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FOCAS® on Emissions Technology
An SwRI-developed apparatus may replace engine-based methods for catalyst aging
By Cynthia Webb and Suzanne Timmons
Southwest Research Institute (SwRI) has developed a new system to rapidly and precisely age catalytic converters at elevated temperatures for extended periods of time, with or without the introduction of oil to simulate engine oil consumption. The patent-pending system, called FOCAS®, is a gasoline-fueled burner with an integrated, computerized control system. FOCAS was designed to realistically simulate the flow of exhaust gas from an engine under a variety of load conditions, allowing full-sized automotive catalyst systems to be rapidly aged.
Catalytic converters became part of the vehicle exhaust system in the mid-1970s to meet emission regulations mandated by the U.S. Environmental Protection Agency (EPA). Since the introduction of the three-way catalyst, engineers and scientists have worked to optimize the performance and durability of these devices to meet increasingly stringent emissions and durability standards.
As catalysts age, their ability to convert exhaust pollutants decreases. Research continues to focus on understanding thermal deactivation and also on formulating improvements to increase the stability of the washcoat, catalyst metals and substrate at higher temperatures, which helps to improve the long-term durability of the device.
Researchers developing new automotive exhaust emissions systems and materials need to create a simulation of the same hot, pollution-laden environment that exists inside a real automobile exhaust for testing and aging of catalysts and sensors. The aging conditions must be controlled to very tight tolerances while the materials, sensors and catalysts are accelerated through their expected service life over days or weeks instead of months or years.
One approach to accelerate the aging of a catalyst is to install the device on an automobile engine on a dynamometer test stand instead of in a vehicle and to run it continuously in a test cell to generate the hot exhaust gases needed to age the system. Engine-based testing is the favored and typical approach used for catalyst aging. However, this approach has limited control over oil consumption (oil is a catalyst poison that contributes to aging). Oil consumption can vary from engine to engine and as the engine wears with age. Additionally, very high aging exhaust gas temperatures (900-1200 degrees Celsius) are difficult to achieve on an engine stand, without additional risk of engine failure and excessive wear (the engine will wear out much faster under high load).
The FOCAS system consists of five main subsystems: a gasoline-fueled burner, capable of continuously operating at rich, lean and stoichiometric air-fuel ratios (AFR); an oil injection subsystem that allows the emission control system (such as catalyst) to be exposed to a precisely controlled amount of lubricating oil within the exhaust stream; a blower; a heat exchanger; and a computerized control system.
The computer controls AFR by modulating the fuel delivered to the injector under an open- or closed-loop control configuration. Burner operation can be adjusted to achieve exhaust gas flow rates ranging from 9.4 to 37.8 liters per second (20 to 80 standard cubic feet per minute) and is stable from AFRs ranging from 10:1 to 18:1 (using a lean-burn injector, FOCAS can achieve AFRs to 30:1).
The FOCAS system software provides the user an interface to develop catalyst-aging cycles, to investigate the effects of oil exposure on catalysts and to run a variety of accelerated thermal catalyst aging cycles (with or without oil addition). The software has a preprogrammed, cold-start simulation mode that allows researchers to duplicate the effects of vehicle cold-start that can be incorporated into an aging cycle test. The catalyst inlet temperature can be adjusted from 400 degrees to well over 1,000 degrees C, allowing for both low- and very high-temperature aging. The FOCAS system has been designed to operate continuously at stoichiometric AFR without damage to the system.
The FOCAS system has undergone a thorough failure modes and effects analysis (FMEA) to identify every potential failure mode conceivable to designers. A safety system has been created to address these failure modes and incorporated to monitor and control the system to a safe shutdown if an accident or malfunction that could damage a test catalyst or test facility is detected.
Equivalence in thermal aging
In theory, thermal aging is predominantly a function of the amount of time spent at a temperature and the AFR to which a catalyst is exposed during aging. The effects of thermal aging increase as the aging temperature increases. Because the mathematical relationship between aging temperature and aging effect is exponential, governed by the Arrhenius Rate Law, researchers can simulate the effects of high-mileage accumulation in a short time by elevating aging temperatures. SwRI researchers reasoned that if a burner system could be used to simulate the exhaust AFR and catalyst inlet and bed temperature profiles that are generated by an engine running an accelerated aging cycle, then a catalyst should thermally age at the same rate and in the same manner when exposed to either exhaust stream.
