# Advances in Diesel-LNG Internal Combustion Engines
## 3 Discussion
### Substitutional LNG
The density penalty is, however, a major issue, as the larger cross-sectional areas needed to accommodate a much larger volumetric flow rate with the LNG translate in a larger and heavier design making more problematic fast actuation. The LNG injector supposed to be used in this paper is not available off-the-shelf, but it may be certainly produced with today's technological capabilities. Starting from the experience of Prof. [PERSON] in the 1980s and early 1990s, Westport has built a considerable strength in converting HDI to LNG and diesel, by using their HPDI injectors. Their ICEs have been tested on dynos by them, and by the many Original Equipment Manufacturers (OEM) who purchased from them the HPDI conversion. There are many thousands of HDT trucks adopting this technology that has been circulating for many years on the road, with good performances confirmed by the truck drivers, the hardest reviewers to convince about ICE development. The Westport HPDI conversion is very well known to produce the same of original diesel steady and transient performances, plus the benefits of using LNG for the economy, CO\({}_{2}\) and PM, by using a pilot diesel and the main LNG injection. The Westport HPDI injector is a single injector for two fuels that is very large and heavy, and it is slow to actuate. Modern diesel injectors permit better injection strategies for more opportunities to shape the heat release rate curve at any speed and load. The injection strategy for the diesel and the LNG can be improved through the use of two dedicated DI injectors, one for the diesel, and one for the LNG. The hard fact that LNG plus diesel 2F works have not to be proven once more in any paper, and thus the proposed results are by no mean not trustworthy as they lack a specific validation. Westport and their customers, either OEM and truck drivers, did already. It is 30 years that the Westport HPDI is a successful product. The novelty of the paper is the use of two dedicated injectors, rather than one single injector for the diesel and the LNG, which is the main feature of the Westport HPDI, moving from a validated diesel-only ICE model. Also, Westport is moving towards mixed modes of combustion, with premixed and diffusion combustion of the LNG, that is injected prior or after the diesel-injection-ignition, as it is also shown in their latest SAE papers. However, they are still constrained by the use of a single fuel injector for both fuels. The proposed two injectors, specifically developed, permit a much better shaping of the heat release rate curve, as explained in the manuscript, thanks to the higher flow rates, the better atomization, and the faster actuation. These improvements are logical consequences of the specific design for one fuel only, and the higher pressures and smaller nozzle areas. The results proposed are quite logical, perfectly aligned with what has been shown by the Westport experience. Thus, the proposed results are backed by the Westport experience, good engineering logic, and simulations with computer codes that have widespread use within the OEM, where changes such as the fuel are easily accomplished with generally satisfactory results. Regarding the injection pressure of the LNG injector, this pressure must be much larger than the 600-bar proposed by Westport to make the injector small and fast-actuating, but with large flow rates and enhanced atomization. As the injection velocity is proportional to the product of the square root of the difference between the injection pressure and the in-cylinder pressure, by the nozzle holes' area, the values being considered are presently about 1600 bar.