The heavy duty transportation sector is expected to play an increasing role in a global economy where logistics will ask for a growing number of freight trade all across Europe. In parallel, public transport and coaches will have to complement more and more personnel mobility in city and inter-city trips to facilitate traffic and improve the fruition of cities.
The transition to alternative powertrains for the heavy duty sector is quite complex as the need in storing high amount of energy on board is fundamental. Replacing the energy equivalent of 100 litres of Diesel fuel would require approximately 3,5 t Li-ion batteries (according to a battery energy density of 280 Wh/kg).
Looking to natural and renewable gas, the ratio of volumetric energy density (kJ/l) between Diesel and CNG (200 bar) is 4,5 while it is 1,6 between Diesel and LNG. This explains why CNG and LNG together can cover both urban-suburban missions and long distance: current LNG trucks on the market are able to ensure more than 1.600 km with torque and power characteristics equivalent to Diesel.
With regard to decarbonization, CNG and LNG engines have to be compared to Diesel engines: so the theoretical 23-24% CO2 emissions reduction is reduced according to the natural gas engine technology. For a 40 t long haulage truck as considered in the Thinkstep study (May 2017 – www.ngvemissionsstudy.eu) spark ignited engines offer a CO2 tailpipe emissions reduction up to 12% compared to Diesel. Considering a direct high pressure gas injection system operating on Diesel cycle, the reduction is equal to 20%, as the engine efficiency is very similar to the Diesel one.
Moving to the so called ‘Well-to-Wheel’ analysis, relative emissions are as indicated in the figure below. LNG being more energy intensive to be produced (Well-to-Tank CO2 equivalent footprint of 19,9 g CO2eq/MJ instead of 12,5 g CO2 eq/MJ considered for CNG) provide an overall CO2 reduction from 6% up to 15% compared to Diesel.
The emissions shown in the visual are calculated as CO2 equivalent, so already including also the contribution from CH4 and N2O emissions.
Future developments in heavy duty CNG/LNG engines have been recently demonstrated in the Horizon 2020 ‘HDGAS’ project (www.hdgas.eu), showing the potential in increasing engine efficiencies by 10% compared to current state of the art technologies. Integration of natural and renewable gas technologies in hybrid powertrain architectures on heavy duty applications is a winning solution, also looking to the vehicle energy management optimization (e.g. in relationship to waste heat recovery systems).
Natural gas is used in the heavy duty sector since a few decades both for bus & coach applications and on freight trucks: vehicle technologies are mature, affordable, safe and ready to integrate the transition towards a more clean and sustainable transport system.
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