Fuel and engine effects on rich-combustion products as an enabler of in-cylinder reforming 2019-01-1144
Onboard reforming has been proposed as a strategy for improving spark-ignited (SI) engine efficiency through knock reduction, dilution limit extension, improved thermodynamic gas properties, and thermochemical exhaust enthalpy recuperation. One strategy for onboard reforming is to combust fuel in the engine under rich conditions producing a hydrogen-rich reformate gas, which can be fed into the engine. Hydrogen is favored in this process due to its high flame speed and knock resistance compared with carbon monoxide and methane.
In this work, the effects of engine operation, fuel composition and water injection were evaluated for their effect on reformate gas composition produced under rich combustion conditions. Engine parameters, including intake pressure, intake temperature, combustion phasing, and cam phasing all had no significant impact on hydrogen yield. However, fuel effects on hydrogen yield were substantial with methanol producing 75% more hydrogen than toluene at the same equivalence ratio. This effect was partially due to the greater hydrogen content of the fuel, and partially due to improved hydrogen selectivity. Production of smoke limited the minimum air-fuel ratio of some fuels contributing to reduced hydrogen yields relative to those fuels which were combustion-stability limited under very rich conditions. Lastly, water injection was shown to boost hydrogen production by 10-60% at the expense of carbon monoxide due to Le Chatlier’s principle in the water gas shift reaction. Toluene exhibited the greatest relative improvement in hydrogen yield due to the lower exhaust water concentrations in the absence of water injection. Hydrogen yield in some cases also exceeded fuel hydrogen content indicating the presence of water gas shift and steam reforming chemistry.
Overall, it was concluded that in-cylinder reforming can be used to produce practical quantities of reformate to improve SI engine performance. This work showed the potential for optimized fuels to improve in-cylinder reforming processes--especially in conjunction with water injection--to produce a hydrogen-rich reformate gas without parasitic losses during the combustion process.