Influence of thermodynamic behaviour on HLA system performances

Abstract

Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.In the recent years, a new interest on kinetic energy recovery (KER) of vehicles arose. In USA, car manufacturers (Ford, Sheep et al.) developed research programs on the use of hydraulic devices to achieve KER, also with economical support of government body (EPA). In 2002 Ford built a very interesting prototype (Ford Tonka) using such recovery system (HLA, Hydraulic Launch Assist), achieving improvements better than 20 % on fuel consumption and pollution, besides increasing of acceleration. At present time, also DIMEG is working about HLA systems, carrying on two work lines: one (based on the building of a simulation program) to verify operation limits and system performances and another experimental (to study particular thermodynamic aspects of cyclic operation of hydro-pneumatic accumulators). In previous works the authors studied application of HLA system on small vehicles, emphasizing the influences of mechanical parameters and kind of mission profile. Here they mean to treat the influences of some thermodynamic characteristics on performances and operation field of such system. The nitrogen behaviour is described by BWR state equation. Particular attention will be devoted to analyze, from a global point of view, the penalties introduced by using a more simple state equation for describing nitrogen, the influences on global performances of the efficiency of expansion and compression transformations and of heat exchange between gas and outside. The analysis will be performed for a commercial vehicle with mass of about 2000 kg.