The present invention relates to a compressed gas supply system for an internal combustion engine which may be used for example for an air-assisted or an air-driven fuel injector.

Air-assisted and air driven fuel injectors have been proposed to produce a finely atomised spray of fuel. However, such injectors require a source of compressed gas. Previously, mechanically driven compressors were used for this purpose but apart from their additional cost such compressors set a lower limit on the idling speed because the air used for atomisation bypassed the intake throttle.

To mitigate the foregoing disadvantages, a compressed gas supply for an internal combustion engine has been proposed that comprises a one-way valve in at least one of the combustion chambers of the engine for bleeding a small proportion of the compressed charge into a plenum chamber and a pressure regulating valve for drawing gas at a lower pressure from the plenum chamber.

In the latter proposal, it is desirable to provide a regulating valve that can allow for the temperature of the engine. One should draw a greater mass of gas during cold starts and warming up of the engine to improve atomisation. However, when the engine is warm, a lesser mass of gas is sufficient to achieve good mixture preparation and continuing to extract gas at the same rate as during cold operation would be wasteful of energy. The provision of a regulating valve that can vary with engine temperature in this way adds to the complexity and the cost of the regulating valve.

According to the present invention, there is provided a compressed gas supply for an internal combustion engine,

comprising a one-way valve in at least one of the combustion chambers of the engine for bleeding a small proportion of the charge compressed in the combustion chamber into a plenum chamber and a capillary tube communicating at one end with the plenum chamber and having at its other end an outlet from which gases can be supplied at a pressure substantially lower than the pressure within the plenum chamber, the capillary tube acting as a temperature dependent flow regulator.

The invention therefore is based on the use of a capillary tube to regulate the pressure drawn from the plenum chamber, which is at a pressure substantially equal to the peak pressure in the combustion chambers. Such a means of dropping the pressure from typically 2000 psi to between 15 and 75 psi, has the advantage of not using any moving parts and automatically permitting higher gas flow rates when the engine is cold because the viscosity of the gases flowing through the cold capillary tube is lower.

To maintain the capillary at the same temperature as the engine, it can be heated by the water jacket of the engine but the same effect can be achieved by forming the plenum chamber within the body of the engine and housing the capillary tube within the plenum chamber.

There is always a pressure drop across a tube that depends on the diameter of the tube, its length and the viscosity of the fluid flowing through it . One can achieve the same pressure drop either by reducing the diameter or extending the length of the tube. In the present invention, a wide range of tube lengths and diameters can achieve the desired effect but other factors place limits on these dimensions. In particular, a very narrow tube could easily be blocked and it would be difficult to calibrate the tube to give identical flow for different engines, taking into account the inevitable manufacturing tolerances. A very long and

relatively wide tube on the other hand is difficult to package within the engine.

To meet these conflicting requirements, it is desirable to form the capillary tube as a coil thereby permitting a long tube of sufficient diameter to be wrapped into the space available within the plenum chamber.

The tube may be of uniform diameter throughout its length but alternatively it is possible for it to be stepped to permit a gradual transition towards the end connected to the supply outlet .

The invention will now be described further, by way of example, with reference to the accompanying drawing, which is a section through en engine formed with a compressed gas supply of the invention.

In the drawing, a cylinder head 10 having four combustion chambers 12 is formed with a long gallery 30 that constitutes a plenum chamber. The gallery 30 is connected to each of the four combustion chambers by a respective small bore drilling 24 having at its end within the combustion chamber a countersunk recess for receiving a one-way valve 20. Each one-way valve comprises a ball 22 held in place by a cap that is press fitted into the countersunk recess. The cap has a small communication hole that is sealed off by the ball 22 to isolate the combustion chamber 12 from the plenum chamber 30 when the pressure in the plenum chamber is the higher. The ball 22 is however unseated by the pressure of the combustion chamber to permit gas flow through the communication hole into the plenum chamber 30 when the pressure in the combustion chamber is the higher. In this way the pressure within the plenum chamber 30 is maintained at or near the peak pressure in the combustion chambers 12 by bleeding off just enough gases to compensate for the flow taken from the plenum chamber 30.

The plenum chamber 30 is closed to the atmosphere by a nut40 through which passes an outlet 38 of a long tube 36, the tube diameter is stepped along its length being formed by a coiled capillary 32 at its inner end and having an intermediate portion 34. Because the capillary 32 is coiled, it can be of a very substantial length and its diameter may therefore be sufficiently large to avoid the risk of it being blocked by impurities in the gases drawn from the combustion chambers.

In operation, gases are drawn from the outlet 38 and supplied to the fuel injectors of an air-assisted or an air-driven fuel injection system. The rate at which gases can be drawn will vary with the viscosity of the gases flowing through the capillary tube 32. The viscosity itself depends on the temperature of the capillary tube 32 which is substantially the same as the engine temperature because of its being housed within the body of the engine. The capillary tube 32 thus acts as an automatic temperature dependent regulator that has no moving parts.

The described construction has many advantages in terms of easy construction and assembly in the techniques used in manufacture need not differ from the manner in which oil galleries and water coolant passages are formed in a conventional engine. Additionally the regulator can be readily extracted for servicing or cleaning.

When the gases drawn from the compressed gas supply of the present invention are used in an air-assisted fuel injection system, they offer the advantage of providing an internal recirculating loop downstream of the intake throttle that does not interfere with the breathing of the engine. For this reason higher flow rates can be used without affecting the engine idling speed.