# Advances in Diesel-LNG Internal Combustion Engines
## 1 Introduction
This makes difficult the fast actuation of the injector, thus preventing multiple injection capabilities and shaping of the injection profile. There is also a problem with the storage on board. The volume of CNG needed for a given amount of energy is much larger than for diesel. The issue of CNG is partially addressed by the storage and the injection of the NG as a cryogenic fluid. LNG gives opportunities but also challenges. LNG has a much larger density than CNG, even if less than the diesel. However, it must be kept at very low temperatures of about 113 K.  
The two opportunities presently available, compressed NG or CNG, with the NG stored gas at (relatively) high pressure (HP), and liquefied NG or LNG, with the NG, stored liquid at extremely low (cryogenic) temperature, target different vehicles for different applications. The first is more suitable for passenger cars, and vehicles that spend the most part of the time parked. The second better fits the needs of long haul HDT, which are mostly traveling long distances. Current turbocharged (TC) DI diesel ICEs have achieved significant power densities and high \(\eta\). Brake mean effective pressures (BMEP) are in excess of 30 bar at about 4500 rpm maximum speed. Peak power \(\eta\) is in excess of 46-47%, and peak efficiency \(\eta\) is about 50%. Additionally, these ICEs deliver high \(\eta\) over the most part of the load range [26; 27; 28; 29]. Same (or even better) performances are possible with 2F operation diesel-LNG. Properties of diesel and LNG are given in Table 1 (data are from [30; 31; 32; 33]). While CNG is stored in the vehicle in HP tanks 200 to 250 bar, LNG is NG that is liquefied by cooling to 113.15 K at atmospheric pressure for an about 2.5 larger volumetric density. Injection pressure here considered, achieved through an LNG injection pump, is 300 bar. Being the LNG a real-world fluid, the actual formula is different from pure methane CH\({}_{4}\), as the percentage of ethane C\({}_{2}\)H\({}_{6}\) is everything but negligible. The formula C\({}_{13.5}\)H\({}_{23.6}\) proposed for the diesel is not representative of the actual composition, which is quite variable across the markets, the seasons, and the suppliers. Figure 1 presents a sketch of a conventional diesel (C\({}_{13}\)H\({}_{28}\)) and a conventional LNG (CH\({}_{4}\)) molecule. Figure 2 then presents the isothermal and isobaric density of methane at 114 K and 300 bars. When injected within the combustion chamber, the pressure reduces and the temperature increases. The temperature increases quickly, translating in the vaporization of the LNG. Replacing diesel by LNG can help reduce CO\({}_{2}\) and PM engine-out emissions from diesel ICEs, with the opportunity of also improving the NOx engine-out emissions, as the injection and heat release curves of an LNG ICE are not constrained by the PM-NOx engine-out emission trade-off of a diesel ICE. LNG-diesel 2F ICEs maintain the higher fuel conversion efficiency \(\eta\) of diesel ICEs while replacing a significant amount of diesel with LNG. The 2F diesel-LNG ICE may perform better than the diesel, being the LNG much easier to vaporize, and to mix with air, allowing relative fuel-to-air ratios \(\varphi=1/\lambda\) larger than the diesel. A diesel ICE can be easily converted to an LNG-diesel 2F ICE, which makes the technology suitable for both new ICE manufacturing and retrofitting existing diesel ICEs.