Cold Start Hydrocarbon Emissions Research Articles

Electrically Heated Catalysts for Hybrid Applications: Mathematical Modeling and Analysis

In view of the significant cold-start hydrocarbon emission reduction potential of the electrically heated converter (EHC) technology for conventional stoichiometric gasoline engines, there is considerable interest in better understanding of the thermal and emission performance characteristics and optimizing the design/operating aspects of an EHC system as applied to plug-in hybrid electric vehicles (PHEVs) and extended-range electric vehicles (EREVs). The application of the EHC technology to these hybrid vehicles is unique in that catalyst cooling to below reaction temperatures can occur during extended periods of electric vehicle driving (with engine off) or during intermittent engine stops/starts, and the EHC can be heated prior to engine start (preheating) for enhanced emission reduction. In this study, the design aspects and heating strategies of an EHC system have been analyzed using a transient monolith converter model which accounts for the resistive heating of an inert metal−substrate monolith placed ahead of a conventional three-way catalytic converter. The results of model calculations presented here quantify the effects of various heating strategies on the emission performance of hybrid vehicles during the first 250 s of the Federal Test Procedure (FTP) drive cycle. It is also shown that there exists an optimum electric heater volume for cases with either preheating only or a combination of pre- and postheating. For the latter case, the emission performance can be further improved by adding a smaller electric heater (downstream of the existing heater) which is capable of heating the gas rapidly and efficiently during postheating.

Recent Solutions for the Abatement of Hydrocarbon Emissions During the Cold Start of Light Vehicles

During the last twenty years the number of patent publications in the field of the control of hydrocarbon compounds during the cold start period of a gasoline engine has strongly increased and seems now that the number of patents tends to increase during the following years. This fact has been related with increasingly stringent legislations all over the world which highlights the importance of developing efficient systems for the control of cold start hydrocarbon emissions. The technologies that show a higher number of patents are the hydrocarbon trap, the close-coupled catalyst and the electrically heated catalyst.

The effects of modified ignition timing on cold start HC emissions and WOT performance of an LPG fuelled SI engine with thermal barrier layer coated piston

In this study, the effect of modified ignition timing of an LPG fuelled spark ignition engine on cold start Hydrocarbon emission (HC) at the wide open throttle (WOT) conditions performance and emission characteristics were investigated when using a thermal barrier layer (TBL) coated piston. The engine was tested at the variable ignition timing. At the first stage cold start HC emission was measured for the first 180 s. Secondly, the performance and HC emissions were measured at the WOT condition. Cold start HC emission in the TBL piston has been decreased considerable. In the case of using TBL piston and variable ignition timing, at the high engine speed the engine torque and brake power have been increased as 15% at the 2700 rpm. Specific fuel consumption (SFC) was decreased at the variable ignition timing expect for 1500 and 1800 rev min-1. Maximum reduction was observed by 8.5% at the 2700 rpm. Key words: Hydrocarbon emissions, cold start, thermal barrier layer, liquefied petroleum gas.

English

The population of spark-ignition vehicles in urban areas is very high and is increasing rapidly due to their convenience for short distance transportation. These vehicles are major sources of urban air pollution compared to vehicles with diesel engines. Catalytic converters are used to control their emissions but they attain their maximum conversion rates of about 80%–90% under optimum operating conditions and are not effective during cold start conditions. The objective of the present work is to demonstrate that an electrically heated catalyst (EHC) in combination with a traditional converter can achieve the Low and Ultra Low Emission Vehicle (LEV, ULEV) standards. Experiments were conducted to investigate the impact of various metal oxides in EHC and design parameters to reduce cold-start emissions of a multi-cylinder SI engine. It is observed that EHC reduces cold-start hydrocarbon and carbon monoxide emissions when used with an existing catalytic converter. The maximum CO and HC reductions were achieved with copper oxide as the catalyst in EHC with air injection of 80 lpm for 40 sec after cold start of the engine.

