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The high efficiency of the diesel engine has made it the primary propulsion source for truck applications worldwide. In Europe, the penetration of diesel engines in passenger cars is growing rapidly in an effort to meet the stringent CO2 future emission targets. However, future regulations call for increasingly lower NOx emission levels. Extremely low engine out NOx emission levels, if at all possible, have significant negative effects on fuel economy and engine life-cycle cost. As such, a viable strategy is to tune the engine for optimum fuel consumption, while relying on an exhaust aftertreatment system to reduce NOx emissions. The Selective Catalytic NOx Reduction (SCR) enables the engine to run in the overall speed/load range at best fuel economy, while the exhaust aftertreatment system reduces the NOx emissions significantly. The NOx reduction efficiency of an “engine map controlled”, open-loop SCR system is about 65 percent.

The application of statistical analysis methods and simulation techniques through the concept stages of a truck engine development process, in order to assist with decision making, is reported in this paper. Aspects of single cylinder engine, combustion system development and the subsequent use of modelling and simulation, to predict multi-cylinder engine behaviour, is described. Finally, the inclusion of vehicle commercial and operational information is shown to provide insights into the likely mix of technical strategies for future truck engines in the UK vehicle parc. It is seen that, in the near future especially in Europe, the likely solution for truck engines will be a mix of EGR and SCR techniques both of which will include the use of particulate filtration. However, the extent of the commercially viable application of this strategy is very dependent upon likely future market prices for the various aftertreatment and fuel technologies.

For the development of new HD Diesel engines which meet the demands of future international emission regulations and, at the same time, ensure economic operation modern development tools need to be used, especially for an optimisation of the combustion principle. To find the optimal engine set up, the variation of a large number of engine and injection system parameters, i.e. injection system, number of nozzle holes and sizes, injection rate profiles, etc. is required. To speed up the design process, modern optical engine diagnostics and 3D-numerical simulation can help to analyse the highly transient in-cylinder processes in detail. These methods provide essential insight to understand the complicated physical and chemical interactions and acting mechanisms during mixture formation, combustion and pollutant formation as well as the function of components of the system.

The combustion process of a heavy-duty DI-Diesel truck engine has been investigated using numerical simulation. The numerical modeling was based on an improved version of the KIVA-2 engine simulation code, employing a modified characteristic time-scale combustion model and a modified Kelvin-Helmholtz spray atomization model. The NO-formation process was modeled using the extended thermal Zeldovich mechanism. The simulation efforts included the effects of different injection characteristics such as varying the injection rate profile or number of injection holes and sizes. The physical sub-models used to improve the simulation of the mixture-formation and the combustion process were validated through comparison with single-cylinder engine experiments. Special attention was given to accurately model the in-cylinder flame propagation of the individual sprays and their effect on thermal NO-formation. All simulations were based on full load cases at medium speed.

For meeting 21st-century exhaust emission standards for HD diesel engines, new methods are necessary for reducing NOx and PM emissions without increasing fuel consumption. The stratified diesel fuel-water-diesel fuel (DWD) injection in combination with exhaust gas recirculation (EGR) is as a means for NOx and PM reduction without any negative effect on fuel economy. The investigation was performed on a charged HD single-cylinder direct-injection diesel engine with a modern low-swirl combustion system, 4-valve technology and high pressure injection. The application of DWD injection combined with EGR resulted in a 60 percent lower NOx emission at full load and a 75 percent reduced NOx emission at part load when compared with present day (EURO II) technology. This was achieved without any fuel economy penalty, but with an additional PM emission reduction.

The significantly poorer efficiency of a spark-ignition engine in the part load range compared to its efficiency at full load is the result on the one hand of the restriction losses and on the other hand of the severe combustion lag. It would be a conceivable proposition to considerably improve part load efficiency (10 % at bmep = 3 bar) by increasing the compression ratio although this is impeded by the risk of knocking combustion at full load. In combination with variable valve timing (this requires variable compression ratio) even 19 % efficiency improvement are achievable (1). This has given rise to a joint development programme between Mahle GmbH and Daimler-Benz. Research aimed at producing a piston with a variable compression height which can thus offer optimum compression ratio at every point of the map.