In the diesel engine market it is common for fuel pumps (primarily rotary gear pumps) pumping low viscosity fluid (as low as 0.9 centistokes (cst)) to be required to run from very low speeds (below 100 RPM) and moderate pressures to relatively high speeds (in excess of 3000 RPM) at increasingly higher pressures. A problem with this is that gear pumps that are capable of running at higher speeds are typically not efficient at low speed operating points and gears pumps that have excellent low speed efficiencies are not typically capable of operating at elevated speeds. This creates a circular problem for the end user because in order for the pump to meet required flow rates at the lower speeds, it must be grossly oversized at the high speed conditions. This causes the end user to have a system that may produce two to three times more flow than they actually need at elevated speeds, requiring all of the excess flow to be dumped back to the system as unusable energy. With increasing demand for cleaner burning engines and more efficient systems, this is a large hurdle that needs to be overcome.
In addition, another problem that affects the performance and repeatability of one pump of the same type when compared to another is the problem of tolerance stack and machining variance from one pump to another. Due to cost and standard machining practices utilized when building and assembling pumps of this type, pump dimensions can vary (within tolerance) from part to part. These variances when added together can cause pump performance to be inconsistent between two pumps of the same design. These inconsistencies can also push pump efficiencies out of the acceptable range. The intent of this invention is to also minimize the effect of these machining variances and to create a more efficient and repeatable pump.
In the past others have tried several methods of improving the low speed efficiency of rotary pumps. Two of the most common methods include reducing mechanical clearances in the pump, and the addition of pressure biased or pressure balanced side plates. Both approaches have issues at low viscosities with elevated speeds and pressures.
When pumping low viscosity fluid, if the clearances in the pump are simply reduced there is a fine balancing act between good efficiency at low speed and enough clearance to keep the pump from seizing as it heats up and thermal expansion takes place. If the clearances are too wide the pump is not efficient. If the clearances are too tight the pump will have a mechanical failure, thus this method is very application specific and usually requires multiple iterations to get a compromised solution. This solution rarely provides an optimum pump sizing for both the low speed and high speed operating points.
The approach of using pressure biased side plates is a common and effective solution especially for low speed and low pressure applications with higher viscosity fluids. With this solution as pressure of the pump increases, the pressure behind the side plate increases, forcing the side plate tighter against the gears, thus closing the clearances in the pump tighter and tighter as pressure increases. This works well for high viscosity, low speed and moderate pressure applications and efficiencies have been shown to increase dramatically. However, with this concept as pressure increases greatly or speed increases greatly there is a large amount of heat generated due to friction. This heat eventually causes the side plates to fail and often seizes the pump.
The same is true with pressure balanced side plate designs. With this type of design the side plate is sized so that the pressure closing the side faces is nearly perfectly balanced so that the side plate does not rub as hard on the rotating gears as pressure increases. This concept works great for high viscosity and low to high pressure ranges, but is limited again to lower speeds operations. As speed increases, even though the side plates are balanced, the clearances remain the same thus heat is generated and the pump eventually fails.
In view of the above, there is a need for an improved gear pump design that improves both the low speed efficiency of the pump as well as creates a design that can still operate at the elevated speeds for extended periods.