Impact of EGR on Combustion Processes in a Hydrogen Fuelled SI Engine

With concerns continuing to grow with respect to global warming from greenhouse gases, further regulations are being examined, developed and are expected for the emission of CO₂ as an automobile exhaust. Renewable alternate fuels offer the potential to significantly reduce the CO₂ impact of transportation. Hydrogen as a spark - ignition (SI) engine fuel provides this potential for significant CO₂ reduction when generated from renewable resources. In addition, hydrogen has advantageous combustion properties including a wide flammable mixture range which facilitates lean burning and high dilution, fast combustion energy release and zero CO₂ emissions. However, the high burning rates and fast energy release can lead to excessive in-cylinder pressures and temperatures resulting in combustion knock and high NOx emissions at stoichiometric operation.

This work examines external Exhaust Gas Recirculation (EGR) as a technique for the reduction of combustion knock and NOx emissions for stoichiometric operation and studies its impact on combustion rates, efficiency, NOx emissions, and other engine operation characteristics. Tests were performed on a single cylinder CFR engine at 900 rpm at a engine load of 410 kPa NIMEP while maintaining lambda (λ) at 1 (stoichiometric operation). Closed loop EGR was estimated and maintained via wide band oxygen sensors placed in the intake and exhaust manifolds. Tests were performed for compression ratios 8, 10 and 12 and various ignition timings. Combustion durations, engine efficiencies, NOx emissions were measured for varying values of EGR over these compression ratios. Comparative results for these tests are provided.

The results show that even low levels of EGR resulted in longer combustion durations and reduced knock intensities. Efficiencies improved with low levels of EGR as we moved away from the knock limits, then reduced because of the longer combustion timings. EGR dilutes oxygen content in intake which finally reduces the combustion rate. This leads to lower in-cylinder peak temperature resulting in reduction of combustion knock and NOx emissions. EGR levels of 0 to 35% by mass were successfully applied.