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High Efficiency, Low Feedgas NOx, and Improved Cold Start Enabled by Low-Temperature Ethanol Reforming
Abstract:
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 NOx, but the
excess oxygen in the exhaust would disable the NOx
reduction function of three-way catalyst (TWC) exhaust
aftertreatment. Use of exhaust gas recirculation (EGR), rather than
lean operation, would preserve the NOx 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.