Jim Henderson wrote:

> There isn't a linear relationship between the rotational speed of the 
> wheels and the speed of the engine.

Well that's news.

> If I take my foot off the gas, the engine revs drop.  The car may or may 
> not slow down (depending on the road and the landscape).

Not true. The engine revs depend on the speed of the vehicle.

If you don't believe me, try rolling down a hill at 30, and then stick 
the car into 1st gear. Watch the revs go through the roof, even though 
you haven't touched the gas.

> Similarly, if I apply the brakes to a full stop, the engine doesn't die, 
> it idles.

Um, yes it does. Unless...

> Then of course if you put the car in neutral, you've completely 
> disengaged the engine from the wheels.

...you disenguage the engine from the wheels. Then there really isn't 
any relationship. But as long as the car stays in the same gear, there 
is a fixed relationship.

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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>> If I put my car into top gear and set the engine to 3,500 revolutions 
>> per minute, my car travels forward at almost exactly 75 miles per hour. 
>> So... how far forward does it travel for a single revolution of the engine??
> 
>   Isn't this laughably simple to calculate?

Yes - *if* you can do things like compute 60/75 *mentally*. :-P And also 
if you know how many yards there are in a mile, and all kinds of other 
random stuff.

[I found myself wondering about this in the car on the way home.]

Now, if you have access to a *computer* - or even better, Google - then 
yes, it becomes quite easy.

>   Given that I'm not sure which whacky units miles are divided into,
> I'll just convert those to more understandable units, so 75/(3500*60)
> miles is approximately 57.5 centimeters.

That's the number I arrived at too.

Google also tells me that 75 mph = 33 m/s. Which frankly seems really 
damned fast. (!) I mean, my car is only, like, 4 m long or something... 
That means the car covers almost 10x its own length in 1 second! o_O

No wonder hitting anything is an instant fatality...

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Jim Henderson wrote:
> There isn't a linear relationship between the rotational speed of the 
> wheels and the speed of the engine.

There is, in a (relatively primitive) manual transmission. There is not, 
however, a linear relationship between engine RPM and gasoline usage.

> Similarly, if I apply the brakes to a full stop, the engine doesn't die, 
> it idles.

When did you last drive a manual transmission? :-)

-- 
   Darren New / San Diego, CA, USA (PST)
     "That's pretty. Where's that?"
          "It's the Age of Channelwood."
     "We should go there on vacation some time."

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On Fri, 16 May 2008 15:20:56 -0700, Darren New wrote:

> Jim Henderson wrote:
>> There isn't a linear relationship between the rotational speed of the
>> wheels and the speed of the engine.
> 
> There is, in a (relatively primitive) manual transmission. There is not,
> however, a linear relationship between engine RPM and gasoline usage.

How many cars use a transmission that would count as "relatively 
primitive"?  But I do agree that there's not a linear relationship 
between RPM and MPG.

>> Similarly, if I apply the brakes to a full stop, the engine doesn't
>> die, it idles.
> 
> When did you last drive a manual transmission? :-)

D'oh!  I was thinking automatic - I learned to drive on a stick....

Jim

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On Fri, 16 May 2008 21:27:22 +0100, Orchid XP v8 wrote:

> Jim Henderson wrote:
> 
>> There isn't a linear relationship between the rotational speed of the
>> wheels and the speed of the engine.
> 
> Well that's news.
> 
>> If I take my foot off the gas, the engine revs drop.  The car may or
>> may not slow down (depending on the road and the landscape).
> 
> Not true. The engine revs depend on the speed of the vehicle.
> 
> If you don't believe me, try rolling down a hill at 30, and then stick
> the car into 1st gear. Watch the revs go through the roof, even though
> you haven't touched the gas.
> 
>> Similarly, if I apply the brakes to a full stop, the engine doesn't
>> die, it idles.
> 
> Um, yes it does. Unless...
> 
>> Then of course if you put the car in neutral, you've completely
>> disengaged the engine from the wheels.
> 
> ...you disenguage the engine from the wheels. Then there really isn't
> any relationship. But as long as the car stays in the same gear, there
> is a fixed relationship.

Perhaps in a manual transmission - in my automatic, though, I'll stick 
with what I indicated....I s'pose I could ask my father-in-law when I 
talk to him next, since his life's work has been working on cars....

Jim

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Jim Henderson wrote:
> Perhaps in a manual transmission - in my automatic, though, I'll stick 
> with what I indicated.

Sure. But the automatic has a clutch built into it. That's why you only 
have two pedals.

-- 
   Darren New / San Diego, CA, USA (PST)
     "That's pretty. Where's that?"
          "It's the Age of Channelwood."
     "We should go there on vacation some time."

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Jim Henderson wrote:
> How many cars use a transmission that would count as "relatively 
> primitive"? 

One that has distinct manually-shifted gears without synchromesh. Lots 
of newer cars have things like variable transmissions.

-- 
   Darren New / San Diego, CA, USA (PST)
     "That's pretty. Where's that?"
          "It's the Age of Channelwood."
     "We should go there on vacation some time."

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On Fri, 16 May 2008 16:06:35 -0700, Darren New wrote:

> Jim Henderson wrote:
>> Perhaps in a manual transmission - in my automatic, though, I'll stick
>> with what I indicated.
> 
> Sure. But the automatic has a clutch built into it. That's why you only
> have two pedals.

True....

Jim

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On Fri, 16 May 2008 16:08:15 -0700, Darren New wrote:

> Jim Henderson wrote:
>> How many cars use a transmission that would count as "relatively
>> primitive"?
> 
> One that has distinct manually-shifted gears without synchromesh. Lots
> of newer cars have things like variable transmissions.

Yeah, that's what I'm thinking of, just didn't know the name of it...

Jim

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>>   Given that I'm not sure which whacky units miles are divided into,
>> I'll just convert those to more understandable units, so 75/(3500*60)
>> miles is approximately 57.5 centimeters.
>
> That's the number I arrived at too.

Sure, 1.885 feet per revolution.

> Google also tells me that 75 mph = 33 m/s. Which frankly seems really 
> damned fast. (!) I mean, my car is only, like, 4 m long or something... 
> That means the car covers almost 10x its own length in 1 second! o_O
>
> No wonder hitting anything is an instant fatality...

Hmm, yeah, 110 feet per second, wear your seatbelt.

0.0027 tsp of gas per cylinder. (other factors ignored).

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