CNG / LPG Engines. I know as much about these things as George Bush knows about global economics.
What kind of noise do they make? Can they be easily modified and how? How to do they stand up to their petrol / diesel counterparts?
Richard.
Firstly, let's deal with road cars. 99.9% of alternative fuel cars will be petrol derivatives that have been converted to run on LPG. This can be a factory dual-fuel model, although aftermarket conversions are more common.
CNG is a very different beast, Saab, I believe, offers CNG compatible models now. CNG filling stations in the UK are very rare, LPG ones less so.
Noise wise they will sound just like the standard car running on petrol.
In the commercial HGV arena there's a growing number of operators who supplement diesel with CNG or LPG. This, I believe, is supplied in relatively low ratios compared to air (which contains oxygen, of course) and compressed along with the air. The diesel ignition process is still triggered by accurately timed fuel delivery. The idea is that less diesel is used because the compressed air/CNG provides optimum power with less use of diesel. Such an engine will sound like a diesel engine, although in 6 or 8 cylinder forms the overriding sound is that of the turbochargers anyway.
IF you are looking at the dual fuel aftermarket option then you need to start with a petrol car. Turbcharged ones are much more expensive to modify to operate on both fuels.
Start with a car that you plan to use for at least 100,000 miles to make the investment truly worthy.