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| blinky |
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blinky
Nitrous Nuisance
Joined: 29 Mar 2009
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 Posted: 03:28 - 02 Mar 2011  Post subject: **ladies & gents, lend me your ears...'Speed Vs Mpg' deb |
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*(que ears to be thrown at me)* 
just curious. was chatting to a mate earlier...
is it generally true that, as power and speed goes up, mpg falls as the compromise?
ie. sacrificing speed for fuel efficiency, but also in relation to small less powerful bikes Vs big (say 400cc and upwards) powerful bikes.
or am i confusingly mixing terms in rgds to power and speed? (forgive me, am quite tired and about to hit the hay!)
do you guys know some good 'high mpg' big bikes that do not neccessarily compromise on speed?
i understand touring bikes generally have 60ish mpg, ...obviously for efficiency over long distances, makes sense!
sports tourers only seem to hit 50-60mpg, other higher mpg bikes, tend to be the smaller bikes tho sadly, but i'm trying to find a selection of good big bikes with high mpgs that can do, say 120mpg comfortably, with a fair bit more to give.
your thoughts?
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Oh i miss my first love, the lovely lil CG!
Rouser 135 2012... Enfield 25... Hornet 600 2002... CG125 2000
Last edited by blinky on 04:01 - 02 Mar 2011; edited 1 time in total |
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Redoko
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lozzypop1
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| Teflon-Mike |
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Teflon-Mike
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Posted: 03:54 - 02 Mar 2011  Post subject: |
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Power = Rate or work done
work done is energy.
energy comes from burning fuel
Theres no compromises of mystical formula in there, its basic physics.
Going a certain speed, you need a certain power to over come the drag, which increases with speed, so faster you go, more power you need to use, hence the more fuel you have to burn to get it.
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Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?' |
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| MinhDinh |
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MinhDinh
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| Posted: 07:42 - 02 Mar 2011 Post subject: |
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Acceleration is just as important as speed.
I am guessing some types of engines, not just the size have different MPG.
Weight will play a part.
If you start and stop and keep on going high speeds, with high revs in a low gear, then it will make your MPG very low.
For example I am sure if you went 100 MPH without stopping, it would be betyer overall MPG than if you were accelerating really fast to 80 and then 0 and back up to 80 every few minutes.
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| Ingah |
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Ingah
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| Posted: 08:13 - 02 Mar 2011 Post subject: |
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The design does matter.
"You pay for power" is largely true though.
And bikes (especially older ones) often aren't really designed for good mpg, unfortunately. Especially the faster ones!
Things like: Type of carburettor, slide vs CV, make differences. Slide is more responsive, CV is more fuel efficient. FI (i.e. not carbs) is more efficient still.
Aerodynamics too (big problem on bikes), i think screens can help. A little. But the design of the screen matters - a little diddy one won't help much. Suspect double bubble may be best as it's supposed to pass the wind over your head...
I don't think you're going to find what you're looking for though, 120mpg is FI 4T 125 territory (a la CBF125).
You can get huge increases in MPG simply by riding in a fuel efficient way. Learn at what revs the peak power is - and stay well away from it 
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| The Tot |
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The Tot
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| Posted: 21:25 - 03 Mar 2011 Post subject: |
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KE = 1/2 MV ^2
It's a non linear relationship. So the faster you go, the more energy you will need, and this rises exponentially.
Just to reiterate Mike's point. Yes it is basic physics.
Energy is neither created nor destroyed, it is simply converted from one form into another.
Chemical Energy -> Kinetic Energy + Heat + A LOT OF NOISE + Frictional losses (of which heat is one, aerodynamic drag is another). The ratio at which the energy conversion is related to the different factors. You can get a light bike that's aerodynamically efficient and powerful but pay over the odds for it! Shaving weight will help a LOT because it's the other primary factor in the KE formula. Get a lighter battery, stop eating McDonalds, Leave your tool kit at home, tell your missus to get a bus! That will help a LOT. Simple things like swapping fairings over to carbon fibre or even fibreglass from ABS will help. Filling your tank half full because you'd effectively save 8kg if you have an 18 litre tank (rough estimate).. you'd have to fill up more often, but you'd carry less weight!
It's a balance game. And energy transfer is never 100%, you always get losses.
Big bikes have a lot of mass they need to haul about. But MPG is also dictated by how efficient you are at using the throttle and conserving momentum. If you're a smooth rider, then MPG can increase since you're not wasting it through braking and accelerating etc (let's just say they're lost as heat).
It can be simple (if you look at it from an energy perspective like Teflon Mike explained) yet complex (thermodynamics of engine combustion of the 4 stroke cycle - never going to be a 100% efficient engine in the carnot cycle).
So  to Teflon Mike's post because that's the simplest answer that explains it best.
