This is what kept me up last night.

[chris-red]

Like a lot of people when I really need to sleep my brain thinks about EVERYTHING.

This puzzled me.

The AF ratio for an engine is ~14-1.
Engines pull fuel in via a vacuum.
The throttle controls how much fuel is allowed in the engine.

Surely with every charge the engine takes in it has to pull the full 'cc' of air/fuel. Does this mean engines run ridiculously lean at part throttle openings?
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Skudd:- Perhaps she just thinks you are a window licker and is being nice just in case she becomes another Jill Dando.
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[please close my account]

Throttle openings dont affect the air-fuel ratio, just the overall quantity of the mixture that goes into the engine.

Carbs/FI ensure that the correct amount of fuel is mixed into the air, for the quantity of air that an engine pulls. Your throttle controls the quantity of air that you pull.

So at half throttle, you're only pulling half of the total amount of air that your engine can take, but you're also pulling in half of the total amount of fuel that would be injected. Roughly speaking that is. AF mixture is not perfect, and theres all sorts of cool things that can be done depending on engine temperature, rpm, speed of throttle opening/closing and if you're trying to get the engine to spin up quicker or pass emissions..

But I may be wrong in all/any of this

[chris-red]

[Aff]

[chris-red]

[weasley]

[MCN]

It is not a perfect science but the attempt is to get it perfect for all throttle settings.
The main issue is how the airflow changes with differing throttle openings as the manifold is of a fixed aperture.
Some manufacturers have variable manifolds to permit more flexibility at different engine speeds.

Engine Management uses spark and air/fuel ratios to produce the most efficient map.
The maps are stored in the memory of ECMs.

Carburettor fueling is not as controllable as FI. Which is why multipoint FI engine can put more power out.

https://www.brighthubengineering.com/machine-design/15235-the-stoichiometric-air-fuel-ratio/
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[Confusion]

[chris-red]

[Aff]

Yes, petrol engines are most efficient at full throttle.
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[chris-red]

[The Shaggy D.A.]

I seem to remember reading that 350cc is the point at which volumetric efficiency starts to decrease for a normally aspirated cylinder; after that if you want to fill the cylinder with fuel/air mix to 100% (or more) you need a turbo or supercharger.
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[bikenut]

14.7 stociometric ( spelling may be wrong there )

atmospheric pressure fills cyl , pushed in rather than pulled...also some port shaping etc and ramming effect at higher revs......

manifold depression??

carb butterfly/slide controls air intake, fueling system controls amount of petrol by weight ( mass = wv so has a bearing )...

acceleration pump as on cg125 for instance, for flat spot/momentary leaness avoiding, raw fuel pumped in via jet etc, works on quick throttle openings, progression holes or jets work for slow openings......

power valve/economy diaphragm works for power demand/cruising light throttle, vaccum operated.....

emulsion tube/air holes.......

altitude compensation, mainly for aircraft etc etc etc .....
FI computer controlled systems are wonderful as all sort of parameters can be softwared in, all sorts of sensors etc etc etc ...when they work right that is! usually poor connection problems.... :karma:
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nuts about bikes

[weasley]

[talkToTheHat]

On a carbed bike throttle position and particularly rapid changes in throttle position casue all sorts of mixture problems. Shut the throttle rapidly and the mixture goes rich. Open the throttle rapidly and you lean out. Unless the carbs have an accelerator pump.

Remember a gas will expand to fill the available volume.

Technically air is pushed in to a the low pressure area of the induction stroke piston by atmospheric pressure. There is no suck, it's all atmospheric push.

The loss from part throttle is called pumping loss, and is why diesel and stratified charge capable direct injection petrol engines are much more efficient at part load, as a diesel has no throttle, controlling engine speed by mixture alone and the latter can operate in a similar manner ti a diesel under light loads.

Efficiency and fuel economy are not always the same thing. At full throttle and high rpm your engine is best at turning chemical energy into kinetic energy. Riding like you mean it means hard acceleration and hard breaking which use more fuel, and higher speeds which means more aerodynamic drag.

As such a small engine running at full throttle is likely more efficient than a large engine producing the same power at part throttle, so for the same task, a smaller engine is more economical.
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Bandit. does. everything.

[Pol Anorl]

as aff said they do run their best at full throttle, and less throttle the pressure will be much less.
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[Teflon-Mike]

Yes, at part throttle, the cylinder only sucks a 'part-fill'.
In fact, few engines EVER manage to suck in their full 'measured displacement' of charge.
Closest they get to it, is not at max-power, but at 'peak-torque' engine revs, WOT.

