Low-speed engines – Concern over wear rates

Almost 80 per cent of the merchant ships built last year were propelled by two-stroke engines. Simplicity, both by design and direct drive, along with low running and maintenance costs have proved the overwhelming reasons for its selection. However, medium-speed engines are becoming far more fuel efficient, and some can match the low-speed on brake specific fuel consumption (BSFC). Shipowners are questioning why they are paying ever-increasing costs for cylinder lubrication on low-speed engines, yet still suffering higher levels of cylinder wear than with medium-speed units.

Figures from The Swedish Club show two-stroke engines have a significantly lower repair cost when it comes to major repairs (over $10,000). As these figures relate to major failures, many engines can go through their life without imposing such costs on their owners. When there is little to differentiate the engines in terms of fuel consumption, other day-to-day running costs have to be considered, and that includes lubrication.

Figures from Castrol indicate that cylinder oil consumption has doubled from the early 1980s as Pmax, BMEP and piston speeds/strokes have increased. MAN B&W currently recommend lubrication levels in the range of 0.9 – 1.6 g/kWh while Wartsila NSD prescribes 0.9 – 1.3g/kWh. However, some owners select to go even higher – for instance first RTA96C will spend its first 7,000 hrs with cylinder oil feed rates above 2g/kWh.