GB2255803A - Supercharged two-stroke engine. - Google Patents

Abstract

Two-stroke piston and cylinder assemblies have crankshafts 10 with respective toothed wheels 12 driving a toothed flywheel 14 directly or through a toothed belt. A centrifugal or vane compressor rotor 14 on the fly wheel shaft 16 supplies air or mixture to the cylinder inlet ports (34, Fig. 1). <IMAGE>

Description

TWO STROKE ENGINE The present invention relates to a two stroke engine having improved characteristics.

The two stroke cycle has been known for many years but in many respects the two stroke engine has never reached its full potential. One problem is that of scavenging, which is the process by which exhaust gas is removed from the cylinder to be replaced by fresh fuel/air charge. Thus, in a conventional two stroke engine, the intake and exhaust ports are arranged so that the fuel mixture is introduced into the cylinder before a complete exhaust cycle has taken place. This results in a diluted intake charge and also to fuel being lost with the exhaust gas. This leads to a reduction in power per stroke and also increases the pollution caused by the engine. In order to minimise these effects, it is desirable to minimise the overlap time when both the inlet and outlet ports are open together.

As well as unburnt fuel, the exhaust gas also contains residues of the oil which is conventionally fed to the engine for lubrication and cooling purposes along with the fuel. Both these effects contribute to the pollution caused by the engine.

The presence of oil in the fuel feed also causes a build up-of carbon within the engine and particularly around the port areas.

Moreover, the engine tends to be noisy as it is susceptible to back pressure in the silencer.

An object of the present invention to provide a single or multi-cylinder two-stroke engine which mitigates some of these problems.

The present invention provides a two-stroke engine, which comprises a cylinder having a reciprocatable piston therein, and having an inlet port and an outlet port; and compressor means connected to the inlet port for compressing air prior to induction into the cylinder.

The compressor can be any compressor known in the art but is preferably a centrifugal or vane-type compressor.

The compressor may be mounted directly on a fly-wheel of the engine. The engine may be arranged to operate according to a petrol cycle or diesel cycle. In the case of a petrol cycle, fuel (which may be petrol or petroleum gas) is preferably injected directly into the compressor where it becomes mixed with the air. This may be achieved by providing the compressor with fuel entry bores in the rotor or, which are supplied by a central bore through the rotor shaft. In a diesel cycle engine, the compressor merely compresses the inducted air, whilst fuel is injected directly into the cylinder.

The benefit of employing a compressor is that the charged gas is at a higher pressure, so that induction into the cylinder may occur later in the exhaust cycle, thereby minimising the overlap between the inlet and outlet ports. This reduces carry over of unburnt fuel into the exhaust gas. In a preferred embodiment, the inlet port is angled in a direction towards the top of the cylinder (preferably at an angle of 30 to 60 to the line of the cylinder) so as to optimise filling of the cylinder by the compressed gas charge. In this way, the degree of overlap can be reduced from the conventional range of about 95% down to 5 to 30%. In a diesel cycle minimisation of overlap is not so critical and some scavanging by inducted compressed air may be desirable.

A non-return valve may be provided between the compressor means and the inlet port.

In a preferred embodiment of the invention, the lubricating oil is contained within a crankcase beneath the cylinder, rather than being introduced with the fuel.

The oil will normally be contained in a sump in the crank case or in a separate container and led to the crank case by appropriate passageways either as an oil mist or as an oil flow. Suitable means may be provided for recycling the oil for cooling and filtering.

As mentioned previously, the engine may be provided with a fly-wheel. This is preferably in the form of a toothed wheel which engages a corresponding cog on the end of the crankshaft. The gear ratio between the crankshaft and the fly-wheel will be determined by the chosen output shaft speed and torque requirements but a tooth ratio on the fly-wheel to crankshaft cog in the region 2:1 to 6:1 is preferred. In a multi-cylinder engine, two or more cylinders and their corresponding crankshafts can be arranged around the periphery of the fly-wheel, each operating independently so as to provide power to the fly-wheel. In principal, any desired number of cylinders may be used, such as 3, 4, 5, 6, 7, 8 etc. At least some of the cylinders may be arranged side by side sharing a common crankshaft if desired.

Preferably, the ignition is activated by fly-wheel magnets, which act on power cells and are adjusted to suit the firing order requirements. In the case of a diesel engine the fly-wheel magnets may be replaced by a cam-operated injectors, the cam being driven off the crankshaft. In multi-cylinder engines, each cylinder may be adjusted relative to other cylinders so as to give the desired power requirements.

