What are the advantages of diesel engines?

[puddle]

Please excuse my ignorance but I have no idea. Are they cheaper to run? What about the environmental impact (this is more important to me)

I have a diesel engine and its more expensive than petrol but it does more miles. I can do 120 miles on 20quid.Not sure about the environmental side of it tho.

It's differently bad for the environment.
The price fluctuates.
I run a diesel car and I've never had a clear anwer on issues of price and environmetal impact either.

Dammit! thought I'd get a simple answer without having to go and research myself!