Why you should never buy a car without a tachometer

On Wed, 14 Sep 2005, Ted B. wrote:

> Otto Cycle engines.
> Otto Cycle.
> 40-45%.
> the Otto Cycle,
> Otto Cycle
> I don't know why I even bother.
> 40%.
> Otto Cycle Otto Cycle.
> Otto Otto Otto, Cycle Cycle Cycle.
> 40%.

That makes all of us. Polly wanna cracker?

> By the way, the link you posted for your source of information did not
> work (probably too long and it got abbreviated when looking here
> through Google). Could you please post it on two lines or with a space
> half way in between so I can copy/paste it together and view the site?
>
>

Nevermind on the link, I got it now. Good site too.

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

Ted B. wrote:
> > What nonsense. I know you want to impress yourself and others with
> > your knowlege of automobile terminology by salting your messages with
> > "Otto Cycle Engine". Lets stick with the basics here. You are
> > asserting that a car that runs at 40% of the RPM indicated the tach
> > will get the best gas milage.
>
> No, I'm not asserting it. I'm simply repeating facts as stated by people
> who know a lot more about the Otto Cycle engine than either one of us do.
> There's a HUGE difference, there.

Unfortunately that is about all you are bringing to this discussion
You are quite adept the copy-and-paste of partial bits of information
but you seem to have little understanding of what they mean.


>
> >For many cars on the road today that
> > would would be well in excess of 90mph.
>
> In some cases, YES, that's possible. I haven't experienced it myself, but
> have no reason to doubt that it could happen. For my current car, the most
> fuel-efficient speed is 3500RPM, 43% of redline, and 78MPH (take your pick,
> all are correct). Several cars I've owned have been most efficient
> somewhere above 70MPH. To think that a car MIGHT be most fuel-efficient
> above 90MPH is not too hard to imagine. That's only about a 13MPH
> difference from my own car, and it's not exactly a sports car.

Good grief man...when was the last time you drive something beyond a
motorized rollerskate!

Taking your impossible example to the extreme if we geared up a car to
a 1:1 ratio just imagine the distance we could cover on a gallon of gas
and the speed...wheee!!! My goodness that would demonstrate how
efficient a passenger car with a Otto Cycle (4 stroke) engine could be.
But why stop at 1:1, lets go further... a .5:1, .05:1, why there is
just no limit to how efficient and fast we can make our cars.

>
> Most automotive authorities
> > I'm aware of believe that it takes more energy to keep a car moving at
> > a constant high rate of speed than a constant lower rate of speed.
> > 50mph vs 90 mph for example. Not surprisingly that has been my
> > experience too.
>
> Yes, but do those "automotive authorities" realize that at optimum RPM of an
> Otto Cycle engine, less energy is wasted as heat, leaving MORE ENERGY
> available to keep the car moving? Again, it's not really that you are
> pulling extra energy out of the engine, it's just that the engine is
> converting more of the available energy to kinetic energy. Thus, the
> greater fuel efficiency.
>
> ON THE OTHER HAND, it's easy to believe that many cars would indeed be more
> fuel efficient at 50MPH than 90MPH, because (NOT coincidentally) some Otto
> Cycle engines are running closer to 40% or so at 50MPH than 90MPH.


For the umpteenth time no it is not.

The very simple reason of why this is so is apparently beyond you.
Just take you car, fill it up with cas, reset the average mpg to zero
and embark of a 350 mile trip at 55mph. Repeat the same drive with a
full tank of gas this time at your favorite 78mph. Use the speed
control to be sure.

And report back the results.

>
> But a blanket statement like slow down to save fuel isn't necessarily going
> to be helpful for all circumstances, unless slowing down brings your Otto
> Cycle engine closer to the 40-45% range where it is most
> uel-efficient. -Dave

On Wed, 14 Sep 2005 wrote:

> By the way, the link you posted for your source of information did not
> work (probably too long and it got abbreviated when looking here through
> Google). Could you please post it on two lines or with a space half way
> in between so I can copy/paste it together and view the site?

