Natural gas (NG) producers and distributors have reserved part of their commercial effort aimed at the land and naval transport sector. Although most of them promote NG as a more sustainable fuel than gasoline and diesel, ideal for the energy transition, the story is not quite like that. In three studies, diesel engines are compared to current gas engines: 1) “Heavy-Duty Vehicles Performance Evaluation”, conducted by the IEA within its Advanced Motor Fuels Technology Collaboration Program (May 2021); 2) “A Comparison of the Life-Cycle greenhouse gas emissions of European Heavy-Duty Vehicles and fuels”, by the International Council on Clean Transportation (February 2023); 3) “Natural Gas as a Fuel for Heavy Goods Vehicles”, by the Center for Transport Studies, Department of Civil and Environmental Engineering, Imperial College London, London, UK (January 2019). In addition to the pure diesel cycle, two of them cover 4 technologies for gas use: 1) spark ignition and stoichiometric air-gas mixture (fired the most common system); 2) spark ignition and stratified mixture (very rare, uses a richer air-gas mixture close to the spark plug; 3) dual-fuel engine maintaining diesel injection and air + NG; 4) HPDI (high-pressure direct injection) – a variation of option 3 – which maintains the injection of diesel (5%) and NG injected at 300 bar directly into the combustion chamber. There is also an option developed by the Swedish company Sekab and used by Scania for over 30 years, the ED95: it uses direct ethanol added to the diesel cycle, without the need for a spark. This fuel was already used in the 1980s in some vehicles in Sweden. Even Brazil received around 50 buses with this engine in 2010, but for some reason they didn’t establish themselves here. The summary could be like this: gas engines are less efficient than diesel engines because of the constant volume cycle (Otto), they have less torque and power which results in greater energy consumption – more MJ/tonne-km. When it comes to emission of the main pollutants (N2O, CO, HC, NOx, and particulates) diesel is worse in terms of NOx despite the selective catalytic reduction system (SCR) while in the NG engine produces a bigger number of fine particles and more unburned HC (mostly methane). CO2 emissions are slightly lower for NG in HDV (3% to 4%), while for medium and light trucks the reduction can reach 10%. The difference should disappear in Euro 7 engines, a universal certification that does not depend on the fuel: the same limits for gasoline, diesel, and gas. The big problem with NG is scope 3 ‘upstream’ emissions: methane leaks in the “well-to-retailer” and “retailer-to-wheel” process. If 1% to 2% of the total volume of transacted gas leaks in any of the stages, CO2e emissions from NG trucks already exceed those from diesel’s emissions. “Methane emissions from the energy sector are 70% higher than official figures” said the International Energy Agency (IEA) on February 23, 2023.
DS: DAIMLER SETRA, Compression Ignition diesel; S: SCANIA Spark Ignition CNG (M3 Class II buses) Results from “Comparison of real driving emissions from Euro VI buses with diesel and CNG fuels”, Universidad de Castilla-La Mancha, Campus de Excelencia Internacional en Energía y Medioambiente, Toledo, Spain. Arántzazu Gómez, Pablo Fernández-Yáñez, José A. Soriano, Luis Sánchez-Rodríguez1, Carmen Mata, Reyes García-Contreras, Octavio Armas*, María D. Cárdenas
