[ ENERGY / IN DETAIL ] ] Introducing the world’s largest gas engine AUTHOR: Mikael Wideskog, General Manager, Gas Plants, Power Plant Tecnology, Wärtsilä Power Plants ENERGY / IN DETAIL DETAIL IN / ENERGY [ 14 in detail WÄRTSILÄ TECHNICAL JOURNAL 01.2011 Wärtsilä has introduced the largest gas engine on the market. Based on the well-proven technology of the Wärtsilä 34SG and 50DF engines, the Wärtsilä 18V50SG has an output of 18 MW and offers an alternative to gas turbines for large power plants. Power plants based on multiple engines gas engine once again makes combustion result in efficiency improvements have many advantages. For example, engines an attractive alternative to of around 0.5 percent per year. since operators need run only as many gas turbines for large power plants. The Wärtsilä 18V50SG is based on engines as are required, they offer flexible the same design principles as the well- power output with high plant efficiency. Design and development proven technology used in the Wärtsilä Operators can also carry out maintenance The Wärtsilä 50SG is a four-stroke, spark- 34SG and Wärtsilä 50DF engines. without shutting down the entire plant. ignited gas engine that works according As the engine was based on existing But as plant sizes increase increase, there to the Otto principle and the lean- technology, its development was very is a need for units with higher output. burn process. The engine runs at 500 or quickly implemented. Development The most engines that Wärtsilä has installed 514 rpm for 50 or 60 Hz applications, started in 2008, and the first 6-cylinder to-date in a single plant, is the 28 Wärtsilä and produces 18,810 and 19,260 kW laboratory engine was built in 2009. 34SG units for a 270 MW power plant of mechanical power respectively. Assembly of the first customer engine in Turkey. This seems to be the point This represents maximum electrical began the following year and its at which operators tend to opt for gas power outputs of 18.32 MW and testing was completed at the Trieste turbines due to their larger unit size. 18.76 MW respectively, at which the facility by the end of 2010. With the introduction of its latest gas Wärtsilä 50SG has an efficiency of 48.6 The engine frame is based on the engine Wärtsilä can now offer a product percent at the generator terminals – Wärtsilä 18V50DF dual-fuel engine, directly competing with the gas turbine 2.3 percentage points higher than the with the same advanced integrated technology. By doubling the output of the smaller Wärtsilä 34SG. This is a big step, lube oil and cooling water channels. Wärtsilä 34SG, the new Wärtsilä 50SG since typically product improvements The combustion system is based on Table 1 – The output of the Wärtsilä 50SG engine. Table 2 – Dimensions and weights of the Wärtsilä 50SG generating set. 50 Hz/500 rpm 18V50SG Lenght mm 18 800 Power, electrical kW 18 321 Heat rate kJ/kWh 7411 Width mm 5330 Electrical efficiency % 48.6 Height mm 6340 60 Hz/514 rpm 18V50SG Power, electrical kW 18 759 Weight tonne 360 Heat rate kJ/kWh 7411 Electrical efficiency % 48.6 in detail 15 [ ENERGY / IN DETAIL ] ] the Wärtsilä 34SG, but has a larger a new technology for Wärtsilä, and bore size to maximize the power thus required more design and research potential of the engine block. work. Furthermore, its performance Increasing the bore size from 340 mm and possible consequential benefits to 500 mm on a spark-ignited engine was were also unknown for this bore size. a key area of technology development. Both concepts were tested and found In a modern gas engine, lean-burn to perform equally well. Based on technology is a necessity in achieving proven reliability and lower cost, it was low emission levels without external decided to opt for the spark ignition ENERGY / IN DETAIL DETAIL IN / ENERGY exhaust gas after-treatment. In lean-burn with pre-chamber technology. [ technology, the charge in the cylinder has far more air than is actually needed Pre-chamber for complete combustion of the gas. The pre-chamber is the ignition Ignition of the extremely lean fuel source for the main fuel charge, and charge is very difficult, and the right is an essential component of a lean- ignition technology is needed to provide burn spark-ignited gas engine. a high-energy ignition source. It should be as small as possible to The choice of ignition technology deliver low NOX values, but big enough most suited for a pure gas engine with for rapid and reliable combustion. a large bore was discussed extensively Extensive calculations and simulations at the beginning of the project. had to be performed to scale-up the Spark ignition with a pre-chamber, size and shape of the combustion also known as SG technology, is used pre-chamber in order to ensure on existing pure gas engines. However, the best combustion process. the