|Layout of the On Board Distillation System. Click to enlarge.|
Researchers at the University of Texas have developed an on-board device that dramatically reduces start-up emissions of unburned hydrocarbons.
The on-board distillation system (OBDS) extracts high-volatility components from gasoline and stores them for exclusive during start-up and warming. New tests show that OBDS reduced cranking fuel requirements by 70%, enabled a 57% decrease in catalyst light-off time, cut the emissions of regulated hydrocarbons (NMOG) by 81%, and delivered an apparent 1% increase in fuel economy. The team reports on its work in a paper in Environmental Science & Technology.
In modern vehicles, 60-95% of all HC emissions occur during the first 90 seconds after a cold start.
The primary reasons for this are twofold: low/unknown fuel volatility and poor catalytic converter efficiency. The relatively low volatility of gasoline (10-30% vaporizes upon injection at 20° C) requires the injection of considerably more fuel (typically 8-15 times) than the stoichiometric amount in order to generate a reliably ignitable fuel/air mixture.
Furthermore, the volatility of the fuel is not known a priori by the engine controller (the powertrain control module, PCM), so the PCM is often calibrated to command fueling rates based upon the expectation of the worst-case volatility fuel. This results in an air/fuel mixture that is overly fuel-rich for most starting conditions and a large amount of liquid fuel that will enter the combustion chamber during the first several cycles. Much of the excess fuel exits the engine unburned or only partially combusted.
A conventional three-way catalytic converter will not reach “light-off” temperature (corresponding to 50% HC conversion efficiency) for 30-40 s or more during the FTP drive cycle. Ironically, the period of highest engine-out HC emissions coincides with the period of lowest catalyst efficiency, making the combination of low fuel volatility with poor catalysis the primary cause of high HC emissions during the cold-start and warm-up period.
(This is also a design consideration for plug-in hybrids, as the longer periods of all-electric drive could be accompanied by more engine cold-start events during the day.)
The OBDS originally was designed to address the cold-start problems inherent in E85-fueled vehicles. The OBDS separated the most volatile fractions of gasoline from E85 for use as a starting fuel. The Texas team of Marcus Ashford and Ronald Matthews then developed a version of the OBDS specifically for gasoline vehicles.
The OBDS intercepts fuel from the main tank. Startup-fuel is extracted by distillation and stored in a separate “light ends” tank for subsequent use; the heaver gasoline fraction is routed back to the main fuel tank. Engine coolant provides the heat source for the distillation process; thus, the start-up distillate is best made when the engine is hot.
|Comparing Emissions Indices. The OBDS Navigator has a lower EI (NMOG) than the two ULEV vehicles and is only slightly higher than that of the PZEV Sentra. Click to enlarge.|
The researchers installed this OBDS on a 2001 Lincoln Navigator equipped with a 5.4-liter 32-valve V-8 engine originally calibrated to meet federal Tier I emissions standards.
With the use of the OBDS, the emissions index for the Navigator was lower than a MY 2000 ULEV Honda Accord and Toyota Camry, and only slightly higher than that of a MY2003 PZEV Nissan Sentra. The emissions index (EI) is a measure of the emissions produced with respect to the fuel consumed in the process. The emissions index effectively normalizes mass emissions by engine displacement and vehicle mass, two factors that tend to negatively impact traditional mass/distance emissions measurements.
The research suggests that a smaller vehicle equipped with OBDS may be able to meet SULEV/PZEV tailpipe hydrocarbon requirements “without having to resort to semi-exotic catalyst and engine control strategies.”
The researchers estimate that the system would add about five pounds to a car’s weight and less than $100 to its cost when in full production. The research was funded by Ford Motor Company, the US Department of Energy, and the Texas Advanced Technology Program.
(A hat-tip to Martin!)
“On-Board Generation of a Highly Volatile Starting Fuel to Reduce Automobile Cold-Start Emissions”; Marcus D. Ashford and Ronald D. Matthews; Environ. Sci. Technol., ASAP Article 10.1021/es051950t S0013-936X(05)01950-4