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Spiderman
03-06-09, 02:43 PM
Calling all sciency, geeky, mathy, smart type of people....

Following on from fuel economy and tyre economy i've always wondered what is really the best speed for fuel economy on a motorway on an SV.
I've always wondered what it might be.

I was told by a very knowledgeable man once that when the fuel was low I should not worry too much about my top speed when getting home as it was hard accelaration that ate the fuel, not a high top speed.

Some of you who are knowledgeable about these things must be able to do some kind of calcualtion and come up with the definative economical speed surley :)

Extra points for showing your working out of course and the winner gets, erm, the satisfaction of knowing;
A) you're well smart
B) we all will ride at your calculated speed from now on and be saving the planet a bit by not filling up as much.

:)

Viney
03-06-09, 02:46 PM
Isnt it meant to be 56mph?

ricky_t
03-06-09, 02:51 PM
The most economical speed is the lowest speed for drag. For the sence of practicality, the lowest speedin the highest gear without laboring the engine. I wouldn't go much slower than 60 on safety grounds. Trucks are not used to overtaking motorbikers!

Spiderman
03-06-09, 02:53 PM
Isnt it meant to be 56mph?I'm sure thats for cars and based on very old technology, isnt it :confused:

Surley for a modern fuel efficient bike with fuel injection and electronics controlling things to a degree this cant be the right figure can it?

Maybe it is, i dunno but i'm hoping someone does and can explain why too.

Brettus
03-06-09, 02:54 PM
I think it does get technical, that was the general rule of thumb but I'd imagine its relative to gearing and revs.
I agree with the hard acceleration and braking though, (braking less so) but it means you wasted the fuel to get you to the speed you are scrubbing off.
Will be interested to see if someone does come up with a decent answer though, I've often wondered what it was, well usually when I'm on a motorway with the fuel light flashing :)

jamesterror
03-06-09, 02:54 PM
Isnt it meant to be 56mph?

I read somewhere 56mph is the speed engines tend to run most efficiently at, think thats the reason for the NSL in America? .. as well as saving on fuel use (1970's). Whether thats correct I don't know.

Quiff Wichard
03-06-09, 02:55 PM
131.8

You're welcome

ophic
03-06-09, 02:56 PM
The most economical speed is the lowest speed for drag. For the sence of practicality, the lowest speedin the highest gear without laboring the engine. I wouldn't go much slower than 60 on safety grounds. Trucks are not used to overtaking motorbikers!
+1 unless the engine has some weird drinking characteristics at low revs.

eg My car will pull top gear at 35mph but its more efficient at 50.

wyrdness
03-06-09, 02:56 PM
I was once told that you get the best fuel economy when the engine is at peak torque. I've no idea if this is correct. Does anybody know?

Jamiebridges123
03-06-09, 02:56 PM
The most economical speed for any vehicle would be where the engine produces the most torque, which in most cases is around 3200rpm in cars, which in general, depending on type of car, is between 55 and 70mph, so hence why most people take 60 to be a good general rule of thumb.

On the SV I just keep to 60 as it's a lovely and rounded 4500rpm.

STRAMASHER
03-06-09, 02:59 PM
If I want to stretch out a tankful I will sit at 65-70 (Usual 80mph).

Big difference in consumption. And still safe and not causing an annoyance.

Got me to the magic 200 mile on my GSX14 from a 22ltr tank with juice to spare.

Only the once mind. I love getting happy on the throttle.:smt045

ophic
03-06-09, 03:00 PM
The most economical speed for any vehicle would be where the engine produces the most torque...
Got any explanations to back that up?

Spiderman
03-06-09, 03:00 PM
Will be interested to see if someone does come up with a decent answer though, I've often wondered what it was, well usually when I'm on a motorway with the fuel light flashing :)

PMSL, to be honest mate i thought it would be you after i saw this post from you in another thread...
Sorry, bit too geeky but my stats collected over the course of a year average out at:
SV650s Sport K6

Fill up: 13.99 (bear in mind that covers when petrol was at 120p +)
Fuel price: 105.1p
litres: 13.8 (so I've usually had around 3.5 left in it)
MPG: 55.1
Miles between fuel stops: 170.4

Pence Per Mile:
Fuel: 9p
Tyres: 2p

Average miles per tank: 206.3 (assuming by calculation that I ran it to empty)

Reserve Capacity:
Flashing (4.5L left so I was told) 54.6 miles
Solid (1.5L left) 18.2

I will usually risk 2-3 trips to work (each way not both) from when it starts flashing and I've not run out yet. closest I got was 1.11L left.

