GB2350153A - Blowerflow injected two-stroke engine - Google Patents

Abstract

Air is blown through the venturi(3) where it increases in velocity into the cylinder to purge it through the inlet port(2) while uncovered by the piston(9). The piston rises, closing the inlet(2) and exhaust(4) ports and compression begins. While the inlet port is closed, air flow is prevented from passing through the inlet manifold into the inlet port. Depression created in the venturi associated with the cylinder being purged is used to draw and relieve air pressure developing in the closed inlet ports of cylinders on the compression and power strokes via inlet transfer ducts(5) to assist purging and charging the cylinder with clean air before compression. An engine management/fuel injection system delivers fuel into the cylinder after the inlet port has closed ensuring that only air is passed to the exhaust system during the purge operation, eliminating the possibility of fuel passing directly to the exhaust.

Description

2350153 BLOWERFLOW INJECTED TWO-STROKE ENGINE The present invention

relates to a two-stroke engine without valves but having an externally driven blower or compressor supplying air to the engine, fuel being provided by a fuel injection and engine management system.

The invention seeks to provide a system that improves control over fuel/air distribution and improves both the purging of exhaust gases and charging of cylinders, while increasing the duration of the power stroke.

In a conventional two-stroke engine, the cycle of operations for each cylinder is completed within one revolution of the crankshaft. Air is induced into the crankcase by the action of reduced pressure generated in the crankcase while the piston is in its upward stroke. The piston in its downward stroke causes an increase in pressure in the crankcase which is relieved into the cylinder by a system of transfer ports, while the gases produced by combustion are forced out of the cylinder by the expansion of the exhaust gas and the scavenging flow of the intake gas. This process is termed loop scavenging.

Many enhancements exist in prior art to improve the performance and efficiency of the two-stroke cycle but a number of problems are common to most loop scavenged two-stroke engine designs to a lesser or greater extent, principally as follows:-

1. Under low-load conditions when a small amount of throttle is used, the scavenging flow tends to be insufficient. This can cause misfiring and unstable operation of the engine.

2. Exhaust gas cannot always be entirely purged from the cylinder. This residual gas degrades the fresh charge of fuel/air entering the cylinder.

3. At certain engine speeds it is difficult to prevent some of the fresh inlet charge passing through the cylinder to the exhaust port. This shortcircuiting has the effect of reducing efficiency and economy and increasing polluting exhaust emissions.

4. There are limits to the increase in scavenging ability of the engine using crankcase displacement alone. This can make it difficult to obtain smooth torque characteristics throughout the speed range and to obtain sufficiently increased torque of the engine under high load conditions.

5. Conventional two-stroke engines rely on a mixture of oil in the fuel/air mix to lubricate the crank, connecting rod bearings and pistons. This oil is then burned in the cylinder which raises pollution levels.

6. Because of the nature of the lubrication method, two-stroke engines are generally fitted with roller bearings on the crank-shaft and connecting rods which are noisier and less able to take higher loads and temperature than the plain, white metal type bearings commonly fitted to four-stroke engines.

f I The loop scavenged engine type is the most commonly used two-stroke engine and has been almost universally adopted for all two-stroke engine applications. This is because it is simple, having only pistons and a crankshaft assembly, and the fact that its power-to-volume and power-toweight ratios are the highest of the possible configurations; but it is by no means the only type. Another form of two-stroke engine known as the uniflow engine dispenses with the action of air displacement in the crankcase to introduce fresh charge to the cylinder in favour of the use of an air blower which drives the fuel/air mixture through a valve in the top of the cylinder. This forces the spent gases out via an exhaust port system at the base of the cylinder. There is less mixing of the intake charge with the spent gases and less possibility of the fresh charge short-circuiting straight to the exhaust. Pollution levels are low but performance suffers because the inlet valve can restrict gas flow through the engine. The uniflow engine has largely been overlooked for automotive applications in favour of the more efficient and simpler loop scavenged two-stroke engine. Nevertheless, it still offers a superior power to weight ratio when compared with a four-stroke engine, having twice the firing frequency per cycle and fewer moving parts. A further configuration of the two-stroke engine exists in prior art of which GB patent application no 9028205.7 is typical whereby air is introduced via a piston-controlled port into the cylinder by an external blower via a rotary valve. A similar rotary valve is also incorporated into the exhaust port side of the engine. This engine is controlled by a complex engine management system which varies the timing and opening of the rotary valves to give optimum engine characteristics under varying operating conditions. Numerous variations of this engine type exist in prior art, each providing enhancements of the basic theory and function of the engine configuration. As those conversant with the two-stroke cycle would know, automotive twostroke engine development has centred around striking a compromise between a high exhaust port for improved gas flow through the engine giving greater 'top end' power and a reduced tendency for fuel/air to short circuit to the exhaust, and a lower exhaust port for greater torque and driveability. As a result of this compromise, many two-stroke engines, particularly those with higher power outputs, have a characteristic commonly known as a'power band'where the engine operates very efficiently at or around a given engine speed but not so well above or below. Automotive two-stroke engine development is largely aimed towards making this 'power band' as wide as possible to improve the useful speed range of the engine. The positioning of the exhaust port also affects the open duration of the port, which again has an effect on the efficiency of the motor. A typical exhaust port open duration for an automotive application is in the region of 2000. This would infer that the compression and power strokes are combined into the remaining 1600.

