Why you should never buy a car without a tachometer

he car's engine's redline is 7000 rpm. Forty percent of that is 2800
> rpm. At 2800 rpm in top gear, the car's speed is about 70 mph. The
> car's fuel efficiency is clearly not as high at that speed as it would
> be when the engine is running at around 1800 rpm which corresponds to
> around 45 mph.
>

Damn, I wish I had more time. I'd be willing to bet you money that you are
wrong. The only condition I would attach is that I'm allowed to inspect the
car (mainly for obvious stuff like proper tire pressure, fluid levels, just
to make sure that it will be safe to test) and allowed to drive it. I'm a
very good driver with a perfect driving record, and yes, I do have a license
(before some smartass asks). Your car would be in very good hands. I'd
baby it, unlike I treat my own car. I'd be shocked if your car actually
got better mileage at 45 than it did at 70. I don't think it will get bad
mileage at 45, but I'm sure it will do better at 70. Most cars would.
Besides which, 1800 is a really LOW RPM for that engine. That's only ~25%
range. It's going to be wasting too much heat energy at that range to get
anywhere near to optimum fuel economy. -Dave

> Only one poster replying to you really belongs in the
> "speed kills" crowd, so that's not it.
>
> Perhaps it is because people who have MPG calculators
> in the dash see different data, they disbelieve you.
>
> Bernard

As they should, if they are only looking at their own vehicle. From what I
read, not all vehicles will get better mileage at higher speeds/RPM. A lot
of them will. But seeing someone say that their own car doesn't really
doesn't prove anything, one way or the other. -Dave

> >The Otto Cycle engine is most fuel efficient at 40-45% of redline RPM.
> >There is no way around that, as that's just how the Otto Cycle engine
works.
>
> Do you really think repeating that assertion over and over again makes
> it any more credible?

Do you think denying physics will make it go away? Don't bother answering
that.

Geez, as I wrote before, keep believing I'm wrong. I'll use my newfound
knowledge to send less of my money to the big oil companies. You all can
feel happy knowing that you're right and that you're going to waste a lot of
fuel proving it, DAMMIT. -Dave

Dave C. wrote:

>
> Do you think denying physics will make it go away? Don't bother answering
> that.
>
> Geez, as I wrote before, keep believing I'm wrong. I'll use my newfound
> knowledge to send less of my money to the big oil companies. You all can
> feel happy knowing that you're right and that you're going to waste a lot of
> fuel proving it, DAMMIT. -Dave
>

Assuming that you're not trolling (starting to look like a big assumption),
and that you're right, dont you think that one would find your fuel saving
rule in all owner's manuals?

(Chuck Tomlinson) writes:

>"Ted B." > wrote:

>>Not really. At best, it's a red herring.

[snip]

>Maximum engine efficiency is *very different* from maximum
>vehicle fuel economy.

Chuck; haven't you worked out that the only way to change his
immutable opinion is by percussion? :-)
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
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"Dave C." > wrote in message
ink.net...
>> Only one poster replying to you really belongs in the
>> "speed kills" crowd, so that's not it.
>>
>> Perhaps it is because people who have MPG calculators
>> in the dash see different data, they disbelieve you.
>>
>> Bernard
>
> As they should, if they are only looking at their own vehicle. From what
> I
> read, not all vehicles will get better mileage at higher speeds/RPM. A
> lot
> of them will. But seeing someone say that their own car doesn't really
> doesn't prove anything, one way or the other. -Dave


I thought I heard somewhere that "all otto cycle engines"
something something or something.

Maybe I didn't hear that?

Bernard

(Matthew Russotto) writes:

>In article ws.net>,
>Ted B. > wrote:
>>
>>The Otto Cycle engine is most fuel efficient at 40-45% of redline RPM.
>>There is no way around that, as that's just how the Otto Cycle engine works.

>Do you really think repeating that assertion over and over again makes
>it any more credible?

Maybe if he puts a GIF anim on a web page. :-)
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
\ / ASCII ribbon campaign | I'm a .signature virus!
X against HTML mail | Copy me into your ~/.signature
/ \ and postings | to help me spread!

