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Within Europe, agreed EEC Directives now exist to control exhaust emissions from heavy duty truck engines. An agreed EEC directive requires that emissions are reduced in two stages, Euro I and Euro II in accordance with current state-of-the-art developments in technology. Euro I standards were implemented in 1992/93 and Euro II standards will be in place for 1995/96. A third step, Euro III is now envisaged for introduction around the 1999 model year. In this paper, results from research work are presented showing how, with an advanced, heavy duty diesel engine, featuring 4 valves per cylinder and a very high pressure, electronic unit injector, effective control of NOx is possible using exhaust gas recirculation (EGR). After optimising the combustion system and air-fuel ratio with EGR, the test data obtained allow the limits for achievable emissions to be explored.