Homemade dynamometer

[varnish]

Hi guys.
I'm interested in building a dyno to test modifications to small engines (lawnmower engines).
I've found this old topic:
https://www.physicsforums.com/showthread.php?t=117800

But I'm having some trouble picturing the design.

I like the idea of using an electric motor and an electric stove.

Would the dyno be as simple as hooking the motor up to the electric motor, then running the electric motor through the stove.

By switching the stove hobs on till the rpm is constant I can get a wattage (and therefor HP figure), and by measuring the rpm I can find torque.

 

[FredGarvin]

The stove is the load. It doesn't have to be that. I have seen very small power dynos that used banks of lights as the loads. Very inexpensive and very easy to get very small load steps.

To get the actual power your engine is putting out, you will have to have an idea of the efficiencies of the system as a whole.

 

[varnish]

I suppose it would be possible to use a stove for coarse adjustment and bulbs for fine adjustment.

I like the stove idea. Cooling could be provided by using a large sauce pan of water.
I can imagine lots of light bulbs being a little unsafe.

But is the idea okay?
I realize it wouldn't be the most accurate device. But would give some idea of power gain.

 

[brewnog]

There's no reason it can't be accurate, if you can measure current and voltage (which is not difficult) you can get a very, very good idea of power being consumed by your 'load bank'. Depending on the size of your engine I tend to favour the light bulb load bank, but there's no reason a stove (or electric fire) wouldn't work; it just becomes more difficult to provide constant cooling. The only question which then remains is your generator efficiency, which you can certainly make allowances for, and you could measure it if you really needed to.

 

[varnish]

I'd like to test somewhere around 5hp, so ~3.7KW

Do you think a 4 hob stove (2*750W, 2*1500W) boiling water for cooling, and a few light bulbs ranging from 100w to 10W (or whatever I can get) would be a reasonable set up?

 

[Mech_Engineer]

That sounds like a reasonable setup to me. If you had 4 gallons of water on the stove at 25 degrees C, not taking into account losses from the environment if you were dissipating a full 3.7kw into the water it would take 21.3 minutes to heat the water to 100 degrees C. That's plenty of time to test and engine under load.

 

[Bob S]

Using the stove as a resistive load will work only if the switching of the stove knobs do NOT control the triggering of triacs.
Bob S

 

[varnish]

Using the stove as a resistive load will work only if the switching of the stove knobs does NOT control the triggering of triacs.
Bob S

Hi Bob,
Could you explain that in layman terms please?

If the stove does have triacs I suppose it would be possible to wire the element directly using a SSR to switch it.

 

[BenchTop]

It means don't use any controls in the circuit, just the nichrome resistance filament - the stove element is acting like a 1000W light bulb.
A regular commercial element used below its rated capacity needs no cooling - in fact, you may wish to characterise its resistance at a stable working temperature for the sake of accuracy.

 

[Bob S]

It is possible that the stove top element is 2 heating elements: 350 W and 650W. So a 3-position stove switch could then select 350 watts, 650 watts, and 1000 watts. It could also put the two elements in series for ~230 W.
Bob S

 

[brewnog]

Good point Bob.