Gas Too Expensive? Try These Options

In article >,
Eeyore > wrote:
>
>Nonsense ! An 'overdrive' is a separate gear box coupled into the drive chain.
>You would perhaps be taking about gear ratios of < 1:1. - that's quite
>different.

Now you're intentionally being difficult. The term "overdrive" has
been used for gears with a ratio of < 1:1 for some time now.

--
There's no such thing as a free lunch, but certain accounting practices can
result in a fully-depreciated one.

In article >,
Free Lunch > wrote:
>
>I had a car that had an overdrive button that would allow the car to go
>into the top gear if it was engaged when set to drive. I have no idea
>why anyone would care if the wheels had more RPMs than the engine.

Whether the _driveshaft_ has more RPMs than the engine. The wheels
still have the final drive ratio in the mix.


--
There's no such thing as a free lunch, but certain accounting practices can
result in a fully-depreciated one.

Matthew Russotto wrote:

> In article >,
> Eeyore > wrote:
> >
> >Nonsense ! An 'overdrive' is a separate gear box coupled into the drive chain.
> >You would perhaps be taking about gear ratios of < 1:1. - that's quite
> >different.
>
> Now you're intentionally being difficult. The term "overdrive" has
> been used for gears with a ratio of < 1:1 for some time now.

It's somewhat fallen out of use over here. More likely to be called a 'high ratio'.

Graham

On Mon, 21 Aug 2006 13:16:07 -0500, in misc.transport.urban-transit
(Matthew Russotto) wrote in
>:
>In article >,
>Free Lunch > wrote:
>>
>>I had a car that had an overdrive button that would allow the car to go
>>into the top gear if it was engaged when set to drive. I have no idea
>>why anyone would care if the wheels had more RPMs than the engine.
>
>Whether the _driveshaft_ has more RPMs than the engine. The wheels
>still have the final drive ratio in the mix.

Er, yeah.

Bill Funk > wrote:

> James Robinson > wrote:
>
>> Bill Funk > wrote:
>>
>>> James Robinson > wrote:
>>>
>>>> Bill Funk > wrote:
>>>>
>>>>> Like I said, argue all you want, but over more than 10 round-trips
>>>>> from Phoenix to El Paso, it consistantly got more than 2mpg better
>>>>> at 70 than at 55.
>>>>
>>>> How often did you really drive at 55 mph between the two cities
>>>> when you could have driven at 70 mph? I can't imagine anyone
>>>> willingly driving 15 mph slower than surrounding traffic and adding
>>>> 100 minutes to the trip.
>>>
>>> See, here's a major problem: what you can't imagine is done quite
>>> frequently, but since you can't imagine it, it's rejected out of
>>> hand. This makes for some rather skewed observations.
>>
>> I would be interested to hear why you frequently drive 20 mph below
>> the speed limit (assuming you drove on the interstate) on a 400 mile
>> drive.
>
> I didn't say I drove frequently at 20mph under the speed limit.

Yes, you have avoided the whole question of how you made the comparison.
To give credibility to your comments, you need to suggest how your
comparison was made. Credible tests have to be tightly controlled.

Did you have the same load? Did you compare the mileage in both
directions? Did you drive on the same roads? Did you repeat the tests a
sufficient number of times to be sure the results were repeatable? Was
the temperature and humidity the same each time? Did you fuel just before
each trip started, and fuel again right at the end? Did you have the same
type of fuel for each test? How did you top up the tank to ensure that
you had an identical amount of fuel each time?

>> I reject the contention that the vehicle uses less fuel at 70 than at
>> 55 simply since the physics don't support it. Air resistance is
>> substantially higher at that speed, and it takes energy in the form of
>> extra fuel to overcome that resistance. It's that simple.
>
> What you think physics supports doesn't really matter.
> it has to do with the efficiency of the entire drivetrain at different
> RPMs and drive ratios, which obviously you don't understand, despite
> your reliance on physics.

But I do understand it. I also understand the effects of air/fuel
ratios, lambda, piston velocity, BMEP, BSFC, entropy, enthalpy, etc. etc.
etc. We can discuss these in more detail if you wish.

The claim of better mileage at speeds greater than 70 mph flies in the
face of all the EPA tests. When such contradictions arise, to convince
myself if such claims are possible, I like to work through the numbers,
since they provide a good basis of comparison, to see if the claim is
within the realm of possibility.

I really don't think you recognize the huge effect that air resistance
begins to play at higher speeds.

Let me put it as simply as I can. The horsepower needed to move a truck
of the size and weight of an F-250 on a level road is double at 70 mph
compared to 55 mph. (about 75 hp at 70 mph, 37 hp at 55 mph.)

All other things being equal, the rate of fuel consumption would also
have to be double with that increase in horsepower, since the two are
directly related.

