Westport Innovations uses diesel fuel to help ignite natural gas with outstanding results.                     

    High pressure direct injection (HPDI) cuts NOx emissions by 40%, particulate matter by 80%, and has the same thermal efficiency as good old diesel.

Basically the injector takes in 1 part diesel fuel and 9 parts methane (natural gas or CH4) and combines the mixture  in the cylinder at high pressure. Since diesel fuel has a lower octance than natural gas it ignites first making a spark for the CH4.

This process is similar to CNG fumigation, used on Jeff Dean's Chevy, in the 2008 Diesel Power Challenge.  Although Westport's injector gives the engine more control to manipulate when the different fuels should enter the combustion chamber.