Microwave Enhancement of Lean/Dilute Combustion in a Constant-Volume Chamber 2019-01-1198

High dilution engines have been shown to have a significant fuel economy improvement over their non-dilute counterparts. Much of this improvement comes through an increase in compression ratio enabled by the high knock resistance from high dilution. Unfortunately, the same reduction in reactivity that leads to the knock reduction also reduces flame speed, leading to the engine becoming unstable at high dilution rates. Advanced ignition systems have been shown to improve engine stability, but their impact is limited to the area at, or very near, the spark plug. To further improve the dilute combustion, a system in which a microwave field is established in the combustion chamber is proposed. This standing electric field has been shown, in other applications, to improve dilution tolerance and increase the burning velocity. The purpose of this study is to determine if a standard microwave generator can be used to couple microwaves into an engine-like combustion chamber, and if it can determine the level of combustion enhancement available at dilute conditions.

For this application, a monopole antenna was designed to couple 2.45-GHz microwave pulses into a constant-volume combustion chamber, creating standing waves that enhanced the combustion of air/isooctane mixtures ignited by a spark plug. Microwave-enhanced combustion (MEC) was quantified by heat-release rate, duration of different combustion phases, and cyclic variability for many combinations of pulse energy, microwave timing, initial pressure, air/fuel equivalence ratio (AFR), and degree of exhaust-gas recirculation (EGR). MEC reduced flame-development time by 24% for AFR = 0.8 and 0% EGR. MEC also effectively improved combustion stability for both lean and stoichiometric mixtures with 20% EGR.