Fuel Efficient Acceleration

[Jack]

An elderly engineer mate of mine claims the most fuel-efficient way to
get a carburetted car to a certain speed is to accelerate at full
throttle to that speed.

At full throttle, the butterfly valve is at maximum air flow and the
jets are providing the most optimum fuel-air mixture. At any setting
less than maximum throttle, the jets are providing an overly-rich
mixture, albeit slight, resulting in a less fuel-efficient acceleration
that also takes longer to reach the speed; a double whammy.

I can sort of see what he says, but it goes against my intuition of
'gently-gently wins the monkey'. What do you think?

[Dene Oehme]

On Tue, 11 Jan 2005 19:25:26 +1100, Jack <noone@here.com> wrote:

>An elderly engineer mate of mine claims the most fuel-efficient way to
>get a carburetted car to a certain speed is to accelerate at full
>throttle to that speed.
>
>At full throttle, the butterfly valve is at maximum air flow and the
>jets are providing the most optimum fuel-air mixture. At any setting
>less than maximum throttle, the jets are providing an overly-rich
>mixture, albeit slight, resulting in a less fuel-efficient acceleration
>that also takes longer to reach the speed; a double whammy.

Your engineer mate has no idea how a carby (other than one on a model
aeroplane engine) works.

[The Interceptor]

Jack <noone@here.com> wrote in message
news:q4fcb2-e3g.ln1@surprise.nick.net...
> An elderly engineer mate of mine claims the most fuel-efficient way to
> get a carburetted car to a certain speed is to accelerate at full
> throttle to that speed.
>
> At full throttle, the butterfly valve is at maximum air flow and the
> jets are providing the most optimum fuel-air mixture. At any setting
> less than maximum throttle, the jets are providing an overly-rich
> mixture, albeit slight, resulting in a less fuel-efficient acceleration
> that also takes longer to reach the speed; a double whammy.
>
> I can sort of see what he says, but it goes against my intuition of
> 'gently-gently wins the monkey'. What do you think?

With the butterfly/s open fully, the pressure losses across the carby are at
their minimum and hence volumetric efficiency can be at its maximum.
However, volumetric efficiency is not necessarily related to thermal
efficiency, so I don't really know why I mentioned that.

I can't see why a carby can't be jetted lean (or rich) at any point in the
rev range. I know my Holley is a bit lean off the bottom (not that it's a
good thing).

Brett

[a9x5l]

On Tue, 11 Jan 2005 19:25:26 +1100, Jack wrote:

> An elderly engineer mate of mine claims the most fuel-efficient way to
> get a carburetted car to a certain speed is to accelerate at full
> throttle to that speed.
>
> At full throttle, the butterfly valve is at maximum air flow and the
> jets are providing the most optimum fuel-air mixture. At any setting
> less than maximum throttle, the jets are providing an overly-rich
> mixture, albeit slight, resulting in a less fuel-efficient acceleration
> that also takes longer to reach the speed; a double whammy.
>
> I can sort of see what he says, but it goes against my intuition of
> 'gently-gently wins the monkey'. What do you think?


Probably depends on the car, when driving a Volkswagen fastback my brother
used to own, the pedal had to be floored just to keep up with the traffic,
whereas in a 400 c.i HQ I had at the time, it would have cost me a new set
of rear tyres for every take off if I tried the same method of
acceleration:-)

--
a9x5l

[Graham W]

Jack wrote:

> An elderly engineer mate of mine claims the most fuel-efficient way to
> get a carburetted car to a certain speed is to accelerate at full
> throttle to that speed.
>
> At full throttle, the butterfly valve is at maximum air flow and the
> jets are providing the most optimum fuel-air mixture. At any setting
> less than maximum throttle, the jets are providing an overly-rich
> mixture, albeit slight, resulting in a less fuel-efficient acceleration
> that also takes longer to reach the speed; a double whammy.
>
> I can sort of see what he says, but it goes against my intuition of
> 'gently-gently wins the monkey'. What do you think?

He's on the right track, but he's got the details all wrong.

The mixture for best economy is slightly leaner than the mixture for
maximum power, given the same airflow.

So a carby is set up to provide the most economical mixture at all
throttle openings except flat out, since if you want a bit more power,
you can push the pedal slightly more and get it with better economy.

So clearly the best economy isn't going to happen at full throttle.

However at anything less than full throttle, the engine is working to
draw it's air in past a restriction - thats why it's called a throttle.

So in principle you get the best possible specific fuel consumption
(unit fuel per unit power) at slightly less than full throttle.

My guess is it's the latter effect that whoever he got the concept from
was thinking of.

