# Advances in Diesel-LNG Internal Combustion Engines
## 1 Introduction  
Natural gas (NG) internal combustion engines (ICEs), both compression ignition (CI), derived from diesel ICEs, and (spark) positive ignition (PI), derived from gasoline ICEs, are popular worldwide [1]. It is reported that there were (as of 31 July 2019) 27,765,376 NG vehicles in the world. While most of these vehicles are PI, the best fuel economies are provided by CI ICEs. The heavy-duty trucks (HDT) sector is projected to be almost entirely covered by CI ICEs within a few years [2]. HDT is the area where diesel-liquefied NG (LNG) ICEs may supply significant advantages in terms of economy, which is one of the major drivers of the change, and emissions, of both CO\({}_{2}\) and PM, that is another major driver, vs. the traditional diesel ICEs. Because of the cryogenic temperatures, LNG is more practical on large HDT continuously in movement than small passenger cars mostly parked. Diesel-LNG engines are becoming popular for ships and heavy-duty truck propulsion [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. A key advantage of LNG over Compressed NG (CNG) as a transport fuel is its energy density [23]. LNG is the preferred solution for HDT used in regional distribution as well as long haul transport [24; 25]. CNG is the preferred solution for limited city distribution trucks, garbage collection trucks, and buses. CNG is the only possibility being considered for passenger cars. Due to their higher reliability and superior fuel conversion efficiency, \(\eta\) hereafter, CI diesel ICEs are the most common. However, diesel ICEs generate carbon dioxide (CO\({}_{2}\)) plus engine-out emissions of particulate matter (PM) and nitrogen oxides (NOx). The PM engine-out emission is the result of the diffusion combustion of the heavy hydrocarbon liquid diesel fuel [23]. Direct injection (DI) of liquid fuels often translates into PM production during combustion. The NOx emission is intrinsic tothe lean-burn operation, with combustion occurring with excess air [23]. The reduction of both PM and NOx is achieved by after-treatment. As the after-treatment is imperfect, the diesel combustion strategies are dictated by the best trade-off between PM and NOx emissions. The use of a fuel with reduced carbon content, that is also gas in normal conditions, such as the NG, has major advantages in both CO\({}_{2}\) and engine-out PM emissions. Even if injected liquid, the NG rapidly vaporizes and quickly mixes with the air. The PM emissions are usually negligible. Not having the constraint of the best PM-NOx trade-off to design the injection profile, the injection profile can be shaped to achieve reduced NOx production. Thus, NG indirectly also permits to reduce the NOx engine-out emissions. NG (CH\({}_{4}\)) has a smaller carbon-to-hydrogen ratio vs. the diesel (C\({}_{13.5}\)H\({}_{23.6}\)) also featuring a higher lower heating value. Less CO\({}_{2}\) is emitted to produce the same output of the diesel with the same \(\eta\). As NG is also gas in normal conditions, this practically reduces to zero the PM emissions. In the dual-fuel (2F) operation, the only PM originates from the pilot diesel. As NG is a low cetane but a high octane number fuel, it is difficult to use alone in a CI ICE The issue is solved by using the 2F diesel-NG design with the diesel-injection-ignition [24; 25]. A small amount of the DI diesel fuel ignites mixed with air within the cylinder. The NG that is injected prior, or after the diesel, then burns premixed, i.e. diffusion. The first phase of combustion translates into a rapid pressure build-up. The rate of combustion of the second phase is determined by the injection rate of the NG. One issue with fuels that are gases under normal conditions is the specific volume, as the density of the gas under normal conditions is much lower than that of liquid diesel or gasoline fuels. This creates problems for the injection system. A fuel such as compressed natural gas (CNG) needs injectors of much larger cross-sectional areas than diesel fuel.