MTU’s 16-cylinder gas engine is unique in its power range (Image: MTU)
When Dutch ferry operator Doeksen sent the first of two new ferries, Willem Barentsz to sea on 1 July, it also marked a historic moment for its main engines.
As The Motorship reported at the time, the ferry is powered by the first pair of Rolls-Royce MTU’s pure gas 16-cylinder Series 4000 engines, each delivering 1,492kW. But in an exclusive interview with The Motorship a few days earlier, Phil Kordic – MTU’s sales manager for the ferry and offshore sectors – said that the event marked more than just the first delivery of a new engine model because the engine’s concept itself is unique.
It is the first high-speed gas engine to offer variable speed in its power segment, he said. All similarly-rated engines are either dual-fuel – and predominately medium speed – or constant-speed genset engines.
Yet MTU’s new engine “has similar performance characteristics to an equivalent high speed diesel engine,” Mr Kordic said. Its transient capabilities give it very good load acceptance and acceleration behaviours, making it suitable for applications that would normally use diesel engines, and it meets IMO’s Tier III NOx emissions standard without requiring any exhaust after-treatment. In addition, because of its variable speed, it can drive fixed-pitch propellers.
This represents a significant change from the ship’s original concept, which included another maker’s constant speed engine and controllable-pitch propellers. But this would not have had the necessary load acceptance characteristics, Mr Kordic said, and changing to MTU’s pure gas option allowed the propulsion arrangement to be reconfigured to Doeksen’s advantage.
An important aspect of the engine’s development was focused on minimising its methane slip. With any LNG-fuelled engine there will inevitably be some methane slip, but the engine’s project manager, Peter Kunz, said they had managed to meet the EU’s Non-Road Mobile Machinery Stage V standards on methane slip.
To achieve this, the combustion process has been optimised and a complex engine control system oversees its operation. This responds to the fuel’s methane number and to the combustion pressure in each cylinder in real time. This, combined with multi-point fuel injection and the engine’s turbocharging arrangement, “is what really gives the engine its outstanding transient behaviour and allows us to maintain stable power output,” Mr Kordic added.
But there is a trade-off between methane slip and acceleration behaviour. To make the engine more dynamic requires a richer mixture of gas and combustion air, but this increases methane slip, “so we had to find out an optimum between these two goals,” Mr Kunz explained.
He believes that the engine’s designers arrived at a balance between those two that secured the minimum methane slip possible from the engine but, as explained in the box below, MTU’s developers are already exploring ways of reducing it further.
Because it uses only gas, it is also a lean-burn engine, which gives it lower firing pressures than dual-fuel engines and contributes to a longer time between overhauls and thus lower maintenance costs.
It is also gas-safe, with features such as double-walled piping with no additional ventilation required in the machinery space. This has an advantage for shipyards, Mr Kordic said, because they can install the engine in a similar way to how they would fit a diesel engine.
As the first 16-cylinder versions of the engine enter service, their eight-cylinder stablemates are not far behind. Two of those 746kW engines have been delivered for installation on a ferry that will be operated by Bodensee-Reederei on Lake Constance, which borders Germany, Austria and Switzerland and is a popular holiday area. That ship is due to be in service by the end of this year.
Once both variants are in service, “the engines will be released [for] full mainstream production,” Mr Kordic said, marking the end of a four-year development journey since MTU unveiled its mobile gas engines for marine propulsion in September 2016. Pre-production 16-cylinder engines were delivered to Vietnam’s Strategic Marine at the end of 2017 where they were installed in Doeksen’s two catamaran ferries. The second, Willem de Vlamingh, is due to enter service shortly and both will operate in the Wadden Sea between Harlingen and the islands of Vlieland and Terschelling, each carrying 600 passengers.
These engines are also significant for MTU’s parent company Rolls-Royce, which sees them as part of its ‘Green and High-Tech’ programme, through which it is investing in technologies aimed at reducing emissions, energy consumption and raw materials use.