Yes it can ...
We did some experimentation on this 10 years ago using injection of LPG vapour up to around 10% into air intake.
Lots of associated problems though. Freezing .. Over-running ..
The efficiency and mpg was definitely improved though marginally. Even 1% makes an impact with commercial truck companies.
There is not enough well-developed equipment around to make it work properly tho. Plus, the equipment has difficulty in the shake-down conditions in commercial truck operation.
The USA have a bit of a scene going ... .. these used to be the folks ...
http://www.bullydog.c...odline=other_products