WO1998051908A1

WO1998051908A1 - Scavenging system for two stroke internal combustion engine

Info

Publication number: WO1998051908A1
Application number: PCT/AU1998/000349
Authority: WO
Inventors: Simon John Moss
Original assignee: Simon John Moss
Priority date: 1997-05-14
Filing date: 1998-05-13
Publication date: 1998-11-19
Also published as: AUPO698897A0

Abstract

A two stroke internal combustion engine comprising at least one cylinder (2) with a piston (1) arranged therein for cycles of reciprocating compression and power strokes. There is a fuel inlet valve (6) for introducing a combustible fuel mixture into the cylinder (2) prior to the compression stroke, an exhaust outlet port (4) for enabling the expulsion of exhaust gases at the completion of the power stroke and a purging valve (5). The purging valve (5) introduces gas into the cylinder under positive pressure during opening of the exhaust port (4) at the completion of the power stroke to flush exhaust gases from the cylinder prior to entry of the fuel mixture for the succeeding cycle. The pressurised gas to the purging valve (5) is provided by a blower fan (11) driven off the crankshaft of the engine.

Description

SCAVENGING SYSTEM FORTWO STROKE INTERNAL COMBUSTION ENGINE

This invention relates to internal combustion engines. More particularly although not exclusively it discloses an improvement to two stroke engines.

While the two stroke engine provides a power impulse per cylinder with each revolution of the crankshaft its efficiency is generally less than that of a comparably sized four stroke. The general principle of a two stroke engine involves shortening the periods during which fuel is introduced into the combustion chamber and exhaust gases expelled to a small portion of a stroke. With the basic two stroke engine the inlet and exhaust valves comprise opposed ports in the cylinder wall. The fuel mixture is introduced through the intake port from the crankcase when the piston is at or close to bottom dead centre at the end of its preceding power stroke. The shape and configuration of the piston head and ports direct the incoming flow of the fuel mixture to both displace the spent gases from the previous power stroke out through the exhaust port and recharge the cylinder for the next cycle. In practice however this purging operation tends to be inefficient. The momentum of the incoming charge is restricted by the limited primary compression of the crankcase. Also, the exhaust mixes with the fuel charge to the extent that some of the fuel is expelled directly out the exhaust port without being burned while on the other hand a significant portion of the spent gases can remain in the cylinder especially at low speeds or if undue back pressure is present in the exhaust system. A further substantial advantage of existing two stroke petrol engines is the requirement that the lubricating oil be mixed with the fuel.

It is therefore an object of this invention to ameliorate the aforementioned disadvantages and accordingly a two stroke internal combustion engine is disclosed comprising at least one cylinder with a piston arranged therein for cycles of reciprocating compression and power strokes, fuel inlet valve means for introducing a combustable fuel mixture into said cylinder prior to said compression stroke, exhaust outlet valve means for enabling the expulsion of exhaust gases at the completion of said power stroke and purging means for introducing gas into said cylinder under positive pressure during opening of said exhaust outlet valve means at the completion of said power stroke to flush exhaust gases from said cylinder prior to entry of the fuel mixture for the succeeding cycle .

Preferably the gas used is air and said purging means comprises an air pump, compressor or blower driven from the crankshaft of said engine and connected to a purging gas inlet valve in said cylinder.

A currently preferred form of this invention will now be described with reference to the attached drawings in which:

Figures 1 to 6 are schematic cross-sectional views of a cylinder and piston assembly showing the power and compression strokes of a two cycle engine according to this concept, and Figure 7 is a schematic cross-sectional view similar to figures 1 to 6 but showing the operation of the poppet valves and air blower.

