Carbon comes in many forms, from the beautiful diamond in a wedding ring to the graphite pencil you used in grade school. When carbon atoms form a diamond, it is a clear transparent hard material and when they form graphite it is an opaque soft material. The unique structure of the carbon atoms allow them to bond with many different chemicals. Carbon has four electrons in its outer most electron shell that can form covalent chemical bonds. A covalent bond is where atoms share electron pairs between one another. This allows carbon to be covalently bonded to one, two, three or four carbon atoms or atoms of other elements or groups of atoms.
Good Carbon and Bad Carbon
Where many of these carbon compounds are very useful, such as the graphite pencil, the carbon compounds that accumulate in the internal combustion engine are not. Even though the carbon compounds that accumulate in the engine are unwanted, carbon is very much a part of the internal combustion engine. This is because the lubricants and fuels that are used in the engine are carbon based compounds.
The lubricant and fuel carbon bonds are formed with hydrogen and produce hydrocarbon chains. These hydrocarbon chains are refined from crude oil and contain various molecular weights. When these hydrocarbon chains are formed to produce lubricating oil they contain heavier, thicker petroleum base stock that have between 18 and 34 carbon atoms per molecule. Lubricating oil creates a separating film between the engine’s moving parts. This occurs in order to minimize direct contact between the moving parts, which decreases heat caused by friction and reduces wear, thus protecting the engine. When these hydrocarbon chains are made for fuel such as gasoline, they contain lighter petroleum base stock that have between four and 12 carbon atoms per molecule. Overall, a typical gasoline is predominantly a mixture of paraffins (alkanes), cycloalkanes (naphthenes), and olefins (alkenes). Fuel is blended to produce a rapid high energy release combustion event that propagates through the air in the combustion chamber at subsonic speeds that is driven by the transfer of heat.
As the internal combustion engine is operated, the fuel’s energy is released in the combustion chamber. This occurs by a chemical change occurring to the hydrocarbon chains. The heat from the ignition spark (gasoline) or from the compression (diesel) breaks the hydrocarbon chains so the bonds between the carbon and hydrogen are separated. This allows the carbon to bond with dioxygen (O2), and the hydrogen to bond with oxygen (O); thus changing the hydrocarbon chains to carbon dioxide (CO2), and water (H2O). However, if there is a lack of oxygen during the burning of the fuel pyrolysis occurs. Pyrolysis is a type of thermal decomposition that occurs in organic materials exposed to high temperatures. Pyrolysis of organic substances such as fuel produces gas and liquid products that leave a solid residue rich in carbon. Heavy pyrolysis leaves mostly carbon as a residue and is referred to as carbonization.
As seen in Figures 1 and 2, these intake ports are from 2004 Honda CR-V engines. The engines are identical except for the mileage and the operating condition that the engine ran under. One of these engines has run for 80,000 miles while the other has run for 130,000 miles. If you examine the carbon within the intake ports, Figure 2 has more carbon accumulation than Figure 1. Your assumption may be that it has more miles as well, however this would be wrong. Figure 1 has 130,000 miles where Figure 2 has 80,000 miles on it. The carbon accumulation within an engine will vary depending on several different variables such as; the type of hydrocarbons that the fuel is made of, the detergents added to the fuel base, the type of hydrocarbons that the motor oil is made of, the operating temperature of the engine, the pressure that the carbon is produced in, the load on the engine, the engine drive time, the engine drive cycle, and the engine design. Each of these variables will affect the type of carbon that will be produced and the carbon accumulation that will accrue within the engine.
It is important to understand that the carbon produced within an engine is not all the same. The carbon in the combustion chamber is produced under high heat and high pressure. Due to the conditions within the combustion chamber the carbon produced is denser and has low porosity; additionally the carbon thickness is usually low. If the carbon accumulations get heavier around the flat outer edge of the piston or head in the squish area, carbon RAP can occur. The clearance between the head and the piston will be minimal in the squish area allowing the carbon accumulation to come in direct contact. This will produce a clatter sound that results in the frequency range of 1kHz to 10 kHz. This clatter sound caused by combustion chamber deposit interference usually occurs at cold start and goes away within five minutes. This usually does not permanently damage the engine but produces an unpleasant noise. These combustion chamber deposits will cause high tailpipe emissions and pre- ignition problems, which can cause serious engine damage. The detergent base that is added to the fuel is designed mainly to control combustion chamber deposits. Tier one fuels such as Shell or Chevron have additional additives in the fuel that work well to control carbon deposits. If these carbon accumulations are large then an in tank fuel additive should be used. These additives are simply poured in to the fuel tank and can be effective in reducing this type of carbon build up.
