From: tcora@pica.army.mil (Tom Coradeschi (FSE) <tcora>)
Newsgroups: rec.motorcycles
Subject: Re: How does a dyno work?
Date: 15 Apr 92 15:34:08 GMT
Organization: Electric Armts Div, US Army ARDEC, Picatinny Arsenal, NJ

dab@vuse.vanderbilt.edu (The Nashville Flash) writes:
>In article <1992Apr14.223117.28744@umbc3.umbc.edu> bmatthew@umbc5.umbc.edu (Bud Matthews) writes:
>>O.K., second try.  How does a rear wheel dyno get a HP estimate
>>from that roller?  Is there some sort of resistance gradient that is
>>being overcome by the rear wheel?  What, exactly, is getting measured?
>>How is HP at a given RPM estimated?
>
>Since nobody else has chosen to walk into the rotisserie on this one, I'll 
>put up the opening bid.  Up front disclaimer: I'm an EE, not an ME.  
>Furthermore, my only dyno experience was more than ten years ago and my 
>memory is fading like drum brakes coming down Pike's Peak.

Good article describing the use of a chassis dyno deleted. There are
two types in common use, engine and chassis. Guess which is which?

Did anyone else notice the blurb in the latest American Motorcyclist?
They have this dyno contest each year at Daytona. Some guy from
?Minnesota? won it on a turbocharged, intercooled GSXR1100.

325 *rear wheel* horsepower. (Wonder if he rode it to Daytona?)

The following is taken from Mark's Standard Handbook for Mechanical
Engineers, pg 16-18, cuz I'm too lazy to try to do all this from memory
(and it's been 10 years since I played with a dyno), and the
scanner/character recognition software are working real well these days:-}

POWER MEASUREMENT

Power is defined as the rate of doing work. Common units are the horsepower 
and the kilowatt: 1 hp = 33,000 ft lbf/min = 0.746 kW. The power input to a 
rotating machine in hp (W) = 2(pi)nT/k, where n = r/min of the shaft, where 
the torque T is measured in lbf ft (N m), and k = 33,000 ft lbf/hp min [60 N 
m/(W)(min)]. The same equation applies to the power output of an engine or 
motor, where n and T refer to the output shaft. Mechanical power-measuring 
devices (dynamometers) are of two types: (1) those absorbing the power and 
dissipating it as heat and (2) those transmitting the measured power. As 
indicated by the above equation, two measurements are involved: shaft speed 
and torque. The speed is measured directly by means of a tachometer. Torque is 
usually measured by balancing against weights applied to a fixed lever arm; 
however, other force measuring methods are also used. In the transmission 
dynamometer, the torque is measured by means of strain-gage elements bonded to 
the transmission shaft.

There are several kinds of absorption dynamometers. The Prony brake applies a 
friction load to the output shaft by means of wood blocks, flexible band, or 
other friction surface. The fan brake absorbs power by "fan" action of 
rotating plates on surrounding air. The water brake acts as an inefficient 
centrifugal pump to convert mechanical energy into heat. The pump casing is 
mounted on antifriction bearings so that the developed turning moment can be 
measured. In the magnetic-drag or eddy-current brake, rotation of a metal disk 
in a magnetic field induces eddy currents in the disk which dissipate as heat. 
The field assembly is mounted in bearings in order to measure the torque.

One type of Prony brake is illustrated in Fig. 35 (No bad ascii drawings from 
me, nosir). The torque developed is given by L(W - Wo), where L is the length 
of the brake arm, ft; W and Wo are the scale loads with the brake operating 
and with the brake free, respectively. The brake horsepower then equals 
2(pi)nL(W - Wo)/33,000, where n is shaft speed, r/min.

In addition to eddy-current brakes, electric dynamometers include calibrated 
generators and motors and cradle-mounted generators and motors. In calibrated 
machines, the efficiency is determined over a range of operating conditions 
and plotted. Mechanical power measurements can then be made by measuring the 
electrical power input (or output) to the machine. In the electric-cradle 
dynamometer, the motor or generator stator is mounted in trunnion bearings so 
that the torque can be measured by suitable scales.

The engine indicator is a device for plotting cylinder pressure as a function 
of piston (or volume) displacement. The resulting "pv" diagram (Fig. 36) 
provides both a measure of the work done in a reciprocating engine, pump, or 
compressor and a means for analyzing its performance (see Secs. 4, 9, and 14). 
If Ad is the area inside the closed curve drawn by the indicator, then the 
indicated horsepower for the cylinder under test = KnApAd where K is a 
proportionality factor determined by the scale factors of the indicator 
diagram, n = engine speed, r/ min, Ap = piston area.

Completely mechanical indicators can be used only for low-speed machines.  They 
have largely been superseded by electrical transducers using strain gages, 
variable capacitance, and piezoelectric principles which are suitable for 
high-speed as well as low-speed pressure changes (the piezoelectric principle 
has low-speed limitations). The usual diagram is produced on an oscilloscope 
display as pressure vs. time, with a marker to indicate some reference event 
such as spark timing or top dead center. Special transducers can be coupled to 
a crank or cam shaft to give an electrical signal representing piston motion 
so that a p-v diagram can be shown on an oscilloscope.
-- 
 
                tom coradeschi    <+>    tcora@pica.army.mil
                     I own two bikes, brand unimportant.
Newsgroups: rec.motorcycles