The composition and gaseous nature of natural gas are a solid backbone to ensure a clean combustion process. Looking to the progressive tightening of pollutant emissions standards, future internal combustion engines, even when fueled with conventional fuels, will be “clean”. But cost and complexity of the gas after-treatment systems for ensuring future standards under real driving conditions will increase, especially for Diesel engines.
Today’s CNG technology is mainly based on spark ignited engines operating under stoichiometric combustion all over the range of engine/load conditions; this translates into full efficiency of the 3-way catalyst that can ensure the best contemporary abatement of THC/CO/NOx emissions.
Natural gas is the simplest hydrocarbon fuel and, thanks to its gaseous nature, provides an immediate benefit in terms of pollutants (THC, NOx, PM/PN) reduction, not only from a quantitative point of view but also from the qualitative one.
For instance, when looking to the effect on the human respiratory tract from the combination of sun radiations with vehicle emissions, the potential in forming ground ozone is playing a fundamental role and this is highly depending on the nature of the fuel.
In a recent study from the Swiss research centre EMPA has compared the emissions from a bifuel Euro 6b vehicle, once operating on gasoline and once on CNG. The formation of ozone promoters is depending on the combination of NOx emissions and the rate of NMHC (Non Methane HydroCarbons), and the benefit from the composition of natural gas is quite clear.
(Source: www.empa.ch)
The study also points out the performance of natural gas with regard to PN (Particle Numbers): even if gasoline is compliant with the emission threshold (6,0E+11 #/km), natural gas provides a reduction factor by 30 compared to gasoline.
Today, the question of ultrafine particle emissions is under discussion: current measurements are done considering only particles with a diameter > 23 nanometer, while future legislation will most probably look also in the range down to 10 nanometers or even lower. In 2016, the European Commission has launched three Horizon 2020 projects (http://www.pems4nano.eu) (http://sureal-23.cperi.certh.gr) (http://www.downtoten.com) to develop new methodologies to measure nanoparticle below 23 nanometer diameter.
The potential generation of ultrafine particles in CNG engines is not due its combustion process but reasoned in its oil consumption (and affected also by its viscosity). In a recent publication (SAE Paper 2017-01-0778) it has been demonstrated that oil viscosity and its ageing are fundamental in the PN generation process and that the precursors for the nucleation of the nanoparticles are mainly sourced by lubricant additives (metals).
Moving now to NOx emissions, it is interesting to observe the results from a test conducted on CNG and Diesel equivalent Euro VI trucks in France (www.projetequilibre.fr).
(Source: www.projetequilibre.fr)
NOx emissions measured under real driving conditions through PEMS (Portable Emissions Measurement System) reveal a clear benefit of CNG trucks over different mission profiles.
An additional advantage from natural gas, especially in the heavy-duty applications, is the reduction of noise emissions. Thanks to the Otto cycle, CNG applications offer less combustion noise, translating in less vibrations and smooth operations. Compared to Diesel, noise reduction is in the range of 3 dB(A), which results in 50% less noise perception.
_
This article is the third publications under the title "Why natural and renewable gas is among the solutions for a clean and decarbonized transport system". Find related publications in the related content section below.
