High Efficiency, Low Feedgas NOx, and Improved Cold Start Enabled by Low-Temperature Ethanol Reforming 2010-01-0621

Two major barriers to wider use of ethanol as an engine fuel are ethanol's low heating value per volume relative to gasoline and higher hydrocarbon emissions at startup. Ethanol provides about one-third lower fuel economy than gasoline on a volumetric basis if the two fuels are utilized with equal efficiency, making ethanol less attractive to consumers. In addition, it is difficult to meet emissions standards such as SULEV when using E85 or hydrous ethanol, because ethanol's low volatility and high heat of vaporization compared to gasoline result in incomplete combustion when the engine is cold.

A catalyst consisting of a copper-plated nickel sponge has recently been developed that enables ethanol to be reformed at around 300°C to a mixture of hydrogen (H₂), carbon monoxide (CO), and methane (CH₄). This low reforming temperature enables heat to be supplied from the engine exhaust. The hydrogen content in the fuel has been previously reported to enable ultralean operation (λ ≻ 2) at high compression ratio (up to 17:1) at part load. As a result, efficiency 40% better than gasoline without dilution was achieved, which would largely offset the fuel economy penalty for ethanol. It would be expected that cold start using stored ethanol reformate, a gaseous fuel, would provide much lower startup hydrocarbon emissions than liquid ethanol or E85.

This paper reports the results of a study performed using a H₂/CO/CH₄ gas mixture simulating the output of a reformer in a single-cylinder research engine at a compression ratio of 14:1. The study addresses the practicality of achieving the desired efficiency improvements in a realistic engine-reformer powertrain. For example, the lean burn strategy used to improve efficiency in the previous study reduced engine-out NO_x, but the excess oxygen in the exhaust would disable the NO_x reduction function of three-way catalyst (TWC) exhaust aftertreatment. Use of exhaust gas recirculation (EGR), rather than lean operation, would preserve the NO_x activity of the TWC. In this study, it was found that use of ethanol reformate enables stable combustion at high cooled EGR rates (30-36%). EGR also provides hotter exhaust than high λ operation, which aids reformer function.

Another question of practical importance is what fraction of the ethanol needs to be reformed in order to maintain high dilution rates. Reforming only a portion of the fuel reduces the demand on the reformer, which would enable a smaller reformer to be installed on a vehicle, reducing cost and reformer startup time. Reforming of 50-75% of the fuel was found to be optimal at 3.5 and 6 bar net mean effective pressure (NMEP) at 1500 and 2000 rpm, although most of the efficiency benefit could be obtained when reforming only 25% of the fuel. At high load (8.5 bar NMEP), reforming is not advantageous.

Engine startup at 25°C using reformate was confirmed to be far smoother than using liquid ethanol fuel and to provide much lower hydrocarbon emissions. The amount of reformate required for startup and catalyst light-off could reasonably be stored on board a vehicle.