Why are there no small turboprops?

Hi George,

Slightly off-topic, why has no-on mentioned diesel engines (that
run on Jet-A1)? This has got to be the way to go, better economy,
better operation at altitude, simpler mechanicals (less to break)
and FADEC/ECU controlled?

Actually, because of the higher compression ratios, the mechanicals
are not simpler.

I know that the forces generated during compression are higher at a
higher compresion ratio, but I cannot imagine that they are higher
than the power forces.
Now whether the power forces (peak) are higher
in a compression ignition engine ... maybe (and probably), but I've
never seen data from conn rod strain gages.

Peak pressure during combustion is about twice as high on a diesel
engine compared to a spark ignition engine (about 160 bar / 2400 psi
vs. 80 bar / 1200 psi). Hence the heavier build of diesels. On the plus
side, diesels run about 200 degC cooler than SI-engines even though
most diesels are turbocharged vs. naturally aspirated gasolines.

However diesels are MUCH more simple in mixture control - there is
none. You just inject the amount of fuel you need to burn to achieve
the desired torque. On gasoline (spark ignition) engines you control
the engine torque by restricting the air flow with a throttle. You then
have to match the fuel flow to the varying air flow pretty precisely.

Also, the entire ignition system, spark plugs, magnets, is omitted on a
diesel. 50% of engine problems on aviation piston engines is related to
ignition problems.

regards,
Friedrich

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