Only twice have I managed to make the bike to less than 50mpg, I got 44.5 on one fill up and the tank would only have got me 166 miles then.


But i'm glad to see its not an obvious answer and i'm not the only one whos wandered it either.

I agree in principle of course with what most of you have said, excpt for Quiff of course, but i'd love to see a calculation proving a certain speed in MPH cos its obviosuly a lot of factors that can change it, like headwind or tailwind etc.

I''m sure the combined talents of the Org will mena there is someone who will know...or can work it out.

Jamiebridges123
03-06-09, 03:04 PM
Got any explanations to back that up?

Read it in Autocar ages ago.

ophic
03-06-09, 03:09 PM
Read it in Autocar ages ago.
Not saying its wrong, just would like to hear the theory behind it before I believe it.

Jamiebridges123
03-06-09, 03:11 PM
I'd guess it's because the engine will "pull" the best, IE you'll need less throttle to go up a hill ETC, basically it just has the most available pulling power with the minimum of throttle input :)

Spiderman
03-06-09, 03:12 PM
The most economical speed for any vehicle would be where the engine produces the most torque, which in most cases is around 3200rpm in cars, which in general, depending on type of car, is between 55 and 70mph, so hence why most people take 60 to be a good general rule of thumb.

On the SV I just keep to 60 as it's a lovely and rounded 4500rpm.

This kind of stuff is just generalisation tbh, which is fair enough when it comes to car engines i guess as they are mostly the same layout.

I wanna know specifically for an SV with its own engine charachteristics what would be the optimum speed cos i'm certain that its not the same as a car!

ophic
03-06-09, 03:15 PM
I'd guess it's because the engine will "pull" the best, IE you'll need less throttle to go up a hill ETC, basically it just has the most available pulling power with the minimum of throttle input :)
but say peak torque is 7krpm - lets say thats 70mph for easy figures

at 35mph in top gear, the engine may be less efficient but the drag on the vehicle has dropped by a factor of 4, so unless the engine is 4 times less efficient at 3500rpm than 7000rpm (which i doubt) then you'll go further on the same fuel at 35mph.

ophic
03-06-09, 03:22 PM
ah-ha i've spotted my own error - the drag is 4x less but the vehicle has to push through it for twice as long. So the engine only has to be 2x less efficient at 3500rpm - i still doubt that its the case.

Jamiebridges123
03-06-09, 03:30 PM
I got 155 miles on 10 liters at between 60 and 75mph with some occasional full throttle bursts riding back from Yorkshire..

So on the SV I'd guess about 60-70 would be fine!

sinbad
03-06-09, 03:41 PM
I think it's fairly easy to "feel" where that sweet point is, particularly on a bike- striking that balance between low rpm and minimal throttle input. The SV cruises gleefully at around 70mph with the merest fraction of throttle in top gear, crank it up by 20mph and the difference is noticeable. It'll do it of course, but not as eagerly.

Similarly much slower than 60 in 6th gear and it's asking you to downshift (at least mine is :) ).

(Standard gearing K4 S).

Daimo
03-06-09, 03:59 PM
I was told by a very knowledgeable man once that when the fuel was low I should not worry too much about my top speed when getting home as it was hard accelaration that ate the fuel, not a high top speed.



I kinda agree from experience. Acceleration is the fuel killer....

Bu then when your at speed, your not on the throttle, but the explosion still happens (hence air n fuel going in).

Rolling off throttle, esp downhill doesn't use fuel, the engine momentum turns the engine over, not the fuel.

But, if i sit at 110mph, or 70mph, doing 11-mph uses a hell of a lot more fuel.. So when running low, i sit at 60mph as i know it'll get me vastly further than sitting at 80mph....

