Truck parts Dynamometer.

[wmcgonegal at rr.etc.ncr.doe.ca]

>What really is involved in putting together a dynamometer - either 
>chassis or engine?

Building a chassis dynamometer is not a trivial task.  I have been 
involved with designing and building complete chassis dynamometers at 
our facility.  Building the rolls is a large task.  They have to be 
machined on large lathes and balanced.  If you go with small diameter 
rolls they must be carefully balanced as they will be spinning faster 
then large rolls.

Our experience with chassis dynamometers is that they are not suitable 
for making horsepower measurements (we use them for emissions work).  
The tires slip on the rolls at higher powers, and there is a lot of 
energy lost by the tires heating up as they deflect at the roll 
surface under sustained high load.

With either a chassis or engine dynamometer you require a power 
absorption unit (PAU) if you want to maintain a constant speed.  Some 
have been suggested in previous postings, including the eddy current 
style PAUs "Telma" and other brakes.  One has to remember that all of 
the power coming out of the engine/vehicle has to be dissipated 
somehow.  To continuously run at a high power output may require a 
large body of cooling water.  One way we get around this with some of 
our engine and chassis dynamometers is to use regenerative DC motors 
as PAUs.  The DC motors operate as generators to absorb the power and 
a power controller converts it to AC power which is fed back to the 
power grid.  The cost of such a system is well beyond that of a DIYer. 
 
Another style of PAU that is used is the water brake.  It consists of 
a rotor that has vanes on it that rotates in an enclosure.  There is 
another set of vanes on the enclosure which are stationary.  Water is 
in the enclosure, and by varying the level of water in the enclosure, 
the load can be changed.  There is also a heat exchanger built into 
the enclose which uses water to keep the unit below a set temperature. 
 The entire enclosure can rotate about the input shaft axis and is 
held in place by a load cell which is used to measure force and 
calculate torque.

For very small engine testing we have set up an alternator to be used 
as a PAU.  The load is changed by altering the alternators field 
current.  The electrical energy that is created is dissipated in a 
bank of resistors.  Again, force is measured with a load cell.  We 
also have set up PAUs that are essentially hydraulic pumps.  The oil 
is circulated by a pump through a heat exchanger and a valve in the 
circuit which is adjusted to vary the load by restricting the flow.

It may be possible to build a hydraulic (oil or water) PAU out of a 
torque converter from an automatic transmission.  Of course you have 
to keep it cool somehow.

I have seen motorcycle dynamometers which consist of a large roll that 
has a lot of inertia.  Knowing the inertia of the roll and by 
measuring speed and the rate of acceleration, you can calculate power.

I think the easiest way for a DIYer to calculate power output from a 
vehicle is (as mentioned in other postings) to accelerate on a flat 
stretch of road while recording speed versus time.  From this 
acceleration can be calculated.  Power is calculated as Vehicle Mass * 
Acceleration * Speed.  If kg, meters/second squared, and meters/ 
second are used the result is in Watts.

Sorry for the length, I just had to say it.

Will McGonegal
Electrical Engineer
Mobile Sources Emissions Division
Environment Canada
wmcgonegal at rr.etc.ncr.doe.ca