Yu Ding
Characterising combustion of diesel engines is not only necessary when researching the instantaneous combustion phenomena but also when investigating the change of the combustion process under variable engine operating conditions. An effective way to achieve this goal is to parameterize the combustion process using a finite combustion stage cylinder process model and then the parameters can be modeled to give a global description of diesel engine combustion.
The main objective of this thesis is getting information how to calculate (simulate) the parameters defining the finite stage cylinder process model using both theoretical and experimental methods. The latter is essential but also complicated.
Heat release analysis is an important tool in the research of the combustion process in diesel engines. An ‘in-cylinder process model’ is described firstly which is the basis of the ‘heat release calculation’ model. Moreover it can be used independently to simulate the in-cylinder process to investigate the main features of the cylinder process of the engine. Then the reversed and anti-causal ‘heat release calculation model’ is presented together with the results for three engine operating points. Then a new smoothing method based on multiple Vibe functions is presented in order to acquire a smoother and more reliable pressure signal.
In order to fit the measured engine cycle to the Seiliger process and to calculate the parameters which define the finite stage cylinder process model, the theory of a basic and advanced Seiliger process is presented. Then a systematic investigation of the Seiliger parameters and the effects on in-cylinder process is carried out. The more important investigation is how to fit the measured engine cycle to the Seiliger process. Several combinations of equivalence criteria are used to make this transformation and set up the applicable systems of equations for the Seiliger parameters. A Newton-Raphson iteration method then is applied to find the solutions of these systems of equations.
Three representative operating points (nominal point, nominal speed and 25% power, low speed and 50% power) are applied in the fitting of the real engine cycle with a Seiliger process firstly. The differences of the Seiliger parameters coming out of several fit procedures for the three operating points were compared and their global behaviour as function of power and speed could be seen. Finally the behaviour of the Seiliger parameters over the full operational range of the MAN 4L20/27 diesel engine is shown based on one extensive series of measurements. Also the error in the equivalence criteria that were not used was presented for the full range of operating conditions in order to decide on the most preferable fit version to use as a basis for modelling and characterizing the combustion.
The analysing approach in this thesis has been verified on an ancient engine, i.e. MAN 4L20/27. However it can be used for characterising combustion in modern engines with late injection timing, high pressure injection equipment, and even common rail since the simulation models and Seiliger model are based on basic principles of physics.