Design proposal for a hydrostatic city bus transmission

Abstract

A city bus is probably one of the harshest operating environments for a vehicle transmission. The average operating speed is low, while frequent stops require swift acceleration from standstill. The options for the placement of transmission and suspension components are highly restricted as the passenger space must be as large as possible, with a completely flat vehicle floor for ease of passenger movement, and ground clearance is kept as low as possible to facilitate the entry and exit of passengers. Therefore, it is common to place the engine and gearbox at the rear end, and power is transmitted to the wheels via a propeller shaft and a rigid rear axle, effectively ruling out any completely level floor design. An improvement is offered by the hydrostatic transmission, using high pressure fluid for the power transmisison. The fluid flows from a pump to a motor via relatively small diameter pipes, enabling the design of a completely level floor. The hydrostatic transmission offers continuous variation of transmission ratios and the possibility that hydraulic motors can be placed very close to the wheels, thus enabling all- wheel drive and increased passenger space. Finally, hydraulic accumulators can be added to a hydrostatic transmission in order to recover and reuse kinetic energy which would otherwise be lost by braking.