Low-speed goes high-tech

Electronics is changing the dependence of low-speed engines on mechanical control.

By far the biggest changes to effect the low-speed engine is the addition of electronic ?intelligence?. This offers increased flexibility, reduced emissions and improved controllability. The first development of this was by MAN B&W for the shuttle tanker engine running on VOC. Development of the software has taken much longer than expected although it is finally approaching completion with class approval tests, covering hardware and software, started in mid-March. Commissioning onboard will take a few weeks and sea trials will follow. The latest situation will be outlined at The Motor Ship Propulsion Conference. Stena has selected MAN B&W?s intelligent engine technology for its Vmax newbuilding. However, as a result of the delays encountered, they will now be delivered as traditional engines with camshafts but prepared for conversion. Another user would be the gas powered low-speed engines for LNG/LPG propulsion, but as yet there are no orders for these.

As part of the safety system the VOC engines require on-line monitoring of the gas and pilot injection fuel and also of the combustion pressure. To do this MAN B&W has developed a system where a drilling, 6-8mm in diameter, is made from the outside of the cylinder cover to about 1cm short of the combustion chamber. A strain pin is inserted into the blind hole and its output calibrated against that of a pressure transducer inserted in the indicator cock. While on-line pressure measurement is a necessary part of the Intelligent Engine package it can also be specified as a stand-alone system for any engine in the low-speed range. The portable PMI system also collects data on combustion chamber pressures, but not on-line, and by using a transducer in the indicator cock. Having collected the data and analysed the results, the system can tell the operator what adjustments to make to redress any imbalance between the cylinders. MAN B&W has sold over 100 units at a cost of about £10,000 ($15,800) each.

Back at the hardware end MAN B&W?s first S90MC-C engine has undergone testing at Hyundai prior to being fitted into a VLCC for Greek owner Kristen Navigation. The six-cylinder engine has a stroke of 3,188mm and produces 4,890kW (6,650 bhp) per cylinder between 61 and 76 rev/min. It is fitted with the slide-type fuel valve, which has the sac volume reduced from around 3 per cent of the injected volume to almost zero. One owner who is trying the slide-type valves says, “It?s the best thing MAN B&W has introduced in the last 10 years. Wear is lower and the appearance of the piston is much improved with fewer particles on the ring lands, and less smoke too.” Fuel consumption is unaltered and the NOx emissions can be 10-25% lower. The slide-type fuel valve will become standard across the range.

Results from the inclusion of piston cleaning rings in two-stroke cylinders are positive, says MAN B&W. One owner who has been testing the rings for two years says there are no figures on wear rates, but they have not suffered the same number of scuffing incidents as in the past. The new computer controlled cylinder lubrication system is also set to become standard on the Danish-designed engines. One engine has been running the lubrication system on selected cylinders for 12,000 hours. Readings show the cylinders with electronically controlled lubrication of 0.9 g/kWh have the same wear rate as the traditionally lubricated ones using a feed rate of 1.0 g/kWh.

Just scratching the surface? Or honed to perfection?

Only in recent years have low-speed engine manufacturers specified honing of cylinder liners.

For many years four-stroke engine manufacturers have specified honing cylinder bores, but it is only recently that the low-speed designers have followed suit. With lubricant injected directly into the low-speed cylinder, honing has not been seen as necessary.

But now with BMEP of 19 bar and P-max reaching 200 bar, much higher piston ring pressure is required to prevent blow-by. This means the liner must have a good running surface from the moment the engine first starts. With a liner made of a hard material, running-in an un-honed liner would take thousands of hours off the piston ring life, and fill the cylinder drain oil with metal ?filings?. If the liner material were soft, the rings would bend the high peaks over and push them into the troughs, filling the areas where the oil collects. This would mean the first piston ring wipes the oil from the surface while the others run dry, and rapid wear will inevitably result.