In Figure 3 an intake port is shown that has very heavy carbon right next to an area with no carbon. This uneven accumulation will create heavy turbulence. Every racer that has ported heads on a flow bench will attest to the very small changes made within the intake runner and intake port creating flow differences, both good and bad. These uneven intake carbon accumulations rob power, torque, and fuel economy. With heavy intake carbon accumulations misfire conditions can also occur. This can be caused by major air disruptions or carbon creating valve sealing issues. Additionally the intake carbon deposits can create cold driveability issues; the intake carbon being very porous allows the fuel to be absorbed into the carbon creating a cold lean run condition.
The multiport injector also can have carbon accumulations occur that disrupt and restrict fuel flow. These carbon accumulations usually occur from fuel droplets forming on the injector tip at hot engine shut down. These droplets can be formed from injector weepage or from the gasoline vapors condensing on the injector tip. The temperature of the injector tip bakes the hydrocarbon within the fuel creating carbon through pyrolysis. This carbon can disrupt the fuel spray pattern and can restrict the fuel flow. These fuel injector carbon deposits create driveability problems, power loss, fuel economy, and increase tailpipe emission.
Dealing With Carbon
The carbon accumulation within the internal combustion engine creates many different problems. One of the biggest problems is determining whether the engine has carbon accumulation present or not. One way is to use a borescope; however this may be very difficult depending on the point of entry. If the fuel injectors are easy to remove this is a good way to check for carbon accumulation. When checking with a borescope you will need to check multiple intake ports. Because intake ports do not accumulate carbon at the same rate, different amounts of carbon will be present in different ports. Also, depending on the type of engine some intake valves and ports may have heavy accumulation while others have light accumulation.
The use of a pressure transducer with an exhaust amplifier is another way to check for carbon accumulation. In Figure 4, a 2000 Toyota RAV4 with 140,000 miles is shown before cleaning the induction system using a pressure transducer. In Figure 5, the same RAV4 is shown after cleaning the induction system. The exhaust pressure changes produced from the pressure transducer before and after cleaning are quite evident. The waveform created by the exhaust pressure makes it clear to see if carbon accumulation is present. The AUTOEKG made by WYNNS, as seen in Figure 6, uses this exhaust pressure technology to check for combustion efficiency, and then rates the carbon accumulation within the engine.
Once you have determined that carbon is present the next step is how to remove the carbon from the engine. This is much harder than you would ever believe! The carbon within the engine is chemically very close to asphalt. So, how difficult do you think it is to remove asphalt from the induction system? To start with it will be important to use quality chemicals. Not all decarburizing chemistries are the same. Most of the chemistries that you can purchase are just light petroleum distillates. These types of chemicals will not clean the induction system. You can use gallons of these chemicals and still not change the carbon accumulation at all! It will be best to use products from companies that have proven themselves over time from names you know.
The method used to apply these chemicals will be very important. Contrary to popular belief, one cannot just pour the chemicals into the engine. First the chemical must be able to reach the carbon deposit and soak it. This will need to be done by pressurizing the chemical and injecting the chemical into the engine. When the chemical is pressurized and injected the droplet size is smaller and can be carried by the energy of the air moving through the engine, and be delivered to the carbon deposit. During the cleaning phase the engine RPM will need to be varied as the cleaner is injected into the engine with multiple snap throttles events. The cleaner will need to be injected for at least 20 minutes; all chemicals take time to work. It will be important to use multiple chemicals to clean with. Many chemical companies make cleaning kits where there are 3 parts, a cleaner, a wash, and a fuel tank additive. These chemistries work together and will make your induction cleaning more successful. In order to clean the induction system you may need to clean the engine multiple times.
For many years, I misunderstood the benefits of cleaning the induction system. My customers would ask me about induction cleaning and I would tell them that we would clean their engine once it began to have trouble. I could not have been more wrong. Over the last five years I have borescoped and cleaned hundreds of engines and I have learned firsthand how hard it is to actually clean these carbon deposits. Just because you put chemical through an engine does not mean you have cleaned it. However, if you do remove the carbon deposits it is amazing how much better the engine runs.
There are numerous SAE papers on the power gains, torque gains, tailpipe emission gains and fuel economy gains from decarburizing the internal combustion engine. I would recommend that you take the SAE engineers’ advice and clean these carbon deposits out of the engine. Your customer will be happy to have a better running engine that produces better fuel economy. The good news is there is new fuel induction cleaning equipment that uses new chemical compounds on the horizon that work much better.
