Method for in-cylinder catalytic oxidation of hydrocarbons in direct injected spark ignition engines

Hydrocarbon emissions from direct injected spark ignition engines are reduced as a result of post-flame oxidation by a catalytic material deposited on the top of a piston disposed in the engine.

BACKGROUND OF THE INVENTION
 1. Technical Field
 This invention relates generally to a method for reducing hydrocarbon
 emissions in direct injected spark ignition engines, and more particularly
 to such a method using a catalytic oxidative coating on a piston for
 post-combustion oxidation of hydrocarbons during an expansion stroke of
 the engine.
 2. Background Art
 There is world-wide interest directed to the development of internal
 combustion engines with fuel injection systems which introduce fuel
 directly into a combustion chamber. Direct injection is advantageous for
 several reasons, including improved efficiency and reduced fuel
 consumption. However, direct injected gasoline engines produce
 significantly higher levels of hydrocarbon (HC) emissions than comparable
 port-fuel injected engines. One of the main mechanisms responsible for
 high HC emissions in direct injected gasoline engines has been found to be
 attributable to the deposition of liquid fuel on the surfaces of the
 combustion chamber (i.e., the piston top, cylinder liner, etc.). This
 phenomenon is generally referred to as combustion chamber wall wetting,
 and is most predominant when fuel is injected late during the compression
 stroke. Most current directed injected, spark ignition (DISI) engines are
 designed so that some wetting of the piston top occurs during light-load
 operation. Recent research, as evidenced in "The Effect of In-Cylinder
 Wall Wetting Location on the HC Emissions from SI Engines", Rudolf
 Stangimaier, et al. 1999 SAE Technical Paper Series, indicates that piston
 top wetting contributes significantly to the hydrocarbon emissions from
 DISI engines.
 Catalytic coatings within combustion chambers have been used for ignition
 of the fuel/air mixture as a result of contact with the catalytic material
 present in the combustion chamber. This form of catalytic-ignition is
 described in U.S. Pat. Nos. 4,773,368; 4,811,707; 4,819,595; and
 4,905,658, all issued to William C. Pfefferle. More recently, U.S. Pat.
 No. 5,806,483, issued Sep. 15, 1998, to Mitarai, et al describes a
 combustion method for preventing the occurrence of knock during engine
 operation by providing a two-stage combustion process which is
 substantially different from the normal spark-ignition combustion process.
 Cracking of carbon-to-carbon bonds in heavier fuel molecules immediately
 prior to combustion is described in U.S. Pat. No. 4,530,340, granted Jul.
 23, 1985, to Millard C. Totman. Totman uses a catalytic coating within the
 combustion chamber to promote the pre-combustion cracking of the heavier
 fuel molecules. Earlier, U.S. Pat. No. 2,978,360 was issued to Samuel W.
 Bradstreet, et al on Apr. 4, 1961 for several specific catalyst
 formulations and methods for generating and applying catalytic coatings to
 combustion chamber surfaces. The coatings are provided for the purpose of
 maximizing the combustion process itself, and as a result of the more
 efficient combustion, reduced overall emissions.
 The present invention is directed to overcoming the problem of piston top
 wetting and the resultant contribution to hydrocarbon emissions. It is
 desirable to have a direct injected spark ignition engine that promotes a
 catalytic reaction at the top of the piston whereby a higher fraction of
 the fuel which impinges on the piston top is oxidized during the expansion
 stroke. It is also desirable to have such an engine wherein the
 detrimental effects of wall wetting on hydrocarbon emissions are reduced.
 SUMMARY OF THE INVENTION
 In one aspect of the present invention, a method for in-cylinder oxidation
 of hydrocarbons in a direct injected spark ignition engine includes
 coating the top surface of a piston with an oxidation catalyst comprising
 at least one metal selected from the group consisting of palladium,
 platinum and rhodium. The coated piston is then assembled in a direct
 injected spark ignition engine, and the engine is operated whereby the
 coated piston is moved between alternating compression and expansion
 strokes. A flow of air is directed into the combustion chamber, and a
 stream of a combustible material comprising a liquid fuel, is injected
 directly into the combustion chamber and onto the coated top surface of
 the piston during a compression stroke. The coated top surface of the
 piston is thereby wetted with a first portion of the liquid fuel. A second
 portion of the liquid fuel is mixed with the air directed into the chamber
 and thereby forms a combustible fuel-air mixture within the combustion
 chamber. An electrical spark is produced within the combustion chamber,
 thereby igniting and combusting the fuel-air mixture. Subsequently, at
 least a portion of the remaining first portion of the liquid fuel present
 on the top surface of the coated piston is oxidized during an expansion
 stroke of the engine by catalytic reaction between the remaining fuel and
 the catalytic coating on the top of the piston.
 Other features of the method for in-cylinder oxidation of catalysts
 embodying the present invention includes the top surface of the piston
 being preferably coated with an oxidation catalyst comprising palladium.

