On Maximizing Argon Engines' Performance via Subzero Intake Temperatures in HCCI Mode at High Compression Ratios

Maximizing the indicated thermal efficiency with minimal amount of emissions is one of the main challenges to overcome in the field of internal combustion engines. The main obstacle that hinders achieving this goal is the typically low thermodynamic efficiency which is the ratio of the positive produced work on the piston to the amount of heat released inside the cylinder. Many concepts and technologies were innovated to maximize the thermodynamic efficiency. One of the main guidelines that have been followed to achieve so, is the ideal Otto’s cycle that predicts that increasing the compression ratio and/or the specific heat ratio of the combustion reactants, will maximize the thermodynamic efficiency. This study combines both high compression ratios and a high specific heat ratio via two of the main approaches used to maximize the thermodynamic efficiency. First, is the HCCI combustion mode. HCCI is typically operated at fuel-lean conditions, allowing to operate at higher compression ratios without having intense knock (pressure waves, generated by undesired autoignition, that can damage the engine). Second, air was replaced by an oxygen-argon mixture to increase the specific heat ratio since argon has a relatively high specific heat ratio. It is also an inert gas with a very low chemical reactivity which makes it a good candidate in terms of emissions control. Therefore, combining HCCI mode with argon seems very appealing. However, having a relatively low heat capacity, argon results in very high in-cylinder temperatures which, in turn, leads to severe knock. Kock phenomenon limits the feasible range of compression ratio before causing a serious damage to the engine. Hence, the advantage of having high compression ratios in HCCI is counteracted by the high in-cylinder temperature caused by the presence of argon. The approach introduced in this study in order to combine HCCI at high compression ratios with the use of argon avoiding intense knock, is to cool the intake to subzero temperatures and hence, avoid the too high in-cylinder temperatures and as a bonus, increase the specific heat ratio due to cooling. This study aims to provide a complete analysis of the effects of cooling the intake and increasing the compression ratio in a CFR argon engine modified to operate in HCCI mode. Various diagrams describing the combustion process will be presented. The objective function is the net indicated thermal efficiency. The main outcomes to be investigated are energy distribution and energy losses and their respective effects on the combustion, gas exchange, and thermodynamic efficiencies.