Cleaning Carbon from Combustion Chambers

 


 

Carbon deposit combustion chamber

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While carbon build-up affects all internal combustion engines to some degree, diesel engines are generally more susceptible to the problems that come with carbon build-up in combustion chambers and on piston crowns. In this article, we will briefly look at why carbon deposits occur in primarily diesel engines, what their effects are, and whether or not carbon deposits can be effectively removed from combustion chambers without dismantling the engine.

In the post-carburettor era, you may seldom go through the process of decarbonising (remove the carbon from inside a petrol engine) maybe with the exception of some direct injection engines, simply because the combustion process on petrol engines is much cleaner than on diesel engines. There are two main reasons for this, the first being that the air/fuel mixture on petrol engines is more tightly controlled than on diesel engines, and that unlike diesel, petrol contains detergents and solvents that inhibit the formation of excessive carbon deposits.

On the other hand, diesel technicians have to deal with excessive carbon deposits in diesel engines on an almost continuous basis, primarily because modern diesel emission control systems are inherently restrictive, in the sense that emission control systems prevent the engine from breathing freely.

In practice, this means that that despite their advanced mechanical design, and sophisticated fuel and emission control systems, diesel engines remain susceptible to issues such as power loss, increased fuel consumption, rough idling, extended cranking times, and increased emissions when carbon deposits reach critical levels.

However, before we can discuss the removal of carbon deposits from combustion chambers, we need to ask the following questions-

What is carbon?  

In simple terms, carbon, as it relates to internal combustion engines, is a residue that consists primarily of unburnt fuel and excess lubrication oil. Collectively, these are known as hydrocarbons, which are the basis of fossil fuels that include petrol, diesel, LP gas, and kerosene.

Why does carbon form in engines?

While the formation of carbon in internal combustion engines is unavoidable due both to the nature of fossil fuels, and the inherent inefficiency of internal combustion engines, the rate at which carbon forms is largely dependent on the following:

Efficiency of the combustion process 

In petrol engines that are in perfect working order, all of the fuel is burned using all of the available air, which (theoretically) leaves no unburnt fuel from which carbon can form. However, since diesel engines always run with excess air, the efficiency of the combustion process depends on the volume of fuel that is delivered during each injection event. In practice, this means that diesel engines often run rich at some points during their operating range and especially during periods of turbo lag when large throttle inputs can cause excess fuel to be delivered, most of which is then transformed into carbon deposits.

Oil consumption rates

While there are many examples, some engines, such as some BMW, Mercedes, and especially VAG group applications that have FSI engines are particularly notorious for their high oil consumption rates. As experienced technicians, we have all had to deal with customer concerns regarding this, but diesel technicians have a much tougher time, since they have to explain problems like loss of power, hard starting, rough idling, and poor fuel economy to sometimes irate customers on the basis of the rate at which their cars consume oil.

The fact is that diesel engines with high oil consumption rates form excessive carbon deposits much sooner than petrol engines with similar oil consumption rates, mainly because diesel fuel does not contain the detergents and solvents that inhibit the formation of carbon in petrol engines. If we add to this that worn turbocharger seals, poor control of the oil film on cylinder walls, the use of high-volatility engine oils, low combustion temperatures, and inefficient crankcase ventilation systems create a hydrocarbon-rich environment throughout the engine, it becomes clear why even low-mileage diesel engines sometimes suffer from a rapid build up of carbon deposits.

Driving style / vehicle usage

Since carbon forms at relatively low temperatures, a vehicle with a diesel engine that is used primarily for short trips during which the engine never, or seldom reaches its optimum operating temperature, that engine will accumulate carbon deposits at a much higher rate than an identical engine that is mainly used at high speeds for extended periods.

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What are the effects of excessive carbon deposits in combustion chambers?    