No other proof other than my own experience form running out of fuel a few times and doing about 50,000 miles in 4 years on bikes :lol:

Dr T
03-06-09, 04:19 PM
I always believed it was lowest throttle angle. Hence less air, hence less fuel. All cars and must be the same for bike have an optimum air to fuel ratio for economy norm 15:1 and performance norm 12.5:1 reguardless of rpm. Cars are normally mapped for economy at constant throttle angles and this is were the lambda probe comes in to hold 15:1(do k8's etc have better consumption?). Under hard acceleration that goes out the window and the car will use the map for optimum perfomance. Also when deaccelerating the throttle valve is shut and the engine stops fuel and air and uses momentum to slow down. So it's better to use engine braking than coasting downhill.
As to the best speed on a motorway not a clue, guessing the lowest throttle angle you can achieve in the highest gear. But that will vary will drag and incline. You should pick up speed going down a hill so you need to use less throttle going up, but you'll slow down when you go up. This is a reason why I find cruise control in cars so poor on fuel.

Dan
03-06-09, 04:40 PM
Just as a point of order:

56mph isn't the point at which a car/bike/bus whatever is running most efficiently, simply one of the speeds at which economy is tested, and quoted, in order to ensure that everything is compared like-for-like.

The most economical speed is that which a vehicle of any kind can maintain using the lowest rpm for any given gear without causing the engine to struggle, which is generally around about the point at which peak torque is produced.

ophic
03-06-09, 04:46 PM
JThe most economical speed is that which a vehicle of any kind can maintain using the lowest rpm for any given gear without causing the engine to struggle, which is generally around about the point at which peak torque is produced.
Not on a petrol engine - most produce peak torque way up the rev range. The engine shouldn't struggle until it gets down near tickover, which is nowhere near peak torque at all.

Jamiebridges123
03-06-09, 04:49 PM
Most normal petrol engines, in cars, produce peak torque around 3000-3500rpm, unless you have some silly little Honda which produces peak torque at 7800rpm... vtak kicked in yo...

ophic
03-06-09, 04:53 PM
Most normal petrol engines, in cars, produce peak torque around 3000-3500rpm, unless you have some silly little Honda which produces peak torque at 7800rpm... vtak kicked in yo...
and they redline at 5-6k? and will still pull at 1200? certainly not starting to struggle at anything lower than peak torque.

ranathari
03-06-09, 05:01 PM
Varies from bike to bike. I read some magazine (damned if I can remember which) that asked the same question and tested it with a Fireblade and a BMW GS - the CBR was actually more economical than the BMW when revved hard purely because the engine was tuned for top-end power rather than the middle of the rev-range, like the BMW.

Bluefish
03-06-09, 05:26 PM
Varies from bike to bike. I read some magazine (damned if I can remember which) that asked the same question and tested it with a Fireblade and a BMW GS - the CBR was actually more economical than the BMW when revved hard purely because the engine was tuned for top-end power rather than the middle of the rev-range, like the BMW.

thats like the test top gear did, with a bmw 5 series or something and a small town car and they both drove around the track at 80/90mph and as the small car was using fill power to maintain the speed and the bm was just ticking over the bm went a couple of miles further than said small car on a gallon of petrol, conclusion buy a big car, haha.

Jamiebridges123
03-06-09, 05:35 PM
and they redline at 5-6k? and will still pull at 1200? certainly not starting to struggle at anything lower than peak torque.

If you put your car in 5th and try to pull at 1200, unless you have a larger engine or something, it'll jump and studder a little :D :smurfin:

andreis
03-06-09, 06:01 PM
Just thought I would throw my 2 cents in:
I figure it's like this: the drag force exerted is (CD * rho * A * v^2) / 2 , where CD is the drag coefficient, rho is the air density, A is basically the area exposed and v is your speed. Assuming you don't change the area (to which you contribute also), as in when you're completely tucked behind the wind screen, and that the CD stays basically the same, it seems the amount of stopping force is squarely dependent upon the speed.
Now that we have one force we just need to figure the other one out, which is at which gear and rpm would the engine produce an equal amount of force for a given speed. Then we would need to also see how that translates to fuel consumption.
Now I don't buy the argument regarding torque, as that is in no way related to engine efficiency (engine efficiency is calculated as which percent of the energy generated by the fuel explosion is actually being put to good use), hence, you might be getting a lot of torque at peak torque, but it does not mean that you are actually using a lot of the fuel to defeat drag force(you might be burning a lot of fuel to get a certain amount of energy).
This is one of the reasons accelerating is so bad for fuel economy: engine efficiency goes down, so defeating drag force and other forces as well e.g. you're own weight going uphill, consumes a lot more fuel.
So let's see how the formula looks.. you take the energy generated by burning fuel, you use it to expand the gas in the engine in a given amount of time. This converts to force applied by the piston to the crankshaft (you lose some force there..), which goes through the gears(you lose some force there..), to the back wheel, where it is divided by the radius of the wheel, encounters friction to the ground and what's left of it fights off drag force..
This generates, depending on the engine, a certain amount of fuel consumed to keep a steady speed (at a certain engine efficiency -to be read rpm + fuel/air mixture- and a certain gear) for .. say 1 sec.
Now, your keeping a certain steady speed, so this consumption per second is then translated to how many miles you do in an hour using how much fuel... Which is then converted easily to the mpg we're all searching for.

