One of the main differences between the “Stavangerfjord” and “Viking Grace” is their engines: gas-only and dual-fuel, respectively. Both engine types are four-stroke Otto Cycle engines, meeting the IGF Code’s requirements for the “inherently safe” engine room. Both engine types, dual-fuel and gas-only, differ in their energy consumption, ignition system and emissions.
All of the four-stroke engines available today are low-pressure engines. The mixture of fuel and air takes place outside of the cylinder behind the turbocharger. This means that the fuel gas pressure is approximately 5 to 6 bar. Nevertheless, the pressure is low and therefore the gas can be provided either directly from a pressurised storage tank or by use of a compressor: If a compressor is used, the specific energy consumption of the compressor is below 1% of the lower heating value of the gas (Hu), even if 10 bar pressure is required. If the gas has to be compressed to a high pressure of 300 bar, the compressor’s specific energy consumption will be much higher.
The self-ignition temperature of natural gas stored as LNG is too high to be reached by the compression cycle in the cylinder. Thus, the combustion must be initiated by an ignition source. Engines running only on gas use a spark plug to initiate the combustion process. The dual-fuel engines use “pilot fuel” to start the combustion process. A small amount of pilot fuel is injected into the cylinder, where it is ignited by the high temperature of the gas air mixture at the end of the compression cycle. Typically, the amount of pilot fuel oil is below 1% of the energy used by the engine.
Compared to heavy fuel oil (HFO), LNG greatly reduces emissions to air. Because LNG does not contain sulphur, these emissions are eliminated completely, complying with ECA regulations. In terms of NOx emissions, the low-pressure Otto Cycle engines reduce these by 85% and the high-pressure Diesel Cycle engines by 40%. Particle emissions are reduced by 95% and more.
DNV GL evaluated the greenhouse gas emissions from production to the tank of the ship (Well To Tank; WTT) and the emissions from the combustion of the fuel (Tank To Propeller; TTP) in two studies in 2012. A comparison of emissions from different fuels indicates that the WTT emissions for HFO, MGO and LNG are similar and small compared to the TTP emissions. The greatest reduction in greenhouse emissions is reached by the Diesel Cycle engines, which reduce the CO2 effect by 26% compared to HFO – thus, meeting EEDI requirements.