The effect of carbon cleaning on your Engine is dependent on how dirty the engine is. This is down to a number of different things, driving style, mileage, fuel used etc.
The below list will give you an idea of whether it is something you would be likely to benefit from:
Constant Low revs driving
Lots of stop-start journeys
Poor quality fuel
The recommended time to have engine carbon cleaning is with your annual service 10,000 miles or 12 months
1) improved MPG, a cleaner running engine will without doubt run more efficiently which in turn saves wasted fuel as more of the fuel is converted into energy by way of higher combustion rates.
2) Reduced Emissions, it goes without saying that a clean engine, together with clean components make for cleaner combustions which in turn keep emissions low. HHO can also be used on a vehicle that has failed its emissions test at its yearly inspection which makes for a vital piece of equipment for every garage that works within this field.
3) Smoother Drive, a clean engine runs smoother than one that has a lot of carbon deposits and this is down to many reasons and factors which vary from a more efficient combustion right through to quieter engine tones.
4) Increased Engine Life, a cleaner engine provides far less stress to the combustion cycle and components. Carbonized components damage internals of an engine as they do not meet in the manor they were designed too. An engine valve for example is designed to sit inside the valve seat perfectly and so a build-up of carbon on one side of the valve will cause the valve to sit off-center which can cause damage to the running gear of the engine. Likewise accessories like the EGR, Turbo Charger and intake also clog with carbon which can cause them to incur damage prematurely.
5) Increased Engine Performance, a cleaner engine will yield more performance. Any blockages will limit either the functionality of the engine or in extreme case limit the amount of air into the engine which in turn throws out the air/fuel mixture. Although by only very tiny amounts, it is still a negative and will lower the efficiency of the engine which increases consumption of gasoline/diesel.
6) Quieter Engine Running, an engine blocked with carbon deposits will be noisy; this is down to the seating of engine components meeting together. When components sit together the sound is far quieter than when they sit slightly off, this is down to the resonance tone of the metallic components the engine is made from causing a type of bell ringing effect.
It is not magic ...
If it is dirty it will clean it, if its not dirty it won't.
However what it will help do is to prevent damaging carbon build up so your engine remains clean. (look at it like preventative maintenance, you are stopping issues before they can start)
An increased burn temperature mixed with a cleaner burn causes a catalyst that breaks down the deposits of carbon that surround the intake system, valves, air pipework, turbo system and exhaust system.
This catalyst changes the carbon into a hydrocarbon which exits the engine through the exhaust system causing the smaller deposits of carbon to immediately exit the system whilst the built up carbon deposits are fractured and broken which in turn causes them to break down during the following engine cycles and load rates.