Field of Invention

The invention relates to an internal combustion engine and, more particularly, to an internal combustion engine which operates in accordance with a two stroke cycle. The invent¬ ion is applicable to any two stroke internal combustion engine such as those having a conventional crankshaft and connecting rod as well as crankless engines.

Background of Invention

Although two stroke internal combustion engines have been known for more than a century, they still have significant drawbacks and they have had limited application. There are heating problems normally experienced with two stroke inter¬ nal combustion engines. Furthermore, the exhaust emissions from such engines include several atmospheric pollutants such as carbon monoxide, hydrocarbons and nitrogen oxides.

Carbon monoxide is a colourless, odourless, poisonous gas, slightly lighter than air. The presence of this noxious gas in exhaust emissions is the result of incomplete combustion of fuel with the carbon partly oxidized to carbon monoxide instead of being fully oxidized to carbon dioxide. This is due to insufficient oxygen in the combustion chamber. Most conventional combustion chambers respond adversely to over- supply of oxygen or to a "lean mixture" as it is normally called.

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The presence of hydrocarbons in exhaust emissions also rep¬ resents unburned and wasted fuel. Generally, the percentage of hydrocarbons is high in emissions from two stroke engines and this is particularly due to the nature in which the engines are scavenged. Although gaseous hydrocarbons at concentrations normally found in the atmosphere are not toxic, they are a major pollutant because of their role in forming photochemical smog.

Nitrogen oxides are produced when fuel is burned at very high temperatures in the presence of oxygen. Nitrogen oxides combine with hydrocarbons to form a complex variety of secondary pollutants called photochemical oxidants which contribute to the formation of smog. The presence of nitrogen oxides in exhaust emissions has become a major problem in all conventional two stroke internal combustion engines, particularly where efforts have been made to reduce carbon monoxide by burning lean mixtures. Some experimental units have been successful in burning lean mixtures, but the excess oxygen in the combustion chamber converts to nitrogen oxides, which in the past have only been able to be removed by installing an expensive catalytic converter.

Among the oxides of nitrogen forming air pollutants, nitric oxide (NO) and nitrogen dioxide (NO ) occur most frequently. Nitric oxide is a colourless, toxic gas formed from nitrogen and oxygen at high temperatures. It converts to nitrogen di-

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oxide, an irritant and poison, in the exhaust of an internal combustion engine.

The formation of nitrogen oxides is a result of excess oxygen, combustion chamber temperatures above 1650 degrees celsius and the dwell period of the piston at top dead centre. The dwell period cannot be reduced in a conventional crankshaft engine. However, it has been found that if the high temperature zone is removed from the location of excess oxygen, the generation of nitrogen oxides is inhibited.

Description of the Invention

It is an object of the invention to minimise harmful emissions from a two stroke internal combustion engine.

It is also an object of the invention to minimise the heating problems normally associated with known two stroke internal con bustion engines.

The invention proposes a two stroke internal combustion engine having a cylinder, a compression chamber in the cylinder, at least one piston adapted to reciprocate in the compression chamber, at least one inlet port communicating with the compression chamber, means to supply scavenging air at high pressure to the compression chamber through the inlet port(s), at least one exhaust port for the discharge of scavenging air and products of combustion from the com¬ pression chamber, a small external combustion chamber having

restricted communication with the compression chamber, means to inject into the combustion chamber a rich fuel/air mixture, which in part passes to the compression chamber to form a fuel lean mixture with the scavenging air therein, and ignition means mounted in the combustion chamber to ignite the fuel rich mixture therein, in turn allowing burn¬ ing fuel to ignite the fuel lean mixture in the compression chamber, the arrangement being such that in operation the temperatures in the compression chamber remain sufficiently low to inhibit the formation of nitrogen oxides therein.

Not only is the formation of nitrogen oxides inhibited, there is substantially complete combustion of fuel due to the abundance of oxygen in the compression chamber thus minimising the emission of carbon monoxide and hydrocarbons. The high volume of scavenging air passing through the com¬ pression chamber is sufficient both to effectively purge and cool the cylinder. Because of the division of fuel between a fuel rich mixture in the small combustion chamber and a fuel lean mixture in the combustion chamber, significant fuel economy characterises the two stroke internal combust¬ ion engine of the invention.

