Roy Douglas

The Development of a Two-Dimensional Transient Catalyst Model for Direct Injection Two-Stroke Applications.

Abstract This paper describes the development of a two-dimensional transient catalyst model. Although designed primarily for two-stroke direct injection engines, the model is also applicable to four-stroke lean burn and diesel applications. The first section describes the geometries, properties and chemical processes simulated by the model and discusses the limitations and assumptions applied. A review of the modelling techniques adopted by other researchers is also included. The mathematical relationships which are used to represent the system are then described, together with the finite volume method used in the computer program. The need for a two-dimensional approach is explained and the methods used to model effects such as flow and temperature distribution are presented. The problems associated with developing surface reaction rates are discussed in detail and compared with published research. Validation and calibration of the model are achieved by comparing predictions with measurements from a flow reactor. While an extensive validation process, involving detailed measurements of gas composition and thermal gradients, has been completed, the analysis is too detailed for publication here and is the subject of a separate technical paper

Measurement of oxygen storage capacity in automotive catalysts

Abstract There is considerable disagreement in the literature on available oxygen storage capacity, and on the reaction rates associated with the storage process, for three-way automotive catalysts. This paper seeks to address the issue of oxygen storage capacity in a clear and precise manner. The work described involved a detailed investigation of oxygen storage capacity in typical samples of automotive catalysts. The capacity has also been precisely defined and estimates have been made of the specific capacity based on catalyst dimensions. A purpose-built miniature catalyst test rig has been assembled to allow measurement of the capacity and the experimental procedure has been developed to ensure accurate measurement. The measurements from the first series of experiments have been compared with the theoretical calculations and good agreement is seen. A second series of experiments allowed the e ect of temperature on oxygen storage capacity to be investigated. This work shows very clearly the large variation of the capacity with temperature.

Performance Characterisation of a Range of Diesel Oxidation Catalysts: Effect of Pt:Pd Ratio on Light Off Behaviour and Nitrogen Species Formation

Computer simulation is now considered to be a crucial stage in the design of automotive catalysts due to the increasing complexity of modern aftertreatment systems. The resulting models almost invariably include surface reaction kinetics that are measured under controlled conditions similar to those found on a vehicle. Repeatability of the measurements used to infer surface reaction rates is fundamental to the accuracy of the resulting catalyst model. To achieve the required level of repeatability it is necessary to ensure that the catalyst sample in question is stable and that its activity does not change during the test phase. It is therefore essential that the catalyst has been lightly aged, or ‘de-greened’ before testing begins. It is also known that the state of the catalysts surface prior to testing has an impact on its subsequent light-off performance and that test history can play an important role in catalyst activity. Suitably pre-treating the catalyst surface can ensure that a reference point is reached prior to a light-off test. The work summarised in this paper includes a study of both the de-greening phase and the state of the catalysts surface on its activity, with the aim of developing a robust test protocol that provides repeatable kinetic data under realistic operating conditions. To establish a protocol for initial stabilisation of new catalyst samples a series of tests were conducted on catalysts that had been thermally aged at 600°C and 750°C respectively until the sample exhibited stability. The activity of the sample was assessed by repeating identical CO light-off tests following each period of time in the oven. A period of 8 hours at 750°C was found to sufficiently stabilise the sample. In the pre-treatment study an investigation into the effect of several pre-treatment protocols on CO light off was carried out on a three-way catalyst of commercial formulation in order to establish a robust protocol to ensure test repeatability. The investigation focused on pre-treating the catalyst sample in hydrogen, oxygen or nitrogen environments at specified conditions of flow and temperature. Following pre-treatment, identical CO light-off tests were performed in order to assess the respective influences of each pre-treatment. A trend of lower light off temperatures was observed for all pre-treatment strategies on the first of two consecutive light-off temperature ramps with a notable increase in light off temperature observed on the second ramp. As a result of this study a pre-treatment protocol was established. The protocol involves treating the catalyst sample in flowing nitrogen up to 600° C using a temperature ramp of 15°C/min.