On most diesel engines, the effects of excessive carbon deposits are directly related to the amount, or total volume of the carbon deposit in any given combustion chamber. If we assume that the engine is in good mechanical condition, the total volume of carbon in each combustion chamber will be the same, or close to the same. So for the purposes of this article we will assume that the engine is in good condition, and the effects of excessive carbon deposits on that engine will be as follows-

Combustion will suffer

Since diesel engines depend on compression for ignition, the total volume of the combustion chamber is of critical importance. Bear in mind that since diesel engines are not throttled like petrol engines, the ECU (Engine/Electronic Control Unit) does not control the volume of the intake air. In simple terms, the ECU assumes that enough air is available to combust the volume of fuel that is delivered to each cylinder, with the volume of fuel being determined on the basis of input data from a variety of engine and driveability sensors.

As a practical matter, one parameter that a diesel fuel control system considers when calculating a fuel delivery strategy is the volume of the combustion chamber at the point of full compression. Thus, if some of that space is taken up by carbon, the ECU cannot compensate for it because it does not control/monitor the volume of the intake air, with the result that there may not be enough air present to combust all, or most of the fuel that is being injected.

The detonation flame front is disrupted

Like petrol engines, the combustion process in a diesel engine depends on a smooth, even, and a predictable propagation of the detonation flame front to achieve effective combustion of the air/fuel mixture, and if a carbon deposit were perfectly smooth, the propagation of the flame front would likely not be affected in any way.  

However, in practice, a carbon deposit in a combustion chamber looks much like a miniature mountainscape, with peaks, valleys, and variations in thickness that can vary from a few microns, to several millimetres. Therefore, as the engine warms up, the carbon deposits also heat up until some parts of it become red-hot, which then act like miniature glow plugs, which in turn, can ignite the air/fuel mixture prematurely.

The practical effect of this is that the detonation flame front propagates in unpredictable ways, and in some cases, the flame front can be extinguished by several, competing ignition events long before the piston even reaches the point where ignition would normally have occurred. While modern diesel fuel control strategies have largely reduced the incidence of premature ignition, these strategies have not eliminated it entirely and if premature ignition is allowed to continue over extended periods, severe engine damage is usually the result.

So, can excessive carbon deposits be removed from combustion chambers?   

Excessive carbon formation in particularly diesel engines has been a feature of these engines ever since they were invented, and dozens, if not hundreds of methods of carbon removal have been suggested, “invented”, proposed, or simply perpetrated on an unsuspecting public, and sometimes, a gullible car repair industry over the intervening decades.

However, since a full exposition on all the fads, gimmicks, and useless methods of carbon removal from diesel engines would fill a small library, we will instead focus on one current method that appears to deliver at least partially positive results, although empirical test and evaluation data in the form of “before and after” test reports has sadly not been forthcoming from either practitioners or purveyors of the technology. Below are some details of the system that seems to be all the rage across the world-

Hydrogen / Oxygen-hydrogen cleaning

In simple terms, the technology involves a process of electrolysis to break up water molecules into their constituent hydrogen and oxygen molecules, and then feeding the resulting hydrogen/oxygen mixture into the engine via the inlet duct for periods of between 20 and 45 minutes while the engine is running. 

The aim of the exercise is to raise the combustion temperature to a point where the hydrogen and oxygen interact with the carbon on the molecular level to form carbon dioxide, which is then expelled through the exhaust system. Since hydrogen burns at around 2800 degrees Celsius in the presence of oxygen, operators of these systems claim that all carbon in the engine, as well as that in the EGR valve, catalytic converter(s), DPF (Diesel Particulate Filter), and other parts of the exhaust system is converted into carbon dioxide as well.

Other claims made by proponents and practitioners of hydrogen cleaning is that no part of the engine or emission control system is damaged in any way, that the engine oil remains unaffected, and that engine performance and fuel economy are restored. 

How effective is removal of carbon with hydrogen?

