Some traces of unburned hydrocarbon have been present and proved experimentally as witnessed in the exhaust from internal combustion engines. What are the reasons behind this phenomena ?
Combustion process in ICE is composed of a sequence of complex processes. Influencing factors of "complete" combustion are the local air-fuel ratio, turbulence, temperature and pressure. In addition, there is a difference between spark ignition and compression ignition combustion. Anyway, the chemical process of combustion represents itself as high dynamic, and not fully complete. Hydrocarbons may occur as a result of local incomplete chemical conversion, which might have different reasons, e.g. lack of time. Escaping hydrocarbons are often result of these (incomplete) combustion processes.
Kind regards,
Mario
I think the causes can be summarized as three aspects:
1. When fuel spray impinges on walls, this part of fuel cannot be oxidized (for diesel and GDI, the wall could be cylinder wall and piston surface, for PFI it is inlet ports).
2. When flame propagate to walls before the fuel is completely oxidized, it quenches.
3. When the fuel/air mixture is heterogenous(which is inevitable for diesel and GDI), regional rich fuel cannot be oxidized completely.
BR
Luo
To complete the preceeding answers. When the fuel mixture is near the wall of the combustion chamber there is the phenomena called the quenching of the flame. Not enough free radicals for reactions.
To add to the above list of reasons: In SI engines, the engine oil film adhering to the cylinder walls also contributes by absorbing HC's (during compression) and desorbing HC's (during expansion) rendering some of the HC's to escape combustion and add up to the UBHC's (unburnt hydrocarbons).
The attached paper shows that exhaust hydrocarbons increase proportionately with content of engine oil added into the combustion chamber.
Article The effect of oil layers on the hydrocarbon emissions genera...
That means at high load conditions or high speeds there will be a great loss of fuel due to this phenomena. Is there any means for catching this unburned HC so as to recirculate it back into the combustion chamber to improve the specific fuel consumption and hence overall engine economy.
In general, this is the right conclusio. The challange for bringing this approach into work can be found in the practical application of a "HC-filter", which should be able so filter the energetic relevant emissions out of the exhaust gas. If this would be possible, the HC emissions could be brought back into the combustion chamber to improve fuel consumption.
There are several research works in the area of Homogeneous charge compression ignition, which deals with a certain share of exhaust gas recirculation. In addition, there has been some research with 2-stroke engines in this topic, e.g. done by Honda. In 2-stroke engines, it is possible to capture unburned fraction of exhaust gas by use of a controlled scavenging process. These high energetic unburned gases can be mixed with fresh gas and be involved in a subsequent combustion cycle.
Some literature, which might be of interest:
@ HCCI:
http://flyingv.ucsd.edu/papers/PDF/112.pdf
http://energy.gov/sites/prod/files/2014/03/f10/ace006_steeper_2012_o.pdf
@ 2-stroke engine research:
http://papers.sae.org/2000-01-1836/
https://en.wikipedia.org/wiki/Honda_EXP-2
http://dwolsten.tripod.com/articles/jan97.html
Kind regards,
Mario
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