# Advances in Diesel-LNG Internal Combustion Engines
## 2 Results
### 2F Diesel-LNG ICEs R&D
For what concerns CI ICEs working with diesel fuel, the ICE of the Audi R18 TDI Le Mans (2016 specs) is certainly worth mentioning. In 2016, Audi stepped up to the 6 MJ hybrid energy and adopted a lithium-ion battery in their diesel-powered R18 for Le Mans. The electric drive system on the front axle had a recharging power of 350 kW. However, the discharging power was less. At Le Mans, the electric drive system only supplied 300 kW during accelerations. The maximum power delivered by the ICE was limited by the fuel flow rate. For a 6 MJ hybrid energy, the maximum fuel flow rate was 71.4 kg/h. This translated, for a 43.4 MJ/kg Lower HeatingValue (LHV) of a nearly pump diesel fuel, in a fuel flow power of 860.8 kW, or 1154.3 HP. For \(\eta=0.45\) at a speed of about 4500 rpm, that is a technological limit of diesel combustion, (this is the peak \(\eta\) at a lower engine speed of the best in class production high-speed diesel ICEs for a passenger car), the maximum engine power at the crankshaft was 387.4 kW or 519.4 HP. The ICE was working full load lean diffusion with an air-to-fuel equivalent ratio of about \(\lambda=1.45\). Coupled telemetry and lap time simulations [55] suggest for the 3.7 L V6 TDI ICE powering the rear axle of the R18 a maximum output of 378 kW or 514 HP during 24h-race conditions. The computed maximum torque was about 850 Nm over the speed range of 3250 to 4250 rpm where the engine typically works over a lap of Le Mans [55]. Figure 3 presents the reconstructed performance curves of a 2015 WEC LMP1-H TDI ICE. In (a) is the maximum BMEP vs. speed, and in (b) the fuel conversion efficiency vs. speed. These curves are reconstructed from the telemetry data of vehicle and engine speed vs. time, plus the maximum torque and power and the fuel flow rate limits as described in [55]. Figure 4 presents the performance curves of a modified Westport 15L HPDI diesel-LNG ICE and the baseline Cummins L15 ISX 450 ICE. Data digitized from images previously published by www.westport.com. This HDT ICE only works up to 2000 rpm. Over the speed range 1100 to 2000 rpm, the full-load performance of the diesel is perfectly reproduced by the diesel-NG conversion. IN terms of torque, the diesel-only has a small advantage at 1100 rpm, and a small disadvantage at 1600 rpm. At 1900 and 2000 rpm, the diesel-NG ICE has some consistent advantage. As reported by Westport, also the fuel conversion efficiencies were comparable, while the PM emissions were dramatically reduced, as reduced where the CO\({}_{2}\) emissions. Figure 3: Reconstructed performance curves of a 2015 WEC LMP1-H TDI ICE. (a) BMEP vs. speed, (b) fuel conversion efficiency vs. speed. Being the combustion of the diesel limited by the time needed by the fuel to evaporate, mix with the air and auto-ignite, the duration of the combustion event is very well known to become prohibitive above the 4500 rpm, reason why also racing diesel ICEs have same, or marginally higher, maximum engine speed of passenger car diesel ICEs, and therefore a small power density compared to gasoline. Slightly larger efficiencies, than the peak power efficiency, are obtained at peak torque. Additionally, by reducing the load by the quantity of fuel injected, the efficiency may be further improved in the medium to high load range about the peak torque speed. DI is superior to port fuel injection (PFI) of LNG or CNG for obvious reasons. The displacement effect, i.e., the volume of air through the intake valves reduced by the volume of the NG, that is minimal with LNG, as the vaporization of the LNG then reduces the temperature of the intake air and thus increases its density, is not the major issue. In the high boost, high compression ratio environment typical of the latest TDI, even a high-octane number fuel such as NG may experience knock under some circumstances. DI during the compression stroke of the NG, even to burn premixed, leaves less time to knock to occur before the diesel-injection-ignition.