Reforming-Controlled Compression Ignition – a method combining benefits of Reactivity-Controlled Compression Ignition and High-Pressure Thermochemical Recuperation 2019-01-0964
This work investigates a new method enabling full HCCI engine control based on a combination of Reactivity-Controlled Compression Ignition and High-Pressure Thermochemical Recuperation, while the engine is fed with a single primary fuel.
Basically, the primary fuel and water flow through a heat exchanger and a reformer heated with the exhaust gases, where they evaporate and then react for creating a hydrogen-rich reformate. The reformate composition depends on the primary fuel and the reformer design (catalyst, geometry, etc.), and contains some other species in addition to hydrogen. Various primary fuels are suitable for the suggested concept. In the reported study, the primary fuel is dimethyl ether (DME), and the reformer is designed to produce mainly hydrogen, carbon dioxide, carbon monoxide, and water. The applied reforming process is steam reforming of DME supported by a bi-functional (metal and acid) catalyst. Both DME (high-reactivity fuel) and the reformate (low-reactivity fuel) are injected directly but separately into the cylinder. Combustion control is achieved by changing the reactivity/composition of the total fuel supplied into the cylinder. This method enables adequate combustion control in the entire range of operating modes, including transients, due to the separate DME and reformate injection.
This work presents results of 1D and 3D CFD simulations. The 1D model includes the whole engine-reformer system and is mainly intended for energy balance and reformer reaction rate calculations. The 3D model is aimed at the combustion process prediction. A reduced chemistry mechanism is applied. The prediction results show system efficiency improvement up to six percentage points as compared to HCCI engine without High-Pressure Thermochemical Recuperation, while the improvement is higher in naturally low-efficiency regimes. At all considered operating modes a shortage in available enthalpy for the reforming is not observed. Suitability of different injection methods to decrease ringing intensity, and hence to expand the high-load operation limit, is investigated.