Design and Testing of Adaptable Piston Damping Mechanism for Rapid Compression Machine

A Rapid Compression Machine (RCM) is, essentially, a piston housed inside a cylinder. Its main objective is to compress a combustible mixture as rapid as possible to a higher temperature and pressure at which its auto ignition characteristics can be studied. Such machines are useful in the study of engine fuels autoignition behaviour because they provide a good representation of a single stroke compression ignited engine. There are various designs and operation techniques of RCM, but they all have to possess a key characteristic of compressing the fuel mixture rapidly while producing minimum heat and mass loss, turbulence and pressure variations due to piston oscillations. For these machines, it is therefore mandatory for the compressing piston to immediately be put into a complete stop after compression stage without any bouncing, any oscillations would result into undesirable change in end of compression volume and pressure. This paper describes a novel design and testing of an improved RCM hydraulic damping mechanism based on the use of groove and ring. The design involved additional return paths from the rear of the damping groove to the oil reservoir, regulated by needle valves. The needle valve made it possible to precisely regulate the oil displacement rate for a specific driving cylinder to combustion chamber pressure ratio. Tests were also conducted using different damping oils with different viscosities which showed improved damping with viscosity. With a combination of damping oil viscosity, driving pressure and needle valves operation, a very good piston damping capability without rebound was obtained for end of compression pressures of up to 2.5 MPa.