23rd International Colloquium Tribology - January 2022 197 In-Bore Engine Component Tribology M. Priest, M. F. Fox Faculty of Engineering and Informatics, University of Bradford, UK 1 Baker, A J S, et.al, Intl.Symp.Mar.Engrs., Tokyo, paper 256, 2.5.59-70, 1973. 2 Dimitroff, E, et.al, Trans.ASME, 406, July 1969 3 Economu, P N, et.al, ‘Piston Ring Lubrication’, Pt.1, ASME, New York 1979 4 Richard, G P, Proc.8th Leeds-Lyon Symp. Tribology, Paper VII(ii), 1982; Bush, G P, et.al, Trib.Intl., p.231, 24, 1991 5 Saville, S B, et.al, SAE 881586, 6 Bagshaw, J, et.al, Trib.Int, 30, 1997. 1. Introduction Only 25% of fuel energy is delivered to light vehicles wheels, 40% for heavy duty transport. Piston and ring pack component internal friction is a major loss, at ~20% of overall efficiency. Total fuel efficiency improvements must be improved in concert with weight reduction, improved thermodynamic efficiency and reduced rolling resistance. Given light vehicle electrification, heavy duty i/ c engines will be required for the foreseeable future. Understanding i/ c engine in-bore component tribology requires knowledge of the lubricant’s role. Conventional wisdom to the 1960’s held that a thin lubricant film at the piston/ bore interface was the same as the sump but at higher temperatures; lubricant above the top ring either evaporated or burned. However, a standing ring of lubricant was demonstrated in the piston ring zone 1 ; inter-ring gas from an operating engine showed extensive degradation of simultaneous oil samples 2 . This changed appreciation of lubricant conditions in the piston led to extensive modelling and testing by Dowson et.al. 3 Figure 1: Piston Ring Lubrication Model This modelling, and the large body of further research it inspired, clearly showed that piston rings operate with a very thin film of lubricant, <1μm towards TDC and BDC, with the upper rings starved of lubricant by the action of the oil-control ring at the base of the piston. The lubricant viscous flow interactions between the rings, Figure 1, and the added effects of gas flow through the ring pack and the inertia of the reciprocating piston, demonstrated that lubricant could reside in this region for some considerable time. Analyses of lubricant samples from the bore wall at the 1 st ring TDC reversal position of diesel engines showed the ring zone lubricant to be severely degraded compared to the sump. The extent of degradation of base oils and additives meant that different lubricant grades could not be differentiated 4 . More measured, differentiated, degradation of base oils and additives was found by continuous internal sampling from the first ring groove of operating engines 5 . Insights gained led to improved lubricant formulation and performance. As one example, ZDDP’s sequentially degraded through several intermediates to the sulphide. A lubricant transport model equilibrated a large (sump) Continuous Flow Stirred Tank Reactor, CFSTR, with a similar (ring zone) reactor and calculated relative base oil and additives degradation rates 6 . Figure 2: Lubricant Transport Model Between Sump and Ring Zone as Two Continuous Flow Stirred Tank Reactors in Equilibrium 198 23rd International Colloquium Tribology - January 2022 In-Bore Engine Component Tribology ‘Residence time’ measurements of lubricant in the ring zone showed how the ring pack controlled flow for both large and small engines, and for different sampling positions. Second ring groove, intermediate land, first groove and crown land samples showed progressive base oil and additive degradation at different rates as the lubricant progressed up the piston face. Degradation of additives was shown to primarily occur in the 1 st ring zone with base oil degradation in the 1 st ring/ crown land region. Continuous measurement of gas and lubricant flow rates as functions of diesel engine speed and load showed plateaus between 40-75% of an engine range 7 , Figure 3. Figure 3: Oil Flow Rates from 1 st and 3 rd Ring Positions, CAT 3406 Engine, 700-1900rpm. 7 C J Jones, PhD Thesis, dMU(UK), 2000 8 Notay, R S, et.al,Trib.Intl, 112, 129, 2019. 9 OME Smith, PhD Thesis, Leeds(UK), 2 Different ring pack designs for the same piston showed substantial changes in oil and gas flow rates, enabling reduced emissions without increased wear. ‘Residence time’ measurements of ring zone lubricant using compatible ‘tracer’ compounds showed how the ring pack controlled flow for both large and small engines, and different sampling positions. For the CAT 3406 engine at 1400rpm, residence times ranged between 6 minutes from the 2 nd ring, 16min for the inter-ring land, 26min from the 1 st ring and 72min for the crown land. ‘Laser Induced Fluorescence’ in the bore wall of both a motored and fired engine measured lubricant film thickness as the piston ring assembly passed the observation point. The changes in oil film thickness between and above the ring pack were consistent with the previous sampling results. Significant differences in oil film thickness were observed between new and ‘end of service use’ oils 8 .‘ An ‘In-Bore Friction Loss Measurement’ system was developed to measure friction losses between an operating piston and bore. A range of organic compounds was added to a standard fuel as potential friction modifiers. Switching between standard fuel and fuels with friction modifier additives showed distinctive patterns of friction reduction. The optimum organic compounds showed friction loss reductions of 4% by introducing lubricants directly into the combustion chamber and piston ring zone as friction modifiers 9 .