Date: Thu, 27 Aug 2009 03:11:53 -0400
Reply-To: rolftruck@AOL.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Rolf Lockwood <rolftruck@AOL.COM>
Subject: Re: Engine to auto-tranny match and the Indy 500
In-Reply-To: <20090826230324.609D81165C1@hamburg.alientech.net>
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Well, you definitely have the advantage over me in terms of physics, Mike. It's a course I successfully avoided in high school and never needed in university. And I do stand corrected on the terminology and a couple of basic principles. I should have looked at a piece I wrote 10 years ago, vetted by several engineers (I'm definitely not such a guy) and subsequently praised on publication by the chief engineer at Cummins, the diesel maker.
Here's a little of what I wrote back then...
"And what’s
torque? It’s pure twisting force — not a measure of how fast the engine can do
work, which is horsepower — but just the bare potential for work arising out of
that twisting motion.
"And the more
horsepower you’ve got, the faster you could haul wood or climb a grade.
It’s a calculated value, directly tied to torque, that measures the rate at
which the work gets done. Oddly enough, it has its origins in Scotland.
"Nearly two
centuries ago, Scottish inventor James Watt decided that the industrializing
world needed a way to measure the output of his steam engine. So he measured
how much work a good horse could do, and found it could lift 330 lb 100 ft in
one minute. Thus the term, “one horsepower”.
"How
much torque is involved there? That’s expressed as 33,000 lb ft. We get that by
multiplying 330 lb (the amount the good horse can move in a minute) by 100 ft
(the
distance he can move it). Put another way, one horsepower is the ability
to do 33,000 lb ft of work in one minute."
You're absolutely right to criticize some of what I wrote rather carelessly, but I think the practical principles remain as I originally suggested -- you want to have your engine cruise speed at least a couple of hundred rpm above the peak torque point so that you fall into torque when you hit a grade and engine speed falls off. Horsepower won't get you over the hill but torque will.
Ask any truck driver. He can't manage that hill at 1800 rpm where peak horsepower is built because rpm and road speed will fall away quickly. Horsepower just can't do the work and torque is at its low point up there. But once the engine falls to about 1500 rpm and lower, he's into torque and he'll feel it pick up the load and move him to the crest. He might still need a downshift or two but he'll always aim to work the hill down as low as 1100 rpm. If I look at the power map for a 2007-spec Detroit Diesel Series 60 515-hp engine, for example, I see those 515 horses developed at 1800 rpm, falling to just 340 hp at 1100 rpm. Torque, on the other hand, rises from 1400 lb ft at 1800 rpm to its peak of 1550 lb ft in a flat line from 1500 to 1100 rpm.
That flat line makes for excellent driveability and I'm guessing that there's nothing even vaguely flat in the torque curve of our little 2.1-liter-or-less engines. But the principles are exactly the same. As I20learn to drive my new old Vanagon, I feel a mighty quick drop in torque below about 3000 rpm but between there and 3200 I feel its grunt. That's a narrow band so it seems to demand a close eye on the tach.
Anyway, sorry for going on so long -- I promise any subsequent posts will be shorter. I'm more likely to be asking questions anyway.
Rolf in Toronto
1990 Westfalia camper
---- Original Message ----
From: Mike S <mikes@flatsurface.com>
To: rolftruck@AOL.COM
Cc: vanagon@GERRY.VANAGON.COM
Sent: Wed, Aug 26, 2009 7:02 pm
Subject: Re: Engine to auto-tranny match and the Indy 500
At 06:15 PM 8/26/2009, Rolf Lockwood wrote...
> Torque is defined as an engine's ability to do work, characterized
> by the twisting motion off the end of the crankshaft.
No. Work is done by power over time, not force (torque). Try lifting a
1000 lb. weight. You can tug and tug, and you will be applying a lot of
force. You might even "work up" a sweat, but unless you actually lift
that 1000 lbs. off the ground, you haven't done any work.
> Horsepower is a purely calculated value that measures only the speed
> at which the work is done. So the ONLY thing that gets you over the
> crest of a hill is torque,
Nope. You need to study some physics. Force only does work when it
actually moves something. Work over time is power.
>Torque is an absolutely 'tangible' value, horsepower
a mere
>calculation.
No. One HP is the amount of power it takes to raise 550 pounds 1 foot
in 1 second. (foot-pounds, not to be confused with pounds-feet, which
is a measure of torque) Power is the rate at which work is done. You're
probably confused by the fact that one can convert torque to HP with
the formula RPM x Torque x 5252 = HP. But you can just as easily start
with HP: HP / 5252 / RPM = Torque.
When a car is put on a dynamometer, it is power which is measured (the
amount of work needed to turn a weight or heat a liquid, and how long
it takes to do so), and torque is calculated.
Moving a Vanagon up a 1000 foot hill is the same amount of work,
whether you're going 10 or 60 MPH. But, to go 60 MPH, you need more
power, which allow you to do the same amount of work in less time.
Here's a pretty good explanation. Don't feel discouraged, it confuses a
lot of people:
http://auto.howstuffworks.com/auto-parts/towing/towing-capacity/information/fpte.htm