DETAILED DESCRIPTION OF A PRESENTLY PREFERRED EXEMPLARY EMBODIMENT
 The present invention is directed to reducing hydrocarbon emissions from
 direct injected spark ignition engines (DISI) by coating the top of the
 pistons with a catalytic material. By promoting a catalytic reaction at
 the piston top, it is possible to oxidize a higher fraction of the fuel
 which impinges on that surface by directly promoting a combustion at the
 piston surface and/or by increasing the reactivity of the hydrocarbon
 molecules so that they can be easily oxidized during the expansion stroke.
 In the preferred embodiment of the present invention, a combustion chamber
 10 of a DISI engine, as represented in somewhat schematic fashion by the
 sole drawing FIGURE, has a spark plug 12 and an injection nozzle 14, each
 of which have a tip, or end portion, projecting into the internal cavity
 defined by the combustion chamber 10. A piston 16 is reciprocatably
 mounted within the combustion chamber 10, and is movable in a vertical
 direction between alternating compression and expansion strokes, or
 alternatively between intake, compression, expansion and exhaust strokes.
 The piston 16 has a coating 18 intimately deposited on the top surface of
 the piston 16 by conventional coating methods. The catalytic coating 18 is
 desirably formed of a material that has excellent oxidizing qualities,
 since the function of the coating is hydrocarbon oxidation or at least
 partial hydrocarbon oxidation. Furthermore, it is desirable that the
 catalytic coating 18 on the top of the piston 16 has high oxidation
 efficiency at low temperature since the piston crown will typically
 operate in a temperature range of from about 250.degree. C. to about
 300.degree. C. during part load operation. Furthermore, it is desirable
 that the catalytic coating 18 be formed of a material that is resistant to
 high temperature excursions under high load conditions. Based on the above
 criteria, a catalyst coating 18 containing one or more metal selected from
 the group consisting of palladium (Pd), platinum (Pt), and rhodium (Rh) is
 desirable. Preferably, the catalytic coating 18 is formed of palladium
 because it has the best low-temperature oxidation efficiency of the three
 metals.
 The method, embodying the present invention, for in-cylinder oxidation of
 hydrocarbons in direct injected spark ignition engines includes coating
 the top surface of the piston 16 with an oxidation catalyst 18 comprising
 at least one metal selected from the group consisting of palladium,
 platinum, and rhodium. The top-coated piston 16 is then assembled in a
 direct injected spark ignition engine whereby the piston cooperates with
 other internal surfaces within the engine to form a combustion chamber 10.
 The engine is then operated such that alternating strokes are produced by
 reciprocation of the coated piston 16. A flow of air is directed into the
 combustion chamber 10 and a stream of a combustible mixture is injected
 through the injection nozzle 14 directly into the combustion chamber 10
 and a portion of the injected fuel impinges onto the coated top surface 18
 of the piston 16 during a combustion stroke. The combustible mixture may
 comprise either a sprayed stream of liquid fuel or a mixture of liquid
 fuel and injection air. As a result of directing the combustible mixture
 comprising a liquid fuel directly onto the coated top surface 18 of the
 piston 16, the coated top surface 18 of the piston 16 is wetted with a
 first portion of the liquid fuel.
 The remaining liquid fuel, comprising a second portion, is mixed with the
 air previously directed into the chamber 10 whereby a combustible fuel-air
 mixture is formed within the chamber 10. The combustable fuel-air mixture
 is subsequently ignited by an electrical spark provided by the spark plug,
 or similar spark ignition device, 12 positioned within the chamber 10.
 Upon ignition, the fuel-air mixture is combusted and a portion of the
 first portion of liquid fuel disposed on the top surface 18 of the piston
 16 is vaporized.
 Combustion of the fuel-air mixture forces the piston 16 downwardly in an
 expansion stroke. During the expansion stroke, at least a portion of any
 remaining unburned or partially burned portion of the first portion of
 liquid fuel present on the top surface 18 of the coated piston 16 is
 subsequently oxidized as a result of catalytic reaction between the
 remaining fuel and the catalytic coating 18 on the top of the piston 16.
 The method for in-cylinder oxidation of hydrocarbons, embodying the present
 invention, requires that liquid fuel be deposited on the top of a piston
 disposed in a direct injected spark ignition engine as a direct result of
 in-cylinder fuel injection. Liquid fuel deposition on the piston top does
 not occur in fully warmed-up port-fuel injected or carburetted engines. It
 is known that the liquid fuel deposition on the piston top of DISI engines
 contributes to the higher emission of hydrocarbons than are emitted from
 comparable port-fuel injected or carburetted engines. In the method
 embodying the present invention, the catalytic coating 18 on the top of
 piston 16, which is disposed in a DISI engine, breaks down the fuel
 molecules and the resulting hydrocarbon molecules have increased
 reactivity so that they are more easily oxidized in a post-flame oxidation
 process.
 Although the present invention is described in terms of a preferred
 exemplary embodiment, those skilled in the art will recognize that changes
 in the exemplary embodiment may be made without departing from the spirit
 of the invention. Such changes are intended to fall within the scope of
 the following claims. Other aspects, features, and advantages of the
 present invention may be obtained from a study of this disclosure and the
 drawing, along with the appended claims.