So, what is decarbonising?
In simple terms, decarbonising an engine involves removal of carbon deposits from the engine, using either mechanical (physical) or chemical methods.
The procedure of mechanically cleaning out carbon deposits entails removal of the cylinder head and then, physically scraping off carbon deposits from the top of the pistons, the combustion chamber and other components (e.g. valves). Two-stroke engines of yore needed frequent decarbonising as the lubricating oil (2-T oil, in common parlance) burnt in the combustion chamber, leaving behind carbon residue.
Opening up the cylinder head of a two-stroke engine is a relatively simple procedure. This isn't the case however, with modern 4-stroke engines and their overhead camshaft(s), injectors, multiple sensors and electronics. There is a lot that can go wrong in the hands of an incompetent mechanic. Therefore, the perceived need for chemical decarbonisation.
Adding certain chemicals like alcohols and terpenes into the conventional fuel supply (petrol / diesel) appears to dissolve and remove at least a part of the carbon deposit formed in various parts of the engine, such as fuel injection systems, piston crowns & rings, combustion chambers, valves, exhaust manifolds, EGR valves, cat-cons and mufflers. The carbon is then ejected out through the exhaust. Various companies make proprietary decarbonising agents, the contents of which are usually trade secrets. A machine, such as the one pictured above, is deployed to meter the decarbonising agent along with fuel into a running engine, where it's supposed to perform its magic.
"Steam cleaning" of engine internals has been put forward at times as a cheap decarbonising method, wherein water is sprayed into the air intake of a running hot engine. The steam thus generated is supposed to dislodge the deposited carbon & remove it via the exhaust. The efficacy of such a method and its positive & negative implications have been discussed at length across the world, but if improperly done, the chances of engine damage cannot be ruled out.
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