Abstract
The world currently faces two kinds of crises from the environmental perspective: the limited availability of fossil fuels and the environmental degradation. Projections indicate that the use of vehicles and consequently the fossil-fuel demand will almost triplicate in 2020. Pollutant emissions are believed to increase excessively as well. Bearing in mind this scenario, it is important to note that the Diesel engine has the advantage of consuming less fuel than the gasoline engine, and hence, of emitting less CO2. Nevertheless, this type of engines produces high NOx and particulate levels. Different actions have been proposed to tackle these difficulties.

Researches found that one solution to this problem is the advanced Diesel combustion. These concepts lead to a reduction of NOx and particulate levels. However, they present the drawback of producing high combustion noise levels. This disadvantage comes from the use of early injections, which promote a burning process of a large amount of fuel in premixed conditions.

On the other hand, alternative fuels have won a great importance in recent years. Biodiesel not only stand out for their ease production, use and storage, but also for its potential to reduce particulate, CO, HC and CO2. However, NOx emissions increase in most operating conditions. Synthetic fuels also reduce emissions significantly, and due to its higher calorific value, reduce the specific fuel consumptions.

From an acoustic point of view, combustion noise is one of the main disadvantages of Diesel engines, since it constitutes the main source of noise in vehicles that use this propulsion plant. In conventional, and especially in new concepts of Diesel combustion, the fuel burnt in premixed conditions causes an abrupt increase of the rate of pressure, thus rise in the cylinder deteriorating the quality of combustion noise.

For the above mentioned reasons, the objective of this thesis is to study the combustion noise of the PCCI (Premixed Charge Compression Ignition) concept, and the use of certain alternative fuels (vegetable and synthetic). For this purpose, experimental parametric studies are proposed, which allow the combustion noise characterization and the establishment of cause-effect relations between the characteristics of the combustion process, and sound quality and the overall noise level. The PCCI combustion was chosen in this investigation due to its straightforward implementation from a conventional Diesel engine, so that the relevant changes in the configuration of the combustion chamber and injection system required by other combustion concepts are avoided.  

The results have clearly shown that the combustion noise deteriorates significantly with the PCCI concept. Published research revealed that the pilot injection is one of the most reliable options to improve combustion noise within conventional combustion strategies. In PCCI strategies with split injection, when the dwell angle exceeds 20º and percentages between 20 and 30% of the total mass of fuel is injected during the pilot injection, is possible to overcome the mark of the conventional combustion, so that the sound quality of engine noise is improved. Furthermore, the use of fuels with lower cetane number as Gasoline/Diesel blends, allows increasing the ignition delay and hence promoting premixed combustion, without the necessity of employing high EGR levels. Mixtures containing 50% of gasoline, can improve the mark of sound quality of combustion noise up to 1.5 compared with standard Diesel fuel.

The analysis of the engine acoustics at several operating conditions with alternative fuels has shown negligible variations in both sound quality and overall level of combustion noise in comparison with those obtained with standard Diesel fuel. To improve the combustion noise with different biodiesel blends, the EGR rate was modified. The results showed that with EGR rates of 35%, it is possible to improve the noise quality to acceptable levels, using rapeseed biodiesel blends in volumetric concentration of 50% and 80%. For blends with lower percentages of biodiesel, the EGR rate is higher than 40%, causing an increase in particles and specific fuel consumption of more than 27%.

In conclusion, the intention of this thesis is to be a contribution to the study of the noise associated with modern combustion modes in Diesel engines, providing guidelines that can be integrated into the overall development process of these engines, highlighting the combustion noise as an important issue to be considered, together with the performance and emissions.