A hybrid hydraulic electronic unit injector (HEUI) assembly, having some characteristics of an accumulator-type assembly and some characteristics of a non-accumulator type assembly, is produced by designing the volume (VACC) of the passageway downstream of an existing blowback prevention check valve of the assembly to be between 1.0 and 10.0 times themaximum quantity QMAX of fuel injected during an injection event. The resulting hybrid assembly 1) exhibits a rising injection rate during at least the initial stages of the injection event and therefore exhibits benefits generally associated with non-accumulator type HEUI assemblies, and 2) exhibits a falling rate injection during at least the final stages of the injection event and thereforeexhibits benefits generally associated with accumulator-type HEUI assemblies. The percentage of the injection event taking place under a falling rate increases generally proportionally with the ratio VACC /QMAX.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to hydraulic electronic unit injector (HEUI) assemblies and, more particularly, to a hybrid assembly havingsome characteristics of a non-accumulator-type HEUI assembly and some characteristics of an accumulator-type HEUI assembly.
2. Discussion of the Related Art
Hydraulic electronic unit injector (HEUI) assemblies have gained increased acceptance in recent years because they permit more precise control of fuel injection rate, timing and quantity than is possible with traditional cam-operated orjerk-type injector assemblies and thus can significantly reduce exhaust emissions and improve fuel economy and power. Both accumulator-type and non-accumulator-type HEUI assemblies are known and both employ pulse width metering or pressure metering to control the timing and quantity of fuel injection. As will now be detailed, however, the manner in which energy for injection is stored and releaseddiffers fundamentally between the two types of assemblies, and each type of assembly is considered by some to exhibit advantages lacking in the other type of assembly.
A HEUI assembly generally considered to be of the non-accumulator type is commercially available from Caterpillar, Inc. of Peoria, Ill. and is characterized by a needle assembly, a pressure intensifier assembly, and asolenoid-actuated poppet valve. The poppet valve is operable to selectively connect a low pressure chamber of the pressure intensifier assembly to a source of fluid pressure and to vent, thus pressurizing or depressurizing a high pressure chamber of the intensifier assembly. The high pressure chamber is fluidically coupled to a fuel supply rail and to a nozzle cavity of the needle assembly. The needle liftsto permit injection whenever fluid pressure in the high pressure chamber increases above a designated level (determined primarily by a needle return spring) and closes whenever the fluid pressure in the nozzle cavity decreases beneath this same level.
Non-accumulator type HEUI assemblies exhibit marked drawbacks and disadvantages.
For instance, injection energy must be transferred veryrapidly, i.e., simultaneously with injection. This rapid energy transfer requires an extremely fast-acting valve with very high hydraulic flow velocity leading to relatively high parasitic losses. Indeed, for peak injection pressures of about 1200 bar, it is estimated that the injector assembly uses about 5% of engine power.
Secondly, a non-accumulator-type HEUI assembly cannot be used in anexpanding cloud injection system (ECI system). An ECI system is one which injects at least a significant portion of each fuel charge at a decreasing rate such that successive fuel droplets have high separating velocities. Injection in this manner prevents droplet agglomeration and stimulates rapid vaporization and mixing of liquid fuel, thus reducing smoke and emissions. The construction and operation...