# Advances in Diesel-LNG Internal Combustion Engines
## 2 Results
### 2F Diesel-LNG ICEs R&D
Furthermore, every homogeneous combustion produces larger heat losses to the wall and incomplete combustion for flame quenching. Hence, best it to inject the NG to burn premixed during the compression stroke at the center of the combustion chamber, to produce a central lean stratified mixture, surrounded by a cushion of air. Opposite to the diesel, the NG is a much simpler hydrocarbon, and even if LNG, it takes less to vaporize and mix with the air than the diesel. Injection of the NG to burn diffusion may only be direct. This occurs after the diesel-injection-ignition has produced suitable conditions within the chamber for the NG to quickly vaporize (if LNG), mix with air, and burn. The Westport HPDI 2F diesel-LNG injector does not offer the same performance for the diesel of a latest-generation diesel-only injector, in both flow rate, speed of actuation, and atomization. A dedicated LNG injector may also supply better performances than the LNG-diesel injector dual fuel. 2F diesel-hydrogen CI ICEs featuring this possibility were studied for example in [56; 57]). In these applications, two DI injectors were used per cylinder, one for the diesel and one for the hydrogen. A diesel ICE converted to a 2F diesel-hydrogen ICE following this approach could have delivered full load efficiencies up to 40-45% and reduced penalties in efficiencies reducing the load. The secrets of the turbocharged TDI CI Diesel ICEs performances are the high compression ratio and boost, the lean bulk combustion mostly diffusion-controlled, in addition to the better use of the exhaust energy. A latest production common-rail diesel ICE could be changed, replacing the diesel injector by a double injector as those proposed by Westport [45; 46] for CNG/LNG, or also by using two injectors, to supply same or better performances of the diesel, [57]. The use of two dedicated, rather than one 2F injector may have advantages and disadvantages. It is more complex and difficult to propose in conversions of existing ICEs. It may deliver much more flexibility in injection  
Figure 4: Performance curves of a Westport 15L HPDI diesel-LNG ICE and the Cummins L15 ISX 450 ICE. Data digitized from images previously published by www.westport.com. shaping. The 2F operation is typically characterized by pilot/pre-diesel injections, about the same of diesel-only operation, followed by the main LNG injection. The power density is increased by increasing the fuel-to-air ratio (i.e., reducing the \(\lambda\)) with LNG. The 2F diesel-LNG ICE may have slightly better-than-diesel \(\eta\) and higher-than-diesel power outputs, same as the diesel-hydrogen ICE of [56], as the NG vaporizes and mixes with air quicker than the diesel, providing about the same mass flow rate is kept unchanged. As discussed later, the LNG has not to be all injected after the diesel-injection-ignition. It may also be injected before or at the same time as the diesel. Thus, it may burn partially premixed and partially diffusion, and not the only diffusion. The LNG injected before the diesel increases the heat released during the premixed combustion, also reducing the amount of the diesel needed to produce suitable conditions for the LNG injected later to burn diffusion controlled. The CI 2F diesel-injection-ignition ICE can be run with different modes of combustion by simply modulating the amount of LNG injected before and after the diesel-injection-ignition. More or less \"controlled\" HCCI modes of combustion are also possible, with the diesel-injection-ignition occurring prior to the expected start of the HCCI auto-ignition, to supply stability to the otherwise unstable HCCI system [58]. It is, however, to be pointed out that HCCI has no advantage vs. the bulk combustion in the center of the combustion chamber surrounded by a cushion of air, as the homogeneous combustion always suffers from larger heat losses at the walls and incomplete combustion for quenching of the flame.