Brief Description of the Drawings

The invention is described with reference to the accompany¬ ing drawings which illustrate two embodiments of the invention. In the drawings:

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Fig.l is a sectional view of a cylinder assembly of a single piston two stroke internal combustion engine according to one embodiment of the invention;

Fig.2 is a sectional view of a cylinder assembly of a two stroke internal combustion engine having opposed pistons according to a second embodiment of the invention;

Fig.3 is a sectional view at right angles to Figε.l and 2; and

Fig.4 is another sectional view at right angles to Fig.2.

Detailed Description of the Embodiments

Illustrated in Fig.l is a cylinder assembly of a two stroke internal combustion engine having a single piston 8 adapted to reciprocate in a cylinder 4. Piston 8 is connected to a conventional connecting rod and crankshaft (not shown). Cylinder 4 is formed with a small external combustion chamber 3 in addition to main compression chamber 6. Combus¬ tion chamber 3 is spaced from compression chamber 6 but has restricted communication therewith along a narrow passage 9. As shown in Fig.3, passage 9 is offset so that it enters compression chamber 6 in a substantially tangential direction.

Spark plug 1 is mounted on the external combustion chamber 3 for ignition of fuel/air mixture therein. The fuel/air mixture is injected into combustion chamber 3 through an inlet port controlled by poppet valve 2. The air is supplied by a blower (not shown).

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A blast of high pressure scavenging air is forced into compression chamber 6 through an inlet port controlled by a poppet valve 5. Scavenging air together with products of combustion exit compression chamber 6 by way of one or more exhaust ports 7. Opening and closing of exhaust ports 7 is effected by piston 8 as it reciprocates in cylinder 4.

To supply a large volume of high pressure scavenging air, a blower (not shown) is provided. The same blower may be used to supply air for the fuel mix as well as air for scavenging purposes, and, if desired for supercharging. The blower may be driven by the engine and, if desired, may be turbo exhaust assisted. The provision of a large charge of high pressure air insures an adequate throughput of scavenging air to not only purge the cylinder but also to cool it. Effective cooling is due in part to the fact that the scavenging air moves at a high velocity in a substantially spiral path down the internal wall of compression chamber 6. This swirling movement of scavenging air in compression chamber 6 is caused by a suitable design of the scavenging air inlet port. Where the port is controlled by poppet valve 5, means such as a fin (not shown) may be provided to appropriately deflect the scavenging air as it enters compression chamber 6.

The inlet port for the air/fuel mixture is relatively small in comparison with the other ports. Preferably, the inlet port for the scavenging air and the exhaust ports 7 are

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larger than those employed in conventional engines of the same size. Larger air inlet and exhaust ports facilitate the supply of an adequate throughput of scavenging air.

The engine operates throughout a complete two stroke cycle in the following manner. It is assumed that the cycle commences with piston 8 at bottom dead centre and commencing to rise in cylinder 4. Poppet valve 5 is closed shutting off the ingress of scavenging air. Compression chamber 6 would then be filled with substantially clean air. Poppet valve 2 would either be open or about to be open. In order to prevent any fuel from escaping to the atmosphere during scavenging, poppet valve 2 would not open until piston 8 had closed or almost closed exhaust ports 7.

When poppet valve 2 is open, a slightly rich fuel/air mixture is injected into the external combustion chamber 3. The fuel/air charge in part proceeds from external chamber 3, along passage 9 and into compression chamber 6. The sub¬ stantially tangential disposition of passage 9 as shown in Fig.3 is arranged so that the charge enters compression chamber 6 in the same direction as the swirling movement of the air therein and is dispersed thereby. The amount of charge allowed to enter compression chamber 6 depends on the power and revolutions required during the operation of the engine. However, exhaust ports 7 close as piston 8 ascends before the fuel mixture is able to descend to that point in compression chamber 6. In any event, it is not necessary for

the fuel mixture to descend more than about the half way point as the only purpose for entry of fuel into compression chamber 6 is to undergo dilution with the remaining scaveng¬ ing air in order to provide a lean mix. The lean mix with excess oxygen in compression chamber 6 promotes complete combustion of all fuel. If a supercharge is required, poppet valve 2 must remain open until piston 8 has completely shut off exhaust ports 7.

After poppet valve 2 closes, piston 8 continues to rise in cylinder 4 on the compression stroke. The air in cylinder 4 is compressed in compression chamber 6, passage 9 and combustion chamber 3. However, the dimensions of passage 9 are such that the fuel/air mix adjacent to spark plug 1 remains sufficiently rich for it to be ignited by spark with a consequent explosion which forces burning fuel along passage 9 into compression chamber 6. The remaining charge, having been made lean by mixing with scavenging air, is ignited by the burning of the richer charge and burns with¬ out detonation. As. the burning fuel is forced into compression chamber 6 in the same direction as the swirling movement of the air therein, the velocity of the air movement is increased and burning is promoted.

