EP0255350A2

EP0255350A2 - High pressure fuel injection system

Info

Publication number: EP0255350A2
Application number: EP87306675A
Authority: EP
Inventors: John Arthur Kimberley
Original assignee: Ambac International Corp
Current assignee: Ambac International Corp
Priority date: 1986-07-30
Filing date: 1987-07-29
Publication date: 1988-02-03
Also published as: JPS63113177A; EP0255350A3

Abstract

A high pressure fuel injection system which utilizes a pressure regulating or control valve (28) in a fuel return tube (26), provides a high residual pressure within a charge chamber (15) of a fuel injector (5) between injections. Maintaining a high residual fuel pressure between injections boosts the actual injection pressure over a wide range of engine speed and load conditions, independant of pump speed. Such injection pressures provide improved fuel atomization, with consequent reductions in particulate emissions and increased fuel efficiency.

Description

This invention relates to fuel injection systems, and more particularly, to a high pressure fuel injection system in which the injection pressure is controllably increased by providing increased residual fuel pressure in the fuel injector between injections, independent of pump speed.
Fuel injection is used in both diesel and gas combustion engines due to the precise control of fuel delivery obtainable, optimizing fuel addition with a consequent improvement in engine efficiency. A typical fuel injection system includes a fuel supply tank, a fuel supply pump (low pressure), an injection feed pump (high pressure), a fuel injector and a control system. Pressurized fuel is supplied by the injection pump to a charge chamber located within the injector, adjacent to a discharge spray nozzle having one or more spray orifices. Generally, such a fuel injector also includes a spring biased valve in the entrance to the spray orifices and a fuel return capability which provides some leakage of the delivered fuel through the fuel injector to prevent pressure equalization within the spring retention chamber, which would detrimentally affect injector performance.
In diesel engines, a problem exists with particulate emissions which are generated over a wide range of engine speeds. Such particulates are usually composed of either carbonaceous solids, condensed and/or adsorbed hydrocarbons, or sulfates, with the solids component of such emissions correlated to smoke opacity. These particulates are formed in the fuel rich regions within a combustion chamber and are believed to result from low pressure fuel injection which produces poor fuel atomization. While over 95% of the particulates formed are subsequently burned as mixing and combustion continues in the combustion chamber, the remaining 5% is discharged with the exhaust to the atmosphere.
While increased injection pressures can reduce both particulate emissions and fuel consumption, it is difficult to achieve the proper injection pressure over the wide range of engine speeds and loads. Generally, an injection pump is engine driven providing a lower rate of fuel delivery at low speeds and a higher rate at high speeds, determined by the pump revolutions per minute. By feeding this varying fuel injection rate through the fixed orifices of a typical discharge nozzle, the pressure of the injected fuel varies with pump speed. At low speed, the pressure is low and at high speed it is high. The injection system, pump and nozzle orifice size are designed around the maximum pressure and flow quantity required at the maximum rated engine conditions. Since this occurs at the maximum load and speed condition, such injection systems generally provide fuel at less than optimum conditions at other engine speeds and loads, thereby reducing combustion efficiency and increasing the amount of particulate emmissions.
One solution to this problem involves modifying the pump to provide higher pressures at low speed conditions. However, this can result in very high pressures at high speed conditions which would overstress the injection system and deteriorate engine performance. A pressure relief device may be provided in the high pressure fuel supply tube to relieve the excess pressure. However, the pump design then becomes more complicated, especially with a multiple injection system. To ensure proper fuel distribution to each engine cylinder would require a separate pressure relief device due to the sequential injection requirements of the engine. Such a complex system would significantly increase the cost of an injection system with a probable decrease in reliability. Utilizing a pressure relief device also reduces pumping efficiency by bleeding off varying quantities of pressurized fuel.
Consequently, what is needed in the art is a fuel injection system which provides higher injection pressures over the wide range of engine speeds without overly complicating the injection system or unduly sacrificing pump efficiency.
It is an object of the present invention to provide a fuel injection system which achieves high injection pressures independent of pump speed for reducing particulate emissions and increasing fuel efficiency.
It is a further object of the present invention to provide a fuel injection system which optimizes pumping efficiency.
These and other objects of the present invention are achieved by a fuel injection system which includes pump means in fluid communication with injector means, including back flow prevention means therebetween, with the injection means including fuel return means for returning a portion of the delivered fuel to the pump means, with pressure influencing means disposed within the fuel return means for variably controlling the residual fuel pressure within the injector means between injections. Such residual pressure takes advantage of the available pressure in the leak off fuel to provide higher pressure fuel injection with improved atomization over the full range of engine speed and load conditions, thereby reducing particulate formation and increasing fuel efficiency.
The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:-

Fig.1 is a schematic illustration of a high pressure fuel injection system in accordance with the present invention.
Fig. 2A and 2B are graphical illustrations of two typical single pumping cycles for a fuel injection system at high and low fuel requirements, respectively, and
Fig. 3A and 3B are graphical illustrations of the beneficial effects of the residual pressure provided by the high pressure fuel injection system of the present invention on two single pumping cycles at high and low fuel requirements, respectively.

