The natural gas has the unique - compared to liquid fuel, capability to be supplied to engine in either high or low pressure. Depending on the supply pressure, engine runs on Diesel (high press) or Otto (low press) heat cycle.
The two cycles have different combustion processes : the diffusion (heterogeneous) burning of Diesel practically ensures that no methane escapes unburned from the combustion chamber. Therefore the engines running on Diesel cycle enjoy the full benefit of CO2 reduction from the use of natural gas, and which is the order of 25% compared to ordinary heavy fuel oil.
And this is truly remarkable !
The premixed (homogenous) principle of combustion in Otto cycle allows some fuel to escape at the exhaust. This is primarily during the gas exchange phase of the cycle, and from incomplete combustion at crevices in the combustion chamber and top piston area.
As a result, the methane slip (or more correct, Total Hydrocarbon (THC) Emissions) is Not a problem inherent to the natural gas, but is related to the applied heat cycle, and the amount of methane slip depends on the technology of each engine.
The Otto engines have covered a long distance in their development and today they are capable to demonstrate a very low THC emissions level, and all included (GWP100=28), succeed to offer a noticeable reduction in CO2 emissions of 10% for the 4 stroke engines, and a even higher for the 2 stroke.
In the past, the THC emissions of engines running on Otto cycle were quite higher. But the case of diesel engine is there to remind us that time is needed for reaching the admirable levels of engine performance and efficiency we enjoy today
For the Otto engines things happen much faster, and they can claim a 65% reduction in THC emissions in only 10 years, while the development still continues.
The future allows for catalysts that will essentially completely remove the THC emissions from the exhaust of an Otto engine, in a manner similar to Diesel engines which require a catalyst (or EGR) for reducing their NOx emissions to levels required by Tier III regulation.
In overall, today's technology advancements ensure that no matter if we run on Diesel or Otto cycle, LNG can guarantee a very significant reduction of CO2 emissions.
By Antonis Trakakis, Chief Technology Officer, Forward Ships
The views expressed in this article are solely those of the author and do not necessarily represent those of SAFETY4SEA and are for information sharing and discussion purposes only.
About Antonis Trakakis
Antonis Trakakis is the Chief Technology Officer at Forward Ships. He has graduated from Mechanical Engineering dept of NTUA and carried out graduate studies at von Karman Institute for Fluid Dynamics in the field of Turbomachinery. Antonis has worked with all kinds of internal combustion engines, ranging from gas turbines, to all sizes of four stroke, and two stroke engines, and one of his special concerns has been the formation and control of emissions, and engine efficiency.