COKING AND MICROCOKING TESTS - BENCH SCRE ENING TOOLS FOR THE DEVELOPMENT AND EVALUATION OF ADDITIVES AND LUBRICANTS

Within the framework of the research done by the ''Essais laboratoire de preselection'' group, which depends on the Groupement Francais de C oordination (GFC), two evaluation tests are described of the tendency of an engine lubricant to form deposits at high temperature : A new mi crocoking test (Fig. 1), which is a static coking test subjecting an o il deposited on an aluminium alloy plate to high temperature gradients that are close to the thermal conditions encountered in the piston-ri ng zone of an engine. At the end of the test, the maximum stablity tem perature of the oil is determined, as expressed by the temperature at which deposits appear (Fig. 2), and the aspect and quantity of deposit s existing in the form of varnish are evaluated by determining a ratin g by means of the rating system described in the CEC M-02-A-78 method for evaluating the cleanness of engine (Table 5 and Figs. 5 and 10). T he association of the two assessment methods results in a effective di scrimination between different level oils. Results of interlaboratory tests concerning European reference oils for gasoline and diesel engin es are given (Table 7 and Figs. 3 and 4). An improvement of a coking t est on an inclined plate (Figs. 6 and 7), in which a given volume of o il flows as a thin film over a stainless-steel plate heated to a very high temperature. The deposits formed at the end of testing are evalua ted according to their nature (carbon or varnish), their amount and al so according to the principles of the CEC rating method (Figs. 8 and 9 ). Likewise, a system of video rating of microcoking plates by color r ecognition is described (Figs. 11 and 12). This system, called ''Video -Cotateur'' can accurately reproduce the visual rating (Fig. 13) while eliminating the subjective factors of this rating. The results obtain ed with this system make up the first phase in the route to the comput er-assisted rating of engine parts (Fig. 14).