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Diesel combustion

Objective

In past work it has been established that squish and the interaction of mixtures with combustion chamber wall are the most important parameters in mixture formation of direct injection Diesel engines. This project will develop a simulation model and carry out experimental work to establish how these parameters influence mixture formation.
1 Harwell, Oxfordshire

Two film thickness measurement techniques were identified and developed. Firstly, a fluorescence/length of line technique in which the length of travel between fluid surfaces of a laser beam passing through a wall film is visualised using a line-scan camera, and secondly a capacitance technique in which the varying capacitance of a sensor submerged in the fuel film is measured. The length of line visible and the sensor capacitance are both proportional to film thickness, which is derived from calibration with films of a known dimension.

The two techniques were proven at elevated temperatures and pressures in the Harwell High Pressure Cell using the spray from a diesel injector. The special spray target plate required for length of line measurements was shown to have no significant effect on film formation through CFD modelling and the film thickness measurements made were verified with laser sheet illumination and photography of the impinging spray. Finally, a sensor array and signal processing/data transmission electronics were produced for use in the Two Period engine at Daimler Benz.

2 Imperial College, London

The present study of spray/wall interactions in Diesel engines has been successfully completed. An extensive literature survey on the droplet impingement process has identified seven different impingement regimes, which are determined by parameters such as droplet size, incident velocity, liquid properties, surface temperature and roughness. It has also uncovered some useful data on transition criteria between the different regimes and the post-impingement characteristics.

A spray impingement model has been formulated on the basis of the literature findings and mass, momentum and energy conservation constraints, using some imperial correlations either taken directly from the literature or further developed therefrom by us. A random procedure is introduced to determine some of the post-impingement characteristics, in view of the stochastic nature of impingement process.

A mathematical model of the wall film formed by spray impingement has been constructed based on some simplifying assumptions. The dynamic pressure due to bombardment by the impingement droplets and the tangential momentum transfer resulting from oblique droplet impingement on the film surface are taken into account in the formulation. The film flow is governed by mass, momentum and energy conservation equations of the boundary-layer type, and an integral equation governing the film thickness is derived using jump conditions at the film/gas interface. It is shown that this set equations can be substantially simplified if local equilibrium occurs and a dimensional analysis is performed to identify the conditions for the applicability of local equilibrium model. The final form of equations suitable for engine wall film flow modelling is subsequently determined by reference to typical engine conditions.

The impingement and film models are incorporated into the EPISO-SPRAY code which solves ensemble-average conservation equations of mass, momentum and energy for three dimensional unsteady coupled gas and droplet phases. The models are subsequently assessed by comparisons with experiments involving normal and oblique spray impingement in bombs as well as in cylindrical-bowl engines.

3 CNPM-CNR

The length of line measurement technique developed at Harwell Laboratories was applied to a bomb designed and installed at CNPM, in order to investigate the formation of a liquid film on the chamber wall following a typical diesel fuel injection. Gas pressure and wall temperature were parametrically changed, while injection conditions were kept constant. Measurements show that a liquid film is produced on the wall for all the tested conditions. Gas-wall temperature was found to be the most influencing parameter, whose increment strongly reduces the film thickness. Gas pressure seems to be not influent at low temperature, while it produced slightly thicker films at high temperature.

The splash of single droplets on a thin liquid film has also been analysed to characterise the different regimes and to obtain a better understanding of the influence of various parameters, such as Weber and Laplace number and liquid film thickness.

4 Daimler-Benz

The refinement of the capacitance film thickness measuring technique at the two period engine was the major part of the work performed during the last investigation period.

Solutions for the problems with the high frequency setup, as there were the mutual influencing of the single data acquirement channels and the interferences by different external sources, have been found. Due to the fact, that the measuring technique showed a dependency on temperature and/or pressure in the combustion chamber, an analysis procedure for the separation of the film thickness and the superimposed temperature/pressure signal had to be developed.

Parametric studies have been performed and the behaviour of the deposited liquid film on the piston bowl wall has been examined.
The results obtained under the EC contracts EN3E-0143-D and JOUE-CT89-0012 are the starting point in this project. It was clearly established that squish (internal mass transfer effect due to the piston movement and the irregular shape of the combustion chamber e.g. bowl in the upper surface of the piston in Diesel engines) and the wall of the piston bowl (in which the fuel is injected) may have the greatest positive influence on mixture formation. The influence of squish on spray propagation and mixture formation is to be investigated on the ZHT (two period engine) at Daimler-Benz. Moreover, Daimler-Benz will carry out single-cylinder tests to determine the influence of varying ratios of wall share/air share on engine characteristics.

Work carried out by CNPR-CNR is aimed at a detailed analysis of the spray structure with and without swirl and of the spray-wall impingement process. Through measurements of the droplet size distributions, the droplet velocity field, and time resolved pictures of the spray development, the interaction fuel-gas phase and the interaction jet-wail will be studied and their effects quantified.

In the study of fuel impingement onto the piston bowl walls and its consequent effect on preparation mixing and combustion by AEA TECHNOLOGY (UKAEA) at Harwell, it is necessary to know whether the fuel "wets" the wall. In addition, detailed measurements of the film thickness and the motion of the film, the waves on the interface and the interaction of the droplets near the wall will be carried out.

A physical and numerical description of the jet/wall interaction and an evaluation of the mechanisms, which are important for the mixture formation will be made by Imperial College; this includes a verification of the computer model with the help of the test results.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Daimler-Benz AG
Address
Epplestraße 225
70567 Stuttgart
Germany

Participants (3)

Consiglio Nazionale delle Ricerche (CNR)
Italy
Address
Via Francesco Baracca 69
20068 Peschiera Borromeo Milano
IMPERIAL COLLEGE OF SCIENCE, TECHNOLOGY AND MEDICINE
United Kingdom
Address
Exhibition Road
SW7 2BX London
United Kingdom Atomic Energy Authority
United Kingdom
Address
353,Harwell
OX11 0RA Didcot - Oxfordshire