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During recent years, the deterioration of greenhouse phenomenon, in conjunction with the continuous increase of worldwide fleet of vehicles and crude oil prices, raised heightened concerns over both the improvement of vehicle mileage and the reduction of pollutant emissions. Diesel engines have the highest fuel economy and thus, highest CO2 reduction potential among all other thermal propulsion engines due to their superior thermal efficiency. However, particulate matter (PM) and nitrogen oxides (NOx) emissions from diesel engines are comparatively higher than those emitted from modern gasoline engines. Therefore, reduction of diesel emitted pollutants and especially, PM and NOx without increase of specific fuel consumption or let alone improvement of diesel fuel economy is a difficult problem, which requires immediate and drastic actions to be taken.

An experimental analysis is carried out to investigate several heat transfer characteristics during the engine cycle, in the combustion chamber and exhaust manifold walls of a direct injection (DI), air-cooled, diesel engine. For this purpose, a novel experimental installation has been developed, which separates the engine transient temperature signals into two groups, namely the long-and the short- term response ones, processing the respective signals in two independent data acquisition systems. Furthermore, a new pre-amplification unit for fast response thermocouples, appropriate heat flux sensors and an innovative, object-oriented, control code for fast data acquisition have been designed and applied. Experimentally obtained cylinder pressure diagrams together with semi-empirical equations for instantaneous heat transfer were used as basis for the calculation of overall heat transfer coefficient.

A difficulty which exists when applying diagnostic techniques on large-scale diesel engines operating on the field, is that usually it is not possible to obtain measurement data at a wide engine operating range due to a number of constraints. In the present work is investigated the possibility to overcome this practical difficulty originating from the test procedure for engines operating on the field (i.e. marine or stationary applications). The main objective is to examine if a diagnosis procedure provides similar results when applied at various engine operating conditions. For this purpose an existing diagnostic technique, developed by the authors, is applied at different operating conditions on a large-scale two-stroke diesel engine used for power generation in a Greek island.

A theoretical study is conducted to examine the effects of oxygen enrichment of intake air and exhaust gas recirculation (EGR) on heavy-duty (HD) diesel engine performance characteristics and pollutant emissions. A phenomenological multi-zone model was properly modified and used to assess the impact of intake air oxygen-enhancement and EGR on the operating and environmental behavior of a HD diesel engine under various operating conditions. Initially, an experimental validation was performed to assess the predictive ability of the multi-zone model using existing data from a HD turbocharged common-rail diesel engine at the 12 operating points of the European Stationary Cycle (ESC) considering certain high-pressure cooled EGR rate at each operating point.

Despite the improvement in HD Diesel engine out emissions future emission legislation requires significant reduction of both NOx and particulate matter. To accomplish this task various solutions exist involving both internal and external measures. As widely recognized, it will be possibly required to employ both types of measures to meet future emission limits. Towards this direction, it is necessary to reduce NOx further using internal measures. Several solutions exist in that area, but the most feasible ones according to the present status of technical knowledge are EGR, water injection or fuel/water emulsions. These technologies aim to the reduction of both the gas temperature and oxygen concentration inside the combustion chamber that strongly affect NOx formation. However, there remain open points mainly concerning the effectiveness of water addition techniques and penalties related to bsfc and soot emissions.

Considering future emission legislation for HD diesel engines it is apparent that it will be probably necessary to employ A/T devices to achieve them. The main problem concerns the simultaneous control of both NOx and particulate emissions at an acceptable fuel penalty. Concerning particulate matter the use of particulate traps is considered to be a proven technology while for NOx emission control; various solutions exist mainly being the use of SCR catalysts or LNT devices. But LNT traps require periodical regeneration, which is accomplished by generating reducing agents i.e. CO and H2. The present investigation focuses on the regeneration of LNT devices through the engine operating cycle. This can be achieved using two techniques, additional injection of fuel at the exhaust manifold (external measures) or operation at low lambda values in the range of 1.0 or lower (internal measures).