Preliminary analysis of organic Rankine cycles to improve vehicle efficiency

This paper presents the background rationale and key findings for a model-based study of supercritical waste heat recovery organic Rankine cycles. The paper’s objective is to cover the necessary groundwork to facilitate the future operation of a thermodynamic organic Rankine cycle model under realistic thermodynamic boundary conditions for performance optimisation of organic Rankine cycles. This involves determining the type of power cycle for organic Rankine cycles, the circuit configuration and suitable boundary conditions. The study focuses on multiple heat sources from vehicles but the findings are generally applicable, with careful consideration, to any waste heat recovery system. This paper introduces waste heat recovery and discusses the general merits of organic fluids versus water and supercritical operation versus subcritical operation from a theoretical perspective and, where possible, from a practical perspective. The benefits of regeneration are investigated from an efficiency perspective for selected subcritical and supercritical conditions. A simulation model is described with an introduction to some general Rankine cycle boundary conditions. The paper describes the analysis of real hybrid vehicle data from several driving cycles and its manipulation to represent the thermal inertia for model heat input boundary conditions. Basic theory suggests that selecting the operating pressures and temperatures to maximise the Rankine cycle performance is relatively straightforward. However, it was found that this may not be the case for an organic Rankine cycle operating in a vehicle. When operating in a driving cycle, the available heat and its quality can vary with the power output and between heat sources. For example, the available coolant heat does not vary much with the load, whereas the quantity and quality of the exhaust heat varies considerably. The key objective for operation in the vehicle is optimum utilisation of the available heat by delivering the maximum work out. The fluid selection process and the presentation and analysis of the final results of the simulation work on organic Rankine cycles are the subjects of two future publications.

Integrating intrinsic and global kinetics as a dual kinetic model for automotive catalysis

The majority of the kinetic models employed in catalytic after-treatment of exhaust emissions use a global kinetic approach owing to the simplicity because one expression can account for all the steps in a reaction. The major drawback of this approach is the limited predictive capabilities of the models. The intrinsic kinetic approach offers much more information about the processes occurring within the catalytic converter; however, it is significantly more complex and time consuming to develop. In the present work, a methodology which allows accessing a model that combines the simplicity of the global kinetic approach and the accuracy of the intrinsic kinetic approach is reported. To assess the performance of this new approach, the oxidation of carbon monoxide in the presence of nitric oxide as well as a driving cycle was investigated. The modelling of carbon monoxide oxidation with oxygen which utilised the intrinsic kinetic approach with the global kinetic approach was used for the carbon monoxide + nitric oxide reaction (and all remaining reactions for the driving cycle).The comparison of the model results for the dual intrinsic + global kinetic approach with the experimental data obtained for both the reactor and the driving cycle indicate that the dual approach is promising with results significantly better than those obtained with only the global kinetics approach.

A Parametric Study of an Exhaust Recovery Turbogenerator on a Diesel-Electric Hybrid Bus

The fuel consumption of automotive vehicles has become a prime consideration to manufacturers and operators as fuel prices continue to rise steadily, and legislation governing toxic emissions becomes ever more strict. This is particularly true for bus operators as government fuel subsidies are cut or removed.In an effort to reduce the fuel consumption of a diesel-electric hybrid bus, an exhaust recovery turbogenerator has been selected from a wide ranging literature review as the most appropriate method of recovering some of the wasted heat in the exhaust line. This paper examines the effect on fuel consumption of a turbogenerator applied to a 2.4-litre diesel engine.A validated one-dimensional engine model created using Ricardo WAVE was used as a baseline, and was modified in subsequent models to include a turbogenerator downstream, and in series with, the turbocharger turbine. A fuel consumption map of the modified engine was produced, and an in-house simulation tool was then used to examine the fuel economy benefit delivered by the turbogenerator on a bus operating on various drive-cycles.A parametric study is presented which examined the performance of turbogenerators of various size and power output. The operating strategy of the turbogenerator was also discussed with a view to maximising turbine efficiency at each operating point.The performance of the existing turbocharger on the hybrid bus was also investigated; both the compressor and turbine were optimised and the subsequent benefits to the fuel consumption of the vehicle were shown.The final configuration is then presented and the overall improvement in fuel economy of the hybrid bus was determined over various drive-cycles.Copyright