z If, as an example, the exhaust open duration were 200' and the compression and power strokes assumed to be 160" + 2 = 800 each, the motor is producing useful power for only 80' of each revolution of the engine.

It is the object of the present invention to provide a two-stroke engine having no valves which gives improved scavenging efficiency and improved torque characteristics over the entire operating speed range of the engine, the engine also having power to volume and power to weight ratios similar to those of the loop scavenged two-stroke engine but with improved fuel economy and reduced polluting exhaust emissions, and with smoother operation.

According to the present invention there is provided a two-stroke engine having a plurality of cylinders, a piston slideable in each cylinder, an exhaust port which is opened and closed by the piston, an inlet port for supplying scavenging air to the cylinder which is opened and closed by the piston and a fuel injector for injecting fuel into the cylinder: said engine also having an externally driven air blower to introduce scavenging air into the cylinder via an inlet manifold system with interconnecting transfer ducts and a throttle assembly. Fuel is provided by an engine management and fuel injection system to optimise fuel mixture and timing.

By way of example, the engine illustrated in Fig. 1,2 & 3, typically has three cylinders each having a reciprocating piston, a fuel injector and, dependent upon fuel type, a spark plug. Each cylinder has an inlet port with a venturi device and an exhaust port in its wall which are uncovered by the piston as it approaches bottom dead centre, and a plurality of inlet transfer ducts connecting between the inlet ports. A throttle valve is also provided close to the venturi in each inlet port.

Air is introduced to the engine by a blower (1) which is connected to the inlet port (2) of each cylinder. Said air travels first through the venturi (3) where it increases in velocity and into the cylinder through the inlet port while it is uncovered by the piston.

As the piston rises, it closes both the inlet port (2) and the exhaust port (4) and the compression phase begins. While the inlet port is closed, air from the blower is prevented from passing through the inlet manifold into the inlet port and pressure tends to build up against the side of the piston, creating a sideways force pushing the piston against the cylinder wall which could, over time, cause wear on one side of the piston and cylinder unless the build up of pressure in the inlet port were relieved by some means.

Using the illustrated engine as an example, the engine has a crankshaft with three crankpins at 1200 intervals.

As the piston rises and falls during the two-stroke cycle, it will perform 3 operations: compression, power and purge, each for a period of approximately 1200 of crankshaft rotation, depending upon port positioning.

Given that the illustrated engine has 3 cylinders, it follows that at any one point during the engine's rotation, one cylinder will be performing each operation.

The depression created by the flow of air through the venturi associated with the cylinder being purged is used to draw and relieve air pressure developing in the closed inlet ports of the two cylinders on the compression and power strokes via the inlet transfer ducts (5) to assist in purging spent gases and charging the cylinder with clean air for the compression operation. The fuel injection system delivers a measured quantity of fuel into the combustion chamber or cylinder through the injector port (7) after the inlet port (2) has closed thereby ensuring that only air is passed to the exhaust system during the purge operation thus eliminating the possibility of fuel short-circuiting to the exhaust (4) and thus helping to keep harmful exhaust emissions low. Dispensing with the use of pressure variation in the crankcase to induce fuel/air mixture into the cylinder with the associated need to add lubricating oil to the fuel/air mixture enables the inclusion of plain white metal type bearings into the design of the crankshaft and connecting rod assemblies, lubricated by an oil gallery and an engine driven oil pump, instead of roller bearings. This method offers the benefits of quieter operation, improved lubrication, improved durability and extended service life. Engine Operation Seguence The engine illustrated has 3 cylinders with crankpins set at 1200 intervals. The cylinder block has 3 cylinders, each cylinder having an inlet port and an exhaust port on opposite sides, the top of the ports corresponding to 1200 after TDC and 1200 before TDC, for inlet and exhaust respectively. Power; 00 - 1200 (fig. 3) The fuel/air mixture in the cylinder is ignited by the spark plug (9) as the piston approaches TDC. The burning mixture rapidly expands forcing the piston down after TDC as pressure increases. As the piston continues downwards it uncovers the exhaust and inlet ports where exhaust gases exit to the exhaust system and clean air is blown in. Purge; 12W - 2400 (fig. 1) As the ports are uncovered clean air is introduced to the cylinder forcing exhaust gas out. This clean air is blown through the cylinder as the piston passes bottom dead centre (BDC) aiding cooling and completely purging the cylinder of exhaust gas. As the piston begins to rise, the inlet and exhaust ports are closed as it passes. The approach angle of the inlet port and shaping to the top of the pistons can be designed to promote swirl in the cylinder to improve both purging and combustion. Compression; 2400 - 36W (fig. 2) As the piston rises and closes the inlet and exhaust ports and the piston continues to rise towards TDC, fuel is injected into the cylinder through an injector port (7) either in the cylinder wall or cylinder head. The mixture is compressed as the piston continues to rise. If diesel or petrol direct injection technology is employed, fuel is injected at the required timing point, usually close to TDC.