The relationship of combustion chamber 3 and compression chamber 6 is such that two separate zones with a significant temperature difference result. In the first place, a high temperature zone is produced near the centre of explosion in

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combustion chamber 3. Secondly, there is a zone of signific¬ antly lower temperature in compression chamber 6. As a con¬ sequence, there is effective separation of the high temperature zone in combustion chamber 3 and the excess oxygen from the leaner mix in compression chamber 6. The temperature of the excess oxygen is sufficiently low to inhibit formation of nitrogen oxides. Furthermore, due to the excess oxygen, substantially complete combustion is achieved resulting in the absence of carbon monoxide and hydrocarbons from the exhaust emissions. Another consequence is that reciprocation of piston 8 in cylinder 4 is a relatively cool operation so that the engine avoids the traditional heating problems normally associated with two stroke engines.

The power stroke then follows with blowdown taking place as cylinder 8 exposes exhaust ports 7. Poppet valve 5 now opens and scavenging air is forced into compression chamber 6 to fully purge it with clean air and to cool cylinder 4.

The cylinder arrangement illustrated in Fig.2 is for use with horizontally opposed pistons. The reference numerals used in this figure and operating principle remain the same as for Fig.l except poppet valve 5 of Fig.l is replaced by open ports 15 which are opened and closed by one of the reciprocating pistons 8. Ports 15 are arranged in cylinder 4 such that exhaust ports 7 are uncovered just before ports 15 so as to allow blowdown. As shown in Fig.4, the

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scavenging air inlet ports 15 are angled so that the air enters compression chamber 6 in a substantially tangential direction to produce a swirling movement of scavenging air therein.

An advantage of this engine is that the exhaust ports 7 may be unrestricted around the circumference of cylinder 4. This is due to the absence of transfer ports, which are required at the exhaust ports of most conventional two stroke engines, but are not necessary with the present design. This allows for more efficient breathing.

The present system also allows conventional two stroke engines to operate with a wet oil sump lubrication as lubri¬ cation oil is not required in the fuel. This permits engine sizes to compete with four stroke engines.

The two stroke engines described may be adapted to employ any one of a wide range of fuels such as petrol, gas and diesel oil and no particular additives are required.

The embodiment described with reference to Fig.l may have the roles of exhaust port 7 and inlet valve 5 interchanged thus reversing the flow pattern of the scavenging air. This arrangement has the advantage of returning to combustion chamber 3 any residual exhaust gases and any gases that may have escaped past piston 8 during the expansion stroke.

In another modification, the embodiments of Figs. 1 and 2 may be provided with an additional inlet port controlled by a poppet valve for the admission of scavenging air into combustion chamber 3. This would allow combustion chamber 3 and passage 9 to be scavenged. This arrangement would be suitable for larger engines.

In a further modification, the additional port in the combustion chamber may function as an exhaust port. Thus any gas escaping past the piston(s) 8 during expansion would be returned to combustion chamber 3. Further, any unburned hydrocarbons would be oxidised while passing through hot combustion chamber3.

In a still further embodiment, compression chamber 6 may be provided with a large exhaust port controlled by a poppet valve so that the other ports in the compression chamber 6, and, if provided, the additional port in the combustion chamber 3, would be arranged to function as inlet ports for scavenging air.

In any of the embodiments described, more than one external combustion chamber 3 may be provided, particularly in the case of large engines. The combustion chambers 3, each with a passage 9, may be spaced around compression chamber 6. Moreover, engines may employ more than one cylinder 4.

As previously described, the substantially tangential disposition of passage 9 as shown in Fig.3 is arranged so that the fuel charge enters compression chamber 6 in the same direction as the swirling movement of the air therein and is dispersed thereby. This would generally be the case. However, for certain capacity engines, it may be necessary for the fuel charge to enter compression chamber 6 in opposition to the direction of swirl to dampen excessive swirling movement of the air. In that case, there would still be effective dispersion of the fuel charge.

Other modifications of the details described would be readily apparent to persons skilled in the art and many changes may be made without departing from the broad inventive concepts herein described. For example, the exterior surface of the cylinder may be provided with radiation fins to assist cooling.