Deactivation of Oxidation Catalysts by Oil-Derived Sulphur

The most common mode of deactivation suffered by catalysts fitted to two-stroke engines has traditionally been thermal degradation, or even meltdown, of the washcoat and substrate. The high temperatures experienced by these catalysts are caused by excessively high concentrations of HC and CO in the exhaust gas which are, in turn, caused by a rich AFR and the loss of neat fuel to the exhaust during the scavenging period. The effects of catalyst poisoning due to additives in the oil is often regarded as a secondary, or even negligible, deactivating mechanism in two-stroke catalysts and has therefore received little attention. However, with the introduction of direct in-cylinder fuel injection to some larger versions of this engine, the quantities of HC escaping to the exhaust can be reduced to levels similar to those found on four-stroke gasoline engines. Under these conditions, the effects of poisoning are much more significant to catalyst durability, particularly for crankcase scavenged derivatives which allow considerable quantities of oil to escape into the exhaust in a neat, or partially burned form. In this paper the effects of oil-derived sulphur on catalyst performance are examined using specialised test apparatus. The oil used throughout the study was formulated specifically for a two-stroke engine fitted with direct in-cylinder fuel injection. The sulphur content of this oil was 0.21% by mass and particular attention was paid to the role of this element in the resulting deactivation. The catalyst was also designed for two-stroke applications and contained a high palladium loading of 300g/ft 3 (28g/l) to prolong the life of the catalyst. It was found that the sulphur caused permanent deactivation of the CO reaction and increased the light-off temperature by around 40°C after oiling for 60 hours. This deactivation was progressive and led to a reduction in surface area of the washcoat, particularly in the micropores of around 5A diameter. By using a validated catalyst model the change in surface area of the precious metal was estimated. It was found that the simulated palladium surface area had to be reduced by a factor of around 7.5 to produce the light-off temperature of the deactivated catalyst. Conversely, the light-off temperature of the C 3 H 6 reaction was barely affected by the deactivation.

Modelling of oxygen storage in automotive catalysts

Abstract The designers of catalytic converters have found that the use of cerium oxide in the washcoat formulation, to store and release oxygen, gives higher conversion efficiencies in three-way catalysts. It has also been shown that oscillating the air-fuel ratio around λ = 1, which effectively oscillates the oxygen concentration, is a useful method of using this oxygen storage capability. This paper describes a detailed mathematical model for oxygen storage, which forms an important part of a complete kinetic model of a three-way automotive catalyst. This model consists of two interdependent submodels: one for the oxidation process and a second for the reduction process. In this study, O2 and NO were used as oxidizing agents and CO, C3H6 and C3H8 were used as reducing agents. The structure and equations of the model are presented and discussed in detail. Some validation work has also been carried out on a purpose-built miniature catalyst test rig. The purpose of this work was to provide the important kinetic factors for the reaction equations. The result of this analysis is included and compared with other published data.

Mathematical modelling of a reciprocating piston expander

Modern internal combustion (IC) engines reject around two thirds of the energy provided by the fuel as low-grade waste heat. Capturing a portion of this waste heat energy and transforming it into a more useful form of energy could result in a significant reduction in fuel consumption. By using the low-grade heat, an organic Rankine cycle (ORC) can produce mechanical work from a pressurised organic fluid with the use of an expander.

Flowrate and heat transfer considerations for oxidation catalysts

This paper presents the results of a study into the effects of flowrate and heat transfer in oxidation catalysts. The analysis was performed using a validated two-dimensional catalyst model, which has been designed to predict the behaviour of an oxidation catalyst during transient warm-up and light-off conditions. It is only during the simulation of transient conditions that the true effects of changes to the flowrate and heat loss to the substrate and surrounding materials can be observed. In addition, the two-dimensional design of the model used allows the reaction intensities to be viewed throughout the catalyst and gives the opportunity to examine these effects in both the radial and axial directions. The study has shown that the heat loss from the upstream section of exhaust pipe not only reduces the rate of increase of the feed gas temperature but also removes heat from the outer edges of the catalyst and delays the onset of reactions in these areas. Consequently, the importance of insulating the upstream exhaust pipe and the catalyst housing is demonstrated. It is also shown that increasing the thermal conductivity of the substrate reduces the light-off temperature by transporting heat to the cooler regions more rapidly. Increasing the void fraction of the substrate also has a signiflcant effect on the light-off temperature as less heat is absorbed from the feed gas. Finally, it is shown that increasing the flowrate, or space velocity, of the gas increases the resulting light-off temperature. For the conditions simulated, it was seen that increasing the flowrate by one order of magnitude increased the light-off temperature by only 13 °C. However, emissions breakthrough was seen to occur at a space velocity around 164 000 h 1.