Fuel Reforming for High Efficiency and Dilution Limit Extension of Spark-ignited Engines

Abstract

Engine efficiency improvement can help combustion powertrains, which include conventional, hybrid, and plug-in hybrid systems, to meet the strict emissions standards and the increasing demand from customers for performance, drivability, and affordability of vehicles. Cooled exhaust gas recirculation (EGR) can reduce fuel consumption and NOx emissions of gasoline engine systems while keeping the capability of using a conventional three-way catalyst for effective emissions reduction. However, too much EGR would lead to combustion instability and misfire. This thesis identified opportunities to improve efficiency in internal combustion engines by high EGR dilution SI combustion by using thermodynamics-based approaches. This goal has been achieved by using fuel reforming in a thermodynamically-favorable way. Exhaust heat was used to drive endothermic reforming reactions to increase the chemical fuel energy to attain thermochemical recuperation (TCR), a form of waste heat recovery, with robust integrated systems and the regular gasoline. Three strategies for fuel reforming, along with the unique designs of corresponding integrated engine systems, a committed in-cylinder reformer, a catalytic EGR-loop reforming system, and fuel reforming by fuel injection during Negative Valve Overlap (NVO), have been proposed and investigated with unique engine system setups and corresponding experimental and simulation research. The concept and the system to use one cylinder to serve as a committed fuel reformer without spark ignition is first demonstrated. The committed in-cylinder reformer engine system achieves 8% brake thermal efficiency improvement through EGR and cylinder deactivation effects, even though there is low fuel conversion. The novel catalytic EGR-loop reforming integrated engine system was designed and tested. The experiments and thermodynamic equilibrium calculations reveal that the suppression of H2 and CO caused by the enthalpy limitation could be countered by adding small amounts of O2 by running one-cylinder lean. As much as 15 volume % H2 at the catalyst outlet is produced when the fuel and air equivalence ratio is between 4 and 7 under quasi-steady-state conditions. It is also found that this catalytic EGR reforming strategy makes it possible to sustain stable combustion with a volumetric equivalent of 45%–55% EGR, compared to a baseline EGR dilution limit which is under 25%. This catalytic EGR-loop reforming strategy results in a decrease of more than 8% in fuel consumption with significant potentials for even higher brake thermal efficiency. This novel design also opens up a new control method to control the amount of fuel reforming and the fraction of the partial oxidation reaction and steam reforming reaction by adjusting the lambda value of the cylinder which is running lean. Through this design, the engine is serving as an active system, which can also be adapted to respond to the needs of the passive catalyst system so that even better more significant benefit can be achieved. The results demonstrate fuel injection during NVO can extend the dilution limit, improve brake specific fuel consumption (BSFC), and reduce CO and NOx emissions on the engine modified with the capability of variable intake and exhaust valve timing and higher compression ratio. A comprehensive comparison of different reforming strategies for engine application and analysis of critical factors contributing to the performance of integrated fuel reforming engine systems is also provided. The research of this dissertation has demonstrated new pathways and scientific outcomes for technology development of internal combustion engine powertrain systems that can operate significantly more efficiently and cleanly.