This paper provides a summary of basic characteristics of a freight pipeline system powered by linear electric motors. Such a concept advocates the separation of freight transportation from human movement and can be very effective in reducing the ever-increasing highway congestion problem. In this paper, aspects of linear electric motor propulsion and aerodynamic drag modeling are explained. Stabilities of a single capsule as well as a multiple capsule system are discussed.
The current highway system is approaching saturation with the ever-increasing traffic in both freight transportation and human movement. Many of the nation's roads are clogged and congestion continues to worsen. The conventional approach of building more roads has ceased to be effective in most areas of the country for both fiscal and environmental reasons. Research is being conducted to enhance capacities of the existing infrastructure. At the same time, it is important to study new transportation systems.
Freight transportation and human movement have different characteristics. Moving people requires flexibility, convenience, and speed; transporting freight requires cost-effectiveness, on-time delivery, and security in transit. Today, most human movement is achieved with a combination of personal automobiles and air flight. On the other hand, freight transportation is mainly achieved by long overhaul trucks. Truck transportation offers more flexibility but not necessarily at a lower cost than trains.
However, the mixture of freight transportation and human movement presents a constraint to the highway capacity and safety. Truck drivers often have to operate the vehicles for a long time and while fatigued. During Minnesota winters, trucks are especially intimidating to travelers in automobiles. At the same time, personal automobiles also present a challenge to trucking operation. Truck drivers have to yield for cars and have to be ever-watchful of small cars.
The separation of freight transportation from human movement will increase the efficiency and safety of both [1]. In Ref. [2], the use of electricity-powered pipelines in efficient freight transportation are examined. The idea is to use capsules for carrying cargos in concealed pipelines powered by linear electric motors. Traditionally, freight pipeline systems employ pneumatic blowers as the power source. Pneumatic pipelines suffer from short haul range, high noise level, and poor energy efficiency. The use of linear electric motors, on the other hand, offers so manyadvantages that the resulting system becomes desirable to freight transportation. This paper summaries the key results in Ref. [2].
Freight transportation via pipelines is not a new concept. Among the first advocates, George Medhurst proposed pipeline systems both for freight transportation and for passenger movement [3]. Small diameter pipelines powered by pneumatic blowers have been in use since before World War II for high-priority movement of documents. After World War II, large diameter pipelines for transporting limestone and garbage were developed in Japan and Russia. Freight pipelines have been the subject of extensive studies [4-9].
Indeed, pipeline systems for freight transportation have many desirable features. Freight pipelines buried underground would have little environmental impact on surroundings once installed. These systems can be fully automated and do not interfere with human movement. Pipeline systems are closed and can thus be operated regardless of weather conditions.
However, freight pipeline systems that have been extensively studied so far use pneumatic blowers as the power source. They move capsules through a duct by using a vacuum or air pressure. Booster pumps have to be used to transport cargos beyond a distance of several miles. Since capsules cannot go through the pumps, it becomes necessary to interpose a setof valves and airlocks through which the capsules can bypass the booster pumps. In addition, pneumatic pipelines suffer from low energy efficiency and high noise level. Coupled with the fact tunneling technologies were very expensive and challenging until recently, these system concepts were deemed impossible and were dubbed ``pipe dream.'' Except for transporting coal and other materials over short ranges, pneumatic pipelines have not been widely used.
Over the last several decades, technological advances in two areas have made a different but similar concept very feasible. Developments in linear electric motors have made them desirable for freight pipelines. Indeed, the use of electric motors would retain all the advantages of a pipeline system and yet eliminate disadvantages of pneumatic propulsion. Pipelines powered by electric propulsion will be able to carry freight with much higher efficiency, and lower noise and pollution. In fact, the use of linear electric motors is always more efficient than that of pneumatic blowers [2]. At the same time, developments in tunneling technology have made underground pipelines much easier to build. As a result, underground pipeline systems powered by electric motors have become quite feasible.
In 1984, the Ampower Company of Alpine, New Jersey proposed and patented a capsule pipeline system powered by linear induction propulsion [10]. Economic analyses were conducted to demonstrate the feasibility of such a concept [11]. However, thetechnical details are yet to be developed. Recently, Japanese researchers have constructed and tested a prototype linear tube transportation system powered by linear synchronous motors [12].
Linear electric motors belong to a special group of electrical machines that converts electrical energy directly to mechanical energy in translational motion [13-20]. While all electric motors operate based on principles of electromagnetic interactions, there are different types of motors. Polyphase synchronous motors and induction motors both use alternating current as input electricity source [13]. Direct current motors are normally used for small horsepower applications. Conceptually, any rotary motor has a linear counterpart. There are linear synchronous motors (with permanent magnet or wound field), linear induction motors, and linear direct current motors. The economically feasible choices for a pipeline system are linear induction motors (LIM) or linear synchronous motors (LSM) [14-20].
A linear induction motor (LIM) consists of a primary and a secondary. When powered by three-phase alternating current, a moving flux is produced in the primary winding. Current induced in the secondary reacts with the flux, producing a mechanical force. Both the primary and the secondary of LIMs are flat structures. The interaction of flux and current moves the secondary linearly. A linear synchronous motor (LSM) has a similar structure, except that its secondary must be either a permanent magnet or a wound field with a direct current. The word ``synchronous'' comes from the fact that the primary magnetic field and the secondary magnetic field in a LSM move at the same speed.
The choice between linear induction motors and linear synchronous motors depends on specific system designs and will be examined further in later research. In general, linear synchronous motors can achieve better energy conversion levels than linear induction motors at the expense of higher costs per motor. In particular, the secondary of a LIM does not require any physical contact with the external power source, while the secondary of a LSM needs direct current to generate a magnetic field or has to use a permanent magnet. A LIM with a solid-iron secondary is very rugged and requires low maintenance.