Using internal research funds, the SwRI team sought to develop a control method for the FOCAS rig to allow the gasoline-fueled burner to simulate the exhaust temperatures, flow and AFR created by an engine during an accelerated thermal aging procedure. The validation portion of the study examined the aging differences seen among six production catalysts, aged using a published accelerated thermal aging method.
The team's approach was to age three catalysts using a gasoline-fueled engine, and three others using a gasoline-fueled burner. The engine was configured to run the aging cycle according to published specifications, while the burner was programmed to create the exhaust conditions that simulated the engine test cycle specifications, providing the same elevated temperature, AFR profile, catalyst space-velocity conditions and bed temperature profile.
Each catalyst's performance was measured at the beginning and at the conclusion of the aging. The performance degradation was then calculated and compared between the two methods. The results showed that the two methods produce statistically equivalent aging effects. A post-mortem analysis then was performed on each catalyst. The tests verified that FOCAS aging provided thermal aging in the absence of non-thermal aging (in this case, oil deposits), thereby creating a means for the definitive isolation of thermal and non-thermal aging effects. Overall, it was found that the FOCAS burner system provided a flexible means for simulating an engine aging cycle and produced thermal aging results similar to the engine aging method.
The impact of oil on catalyst aging is typically an uncontrolled factor due to the variation in oil consumption of the aging test engines. Engine oil contains zinc and phosphorus in the form of zinc dialkyldithiophosphate, an engine anti-wear additive. The phosphorus from this additive is a catalyst poison because it tends to form deposits on the catalyst surface. FOCAS offers consistent oil consumption rates and characteristics. The main impact of non-thermal aging (phosphorus and zinc deposition) on the catalyst surface typically manifests itself as delayed catalyst light-off times and lower temperatures as the poisons tend to accumulate on the inlet face of the catalyst and block active catalytic sites. Thermal aging typically reduces the catalyst surface area, which reduces the total surface area of the active catalytic sites and results in lower conversion efficiency for all regulated emissions, with oxides of nitrogen (NOx) typically being impacted most significantly.
The FOCAS apparatus has also been used to develop oil-aging cycles. Under a separate study, a catalyst oil-poisoning procedure for evaluating and differentiating the effects of engine oil formulations on catalyst performance was developed. The nonthermal aging procedure that was developed utilized the FOCAS oil injection subsystem in conjunction with the burner. The oil system precisely meters the consumption rate and oxidation state of the oil (unburned, partially burned or fully burned) that is delivered to the catalyst.
There were four main components to this study: developing an aging cycle that included a variety of operating modes, including a cold-start simulation; using this cycle to age several catalysts using two oils that have field-proven performance differences; evaluating the FOCAS-aged catalysts to assess the impact of the two oils; and analyzing the oil deposits of the aged catalysts and comparing them to field-aged catalysts.
During the test, four catalysts were aged and tested using the two candidate oils. All the catalysts were emissions-tested at the conclusion of aging and the performance degradation of each was ranked and compared to the field-aged catalysts. The performance degradation observed on these catalysts ranked the same as the field-aged catalysts. A chemical analysis was then performed on the catalysts to assess the phosphorus loading and to profile the deposits on each catalyst.
The low-phosphorus oil profiles compared very well, matching the levels measured on the field-aged catalysts (presented in an earlier study) very closely. The high-phosphorus oil profile comparison showed more deposit on the FOCAS aged catalyst than the field-aged catalyst. However, when these profiles were normalized, the shape of all the phosphorus profiles proved to be similar. Therefore, under this study, FOCAS was successfully used to differentiate the emissions impact of two different engine oils and was able to simulate the oil deposition profile measured in the field.
Using FOCAS simplifies the lab preparation procedures to reduce inconsistencies from lab to lab and test to test in such factors as fuel properties, mass air flow, exhaust AFR, oil injection rate, distance from oil injection point to catalyst inlet, exhaust temperature at the oil injection point, catalyst inlet temperature, and catalyst bed temperature during the thermal excursion.
FOCAS is currently undergoing refinement in preparation for production and sale. Some of the technical goals currently being addressed include:
Comments about this article? Contact Phil Weber at (210) 522-5872, or Phil.Weber@swri.org. Additional information is available in the Society of Automotive Engineer papers, SAE 2003-01-0663 and SAE 2003-01-1999.
Published in the Winter 2004 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.