[Graham W]

> Jack wrote:
>>An elderly engineer mate of mine claims the most fuel-efficient way to
>>get a carburetted car to a certain speed is to accelerate at full
>>throttle to that speed.
>>
>>At full throttle, the butterfly valve is at maximum air flow and the
>>jets are providing the most optimum fuel-air mixture. At any setting
>>less than maximum throttle, the jets are providing an overly-rich
>>mixture, albeit slight, resulting in a less fuel-efficient acceleration
>>that also takes longer to reach the speed; a double whammy.

Dene Oehme wrote:
> Your engineer mate has no idea how a carby (other than one on a model
> aeroplane engine) works.

Thats a bit harsh. It sounds more like he once knew and he's forgotten.
He's vaguely near right, but all mixed up.

[Fred Ferd]

"Jack" <noone@here.com> wrote in message
news:q4fcb2-e3g.ln1@surprise.nick.net...
> An elderly engineer mate of mine claims the most fuel-efficient way to
> get a carburetted car to a certain speed is to accelerate at full
> throttle to that speed.

bullshit.


The simple fact is that when you put the engine rpm's up to higher rpm's
they spew unburnt petrol out the back.

And if you put lots of fuel charge in at low rpm's spew unburnt
hydrocarbons out the back and ping and cause valve damage.

> At full throttle, the butterfly valve is at maximum air flow and the
> jets are providing the most optimum fuel-air mixture.

Thats totally bogus. the main way to tune the mixture is to have it economic
up to half way and have it set for power after half way.

The mixture is adjusted however the carby wants to adjust it !!! thats what
a carbo is for, right ?
to give you good economy ? to give you good power and torque ?

To save the valves and avoid overheating ?

Well its got a lot of reasons to be designed in such a a way as to vary
mixture based on air flow..
which is all compromise of course , and that means there is no one way to
set it.



And mixture is very irrelevant here. The thing is that at high rpm's the
fuel doesnt get time to burn - no engine management here to get the timing
advanced enough to get better economy from putting the pedal to the metal.
Doesnt really matter what the subtle changes in mixture are - the thing is
that if you floor it you dont burn all your fuel.

If the car is manual and you change gears before getting to high rpm,then
you do have better burning of the fuel. but then you have to be quite good
at racing gear changes and risk damanging things if you get it wrong.


>
> I can sort of see what he says, but it goes against my intuition of
> 'gently-gently wins the monkey'. What do you think?


You intuition has been programmed by 100 years of car driver experience
being told to you.

If you dont know much about carbie's then you would be correct in not
putting a lot of trust in this intuition.

it just happens to be correct in this case, because

[Michael C]

"Fred Ferd" <fred@ferd.com> wrote in message
news:41e3f1fa@news.comindico.com.au...
> The simple fact is that when you put the engine rpm's up to higher rpm's
> they spew unburnt petrol out the back.

The OP never said anything about high RPM. I've heard from an engineer that
an engine is at it's most efficient at full throttle but low rpm.

Michael

[Forg]

Michael C wrote:
....
> The OP never said anything about high RPM.
> I've heard from an engineer that an engine
> is at it's most efficient at full throttle
> but low rpm.
....

I suspect, therefore, that a CVT would be required to make the OP's
engineer-mate close to accurate

I do think it most likely it would've been a purely theoretical musing
though, probably not something to take seriously in the real world. Or
maybe he was a Ford Engineer?


--
---
Forg! -DUH#6-

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[Michael C]

"Forg" <Forg@zip.com.au> wrote in message
news:41e448a3$0$27133$afc38c87@news.optusnet.com.a u...
> I suspect, therefore, that a CVT would be required to make the OP's
> engineer-mate close to accurate

Manual trans would do.

> I do think it most likely it would've been a purely theoretical musing
> though, probably not something to take seriously in the real world. Or
> maybe he was a Ford Engineer?

I think this is the basis behind V8s that shut down cylinders and possibly
why 4 cylinder engines use less fuel. It's more fuel efficient to have less
cylinders working harder to put out the same amount of power (to a point of
course).

Michael

[Forg]

Michael C wrote:
....
>> I suspect, therefore, that a CVT would be
>> required to make the OP's engineer-mate
... close to accurate
....
> Manual trans would do.
....

Doesn't changing revs cause all sorts of efficiency problems?

I thought that's why trains were generally diesel-electric (or, if
French, turbines running electric motors); no throttle, keep the motor
at it's most efficient revs at all times, and vary speed by varying the
supply to the electric motors. The extra efficiency in running the
engine at a constant speed maks up for the losses in converting kinetic
energy to electricity & back again.
???

