# Advances in Diesel-LNG Internal Combustion Engines
## 2 Results  
### 2F Diesel-LNG ICEs R&D  
The principles of multijet diesel ICE operation are well-known [41]. In the past, diesel ICEs were designed by using a single injection by a mechanical injector [42]. This single injection produced significant issues despite the fact the flow rate was relatively small, as the premixed combustion after the ignition delay produced a relatively large pressure build-up (dp/d\(\theta\), with p pressure, and \(\theta\) crank angle), as well as maximum cylinder pressure, with the subsequent diffusion combustion often insufficient to allow a proper shape of the heat release curve and thus of the pressure curve. pre-chamber diesel ICEs were often preferred to DI diesel ICEs due to the injection and combustion issues. Things drastically improved when FIAT replaced this single diesel fuel injection of the past, with the Unijet [41]. In this case, the fuel injection was separated by introducing a pilot injection before the main injection. A common rail was delivering at relatively HP the pressurized diesel to the different injectors, and solenoid actuation was allowing relatively fast (for the times) speeds of actuation. The Unijet innovation was so successful that the share of diesel passenger cars in Europe increased from about 10% to more than 50%. The Unijet technology then evolved in the more recent Multijet (and comparable products) injection system featuring multi injection strategies, where the number of injection events has been made larger, with actuation also made much faster, injection pressures dramatically increased, and fuel atomization drastically enhanced. The diesel ICE dramatically improved over the last three decades, mostly because of the turbocharger and the Multijet technology. These two key developments improved efficiency, power density while delivering less noisy operation, fewer vibrations, within acceptable mechanical stresses due to both peak pressure and rate of pressure build-up and reduced out emissions of smoke and nitrous oxides (NOx). The Unijet fuel injection system of the 1990s was using a pilot injection for noise reduction and the main injection for torque control. The Multijet fuel injection system of the early 2000s increased the number of injections to a total of five for more refinement and performance. With Multijet, the injection is divided into multiple smaller injections, pilot and pre for noise reduction, to produce suitable conditions for the main injection fuel to easily and quickly burn diffusion-controlled, one main injection, plus after and post injections for catalyst light-off, soot reduction, and diesel particulate filter regeneration. In 2009, the further evolved Multijet II now included up to eight injections per cycle. With Multijet, the injection is divided into multiple smaller injections, pilot and pre for noise reduction, to produce suitable conditions for the main injection fuel to easily and quickly burn diffusion-controlled, a first main injection for noise and NOx reduction to further improve the in-cylinder conditions to accept the most part of the fuel in a second main injection, a second main injection to further control the torque within constraints of noise and NOx, plus after and up to three post injections for catalyst light-off, soot reduction and diesel particulate filter regeneration with reduced oil dilution. The injection rate shaping resulted in better combustion control allowing to also control emissions and improve fuel efficiency and power density while limiting noise and vibrations. As NG needs a higher ignition temperature than diesel, the temperatures about the top dead center (TDC) of typical diesel ICEs are not enough to allow the combustion of the NG diesel-like. While a properly redesigned ICE could work diesel-like also with high octane number gases such as NG or hydrogen [43; 44], this may require a different control of pressures and temperatures within the chamber, as well as wall temperatures, to properly work, and therefore significant research and development effort may be needed.