Motor Magazine Carbon Deposits: Cleaning Up What’s Left Behind http://www.motor.com/article.asp?article_ID=1317 By John Thompson | May 2008 Even if a customer drives only 12,000 miles per year, each injector on the engine will need to pulse approximately 18 million times. [..] Given time, contaminants in fuel tanks, fuel lines or the fuel rail—or even in the fuel itself—will always restrict injector flow; that’s a fact. Foreign particles such as rust will also accumulate within the injector filter or fuel filters to effectively reduce fuel flow. Extremely small rust particles may even pass through the tiny injector filter itself, causing altered spray patterns as well as reduced injector volume; they may even prevent the injector pintles from seating properly (see photo 1 on page 50). Whether a pintle is sticking on or off its seat, overfueling of cylinders will always occur. If an injector’s pintle is off its seat, not only will the corresponding cylinder be flooded with fuel, but also the PCM (via O2 sensor feedback) will reduce fueling to other cylinders, causing a lack of performance (and a reduction in fuel economy), and creating the potential for engine, piston or ring damage. On the other hand, if a stuck pintle never opens, that cylinder will receive no fuel at all and the PCM will try to correct a lean bank issue by overfueling the rest of the cylinders on that O2 sensor bank. These scenarios are common on vehicles whose fuel systems have not been regularly maintained. Injectors need to be very clean for optimum system performance and fuel economy. Although a PCM (in closed loop) can alter injector flow by reducing injector pulse width, it cannot control a single faulty individual injector. Just one inefficient injector will affect the overall performance and fuel efficiency of an engine. Aside from issues relating to fuel quality, the environmental heat injectors are subject to will invariably cause internal as well as injector tip clogging. Every day, unburned fuel additives adhere to injector pintles and orifices and will eventually alter injector flow volume and fuel spray patterns. After an engine is stopped, the injector tips become a heat sink and will bake residual fuel and/or fuel additives onto the nozzle tips. Eventually, this will cause such symptoms as lack of engine performance, leaking injectors and damage to other components such as O2 sensors and catalytic converters when multiple cylinders are overfueled to compensate for one or more underfueled cylinders as the PCM attempts to maintain stoichiometry. But way before these issues become severe, a significant reduction in your customer’s fuel economy will occur. Part of the fuel injector’s job is to atomize fuel by physically turning the liquid fuel supplied to the fuel rail into very tiny droplets. But in order for the fuel to be fully combusted and release as close to 100% of its energy as possible, it must be vaporized by the back of a hot intake valve. Only after vaporization can the fuel effectively mix with oxygen to form an efficient combustible mix. Even in a brand-new engine, total vaporization of fuel will never take place. Over time, the problem of inefficient atomization from restricted injectors will build carbon deposits on the valves. Because carbon deposits are a very poor heat conductor, the fuel vaporization process eventually will become less and less effective and, as a consequence, will reduce individual cylinder combustion efficiency, waste fuel, decrease performance and create undesirable emissions. So exactly how and why does carbon residue accumulate? The singular reason is that there’s always some degree of combustion inefficiency in the chamber to begin with. But the wasted energy from incomplete combustion that results in carbon accumulation in the first place (photo 2) can also accelerate and compound the waste of fuel energy. Hexane is the primary chemical compound found in gasoline. Hard carbon deposits that accumulate in a gasoline engine are always an indicator of wasted energy from incomplete conversion of a specific type of hydrocarbon (hexane) to carbon dioxide. Like any other chemical, hexane can be separated into other substances only by a chemical reaction. In the case of an internal combustion engine, that reaction is known as combustion. When the hydrocarbons (HCs) contained in gasoline burn, the chemical reaction involves molecular oxygen. Theoretically, this type of combustion should have only two byproducts left over—carbon dioxide (CO2) and water (H2O). Of course, in the real world of a gasoline engine’s four-stroke process, the reaction that takes place will never be total and complete. During the combustion process, heat transforms unconsumed vaporized HCs into a solid or hard substance known as an activated carbon. Activated carbon will accumulate on hot components within the combustion chamber with an exceptionally grainy composition containing many small cracks and edges exposed at its surface, making it extremely porous and a natural absorbent of additional raw or unreacted hydrocarbons. Obviously, PCM cold enrichment strategy is required even in the case of a brand-new engine because sufficient vaporization of atomized fuel on the backs of cold inlet valves is impossible to achieve. But the inevitability of carbon buildup accumulating on the valves will eventually result in cold (and sometimes even warm) engine performance issues such as stumble, sag, stalling, etc. Injectors spray their fuel volume very close to the beginning of an intake stroke; it’s only later in the stroke that the inlet valve actually opens in order to draw air and fuel into the cylinder. Small portions of the