You buy a fast bike to go fast and not care about how much you burn. If you know you've got a supersports, it's not mpg you're counting, it's SPG! Smiles Per Gallon... and that's how I'm going to stay. If i wanted mpg, I would have got myself an XR125 again and learn never to to take it to the redline and stay below 50mph...
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| Teflon-Mike |
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Teflon-Mike
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| Posted: 22:54 - 03 Mar 2011 Post subject: Re: **ladies & gents, lend me your ears...'Speed Vs Mpg' |
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| blinky wrote: |
do you guys know some good 'high mpg' big bikes that do not neccessarily compromise on speed?
but i'm trying to find a selection of good big bikes with high mpgs that can do, say 120mpg comfortably, with a fair bit more to give.
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The only things that actually MANAGE to get 120mpg are lightweight sub 125cc motorcycles. Of them, the only ones that can actually achieve it are the lower powered commuter bikes, with around 8 or 9bhp at MOST, and often THEN only under favourable conditions.... like nice easy roads with few junctions, or sharp bends where they can maintain a fairly constant speed.... UNDER 50mph....
THAT is the reality here.
Power = Force x Speed.
On one end we have the engine making Power, from pressure on a piston, so many revolutions per minute.
On the other end we have DRAG providing an opposing force from friction and wind resistance.
Power IS fuel consumption. The energy to make the force to oppose drag can ONLY come from burning fuel.
Now, you cant get out more than you put in.....
And withing the practical limitations of motorcycle design, many variables aren't that variable.
Weight is one thing... strickly doesn't make much difference rolling at constant speed, kenetic energy doesn't change unless speed or mass does, so unles you are accelerating or braking mass doesn't come into the equations.
And practically, the range of weights for a motorcycle, go from about 100Kg for a typical moped or very light weight light weight, up to around 300Kg for a REALLY heavy full dress tourer.
Simple reason, practically thats roughly as light as we can make a motorcycle sized structure, without really exotic materials and expense, or as much as a human leg can realistically balence up the other end, without mechanical assistance.
People weights range from perhaps 50 Kg to around 160Kg.... but you have to have at least a rider.
In the real world, this means that a heavier bike, and heavier rider, will take more power to accelerate to a certain speed, but coasting back down again, you'll get most of it back.. unless you use the brakes.... which is why cornering and junctions aren't good for ecconomy, nor is sitting in traffic burning fuel going no where.
So, if you want ecconomy, light weight, and as little braking or waiting as possible.
And low speeds.
Power is used to over come drag. Drag is mostly from wind resistance, which is pretty much dependent on the area of air the vehicle has to shove asside to get through it. Bigger frontal area, more drag, smaller frontal area less drag.
A motorcycle has, pretty much a person sized and shaped frontal area. Sat a bike you can reduce it a bit crouching over the bars rather than sitting up straight, and that can have a fairly big significance.
BUT, more significant is speed. Drag increases EXPONENTIALLY with speed, ie more than double teh drag for less than double the speed.
In FACT, 3bhp for a typical motorcycle, will just about over come the drag at 30mph, but to achieve 60mph you need about 9bhp. 27bhp, just about gets you to 90, so the power needed to overcome drag roughly tripples each time you add another 30mph...
Working this the other way then....
Travel at 30mph you are using 3bhp and returning about 150mpg
Travel at 60mph, you are using 9bhp, and return about 75mpg
Travel at 90mph, you are using 27bhp return about 35mpg
The exponential of fuel consumption is flatter than the exponential of power, mainly because power = the rate of energy transfer, or the SPEED you sup fuel. MPG is the actual DISTANCE you cover for that fuel.
So traveling at higher speed, you burn fuel an AWFUL lot faster, but you are also covering ground faster, so although your burning more fuel per mile, its multiplied by covering those miles more rapidly.
But, look at the numbers above. You are looking at the super economy end of the scale... if you want to achieve 120ish mpg figures, you will have to be looking at smaller lighter machines, with smaller, less powerful engines, and even THEN restricting the amount of the limited performance they have, keeping speeds down below 50mph, and enforcing conciouse driving for ecconomy techniques to maintain a good average without wasting energy in acceleration or braking or waiting.
You cant get a macghine to do it FOR you, YOU would have to conciousely ride the machine in a way to achieve those figures.
You sure as HECK aren't going to be able to miraculousely double the overall efficiency of a typical 60mpg machine, which is still no street scorcher looking for 'efficiency gains' though more advanced engine design, or better aerodynamics.....
The engine we have does NOT have the scope to have its efficiency improved THAT dramatically, and nor does streamlining.
The benefit of REALLY really good streamlining on a a motorcycle over NO streamlining is MAYBE 25% AT BEST, and that would be using full tear drop enclusures. You ACTUALLy would reduce drag MORE significantly from the pottentially 50% reduction in frontal area leaning flat over the handlebars.......