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.

A major challenge for researchers and engineers in the field of diesel engine development is the simultaneous reduction of both NOx and soot emissions from diesel engines to comply with strict future emission legislation. One of the promising internal measures that focus on the reduction of soot emissions is post fuel injection which does not have a serious effect on NOx emissions. The main parameters involved when using this technique are post fuel quantity and dwell angle between the main and the post fuel injection events. In the present work a detailed computational investigation has been conducted to determine the effect of post fuel injection on engine performance and pollutant emissions (NOx and soot). To this scope, a phenomenological multi-zone combustion model has been used, properly modified to take into account the interaction of post and main injected fuel amounts.

In this paper, the results are presented from the analysis of the second stage of an experimental investigation with the aim to provide insight to the cyclic, instantaneous heat transfer phenomena occurring in both the cylinder head and exhaust manifold wall surfaces of a direct injection (DI), air-cooled diesel engine. Results from the first stage of the investigation concerning steady-state engine operation have already been presented by the authors in this series. In this second stage, the mechanism of cyclic heat transfer was investigated during engine transient events, viz. after a sudden change in engine speed and/or load, both for the combustion chamber and exhaust manifold surfaces. The modified experimental installation allowed both long- and short-term signal types to be recorded on a common time reference base during the transient event.

Further reduction of brake specific fuel consumption (bsfc) in heavy-duty diesel engines, which are used for vehicle applications, is of utmost importance due to high fuel prices, global warming issue (CO₂ emissions) and continuously stringent environmental regulations. Specifically, the necessity for further reduction of specific diesel oil consumption and increase of vehicle mileage, respectively, is more pronounced in large haul diesel trucks due to technical, environmental and economical reasons. Heavy-duty (HD) direction injection (DI) diesel engines are used in these vehicles, which indicate a rather high power output in the range of 200-400 kW. During recent years, various measures have been proposed from engine manufacturers and researchers for improving combustion process and through that, increasing the fuel economy of diesel engines.

The past decade significant research effort has concentrated on the DI diesel engine due to stringent future emission legislation which requires drastic reduction of engine tail pipe pollutant emissions, mainly PM and NOx, without significant deterioration of specific fuel consumption. Towards this effort, the important role of modeling to investigate and understand the impact of various internal measures on combustion and emissions has been widely recognized. Phenomenological models can significantly contribute towards this direction because they have acceptable prediction capability and the advantage of low computational time. This enables the production of results, on a cycle basis, that indicate the effect of various parameters on both engine performance and emissions. Therefore their use can significantly reduce engine development time (i.e. reduction of experimental effort) and cost.

In the present work is presented a detailed evaluation of an advanced diagnostic technique, developed by the authors, for the determination of diesel engine condition and tuning. For this purpose, an extended experimental investigation has been conducted on a prototype test engine installed in the author's laboratory. During the measurements various operating parameters (i.e. torque, fuel consumption, injection pressure, cylinder pressure, peripheral temperatures etc.) have been recorded at various operating conditions (i.e. engine speed and loads). Initially the engine operated at its normal conditions (i.e. reference state). Then, two “virtual” faults (i.e. reduction of injector opening pressure and increase of cylinder mass leakage) were introduced, that affected engine operation.

Engineers use phenomenological simulation models to determine engine performance. Using these models, we can predict with reasonable accuracy the heat release rate mechanism inside the engine cylinder, which enables us to obtain a prediction of the pressure history inside the engine cylinder. Using this value and the volume change rate of the combustion chamber, we can then estimate the indicated power output of the engine. However, in order to obtain the brake engine power output we must have an indication for the mechanical losses, a great part of which are friction losses. Up to now various correlations have been proposed that provide the frictional mean effective pressure as a function mainly of engine speed and load. These correlations have been obtained from the processing of experimental data, i.e. experimental values for the indicated and brake power output of engines.

