Different systems for achieving short-route cooled EGR on turbocharged and aftercooled heavy-duty diesel engines have been tested on a 12 litre 315 kW engine with 4 valves per cylinder and an electronically controlled unit pump fuel injection system. In all of these systems the exhaust gas was tapped off before the turbine, cooled and mixed with the intake air after the compressor and aftercooler. The systems differed (mainly) in the method used to set up a positive pressure difference across the EGR circuit. This was done either by the use of an exhaust back pressure valve in the exhaust, or by using a turbocharger with variable nozzle turbine (VNT) geometry, or by combining such a VNT turbocharger with a venturi-mixer that was positioned in the intake manifold such as to provide extra suction power to the EGR gas.
The emissions behaviour and efficiency with these different EGR systems were tested in a number of engine working points, including key points of the AVL 8-mode US FTP cycle simulation. The results indicate that for achieving 1998 emissions levels, a VNT only would be the most efficient solution. For achieving 2004 emissions levels, combining the VNT with a venturi-mixer could give a further fuel consumption benefit. Particulate matter emissions will however be unacceptable, making the use of additional particulate reducing technology necessary.
A measure of venturi-mixer efficiency is proposed and a new, compact and efficient venturi-mixer design is presented. Results from flow tests are given and compared with results from CFD calculations.