Frank van der Hulst wrote:

A natural gas powered airplane would be interesting, but the fuel storage
would be too heavy.

I used to work for a company that produces stuff for the alternative
fuel car market here in NZ. CNG used to be very popular here, especially
in the 70s & 80s.

Weight of the fuel isn't a big problem in itself. I have a couple of old
CNG cylinders (I used one to build a compressor) at home... they're not
that heavy. However, CNG has several disadvantages over petrol:

1. It doesn't produce as much power from the same engine
2. A tankful (where the tank is about the same size as the petrol tank
it replaced) gets you about half the distance
3. Getting CNG to the airport would be difficult
4. Ignition timing needs to be different

Because of #2, and because of the relative unavailability of CNG gas
stations, most cars were 'dual fuel'... they had both a petrol tank and
a CNG tank. There was a switchover system, including a 'dual-ignition
controller' to cope with #4. However, this meant that the carb couldn't
be tuned perfectly for CNG, which made point #1 even worse.

Incidentally, the company now develops CNG systems for diesel engines.

Frank.

 
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