Why you should never buy a car without a tachometer

"Ted B." > wrote:
>>
>> I'm sitting here thinking about my brother-in-law and his 1995 vette.
>> As I remember it is loafing along at about 1500 rpm or maybe a little
>> slower at 60mph. By your formula he should be getting great milage
>> somewhere above 100mph right? Wonder why it doesn't work that way.
>
>Good question . . . does it or does it NOT have an Otto Cycle engine?

It does.

>Does Chevy use variable valve timing?

Not in that engine (which is identical to my car's).

>There are certain modifications that can
>be made to an Otto Cycle engine to make it waste less heat energy at RPMs
>outside of the optimum 40%. But I did not design the Corvette engine, so I
>have no idea why it is not acting like an Otto Cycle engine SHOULD act.

IMHO, the Corvette engine acts just the way an Otto Cyle
engine should act (see below). The key issue here, I think,
is the difference between _engine efficiency_ and _vehicle
fuel economy_.

>I would ask this though... has anybody ever run a Corvette at a 40% speed
>"above 100MPH" for a few hundred miles to measure the fuel economy? That's
>a serious question, BTW.

There's no need. As I mentioned before, '92-up Corvettes
can display "instantaneous" and distance-averaged mpg, which
have proven quite accurate in my car. On several road
trips, I've set the cruise at 100 mph while driving across a
desert (passing dry & salt lake beds that are very close to
level grade), zero the average mpg, and check it when I
finally have to drop out of 100 mph cruise (in those
conditions, it converges to an average in well under a
minute). My car consistently returns 20-21 mpg at 100 mph.

Using the same technique at other speeds, I've found that my
car gets 28-29 mpg at 70 mph, and ~40 mpg at 40 mph (all in
top gear). With this information (and approximations to my
car's mass, aero properties, and rolling resistance), it's
easy enough to estimate engine efficiency at each of these
cruising speeds. The results:

40 mph, 9 hp, 40 mpg, 1.0 gal/hr, 0.66 (lb/hr)/hp
70 mph, 27 hp, 28 mpg, 2.5 gal/hr, 0.56 (lb/hr)/hp
100 mph, 62 hp, 20 mpg, 5.0 gal/hr, 0.48 (lb/hr)/hp

The last number in each row is a common measure of engine
fuel efficiency, the brake specific fuel consumption (BSFC),
expressed as fuel mass flow rate divided by output power.
Lower is better, for BSFC. For gasoline engines, minimum
part-throttle BSFC tends to be about 0.38 lb/hr/hp (more or
slightly less).

Anyway, you can see that the engine efficiency is increasing
(lower BSFC) even as the fuel economy is decreasing.

That's how Otto-cycle engines typically behave in passenger
vehicles, and that's why you *cannot* predict the rpm for
maximum _vehicle fuel economy_ based on the rpm (and load)
of maximum _engine efficiency_.
--
Chuck Tomlinson

Chuck Tomlinson wrote:
> The key issue here, I think,
> is the difference between _engine efficiency_ and _vehicle
> fuel economy_.

Very good point!

> 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.

>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.

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.

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

"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. Unless it's a
hybrid with electric motor(s), but that's a whole other topic.

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

Ted B. wrote:

> 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.

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.
>


I think you missed the part about me writing engine simulation software
in our earlier exchange. If you'd like to discuss the Otto Cycle and
what that precisely means and entails, and how it might or might not
have any bearing on what you currently believe about engines and fuel
economoy, feel free to ask.


> >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.
>


If I'm not mistaken, what you're stating is that one can multiply the
redline by around 40% and take a look at the speedometer when
travelling at that engine speed in top gear. Whatever speed you're
running at that point is going to give the optimum fuel mileage,
correct? And the reason for this is because of the scientific fact
that an Otto Cycle engine attains peak fuel economy at that engine
speed (40% of redline). (Note that that figure is most likely
referring to engine speed and fuel conversion efficiency, not that in
combination with vehicle speed and fuel mileage.)

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.

This reminds me of a time in high school when a buddy wanted to put a
2.32 rear end ratio in his Camaro so he could cruise along at 2000rpm
in excess of 100mph. Unfortunately he did not realize that the engine
did not produce enough power at 2000 rpm to allow that speed to be
reached, even at full throttle. But I digress.

Anyway, this should continue to be a fun debate so long as nobody here
is the sort of person who would insist his MD was wrong about his
"broken leg" diagnosis. ;-)

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?


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

On Wed, 14 Sep 2005, 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.
> 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.

Horseapples. There are so many design, implementation and operational
factors that go into fuel efficiency that picking a random percentage of
"redline RPM" (for which there is absolutely zero standard definition
other than "wherever the carmaker prints the red line on the tach, if so
equipped") and calling it the most efficient speed is grossly untenable.

On Wed, 14 Sep 2005, 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.

You're asserting it, calling it a "fact" without a shred of evidentiary
support, and telling us we should believe you because you heard it from
really knowledgeable people.

You will have to do a great deal better than that if you wish to be taken
seriously. Just saying "Otto cycle! Otto cycle! Otto cycle! Otto cycle!"
isn't going to cut it.

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

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

> > I and others are waiting for you to post your methodology for
> > measuring your mileage. It would also help for you to post some
> > engineering reports supporting your 40% assertion.

>
> 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".