Ultrasonic measurements of the piston ring lubricant film thickness in three different sized engines

Internal combustion engines are undisputedly one of the most impactful engineering achievements and are the predominant power source for a range of industries. Modern developments focus on producing cleaner and greener engines in part to meet emission legislation and growing customer demand for more environmentally friendly options. Large sections of engine research focus on raising the mechanical efficiency by reducing frictional losses. The greatest contributing factor to frictional losses is the contact between the piston and piston ring pack with the cylinder liner. This is due to the reciprocating motion of the piston in which the piston rings form a seal between the high temperature/pressure combustion chamber and the engine crankcase. Optimisation of the interaction between the piston and liner, therefore, has the potential to reduce engine emissions whilst simultaneously reducing frictional losses, raising the efficiency of the engine.

This thesis developed novel ways to use ultrasound to monitor the lubricating oil film between the piston/piston rings and the cylinder liner in engines from various sectors (passenger, marine and agricultural). The ultrasonic method is based on studying the portion of an ultrasonic wave that is reflected from a boundary which can be correlated to oil film thickness. A new data processing technique has been created to define the reflection coefficient from the Hilbert envelope, this novel technique simultaneously removed several undesired factors in the results, and revealed trends hidden by the previous data processing technique. The novel data processing technique calculated oil films typically 20% greater than those from the previous technique as it was subject to ultrasonic reflections from an unaligned piston ring leading to an underestimation of the lubricant film thickness. In addition to this, spectrograms and B-scans have been applied to ultrasonic piston ring reflections for the first time and were able to identify a series of trends, such as the detection of a piston ring outside of the alignment zone.

The ultrasonic sensors have successfully been used to quantify the piston ring lubricant film thickness in a series of engine conditions not previously studied. These include: during start-up and shut-down of an engine, residual oil films on the cylinder liner, live variation in the film thickness with varying engine loading/lubricant feed rate etc. and the detection of metal-metal contact between the piston ring and liner. The sensors have also been used for the first time in the design phase of an engine. In the marine diesel engine, ultrasonic sensors were used to evaluate a series of lubricant injector configurations and led to the identification of a configuration that consistently produced thicker lubricant films.