An investigation into piston ring blowby and its effect on biogas engines
This study has investigated the severe corrosion of Biogas engines by the blowby gases. The formation of blowby its composition and flow rate have been measured and simulated. The nature of the piston ring sealing, lubrication and breakdown has been examined. A study of Biogas engines showed that Copper corrosion of the small end and camshaft bearings by HZS gas was the reason for engine failure. H2S is present in all Biogas at a concentration of usually less than 1%, but succeeds in chemical attack despite its good combustion properties, and the expected reaction with the bases present in the lubrication oil. The HZS was corroding in its gaseous state, but only those bearings with indirect lubrication. The solution to this problem is either to adopt force fed lubrication of the bearings, or to replace the alloy with Aluminium-Tin. The experimental work used four engines of differing wear. The constant speed work showed that the fuel content of blowby gas increases with load despite any increase in fuelling rate. This trend was consistent for all gaseous fuels present including H2S. A series of computer calculations of piston ring blowby were completed, using conventional and novel input data. The resultant blowby flow was within an order of magnitude, confirming that two blowby mechanisms, ring gap blowby and ring seal breakdown, are present on worn engines. The composition results showed that the fuel content of blowby is subject to the complex nature of the quenching process in the combustion chamber. A study of the oil present at the top ring showed that the oil is greatly modified when compared with the sump oil, as a result of thermal degradation and base depletion. The oil has a high acid TAN, which suggests it could encourage corrosive wear of the cylinder liner.