Variation in LNG composition and dual-fuel engines

In the course of their experiments, CO2 was introduced into the base gas to increase the methane number (MN). Subsequently, CO2 was reduced and propane added to significantly lower the MN. This requires precise control of the fuel gas flow to maintain a consistent mean indicated pressure (pmi) due to changing heating values in the series.

This was achieved by adjusting the gas valve’s injection duration. Additionally, the centre of combustion (CoC) was maintained at constant values during the measurements by adapting the start of energisation of the pilot fuel injector. This helps increase the ignition delay and, consequently, keeps the CoC stable despite variations in fuel gas composition, which affect combustion behaviour.

The findings revealed that as methane numbers fall to 70 and below, there was a significant increase in maximum in-cylinder pressure and knocking. Interestingly, as knock levels rise, the coefficient of variation of pmi decreased to values below 1. Despite these variations, the duration of combustion remains stable, resulting in only minor reductions in exhaust temperatures. To assess the environmental impact, the greenhouse gas emissions (GHG) were corrected by considering the added CO2 within the fuel gas. A CO2-factor of 3.3 was assigned to propane and 28 to methane. As the measurement series progressed, methane emissions decrease, although the propane content did not rise to the same extent. Consequently, GHG emissions declined.

Given the diverse qualities of LNG available, dual-fuel engines for marine applications must adapt to changing gas compositions. Methane numbers can vary from approximately 65 to 100. The control strategy presented at CIMAC would allow ship operators to use the same dual-fuel engine worldwide, irrespective of the next bunker source, without the need for hardware modifications. Software updates to the engine control unit, primarily focused on the pilot injector, are the only requirement, making this approach cost-effective and feasible in a short timeframe.

These studies have employed a piezo-electric sensor for real-world applications, demonstrating that knowledge of the precise methane number is not essential. A self-tuning control loop could constantly monitor and detect changes in gas quality. By combining various thermodynamic parameters, the direction and speed of changes, particularly in relation to methane numbers, can be inferred. As the maritime energy landscape evolves with a broader range of fuel gases, this control approach becomes increasingly relevant.

In summary, the experimenters approach, which leverages an ECU-controlled combustion adjustment via the pilot injector, offers a promising solution for the global operation of dual-fuel engines, ensuring adaptability to varying gas qualities without the need for extensive hardware modifications. This approach holds the key to a sustainable and efficient future for LNG-fuelled marine engines.

* Manuel Glauner, University of Rostock

Björn Henke, University of Rostock

Sebastian Cepelak, University of Rostock

Jules Dinwoodie, University of Rostock

Bert Buchholz, University of Rostock

Martin Theile, FVTR GmbH