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The Diesel engine and especially the direct injection type one is considered to be one of the most efficient thermal engines known to man up to now. It has an efficiency that in some cases is 30 to 40% higher than its competitor the spark ignition engine. The efficiency of the direct injection diesel engine has been considerably improved during the last decade resulting to low fuel consumption and lower absolute values of pollutant emissions. If we consider the green house effect caused by the emitted CO2 it is revealed the environmental importance of high engine efficiency. In the present work a theoretical investigation is conducted using a detailed simulation model for engine performance prediction, to examine the possibilities for improving engine efficiency. The simulation model used is a complete open cycle model for the engine and its subsystems. Such phenomenological models are very suitable for the prediction of engine performance.

The compression ignition engine of the dual fuel type has been employed in a wide range of applications utilizing various gaseous fuel resources, while minimizing soot and nitric oxide emissions without excessive increase in cost against that of the conventional direct injection diesel engine. Fumigated dual fuel compression ignition engines are divided into two main groups: the conventional dual fuel engines where part of the liquid fuel is replaced by gaseous one and the pilot ignited ones where a pilot amount of the liquid fuel is used as an ignition source. Due to the high auto-ignition temperature of the natural gas, it can be used as a supplement for the liquid diesel fuel in conventional diesel engines operating under dual fuel mode. Moreover, the use of natural gas as a supplement for the liquid diesel fuel could be a solution towards the efforts of an economical and clean burning operation.

Homogeneous Charge Compression Ignition (HCCI) engines have the potential of reducing NOx emissions as compared to conventional Diesel or SI engines. Soot emissions are also very low due to the premixed nature of combustion. However, the unburned hydrocarbon emissions are relatively high and the same holds for CO emissions. The formation of these pollutants, for a given fuel, is strongly affected by the temperature distribution as well as by the charge motion within the engine cylinder. The foregoing physical mechanisms determine the local ignition timing and burning rate of the charge affecting engine efficiency, performance and stability. Obviously the success of any model describing HCCI combustion depends on its ability to describe adequately both the chemistry of combustion and the physical phenomena, i.e. heat and mass transfer within the cylinder charge. In the present study a multi-zone model is developed to describe the heat and mass transfer mechanism within the cylinder.

Considering future emission legislation and the global thermal problem, two are the main issues that are of specific concern for the future of the diesel engine, specific gaseous pollutants and CO2 emissions. Both parameters are related to engine bsfc consumption directly or indirectly. The last is becoming even more important considering current fuel prices and the projection for the future indicating a trend for increasing fuel prices. The last decade significant improvement have been accomplished in the field of diesel engine efficiency that has resulted to considerable reduction of engine bsfc. It is obvious that despite improvements in diesel engine efficiency still a considerable amount of energy is rejected to the environment through the exhaust gas. Approximately 30-40% of the energy supplied by the fuel is rejected to the ambience.

Pollutant emissions and specifically NO and soot are one of the most important problems that engineers have to face when developing heavy duty DI diesel engines. Two main strategies exist as options for their control, reduction inside the engine cylinder using advanced combustion and fuel injection technologies and use of after-treatment systems. In the present work it is examined the use of EGR to control the formation of NO inside the cylinder of an engine with extremely high peak pressure. The work is applied on a single cylinder truck test engine developed under a project funded by the European Community focusing on the improvement of heavy duty DI diesel engine efficiency using increased injection timing. Use is made of a simulation model to predict the effect of more advanced injection timing on engine performance and emissions. The model has been modified to include the effect of EGR used to c ontrol the formation of NO which is considerably increased at high injection timings.

A major challenge in the development of future heavy-duty diesel engines is the reduction of NOx and particulate emissions with minimum penalties in fuel consumption. The further decrease of emission limits (i.e., EPA 2007-2010, Euro 5 and Japan 05) requires new, advanced approaches. The injection system of DI diesel engines has an important role regarding the fulfillment of demands for low pollutant emissions and high engine efficiency. One of the injection system parameters affecting fuel spray characteristics, fuel-air mixing and consequently, combustion and pollutant formation is the geometry of the nozzle hole. A detailed experimental investigation was conducted at UPV-CMT using three different nozzle hole types: a standard, a convergent and a divergent one to discern the effect of nozzle hole conical shape on engine performance and emissions.

The direct injection diesel engine is one of the most efficient thermal engines known to man. For this reason DI diesel engines are widely used for heavy-duty applications and especially for the propulsion of trucks. Even though the efficiency of these engines is currently at a high level there still exist possibilities for further improvement. One way to accomplish this is by increasing the injection timing which usually improves, depending on the operating conditions, the indicated efficiency of the engine. On the other hand advanced injection timing has a negative effect on peak pressure causing a serious increase of its value, a negative effect on NO emissions which are also seriously increased and a positive effect on Soot emissions which are reduced. In the present work a theoretical and experimental investigation is presented to determine the effect of more advanced injection timing on engine performance and pollutant emissions.