Engines that are capable of running on LNG, LBG or e-methane, can on a high level be divided into two categories. One comprises gas injection into the intake port at a low pressure. The other has a high-pressure gas injection towards the end of the compression stroke or just before the top dead centre. In both categories, a small amount of easily
ignitable liquid or pilot fuel ignites the air-gas mixture. In the Figure 1, the conceptual difference between the two is illustrated.
Figure 1. Visualization of the Otto cycle (gas mode) and diesel cycle (Bui 2011) Whilst high pressure gas engines are typically straight forward in terms of gas quality variations and related controls, low pressure gas engines require a more sophisticated combustion control system, enabling numerous advantages:
- No fuel gas compressor is needed, reducing capital costs and parasitic load
- Higher engine efficiency
- NOx emissions compliant with IMO Tier III level without exhaust gas aftertreatment
Low pressure gas engines do have an unwanted feature called methane slip, i.e. a minor portion of the fuel gas passing unburned through the cylinder to the exhaust. This has been a focus area in Wärtsilä R&D activities during the past 25 years with substantial advancements, and further improvements are on the way including both in-engine development and catalyst solutions. With the latest low pressure gas engine technology, the methane slip is typically below 2 g/kWh, corresponding to 10% of the GHG emissions from a gas engine running on natural gas.
Wärtsilä offers both spark ignited (SG) and dual fuel (DF) low pressure gas engines. The controls in this report are relevant for both on a conceptual level, whilst both deeper analysis, development and upcoming testing activities within this task will be focused on DF engines. Thanks to the fuel redundancy and the robust ignition source, DF is namely dominant in the marine market. Wärtsilä’s DF engine portfolio for the marine market ranges from the 1 MW 6L20DF up to 20 MW with the 16-cylinder 46TS-DF. This development focuses on the new portfolio engines Wärtsilä 31DF and Wärtsilä W46TS-DF, both being suitable candidates both for cruise and bulker depending on the power demand. To make it possible to reach higher engine efficiency without engine knocking, larger low pressure gas engines are typically designed for so called lean burn combustion, meaning that the air intake is higher than what´s required for stoichiometric combustion of the injected fuel. Oftentimes the lambda can be around two, meaning that there is double amount of air compared to what is needed for theoretical complete combustion.
