City Disel powered Power Generating Plants have been run on Natural Gas.

Since they are big Engines and use a lot of Fuel connecting them to the Cities Gas Pipes makes a lot of sense.

As stated in another post sometimes a small amount of Diesel Fuel still has to be used as a Pilot Fuel to ignite the Natural Gas.
This is genreally done where the Natural Gas comes in with the intake Air.

Otherwise the Natural Gas needs to be compressed and injected into the Cylinder.
The above is what I was told in Tade School back in 1973.

Of course technology is different now.

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84 300D, 82 Volvo 244Gl Diesel