atomized hydrocarbons sprayed by injectors onto the backs of the closed inlet valves will invariably be absorbed and transformed by heat into additional activated carbon residue. Heavily carboned valves become a very effective fuel sponge, absorbing greater and greater quantities of raw hydrocarbons before they open. This effectively causes a lean air/fuel charge to be drawn into the chamber, resulting in a less efficient combustion stroke with additional unconsumed HCs available to be transformed into activated carbon deposits. Over time, increasingly leaner-than-desired air/fuel mixtures will be created through absorption of raw HCs to preexisting activated carbon during each successive intake stroke cycle. Carbon residue expands more and more, growing like a fungus and all the while wasting energy and creating the potential for other issues such as preignition or poor valve sealing or sticking. While it’s normal to expect that some degree of unconsumed hydrocarbons (and resulting hard carbons) will remain from even the most efficient results of an inherently imperfect combustion process, you should also take the time to look at and point out to your customers what is not “normal.” The tailpipe can be a barometer of how much carbon “waste” (and buildup) has been occurring inside the combustion chamber. Obviously, a black and sooty tailpipe indicates greater combustion inefficiency (and fuel waste). Carbon buildup in the combustion chamber will also affect heat transfer. You might already be aware that an additional heat buildup of just 30° to 40°F from excessive combustion chamber carbon deposits can cause preignition, resulting in a reduction in fuel economy, and that PCM-adjusted timing retard from an active knock sensor signal will cause even greater loss of engine efficiency. But did you know that excessive hard carbon deposits also effectively reduce an engine’s volumetric efficiency? During the combustion and exhaust strokes, the cylinder head and piston rings that contact the cylinder walls absorb some portion of the heat of cylinder combustion; however, the piston crown acts as the primary heat sink. Depending on the heat transfer characteristics of a particular engine, the amount of heat initially absorbed (and temporarily stored) by the piston during the combustion and exhaust portions of the engine strokes can be significant. A portion of this stored heat is inevitably transferred to the air/fuel charge during the intake and compression strokes. Heat transferred to the induction charge should be enough only to improve evaporation of the fuel to avoid condensation on the bore walls. Heavily carboned piston and combustion chamber surfaces that inordinately raise the temperature of the incoming intake mixture into the combustion chamber result in air/fuel mixtures that attain relatively higher temperatures at the end of the intake stroke than at its start, and this in turn can reduce volumetric efficiency. So just like restricted injectors issues, carbon deposits are undesirable, but over time become unavoidable. These energy-absorbing deposits build up not only on components directly exposed to the combustion chamber—such as pistons, rings and valves—but also on injector tips, throttle bodies and EGR passages. Deposits create cold performance and fuel economy concerns long before they show up as a severe driveability issue. There are other engine components vulnerable to hard carbon accumulation: Rings. Many of today’s engines use aluminum pistons. Since aluminum pistons experience higher thermal expansion characteristics than cylinder bore walls, they must be designed to have sufficient clearance at the most extreme temperature conditions. Naturally, the expansion rates between the pistons and cylinder bore walls will be most extreme under full-load engine conditions, so under part-load operating conditions, the aluminum piston-to-bore clearance must be greater than ideal. This in turn increases the space between pistons and bore wall, increasing the likelihood of carbon buildup in the ring area. Injectors. Aside from the injector plugging issues from fuel contaminants mentioned earlier, carbon deposits (from heat soak) that build up on fuel injector tips will inevitably cause an uneven fuel pattern spray. As a conical spray pattern deteriorates to unevenly atomized type patterns, an increase in activated carbon buildup will also naturally occur. EGR. Since no engine is 100% combustion-efficient, some hard carbons will naturally exit through the exhaust system. Activated carbon “waste” will then be reintroduced through the EGR system and tend to accumulate and clog EGR passages. Engines suffering from excessive oil consumption issues can also add to the problem. Oil-based carbons can build up when piston rings become worn, enabling oil to leak past the rings from the crankcase. Oil can also be drawn directly into the combustion chamber from worn intake valves or guides. Oil-based carbon deposits will appear to have a gummy and tarlike consistency, as opposed to the drier activated carbon deposits from an inefficient or incomplete combustion process. Spark Plugs. According to at least one spark plug manufacturer, carbon fouling accounts for around 90% of all spark plug troubles. NGK states that carbon deposits that build up on the firing end of the insulator nose of a spark plug will form a conductive path from the center electrode and down the insulator nose to where the insulator meets the metal shell for the electrical current to leak through. When voltage is applied, under certain conditions, the carbon path may sink enough current to prevent sufficient voltage to build up at the gap, and ignition