Vehicle and Drive Cycle Simulation of a Vacuum Insulated Catalytic Converter

Vehicle and Drive Cycle Simulation of a Vacuum Insulated Catalytic Converter Rohil Daya 9 th November 2015

Introduction The drive to control automobile emissions began with the enactment of the first emissions regulation by California in 1959, and there has been a progressive reduction in the statutory limits for tailpipe emissions [1,2] Furthermore, it has been shown in [3] that for the average drive distance (trip distance) of real-life traffic, nearly all of the CO and HC exhaust emissions for gasoline engines occur during the cold-start phase Since tailpipe emissions are inversely related to converter temperature, several techniques have attempted to promote catalyst light-off by supplying heat Techniques like the Electrically Heated Converter (EHC) and fuel fired burner systems require some form of power input One of the most important alternatives in this regard is the Vacuum Insulated Catalytic Converter (VICC) 2 11/2/2015 Contains Proprietery Information

Introduction This technology uses vacuum insulation, metal bellows, radiation shields and phase-change material (PCM) to keep the converter above light-off temperature following long periods of cold-soak The VICC was originally developed at the National Renewable Energy Laboratory (NREL). Analysis by NREL and Benteler Automotive [4,5] demonstrated the heat retention capacity of the VICC, with NMHC and CO emissions reductions of 66% and 65% respectively 3

Introduction In this research, a VICC-type converter has been modeled and simulated in GT-SUITE This aftertreatment model simulated drive cycle runs followed by long periods of vehicle soak A vehicle-aftertreatment model was developed to simulate the VICC for a variety of drive cycles, ambient conditions and vehicle types A HEV-aftertreatment model was also modeled and simulated to analyze the benefits of the VICC This simulation is an attempt to emphasize the ability of the VICC to achieve real world cold-start emissions reductions 4

Cold-Start Converter Modeling Standalone Aftetreatment Model 5

Cold-Start Converter Modeling Standalone Aftetreatment Model 6

Cold-Start Converter Modeling Standalone Aftetreatment Model Section-II consisted of two stainless-steel converters placed in series The PCM used for the present simulation was a Magnesium-Zinc- Aluminum (Mg-Zn-Al) metal eutectic alloy The latent heat capacity of the PCM was incorporated into the specific heat using the effective heat capacity method [7] Some properties of Section-II of the VICC are listed below 7

Cold-Start Converter Modeling Standalone Aftetreatment Model Several assumptions were made in the model development to simplify the analysis while capturing the key heat transfer effects: 1. The metallic bellows present in the prototype to increase the conduction heat path were modeled as straight pipes 2. The dimensional change of the phase-change material (PCM) on melting/solidification was not considered 3. The thermal properties of the PCM were assumed to undergo a step change during melting/solidification 4. The copper foil radiation shield was modeled implicitly as a reduction in the surface emissivity of the stainless-steel material above which it was present (from 0.1 to 0.05 for simplicity) 8

Cold-Start Converter Modeling Steady-State Vehicle-Aftertreatment Model 9

Cold-Start Converter Modeling Steady-State Vehicle-Aftertreatment Model Vehicle-aftertreatment models were developed to analyze the VICC over different drive cycles and ambient conditions While the transient model relied on second-by-second data over an FTP, the steady-state model calculated these inputs using engine maps The drive-cycle speed (in km/hr) was imposed using the vehicle kinematic analysis (VKA) object This model used an imposed drive cycle vehicle speed to back calculate engine speed and load, a method known as backward kinematic analysis The engine-state component was modified to introduce the appropriate fuel properties and steady-state maps. For stop-start vehicle simulations, the engine-state was reconfigured to introduce fuel shutoff An identical model was created for ceramic conventional converters 10

Cold-Start Converter Modeling HEV-Aftertreatment Model 11

Cold-Start Converter Modeling HEV-Aftertreatment Model Hybrid Electric Vehicles (HEVs) represent an important field of application for the VICC, since they experience periods of engine shut and catalyst cooldown during vehicle operation The base model used as a starting point was a power-split HEV configuration with steady-state engine maps No modifications were made to the default powertrain configuration in the model The engine-state was modified and connected to the aftertreatment subassembly in an arrangement identical to the conventional vehicleaftertreatment model 12

Simulation Results and Discussion Steady-State Vehicle-Aftetreatment Model Steady-state engine-out maps for flow rates, emissions and exhaust temperatures were obtained from an automotive OEM The figure below shows the substrate outer wall temperature variation during the FTP. The reduced temperature drop for the VICC during idle periods of the FTP demonstrated the ability of the vacuum-pcm insulation to accumulate thermal energy during converter warmup 13

Simulation Results and Discussion Steady-State Vehicle-Aftetreatment Model The increased engine-out temperatures associated with naturally aspirated engines led to faster melting of the PCM, and simulation results showed that the PCM had fully melted by the end of the first FTP The figures below show the radial and axial temperature distribution inside both converters near the end of the FTP prep-cycle 14

Simulation Results and Discussion Steady-State Vehicle-Aftetreatment Model The figure below shows the convertor cooldown curves during soak simulations. The cooldown rate was calibrated to 27.5 C/hour, as was obtained from experimental investigations in [5] Rough estimates of vehicle soak times for common activities (such as grocery shopping, going to the movies etc.) are included in the figure 15

