The Effect of Oil Availability on the Operation of a Piston-Ring in a Large, Two Stroke Marine Diesel Engine

Abstract

Although transport of freight by sea has been found to be between 10 to 100 times less polluting than road and air transportation [1], in 2005 about 29,045 bn ton miles of trade was conducted by sea transport [2] and this level of activity leads to significant emissions. It has been estimated that marine transport emitted around 800 million tonnes of CO2 in the year 2000 contributing about 2.4% of anthropogenic emissions in that year [3]. A large proportion of marine emissions are due to the combustion of fuel, but in the case of large two stroke engines, the type which are mostly used to propel freight carriers, a significant proportion also comes from the emission of combustion products of the lubricant which is used to lubricate the piston rings in the engine.
Large, two stroke engines typically have cylinders up to 3m in diameter and may have power output exceeding 100,000 KW. Lubricant is supplied to the cylinder to lubricate piston-rings, at a rate which depends on engine load, by a number of injectors located at intervals in the liner. This lubricant also incorporates a basic additive to neutralize acidic products of combustion to limit corrosion of the cylinder wall and this partly explains why the supply rate is linked to engine load / fuel supply rate. Delivery rates typically fall between 0.9 gKWHr-1 and 1.2 gKWHr-1. Inconveniently from an environmental perspective, the arrangement is virtually a total loss system with almost all lubricating oil burnt in the combustion process. Using data supplied by Lloyds we have estimated that there are about 50,000 ships using engines of this type across the world and that annual consumption of lubricant by the global fleet approaches 250,000 tonnes and costs range between £150m and £220m annually. As a consequence, if it is possible to reduce the quantity of lubricant used for this purpose both economic and environmental benefits will ensue.
This presentation will examine the effects of varying the amount of oil supplied to the cylinder wall in a marine diesel engine and determine the parameters required to minimise oil supply whilst maintaining adequate lubricating film thickness.