Now, lets talk wind-resistance; when you are riding, at walking pace, winds nice and soft, you don't even notice it; get up to about 15mph and you start to feel it as a bit of a breeze; 30mph and you get a bit of beffet, its starting to move you around; 60mph and you are hanging on against it; faster you go, stronger it gets; and its an exponential 'thing'; only takes about 3bhp to go 30mph, but takes near 9bhp to go 60, takes near 27bhp to get near 90... double the power, you don't double the speed, right?

Now, work that back to front and inside out... In an engine, instead of trying to push a 'lump' through still air... you are trying to drag air through holes in a lump... this wind resistance thing is going to work just the same.

OK.. Hyabusa.. 172mph? As bikes started getting up over 160, so the significance of aerodynamics became that much more critical to going any faster; that is about the speed that this exponential 'thing' is becoming a bit abrupt, and you are hitting the rule of diminishing returns, where to go ever so slightly faster, you need to find inordinately more power to do so.

250cc 'measured displacement' cylinder... lets give it a reasonably sized carb, say 25mm, that's aprox 5cm2 in cross section, so to 'fill' this 250cc engine, on one cycle, you need to pull a column of 'charge' 250cc's in volume, through a hole 5cm in cross section; so it would be 250cc / 5cm2 = 50cm long.. half a meter.... Yeah?

And you need to pull one half meter long column of air through the carb every cycle..... it's a 4-stroke engine, so it makes one cycle in two crank revs... lets say it's turning 6,000rpm, that's 100 revs per second, BUT only 50 'cycles'... so you have 1/50th of a second to get half a meter of air through this orifice, right? So, 0.5m in 1/50s means an air-speed of 25meters per second... that's about 55mph, it's not exactly 'slow' is it?

But, its worse than that; because you DON'T have, 1/50th of a second to get that column of charge into the engine; that's how long the engine takes to make a full cycle, all four 'strokes'.. but it's only 'sucking' on one of them, the induction stroke! so you have 1/4 of 1/50s to shift that half meter of air... so, you to get it all in, you are going to have to move it four times as quick, aren't you? 100M/s or 220mph.... oooh! That's JUST a little bit faster than a Hyabusa, struggling to shift air past its dolphin-beak, isn't it?

Now, you could ask why we just don't fit a bigger carb; double the choke area, halve the air-speed... which is reasonable enough, except that the carb is probably not the 'pinch-point'; that will usually be at the inlet valves, where the diameter of the cylinder limits just how big a hole you can make to let air through, and how much metal by way of valves you have choking it; AND if you want to get down to the nitty-gritty, you also have the matter of the cam timing and profile; valve doesn't open instantly, nor close instantly; so that column of air starts at a dead stop, had to be accelerated to some peak velocity, then slowed and stopped, to give an 'average' air-speed, in the order of a maxed out Hyabusa..

So the point IS, that even at relatively 'modest' crank-speeds, 6K is not particularly 'high' even for a push-rod CG engine... the air-speeds you need to attain to get a 'full-fill' of charge into the cylinder, are well up there, working against the exponential of wind-resistance, to which conclusion, getting a 'full-fill' even at the usually lower rpm of 'peak-torque' is unlikely.

Now, what we are talking about is a thing called "Trapping Efficiency", in short, the % of the 'measured' cylinder displacement, you actually manage to get, and as importantly, keep in the cylinder, on the compression stroke.

Going back to that cam timing a second; induction stroke starts at the end of the exhaust stroke; if the engine hasn't purged all the exhaust gas from the last cycle from the cylinder, when the piston starts going down to suck in 'new' charge, some of the space it makes is likely to be filled with old exhaust gassed, which, still hot, possibly expand, filling the cylinder with smoke, faster than new charge can get in... will also contaminate the new charge and 'damp' it's 'burn' when the time comes, so its not 'good', but for now, you will always have some smoke left in the pot, taking up space new charge cant fill. Next up; if you don't open the inlet valve before the exhaust stroke ends, then you wont have such a big hole to suck charge through when it can, so you are back to that 'problem' of wind-resistance strangling flow; on the other hand; open the inlet 'early, before the exhaust stroke has ended.. exhaust smoke is as likely top be pushed 'out' that hole as the other, and the first bit of your column on incoming gas, when the piston starts to suck will be smoke, not charge.... and we have 'over-lap'.. closing the exhaust valve 'late', so as to give most time and valve area to get the smoke 'out', means more time both valves are open at the same time, and some new charge likely to bugger off down the exhaust pipe, without doing anything..... And it all starts to get complicated, and perverse as effects compliment or counteract each other, which is where the art of 'tuning' lies.....

