Two injectors, one is the main fuel injector used for diesel mode and another is the micro-pilot fuel injector used for gas mode, are equipped in the cylinder head, the engine can switch between diesel mode and gas mode freely with certain operating condition restrictions.
The engine is optimised for running on gaseous fuels and diesel fuel is used for back-up fuel operation. Instead, the gas fuel is injected into the compressed air in the same way as diesel fuel, and then ignition is achieved by injecting fuel via the micro-pilot fuel injector see Fig.
Knocking can be detected by using a vibration sensor to find changes in combustion vibration or detected directly from the internal cylinder pressure waveform. However, if the thermal load and combustion pressure in the cylinder increase for some reason, such as an increase in load, the unburned fuel mixture may ignite spontaneously prior to being reached by the propagating flame.
If spontaneous ignition sets off a chain reaction, this can result in serious damage to the engine due to severe pressure or temperature increases. That is, there is a need secure provision of backup for the case of gas-fueled operation being lost due to a problem in the fuel supply system or engine.
In addition to the main intake passage, it equips a bypass that returns some of the intake air to the turbocharger inlet.
This is why the knocking margin would be reduced in natural gas fuel mode due to the build-up of alkalis deposit and other combustion deposit on the cylinder walls that act as a heat insulator causing the combustion temperature to increase.
Fuel characteristics are a problem specific to dual-fuel marine engines. Many different types of dual-fuel engines have been developed since then, many of which have been either diesel-ignited gas engines that use liquid fuel to ignite the gas fuel, or mixed combustion engines in which LNG tanker boil-off gas or the byproduct natural gas from oil extraction is used together with another fuel.
Knocking occurs when output is high in the region where the excess air ratio is around one, whereas the region in which misfiring occurs is at higher excess air ratios.
This article describes the history, design, and technical challenges of dual-fuel engines designed for use in ships to improve their environmental performance.