Referring to Fig. 1, a schematic illustration of the fuel injection system of the present invention is shown, illustrating the basic components of a single cylinder injection system. While most applications will involve multiple injectors, a single cylinder injection system is illustrative of the features and advantages of the present invention while avoiding undue complexity. For ease of illustration, the particular details of the engine will not be discussed. Suffice it to say that the engine is a piston type engine which requires combustion of a compressed fuel oxidizer within a combustion chamber to derive useful work. Such an engine includes one or more pistons and cylinders as well as fuel storage and supply systems.
Referring to Fig. 1, an injection pump 1 includes a metering plunger 2 which is reciprocally and rotatably movable within a barrel 3. For illustrative purposes, the pump 1 is a diesel fuel injection pump such as a model 300 pump produced by United Technologies Diesel Systems, Springfield, Massachusettes. The pump 1 supplys a fuel 4 to an injector 5 through a fuel supply tube 6, with the fuel 4 delivered to a fuel sump 7 of the pump 1 by a supply pump 8 which is connected to a fuel supply tank 9. The pump plunger 2 draws the fuel 4 from the sump 7 into a sloped fuel draw chamber 10 sculpted on the plunger. The fuel is metered by being drawn into the fuel draw chamber 10 which varies in volume in response to the rotation of the plunger. Reciprocal movement of the plunger pressurizes the fuel in a discharge chamber 11 and then delivers the fuel 4 to the injector 5 through the tube 6. A check valve 12 is disposed in the entrance to the fuel supply tube 6 to prevent back flow from the injector 5 to the pump 1, and thereby prevents pressure from bleeding off through the pump 1.
The injector 5 includes a body 13, and an injector plunger 14 which is reciprocially movable within a fuel charge chamber 15 within the injector 5, with a fuel supply passage 16 providing fluid communication to the charge member 15 from the supply tube 6. For illustrative purposes, the injector 5 is a diesel fuel injector such as a model NHM 780352, produced by United Technologies Diesel Systems, Springfield, Mass.. A spring 17 is disposed within a spring retention chamber 18, which resiliently biases the plunger 14 downwardly. The plunger 14 includes a valve end 19 which mates with a valve seat 20, together comprising a valve assembly 21. Below the valve assembly 21 is a spray chamber 22 which includes one or more spray orifices 23. The plunger 14 also includes a valve face 24 located on a portion of the plunger disposed in the fuel charge chamber 15. A fuel by-pass duct 25 provides fluid communication between the fuel chamber 15 and the spring chamber 18, with the spring chamber 18 in fluid communication with a fuel return tube 26 via a conduit 27. The fuel return tube 26 provides means for returning a portion of the delivered fuel to the fuel supply tank 9. While a by-pass duct is illustrative, any injector which provides means to allow fuel leakage by the plunger can benefit from this invention. Generally, such leakage occurs in the clearance provided between the plunger and nozzle body.
A pressure influencing device 28, preferably a regulating valve, is disposed within the return tube 26 and variably restricts the return flow, thereby variably controlling the residual pressure within the fuel charge chamber 15. While a valve is illustrative of the present invention, other pressure influencing devices may also be used. For example, a small recipricating pump may be included in the return tube 26 to controllably boost the residual pressure in the fuel chamber between injections. Other auxilliary devices, such as a fuel accumulator to dampen pulsations, may also be utilized in the return tube without detrimentally affecting the pressure benefits derived from the present invention.
In operation, the injection pump 1 is engine driven and provides periodic pressurized pulses of metered fuel to the injector 5 through the supply tube 6 and the passage 16 to the fuel charge chamber 15. An individual pressure pulse causes a pressure build up in the chamber 15, which acts against the valve face 24 of the plunger 14, with the spring 17 biasedly opposed to this pressure, preventing discharge of the fuel through the spray orifices 23. When the pressure in chamber 15 has built up suffient force to overcome the spring bias, the plunger 14 is lifted, opening the valve assembly 21 and allowing pressurized fuel to pass through the spray chamber 22 to the orifices 23. During the injection cycle, when the pressure in chamber 15 is high, fuel is allowed to leak through the by-pass duct 25 into the spring chamber 18. To prevent pressurization within the spring chamber 18, which would alter the spring opening and closing rates, this leaked fuel is passed through the spring chamber 18, through the conduit 27 and the return tube 26, with eventual return to the fuel supply tank 9.
In a conventional fuel injection system, the nozzle opening and leakage combine with features of the check valve 12 to reduce the residual pressure between injections to between zero and the lifting pressure of the plunger. Referring to Fig. 2A and 2B, conventional pressure curves for a single injection cycle are shown for two different fuel requirements. From Fig. 2B, it is seen that at a requirement of 30 cu mm, the injection pressure begins at zero, rises to about 5 kpsi (352 Kg/cm²), and then drops back to zero. Such low pressure fuel injection results in reduced combustion efficiency and increased particulate emmissions.
By adding a pressure influencing device 28, preferably a regulating valve, in the fuel return tube 26 and a check valve 12 in the entrance to the supply tube 6, the fuel in the fuel supply tube 6, the passage 16, the charge member 15 and the spring retention chamber 18 is isolated and controlled to achieve a residual pressure which is greater than the conventional nozzle opening pressure, increasing the pressure in the injection line between injections. Consequently, the entire injection cycle pressure curve is shifted higher, providing higher pressure injection independent of speed over all engine ranges. Such high pressure injection increases atomization, improving mixing within the combustion chamber and thereby reducing particulate formation and emissions.
In addition, such pressure control neutralizes the effect of the pressure boost on the valve face 24 in the charge member by equalizing the pressure in the spring chamber, such that the valve assembly opening and closing rates respond to spring pressure variations alone, with only a small deviation effected by the increased residual fuel pressure. This allows utilization of coventionally designed fuel injectors without altering spring settings, and with little increase in impact seat loading at nozzle closing even though the nozzle closing pressure has been substantially increased.
Referring to Fig. 3A and 3B, the stepped up pressure curves are shown for an injection system which provides a constant residual pressure between injections of 10 kpsi (703 Kg/cm²). From the graphs it is seen that at a fuel requirement of 30 cu mm, the fuel is injected at up to 20 kpsi (1406 Kg/cm²). While the residual pressure after injection is not precisely 10 kpsi, the pressure recovery after injection is a function of the speed of valve closing which in turn depends on the sizing of the by-pass duct. The larger the duct or clearance between the plunger and the injector body, the quicker the fuel pressure will equalize and assist in valve closing. For the illustrated injector, the clearance is 2.032 x 10⁻⁴ mm (8.0 x 10⁻⁵ inches), which is the conventional clearance for a model NHM 780352 injector.
While a simple self contained pressure regulating valve, which senses residual pressure and responds by variably restricting the fuel return flow, could be used to deliver a constant boost in injection pressure over the full range of engine speeds, a variable control valve may also be used, acuated by an engine control system which monitors and controls engine operation, thereby optimizing the reduction in particulates and maximizing fuel economy. Such valves could also be replaced or supplemented by a pump, disposed in the return tubing, which could also achieve either a constant pressure boost over the range of engine speeds or respond variably to a signal from an engine control system. Of course, the choice of pressure influencing device and degree of control desired will vary with each particular application.
A particular advantage of the present invention is that a single pressure influencing device could be used to boost the residual pressure in a multiple injection system. The return tubes could be connected to a common return tube which includes the valve or pump, with the residual pressure boosted for all the injectors regardless of engine timing. This significantly simplifies the modifications required in the injection system as well as the control system requirements.
While the injection system of the present invention is described in relation to a seperate pump and injector system, it will be understood by those skilled in the art that this invention is applicable to unitary injectors which employ integral pumps.

