Catalyst deterioration over the lifetime of small utility engines.

ABSTRACT

In this paper, the deterioration of catalysts in small, four-stroke, spark-ignition engines is described. The laboratory testing performed followed a proven test method that mimics the lifetime of a small air-cooled utility engine operating under normal field conditions. The engines used were single-cylinder, 6.5-hp, side-valve engines. These engines have a nominal 125-hr lifetime. The effectiveness of the catalysts was determined by testing exhaust emissions before and after the catalyst to determine the catalyst's efficiency. This was done several times during the lifetime of the engines to determine the deterioration in the performance of the catalysts at lowering pollutant emissions. Additional testing was performed on the catalysts to determine wear patterns, contamination, and recoverable activity. The results indicate that considerable catalyst deterioration is occurring over the lifetime of the engine. The results reveal that soot buildup, poisons, and active surface loss appear to be the contributing factors to the deterioration. These results were determined after analyzing the exhaust emissions data, scanning electron microscope results analysis, and the impact of regeneration attempts. An ANOVA statistical analysis was performed, and it was determined that the emissions are also impacted, to some degree, by time and the engine itself.

INTRODUCTION

Internal combustion engines come in a variety of sizes, designs, and are used in many different applications. However, all internal combustion engines produce some undesirable air pollutants. The three most-common air pollutants produced by spark-ignition internal combustion engines are hydrocarbons (HC), oxides of nitrogen (N[O.sub.x]), and carbon monoxide (CO). These emissions can cause serious health problems and harm the environment. As a result, internal combustion engines have long been under regulations; however, only relatively recently has the small engine industry become the focus of more stringent regulation. (1)

There are many difficulties in trying to achieve a high level of emissions control for small utility engines. Two main problems impacting the emissions controls in small engines are the technology behind the engines and the market. Much of the engine technology used in the small engine market has long been replaced in other markets, such as automobile engines. For example, most small engines use carburetors, and carburetors do not automatically adjust fuel delivery rates to provide the most efficient or cleanest combustion. The amount of pollutants an internal combustion engine generates varies with the operating conditions of the engine and is influenced primarily by the air-fuel ratio in the combustion chamber. (2) Secondly, small engines are usually built to be very affordable; therefore, it is undesirable to increase the product's cost through potentially expensive modifications such as new emissions controls. In addition, small engines operate differently than automobile engines, as they generate higher temperatures and more vibrations at the catalyst when compared with the remote-mounted automotive catalyst. These factors need to be accounted for in the application and design of after-treatment systems. (3)

After-treatment of exhaust gases reduces emissions by use of secondary-air and a catalyst. Currently, catalytic after-treatment systems are not commonly used on lawn mowers. Engine manufacturers are evaluating these systems as part of their overall strategies to comply with future emissions regulations. (4) Part of this evaluation concerns the choice of specific catalysts for the systems, and part of the evaluation concerns the structure, geometry, and mounting of the catalysts. This study is concerned primarily with the long-term effectiveness of the structure and mounting of the converter than the actual catalytic material chosen. The catalysts used, although an alternative under consideration, may not be the ideal catalysts for this application; such a determination is still to be made by industry. The structural design of the converter is one that is under widespread consideration.

There are several different kinds of catalysts. The most commonly used type of catalyst, especially on internal combustion engines, is the three-way catalyst. These are called three-way catalysts because they promote reactions that are designed to oxidize HC and CO to carbon dioxide (C[O.sub.2]) and reduce N[O.sub.x] to [N.sub.2]. A three-way monolith catalyst is used as the catalyst under study in this project. The monolith design consists of three components. There is the substrate or core, which is a honeycomb structure and is typically made of a ceramic. The substrate is then coated with a washcoat to increase surface area by creating a rough surface. Then the washcoat has precious metals impregnated on the surface. (5) The precious metals commonly used are platinum, palladium, and rhodium. Different combinations of these precious metals can affect the impact of the catalysts on the emissions. These metals have been proven to be able to withstand large temperature ranges and emission concentration levels. The monolith design features multiple parallel channels that make for a large number of single pass reactors, which minimizes restriction in the flow; however, it also allows poisons to quickly contaminate the surfaces.

There are three main causes behind catalyst deterioration: thermally induced deactivation, poisoning, and washcoat loss. Thermally induced deactivation happens when sintering occurs. During sintering, the precious metals can move and form together in one area, reducing the total area of active sites. Another problem that can result from sintering is when the washcoat expands, closing off the precious metal sites and reducing the active area. Poisoning occurs when substances such as lead, zinc, sulfur, or phosphorous find their way to the catalyst. Many of these substances can be found in lubricants. Poisoning can reduce catalyst efficiency by reacting directly with the active site and rendering it inactive or by simply covering the area, which blocks the active site from being able to promote reactions. A third cause behind catalyst deterioration is washcoat loss. This is a result of expansion differences or lack of adhesion. When the washcoat breaks off, it reduces the amount of active surface area, reducing the catalysts' overall efficiency. (2)

In this paper, the results of a study of the effects of engine operation on the catalyst are presented. In this study, five 6.5-hp, single-cylinder (with a displacement of 190 [cm.sup.3],) side-valve engines were modified to have a honeycomb catalytic converter installed inside the muffler. The muffler was specially designed and built to hold the catalytic converter, and the muffler and mounting were identical on each engine. As these engines tend to operate fuel-rich, ports were drilled into the muffler to provide secondary air for use in the converter. The engines then underwent a laboratory aging process, and the impact of the aging on the performance of the catalyst was tested at several different points over the engine lifetime. At different stages of the test, one engine was removed from the test so that the physical deterioration of the catalyst could be more thoroughly examined. The results of the catalyst performance as gauged by both emissions tests and the additional test procedures will be described.

EXPERIMENTAL PROCEDURES

Characterization of Engine Emissions

Determining the emissions of an engine over the course of the engine's lifetime involves many steps. The first step is the engine break-in period. During this period, the engine is mounted onto the dynamometer and follows a prescribed engine loading and speed chart. The engine is started and idled at 2400 revolutions per minute (rpm) for 1 min. The engine speed is then increased to 3060 rpm and the load is set to 10% of full load for 5 min. The load is then increased to 25%, 50%, 75%, and to 100% of full load for a duration of 5 min each. Once 100% load is achieved, the engine load is cycled between the 100% and 50% loads for 5 min each. The total break-in period should be 121 min. The dynamometer used is a vertical/horizontal shaft eddy-current small engine dynamometer.

The emissions testing follows the standard SAE J1088 test procedure. (6) The SAE J1088 test procedure is a detailed method used to determine small utility engine emissions. The concentration emissions values can be converted to mass values by use of the fuel flow method found in Section 6.2.2.1 of the SAE J1088 test procedure. This procedure involves holding the engine at 3060 rpm and starting with 100% load. Then once the oil temperature stabilizes, the emissions are sampled for 3 min. Once sampling is completed the load is reduced to 75%, 50%, 25%, and 10%. The emissions are sampled at each load point. In addition to measuring the CO, HC, and N[O.sub.x] emissions, measurements of [O.sub.2] and C[O.sub.2] emissions as well as temperature were performed.