Speed Profiles in a Dual Mode System
By Palle R Jensen
RUF International, www.ruf.dk
Speed is very important in order to attract interest from politicians because they know that their voters want speed. Capacity is equally important because commuters want to get rid of congestion problems.
Some dualmode designers claim very high speeds and very short headways (distance in time between following vehicles). What is realistically obtainable in a network of dualmode guideways ?
The critical points in a typical network of guideways (mesh size 3 x 3 miles) are the junctions. If two guideways have to meet at a 90 degree angle, and the vehicles have to be able to drive from one guideway to the other at high speed, the radius of curvature has to be very large (630 m at 100 mph). This is unrealistic in most cases when we are talking about existing cities. The curves cannot be made smaller because the human nature dictates that lateral accelerations must not exceed 0.25 G in order for the ride to be comfortable.
A realistic case (as in the RUF system) is to lower the speed in the junction to 20 mph. This will lower the radius of curvature to 26 m. It is possible to have this curve on a guideway using a wide street (20 m width) without having to interfere with existing buildings.
The average speed will still be high (75 mph) and the capacity will be between 3,600 and 7,200 ruf cars per hour per direction depending on how the junction is organized.
If the system is used for public 10-passenger maxi-ruf busses also (50%) the passenger capacity can be more than 20,000 seated passengers per hour per direction.
This speed and capacity is more than enough to make the system very interesting to decision makers and the response from them has been very positive.
Some system designers claim that a network with vehicles running at constant speed all the time is much simpler to control. This may be true if it was realistic to do so, but as stated earlier, it is only realistic to have constant speed if it is low all the time (20-30 mph). This is the case in single mode systems (PRT) where individual vehicles run separated by a safe distance in a network with very frequent stations (junctions).
In the RUF system, the speed profile will vary according to the network structure.
Between cities (more than 15 miles between junctions) the top speed will be maximum (125 mph) and average speed will be very high (90 mph). Vehicles will normally form long trains (more than 10 vehicles).
In the main part of the city (low density) top speed will be medium (95 mph) and average speed will be very attractive (75 mph). Trains will be 2-10 vehicles long dependant on traffic volume.
In the dense parts of the city the network will become denser and the speed will be constant at 20 mph. Vehicles will run individually (PRT mode).
All dualmode vehicles require electric motors to be able to drive along the normal roads. The power of these motors needs to be large enough so that they can safely follow the normal traffic. The RUF standard requires that the ruf has to be able to drive at least 50 mph along ordinary roads powered by its own batteries (or fuel cells or hybrid drive).
When the ruf enters the guideway and becomes part of a train, the same power can propel the vehicles at much higher speed because air resistance and rolling resistance is lowered significantly. The air resistance is lowered due to train formation (platooning) and the rolling resistance is lowered because of the separation of carrying wheels and the braking system (unique to the RUF system).
Platooning can easily be obtained using normal electric motors. There is no need to use expensive Linear Synchronous Motor systems. The platoons are created at low speed (just after the junction) and they are dissolved just before entering the next junction. Keeping them together is no problem. It has been demonstrated in the AHS system tests and in the case of RUF, the situation is much simpler and safer due to the close coupling and the fact that the RUF vehicles "ride" safely on top of a triangular monorail. Also electric motors are much simpler to control than traditional engines involving gear shifts during acceleration.
The advantage of using variable speed is that merging can be done at low speed (high safety), platooning take place when all vehicles are "riding" safely on the triangular monorail. After 10 seconds, 10 rufs can be joined safely together in a platoon which will then accelerate to 95 mph.
If constant speed control was used, the platooning distance would have to be the same as the merging distance. This would prevent the system from taking full advantage of the aerodynamic drag reduction involved in close coupling of vehicles. In a time where most of the world has agreed upon the Kyoto protocol, every means of energy savings will count.
Last modified: July 27, 2001