An embodiment of the present invention will now be described by way of example only with reference to the drawings wherein; Figure 1 is a side elevation of a two-cylinder two-stroke engine partially cut away; Figure 2 is a rear elevation; Figure 3 is a view from above; and Figure 4 is a view in the direction of the arrow shown in Figure 1.

The-engine comprises a pair of two-stroke cylinder units 2, 4 mounted on a housing 6 containing a vane-type compressor 8, each unit has a respective output shaft 10 provided with a toothed wheel 12 in engagement with a toothed fly-wheel 14 mounted on a common shaft 16 with the compressor rotor 18. Alternatively, the toothed wheels could be interconnected by a flexible toothed belt.

Each cylinder unit comprises a crank case 20 within which a crankshaft 22 runs in bearings. A conrod 24 is attached at its big end 26 to a web of the crankshaft and at its little end 28 to a piston 30. A cylinder head 32 is also provided. An oil control scraper ring 33 is provided in a groove in the cylinder bore.

The cylinder has an inlet port 34 which is angled towards the combustion chamber defined between the piston and the flat cylinder head. A conventional exhaust port 35 is also provided. The inlet port is smaller in area than the exhaust port to minimise back-pressure.

The inlet is generally positioned at a point in the exhaust cycle (for slowest speed running) where the exhaust gas pressure is substantially equal to the atmospheric pressure in the exhaust system, so that the pressure in the cylinder is never sub-atmospheric.

In the embodiment shown, air and fuel mixture from a carburetor is inducted into the compressor via a drilling 36 in rotor shaft 38 which communicates with radial extending drillings 38 which exit between vanes 40 of the compressor. Outlets for compressed gas/fuel mixture are arranged tangentially from the compressor. Outlet 42 extends to a cylinder unit 4 and the compressed charge enters via non-return valve 44.

Oil for the engine is contained within the crankcase 20.

For the petrol cycle shown, conventional spark plugs are mounted in the cylinder head (not shown) and actuated by means of magnets 46 of a magneto unit 48 mounted on the opposite end of crankshaft 10 to the tooth cog 12.

In the above embodiment, the same reference numerals are used for analogous parts in the two cylinder units.

If desired a number of such engines as shown in Figure 1 can be connected side-by-side, possibly on a common rotor shaft or connected via clutch units.

The engine may be operated as follows. The rotor 14 is rotated by means of a suitable starter motor (not shown) so as to operate the compressor and produce a pressurised gas charge. Rotation of the compressor draws fuel through the bores in the rotor and mixes it with the compressed gas. The compressed gas charge is fed through the non-return valve into the inclined inlet port leading to the combustion chamber of each cylinder unit, where the charge is introduced late in the exhaust cycle. After introduction of the charge, the continued rotation of the fly-wheel 14 rotating the crankshaft causes the piston to move upwards and compress the charge. The compressed charge within the cylinder is ignited by means of a spark plug operated via the magneto. As the piston moves downwardly, exhaust gas is exhausted and the process repeats itself.

Claims (10)

1. A two-stroke engine, which comprises a cylinder having a reciprocatable piston therein, and having an inlet port and an outlet port; and compressor means attached to the inlet port for compressing air prior to induction into the cylinder.

Arneocknents to the cIakns hm'e been fSed as folows 1. A two-stroke engine, which comprises a plurality of cylinder assemblies each assembly comprising a cylinder having a reciprocatable piston therein connected to a crankshaft, and having an inlet port and an outlet port; compressor means connected to each inlet port for compressing air prior to induction into the respective cylinder; and a flywheel, the crankshaft of each cylinder assembly being in driving engagement with periphery of the flywheel.

2. An engine according to claim 1 wherein the compressor means is provided with fuel entry bores such that a mixture of fuel and air is compressed.

3. An engine according to claim 1 which further comprises a fuel injector mounted on each cylinder for direct injection of fuel into the cylinder.

4. An engine according to any preceding claim wherein the inlet port is angled 30 to 600 to the line of the cylinder in a direction towards the top of the cylinder.

5. An engine according to any preceding claim wherein the overlap between the opening of the inlet and exhaust valves is 5 to 30%.

6. An engine according to any preceding claim which comprises an oil sump for lubricating oil located in a crankcase beneath the cylinder.

7. An engine according to any preceding claim wherein the flywheel comprises gear teeth arranged around the periphery thereof in engagement with cogs provided on the respective crankshafts.

8. An engine according to any preceding claim wherein the gear ratio between the flywheel and the crankshaft of each cylinder is in the region 2:1 to 6:1.

9. An engine according to any preceding claim which comprises two to eight cylinder assemblies.

10. A two-stroke engine substantially as described in conjunction with the drawings.