Mohamad, T., Yusoff, A., Abdullah, S., Jermy, M. et al., "The Combustion and Performance of a Converted Direct Injection Compressed Natural Gas Engine using Spark Plug Fuel Injector," SAE Technical Paper 2010-32-0078, 2010, doi:10.4271/2010-32-0078.
Compressed natural gas (CNG) has been widely used as alternatives to gasoline and diesel in automotive engines. It is a very promising alternative fuel due to many reasons including adaptability to those engines, low in cost, and low emission levels. Unfortunately, when converting to CNG, engines usually suffer from reduced power and limited engine speed. These are due to volumetric loss and slower flame speed. Direct injection (DI) can mitigate these problems by injecting CNG after the intake valve closes, thus increasing volumetric efficiency. In addition, the high pressure gas jet can enhance the turbulence in the cylinder which is beneficial to the mixing and burning. However, conversion to direct fuel injection (DFI) requires a costly modification to the cylinder head to accommodate the direct injector and also can involve piston crown adjustment. This paper discusses a new alternative to converting to DFI using a device called Spark Plug Fuel Injector (SPFI). It is a combination of a fuel injector and a spark plug which fits into the engine block through the existing spark plug hole. With SPFI, conversion to DFI is simple, cheap and requires no modification to the original structure of the engine, except minor calibration to the ECU. The SPFI was installed on a 0.5 liter single cylinder engine and run at 1100 rpm, WOT and stoichiometric air-fuel ratio. Results showed that engine running with SPFI gained the advantage of significant increased volumetric efficiency, faster burning rate, higher output power and improved fuel conversion efficiency compared to port injection operation in the expense of reduced effective compression ratio.