Now I don't think I'll take the actual calculations to an actual exact formula. That would be pointless, as many factors vary by much - engine efficiency goes down through time, the amount of force you lose to crankshaft and gear box increases over time and tire condition comes to play (because that affects the amount of friction you encounter at the wheels - both front & back). drag force doesn't really change in time (unless you stick your head above the windshield), but that's one constant in a sea of variables...

For keeping a high mpg I'm positive you have to keep tucked behind the windshield (closing a car's windows increases fuel economy by 10% - 15%), but other than that my post is absolutely worthless..

Well, that was it.. if anyone feels like it, go ahead and do the formula :p

Brettus
03-06-09, 06:11 PM
For keeping a high mpg I'm positive you have to keep tucked behind the windshield (closing a car's windows increases fuel economy by 10% - 15%),


I can see it now, half the org laying on their tanks, feet tucked up over the pillion pegs, streamlining on an SV :D


Excellent post though Andreis, outlines all the factors that need to be taken into account, I tried googling but I was being harassed by muppets at work so didn't find anything useful just the usual conjecture and postulation :-) (along with OMG U R teh SUCK, LOL etc :D)

andreis
03-06-09, 06:16 PM
Thanks, but I think I missed a big one though.. Engine efficiency is also highly dependent on engine temperature, but as we're talking highway running, it shouldn't be a problem. But for those of us interested in work commuting, this has a big impact as you don't typically get the engine warm enough in a 3-5 miles drive.. This is probably why someone using the bike for a lot of short distance travels gets bad mileage..

trumpet
03-06-09, 06:22 PM
A big fat 14 stone me in a floppy suit would have to travel slower for the same force than someone 10stone + in a leather suit'
I'm sure i got a few miles more per tank when I could fit in my leathers in summer

V-twin
03-06-09, 06:48 PM
My sv on long motorway runs has achieve 70mpg @ 70mph.....then again it is restricted. It works out at a theoretical 256 miles to a tank. I haven't put that theory to the test yet.........256 miles on one full tank.

I've got too much free time

sarah
03-06-09, 07:05 PM
It's not just drag you have to take into account. Don't forget the friction between road and (hopefully) tyres.

andreis
03-06-09, 07:15 PM
Hmm.. I did account for that. Which is to say I mentioned it. Nothing is actually accounted for as there is no formula posted yet..
As Trumpet said, we'll all get on our bikes, from a standstill with a full tank. Divide into .. say 4 groups of at least 5 (that would mean at least 20 drivers). Group 1 will do 3rd gear, group 2 will do 4th and so on. We'll divide the range speed of each group into 5 equal sections and each rider will ride into his/her own section.
The one to get the furthest answers Brettus' question...

But better yet, we just keep riding safe for years and figure it out in time :D
Except for those special days(most of them) when you say "the hell with mileage" and twist it for the extra smile

sarah
03-06-09, 07:25 PM
Hmm.. I did account for that. Which is to say I mentioned it. Nothing is actually accounted for as there is no formula posted yet..


oops, soory, missed the last page of the thread. just thought i'd mention it as i hadn't noticed it elsewhere and my mech eng background got the better of me.

yorkie_chris
03-06-09, 07:38 PM
Got any explanations to back that up?

Peak torque happens at the point in rev range where BMEP is at maximum. BMEP is a function of how efficiently the fuel is burned and the volumetric efficiency of the engine. Of course, frictional losses also come into play, so cruising at 10krpm on a "hot" engine won't be good either.