Hydrocarbon storage modeling for diesel oxidation catalysts

A simple hydrocarbon (HC) storage model was proposed which represents the adsorption–desorption processes in zeolites incorporated in a diesel oxidation catalyst (DOC). Using four experiments in which the HC was stepped up and the reactor run until steady state, a Langmuir isotherm was generated that was sufficient to represent the equilibrium data, and the remaining rate constant capturing the adsorption time scale was obtained by fitting. The model thus developed was validated using reactor data which was obtained by stepping the inlet HC concentration to zero after outlet reached equilibrium. A typical DOC which contained both a storage component (zeolite) and noble metal component (for oxidation) was studied using a full scale 1D reactor model which includes the storage kinetics developed here with the oxidation kinetics developed in our previous work [ Sampara et al., 2008. Global kinetics for a commercial diesel oxidation catalyst with two exhaust hydrocarbons. Industrial & Engineering Chemistry Research 47, 311–322]. Simulations of a simplified warm-up process indicated that the zeolite storage component reduces the overall cold start HC emissions by at least a factor of two if the warm-up rate achieves 45 – 65 ∘ C min - 1 , a range commonly observed during start-up. Modeling results also showed that the HC oxidation for these reactors commonly starts at the rear end of the reactor due to reduced CO inhibition. The rates of the individual processes during cold start were analyzed in detail and compared with the rate of inlet temperature increase provided from the exhaust.

Sub-ambient cold-start emissions reduction using SwRI® PO x catalyst technology

This paper describes the application of SwRI’s cold-start PO x catalyst technology to reduce cold-start hydrocarbon emissions from a US Tier 2 vehicle at −7 °C. A reduction in −7 °C (20 °F) cold-start hydrocarbons will help US Tier 2 vehicles meet the proposed EPA NMOG standards. Improvements in cold temperature hydrocarbon emissions would also be beneficial in many parts of Europe during the winter months. In this work, a total hydrocarbon reduction of 19% was realized at 24 °C, in line with previous results, but only up to 3% at −7 °C. Insufficient oxygen in the engine-out exhaust gas at −7 °C was determined to be the reason why the PO x catalyst failed to significantly reduce HC emissions. Addition of supplemental oxygen to the exhaust during the cold-start, to simulate an adjustment in the engine calibration to less rich operation, resulted in a total hydrocarbon reduction of 18% with the PO x catalysts in place, but no benefit when the PO x catalysts were removed. Hence, the PO x catalyst approach can be used to good effect, even under sub-ambient cold-start conditions.

Alternative Catalytic Approach for Reduction of Cold-Start Hydrocarbon Emissions

This paper describes an alternative approach to reduction of cold-start hydrocarbon emissions that allows the engine to operate under rich conditions for stable operation, and makes use of that rich condition to reduce overall hydrocarbon emissions. The approach requires only the addition of a small catalyst with particular catalytic properties. No additional hardware is required, and secondary air pumps, hydrocarbon adsorbers, electrically heated catalysts, heat storage devices, etc. can be removed from the vehicle. Reductions of 30–40% in hydrocarbon emissions were realized over the US FTP-75 test, without changes in calibration, when compared to the base catalyst systems.

Real-world and type-approval emission evolution of passenger cars

The investigation of several passenger car generations with gasoline engines shows that the emissions depend very strongly on the driving cycle. Official type approval cycles allow just very inaccurate predications about their real-world emissions. The measured gasoline vehicles have up to factor 11 higher real-life emissions than in type approval cycles. However, a clear reduction of real-world emissions can be seen over the different investigated generations of gasoline cars. In addition, it can be seen that the cold start emissions depend strongly on ambient temperature levels for all generations of cars and that the cold start accounts for an increasing part of the total pollutant emissions. As an extreme example, the cold start hydrocarbon emissions of Euro-3 cars at -20°C ambient temperature correspond approximately to those of 1,000 km driving with warm engines.

One-dimensional zeolites as hydrocarbon traps

Several zeolites with varying pore dimensionality have been investigated as hydrocarbon (HC) traps by studying the temperature programmed desorption of propane and toluene mixtures. In one-dimensional (1-D) zeolites, the diffusive motion of propane is blocked by the more-strongly adsorbed toluene thus raising the desorption temperature of propane above its single-component desorption temperature. In the 1-D zeolite EUO, propane is trapped within the pores by the toluene and is only able to desorb to the gas phase once toluene begins to desorb. This leads to an increase in the propane desorption temperature of 35 °C over the single-component value. In the 1-D zeolite mordenite, propane desorbs in the presence of toluene at an intermediate temperature that is 100 °C larger than the single component case. Toluene then desorbs at an even higher temperature. It is suggested that propane undergoes an activated process of passing toluene within the 1-D pores that requires the intermediate temperatures. This HC trapping is not observed in the three-dimensional pore zeolites ZSM-5 or Y. Propane in the binary mixture with toluene in these zeolites desorbs at the same temperature as in the single component cases, suggesting that propane is able to diffuse around the more-strongly adsorbed toluene due to the connectivity of the pores. Therefore, it is the one-dimensionality of the zeolites that allows the trapping of propane to higher temperatures in the presence of the more-strongly adsorbing toluene. This phenomenon may find application in reducing cold start HC emissions from automotive exhaust.