Referring first to figures 1 to 6 the engine is shown initially with the piston 1 at or near top dead centre in the cylinder 2. While only one cylinder is shown in these illustrations the principle is identical for multi- cylinder engines. At the piston position shown in figure 1 ignition of the fuel mixture 3 occurs electronically by way of a spark plug. During the subsequent power stroke (figure 2) the piston moves downward under the force of the burning expanding fuel mixture until the exhaust port 4 is uncovered (figure 3) . As shown in figure 4 the purging gas inlet valve 5 opens at or near bottom dead centre whereby the cylinder is flooded with pressurised air to flush out the residual exhaust gases. Immediately following this flush out a second inlet valve 6 opens to allow a fresh charge of fuel mixture into the cylinder prior to compression and start of the next cycle. Preferably as shown in figure 7 the two inlets 5 and 6 comprise cam actuated poppet valves. More specifically there may be two overhead cam shafts 7 and 8 which are driven off the crankshaft 9 by a timing belt 9A. A spark plug 10 is also shown in this particular embodiment which may operate on the Otto cycle. The pressurised air to the inlet valve 5 is provided by a centrifugal blower fan 11 also driven by a belt 12 off the crankshaft. Atmospheric air is drawn into this blower and then maintained at a predetermined positive pressure for forced entry into the cylinder at the appropriate time when valve 5 is open as described earlier. With this embodiment both valves 5 and 6 operate from a common intake manifold 13 so that the same blower air supply can be used to subsequently charge the cylinder through valve 6 with the addition of a fuel spray from injector 14.

It will thus be appreciated that this invention at least in the form of the embodiment described provides a novel and useful improvement to two stroke internal combustion engines. As well as improving the operating efficiency of such engines and thus reducing fuel consumption it is considered that the flush out of exhaust gases using air or possibly a very lean air/fuel mixture would enable more exhaust pollutants such as unburned fuel and carbon monoxide to be post combusted. Also, as the fuel is introduced into the cylinder via an injector and blower air supply the crankcase is available for use as a lubricating oil sump. This allows unmixed fuel to be used. Clearly however the example disclosed is only the currently preferred form of this invention and a wide variety of modifications may be made which would be apparent to a person skilled in the art. While a combination of cylinder port for the exhaust, and cam operated poppet valves for the inlets are currently proposed other arrangements may be employed. For example the inlet poppet valves may be solenoid actuated or cylinder ports used as an alternative.

The timing, duration, flow volume and pressure levels used for the purging gas are currently the subject of further research by the inventor.

Claims

The claims defining the invention are as follows:

1. A two stroke internal combustion engine comprising at least one cylinder with a piston arranged therein for cycles of reciprocating compression and power strokes, fuel inlet valve means for introducing a combustable fuel mixture into said cylinder prior to said compression stroke, exhaust outlet valve means for enabling the expulsion of exhaust gases at the completion of said power stroke and purging means for introducing gas into said cylinder under positive pressure during opening of said exhaust outlet valve means at the completion of said power stroke to flush exhaust gases from said cylinder prior to entry of the fuel mixture for the succeeding cycle .

2. The two stroke internal combustion engine as claimed in claim 1 wherein said gas is air.

3. The two stroke internal combustion engine as claimed in claim 2 wherein said purging means is connected to a purging gas inlet valve means in said cylinder.

4. The two stroke internal combustion engine as claimed in claim 3 wherein said purging means comprises an air pump, air compressor or air blower.

5. The two stroke internal combustion engine as claimed in claim 4 wherein said purging means is driven off the engine exhaust.

6. The two stroke internal combustion engine as claimed in claim 4 wherein said purging means is driven off the crankshaft of said engine.

7. The two stroke internal combustion engine as claimed in claim 4 wherein said purging means is driven off an electrical system of said engine.

8. The two stroke internal combustion engine as claimed in claim 4 wherein said fuel inlet valve means and purging gas inlet means are poppet valves located in the head of said cylinder.

9. The two stroke internal combustion engine as claimed in claim 8 wherein said poppet valves operate from a common intake manifold and a fuel injector is associated with said fuel inlet poppet valve.

10. The two stroke internal combustion engine as claimed in claim 9 wherein said poppet valves are located on opposite sides of the cylinder head and are actuated by cam shafts driven off the crankshaft of said engine.

11. The two stroke internal combustion engine as claimed in claim 9 wherein said poppet valves are actuated by solenoids.

12. The two stroke internal combustion engine as claimed in claim 4 wherein said purging gas inlet valve means comprises a port in said cylinder.

13. A two stroke internal combustion engine substantially as described herein with reference to the drawings.

14. A power driven apparatus which includes the two stroke internal combustion engine as claimed in claim 1.