A Study on the Performance and Emissions of HCCI Oxy-Fuel Combustion in a CFR Engine with Recirculated Carbon Dioxide

Stringent emission regulations and the anticipated climate change call for a paradigm shift in the design of the conventional internal combustion engines. One way to combat this problem is oxy-fuel combustion in which the combustion products are mainly water vapor and carbon dioxide. Water vapor can be easily separated by condensation and carbon dioxide is then easily captured and stored. However, many technical challenges are associated with this mode of combustion. There are many challenges facing oxy-fuel combustion before it find its way to commercial production especially for internal combustion engines. One such challenge is the relatively high temperature of the oxy-fuel combustion. A solution to this problem is the recirculation of the generated CO₂ to moderate the in-cylinder temperature. Therefore, careful study of the effect of recirculating the CO₂ back to combustion chamber is needed before the implementation of such a concept. This study is a continuation of a previous work published by the authors. In the previous study, the performance and emissions of an oxy-fuel HCCI engine were investigated. The in-cylinder temperature was moderated by supplying the engine with fresh CO₂ from gas cylinders.

In this present work the performance of the same oxy-fuel HCCI engine is investigated but the difference is that the in-cylinder temperature is regulated by recirculating the generated CO₂. The experiments were conducted in a variable compression CFR engine that was modified to operate in oxy-fuel mode with recycled carbon dioxide. The emissions were also measured using an FTIR exhaust analyzer. The results showed that the implementation of this concept is possible. However, the indicated thermal efficiency deteriorates due to lower combustion and gas exchange efficiencies. Also, the CO₂ recirculation resulted in higher CO and unburned hydrocarbon (UHC) emissions.