Scientists are investigating the source of the stochastic “super-knock” process in engines that use hydrogen/methane fuel.
Small turbo-charged internal combustion engines can be “knocked” without breaking down for long periods of time. Knocking occurs when fuel burns earlier than expected. Even more dangerous is the “super-knock”. Unlike normal knocking, heavy knocking is caused by a detonation wave due to a reaction wave between the release of heat associated with the flame and the pressure inside the engine cylinder.
One fuel, hydrogen, is more prone to knocking than other fuels because of the ways in which engines must run efficiently on hydrogen. Another solution is fuels made from a mixture of hydrogen and methane. Adding methane to hydrogen fuel can burn more easily and reduce other emissions. The combustion of engines is very complex, so researchers need to study the hydrogen/methane fuel reaction to aid in engine design.
Using a mixture of hydrogen and methane as fuel in internal combustion engines is one of the most promising strategies to reduce carbon dioxide emissions. Another concern when burning these fuels in spark-ignition engines is the transition from the “desired” combustion curve to the detonation wave pattern. In a spontaneous fire, the fuel’s combustion wave (called a deflagration) spreads away from the source of the spark.
Under the wrong conditions, this combustion can lead to the formation of a detonation wave. This wave uses up all the fuel very quickly and causes a strong wave called “super-knock in engines”. In order to avoid knocking too much, scientists studied the causes of deflagration-to-detonation. This research will help pave the way for the use of more efficient alternatives to fossil fuels in internal combustion engines.
The researchers investigated the effect of non-thermal chemistry on the propagation of flame elements for H.2-CH4 mixture of fuel burning in air in a confined space, representing an ideal engine cylinder.
Previous work has shown that in some burning environments, H + CH3 and H + OH radical-radical recombination with H + O2 A radical-molecule association reaction can form long-lived excited intermediates (such as CH4*, H2O*, HO2*) which can have the following reactions with H, O, OH, and O2 before facing collision stability (to CH4H2Oh, HO2).
An international team led by Sandia National Laboratories included a non-flammable “termolecular” reaction in the model, studying the effects of such radical-radical recombination and radical-molecule association.
The team used the S3D direct number simulation (DNS) code, with a spatial resolution of 1 micrometer, to show that the addition of non-thermal chemicals to the surface induces chemical changes. chemical during high pressure H.2-CH4 burning and changing sides of the deflagration to high pressures, moving fast.
On the other hand, the inclusion of the chemical explosion analysis method (CEMA), a reliable flame analysis tool to systematically detect the important types and actions performed during a fire, has shown that, regardless of the presence of non-thermal activity, temperature and oxygen pressure. are still the two main variables that affect the detonation structure in H2/CH4-air mixtures under ideal engine conditions.
The researchers noted that this view can change with different types of H2/CH4 combining elements. The researchers’ proposed model appears in the figure above. First, the flame produced by the spark spreads outward with speed ‘Sf‘ accompanied by a pressure wave traveling at speed ‘a’, where ae is greater than Sf (a >> Sf). Without termolecular references, the unburned gas near the cylinder wall ignites suddenly before being ignited by the flame produced by the spark.
After that, the radiation front spreads outwards with a speed ‘SSp’ such that it remains separated from the pressure wave (SSp >> a), thus leading to the formation of normal knocking. However, in the presence of termolecular reactions, contact between the flame generated by the spark and the pressure wave occurs with ‘a’ approximately equal to ‘S.f.’
Perfect contact between the pressure wave and the flame front produced by the spark results in a deflagration-to-detonation transition ie, a super-knock without spontaneous combustion of the unburned gas. The researchers suggest that their non-thermal chemistry needs to be included when modeling the combustion of H.2-CH4 compounds to accurately predict important aspects of flame behavior.
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Scientists Search for Source of Stochastic Super-Knock Event in Hydrogen/Methane Fuel Engines, 31 January 2023