Increasingly, the name of the diesel game is fuel-flexibility. Dual-fuel diesel engines that can burn gas are a key to success in markets where fuel supply is uncertain.
The key developments in diesel and gas engines have been dual-fuel capability and lean-burn capacity. Wärtsilä has developed its 32DF, which is just such a machine, with two plants having recently been installed in Turkey. The engine can run on both gaseous and liquid fuels. In gas mode, the engine works according to the Otto cycle, where a lean air-fuel mixture is fed to the cylinders during the suction stroke. When running on liquid fuel, the engine operates on the traditional diesel cycle.
In a lean-burn gas engine, the mixture of air and fuel is lean, ie, there is more air present in the cylinder than is necessary for combustion. At leaner combustion, less NOx is produced, and the efficiency can be increased by using a higher compression ratio and a higher Break Mean Effective Pressure (BMEP). A lean mixture also reduces knocking or self-ignition. A powerful ignition source is needed to stabilise the ignition and combustion of the lean mixture and to avoid misfiring.
Lean-burn combustion is a sensitive process, and variations in ambient conditions, gas quality, wear of engine components etc may cause an individual cylinder to run either too lean (tending to cause misfires) or too rich (tending to result in self-ignition). It is thus of the utmost importance to have an intelligent control system.
A key issue is the control of the combustion process in each cylinder. It is important to stay within the operating window and to ensure optimal performance for all cylinders in all conditions. Cylinders operating beyond the knock-limit will cause damage to cylinder components.
Wärtsilä 32DF engine design
The engine is optimized to run on gaseous fuels, with diesel as a back-up. In gas mode, it can transfer instantly to back-up fuel operation at any load. The diesel fuel injection valve, in the centre of the cylinder head, is used for both pilot fuel and normal diesel fuel injection.
Gas admission system
Gas is supplied through common pipes running alongside the engine, continuing with individual feed pipes to each cylinder. The gas is mixed with the intake air immediately upstream of the inlet valve in the cylinder head. Since the timing of the gas valve is done independently of the inlet valve, scavenging of the cylinder is possible without the risk of unburned gas being able to escape directly from the inlet to the exhaust system.
The gas admission valves located on the cylinder heads function as the engine speed regulator and the valves control the amount of gas fed to each cylinder. The gas valves are solenoid operated. The engine control system adjusts the amount of gas fed to each cylinder individually when the engine is running.
Pilot fuel system
The pilot fuel system consists of a pump unit, common rail, feed pipes and injection valves. The pump unit elevates the pilot diesel pressure to the required level. The pump is located on the engine’s common base frame. The pump unit is controlled by the engine control system. It receives stop, start and pressure signals from the control system, and also automatically runs start and stop pressure ramping.
The high pressure pilot fuel is delivered to a common rail and further to the feed pipes for each injection valve. The injection valve is a two-needle design, one for full load diesel operation, and one for pilot fuel in gas mode operation. The pilot fuel needle is solenoid-operated, allowing exact timing and duration for each cylinder. It is also adjusted individually during operation for optimal performance.
Cylinder control system
Parameters such as load, speed, combustion performance and exhaust gas temperatures are monitored and used as inputs to the control system. Each cylinder is equipped with a combustion sensor. The amount of gas and timing, and amount of pilot fuel and timing, are individually controlled. As a result of this, each cylinder is at the correct operating conditions in order to avoid knocking and misfiring. This ensures reliable and safe operation, as well as high efficiency and low emissions.
Air-fuel control system
To maintain optimal performance of the engine, it is essential to have the correct air-fuel ratio in all situations. The lean-burn engine uses an exhaust gas wastegate valve and a charge air throttle valve to adjust the air-fuel ratio. The exhaust gas wastegate valve bypasses part of the exhaust gases past the turbocharger. Working as a regulator, it adjusts the air-fuel ratio to the correct value, regardless of varying site conditions. The wastegate valve is located at the turbocharger turbine side.
The charge air throttle valve is used in low load where the low exhaust gas pressure reduces the effect of the wastegate valve and is controlled by the engine speed and load. The charge air throttle valve is located after the charge air cooler.
Engine control and monitoring system
Engine speed and load are controlled by the Wärtsilä Engine Control System (WECS). This collects signals from various engine sensors at different locations on the engine, and processes and compares them with the control parameters given for the combustion process. The gas feed pressure, duration and timing, the pilot fuel duration and timing, and the air fuel ratio are immediately adjusted to meet load demands or other conditions.
WECS also automatically controls the start and stop sequences of the engine and the safety system. If any input signal shows an abnormal value, the monitoring system first gives an alarm. If the signal continues to deteriorate, a load reduction or a transfer to diesel operation is made automatically if running in gas mode. If the situation continues to alarm, engine shutdown will follow. Parameters handled by WECS are transferred to the operator interface and presented in graphs for information and preventative maintenance use.
WECS is a physically distributed system that consists of the following modules: main control module, cylinder control module, monitoring module and sub-system module. The main control module is located in the WECS cabinet at the flywheel end of the engine, and the rest at various locations close to the engine sensors or controllers they are reading or controlling.
The dual fuel concept has been demonstrated on a laboratory engine. In addition, four engines are in operation in Turkey.
The Turkish corporation Camis Elektrik Otoproduktör signed two contracts with Wärtsilä NSD for engineering, procurement and construction of two power plants. Camis Elektrik Otoproduktör is a branch company of the Turkish Sisecam industrial group, well-established in the glass industry.
The plants are located at two of Sisecam’s industrial facilities in Istanbul. One is in the Topkapi district, on the European side of the Bosphorus, and close to the centre of Istanbul city, and the other is on the Asian side in the Cayirova industrial area.
Camis Elektrik Otoproduktör produces and distributes power primarily for use by the factories in the Sisecam group.
The two contracted plants required two 18-cylinder Wärtsilä 32DF engines per plant. Each plant has a capacity of 11.6 MWe when operated on gas, and 12.2 MWe in diesel mode.
Fuel flexibility of the prime movers was an essential need, and one of the items strongly requested by the client. In Topkapi, a gas supply was in principle available all year around, with diesel operation planned mainly for use as a back-up in case of gas supply disturbances. In Cayirova, gas supply was only assured between April and November. As a result of this, the plant will be run in diesel mode for the rest of the year.
Fuel flexibility has already shown itself to be valuable at Cayirova. In November 1998, the supplier suffered distribution difficulties, and shut off the gas supply to the plant, with just a few hours notice.
The four engines are the 32DF engines of the dual-fuel type to be delivered by Wärtsilä NSD. The engines operate on gas with a supply pressure of 3.8-5.0 bar. In gas operation, the engines have an output of 5777 kWe, and on LFO, 6100 kWe. The efficiencies are 41.6 per cent in gas mode and 40.5 per cent in diesel mode.
The plants are specified for baseload operation in parallel with the grid, but can also operate in island mode when disturbances occur on the grid.