motor is lugging, like it has no power in a higher gear while trying to keep your vehicle moving at a certain speed then it is putting more strain on it then. The key to getting the best mpg at a certain speed is to keep the car in a gear that will give it enough power to not put strain on the motor at that speed while keeping as low an rpm as possible.
StealthZX9R is offline  
post #6 of 20 (permalink) Old 10-10-2007, 07:10 AM
Registered User
 
Join Date: Jan 2007
Location: Sacramento, CA USA 96814
Posts: 369
It's a little more complicated than that.

1. Drag losses increase with speed, exponentially. The slower you go, the less drag you have, and less energy is required to go the same distance. That is of course, obvious.

2. Engines are most efficient not at lower RPM's, but rather at specific RPM's, AND loads. If you run an engine at a higher, or lower load, or at a higher, or lower RPM than that which results in the highest BSFC (brake specific fuel consumption) then you are sacrificing efficiency. For most engines that means a nearly fully loaded engine running at somewhere around its peak torque curve.

3. Obviously, a fully loaded sportbike engine running at its peak torque curve would be driving that bike at a very high speed, resulting in very inefficient running conditions, even though the engine itself may be operating at its optimal BSFC. Conversely, cruising around in third gear, slightly off idle going 25 mph results in another inefficient running condition, since engine efficiency is very low, even though minimal power is required to propel the bike.

4. So there is a tradeoff, and an optimal speed for the most efficient running conditions. For most cars and light trucks, that speed is by design somewhere around 50-60 mph. This translates in to running the vehicle in the highest gear without lugging the engine, and then a slight amount. Again, that's still an inefficient running condition with respect to the engine, but the optimal tradeoff between engine efficiency and vehicle drag.

5. I notice myself, that there is surprisingly little difference in mileage on my bike, whether I cruise at 60 mph, or 90 mph. Obviously, there is a whole lot more incremental drag at 90 mph, but on the other hand the engine is running much closer to peak efficiency, which provides some degree of offset.

Last edited by Mister Tee; 10-10-2007 at 07:12 AM.
Mister Tee is offline  
post #7 of 20 (permalink) Old 10-10-2007, 07:33 AM
 
Join Date: Jun 2005
Posts: 892
i have always heard thats what 5th gear was for....there no real powerband in it. heard that you'll get better mileage in it opposed to 4th. but thats pure heresay.
Meat_Shield is offline  
post #8 of 20 (permalink) Old 10-10-2007, 07:42 AM
Registered User
 
Join Date: Jan 2007
Location: Sacramento, CA USA 96814
Posts: 369
Quote:
Originally posted by Meat_Shield
i have always heard thats what 5th gear was for....there no real powerband in it. heard that you'll get better mileage in it opposed to 4th. but thats pure heresay.
Very possible, depending on your bike, cruising speed and sprocket gearing.
Mister Tee is offline  
post #9 of 20 (permalink) Old 10-10-2007, 07:56 AM
 
Join Date: Jun 2005
Posts: 892
ya i always use 5th when im on the hiway. unless there is traffic in that case im in no higher than 4th just in case i need the instant power due to a jackass cager. but i do notice better mileage in 5th....of course that could be just because they are hiway miles.
Meat_Shield is offline  
post #10 of 20 (permalink) Old 10-10-2007, 10:57 AM
Registered User
 
Join Date: Jan 2007
Location: Sacramento, CA USA 96814
Posts: 369
Quote:
Originally posted by Meat_Shield
ya i always use 5th when im on the hiway. unless there is traffic in that case im in no higher than 4th just in case i need the instant power due to a jackass cager. but i do notice better mileage in 5th....of course that could be just because they are hiway miles.
Actually I had a brain fart - I re-read your post. What I meant was that it's possible but not likely that 5th may give you better mileage than 6th, if you have a six speed transmission. I suppose the same could be said of using 4th as opposed to 5th, but that's more unlikely.

I never drop a gear for cruise. I might if I'm lane splitting, or in really heavy traffic.
Mister Tee is offline  
Sponsored Links
Advertisement
 
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Sportbike Forum: Sportbike Motorcycle Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome