> Does anyone have any ideas on how to build a relatively low-buck DIY
> engine dyno? I want to examine the expense/feasibility of building one
> myself.
>
> The basic idea would be a water-brake using a jet ski impeller (or
> similar) and some sort of torsional load cell driveshaft. My main
> questions and concerns would be instrumentation. How to setup and
> collect data from a single (or series of) strain gauges on a metal
> shaft and take periodic accurate readings from them while also reading
> shaft RPM. I'm sure it's possible to buy instrumentation for <$200 if
> you look in the right places or >$2000 if you look in the wrong ones.
>
> Makes me wish my degree was in electrical engineering.
>
> -Chris
>
> Water brake dynos heat the water. 800 gallon tanks are typical.
> 250 HP is 186 Kilo Watts. Here is one that heats the air.
>
> CALIBRATING THE DYNO
> Calibrate the torque system with a large torque wrench. With a two
> foot lever arm double the scale reading to get foot pounds of
> torque.
>
> TESTING
> Mount the engine and PSRU with an adjustable pitch prop. Balance the
> rig about the axle shaft so the engine does not rotate on it's own
> and zero is showing on the bathroom scale. Run the engine up to max
> RPM allowed by the pitch chosen. You now know the torque and prop
> RPM so the HP can be calculated. Stop the engine and lower the
> pitch until the engine will run at max HP RPM. Usually around 7500
> RPM for a geared engine and around 2700 RPM for an aircraft engine.
> You now know the max HP. Repeat this process for any RPM of
> interest.
>
> MEASURING FUEL CONSUMPTION
> Make a fuel consumption gage out of a long piece of clear plastic
> hose mounted vertically. Measure how much fuel the hose will hold
> between marks using a chemistry lab calibrated beaker. Time the fuel
> consumption with a stop watch. Use an electric pump and an on/off
> switch to fill the hose again to repeat the fuel consumption test.
> Don't forget to correct for the temp. of the fuel. You now know the
> gallons per hour and the BSFC can be calculated from that
> information and the HP. Brake Specific Fuel Consumption or BSFC is a
> measure of the efficiency of the engine. It is in units of pounds of
> fuel burned for each HP for one hour. A good number for an aircraft
> engine is .45 pounds per HP hour. Some car engines are about .45 to
> .55. Hook up a manifold pressure gage and record that as a function
> of HP. Knowing manifold pressure is very important to be sure your
> engine is developing the maximum possible HP at any RPM.
>
> Omega.com has load cells if you don't want to use a bathroom scale.
>
> Paul Lamar
Hello All/Paul
long time lurker,first time poster here........I've been reading and
stewing about how to build a dyno for a sachs km48 engine for possible
use in a cri cri aircraft? still not sure it is going to work out but
that's what experimenting is all about "a"....anyway i was wondering if
anyone had any thoughts for building a small dyno, say 20hp max. i seen
on Utube a guy had used electric motors as the energy absorption method,
basically a generator. I could get an old generator off craigslist
easily and cheaply enough but I'm not sure if its a good way to go or
not? also as an fyi type thing i seen on ebay there are quite a few
shipping/postal scales available some even have a removable digital
readout that would make for a nice install on the test rig, just
something i seen,maybe a little more scientific than the bathroom scale?
anyway if anyone has ideas on my small scale dyno comments would be
greatly appreciated.
thanks,John Durant
A generator large enough to absorb 20 HP would certainly work.
My guess is it would have to be able to put out about 10 to 12 KilliWatts.
The load could be ten 1.2 KW room heaters or one hundread 100 watt light bulbs.
You could then measure the torque and RPM necessary to drive the generator.
Paul Lamar
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