....
> I think this is the basis behind V8s that
> shut down cylinders and possibly why 4
> cylinder engines use less fuel. It's more
> fuel efficient to have less cylinders
> working harder to put out the same amount
> of power (to a point of course).
....

Hmmm ... again, while you could be right theoretically, the engines that
shut cylinders down only do it under pretty-much zero-load conditions;
it's really only for highway cruising & coasting. That's not to say
you're wrong about the efficiency, however, it might just be that the
technology isn't up to coping with 100kW of forces/heat/gasflow/etc in
one half of the engine & squat in the other half.

4cyl engines don't really use less fuel, in these days of smart
electronic control it's more to do with how much power the engine is
making & how much fuel it has to waste under zero-load conditions. A
heavier car will force an engine to make more power & burn more fuel all
the time when driven normally; a larger engine will usually increase the
weight of the car; and when idling/coasting a larger engine must consume
more fuel than a small one. To give an example, in around-town use
there'd be almost no difference in around-town fuel consumption between
a Commodore with a 6.0 V8 up front, and a Commodore with a 3.0 inline 4
(assuming the 4 is made from half of the 8, so head design issues don't
come into it); assuming they're both driven in much the same "normal"
way (ie. not being used as getaway cars). Problem is that with an LS2
in your Commodore you're having too much fun ... oops, leadfoot = fuel
go bye-byes. :)


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[Michael C]

"Forg" <Forg@zip.com.au> wrote in message
news:41e44f3a$0$2590$afc38c87@news.optusnet.com.au ...
> Doesn't changing revs cause all sorts of efficiency problems?

I imagine it would and CVT would be better but a manual still allows you to
keep the throttle close to wide open with the rpm low.

> I thought that's why trains were generally diesel-electric (or, if French,
> turbines running electric motors); no throttle, keep the motor at it's
> most efficient revs at all times, and vary speed by varying the supply to
> the electric motors. The extra efficiency in running the engine at a
> constant speed maks up for the losses in converting kinetic energy to
> electricity & back again.
> ???

Sounds right. It would be interesting to see how they run their engines for
peak efficiency.

> Hmmm ... again, while you could be right theoretically, the engines that
> shut cylinders down only do it under pretty-much zero-load conditions;
> it's really only for highway cruising & coasting. That's not to say
> you're wrong about the efficiency, however, it might just be that the
> technology isn't up to coping with 100kW of forces/heat/gasflow/etc in one
> half of the engine & squat in the other half.

It could also be for marketing reasons. Who would want a V8 that sounds like
it is struggling when it's putting out 100kw and taking it easy at 120kw.

> To give an example, in around-town use there'd be almost no difference in
> around-town fuel consumption between a Commodore with a 6.0 V8 up front,
> and a Commodore with a 3.0 inline 4 (assuming the 4 is made from half of
> the 8, so head design issues don't come into it); assuming they're both
> driven in much the same "normal" way (ie. not being used as getaway cars).
> Problem is that with an LS2 in your Commodore you're having too much fun
> ... oops, leadfoot = fuel go bye-byes. :)

I presume that is because most of the time is spent at very light loads and
not showing the true efficiency of the engine. A test would really need to
be done to determine the efficiency of both engines at different throttle
openings and different loads. Actually you wouldn't need to test both
engines, just test the one and divide the figures by 2 to simulate the 4
cylinder.

Greater efficiency at low rpm wide open throttle does make sense,
compressing the fuel to the maximum possible pressure and giving it as long
as possible time to transfer it's energy without collapsing. I guess some
tests would need to be done though.

Cheers,
Michael

[David]

> Greater efficiency at low rpm wide open throttle does make sense,
> compressing the fuel to the maximum possible pressure and giving it as long
> as possible time to transfer it's energy without collapsing. I guess some
> tests would need to be done though.
>
> Cheers,
> Michael
>

This article is very interesting, the tests have been done in the aviation
environment although there is considerable debate about the various techniques.
The bits about efficiency, pressure and detonation issues occur about 1/3 to
3/4 of the way through.

[David]

Of cource a link would be good, wouldn't it?

http://www.avweb.com/news/columns/182084-1.html


"David" <dave@mail--swap.bpa.nu> wrote in message
news:xY%Ed.114816$K7.27895@news-server.bigpond.net.au...
>
> > Greater efficiency at low rpm wide open throttle does make sense,
> > compressing the fuel to the maximum possible pressure and giving it as long
> > as possible time to transfer it's energy without collapsing. I guess some
> > tests would need to be done though.
> >
> > Cheers,
> > Michael
> >
>
> This article is very interesting, the tests have been done in the aviation
> environment although there is considerable debate about the various
techniques.
> The bits about efficiency, pressure and detonation issues occur about 1/3 to
> 3/4 of the way through.
>
>
>

[Graham W]

> "Fred Ferd" <fred@ferd.com> wrote:
>>The simple fact is that when you put the engine rpm's up to higher rpm's
>>they spew unburnt petrol out the back.

