# Advances in Diesel-LNG Internal Combustion Engines
## 3 Discussion  
### Substitutional LNG  
As shown by [68] studying a 2F diesel-LPG 1.6-L 4-cylinder ICE derived from a passenger car ICE, the amount of diesel needed in these operating conditions may be larger. As the baseline ICE model of the present study was developed targeting the high speed and load operation as customary in racing ICE applications, the low-speed low load definition of the pre/post injections was not of particular interest. The reference experimental definition of the pre/pilot mass flow rate of the small diesel 1.6 L is proposed in Figure 11.  
Figure 11: Reference mass flow rate of diesel pre/pilot for a small 1.6-L diesel ICE for passenger cars. (**a**) fuel flow rate. (**b**) the ratio of pre/pilot to pre/pilot plus main flow rates. Especially in the low-speed operating points, where turbocharging does not produce enough boost, the amount of diesel needed for the pre/pilot injections may be large, also 50% of the total mass injected in the worse cases. With super turbocharging or electrically assisted turbocharging, the issue is less relevant. With more complex strategies of injection of the LNG, suitable conditions for the main injection of the LNG to progress diffusion-controlled may also be obtained by injecting a small amount of LNG prior or contemporary with the diesel-injection-ignition. This option may permit to drastically reduce the diesel needed. This is a major advantage vs. the Westport HPDI. Where convenient, the amount of the LNG injected before the diesel-injection-ignition may be increased to almost any value, thus producing a very fast combustion, close to an HCCI combustion, but with the advantage of reduced heat losses, as the LNG premixed with air may be concentrated at the center of the chamber where it is ignited by the diesel-injection-ignition, with a cushion of air surrounding this area and limiting the heat transfer to the walls. In addition to assisted turbocharging, air cooling, and injection shaping, exhaust gas recirculation (EGR) is a control option that is effective to reduce the NOx engine-out emissions [52; 69; 70]. The EGR reduces the NOx lowering the oxygen concentration in the combustion chamber, as well as absorbing heat. As the present work has only focused on performances and not emissions, EGR has not been considered. It must be added that the injection of liquid CH\({}_{4}\) after the diesel under certain conditions may quench the burning of a too-small quantity of diesel that consumes at relatively low pressure and temperature. Similarly, the injection of liquid CH\({}_{4}\) before the diesel if excessive for the pressure and temperature of air and residuals in the chamber can also make difficult the autoignition of the diesel. While this is unlikely to occur in the conditions here examined of a high boost high compression ratio ICE, these opportunities are certainly worth further studies preferably through experimental approaches. The proposed performances of the 2F diesel-LNG CI ICEs may be further improved. As shown in [65], a more aggressively designed ICE, with higher boost obtained by a larger mechanically assisted turbocharger, larger compression ratio, and fuel-to-air ratio slightly less lean of stoichiometry, may have larger BMEP and power density values (up to 30% larger), albeit without any significant improvement of efficiencies. The diesel ICE model was developed in 2013 and 2014 for FIA WEC applications. The injector was a 2850-bar diesel DI injector allowing fast actuation and high flow rates and atomization. The proposed 2F diesel-LNG needs of a proper LNG injector allowing about the same of the diesel actuation times and energy flow rates. The LNG has a drastically reduced density compared to the diesel, but a slightly larger lower heating value. To help the design of the requested DI LNG injector, the LNG vaporizes and mixes with air much quicker than the diesel.