Effects of reformer gas addition on the laminar flame speeds and flammability limits of n-butane and iso-butane flames

One of the many difficulties associated with reducing unburned hydrocarbon emissions during a cold-start from port-fuel injected engines is that the engine must be operated fuel rich to avoid partial burns and obtain suitable idle quality. If the dilution tolerance of the engine can be enhanced during the cold start, it would be possible to operate closer to stoichiometric or even fuel lean and thereby obtain significant reductions in hydrocarbon emissions. The overall objective of this study is to computationally explore the feasibility of extending the dilution limit of an engine during cold-start conditions by adding reformer gas to the inlet manifold of the engine. Because the gas mixture produced by reforming a hydrocarbon fuel contains large amounts of H 2, its addition to the hydrocarbon/air mixture holds the potential to enhance both the ignitability and flammability. n-Butane and iso-butane are chosen as surrogate hydrocarbon fuels because of their relevance to gasoline. In addition, comprehensive chemical kinetic models are available for these fuels. Numerical computations of the effect of reformer gas addition on the laminar flame speeds, lean flammability limits, and auto-ignition delay times of butane/air mixtures are performed with detailed chemistry and transport properties. Results show that addition of a first-stage reformer gas, H 2/CO/N 2 with volume percentages of 30/25/45, can increase the flame speed, extend the lean flammability limit, and reduce the ignition delay of butane/air mixtures over a wide pressure range. Although the second-stage reformer gas, H 2/CO 2/N 2 with volume percentages of 45/20/35, contains more hydrogen than the first-stage reformer gas, its addition is shown to have an adverse effect on the burning rate due to the larger concentrations of N 2 and CO 2 diluents. The present study demonstrates that the addition of a first-stage reformer gas, H 2/CO/N 2, in the intake of an engine would be a viable approach to reduce cold-start hydrocarbon emissions.

00/01002 Automotive spark-ignited direct-injection gasoline engines

The development of four-stroke, spark-ignition engines that are designed to inject gasoline directly into the combustion chamber is an important worldwide initiative of the automotive industry. The thermodynamic potential of such engines for significantly enhanced fuel economy, transient response and cold-start hydrocarbon emission levels has led to a large number of research and development projects that have the goal of understanding, developing and optimizing gasoline direct-injection (GDI) combustion systems. The processes of fuel injection, spray atomization and vaporization, charge cooling, mixture preparation and the control of in-cylinder air motion are all being actively researched, and this work is reviewed in detail and analyzed. The new technologies such as high-pressure, common-rail, gasoline injection systems and swirl-atomizing gasoline fuel injectors are discussed in detail, as these technologies, along with computer control capabilities, have enabled the current new examination of an old objective; the direct-injection, stratified-charge (DISC), gasoline engine. The prior work on DISC engines that is relevant to current GDI engine development is also reviewed and discussed. The fuel economy and emission data for actual engine configurations are of significant importance to engine researchers and developers. These data have been obtained and assembled for all of the available GDI literature, and are reviewed and discussed in detail. The types of GDI engines are arranged in four classifications of decreasing complexity, and the advantages and disadvantages of each class are noted and explained. Emphasis is placed upon consensus trends and conclusions that are evident when taken as a whole. Thus the GDI researcher is informed regarding the degree to which engine volumetric efficiency and compression ratio can be increased under optimized conditions, and as to the extent to which unburned hydrocarbon (UBHC), NOx and particulate emissions can be minimized for specific combustion strategies. The critical area of GDI fuel injector deposits and the associated effect on spray geometry and engine performance degradation are reviewed, and important system guidelines for minimizing deposition rates and deposit effects are presented. The capabilities and limitations of emission control techniques and aftertreatment hardware are reviewed in depth, and areas of consensus on attaining European, Japanese and North American emission standards are compiled and discussed. All known research, prototype and production GDI engines worldwide are reviewed as to performance, emissions and fuel economy advantages, and for areas requiring further development. The engine schematics, control diagrams and specifications are compiled, and the emission control strategies are illustrated and discussed. The influence of lean-NOx catalysts on the development of late-injection, stratified-charge GDI engines is reviewed, and the relative merits of lean-burn, homogeneous, direct-injection engines as an option requiring less control complexity are analyzed. All current information in the literature is used as the basis for discussing the future development of automotive GDI engines.