So, an engine with hot cams (bike engine), at 2000rpm will spit loads of fuel out through the exhaust ports and generally stutter and whinge. You'll need loads of throttle to get anywhere at reasonable speed, and because of that, you'll use loads of fuel.

When you want to accelerate, use light throttle a little higher in the rpm, where the cams are going to be about right. When cruising, use the gear that means least throttle opening. Probably the highest one to put the engine at the lowest revs where it 'feels' happy.


Rolling off throttle, esp downhill doesn't use fuel, the engine momentum turns the engine over, not the fuel.

There's a valve on the carbed bike which throws loads of fuel in on the overrun. You are right in that the momentum turns the engine over, but the fuel flow is still there, it just doesn't have the air there to actually burn.

and that the CD stays basically the same,

Cd is also a function of Reynolds number so it also varies with speed, but since it's only ^1 and the velocity is ^2 then velocity will be the main defining factor.

yorkie_chris
03-06-09, 07:40 PM
Hmm.. I did account for that. Which is to say I mentioned it. Nothing is actually accounted for as there is no formula posted yet..
As Trumpet said, we'll all get on our bikes, from a standstill with a full tank. Divide into .. say 4 groups of at least 5 (that would mean at least 20 drivers). Group 1 will do 3rd gear, group 2 will do 4th and so on. We'll divide the range speed of each group into 5 equal sections and each rider will ride into his/her own section.

So long as everybody has same drag, same tyres, same state of tune, equally healthy chain (couple of bhp can go into a f#cked chain), no brake bind, same clear run at the roads... What a pain in the rse...

ophic
03-06-09, 08:44 PM
If you put your car in 5th and try to pull at 1200, unless you have a larger engine or something, it'll jump and studder a little :D :smurfin:
Old MGF 1.8 we've got pulls smooth right down to tickover - about 900rpm. Corsa 1.2 I had as a courtesy car would actually stay smooth at below tickover, would run down to about 500rpm - altho not a lot of power there admittedly. My old 1L vauxhall triple was somewhat rougher but also didn't have a rev counter so... no idea :)

Jamiebridges123
03-06-09, 08:46 PM
Guess it's just our manky cars. :D

ophic
03-06-09, 08:57 PM
stuff
Ta YC. The "peak torque = max efficiency" just didn't add up for me.

Spiderman
04-06-09, 11:09 AM
Hmmm interesting discussion guys but no one has come up with the magic number yet :)

I appreciate there are loads of variables but for arguments sake lets say this test is done on an SV650 with tyres/engine etc all at optimum performance and is condcted on a flat and empty section of motorway and the rider stays tucked in at all times.
What would the optimum speed be then?

ophic
04-06-09, 11:16 AM
we've established (or YC has) that the most efficient riding is done at constant throttle in top gear at the lowest revs that the engine is comfortable with.
the gearing commander site says that 3600rpm is 50.4 mph in top. I think my SV would be comfortable at those revs so i'm gonna say 50mph.

Grinch
04-06-09, 11:29 AM
All I know is that if I ride my SV at 80mph in top gear, about 6k I think I will get about 55mpg.
If you want high cruising speed at less revs you could change your sprocket ratios.

I always though the SV's optimal torque ranges where between 5 and 8k.

On long trips tucking in normally added about 10 miles to the tank, this is over a motorway trip of 140 miles plus.

Quiff Wichard
04-06-09, 11:42 AM
main variable must be riders weight>> ? thats gotta take up more juice to pull a fat lad..

ophic
04-06-09, 11:46 AM
main variable must be riders weight>> ? thats gotta take up more juice to pull a fat lad..
a bit - but greater weight increases rolling friction, which tends to stay constant regardless of speed, and quickly becomes insignificant with respect to drag, and will therefore have no impact on the most efficient speed.

sinbad
04-06-09, 11:50 AM
main variable must be riders weight>> ? thats gotta take up more juice to pull a fat lad..

Around town it would make a difference because you are accelerating and slowing frequently, although a couple of stone isn't a huge increase in the overall weight of bike and rider.

Once you have reached motorway speeds, though, and that extra mass is no longer being accelerated, the difference would be negligible (as long as tyres are inflated appropriately).

