# Advances in Diesel-LNG Internal Combustion Engines
## 1 Introduction
\begin{table}
\begin{tabular}{c c c} \hline \hline  & _Diesel_ & _LNG_ \\ \hline _Chemical Formula_ & C\({}_{13.5}\)H\({}_{23.6}\) & CH\({}_{4}\) \\ _Carbon Atoms per Molecule_ & 13.5 & 1 \\ _Hydrogen Atoms per Molecule_ & 23.6 & 4 \\ _Molecular Weight_ & 186 & 16 \\ _Physical State at 15 \({}^{\circ}\)C and 1 bar_ & Liquid & Gas \\ _Injection pressure bar_ & 2850 & 300 \\ _Injection temperature K_ & 298 & 113 \\ _Density kg/nr\({}^{3}\)_ & 837 & 443.5 \\ _LHV, MJ/kg_ & 43.25 & 50.00 \\ _HHV, MJ/kg_ & 45.60 & 55.00 \\ _Latent Heat of Vaporization, MJ/kg_ & 0.25 & 0.50 \\ _Critical Temperature, K_ & 569.4 & 190.4 \\ _Critical Pressure, bar_ & 24.6 & 46 \\ _Physical State_ & Liquid & Cryogenic Liquid \\ _Cetane Number_ & 40–55 & N/A \\ _Octane Number_ & N/A & 120+ \\ _Flash Point K_ & 347 & 425 \\ _Auto-ignition Temperature K_ & 589 & 813 \\ _Lower Flammability limit, volume \%_ & 1 & 5.3 \\ _Higher Flammability limit, volume \%_ & 6 & 15 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Reference properties of LNG and diesel fuel. Differences exist between sources, also depending on the actual composition of the real fluid. Figure 1: Diesel and LNG molecules. Some known issues of the 2F diesel-LNG ICEs developed so far are the limited replacement ratio, efficiency deterioration, and the incomplete combustion of the LNG at low load conditions [34, 35]. These issues may be addressed by assisted turbocharging, where the speed of the turbocharger can be altered from the balanced operation of turbine and compressor, by fitting an electric motor-generator to the turbocharger shaft (the FIA F1 style MGU-H, [26, 36]) or connecting the turbocharger shaft to the crankshaft via gear and a variable speed transmission, that is an evolution of the Torotrak V-Charge supercharger, [37, 38, 39], for super turbocharging, [26, 27, 40], and obviously with a better injection system. The assisted turbocharger control, coupled to the air-cooling control, allows to produce the most suitable values of pressure and temperature at intake valve closure (IVC) for every speed and load of the steady-state map, and additionally address the transient issues, for both acceleration and deceleration. Further improvements of the injection system are however fundamental. The success of the latest diesel ICEs in terms of power density and \(\eta\) stems from the opportunity to inject the diesel at extremely elevated pressures (2850 bar already in 2014) with high flow rate, high atomization, and using multiple injection events, precisely defined in time thanks to the fast actuation, often piezo rather than solenoid. The shaping of the injection translates in the shaping of the heat release curve, and thus the construction of a pressure curve where the rate of pressure build-up, the peak pressure,  
Figure 2: (a) Isobaric density of methane at 300 bars of pressure. (**b**) The isothermal density of methane at 114 K of temperature. Data from [30]. and the crank-angle location of the peak pressure, can be finely tuned. The shape of the temperature curve is also relevant, for example for NOx production within the cylinder. Similar capabilities are impossible with present CNG or LNG injectors that do not have the same speeds of actuation and mass flow rates of the latest diesel injectors. The purpose of this paper is to focus, after a review of recent diesel and diesel-LNG achievements, and the latest diesel and diesel-LNG developments, on the improvements needed in CI LNG-diesel 2F ICEs using pre and post injections of LNG coupled to the diesel pre/pilot injections. The manuscript covers fundamental injection and combustion strategies, with advanced control of turbocharging and air cooling, as well as injection shaping for both fuels, diesel, and LNG, to achieve better-than-diesel-only \(\eta\) and power density within constraints of pressure build-up rate, peak pressure, and peak temperature. The work will propose first a review of the literature and the market data of diesel-LNG ICEs.