Natural gas has been extensively used in automotive engines due to its abundance availability, adaptability to existing engine design, cleaner emission and competitive price. However, converting engine to natural gas always results in power drop. One of the most practical solutions is direct fuel injection. Research on natural gas direct injection engines proved that engine power similar to gasoline is achievable but with the penalty of engine retrofitting costs (cooling water jacket and piston crown modification). Spark Plug Fuel Direct Injection (SPFI) offers a cost competitive and a...