Quiff Wichard
04-06-09, 11:50 AM
a bit - but greater weight increases rolling friction, which tends to stay constant regardless of speed, and quickly becomes insignificant with respect to drag, and will therefore have no impact on the most efficient speed.


oooo

thank you Johnny Ball. !! :D



I know I shudda listened in Science instead of looking at Mandy Olliers legs.. ...............

ophic
04-06-09, 11:51 AM
oooo

thank you Johnny Ball. !! :D
i once had his chemistry set :takeabow:

Grinch
04-06-09, 11:57 AM
Nothing better then a bit of rolling friction... ;-)

Spiderman
04-06-09, 12:11 PM
So the 90mph that i usualy cruise at isn't better than the legal 70mph?

Dpes the fact that i get to my destination quicker therefore use the bike for a shorter period of time not negate the small loss of fuel efficiency between the 2 speeds?

ophic
04-06-09, 12:16 PM
So the 90mph that i usualy cruise at isn't better than the legal 70mph?

Dpes the fact that i get to my destination quicker therefore use the bike for a shorter period of time not negate the small loss of fuel efficiency between the 2 speeds?
No because efficiency doesn't include time - simply work done for given amount of input energy. The work done is the same - the distance travelled. If you travel at any speed other than the most efficient, you will use more fuel.

ThEGr33k
04-06-09, 12:41 PM
I was once told that you get the best fuel economy when the engine is at peak torque. I've no idea if this is correct. Does anybody know?


Yes it does...

Problem is that often in top gear when at max torque the drag is so large it negates the effect. So unless you go around in a lower gear then no. :(

Stu
04-06-09, 01:57 PM
Originally Posted by wyrdness http://forums.sv650.org/images/buttons/viewpost.gif (http://forums.sv650.org/showthread.php?p=1929589#post1929589)
I was once told that you get the best fuel economy when the engine is at peak torque. I've no idea if this is correct. Does anybody know?

Yes it does...

Problem is that often in top gear when at max torque the drag is so large it negates the effect. So unless you go around in a lower gear then no. :(
But what is peak torque?
From these Dyno charts http://forums.sv650.org/showpost.php?p=1922172&postcount=8 and http://forums.sv650.org/showpost.php?p=1922261&postcount=22 for SV's
In the first diagram the torque curves are so flat that I would imagine peak efficiency would be quite low down to reduce the effect of drag (which is squared as speed rises) Also I can't believe the VTEC engine would be most economical above 7000 revs when the all the VTEC kicks in and yes torque increases, but is that not just a function of more fuel being fed into the engine?

Dicky Ticker
04-06-09, 02:28 PM
My car is VTEC and I find the most efficient motorway speed for fuel economy is about 500rpm before the Vtec kicks in
i.e 3500rpm = 70+mph = 40+mpg
Vtec 4200rpm = 80+mph = 33mpg

Vtec in 4th gear,max revs 115+mpg and you can watch the fuel gauge dropping and completely illegal so I don't do it;)

sinbad
04-06-09, 03:19 PM
Vtec in 4th gear,max revs 115+mpg and you can watch the fuel gauge dropping and completely illegal so I don't do it;)
Optimistic typo! :)

Ceri JC
04-06-09, 04:11 PM
In my experience, it's the acceleration and braking of stop-starting that burns fuel. I've had better mpg on runs with an average speed nudging 3 figures when the roads are clear, compared to ones where my speed has fluctuated between 35-80 (probably averaging 60mph) due to heavy traffic.

Jamiebridges123
04-06-09, 04:13 PM
Talking of fuel, had a heartstopping moment, nearly ran out of the stuff today! Had do do 15 minutes of filtering and oh my god traffic on solid light. :|

stevoA4
05-06-09, 12:59 AM
On a car ecu i have read about a 'closed loop' state that an engine enters into and maintains 15-1 air fuel ratio once on constant throttle

Does an sv with efi use this?

Sid Squid
05-06-09, 07:04 AM
Peak torque = max efficiency, not max economy.

Do not confuse economy with efficiency. In essence max efficiency is getting the most back for whatever you put in, max economy is simply using the least amount.

56mph is simply the Imperial equivalent of 90kmh - sort of the NSL for metric users - and thus the speed used when calculating the out-of-town fuel consumption so that various manufacturers vehicles can be compared.

ophic
05-06-09, 09:27 AM
Do not confuse economy with efficiency. In essence max efficiency is getting the most back for whatever you put in, max economy is simply using the least amount.
That's a bad description, Sid, although you're right.

