Internal-combustion engines – Two-cycle – Whirl through piston-controlled ports
Reexamination Certificate
2001-05-10
2002-06-04
McMahon, Marguerite (Department: 3747)
Internal-combustion engines
Two-cycle
Whirl through piston-controlled ports
Reexamination Certificate
active
06397795
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to improved internal combustion engines, and especially to two-stroke engines having improved lubrication, scavenging, charging and exhaust port timing to reduce polluting emissions and improve engine performance and fuel efficiency.
BACKGROUND OF THE INVENTION
The present invention recognizes the global need for reduced hydrocarbon emissions from small power-producing engines, especially as relates to the rapidly growing demand for agricultural and light industrial power in developing economies. In these economies, the low weight and low cost of two-stroke engines will be difficult to ignore, and it may be expected that two-stroke engines will be widely used. Two-stroke engines have inherently high levels of unburned hydrocarbon emission due to their operating principle, in which burned exhaust gases are expelled from the engine's cylinder at the same time that a fresh fuel/air charge is brought in, leading inevitably to mixing between the two and inadvertent expulsion of unburned charge with the exhaust gases.
Furthermore, two-stroke engines pass their fuel/air charge through the crankcase in order to allow a slight pressurization, caused by the descent of the piston, to assist the flow of charge into the cylinder. As it passes through the crankcase, the charge entrains lubricating oil droplets, which are splashed on the crankshaft main and rod end bearings and sprayed on the cylinder walls and wrist pin. (Alternately, oil is mixed with the fresh charge before entering the crankcase, in which case the charge is used as an agent for transporting oil to the surfaces requiring lubrication.) Lubricating oil entrained in the charge is inducted into the cylinder, where it either flows through into the exhaust, creating more unburned hydrocarbon emission, or stays in the cylinder and is burned, creating a more noxious set of pollutants than would stem from the combustion of the engine fuel itself.
The pollution disadvantages of conventional two-stroke, spark-ignited engines (overlap of intake and exhaust flows and crankcase charge compression) lead to its advantages in day-to-day applications. Since the exhaust and intake strokes are not separate, for a given requirement for engine power and speed, at a gas constant compression ratio, a two-stroke engine requires only half the displacement of a four-stroke engine. The weight of the two-stroke engine would also be little more than half of the weight of a power-equivalent four-stroke engine and cost much less to produce. These advantages will prove very difficult to ignore in a developing economy, and thus, if two-stroke engines retain their conventional form, there is a great potential for globally significant increases in engine-related air pollution.
The present invention retains the engine size advantage of the two-stroke engine, the cost advantage of the carbureted two-stroke engine and reduces its unburned hydrocarbon emissions and lubricating oil combustion characteristics to levels comparable with the most advanced direct injected, two-stroke, dry-sump engines. This is accomplished with a relatively minor increase in cost for the inclusion of new parts and new machined or cast features on conventional parts. These parts and features allow the present invention, an improved two-stroke, spark-ignited engine, to operate with very little unburned fuel emission and with very little lubricating oil combustion.
SUMMARY AND OBJECTS OF THE INVENTION
Nearly complete reduction in unburned fuel emission is achieved in the invention by separating the air flow from the crankcase to the cylinder into two separate tubes. One tube contains air only and scavenges the burned gas out of the cylinder through the exhaust port. The top of the port for this scavenging tube is located relatively high on the cylinder wall and is uncovered sooner on the piston down-stroke. The other tube contains air and fuel and charges the cylinder. The top of the port for the charging tube is located relatively lower on the cylinder wall and is uncovered later on the piston down-stroke than the scavenging tube port. This timing of the ports will allow the air-only scavenging flow time to purge the cylinder of burned gas before the air/fuel charge flow is initiated. Fuel will not be mixed with air on inlet to the crankcase, as is the case in conventional two-stroke spark-ignited engines, but rather is mixed on the passage from the crankcase to the cylinder through the charging tube. Fuel is mixed with air only on its passage through this charging tube and not on its passage through the scavenging tube.
Not all of the scavenging air is exhausted from the cylinder, and the remainder is mixed with the air/fuel charge. Therefore, in order to maintain an appropriate overall air/fuel ratio in the cylinder, the charging air/fuel mixture must be rich in fuel. This rich charge can be directed, with appropriate port and tube design, towards the spark plug. Combustion is then initiated, at the plug, in a rich mixture (mostly rich charge and a little scavenge air) and burns out, away from the plug, into a lean mixture (mostly scavenge air and a little rich charge). This is precisely the principle behind stratified charge ignition, a widely recognized enhancement to combustion efficiency, pollution reduction and engine cycle efficiency. This sort of rich-lean combustion cannot be achieved in a conventional two-stroke, spark-ignited engine. It is achievable in the present improved two-stroke, spark-ignited engine because of the novel features of the invention. This advantageous form of combustion is also achievable using advanced, direct-injected, two-stroke engine technology; however, direct injection of fuel into the cylinder is costly, and such a system would be difficult to acquire and maintain in a developing economy. The present invention allows the advantages of stratified combustion while using only easily achieved, relatively low-cost technologies.
A further achievement of the invention's separated charging and scavenging flows is that the engine may be controlled by throttling only the charging flow. As a result, the present improved two-stroke spark-ignited engine will have higher partial-load efficiency than conventional two-stroke spark-ignited engines. Conventional spark-ignited engine control is achieved by throttling the intake flow, which reduces the amount of fuel entering the engine and also reduces the amount of air intake. These reductions are achieved by partially blocking (throttling) the intake flow, leading to large pressure drops in the intake flow and reduced engine efficiency due to the piston-cylinder pumping power needed to overcome this pressure drop. In the present improved two-stroke, spark-ignited engine, intake flow is divided into separate charging and scavenging flows. At partial-load only the charging flow needs to be throttled, leaving the scavenging flow without any pressure drop, and reducing the total amount of pumping power needed at partial-load, thus, increasing the engine's efficiency. This advantage in engine efficiency is also achievable using advanced, direct-injected, two-stroke engine technology; however, direct injection is costly and would be difficult to acquire and maintain in a developing economy. The present invention allows high efficiencies while using only easily achieved, relatively low-cost technologies.
Nearly complete reduction in lubricating oil combustion is achieved in the invention by using a novel system for dry-sump lubrication, in which oil is circulated by piston pumping action (assisted by a crankshaft-mounted pump if necessary) from a reservoir that is segregated from the crankcase by seals. The oil passes through and lubricates bearings in the crankcase via a system of sealed passages or conduits. Oil is pumped through these conduits by the novel arrangement of an oil sleeve mounted between the cylinder and the crankcase, a fixed oil seal positioned between the oil sleeve and the cylinder and a moving oil seal mounted on the piston. An annular oi
Ali Hyder
McMahon Marguerite
Synnestvedt & Lechner LLP
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