Anyway; back to this wind resistance thing; IF we presume we will NEVER get a full 'measured' displacement 'fill' of charge... then the actual measured displacement becomes something of an irrelevance... what matters is the volume of charge we can turn into motion...

And, here lies the big danger talking 'efficiency', because its a %, and when you start talking 'volumetric' efficiency, you are linking that back to an irrelevant dimension.....

If you want, 9bhp to be able to go 60mph, does it matter if you get that from a 50cc engine turning 18,000rpm, or a 125cc engine turning 9,000rpm, or a 750cc engine turning 1,500rpm? To make 9bhp, they are all burning the same amount of 'charge' in the same time, to do the same job? Only you'd probably argue that the 50cc engine has an infinitely greater 'volumetric-efficiency'... but only because you are dividing the same result by a much smaller arbitrary reference parameter!

And same goes for so many other elements of 'engine efficiency'... looking at small 'slices' of the overall system, and taking arbitrary comparitors to calculate your %... its statistics... the art of lying with numbers! You can make them say almost anything!

BUT, it's to hint at the possible 'error' behind two common suggestions; first engines are 'more' efficient at Wide-Open-Throttle, and that they are 'most' efficient at peak torque revs... IF....

The peak-torque figure of the power-trace, shows where an engine is making maximum cylinder pressure; Torque is force x leverage; leverage on the crank doesn't usually chance, so peak torque = peak piston force; likewise the piston area doesn't normally change, and pressure is force over area, SO peak torque=peak cylinder pressure.And, you will normally get most cylinder pressure, when you have the most 'fresh' charge in the pot going 'bang'.... So it IMPLIES that is when you have peak 'volumetric efficiency', and the best % of cylinder fill.... and you WILL only get THAT at 'Wide-Open-Throttle' when there is least impediment to getting that 'fill'...

BUT, it is only true under THOSE specific circumstances; peak-torque revs AND Wide-Open-Throttle.. to give 'max volumetric efficiency'

Which, I have already established is likely something of an irrelevance.

Overall system efficiency? Of an internal combustion engine is CRAP.. end of!

If you are lucky, you might get around 25% of the 'heat' energy released from the burning charge, turned into kinetic energy, motion, at the piston top. How much of that gets anywhere useful after that, is any-ones guess, but, turning the cam, making valves open, pumping oil, driving a generator to make sparks, all that, before you start looking at bearings and gears and stuff down through the transmission, likely 'rob' 5-10% of the K.E. you get at the piston; transmission losses on a bike, typically another 10-20%.. BUT how much 'energy' did you put in to start with? Efficiency is "Gross-Input/Net-Output"... if we took the chemists laboratory derived 'calorific' value of our fuel, in an engine we'd be doing well, in the far from perfect conditions of combustion in an engine, rather than a laboratory, to get more than about 2/3 of the specified Calorific Value of the fuel, turned into heat... so we get 80& of what comes out the crank, which is getting 90% of what the pistons caught, which was only 25% of what was released in the combustion chamber, that was only 70% of what we stuck in by way of 'fuel'...

AND that is when we have this 'ere engine working at its 'best' efficiency we get... 12.5% 'system efficiency'

Of which, the influence of how much cylinder fill we get, and how much we might loose, not running WOT, is going to be a tiny TINY % difference of a pretty negligible % in the bigger scheme of things...

So... bottom line... Gross-in / Net-Out.....

Well, I put petrol in.. physically, BUT that costs me money, and I really don't GAS how many liters of the stuff I get, or how many calories is in them... nor when I top up the Rangie that burn LPG rather than petrol, OR my mother, when I have gone to fill her French-Barge, reminding me it's a bludi AGA-engine, and 'how much' more 'efficient' diesel is...

Bottom Line - Quids-In/Miles Out... THAT is the overall efficiency...

But even then, MPG is taking a snap-shot... Most vehicles I have owned, I have managed to get the MPG to vary by as much as twice the 'book' spec, to as little as half it; VF1000, book said 30mpg.. I could get it up to 60, or see it down to 15; Rangie; book says 15mpg.. I have had it up nudging 30... I have NOT 'quite' got it down to single figures.. my wallet kept leaping out of my pocket and trying to strangle me... but the ex-missus DID!

