1. Field of the Invention
This invention relates to pump systems for fuel injection systems.
2. Description of the Related Art
Engine exhaust emission regulations are becoming increasingly restrictive. One way to meet emission standards is to precisely control the quantity and timing of the fuel injected into the combustion chamber to match the engine cycle. For certain engine operating conditions, effective injection rate shaping may result in reduced levels of particulates and oxides of nitrogen in the engine exhaust. Such rate shaping may comprise multiple injection events including early pilot, close pilot and early and late post injection in addition to shaping the main injection event.
Some existing rate shaping techniques attempt to control injection rates by making various modifications to the injector nozzle assembly. Another rate shaping technique utilizes a separate control valve to provide more precise rate shaping than with modified injector nozzle assemblies. An example of such a control valve is disclosed in U.S. Pat. No. 6,276,610 to Spoolstra et al.
Fuel injection systems that utilize one or more control valves in unit pumps or unit injectors typically have cam driven pumps that provide good pressure capability, smooth beginning of injection characteristics, compact size and lower cost. However, among the disadvantages of unit pumps or unit injectors are design constraints imposed by the cam, and complexity in valve design and valve actuation.
Another rate shaping technique utilizes a common rail system where high-pressure fuel is stored in a common rail and injection events are controlled by a single needle control valve in each injector. Common rail systems are highly effective for multiple injection events, yet they too have some disadvantages. Common rail systems typically have a large number of high-pressure connections that increase opportunities for leakage. Also, the distance between the rail and the injector nozzle results in pressure waves that inhibit repeatable valve behavior.
One proposed solution is disclosed in DE 199 63 219 where an accumulator is provided between the pump and the injector for each cylinder. One problem with this solution, however, is that pressure in the accumulator is still constrained by the design of the cam and no direct controls are provided to regulate the pressure in the accumulator.