Steady-State Vehicle-Aftetreatment Model Following prep-cycle and soak simulations, the tailpipe emissions for the VICC were examined on different kinds of driving conditions Three parameters were varied to analyze the behavior of the VICC for a range of conditions. They included: a) Drive Cycles b) Ambient Conditions c) Vehicle Types Simulation Results and Discussion In order to prevent an unnecessarily large number of simulations, a reference case was required. This was taken as a conventional vehicle on an FTP drive cycle with an ambient temperature of 21 C All emissions results were compared with results obtained for a conventional ceramic converter 16

Simulation Results and Discussion Vehicle-Aftetreatment Model: Drive Cycle Varations Five different drive cycles were considered for emissions data analysis The figure below shows the substrate outer wall temperature variation during drive cycle runs following 24-hour vehicle soak. The influence of drive cycle speed profiles becomes significant following long periods of vehicle soak, since the VICC falls below light-off temperature and will be slower to warmup as compared to conventional converters 17

Simulation Results and Discussion Vehicle-Aftetreatment Model: Drive Cycle Varations Following 12 hours of vehicle soak, CO, HC and NO x emissions improved by 92%, 61% and 84% respectively over the first bag of the FTP High speed cycles like the HWFET and US06 gave lower % point improvements due to the increased rate of temperature increase for the ceramic CC, and correspondingly faster light-off 18

Simulation Results and Discussion Vehicle-Aftetreatment Model: Ambient Conditions Following 12 hours of vehicle soak, the benefits of the VICC were seen for all ambient temperatures, and the % points improvement in emissions was nearly identical for all cases The influence of ambient conditions was significant once the converter fell below light-off temperature for a particular exhaust species 21

Simulation Results and Discussion Vehicle-Aftetreatment Model: Vehicle Types The benefits of VICC s thermal storage capacity should be seen whenever the converter temperature drops during vehicle operation. This is the case for stop-start vehicles at idle and hybrid electric vehicles (HEVs) in electric mode operation The figure below shows the FTP prep-cycle warmup curves for the HEVaftertreatment simulation 24

Simulation Results and Discussion Vehicle-Aftetreatment Model: Vehicle Types Simulation results suggest that the VICC does relatively worse for stopstart vehicles as compared to conventional vehicles HEV simulations following 12 hours of vehicle soak showed that the VICC did noticeably better than a ceramic CC. FTP first bag emissions reductions of 98%, 72% and 93% were seen for CO, HC and NO x 25

Conclusions and Future Work 1. One FTP prep-cycle was enough to melt the PCM for simulations using engine maps representative of a naturally aspirated engine 2. The vacuum-insulation and PCM outer layers were successfully able to keep the converter above 300 C for at least 12 hours of vehicle soak. 3. Due to the high thermal mass of the PCM, the VICC was noticeably slower in reaching light-off temperatures, and this led to increased baseline cold-start emissions 4. For all simulated drive cycles, CO and HC emissions were reduced significantly following 12-hour vehicle soaks 5. As long as the VICC remained above light-off temperature following vehicle soak, the benefits in emissions were identical for a range of ambient temperatures. Following 24-hours of vehicle soak, the VICC did relatively worse for lower ambient temperatures 28

Conclusions and Future Work 6. Stop-start vehicle operation was not affected by the VICC 7. The heat retention capacity of the VICC led to higher emissions reductions for HEVs compared to conventional vehicles following 12- hour vehicle soaks Following on from this work, detailed analysis of real world emission benefits of the VICC will be performed. A range of driving data will be analyzed for soak time distributions and average number of soaks per driving cycle. This kind of information will be useful in determining, among other things, the probability that the PCM will melt early enough in the driving cycle to give significant reductions in emissions 29

References 1. Eastwood, Peter. Critical topics in exhaust gas aftertreatment. 2000. 2. Mondt, J. Robert. Cleaner Cars: The history and technology of emission control since the 1960s. 2000. 3. Weilenmann, Martin, Jean-Yves Favez, and Robert Alvarez. "Cold-start emissions of modern passenger cars at different low ambient temperatures and their evolution over vehicle legislation categories." Atmospheric environment 43, no. 15 (2009): 2419-2429. 4. Burch, Steven D., Matthew A. Keyser, Chris P. Colucci, Thomas F. Potter, David K. Benson, and John P. Biel. Applications and benefits of catalytic converter thermal management. No. 961134. SAE Technical Paper, 1996. 5. Burch, Steven D., and John P. Biel. SULEV and Off- Cycle Emissions Benefits of a Vacuum-Insulated Catalytic Converter. No. 1999-01-0461. SAE Technical Paper, 1999. 6. "Transportation Secure Data Center." (2015). National Renewable Energy Laboratory. Accessed September 11, 2015: www.nrel.gov/tsdc. 7. Lamberg, Piia, Reijo Lehtiniemi, and Anna-Maria Henell."Numerical and experimental investigation of melting and freezing processes in phase change material storage." International Journal of Thermal Sciences 43, no.3 (2004): 277-287. 33