Which, makes a BIG point, that the biggest variable in the 'total-efficiency' equation, is the driver/rider... NOT the engine, not the engine size, nor the engine revs, number of cylinders, or valves, type of carburation system, or any other elements of it's architecture. Even more so, when you look further at the £'s-IN, and fuel is only one cost; there's the purchase cost, the maintenance costs, tax and insurance etc, where variability of insurance is a huge factor in the sum and dependent on an individual's circumstances.

Which is a long way from "does an engine fill its cylinder each 'bang'.. to which the short answer is "No - it hardly ever fills its pot"

But, VERY easy to get lost in forest, unable to see the wood for the trees when you start talking 'efficiency', and looking at elements of it, only in certain areas.

Engines, in real use, rarely run at either peak-torque or peak power revs for sustained periods, nor do they run, for sustained periods at WOT, they are for more often in a 'transient' condition changing engine revs, changing load, when a lot of the neater 'theory' starts getting banged about.

Your original 'idea' about the 14:1 stoic ratio, for example.... during acceleration, carburation needs to 'richen' the charge above stoic; when decelerating, it can 'weaken' it... So, if you have an engine accelerating through low RPM, might be able to more readily fill it's pots, BUT at WOT, you could be over enriching the charge and loosing overall system efficiency for it, compared to dropping a gear, getting the revs up, which would intrinsically have a negative impact on system efficiency, making more motion to do the same work, BUT, not having the throttle so Wide-Open, with less enrichement, circles of optimisation co-inside to give you a greater 'overall' efficiency in THAT situation.

Which is what it all comes down to; circles of optimisation, and actual specific circumstances in a constantly changing 'dynamic' where the neat 'steady-state' theory can start to take a pretty big knock..

And THIS is what your carburetors do.... try and keep tabs on whats going on, try and control whats going on, and deliver a mixture strength that's as 'optimal' as it can be, for the ACTUAL specific conditions you are making the engine run in... and it's pretty 'fuzzy'!

And I haven't mentioned variable compression-ratio engine's or two-strokes, let alone 'hook-curves'!! Another day, perhaps
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[Pol Anorl]

^what he said probably.. i didnt read it but that.
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[orac]

All that and still no mention of the venturi effect. Kind of important fit the function of carbs
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[Teflon-Mike]

[Vincent]

[MCN]

[stevo as b4]

Teff, Did you at some point work on engine development and configuring tunes and mapping etc?

One massively way off topic point about bike engines being less economical than they could be is, that I do think the vast majority are set up to run on the rich side to eek out the last few bhp and have the best throttle response. I'm talking in relation to car engines, as if we say 70% of bikes on the road are performance/sports bike orientated, then performance is much higher up the agenda, than economy or emissions as bikes have had it so easy emissions wise for decades.

That's one reason why I believe that 2D ignition advance curves even on quite a few EFI bikes were used for so long.

I also imagine that with car engine tunes that economy after meeting emissions is always the top line on the list of priorities. I can't think of many factory car's that were ever set up to run for the most performance possible at the expense of the other factors.

You could say that bike engine design and tuning in the late 80's-early 2000's was easy to do with less constraints or complication than car engine development?

[Teflon-Mike]

[talkToTheHat]

With sufficient valve overlap and carefully designed exhaust and intake, you can get more than the cylinder volume at atomospheric pressure drawn in each induction stroke becasue air has momentum.

As the efficency of a petrol engine is tied to pumping losses and compression ratio, an induction charge at already elevated pressure mitigates pumping losses, increases effective compression ratio and imcreases overall efficiency.

Also consider a two stroke exploiting helmholtz resonances or ram air systems.

Now consider forced induction systems, a supercharger uses kinetic energy from the crankshaft to increase charge pressure, and in some circumstances is more efficent if the combustion efficency gain is more than the load on the crank.

A turbocharger uses the energy otherwise wasted from the exhaut to drive the compressor, thus does not make the same trade off, but works well under a narrower range of conditions, often introducing a slight delay between torque demand and torque supply unless there is a secondary turbine drive method such as those found in current f1 cars.

Oh and drag isn't expomential in most cases, it's polynomial. For car/bike/person size objects drag is approximately proportional to the square of the relative velocity. Thus the power to overcome drag is approximately proportiomal to the cube of the velocity. Tef's numbers are good, so I can only conclude he has googled hard but not quite understood the long essays he is producing and is hoping to disguise the lack of understanding with verbosity.

I suggest a brief reading of undergrad heat and mass transfer, and fluid dynamics books. There will be plenty of answers in there if you can keep up with the maths, otherwise, i suggest a copy of Mathematics for Engineers which is either a useful tool, insomnia aid ir doorstop depending on aptitude.
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