Only if there's something horrid wrong with the motor, either through
design or lack of maintenance.

Michael C wrote:
> The OP never said anything about high RPM. I've heard from an engineer that
> an engine is at it's most efficient at full throttle but low rpm.

Volumetric efficiency is best at low revs and full throttle. However at
full throttle mixture is set for maximum power, not for maximum
efficiency. Slightly off full throttle you get efficient mixture
combined with good volumetric efficiency.

[John_H]

Michael C wrote:

>
>The OP never said anything about high RPM. I've heard from an engineer that
>an engine is at it's most efficient at full throttle but low rpm.

Most engines give their best fuel efficiency (power produced for fuel
consumed) at around 80% of peak power output -- which usually means
reduced throttle at something above max torque revs and something less
than peak power revs.

The way aircraft engines are normally operated is intended to deliver
the best fuel efficiency (propeller pitch is varied to provide the
correct load, based on revs and manifold vacuum).

--
John H

[John McKenzie]

Jack wrote:
>
> An elderly engineer mate of mine claims the most fuel-efficient way to
> get a carburetted car to a certain speed is to accelerate at full
> throttle to that speed.
>
> At full throttle, the butterfly valve is at maximum air flow and the
> jets are providing the most optimum fuel-air mixture. At any setting
> less than maximum throttle, the jets are providing an overly-rich
> mixture, albeit slight, resulting in a less fuel-efficient acceleration
> that also takes longer to reach the speed; a double whammy.
>
> I can sort of see what he says, but it goes against my intuition of
> 'gently-gently wins the monkey'. What do you think?

He's full of shit, air correctors, metering rods and indeed power valves
are all parts of a car which alter the a/f ratio delivered at various
flows (most of them increase richness with progressive flow increases)

So basically unless the carb was optimised by a complete fucking idiot,
the richest mixtures will be at full throttle.

Is he aware that without air correctors at all, the mixture will get
expoentially rich from about 70% of max flow through the particular
sized venturi, and the air correctors job is to partially address this
phenomena?

It's not all so cut n dried though. At higher throttle openings there's
more volumetric efficiency, so far more dynamic compression, and hence
even at the exact same a/f ratio there might be a more favourable brake
specific fuel consumption (i.e the amount of fuel actually burnt per hp
produced)

On two identical engines, it's of less consequence, but what it does
mean is that you can potentially use less fuel in a transport situation
(admittedly this would be less likely to be petrol powered in the first
place) by having an engine capacity only sufficient in size to produce
the power needed. I.e. a 4 litre engine that can handle the towing would
likely use less fuel per 1000km than a 6 litre engine, of similar design
in the same application, since the latter would run with less throttle
opening, and less dynamic compression. Of course you'd absolutely need
to have the timing, combustion chamber design, inlet manifold and indeed
carb setup ideally for each. On the other hand, you'd likely get better
mileage in a 'not all else equal' situation above with a revised diff
ratio.

Bottom line is that there's far too much in it to narrow it down to one
point, it's the whole combination that must be taken into consideration.

--
John McKenzie

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[hippo]

Graham W wrote:

>> Jack wrote:
>>>An elderly engineer mate of mine claims the most fuel-efficient way to
>>>get a carburetted car to a certain speed is to accelerate at full
>>>throttle to that speed.
>>>
>>>At full throttle, the butterfly valve is at maximum air flow and the
>>>jets are providing the most optimum fuel-air mixture. At any setting
>>>less than maximum throttle, the jets are providing an overly-rich
>>>mixture, albeit slight, resulting in a less fuel-efficient
acceleration
>>>that also takes longer to reach the speed; a double whammy.

>Dene Oehme wrote:
>> Your engineer mate has no idea how a carby (other than one on a model
>> aeroplane engine) works.

>Thats a bit harsh. It sounds more like he once knew and he's forgotten.
>He's vaguely near right, but all mixed up.

Nearly 30 years ago, my girlfriend at the time had a 2250 auto Torana with
a vacuum gauge. Driving it wfo everywhere, the gauge consistently showed
'eocnomy', as it does in that situation for reasons we don't need tg into
now, but the car returned 8.5 mpg. Hmmmmmm... Cheers