Miata Mailing List: November 1996, Message #93
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   From: "Gordon J Choate" 

Subject: Re: Tech question about dynos

   Date: Fri, 1 Nov 1996 19:39:48 -0500











Keith asked:



>Okay, how the heck does a dynamometer work?  As far as I know, you put the

car up on >rollers and floor it.  What are the rollers doing?  The gearing

of the car must provide a >significant difference.  C'mon, who knows the

answer to this one?





I can provide a partial, simple, vague answer, since my mech engineering

university dyno days were 15 years ago...



The rollers are providing resistance to the wheels via a brake - the dyno

measures how much work is required to brake the rollers to a constant

speed, and that work is translated to (torque, which is translated to)

BRAKE horsepower (bhp).  The dyno operator controls the speed of the

rollers via the dyno brake, which controls the speed of the engine.  So,

you strap the car down, floor it, then the dyno operator takes readings as

he gradually (in increments) allows the speed to increase.  My experience

was with engine dynos, where the crankshaft was connected directly to the

brake dynamometer.  If I recall correctly, those dynos used water brakes...

I don't know how the roller dynos are braked.  I'm guessing about this next

part - the amount of work being applied to the rollers can be calculated at

any rpm, so the rollers probably don't care whether they receive 130 hp in

2nd gear or 5th.  The dyno operator will care, however!



History pop quiz - who remembers the early 60s Rover gas turbine powered

cars?  Rover actually entered a couple of them in LeMans a couple of years,

as a test program for the gas turbine development.  Anyway, one of the test

engines in our dyno lab at the University of Manitoba was a Rover gas

turbine.



Gordon Choate and Artoo (who says pistons suit him just fine)

Calgary, Alberta

96 BRG/tan















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