Or, much better and less cumbersome, make the long URL into a short one
before posting it, by visiting www.tinyurl.com .

wrote:
> Ted B. wrote:
> > "223rem" > wrote in message
> > news:SiXVe.329298$x96.300841@attbi_s72...
> > > 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. Outside of the engine, no other
> > component of the car is adding kinetic energy to the car.
>
> At a constant 78mph nothing is adding any kinetic energy to the car at
> all, INCLUDING the engine. :-)
>
> >
> > 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.
> > Change any other component of the car, and you might lower or raise gas
> > mileage. But if the Otto Cycle engine is still allowed to operate at about
> > 40% of redline RPM, that is STILL going to be it's most fuel-efficient
> > speed. -Dave
>
> 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. It does not give any
> indication of fuel mileage at all.
>
> Todd Wasson
> Performance Simulations
> http://www.PerformanceSimulations.com

Now, now Todd. Yau are trying to confuse him with facts, something
that he apparently has very little association with. He thinks that
repeating Otto cycle and 40% repeatedly somehow proves his assertions
(and that is all they are).

Harry K

"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.

[...]
>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.

You don't seem to understand that engine efficiency (i.e.,
BSFC) is a function of both load *and* rpm. In fact, over
an engine's normal operating speed range, BSFC varies at
least as much with load as with rpm. But you choose to
ignore varying loads and base your conclusions solely on
rpm. That's not valid!

Here's a simple question: Do you think that your engine has
the same efficiency at 40% of redline in 1st gear as it does
at 40% of redline in top gear? If not, why not?

>Change any other component of the car, and you might lower or raise gas
>mileage. But if the Otto Cycle engine is still allowed to operate at about
>40% of redline RPM, that is STILL going to be it's most fuel-efficient
>speed. -Dave

IMHO, you need to understand two things:

1. Engine efficiency (BSFC) is a *strong* function of engine
load, as well as engine rpm.

2. Vehicle road load (i.e., the power required to hold a
steady speed) increases quickly at highway speeds. For
typical passenger cars and highway speeds, required power is
roughly proportional to speed squared (the exponent is
usually slightly larger than 2, actually...).

Items 1 & 2 can be used to show item 3:

Maximum engine efficiency is *very different* from maximum
vehicle fuel economy.
--
Chuck Tomlinson

> > The Otto Cycle internal combustion engine is most fuel-efficient near
40% of
> > redline.
>
> It isn't. My car will probably get around 40 mpg at 40 to 45 mph. At
> 60 to 65 mph it gets around 26 to 27 mpg. At 75 to 80 mph it gets
> around 23 to 24 mpg.
>
> My car has both instantaneous and average mpg readings with which to
> verify these figures.

OK, again, yet someone else who is posting data points based on MPH. Where
is the 40% RPM speed, and what does it actually DO there (as opposed to what
a car computer estimates) -Dave

> >
http://home.earthlink.net/~graham1/M...stion.htmQuote
> > from above:" A modern Otto cycle engine tends to be most efficient at40%
> > to 45% of its"red-line" r.p.m.
>
> Ah. You read it on the interweb, therefore it's true.
>
> Just so we're clear on the source you're citing as "authoritative".
>

Well I've read it several places. I have no reason to doubt that it's true,
when it perfectly agrees with what I have observed over the past few
decades. It seems to be the speed kills crowd that is most determined to
spin this into something *I* claim is true, as opposed to facts as
documented by people more knowledgeable than me. -Dave

> 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

In article >, Chuck Tomlinson wrote:

> 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?

Just to back this up.

This should be expected because engine efficiency is measured as useful work
out over energy in and fuel economy is measured in distance traveled over
volume of consumed.

For instance, an engine can be at peak efficiency, accelerating a vehicle,
but mpg will will suffer because the distance traveled isn't as much had
the vehicle used the same volume of fuel at a constant speed. Even though
in the later case, the engine may not be operating as efficently given
load, rpm, etc.