Claims

1. A fuel injection system for providing a controllable residual fuel pressure within a fuel injector between injections, comprising fuel delivery means (8, 1) connected to a fuel supply (9), and injector means (5) for periodically injecting a fuel (4) into an engine, said injector means (5) being connected to said fuel delivery means (8, 1) such that fuel (4) is delivered thereto, characterised by back flow prevention means (12), disposed between said fuel delivery means (8, 1) and said injector means (5) to prevent back flow to said fuel delivery means (8, 1), fuel return means (26-28) for returning a portion of said delivered fuel to said fuel supply (9), and, pressure influencing means (28), disposed within said fuel return means, for variably controlling the residual fuel pressure within said injector means (5) between said periodic injections.

2. The fuel injection system of claim 1, wherein said pressure influencing means comprise a valve (28) which variably restricts the amount and rate of fuel return.

3. The fuel injection system of claim 2, wherein said valve (28) is a pressure regulating valve which senses the residual pressure within the injection means (5) and responds thereto, providing an essentially constant residual pressure between injections.

4. The fuel injecting system of any preceding claim, further comprising engine control means which monitor and control the engine operation, wherein said valve is responsive to a control signal issued from said control means.

5. The fuel injection system of claim 1, wherein said pressure influencing means (28) comprise a pump.

6. The fuel injection system of claim 5, further comprising engine control means which monitor and control the engine operation, wherein said pump (28) is responsive to a control signal issued from said control means.