Engine carbon cleaning in progress

In the total absence of objective and empirical “before” and “after” test data from independent third parties, we are left with thousands of anecdotal positive “reviews” on a multitude of motoring forums and other web resources that may or may not be true. Note however, that despite a diligent, several-hour long search by this writer, no negative reviews or comments on the efficacy of hydrogen as a means to remove carbon from combustion chambers could be found, which may or may not be indicative of a degree of marketing manipulation. Moreover, and perhaps more telling, is the fact this writer could find no reviews – positive or otherwise – of the technology by persons who can be identified as automotive mechanics or engineers.

Nonetheless, posters to forums who have observed the process being performed on their own vehicles (both petrol and diesel), all report that the idle quality improved within minutes, that the idling speed increased noticeably, that all previously-heard engine noises disappeared, and that either billowing clouds of smoke, or large volumes of black water (or both) emerged from the tail pipe.  Additionally, all posters report that the overall performance and especially the throttle response of their vehicles had improved dramatically after the hydrogen treatment. 

Does the above then mean that all the carbon had been removed from the combustion chambers on the vehicles that had undergone the treatment, and without any damage to any parts or components having been the result?  This writer cannot say for certain, but we can explore two possibilities; one being that at least some carbon had been removed, and that if the technology works as advertised, it is unlikely not to cause damage to engine and exhaust system components. Let us start with the possibility that-

Some carbon may have been removed

If we take reports of smoke and black water emerging from the tail pipe during treatment at face value, we can only conclude that carbon deposits in the engine had not been converted into only carbon dioxide, since carbon dioxide is invisible.

What is more likely to have happened is that the oxygen and hydrogen molecules somehow recombined in the engine to form water, and hence, steam (mistakenly interpreted as smoke) during the normal combustion process. This will also explain the presence of large volumes of water emerging from the exhaust pipe. Note that although water vapour is also formed when a mixture of hydrogen and oxygen burns, the amount of water vapour formed does not explain the actual amounts of water that have been observed during hydrogen cleaning procedures.

As experienced technicians, we have all seen how engine coolant can remove carbon from piston crowns when small amounts of water/coolant enter a cylinder though a leak path in a head gasket. If water had formed in the engine during the cleaning process, then some carbon may have been removed from the combustion chambers, but there is no way of telling how much had been removed without either tearing down the engine, or without having performed “before” and “after” inspections with a boroscope.    

The technology does not work as advertised

It will be remembered that hydrogen burns at a maximum temperature of about 2800°C in the presence of oxygen, which makes it almost inconceivable that any part of a normal internal combustion engine can survive this temperature for up to 45 minutes. This must be seen in light of the fact that the average cylinder temperature in a diesel engine is only about 600°C (briefly peaking to about 2600°C during actual combustion), and that for a gasoline engine is about 1100°C during combustion.

In the case of diesel engines, the roughly 200°C difference between the maximum combustion temperature and the temperature at which hydrogen burns, is more than sufficient to overwhelm even the most efficient engine cooling system within a matter of seconds. Not to mention the effects of the huge differences between those that occur in petrol engines, and the temperature at which hydrogen burns, which will overcome the best efforts of the cooling system in very short order.

 There are other issues as well. For instance, cast iron (from which exhaust manifolds are made) melts at between 1100°C and 1500°C, aluminium pistons melt at between 600°C and 1000°C, while cordierite catalytic converter cores melt at only about 1200°C. Thus, faced with reports that no damage to any of the above parts has been observed during hydrogen cleaning of carbon deposits, we can only conclude that no hydrogen is actually burnt or combusted during the cleaning process. 

Conclusion

Based on available information, it is impossible to state unequivocally that hydrogen as a cleaning agent is effective (or otherwise) as a means of removing carbon deposits from combustion chambers on modern internal combustion engines.

However, as experienced technicians that always have the best interests of our customers at heart, it is incumbent on us not to recommend any carbon removal procedure unless we can prove and/or demonstrate that it works, is cost effective, and does not cause harm to engines or other parts/components, lest we be accused of peddling snake oil. 

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