4 Manifold Action As the crankshaft rotates, each cylinder carries out each phase in turn.

Cylinder 1: at or near MC on compression stroke with both ports closed by the piston. Air flow in the manifold is halted by the piston. Air pressure building in the inlet port is drawn by venturi action through the inlet transfer ducts (5) to an adjacent cylinder on its purge phase. (in this example 2) Cylinder 2: at or near 1200 after top dead centre (ATDC) on firing phase, where purge phase begins and both ports begin to open, allowing exhaust gas to escape and the ingress of clean purging air. Airflow through the cylinder is unimpeded and draws air from cylinders 1 and 3 by venturi action via the inlet transfer ducts (5).

Cylinder 3: at or near 120" before top dead centre (BTDC) at end of purge phase where compression phase begins. Both ports are closed and the piston is rising. Air pressure building in the inlet port is drawn to cylinder 2 which is purging. Fuel is injected into the cylinder after the ports close but before the ignition point.

As each cylinder in turn begins purging, air is drawn to the purging cylinder as the venturi creates low pressure in the manifold, relieving lateral pressure on the cylinders with ports closed by the piston and increasing the purging air flow to aid cooling of the purging piston and cylinder and the exit of exhaust gases. The relief of lateral pressure reduces piston and cylinder wear.

The effective power stroke for an engine according to the invention will be approximately 12W which is considerably longer than that of a conventional two-stroke engine, (typically 80'-850), and only marginally shorter than a fourstroke engine, (typically 1250-1350). The power strokes will follow each other with a very small gap making the engine smooth in operation. If the effective power stroke of a four cylinder four-stroke engine is 130", and the firing frequency is 1800, the engine is doing nothing for up to 500 of every firing stroke. The same can be said for a four-stroke six cylinder engine except that, firing every 120', there is an overlap of the power strokes which makes a six cylinder engine much smoother in operation than a four cylinder engine.

Given that a three cylinder engine according to the invention would also give a firing stroke every 1200, a smooth power and torque output would be attainable from this engine similar to that of a 6 cylinder four- stroke engine of twice the engine capacity. Although this example illustrates the action of a three cylinder engine, the overlapping of power strokes which would occur with a greater number of cylinders would further enhance torque output and smoothness.

While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that the disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.

Claims (8)

1. A blower assisted two-stroke engine without valves having a plurality of cylinders, each having a piston slideable in the cylinder, an inlet port incorporating a venturi device and an exhaust port which are opened and closed by the piston, an air blower or air compressor connected to said inlet port for supplying purging air into the cylinder, and a fuel injector for injecting fuel into the cylinder. said engine having an inlet manifold system incorporating interconnecting transfer ducts between the inlet ports whereby the open inlet port of a purging cylinder draws air by venturi action from the closed inlet ports of the other cylinders thereby enhancing airflow to the purging cylinder and relieving lateral pressure against the pistons of other cylinders.

2. A blower assisted two-stroke engine as claimed in claim 1, wherein the duration of the power stroke is increased to improve torque characteristics across the engine's operating speed range.

3. A blower assisted two-stroke engine as claimed in any preceding claim, wherein the duration of the power stroke is increased without risk of fuel/air mixture passing directly to the exhaust system.

4. A blower assisted two-stroke engine as claimed in any preceding claim, wherein only air is forced into the cylinder while the inlet and exhaust ports are open, thus preventing the possibility of fuel passing directly to the exhaust port.

5. A blower assisted two-stroke engine as claimed in any preceding claim, wherein a volume of air relative to throttle position is induced into the cylinder to improve purging and charging of the cylinder.

6. A blower assisted two-stroke engine as claimed in any preceding claim, having a purging flow which varies as a function of engine speed and load.

7. A blower assisted two-stroke engine as claimed in any preceding claim, with crankshaft bearings of the plain white metal type for improved load bearing capacity, durability and extended service life.

8. A blower assisted two-stroke engine as claimed in any preceding claim, wherein an oil pump and lubricating gallery is provided for the purpose of lubricating and cooling the engine components.

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