Investigations on thermodynamic processes of a novel pneumatic drive asymmetric Gifford-McMahon cycle cryorefrigerator

In this paper, a numerical and experimental investigation is conducted on a novel pneumatic-drive asymmetric Gifford-McMahon cycle cryorefrigerator for the first time. In the pneumatic-drive asymmetric Gifford-McMahon cycle cryorefrigerator, the duration of the assistance space exhaust process is kept higher than that of the assistance space intake process. Therefore, the displacer moves faster at the lower dead centre and slower at the upper dead centre inside the expander cylinder, which makes the duration of expansion process longer. The numerical model solves the governing equations of the refrigerant and dynamics of free-floating displacer iteratively to illustrate the refrigeration mechanisms. Additionally, the model computes the performance parameters of the cryorefrigerator, like refrigerating capacity, and specific refrigerating capacity. By adopting the numerical model, the impact of the loitering time on the thermodynamic processes is elaborated. It is perceived that, both refrigerating capacity and specific refrigerating capacity reduces with an increase in the loitering time. The experimental cooling characteristics are studied for different values of discharge to suction pressure ratios of helium compressor.

Experimental investigation of the dynamics of a slider-crank mechanism with local linear force input

Conventional implementation of slider-crank mechanisms result in high loads transmitted through the mechanical structure, inhibiting the design of compact and oil-free machines. Therefore, this research proposes to step away from the conventional, i.e. rotative, actuation and to investigate local linear actuation on the slider-component directly, while maintaining the kinematic link of the slider-crank configuration. In this work the local linear actuating principle is evaluated experimentally where the goal is to obtain a continuous movement of the slider mechanism where Top Dead Centre & Bottom Dead Centre are reached and to minimise the loads transmitted through the mechanical structure. The non-isochronous transient behaviour of a slider-crank mechanism loaded with a spring-damper element is detailed as well as the optimal working conditions at steady state to achieve a reduced loading of the kinematic structure. By matching the operating frequency and resonance frequency of the system, a reduction of the loads transmitted through the system by 63% of the nominal spring load can be achieved. Further experimental (and multibody mechanical) investigation on the influence of flywheel exposes a clear trade-off between the sensitivity of the system and the transmission of the actuation force through the kinematic link.

Influence of Surface Texture Parameters on Friction Characteristics Under Starved Lubrication

A cylinder liner and piston ring running under starved lubrication near the top dead centre (TDC) and bottom dead centre (BDC) cause abnormal friction and wear during operation of a marine diesel engine. The method of laser texturing is proposed to improve the surface friction property under this condition. Spherical crown pits with different parameters were formed on the surface of samples by femtosecond laser processing. The BDC and TDC conditions of oil starvation were simulated in a reciprocating friction and wear experiment, and a numerical model of surface texture lubrication based on the Reynolds equation was established. The influence of the distribution density, diameter, and depth parameters of the texture on the surface properties was studied. In the BDC condition, compared with the untextured surface, the average coefficient of friction (COF) can be reduced by up to 24% and the average friction force can be reduced by up to 18%. In the TDC condition, the COF can be reduced by up to 19%, and the average friction force can be reduced by up to 18%. Therefore, the textures with various parameters should be arranged in different positions on the cylinder liner; more attention should be paid to the optimisation of diameter in the texture of the cylinder liner near the BDC, whereas more attention should be paid to the optimisation of distribution density in the texture of the cylinder liner near the TDC

Atomic force microscopic measurement of a used cylinder liner for prediction of boundary friction

Accurate simulation performs a crucial role in the design and development of new modern internal combustion engines. In the case of piston rings, simulations are used to effectively predict generated friction and power loss of proposed designs. These are consequences of viscous shear of a thin lubricant film, likewise boundary friction caused by direct interaction of piston rings with the cylinder liner/bore surface. The most commonly used model for determining boundary friction is that of Greenwood and Tripp. The model requires the pressure coefficient of boundary shear strength of asperities from the softer of the contacting surfaces as an input. This parameter needs to be measured. The paper describes the process of measurement using an Atomic Force Microscope (AFM), both for a dry surface and that wetted by the presence of a lubricant layer. For realistic results, the investigated specimen is a used, tested engine cylinder liner where boundary active lubricant additives are bonded to its surface as well as combustion products. This approach is as opposed to the previously reported works using new flat surfaces with base oil or partially formulated lubricants and has not been previously reported in the literature. The results show that for used cylinder liners, the measured boundary shear strength of asperities varies according to location along the stroke. Results are reported for the top dead centre, mid-stroke and bottom dead centre locations. The measurements are subsequently used with two-dimensional Reynolds solution for a top compression ring-liner contact, where it is found that accurate localised predictions of generated friction and power loss can be made instead of the usual average value approach reported in the literature.