Automotive spark-ignited direct-injection gasoline engines

The development of four-stroke, spark-ignition engines that are designed to inject gasoline directly into the combustion chamber is an important worldwide initiative of the automotive industry. The thermodynamic potential of such engines for significantly enhanced fuel economy, transient response and cold-start hydrocarbon emission levels has led to a large number of research and development projects that have the goal of understanding, developing and optimizing gasoline direct-injection (GDI) combustion systems. The processes of fuel injection, spray atomization and vaporization, charge cooling, mixture preparation and the control of in-cylinder air motion are all being actively researched, and this work is reviewed in detail and analyzed. The new technologies such as high-pressure, common-rail, gasoline injection systems and swirl-atomizing gasoline fuel injectors are discussed in detail, as these technologies, along with computer control capabilities, have enabled the current new examination of an old objective; the direct-injection, stratified-charge (DISC), gasoline engine. The prior work on DISC engines that is relevant to current GDI engine development is also reviewed and discussed. The fuel economy and emission data for actual engine configurations are of significant importance to engine researchers and developers. These data have been obtained and assembled for all of the available GDI literature, and are reviewed and discussed in detail. The types of GDI engines are arranged in four classifications of decreasing complexity, and the advantages and disadvantages of each class are noted and explained. Emphasis is placed upon consensus trends and conclusions that are evident when taken as a whole. Thus the GDI researcher is informed regarding the degree to which engine volumetric efficiency and compression ratio can be increased under optimized conditions, and as to the extent to which unburned hydrocarbon (UBHC), NO x and particulate emissions can be minimized for specific combustion strategies. The critical area of GDI fuel injector deposits and the associated effect on spray geometry and engine performance degradation are reviewed, and important system guidelines for minimizing deposition rates and deposit effects are presented. The capabilities and limitations of emission control techniques and aftertreatment hardware are reviewed in depth, and areas of consensus on attaining European, Japanese and North American emission standards are compiled and discussed. All known research, prototype and production GDI engines worldwide are reviewed as to performance, emissions and fuel economy advantages, and for areas requiring further development. The engine schematics, control diagrams and specifications are compiled, and the emission control strategies are illustrated and discussed. The influence of lean-NO x catalysts on the development of late-injection, stratified-charge GDI engines is reviewed, and the relative merits of lean-burn, homogeneous, direct-injection engines as an option requiring less control complexity are analyzed. All current information in the literature is used as the basis for discussing the future development of automotive GDI engines.

Electrically heated converters for automotive emission control: determination of the best size regime for the heated element

In view of the significant cold-start hydrocarbon emission reduction potential of the electrically heated converter (EHC) technology demonstrated in recent studies, there is considerable interest in better understanding the behavior and design aspects of an EHC during the cold-start portion of actual vehicle emission tests. Computer simulations based on the EHC model developed previously (Oh et al., 1993, Industrial and Engineering Chemistry Research 32, 1560–1567) show that although decreasing the volume of an electric heater generally improves the emission performance of the EHC system, there is a critical heater volume below which no significant emission benefit is obtained. This paper examines the EHC sizing issue in more detail by analyzing a simplified version of the EHC model to investigate why the emission benefit of decreasing electric heater volume eventually disappears and how the critical heater volume where this occurs is related to system parameters. Analysis of the simplified EHC model identifies this critical volume, V h , which is proportional to the exhaust flow rate through the EHC and inversely proportional to the product of the interphase (gas–solid) heat transfer coefficient within the heated element and its geometric surface area per unit volume. It is shown that the recommended heater size regime must be where the physical heater volume, V, and V h are comparable in magnitude. When V≫ V h , the electric heater is clearly too large because the emission performance suffers from the slow heat-up rate caused by the large thermal mass. Also, decreasing the heater volume to values much smaller than V h (i.e., V≪ V h ) is not desirable because it would simply lead to unnecessarily higher (potentially damaging) solid-phase temperatures without producing additional emission reductions. In this smallest size regime, the emission performance of the EHC system becomes insensitive to heater size variation because the higher solid-phase temperatures within the smaller heater tend to be compensated by its poor interphase (solid-to-gas) heat transfer arising from the insufficient heat transfer area available.

Hydride-based cold-start heater for automotive catalyst

A novel, closed-cycle, hydride-based cold-start heater for automotive catalysts is described. The device consists of coupled AB 5 and β-Ti alloy beds and is designed to store a small part of the automotive exhaust heat for later release during cold-start. Without the significant use of electric power, it can heat the catalyst bed to operating temperature in a few seconds thereby greatly reducing cold-start hydrocarbon emissions. A full-sized prototype was built and tested both on the bench and in an FTP automobile. ULEV performance was achieved with a 2.2 L, 4-cylinder vehicle.