I've been reading a lot on adding CNG to your diesel. Fascinating stuff! There are some great threads out there including this one on CNGchat.com
Anyway, I've been trying to figure out what engine would be best for CNG, an electronic one or a mechanical. I guess CNG blending can more than double your gas mileage by basically putting CNG in place of it. It's flash point is higher than gasoline (somewhere around 1200' I think). So it basically detonates with the diesel fuel injection. Not only do you get something like 100 hp and better efficiency but good milage too. The tank is cumbersome but if you had a large pickup for example I think you're OK.
According to that thread (towards the end) there are something like 4 or 5 manufacturers making both EPA and "off-road" certified kits for CNG into diesels. It's definitely something I would be into especially if I were commuting...
The way they work is its based on vaccum. CNG is pulled in according to draw from the turbo (or intake for NA engines) before the turbo. At idle it runs on diesel only and then accordingly added depending on how much vacuum you have happening. You can then adjust from 25 to 50/60% safely (I've been reading as much as 80%) and somehow it knocks down your fuel consumption as well...
So that would be the idea, I'd want to get screamin diesel mileage with a smaller CNG tank like a "12 gallon" or so which would mostly be used for commuting to and from work. The CNG directly offsets whatever the diesel consumption would be so a 25% blend would reduce the diesel use by 25% (so a 16 would get 21/22 or so MPG) and your CNG use would be 12 gallons @ 16 mpg total consumption / divided by 4 (or x .25). If that makes sense. But the kicker is it adds horsepower and I've been hearing all sorts of numbers.. And finally, how do you control your diesel fueling? Pretty funny and pretty simple. It's as far as just letting off the throttle.
One thing I've noticed is both electronic and mechanical diesels have "boost readers" that either go directly into the injection pump, or to an electronic sensor that then provides input to the computer.
It is pretty interesting uh?