consequences on performance In addition to the size and shape when scaling the technology to a of the pre-chambers, some of the key bore of 500 mm were unknown. design parameters considered were: the An alternative option of using a micro mixing of air and fuel; gas velocities pilot liquid fuel ignition through a pre- and turbulence at the spark plug; chamber was also proposed. This was cooling of pre-chamber and spark 16 in detail WÄRTSILÄ TECHNICAL JOURNAL 01.2011 plug; and the selection of material. Engine frame Advanced three-dimensional, The engine frame is based on the computerised fluid dynamics were proven design of the Wärtsilä 50DF, used during design to deliver: and the block is made from cast O Reliable and powerful ignition iron. The engine has an under slung O High combustion efficiency crankshaft, which imparts high stiffness and stability to the engine block and provides O Extended spark plug life excellent conditions for maintenance. O Very low NOX levels. The engine block has large crankcase doors to enable easy maintenance. The engines use a ported gas admission system, whereby gas is admitted Cooling system to the pre-chamber through a The Wärtsilä 50SG is designed with a mechanical, hydraulic-driven valve. Wärtsilä open interface cooling system The gas admission valves are for optimal cooling and heat recovery. located immediately upstream, The system has four cooling circuits: the and are electronically actuated cylinder cooling circuit (jacket), the low and controlled to feed the correct temperature charge air (LTCA) and high amount of gas to each cylinder. temperature (HTCA) cooling circuits, Since the gas valve is timed and the circuit for the lube oil cooler independently of the inlet valve, the (LO) built onto the auxiliary module. cylinder can be scavenged without The LTCA cooling circuit and jacket risk of the gas escaping from the cooling circuit have water pumps inlet directly to the exhaust. integrated within the cover module at Various parameters, such as engine the free end of the engine coolers, and load, speed, and cylinder exhaust the temperature of the water exiting temperatures, are monitored and used the jacket cooling circuit is controlled as input to the Engine Control System by external thermostatic valves. (ECS). The ECS is Wärtsilä’s latest The default cooling system is a single- UNIC (Unified Controls) C3 system, circuit radiator unit whereby the cooling which controls the entire engine. circuits on the engine are connected in This solution has proved to be series. For heat recovery applications, extremely reliable and it results in an each cooler can be individually connected excellent mixture in the pre-chamber. to an external cooling system. Ignition system Lubricating oil system The Wärtsilä 50SG ignition system has The engine has an engine-driven been specifically designed for the new lubricating oil pump and is provided with engine and is closely integrated with the a wet sump oil system. Before entering the ECS. The ignition module communicates engine, the oil passes through a full-flow with the main control module, which then automatic back flushing filter. A duplex determines the global ignition timing. cartridge filter is installed in the back The ignition module controls the flushing line, and both filters are equipped cylinder-specific ignition timing based with differential pressure switches. on the combustion quality. The cylinder- A separate pre-lubricating system is used specific control ensures optimum before the engine is started to avoid combustion in every cylinder with engine wear. respect to reliability and efficiency. The ignition coil is located in the Pistons cylinder cover and is integrated into the The pistons are of the low-friction, spark plug extension. The coil-on-plug composite type, with a forged steel top design minimizes the number of joints and nodular cast iron skirt. Their long between the spark plug and the ignition life is ensured through the use of a skirt- coil, and thus increases reliability. lubrication system, a piston crown with The spark plug has been specially shaker-cooling, hardened piston ring developed for long life, and to withstand grooves, and low-friction piston rings. the high cylinder pressure and temperature resulting from the high engine output. in detail 17 [ ENERGY / IN DETAIL ] ] Piston ring set corrosion-resistant precision bearing. The two compression rings and the oil control ring are located in the piston Crankshaft and bearings crown. This three-ring concept has The crank gear has to be able to operate proven its efficiency in all Wärtsilä reliably at high cylinder pressures. The engines. Most of the frictional loss from crankshaft must be robust and the a combustion engine originates from specific bearing loads maintained at the piston rings.