Sankesh, D. and Lappas, P., "Natural-Gas Direct-Injection for Spark-Ignition Engines - A Review on Late-Injection Studies," SAE Technical Paper 2017-26-0067, 2017, doi:10.4271/2017-26-0067.
Significant research has been made on traditional pre-mixed charge Spark-Ignition Natural-Gas engines which have seen widespread usage across the automotive sector. Many researchers including those in industry are now exploring the Direct-Injection concept for Natural-Gas Spark-Ignition engines. Direct-Injection has significant performance benefits over port-fuel injection, primarily due to increased volumetric efficiency as a result of injecting the fuel after intake valve closure. This could lead to enhanced driving performance over port-fuel injection comparable to gasoline engines. Furthermore, Direct-Injection with increased compression ratio in conjunction with downsizing concepts has the potential to increase thermal efficiency while exhibiting significantly lower CO2 emissions. Advanced combustion strategies like stratified mixture combustion has been widely studied for gasoline and proven to increase the low load thermal efficiency over homogeneous stoichiometric combustion. If successfully implemented, similar benefits can be expected with Natural-Gas as well. However, currently there is no Direct-Injection Natural-Gas vehicle in production and there are gaps in the literature as research in this area is relatively new. Mixture formation, ignition and combustion processes are not fully understood. Nevertheless, attempts have been made to explore the potential of stratified combustion. This paper provides a comprehensive literature review on such studies and highlights the challenges involved in developing such combustion systems.