New ways of fluid flow control in automobiles: Experience with exhaust gas aftertreatmetn control 2000-05-0299
Flow control by fluidic devices - without moving parts - offers advantages of reliability and low cost. As an example of their automobile application based on authors'' long-time experience the paper describes a fluidic valve for switching exhaust gas flow in a NOx absorber into a by-pass during regeneration phase. The unique feature here is the fluidic valve being of monostable and of axisymmetric design, integrated into the absorber body. After development in aerodynamic laboratory, the final design was tested on engine test stand and finally in a car. This proved that the performance under high temperature and pulsation existing in exhaust systems is reliable and promising. Fluidic valves require, however, close matching with aerodynamic load. To optimize the exhaust system layout for the whole load-speed range and reaching minimum counter- pressure, both the components of exhaust system and control strategy have to be properly adopted. Conclusions gained for successful design: the system has to be fully described by pressure- flow-rate characteristics (aerodynamic load); matching condition for the fluidic valve - the pressure drop across the valve roughly comparable to that between the branches being switched - should be respected for the design of system and control strategy; places of control-flow taps should be found with respect to lowest flow loss possible; demands for 100% diverting efficiency typical for mechanical valves, might be in many cases modified, e.g., by the use of auxiliary reactors in bypasses, treating so spill- off gas; any flow acceleration should be taken into account as possible place associated with flow deflection; pressure recovering diffusers have to be matched to a nozzle designed for minimum velocity required to overcome pressure difference between the branches; low pressure differences between the branches switched may be achieved just using latter possibility. Then the potential of fluidic elements may be fully utilized. The advantages gained are maintenance free, unlimited life, and highly reliable operation.
Václav Tesar, Axel Konig, Jan Macek, Pavel Baumruk
University of Sheffield, Volkswagen AG, Czech Technical University in Prague