During the recent years, extensive research conducted worldwide in the field of Heavy Duty Diesel engines has resulted to a significant improvement of engine performance and emissions. These efforts have been assisted from simulation models providing good results. Towards this direction a multi-zone model developed by the authors has been used in the past to examine the effect of injection pressure on DI diesel engine performance and emissions. The attempt was challenging since no experimental data existed when the calculations were conducted, to support the findings. Eventually, experimental data concerning engine performance and emissions became available using slightly different operating conditions and injection pressure data. In the present study an attempt is made to evaluate the prediction ability of the multi zone model by comparing the theoretical results with experimental data and explain any discrepancies between them.

The reduction of brake specific consumption and pollutant emissions are issued as future challenges to diesel engine designers due to the depletion of fossil fuel reserves and to the continuous suppression of emission regulations. These mandates have prompted the automotive industry to couple the development of combustion systems in modern diesel engines with an adequate reformulation of diesel fuels and have stirred interest in the development of “clean” diesel fuels. The use of oxygenated fuels seems to be a promising solution towards reducing particulate emissions in existing and future diesel motor vehicles. The prospective of minimizing particulate emissions with small fuel consumption penalties seems to be quite attractive in the case of biodiesel fuels, which are considered as an alternative power source. Studies conducted in diffusion flames and compression ignition engines have shown a reduction of soot with increasing oxygen percentage.

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.

Thermodynamic, dynamic and design parameters have a significant and often conflicting impact on the transient response of a compression ignition engine. Knowing the contribution of each parameter on transient operation could direct the designer to the appropriate measures for better engine performance. To this aim an explicit simulation program developed is used to study the performance of a turbocharged diesel engine operating under transient load conditions. The simulation developed, based on the filling and emptying approach, provides various innovations as follows: Detailed analysis of thermodynamic and dynamic differential equations, on a degree crank angle basis, accounting for the continuously changing nature of transient operation, analysis of transient mechanical friction, and also a detailed mathematical simulation of the fuel pump. Each equation in the model is solved separately for every cylinder of the 6-cylinder diesel engine considered.

This work is a part of an extended investigation conducted by the authors to validate and improve a newly developed quasi-dimensional combustion model. The model has been initially applied on an old technology, naturally aspirated HSDI Diesel engine and the results were satisfying as far as performance and pollutant emissions (Soot and NO) are concerned. But since obviously further and more extended validation is required, in the present study the model is applied on a new technology, heavy-duty turbocharged DI Diesel engine equipped with a high pressure PLN fuel injection system. The main feature of the model is that it describes the air-fuel mixing mechanism in a more fundamental way compared to existing multi-zone phenomenological combustion models, while being less time consuming and complicated compared to the more accurate CFD models. The finite volume method is used to solve the conservation equations of mass, energy and species concentration.

Simulation models are widely used from research engineers to investigate the combustion mechanism of DI diesel engines. These models can be used, as tools to either comprehend information provided by experimental data or to perform predictions and assist the development process. As widely recognized a valuable source of information for engine performance and emissions studies is the cylinder pressure trace. It can provide after processing information concerning the combustion rate of fuel injected inside the combustion chamber. Often it is also used to calibrate simulation models or even to derive correlations to represent the combustion rate of fuel inside the combustion chamber. The present research team has during the development process of a simulation model for the description of DI diesel engine performance and emissions realized that there exists a serious problem.

During the last years a great deal of efforts have been made to reduce pollutant emissions from Direct Injection Diesel Engines. The use of gaseous fuel as a supplement for liquid diesel fuel seems to be one solution towards these efforts. One of the fuels used is natural gas, which has a relatively high auto - ignition temperature and moreover it is an economical and clean burning fuel. The high auto - ignition temperature of natural gas is a serious advantage against other gaseous fuels since the compression ratio of most conventional diesel engines can be maintained. The main aspiration from the usage of dual fuel (liquid and gaseous one) combustion systems, is the reduction of particulate emissions. In the present work are given results of a theoretical investigation using a model developed for the simulation of gaseous fuel combustion processes in Dual Fuel Engines.