Problem Definition
3.1
Problem Scope
Transfer of shaft torque to drive a fuel injection pump for LPG/DME while eliminating any
leakage by shaft seal is the objective of our project. The design of the pump is external to the fuel
tank to allow ease of access for service and replacement. A magnetic coupler is used to transfer
torque through a diaphragm; the transfer by the coupler must meet the torque needs of each type
of pump while the diaphragm must withstand a burst pressure of 1750 psi. The LPG pump must
transfer 2 ft-lbs of torque and the DME pump requires 10 ft-lbs of torque. Our system to transfer
torque to the pump must meet all pertinent safety requirements when working with LPG.
3.2
Technical Review
Current designs for LPG/DME fuel tank assemblies use a centrifugal pump mounted within the
tank similar to modern gasoline tanks. This design has a major flaw in the fact that the electrical
power has to be passed through a wire that needs to penetrate the tank wall. Because of this,
servicing the pump can take four or more hours to drain the LPG/DME from the tank before
breaking the seal and accessing the pump. This design and its maintenance techniques form the
current industry standards for this type of system. Currently there is no need in LPG/DME
pumps for an external mechanical drive force that would require shaft seals in the tank.
However, in the switchover to direct injection LPG/DME engines, (which causes much higher
tank pressures) there is a need for more mechanical input to the pump via an electrical motor.
The drive shaft and electrical wiring would require seals similar to other systems that employ
pressurized fluids and require external mechanical input.
In our system the fuel pressures are too great for the seals to hold thus requiring a focus on an
external motor with a non-physical interaction between the electric motor and the pump within
the fuel tank. The only similar systems involve magnetic torque transfer. There are relatively
few of these designs in production and none in the field of LPG/DME fuel tank systems. The
closest magnetic torque transfer design relevant to our application is in the new Toyota Prius’s
magnetically driven water pump. Other designs that transfer torque are usually found in random
other mechanical applications and Rube Goldberg machines.
In using magnets for torque transfer it was necessary to use previous knowledge of magnetic flux
transfer and the effects of rotational movement on magnetic flux. Professor Randall Victora of
the University of Minnesota was consulted in this area as a resident expert in magnetism. From
our consultation with him, we learned that the magnetic flux through non-magnetic materials
(such as Aluminum and Titanium) was approximately the same as through open air of the same
distance. Also, it was determined that the spacing between two attracted magnets is ruled by the
fact that the attractive strength falls off at a rate of 1/r^3 where r is the spacing between the
magnet surfaces. Another important consideration at high speeds of rotation discussed were
induced eddy currents in the diaphragm material. Reductions of the eddy currents can be
achieved by doping the metals.
Pressure vessels were another important aspect to the project. The diaphragm proposed would
have to pass the NFPA 58 Propane Fuel Pressure Standard. The governing equations dealing
with pressure vessels include directional stresses in cylindrical and spherical pressure vesicles.
The main material properties in these equations are the yield stresses of the materials used. In
order to reduce thickness needed and deformation of the vessel, stronger non-magnetic materials
were investigated.
In our research we found some patents that were similar to our design and they included patent
numbers; 3,890,515, 5,204,572, 7,024,963, 7,029,246 and 11,783,576. None of the magnetic
coupler patents we found mentioned torque transfer through a wall in which a high pressure
differential is present. Copies of these patents may be found in our appendix.
3.3
Design Requirements
1. The Diaphragm and Apparatus Must Not Leak: The diaphragm that separates the pressurized
fuel from the atmosphere must not, under any circumstances, leak. This requirement is
important for many reasons. First, LPG is an expensive fuel. Our product is intended for initial
use by the industrial market, which paid a substantial capital investment to have their fleet
converted to run on LPG with hopes to save money in the long run. Leaking fuel decreases the
overall savings, possibly eliminating any possibility of overall long term savings. LPG is also a
volatile liquid. It is denser than air so it can stay stationary for relatively long periods of time. It
can be a dangerous fire hazard at the point of leakage, and any places it may have settled after
leaking. The diaphragm must be able to withstand a constant operating pressure of 350 psi and
pass a one-time burst test of 1750 psi.
2. The Coupler Must Meet the Torque Requirements: The coupling system uses a static sealing
surface and eliminates the need for a dynamic shaft seal. However, a design that is leak proof
does not succeed if we are not able to transmit enough torque through the diaphragm to drive the
pump. We were given a target torque capacity along with the project. This represents the
required torque to achieve the needed pressure and flow characteristics to accomplish direct
injection of LPG. The initial design must be able to transmit 1 ft-lb of torque from the outer
rotor to the inner rotor. It is ideal, but not required, that the design be easily scalable to achieve
torque transmissions as high as 10 ft-lb. To verify this requirement we will have a prototype on
a test stand and perform a static torque breakaway test.
3. The Magnets Must be Fixed Reliably to the Apparatus: The problem of how to attach the
magnets must be solved elegantly. The use of epoxy or other chemical bonding agents is not
advised, because of troubles that arise from the different thermal expansion properties of the two
materials. The rotor is to be constructed out of aluminum alloy and the magnets are neodymium
boron magnets. It is also imperative that the magnets are held with enough strength to achieve
the required torque.
4. Ease of Machinability and Manufacturability: Reactions between magnets can be very
complex; to be certain all of our design requirements are met a prototype must be built. Very
complex rotor designs could be extruded at a large scale for retail sale, but the time scale on
processes such as this is outside of our allowable time to complete the project. We were careful
throughout the design process that our designs could be easily machined by us with the
equipment available or outsourced with a quick turn-around. We kept large scale manufacturing
costs in mind throughout.
5. The Pump Apparatus Must Fit Space Constraints: The final design must be able to fit within
the space constraints of the vehicle. The coupler must be tank-mounted and mount with a flange
of 3 inches or less in diameter. It also must not be excessively bulky, so that it can be mounted
low in the tank without interfering with the other vehicle systems of the typical delivery vehicle.
Mounting the tank pickup low insures that all of the fuel can be drawn out of the tank easily and
efficiently.