Five power transmission systems for heavy vehicles have been studied in this work. These are a multispeed gearbox, a hydrostatic transmission and three split power transmissions. The three split power transmissions are hydromechanical transmissions and each consists of a hydrostatic transmission and an epicyclic gearbox drive. One of them the dual mode system was investigated as a main vehicle drive and also as a steering transmission for tracked vehicles. Four transmission systems were simulated on a digital computer and were also incorporated in a larger simulation which included a typical engine, the vehicle and the torque-speed characteristics of several specified test routes. It was possible to compare the fuel consumptions of these four transmissions, for various specified times to complete the test routes by utilizing a transmission ratio change strategy that selects the lowest engine running speed, It was found that on this basis, the multispeed gearbox had better fuel consumption than the split power transmissions, especially at high engine power levels when the specified times to complete the test route were low. If the different control strategy of maximising the overall efficiency by varying the transmission ratio is adopted, it results in each case, in superior fuel consumption. There was a 9% improvement with the % multispeed gearbox, while the improvements with the split power transmissions ranged from 21% to 29%, The difference in economy for the four systems studied depended on the duty, for instance, very little difference would be discerned between the four systems on the long distance route, However, on a typical U.S.A. urban route, wide range hyrostatic split power systems would show lower efficiencies than both gearboxes and narrow range transmissions, if similar control strategies were possible for these latter systems. In order to investigate the feasibility of this type of optimising control system, for continuously variable transmissions, a system was developed for a vehicle with a simple hydrostatic drive. Air consumption was used as an indirect measure of engine power and the control system scheduled engine speed so that the fuel consumption was minimised. This control system was designed, analysed and installed in a vehicle and comprehensive tests were carried out which show that such control systems are realisable and that the optimizing procedure does not, in any way, impair the controllability of the vehicle. The performance characteristics of a steering system for tracked vehicles, operating as a simple hydrostatic transmission at low track speed differentials and as a shunt transmission at high speed differentials has also been investigated. In the original design, sadden changes of track speed occurred when the mode was changed and it was shown that these changes could be minimised by changing the gear ratio in the mechanical path and also destroking one of the hydrostatic units. Increased vehicle inertia resulted in an increase in the sudden changes of track speed when the mode was changed.
|Date of Award||1978|