Sunday, 24 February 2008

Another way to test the compressor

Bloody infuriating!

It still refuses to run -just!.

Once I get it spinning and ignite the gas, I can back off the hairdryer until I am standing two feet away, just playing the wimpy little hairdryer on the air intake from that distance. The engine keeps running with just that gentle draught, but if I switch off the hairdryer it slowly winds down.

Now I've run out of propane and have to wait for a delivery.

Later that same day...

It occurred to me that since I know how much airflow it takes (1 hairdryer's worth) to spin my turbine at a particular speed (4000 rpm close-up), if I spin the compressor at that speed, in a perfect engine I should be able to get the same airflow. (In a perfect engine, the massflow produced by the compressor would be precisely equal to the that required to spin the turbine - it's a perpetual motion thingy)

Obviously, the engine isn't ever going to be perfect, but I'll get a figure for the relative 'goodness' - an overall efficiency figure.

So I removed the turbine wheel and set up an electric motor to drive the engine shaft at 4000rpm Then I taped a big plastic bag over the exhaust and switched on the motor.

It took 30 seconds to completely inflate the bag. Motor speed was a little higher than I intended (about 4500 rpm), so call it 35 seconds to compensate for that.

Then I repeated the experiment, using the hairdryer shoved up the air intake instead of the electric motor.

That took 15 seconds to inflate the bag.

So the compressor supplies a little less than less than half the airflow that would be required to make the engine perfect.

That's not as bad as it sounds, an overall 50% efficiency figure would actually be acceptable. If the turbine and compressor are 70% efficient each, that's the resultant efficiency you get (70% of 70% is 49%). According to the books, those are typical figures. It looks as if my gut feeling that it's almost there is bolstered by the measurement - it comes out at 43% in this case.

Dimensionless constant

In turbo theory there is a number of formulae, used for describing compressors and turbines that are known as 'dimensionless'. This is because they correspond to constants in the theory, which can be used to compare and model different designs.

One of these is the 'supply coefficient', which defines the perforance of a compressor in delivering a flow of gas.

It's defined as the radial flow divided by the rotational speed and it's a constant for a given design. Turbocharger compressors typically have values from 0.26 to 0.30

Schreckling gives figures in his book which allow the value of the supply coefficient (Sc) for his compressor to be calculated. It comes out as 0.23.

Sc can be written as : volume flow/(rotational speed * wheel diameter3)

In Schreckling's case, that's 0.1/(1250 * 0.073) = 0.23

I measured the volume of my plastic bag and it came out to 0.1 m3.
I also did several more runs at different speeds and came up with a consistent value of 60 seconds to fill the bag at 33 revs/sec [proportionally, at 132 rev/sec it was 15 seconds].
So my supply coefficient is 0.1/(60*33*0.063) = 0.23

It's absolutely spot on!

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