P REVIOUS W ORK

In this section, the previous work carried out within City‘s research group, previously

based at Imperial College, is highlighted. Ostovar (1996) developed a single-ring test rig to simulate under idealised conditions the lubrication action between the piston ring and the cylinder liner in a reciprocating engine. The test rig allowed the measurement of oil film thickness, using the capacitance method, and friction measurement between the ring and liner to be recorded simultaneously. A numerical method had been proposed for the prediction of the lubricant film parameters which allows the Swift-Stieber, separation, Coyne and Elrod and Floberg boundary conditions to be employed by simply changing a in the equation set; the model was assessed against the results obtained in the test rig. A cavitation delay mechanism was found to be responsible for the thinner film, after reversal points of reciprocation, than that predicted by the Reynolds boundary condition. Furthermore, the surface roughness models were found to be very sensitive to the roughness parameters used. A dual Laser Induced Fluorescence and interferometric technique had also been developed which, in theory could permit the dynamic calibration of the oil film thickness in an engine.

Duszynski (1999) employed the Laser Induced Fluorescence technique developed by Ostovar (1996) in a firing four-stroke diesel engine but the interferometric technique proved to be inadequately accurate to be used for this application. Optical fibres were installed in the liner and a static calibration method was developed. He performed a quantitative study of the temperature effect on the calibration coefficients of all the engine oils tested with a custom made calibration apparatus. It consists of 20 x 20 x 100 mm block of fused silica on which the calibration standard was in the form of grooves of known depth. The selection of the material was made on the base of its physical properties and, in particular, its low thermal expansion. The grooves etched into the

surface of the block were intended to be incremental in depth between 2 and 10 μm, in

steps of 2 μm. From the early engine tests, it was found that the oil film thickness under

the piston rings varied within that range. Various oils were examined, varying in additive concentration and viscosity. At top dead centre, chemistry-related effects were found to be more significant than those related to viscosity. The Laser Induced Fluorescence technique was later applied to a two stroke engine and the results showed higher oiling rates associated with thicker films.

64 Pyke (2000) carried out experiments to characterise piston ring lubrication in both a firing diesel engine and in an idealised test rig. Oil film thickness was measured on the liner by employing the LIF technique. Sets of experiment were carried out in the engine at different operating conditions. Repetitive measurements of oil film thickness on the liner did not prove to be satisfactory except on the ring where results were found to be more consistent. Complications have risen in comparing lubricants due to issues related to the calibration of the LIF technique. Despite the difficulty in some cases to decouple the various parameters, the expected trends in the film thickness variation with engine conditions were confirmed. Experiments at engine start up were also conducted where the behaviour of the oil film thickness was analysed. The build up of the oil film was identified to be a four- stage process and the effects of cylinder pressure, engine speed, liner temperature and lubricant availability on the film thickness were all visible during the first two hundred cycles. The test rig built by Ostovar (1996) was modified by inserting an optical fibre where the LIF technique was used to identify cavitation under the ring for a range of lubricants. However, uncertainty in the variation of cavitation along the ring has made comparison difficult. An investigation about the surface roughness orientation and its effect on friction was also carried out. The results clearly show that transversely orientated roughness produces a reduction in friction when the lubricant is mixed and while for hydrodynamic conditions the opposite is true.

Dellis (2005) utilised the purpose built test rig to measure the oil film thickness using both the capacitance and the LIF techniques. Various methods for calibrating the LIF technique were employed. A dynamic technique has been proposed whereby a groove of known depth on a piston ring specimen is used to provide in-situ dynamic calibration, circumventing the uncertainties in static calibration of oil film temperature and properties variation during the cycle. A micrometer-based static calibration method has been compared directly to the dynamic one. The power output of the fibres had been taken into account as well as the background noise and resident voltage on the fluorescence detector. However, none of the above mentioned methods proved to be

satisfactory in comparison with the capacitance‘s results. Therefore, a ring profile fitting

was being used as the standard calibration method. Parametric studies on oil have been conducted where the oil film thickness, friction and oil film pressure measurements were measured and compared for different speed, load, temperature, lubricant type and

65 piston-ring geometry. The test rig has been modified in order to provide optical access, between the specimen ring and the liner, to visualise the onset and development of cavitation with a CCD camera. Different types of cavities had been identified which already have been discussed above in section 2.3. The single cylinder diesel engine used by Pyke (2000) has been modified to incorporate quartz windows for visualisation experiments. The tests were run at motored conditions where similar forms of cavity to that of the test rig were identified and an approach towards their formation has been suggested. On the other hand, the lack of pressure measurements in the engine does not allow confirmation of the presence of cavity provided by the imaging results alone and this raises some important questions. Although the top compression ring operates under starved lubrication conditions, oil droplets were captured by the camera emerging from the ring side clearances that contribute to oil transport. Higher magnification images provided further evidence and these were compared with previous engine research on oil transport by other investigators.