Seizure Recovery System for Fuel System Distributor
Project Background:
Cummins has a fuel system in production that uses a distributor to route the fuel from a high pressure accumulator to the engine cylinders. The distributor is driven via a shaft from the gear pump that supplies fuel to the high pressure pump. the problem to be addressed is that in certain applications, due to either debris ingestion or overheating, the distributor rotor seizes. In order to prevent progressive damage, the shaft driving the rotor shears when the rotor seizes. Recently, a breakaway coupling was introduced to minimize the repair cost. But, this is a mission disabling failure with a very high warranty cost.
However, another opportunity exists within the drive system of the distributor. The nature of of the distributor is such that almost all seizures do not do sufficient damage to the rotor and bore surface to permanently lock the rotor in the bore. If the drive could be made such that:
At a given torque level ( corresponding to the onset of seizure), the drive could rotate freely, then
as the rotor torque falls below the threshold value, the drive re-engages in the proper timing, then
the distributor seizure would no longer be mission disabling. At the present failure rate and cost for 2000, this would result in a significant reduction in Cost per Engine. In fact, past experience with repeat lab failures and production bore conditioning cycles indicates that distributors can increase in seizure margin after seizure events. If this holds true for some portion of the pumps in the field, rotor seizure may not event be noticed if the drive re-engages quickly enough.
The objective of this design request is to design and validate a mechanism that will enable the rotor to break away from the gear pump shaft when the rotor seizes and re-engage when the rotor torque falls below a threshold level. At re-engagement , the timing of the rotor must be maintained.
Design Instructions:
The design considerations for the seizure recovery system are:
The target disengagement torque should be in the 30+ in-lb range (subject to revision given recent torque level information at high idle), with a drag torque while disengaged of less that 5 in-lbs.
The drive should be capable of withstanding normal (non-seizure) operation indefinitely. Torsional fatigue life to 10 million cycles at +/- 20 in-lbs is recommended.
The drive should be capable of disengaging/ re-engaging for a t least 100 seizures without appreciable loss in disengagement torque.
The drive should have wear resistance as good as or better than the current design.
The drive should have angular alignment as good as or better than the current gear pump drive shaft design or as determined acceptable through negotiation with the customer and performance testing.
Finally. The drive should be backwards compatible with the current distributor and gear pump components(with the exception of the drive shaft, of course), including the breakaway coupling.
Technical Profile:
Both the technical profile and potential DVP&R testing necessary can be distilled from the design instructions above. The only formal requirement on the current technical profile is the 100 in-lb on failure torque.
Other Design Criteria:
This design change will probably result in a cost increase. Since there is a substantial warranty issue with the current design, there is some room for a piece-part price increase. However, price increase should be kept to a minimum.