Max efficiency = max power output for least fuel used.
Max economy = furthest distance travelled for least fuel used.

The difference is that at peak efficiency, the engine is chucking out quite a lot of power - you'd be accelerating quite hard or maintaining a high top speed, where the effects of drag would prevent good economy.

I spose it could be argued that you should accelerate using best efficiency to the best economic speed.

yorkie_chris
05-06-09, 12:01 PM
Efficiency for heat engine = what you want / what you pay for = power out / fuel used

It's always going to be a trade off between getting there cheaply and getting there sometime before next christmas. I get pretty decent economy with 85-90ish cruising so I stick with that...

Sid Squid
06-06-09, 08:22 AM
That's a bad description, Sid, although you're right.I think calling it bad is rather harsh :D.

But it was intended to be a simplistic explanation.

JohnMcL7
06-06-09, 12:53 PM
In my experience, it's the acceleration and braking of stop-starting that burns fuel. I've had better mpg on runs with an average speed nudging 3 figures when the roads are clear, compared to ones where my speed has fluctuated between 35-80 (probably averaging 60mph) due to heavy traffic.

I similarly find that in practice keeping a constant speed is more important than the actual speed itself (within reason), years ago when I was a poor student trying to squeeze as much as possible out of every drop of fuel in my Metro 1.1 I found it my mpg was better at 60 than 50 or 55 simply because I found it easier to keep a constant speed. At the lower speed I found I was having to accelerate harder for hills and similar to keep my speed reasonably constant.

My current car seems to be much the same, it can sit at around 70-80mph at 2,000 revs and there's no noticeable increase in economy going down to 60 where it's not much above idling speed. The mpg takes a noticeable hit if I'm spending most of my time at 50-60mph and having to overtake frequently.

John

embee
06-06-09, 03:06 PM
A couple of things -

As Sid said, the old "56mph" economy for cars thing was purely a legislative speed for fuel economy testing (Euro used to be "urban", 56mph (=90kph), 75mph (=120kph), weighted 40/50/10% to give a combined fuel economy, USA used to be city/highway weighted 55/45), it is now done on a drive cycle consisting of "urban(x4) plus extra-urban" as here
http://i258.photobucket.com/albums/hh268/Hi-vis/600px-New_European_Driving_Cycle_sv.png
but that's by-the-by, and doesn't apply to bikes anyway. It just happened to be that of the test conditions used, the constant 56mph was the best economy, but didn't pretend to equate to the best economy point for the vehicle.

As for economy generally, it depends primarily on where in the load/speed range the engine is working and what the demand is.
Brake Specific Fuel Consumption (BSFC) graphs show how the amount of fuel required to give each kW output varies, it is a function of the various efficiencies (volumetric, thermal, mechanical, chemical) involved.
This one is for a VW 90PS diesel, but shows the principles (I chose this map because it already had the constant power lines shown).
http://i258.photobucket.com/albums/hh268/Hi-vis/tdi-bsfc.png

If the road load of the vehicle is, say, 20BHP (15kW, typical of around 60mph), it could run at nearly 4000rpm in a low gear and use 400g/kW.hr of fuel, or it could run at 1750rpm in a high gear and use 230g/kW.hr and get much better economy, 6kg/hr vs 3.5kg/hr, and if it was indeed 60mph that would be something like 36mpg vs 58mpg.

Road load is the total power required to overcome drag. Rolling resistance for a car is typically something like 1.5% of the weight, so a 1500kg car needs about 20-25kgf (200-250N) to roll it and is roughly constant, then the aerodynamic drag rises with the square of the speed,
F=(.r.v2)Cd.A ,
where Cd is drag coefficient (car around 0.3) and A is frontal area (car around 2m2), r is density of air (1.2).
So at 60mph or around 25m/s a typical car road load is around
P(kW)= 25m/s x (250N + (0.5 x 1.2 x 25 x 25 x 0.3 x 2)
Road Load = 12kW which might not sound much but is about right. Roughly half and half rolling resistance and aerodynamic.
A bike has much worse drag coefficient, more like 1.0 I believe.

Basically unless you know the BSFC map and road load curve, you're guessing, but as the others say, lowest speed comfortable in top gear usually gives about the best economy.

sunshine
06-06-09, 04:19 PM
max fuel economy? whats your top speed? and i beleive thats the best on motorways