Even though the rate of fuel consumption is doubled, the amount of fuel
used for the whole trip is not double, since you get there faster, and
therefore aren't running the engine for as long. Given that you would
take 21% less time to get to the destination, the total amount of fuel
has to be reduced by that amount. Thus, the total fuel would be 57%
greater for a trip when driving at 70 compared to 55, again all other
factors being equal.

Now, you are claiming an improvement in fuel economy of about 10 percent
with the higher speed. That means the engine would have to be much more
efficient at the higher RPM to compensate for the extra horsepower
required. Not only does the improvement in engine efficiency have to be
at least 57% at the higher speed just to keep the consumption the same,
but it also has to improve on that by an additional 10 percent to meet
your claims, or over a 60 percent improvement in efficiency with that
slight speed increase. Do you see why I have a problem?

I'm looking at a map of engine performance for a typical spark ignition
engine, which I unfortunately can't share in a text message. It relates
fuel consumption to a variety of factors, including RPM, load factor, and
mean effective cylinder pressure. It suggests that the typical engine
will see an improvement of about a 30 percent in fuel efficiency at the
RPMs and horsepower we are talking about. Therefore, with an ideal
design, I would expect that when driving at 70 mph the mpg would drop by
about 30 percent. It would not improve. Not even close.

There is no mystery to this stuff, all the factors are well known and
documented. It's how the vehicle designers do their jobs.

>> Further, as I mentioned in another post, the manufacturers would be
>> optimizing fuel economy for speeds below 60 mph to meet the CAFE
>> requirements.
>
> Not on vehicles which don't need to adhere to CAFE rules.

My leap of logic is probably too big. Yes, I realize the F-250 is has a
high enough GVW to be exempt from the CAFE requirements. My point is
that there is great commonality between engines, fuel injection systems,
and Ford would not have a separate set of designs with separate settings
for the limited market of heavy pickup trucks. What would even possess
them to optimize a heavy truck for 70 mph operation when it is normally
used for commercial applications where the speeds would be much lower?

> Again, that which you have no experience of shouldn't be dismissed out
> of hand.

I have no experience with flying pigs either. I am nevertheless confident
I shall never see one.

Even when I do a search on the web for similar testimonials about the
fuel efficiency, I find people saying that they get between 10 and 20
percent poorer mileage at speeds greater than 50 mph:

http://www.ford-trucks.com/forums/22...mileage-2.html

(Yes, it's for a diesel engine, but their experience is closer to the
calculations)

Eeyore > wrote:

> Aidan Stanger wrote:
>
> > Eeyore > wrote:
> >
> > > Aidan Stanger wrote:
> > >
> > > > Eeyore > wrote:
> > > >
> > > > > A modern ECU will deliver *no* fuel to an engine that's being used
> > > > > for braking / deceleration. The engine simply has clean air going
> > > > > through it. This is easily seen in any car with an instantaneous
> > > > > mpg readout.
> > > > >
> > > > > However, to keep an engine idling ( as when coasting ) *does*
> > > > > require fuel. So coasting is actually very bad not only for fuel
> > > > > consumption but also for vehicle control too.
> > > >
> > > > Coasting and putting the vehicle into neutral are two very different
> > > > things.
> > >
> > > What has that got to do with it ? Do please explain your point.
> >
> > 'Tis only when the engine's going slowly that it will require fuel to
> > keep it going. If you're coasting in gear, it will take a while to get
> > it slow enough.
>
> Absolutely wrong !
>
> If you have a car with an instantaneous mpg teadot from the ECU you wil
> see that even when only slightly decelerating in gear with your foot off
> the accelerator, the fuel used is zero.
>
WHICH IS EXACTLY WHAT I WAS SAYING!!!

Fuel use will be zero from the time you take your foot off the
accelerator until it is going so slowly that the ECU will supply some to
keep it going.

> > You stated yourself that the engine gets no fuel when it's being used
> > for deceleration.
>
> Yes.
>
> > It IS doing that in a coasting vehicle, except at low
> > speeds and when it's in neutral.
>
> NO.
>
Why not? How can an engine that's in gear possibly not be decelerating a
vehicle when it's coasting?

--
Aidan Stanger
http://www.bettercrossrail.co.uk

Bill Funk > wrote:


> Like I said, argue all you want, but over more than 10 round-trips
> from Phoenix to El Paso, it consistantly got more than 2mpg better at
> 70 than at 55.

Would I be correct in deducing that you had the air conditioner on full
blast?

On Wed, 23 Aug 2006 01:01:45 +0930, (Aidan Stanger)
wrote:

>Bill Funk > wrote:
>
>
>> Like I said, argue all you want, but over more than 10 round-trips
>> from Phoenix to El Paso, it consistantly got more than 2mpg better at
>> 70 than at 55.
>
>Would I be correct in deducing that you had the air conditioner on full
>blast?

No, not in cool weather; panel (outside air) only then.
And even in the heat, full blast isn't needed, not even Max AC
(recirculate only), after initial cooldown.
--
Bill Funk
replace "g" with "a"