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 approach to onboard fuel reforming is to combust fuel in the engine cylinder under rich conditions, producing a hydrogen-rich reformate gas--which can subsequently be recirculated into the engine. Hydrogen is the preferred product in this process due to its high flame speed and knock resistance, compared with other reformate constituents.
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 valve timing all had no significant impact on hydrogen yield at a given equivalence ratio. Fuel effects on hydrogen yield were more significant--with methanol producing 75% more hydrogen than toluene at the same equivalence ratio. The greater hydrogen yield was due to greater hydrogen content of the fuel, although the benefit was shown to be partially offset by lower hydrogen selectivity and conversion. Production of smoke limited the minimum relative air-fuel ratio of some fuels contributing to reduced hydrogen yields. Upstream water injection was shown to boost hydrogen production by 10-60% (rel.) at the expense of carbon monoxide due to steam reforming reactions and 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. In the presence of water injection, hydrogen production in some cases exceeded fuel hydrogen content indicating the presence of water gas shift and steam reforming chemistry occurring.
Using the speciated exhaust data, a correlation was developed using measured exhaust hydrogen content to predict hydrogen concentration from carbon monoxide and relative air-fuel ratio. The correlation developed improves upon previous correlations by explicitly including the hydrogen content of the fuel, and thus allowing more accurate prediction. Lastly, the energy balance was calculated under rich combustion conditions from the indicated power and chemical potential energy of the reformate. The energy balance analysis suggests that in-cylinder reforming is a net endothermic process, with some exhaust heat being converted into chemical potential energy.
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, in conjunction with water injection, to produce a hydrogen-rich reformate gas without parasitic losses.