Combining the two, you COULD get close, and doing EVERYTHING, achieve something pretty impressive..... the speed record for a 50cc streamliner motorcycle, with the frontal area hardly any bigger than the top of a crash helmet, a full teardrop enclosure over the rider thirty foot long, is about 150mph.... but not from a 3bhp engine, but a super tuned one making arond 9, more like a typical 1oocc comuter......
And the thing about streamlining is that due to the exponential increase in drag as speed increases, so the significance of streamlining increases with speed.... unfortunately that means that its significance at LOW speeds starts to become negligable, and at the sort of sub 50mph speeds needed to maintain high mpg figures over 100mpg, its almost non existant....
If you want an idea of how far you COULD go for as little fuel as possible, then there are again world records for it, but I think that the best ever fuel eccomomy achieved from again, another barely crash helmet area streamliner, was around 300mpg.... doing an almost constant speed that a fit cyclist could beat around a flat oval cycle track sheltered from any wind!
Real world motorcycles, work in the realms of the figures I have mentioned.
If you want more performance you pay for it with exponentially reduced fuel ecconomy.
If you want fuel ecconomy, you pay for it with exponentially reduced performance.
The two are practically mutually exclusive, and yes,m you can talk about five or ten or fifteen percent here and there, in 'efficiency losses'.... firs off you'll never ELIMINATE efficiency loses, merely reduce them. And each small efficiency gain made, means that each successive small efficiency gain found is having a smaller effect on the overall system efficiency.
Eg: if you start loosing 50 out of 100 units of energy, your overall system efficiency is 50% right?
So, you find an area where you have an efficiency loss of 15%, or 15 units.... and make a 30% improvement.... but 30% of 15% is only 5%, you are now only loosing 45 units, and have a 55% overall efficiency.
Move onto another area where you HAD a 10% efficiency loss, you WERE loosing 10 units of energy, but NOW, having already made a 30% gain in one area, this ones now only loosing 9 units not 10, so making ANOTHER 30% improvement in efficiency you now only gain 3 units of energy back, and overall efficiency has increased to 58%
In other words two 30% effieciency 'gains' compounded, only ACTUALLy increase efficiency overall, by 8%.... and each successive efficiency gain, has similarly deminished effect.
The thermodynamnics of the internal combustion engine, particularly define the maximum efficiency that could EVER be achieved, becouse they are proportional to the temperature difference between the combusting fuel, and the ambiant temperature....... Ambient air is generally around 20Deg c, lets call it zero.... its not THAT significant in comparison to petrol burning at around 12, THOUSAND degrees C.......
When you have that much heat, its going to go wherever it can as quick as it can.... and you can only hope to capture a fraction of it.
So unless you can find some way to make a 'cold' combustion engine, your pretty much onto a looser there......
So, 120mpg, possible.. carefully ridden YBR125.... but get stuck in traffic or get a bit eager with the throttle and do more than 60, you wont get the mpg.
Following newbies about on my 750, I can sort of get bus beater mpg up in the 70's, mainly from not going very quick and riding really smooth...... but double the weight of a YBR, even riding smoothly you see how the extra weight and loss of efficiency lugging a big, and largely unused engine about.... can nearly HALVE whats achievable........
Sorry... but your expectations are just a LITTLE bit unattainable.....
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?' |
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| Kickstart |
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Kickstart
The Oracle
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| Posted: 11:00 - 04 Mar 2011 Post subject: |
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Hi
To go faster you do need more power, which means more energy so more fuel used. But most of the time an engine is bimbling along using a tiny amount of throttle, resulting in large pumping loses on a petrol engine (as the engine tries to draw in air through the tiny remaining throttle) which will cost you in efficiency.
Add in other quirks of particular engines (eg, the Clio we used to have that couldn't maintain 70mph in top gear on a motorway when any hills came along so needed to change down to 4th, yet could maintain 80 in top on the same motorway) and it is not that clear cut.
I suspect one of the larger factors in the difference in fuel consumption with speed is that in typical traffic the higher the speed you are aiming at the more often you will have to slow down when blocked and then accelerate.
All the best
Keith
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c-m
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Kickstart
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Gerrard
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| Scotsman37 |
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Scotsman37
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| Posted: 14:56 - 04 Mar 2011 Post subject: |
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A well maintained bike with properly inflated tyres and not being run at very high/low speeds will provide the best mpg which that particular bike can offer the rider!
Fuel efficiency v Performance which is a hard thing to do these days to find when the cost of fuel keeps rising to make it worth while on your income to keep continuing with a particular bike that truly gobbles up fuel at very low/high speeds!
Motorcycle Fuel Consumption & Real World Performance Guide
https://www.motorcyclefuelconsumption.com/
Total Motorcycle Fuel Economy Guide
https://www.totalmotorcycle.com/MotorcycleFuelEconomyGuide/index.htm[/url] |
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