There are two choices in using LIMs in pipelines. (1) One can put primaries of LIMs on the freight capsules. This configuration requires complicated windings on the capsule cart, and electrical current transfers between the traveling capsules and the stationary pipetubes via some sliding connection. (2) Alternatively, one can put primary windings on the pipetubes and build capsules as secondaries. The latter configuration makes the system much simpler. If the number of capsules is very small, the second configuration is less efficient than the first one because primaries produce flux without moving secondaries. The proposed pipeline transportation concept is envisaged to move a block of capsules at a time. We assume the second configuration in the current analysis. In fact, German researchers also recommend the second configuration for use in the high-speed train TRANSRAPID system [21-26].
To use LIMs in a pipeline transportation system, we need to be able to control the thrust force. The thrust force of an LIM can be controlled by changing one of the three things: the distance between two winding poles in the primary, input voltage, and/or input frequency. A practically feasible method of thrust control is by altering the input frequency while keeping the input voltage constant or variable. If the voltage is changed in proportion to the frequency, the flux will remain constant resulting in no change in force.
Electric motors can be used for stopping the capsules as well. Electrical braking occurs when the thrust is in the opposite direction of velocity [14-20]. There are three braking methods available on a linear induction motor. Details can be found in Ref. [2].
Aerodynamic drag constitutes an important force on a capsule in a pipeline system. In Ref. [2], the total aerodynamic drag on a capsule inside a pipetube is determined by summing up the following components: the inviscid pressure drag caused by high pressure on the capsule nose and low base pressure at the rear, a viscous contribution caused by shear stresses in the high speed annular gap region between the capsule and the tube wall.
While in most cases the capsule has a fairly low velocity compared to the speed of sound, the velocity in the gap is much larger and can approach sonic if the gap is small. When the gap velocity reaches the speed of sound the mass flow through the gap is restricted and the flow is said to be ``choked'' [27-30]. Calculations reported by Hammitt [7] show that the drag coefficient stays close to the incompressible value until the gap Mach number nearly reaches one and then rapidly increases to a value which is easily double the incompressible value. This can occur at moderate capsule speeds if the gap is small.
There is a tradeoff in selecting a proper blockage ratio. Blockage ratio is define as the cross-sectional area of the capsule divided by the inside cross-sectional area of the pipetube. As the blockage ratio increases, the aerodynamic drag on the capsule increases and so do the electrical power required to overcome the drag. On the other hand, a large blockage ratio provides better protection on capsules in case of power failure, so that capsules would not collide with each other too hard. Blockage ratios should be selected such that capsules will not collide at all in case of power failure.
The choking phenomenon may be used as an aerodynamic braking technique. Specifically, fins that can be extended may be mounted on the capsule surface. These fins are held under the capsule surface during normal operation and can be extended to stop the capsule. The extensions can be controlled remotely in the operation station. They can also be triggered by a sensor mounted on the capsule nose to avoid collisions, or by some mechanical device at the exit station to stop the vehicle. The proposed concept of aerodynamic braking is similar to the concept discussed in Tsuji, Morikawa \& Seki [31].
In an operational pipeline system, capsules should travel at specified speeds despite possible imperfections and disturbances in the system. Ref. [2] shows that a single capsule is neutrally stable in speed, but unstable in position. A multiple capsule is unstable in that capsules can not maintain their specified relative separations without active feedback control.
This paper studies a freight pipeline transportation system powered by linearelectric motors. A concealed pipeline transportation system does not interfere with human movement and has little effect on the environmental surroundings, and thus provides an ideal system for freight transportation. However, traditional pipeline systems use pneumatic blowers as the power source. Pneumatic pipelines suffer from short travel range, high noise, and low efficiency. The use of linear electric propulsion has many advantages and makes the pipeline concept truly desirable. Linear electric propulsion is always more efficient than that of pneumatic blowers in terms of energy. Furthermore, it can carry freight over a much longer distance and produces little environment pollution.
Aerodynamic drag on a capsule can be determined as a sum of three drag components: pressure force on the frontal surface, pressure drop caused by friction along the capsule length, and pressure force on the back of a capsule. Drag modeling leads to the understanding of a phenomenon called ``choking.'' If the annular gap between a capsule and the pipeline is small, the flow speed along the gap could become sonic and restrict the flow through the gap. Under these conditions, the flow is said to be ``choked.'' The capsule diameter can be selected properly for a given pipe diameter (blockage ratio) so that the capsules won't crash into each other in case of any power malfunctions.
An uncontrolled multiple-capsule system is unstable. In other words, capsules would approach each other and may collide with each other if left uncontrolled. Active capsule position control is needed.
This research is supported by the Minnesota Department of Transportation. We thank the following individuals for many useful suggestions and discussions: John Sampson, Larry Foote, Gregg Paul Busacker at Mn/DOT Environmental Services, Richard Lambert at Mn/DOT Office of Railroads & Waterways, Don botz at Mn/DOT Office of Research Administration, Prof. D.D. Joseph at the University of Minnesota and Mr. W. Vandersteele at Ampower, Inc.
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Yiyuan Zhao is a faculty member at the Department of Aerospace Engineering and Mechanics at the University of Minnesota, Minneapolis, Minnesota, 55455. This paper was presented at the International Conference on PRT and Other Emerging Transportation Technologies held in Minneapolis in November, 1996. Other technical papers on this topic will be forthcoming soon from Professor Zhao.
Last modified: August 13, 2002