Dave C. wrote:
> > Dave, this 40% of redline RPM efficiency value you read about refers to
> > "fuel conversion efficiency," not "fuel mileage." They are two vastly
> > different things and are not equal or directly representative of each
> > other by any stretch of the imagination. In fact they have different
> > units.
> >
> > All that the "fuel conversion efficiency" value means is that for every
> > gallon of fuel you're burning, you're getting the most possible power
> > out of the engine at that particular RPM.
>
> And meanwhile, the car is? . . . (class, help him out a little here)
>
> Nobody?
>
> Moooooooooooooving. Eating up miles. -Dave

Dave, I am flabbergasted that you're arguing with me about this. For
the past several years I've made my living writing vehicle dynamics
software, including a couple of engine simulation packages, one of
which models pressure wave dynamics and their full interaction between
cylinders throughout all intake and exhaust channel junctions. In
other words, I do all the mathematics that predicts exactly the sort of
things we're discussing in this thread and put it into software form
for people to use. You're thigh is snapped in two places and sticking
out of your skin, yet you're telling the MD your leg isn't broken.

Todd Wasson
Performance Simulations
http://www.PerformanceSimulations.com

Dave C. wrote:
> > Consider the consequences of that if it is or were indeed true. This
> > literally means that if you swap out your transmission for a direct
> > drive unit (1:1 ratio), and assuming your car came stock with a 3:1
> > differential ratio, your "40% of redline" speed has now tripled. Your
> > optimum speed was 78mph before, which I still maintain is rather
> > coincidentally tied to your 43% value.
> >
> > With the new driveline unit your optimum fuel mileage now occurs at
> > 234mph if what you're saying is correct. Congratulations, the fuel
> > crisis has now been put off for quite some time. ;-) All we needed
> > were higher gears all along, go figure.
>
> Funny. Your example is rather extreme, but in an odd way, it does make
> perfect sense. Take my car for example. If the gear ratio of the various
> manual tranny gears was changed a bit, I could move the most fuel-efficient
> speed from 78 down to 70, which would be more useful. I doubt if I could
> move it up to 234, though. At some point, lack of horsepower would be a
> HUGE problem. If the car isn't moving at all, it's obviously not going to
> be fuel-efficient. But if you triple the size of the engine at the same
> time? Yes, maybe you could indeed push the most fuel-efficient speed up to
> 234. I'm not gonna drive that beast, but I'm sure there will be some
> takers. -Dave

Dave, "fuel efficiency" is not the same thing as "fuel mileage."

"Fuel mileage" is expressed in miles per gallon. That, as you know,
means "how many gallons of fuel are consumed per mile travelled." This
is really the more important thing to consumers and saves them money
when maximized.


"Fuel EFFICIENCY" is expressed as a PERCENTAGE. From your own source:
"Argonne Labs measured the efficiency of the Japanese Prius engine to
be 34% (good for any engine at its peak) at only 13.5 hp.
"

So, Dave, how many miles per gallon is 34% fuel efficiency equal to?

If you're getting 34% fuel efficiency, how many gallons of fuel are you
burning every hour?

Chuck Tomlinson wrote:
> "Ted B." > wrote:
> >"223rem" > wrote...
> >> Chuck Tomlinson wrote:
> >>> The key issue here, I think,
> >>> is the difference between _engine efficiency_ and _vehicle fuel economy_.
> >>
> >> Very good point!
> >
> >Not really. At best, it's a red herring.
>
> Interesting. I offered you information (actual data that I
> gathered from vehicle instrumentation) that shows how fuel
> economy can decrease while engine efficiency is increasing.
> Do you need clarification of the data? Assumptions? Math?
>
> My data shows that your premise is wrong. Furthermore, it
> also shows (quite clearly IMHO) that engine efficiency
> (BSFC) and fuel economy (e.g., mpg) are *completely*
> different.
>

<snip excellent stuff>
> Chuck Tomlinson

Dave, Chuck is 100% correct here. You're confusing "fuel economy"
(mileage) with "fuel efficiency" (percentage of fuel's heating value
that is converted to mechanical power).

One is "miles per gallon," the other is a percentage. They mean
completely different things. If they were the same thing, they would
not have different definitions and be expressed in different units!

Todd Wasson
Performance Simulations
http://www.PerformanceSimulations.com