While light vehicles have successfully integrated efficient engines that respect the environment, large vehicle engines have not jumped on the bandwagon yet due to lagging knowledge in this area. The EU-funded project 'Green heavy duty engine' (GREEN) researched ways to develop a heavy-duty powertrain based on novel engine concepts. The proposed powertrain would feature improved combustion, flexible components, model-based closed loop emission control, high-power density and exhaust treatment. It would also boast improved cost efficiency, the highest diesel fuel conversion efficiency and near-zero emissions, implying a strong reduction in carbon dioxide (CO2) pollution. Research priorities for the project included developing a gas engine with maximum thermal efficiency and evaluation of tailored exhaust after-treatment systems based on novel fuel injection and variable valve actuation (VVA). They also covered development of a new combustion process and the first step to model-based closed loop powertrain control, as well as an innovative brake mean effective pressure (BMEP) engine. After 39 months of work, the project met all its objectives successfully, having tested a viable prototype for a heavy-duty engine based on flexibility, multiple fuel injections and variable valve timing. Emission targets have also been achieved in gas and diesel engine applications, realising significant fuel savings. Project partners have recommended more work in the future on related subsystems such as the air management and cooling systems to achieve maximum benefit from the new combustion engine.