Improving fuel efficiency with laser surface textured piston rings

Abstract

An experimental study is presented to evaluate the effect of partially laser surface textured piston rings on the fuel consumption and exhaust gas composition of a compression–ignition IC engine. Dynamometer tests were performed with a Ford Transit naturally aspirated 2500 cm³ engine at a wide range of engine speeds under near-half-load conditions. A comparison was made between the performance of reference non-textured conventional barrel-shaped rings and optimum partial laser surface texturing (LST) cylindrical-shape rings. It was found that the partial LST piston rings exhibited up to 4% lower fuel consumption, while no traceable change in the exhaust gas composition or smoke level was observed.

Introduction

Developing ever more fuel-efficient and compact automobile engines with reduced environmental impact is one of the biggest challenges that tribologists are facing in the era of global warming [1]. The friction loss in an internal combustion engine is a major factor in determining the fuel economy and performance of the vehicle utilizing the power of the engine. Some 20–30% of the friction losses in a compression-ignition engine are due to the piston/cylinder system, of which a large part is attributed to the piston rings, e.g. [2], [3], [4], [5], [6]. Proper lubrication and surface texture are key issues in reducing friction in a piston/cylinder system and, hence, have received a great deal of attention in the relevant literature. Surface texturing as a means for enhancing tribological properties of mechanical components is well known since many years. Perhaps the most familiar and earliest commercial application of surface texturing in engines is that of cylinder liner honing [7], [8]. The honing technology is still the leading one in cylinder texture preparation and an interesting recent attempt to model its effect on friction reduction is presented in Ref. [9]. More recently laser surface treatment has also been applied to cylinder liners, e.g. [10], [11] showing up to 4.5% improvement in the torque compared with a standard liner [11].

Surface texturing in general and laser surface texturing (LST) in particular has emerged in recent years as a potential new technology to reduce friction in mechanical components [12]. More specifically for the piston/cylinder system, Ronen et al. [13] developed a theoretical model for LST cylindrical face piston rings. The authors studied the potential use of piston rings with a surface micro-structure in the form of spherical micro-dimples to reduce the friction between rings and cylinder liner, where the entire ring face in contact with the cylinder liner was textured. An optimum value of the ratio of micro-dimple depth over diameter was found, which yields a friction reduction of 30% and even more compared with an un-textured ring. The model prediction was experimentally verified on a laboratory reciprocating test rig [14]. A significantly lower friction with optimum partial LST cylindrical face piston rings, where only a portion of the ring face width is textured, was theoretically predicted [15] and experimentally verified [16]. The partial LST is based on a “collective” effect of the dimples [17] that provides an equivalent converging clearance between nominally parallel mating surfaces similar to the “inlet roughness” concept in [18].

Some real firing engine tests that were performed with LST barrel-shape rings showed very little friction reduction compared with the same un-textured rings at low speeds below 2000 rev/min. Above 2000 rev/min this little benefit of the LST vanished completely. It seems that the barrel shape, which presumably was arrived at by trial and error experience over many years [19], is not a good candidate for LST. The crowning of the ring face by itself provides strong hydrodynamic effect that masks the weaker hydrodynamic effect of the surface texturing especially at high speeds. Indeed, a more appropriate comparison between the performance of non-textured barrel shape and optimum partial LST cylindrical shape rings, which was performed recently on a laboratory reciprocating test rig [20], showed that a friction reduction of up to about 25% can be obtained with partial LST cylindrical face rings. The aim of the present paper is to present a similar comparison that was made with a firing engine. An experimental study is presented to evaluate the effect of partial laser surface textured piston rings on the exhaust gas composition and fuel consumption of a compression-ignition IC engine.