Always wondered...Fuelling question

[The Artist]

Say on a 4t carb'd engine where the carb has a slide style throttle.

if you are in top gear at 4000rpm and with the throttle quite a bit open, you are currently doing 30mph.

Or

In a lower gear at higher rpms, still 30mph but the throttle is not as open as before.

Which uses more fuel?

Thanks

[carlperkins00...]

i dont know the answer to this question....

but i try to think of riding a motorbike like riding a mountain bike.

(imagine your legs are an engine and energy is fuel.)

if your going up hill you want to be in a lower gear as you wont want to be pushing your legs too hard. Therefore less energy needed.

If your on a level or downhill road you want to be in a high gear because if you were in a low gear your legs would be going like the clappers and you would still be using alot of energy.

that makes sense in my head, dont know if it will work for anyone else

[strag]

The first one.
____________________
carbon bling

[The Artist]

[tone]

A higher gear means less revs which equals better fuel consumption, a lower gear means more revs for the same speed & more fuel consumption...........simples
____________________
Of all the things ive lost , i miss my mind the most

[user]

I guess the 'ideal' combination of throttle position / rpm varies a far bit depending on engine, but I would guess most would be more economical in a higher gear. At a constant 30 the throttle will hardly be open whatever gear you use. The rpm could vary a lot, depending on the gearing of the bike (and if it's running 4000rpm in top, I guess it's pretty low).

[strag]

I misread that and got it the wrong way round sorry

My thinking is that if you are in 6th at a low speed, the engine will be 'labouring'. In 3rd the engine will be smoother and running more efficiently.
____________________
carbon bling

[salty21]

but your not taking into account the wide throttle opening at 4000 rpm as opposed to the small throttle opening but higher rpm. i would have thought the less open the throttle is(regardless of rpm) the less fuel is used .

I've often wondered this too so im glad you asked, just need keith to come along now and clear it up
____________________
04 NSR 125(sold) ---- 03 CBR 600rr(sold) ----90 pan euro ST1100 '' ----02 CG 125
94 CB400 Super Four ---- 2000 VTR SP1 (sold) ---- 08 ninja p8f(sold, meh) ----05 CBR600rr

[user]

After thinking about it more, I'm less certain than I was a minute ago.

To produce enough power to maintain a constant 30, the engine theotetically burns the same amount of fuel, either in small frequent amounts (high rpm) or large infrequent amounts (low). But the engine burns fuel more efficiently at higher rpm (therefore needing less), so a low gear might be more efficient.

[Pete.]

[tone]

Right

On a cv carb the throttle slide is opened by the vacum caused by the piston going up & down (EG the fuel is sucked in) therfore the amount of fuel used is directly connected to the revs (one rev equals one small dose of atomised fuel/air mix) the cable only controlls the air flow via the butterflies & is not connected to the slide this is why you will sometimes get a small lag if you whack the throttle hard open, the vacume & the slide need to catch up

Presuming the carb is correctly set the slide will not be open & lift the needle more than is needed to supply the correct amount of fuel/ air mix

Big throttle openings mean big revs (more sucks & more fuel for a given speed) smaller throttle openings when you are in a higher gear mean less revs ( sucks ) & less fuel for the same speed

On most 4 cylinder 4 stroke bikes with cv carbs you will only just be coming off the pilot jet at 3500/4000 revs & the engiine will labour in a too high gear this is because the vacume is not yet strong enough to properly atomise the fuel & in this situation you will be using a little more fuel
____________________
Of all the things ive lost , i miss my mind the most

[T0MMY]

I always work on the principle that to save fuel you should be in as high a gear as you can without getting to the point where the engine is labouring. Well actually, the best way to save fuel is to plan well ahead but in terms of engine revs alone...

Also, the bicycle analogy is good to help understand why labouring your engine is so damaging. Imagine you are on a bicycle, get to a steep hill and are in top gear. Think how much force you are pushing down with to turn the cranks, that's an awful lot of force trying to break your cranks and snap your chain etc. On a motorbike in too high a gear the piston is trying to withstand each explosion in the cylinder with a great deal of force, putting lots of pressure on crank and bearings etc as well as the transmission. This is of course why when your clutch is on it's way out it slips in high gears and not in lower gears.
____________________
ZX6R + CBR600RR

[tutton]

I thought clutch slip was also lower gears, like if you nail it in a low gear while clutch is on way out itll slip all the way up? or is that when the clutch isnt good enough spec for the power your running
____________________
Past: '05 Peugeot XPS '99 Aprillia RS125 '94 FZR600R
Current: '02 ZX636 A1P '51 CCM R30

[The Artist]

[calyx]

In high gear the engine produces more HP to keep the same speed.
Yes it does produce less tork but it is the HP decides fuel consumption.

If you are a good rider, then you know how to convert horses to torks efficiently.
____________________
Boris - London

[iooi]

Lets put this in real terms that i have found from my fuel consumption.

Stick to max of 4K revs and i can get 70 MPG
Stick to max of 5K i get 67 MPG
Stick to max of 6K i get 65 MPG
Let it go to red line when i want ( not often ) around 60-62MPG
____________________
Just because my bike was A DIVVY, does not mean i am......

[T0MMY]

[calyx]

[T0MMY]

Well, if you're idling in neutral at a standstill then fuel consumption in terms of MPG will be infinite. If you mean in terms fuel flow rate then I can see how that's possible too, need to burn fuel to keep the engine ticking over, maybe more than if you're on a VERY light throttle cruising in high gear and conserving your momentum.
____________________
ZX6R + CBR600RR

[Kickstart]

Hi

Suspect there are no hard and fast rules.

In theory an engine is at its most efficient at full throttle (min pumping losses) at the point of peak torque, but in practice any modern engine will run a richer mixture at full throttle thinking that you want more performance which will massively outweigh this.

I do seem to get fairly decent fuel consumption and I also will quite often be sat in a lower gear waiting for opportunities to get past an obsticle.

In top gear cruising I suspect that massive increase in drag with speed (and so more power required to overcome it) renders any comparison of fuel consumption at different cruising rpm in top gear to not really be important to the original question.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Pete.]

[daemonoid]

[Pete.]

[Kickstart]

Hi

Whether it is more efficient at high or low rpm probably depends on the design of the engine, with stroke, con rod length and combustion chamber design being important.

While it might well be more efficient at high rpm, if those higher rpm mean less throttle then you have more resistance to incoming air and so higher pumping losses.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[The Artist]

I was thinking in terms of the carb slide itself being more open or not, needle circumference changes the more you open the throttle. I was wondering what relation this has to fuel consumption at different positions.

Wide open but low rpm etc so I assume the difference is negligible if there was no friction etc but as there is then lower rpm, higher gear would use less fuel for the same speed?