Two stroke cycle internal combustion engines

Details Two stroke cycle internal combustion engines Two stroke cycle internal combustion engines

1997-01-15

2001-06-19

Kamen, Noah P. (Department: 3747)

Internal-combustion engines

Two-cycle

Whirl through piston-controlled ports

C060S302000

Reissue Patent

active

RE037230

ABSTRACT:

This invention relates to the controlling of exhaust gas emissions from two stroke cycle internal combustion engines by the use of catalysts to treat the undesirable components of the engine exhaust gas.
Environment protection authorities in various countries prescribe limits to the emissions in the exhaust gases of motor vehicles and usually the limits placed on emissions from private cars, motor cycles and light commercial vehicles are progressively becoming more strict. In most countries the permissible levels of emissions for automotive vehicles are stipulated on the basis of the weight of various components of the exhaust gas per mile or kilometer travelled, the limits
applyomg

applying
irrespective of the weight of the vehicle or the size of the engine thereof. Accordingly, there has been a trend towards the production of small motor vehicles of relatively light weight whereby the rate of consumption of fuel can be reduced with a corresponding reduction in the weight of the various exhaust gas components generated per unit distance travelled.
The three major components of the exhaust gas which must be controlled are hydrocarbons (HC) oxides of nitrogen (NOx) and carbon monoxide (CO). NOx is normally treated by a catalyst that requires the establishment of a reducing environment to separate the oxygen from the nitrogen, whereas an oxidising atmosphere is required to treat the HC and CO.
The use of catalysts in the exhaust systems of vehicle internal combustion engines is widely known and practised in the control of exhaust emissions. It is customary to locate the catalyst in the exhaust system, somewhat downstream from the actual exhaust ports of the cylinder or cylinders of the engine, where the exhaust gases from any one cylinder or from a number of cylinders have been subject to a degree of mixing between the time of leaving the cylinder and reaching the catalyst. As a result of the mixing the distribution of the various components of the exhaust gas is approaching a homogeneous mixture when presented to the catalysts. Accordingly a catalyst system must be used that is able to treat the
HC

HC,
NOx and CO components of the generally homogeneous exhaust gas mixture.
It is known, as shown in SAE Paper 872098 relating to investigations by the Toyota Central Research and Development Laboratories, Inc., that the efficiency of conversion of a three-way catalyst system applied to an automotive type four stroke cycle engine is substantially dependent upon the air/fuel ratio of the exhaust gas presented to the catalyst. In particular it is known that the efficiency dramatically changes as the air/fuel ratio transits the stoichiometric ratio. As can be seen from the graph constituting
FIG. 1
of the accompanying drawings, the efficiency of conversion of HC and CO increases as the air/fuel ratio of the exhaust gas increases, that is as the mixture gets leaner. In contrast, the efficiency of the conversion of NOx is high, whilst the air/fuel ratio is low, that is, with a rich exhaust gas mixture, but drops most dramatically as the air/fuel ratio passes through stoichiometric from a rich to a lean mixture.
Engines operating on the two stroke cycle have presented a substantial problem in the control of the level of exhaust emissions, particularly in engines where fuel is entrained in the air charge when it enters the engine cylinder, as part of that fuel passes unburnt through the exhaust port during the conventional scavenging process. This escape of unburnt fuel, that contributes to HC and CO in the exhaust gas, can be reduced by modern electronically controlled fuel injection systems that inject the fuel directly into the engine cylinder rather than the fuel being carried into the cylinder with the incoming air charge. However, the direct injection of the fuel does not in itself contribute significantly to the control of the generation of NOx, particularly as the rate of fuel consumption increases with the increase in size of the vehicle resulting in a corresponding increase in the level of NOx emissions when measured on a mass per unit distance travelled basis. Although other combustion control techniques can also be employed to contribute to the control of the level of NOx in small horsepower engines, the multiplication of the level of NOx generated with increasing power output, leads to a situation where the control NOx is best exercised by way of catalytic treatment of the exhaust gases, particularly from the considerations of cost and stability of operation.
In a direct injected two stroke cycle engine the fuel free fresh charge which enters the engine cylinder, whilst the exhaust port and inlet port are both open, results in a dilution of the subsequently expelled exhaust gases to an air/fuel ratio considerably above stoichiometric. This results in oxidising conditions existing in the exhaust system which are in direct conflict with the desired reducing conditions necessary to achieve effective reduction of the NOx by catalytic treatment. Accordingly the provision of a conventional three-way catalyst system in the exhaust system at the conventional location to treat homogeneous exhaust gases would only result in a lowering of the HC and CO, but would not break down the NOx.
It is an object of the present invention to improve the performance of the catalyst system in the treatment of the exhaust gases in a two stroke cycle internal combustion engine.
With this object in view, there is provided a method of operating a two stroke cycle internal combustion engine wherein for each combustion chamber there is provided means to supply fuel to the combustion chamber, an exhaust port through which gases pass from the combustion chamber to an exhaust system, and at least one inlet port through which a fresh charge of air enters the combustion chamber, the inlet and exhaust ports being arrange so that the inlet port opens prior to the closing of the exhaust port, the method being characterised in that during an exhaust port open period of each combustion chamber a first portion of the gas exhausted from said combustion chamber during the exhaust port open period is directed into contact with a first catalyst means of a first catalytic character, and a subsequent second portion of the gas exhausted from said combustion chamber in the same exhaust port open period is directed into contact with a second catalyst means of a differing catalyst character to the first catalyst means.
In a two stroke cycle internal combustion engine with direct fuel injection the exhaust gases in said first portion are the gases that pass through the exhaust port when it first opens, and are often stoichiometric or richer in fuel and thus capable of sustaining a reduction process, whereas the exhaust gases in said second portion which pass through the exhaust port later are leaner in fuel and capable of sustaining an oxidation process. The stoichiometric or chemically reducing gases are those gases which generally contain most of the mass of nitrogen oxides produced in the combustion chamber. By the term chemically reducing gases is meant gases which have an oxygen deficiency for stoichiometric combustion. In other words, the gases may still contain free oxygen although an insufficient quantity to fully oxidise the unburnt fuel and products of partial combustion in the gases. Similarly, by the term chemically oxidising gases is meant gases which have an oxygen surplus for stoichiometric combustion.
Conveniently the first catalyst means is located at or adjacent to the exhaust port of the particular combustion chamber. The location of the first catalyst means is selected so that during each exhaust port open period the first catalyst means will receive the exhaust gas that passes through the exhaust port during the initial portion of the exhaust port open period which is chemically reducing exhaust gas
, and the

. The
first catalyst means includes an active catalyst material of a nature to reduce oxides of nitrogen (NOx) in the gases received from the exhaust port.
The location of the first catalys

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