I have seen LPG powered petrol cars which can run on both the fuels. But I have not seen any Diesel engine car running on LPG. Is it possible to run a diesel engine on LPG?
Yes, but not very easily or effectively. To convert to 100% lpg means reducing the compression ratio and providing a spark. The lower compression ratio takes away diesel as a fuel and providing a spark means introducing a spark plug into the head - with machining issues : space, depth etc.
Using lpg with diesel - called fumigation can improve deisel economy but the cost of the lpg equipment is also a factor.
As the diesel engines are designed with a lower rev range compared to petrol engines, it seems obvious why it is not popular....
More info here : http://www.go-lpg.co.uk/diesel.html