Conceptual Model for the Start of Combustion Timing in the Range from RCCI to Conventional Dual Fuel

In the challenge to reduce CO₂, NO_x and PM emissions, the application of natural gas or biogas in engines is a viable approach. In heavy duty and marine, either a conventional dual fuel (CDF), or a reactivity-controlled compression ignition (RCCI) approach is feasible on existing diesel engines. In both technologies a pilot diesel injection is used to ignite the premixed natural gas. However, the influence of injection-timing and -pressure on the start of combustion timing (SOC) is opposite between both modes. For a single operating point these relations can be explained by a detailed CFD simulation, but an intuitive overall explanation is lacking. This makes it difficult to incorporate both modes into one engine application, using a single controller. In an experimental campaign by the authors, on a medium speed engine, the lowest emissions were found to be very close to the SOC corresponding to the transition from RCCI to CDF. This highlights the relevance of having one overall control-scheme for the start of combustion. Principally the start of combustion boils down to having the required equivalence ratio (ϕ) and temperature (T). By using a 1D spray simulation the distribution of ϕ was investigated. Combined with a kinetic scheme and a thermodynamic engine analysis the earliest occurrence of an ignitable ϕ - T combination could be depicted. Graphically shown as an ignitability island during the engine cycle, this gives an intuitive prediction of SOC and its relationship with the start of injection (SOI), rail pressure, and air- and fuel-amount. It also shows the distinction between CDF and RCCI. For CDF the combustion starts when the ignitability island is approached from the rich side. For RCCI it is approached from the lean side. The described relationships could make it possible to incorporate both RCCI and CDF modes into one engine controller.