misfire will occur. Carbon deposits can also accumulate on the throttle body and intake manifold as well as in the catalytic converter and on oxygen sensors. Underlying component faults that cause cylinder combustion efficiency to be any less than what the engine was designed to deliver when new will accelerate the ticking of the carbon time bomb. For example, if the ignition system produces lower-than-normal spark kV in one or more cylinders, less HC will be combusted and increased deposits will accumulate. Too much fuel in the chamber (running rich), EGR system faults and dirty, dripping or clogged fuel injectors all will lead to combustion inefficiency and more wasted energy that will accumulate in the form of uncombusted and activated hard carbon deposits in the combustion chamber. That’s why you should always recommend a good decarbonization procedure after performing an emissions-related repair that your customer has neglected for some time. [..] Fuel octane and the quality or type of fuel used in an engine can also be an area of concern. Driveability Index (DI) is a measure of gasoline’s total volatility, or tendency to vaporize completely. A high DI number is less volatile than a low number. Premium grade gasoline is rated at a higher DI (less volatile) than regular or midgrade gasoline. Since fuels with a higher DI number or octane burn more slowly, higher compression ratio engines typically use higher octane fuels to avoid heat-induced preignition. Conversely, when using a high-octane (less volatile) fuel than an engine was designed for, fuel will burn too slowly, resulting in incomplete combustion, increased carbon deposits and driveability concerns such as increased cold start, warm-up sags, hesitations and stalling at moderate ambient temperatures. [..] Engines shake because the relative combustion inefficiencies between individual cylinders also create an imbalance in the power of their respective combustion strokes, and the degree of the imbalance directly relates to the intensity of the quiver. The subsequent exhaust strokes of inefficient individual cylinders will likewise produce asynchronous pressure pulses exiting through the tailpipe. [..] So how will you service your customers’ fuel injector and carbon issues? A variety of carbon-cleaning equipment is available, and a list of suppliers is provided on page 48. One of the simplest methods is a chemical additive that’s introduced to the plenum and fuel rail through a delivery system suspended from the hood by a hook, such as BG Products’ Inject-A-Flush (photo 3 on page 50). This type of equipment is pressurized by shop air to introduce strong chemical solvents to the fuel rail and induction systems in order to clean fuel injectors and help remove upper engine deposits. A second option includes on-car cleaning machines that are connected to the vehicle’s fuel system inlet and return lines with vehicle-specific adapters (photo 4 on page 52). This type of machine bypasses the fuel supply from the vehicle tank, replacing it with the fuel/solvent tank located inside the machine. A mixture of chemical cleaning solution and gasoline is supplied to the fuel rail to pass through the injectors and run the engine. Carbon and other contaminants in the injector nozzles, on the intake valves, in the combustion chamber, on the O2 sensor and in the catalytic converter are removed and exit through the exhaust system. Even this type of cleaning is typically only about 75% effective (or less) in cleaning fuel injectors. For this reason, both the first and second type of injector-cleaning equipment may be best suited for preventive maintenance types of services rather than for solving driveability issues arising from high-heat-soak engines or from injectors clogged by sediments such as rust or water contamination of ethanol blend fuels. Introducing solvents to an engine to chemically remove carbon does do a reasonably effective job in cleaning the tops of intake valves, but potentially plugged or disintegrating injector pintle baskets are not replaced and you have no way of knowing their condition. The high-heat-soak conditions typical in the drive cycles of today’s traffic-challenged commuters harden deposits trapped in injector inlet screens, and the injectors themselves make a totally effective chemical cleaning impossible. Even though some contaminants may become soft enough for chemicals to dislodge, some or all of the injectors may not be cleaned. Leaking injectors, weak pintle springs and poor spray patterns, among other potential problems, may still exist. The most thorough cleaning and evaluation of fuel injectors can be performed only by physically removing injectors from the engine, followed by cleaning without using caustic chemicals. Off-car cleaning equipment utilizes ultrasonic baths (photo 5 on page 52) that produce sound waves well above the range of human hearing (33 to 40kHz), achieving total injector restoration. This method immerses the injectors in a nonflammable ultrasonic cleaning agent (typically linear alcohol and sodium silicate) contained within a basin. Contrary to what you might assume, the application of sound waves at extremely high intensity and high frequency does not directly “shake” the dirt and debris loose from the injectors. Ultrasonic frequencies cause air bubbles to form within the bath. The energy released from the collapse of millions of microscopic cavitations while the injectors are electronically pulsed is what actually cleans the dirt from the injectors. As the bubbles formed in the cavitating fluid collapse, they form tiny but powerful jet streams of pressure directed at both internal and external injector surfaces. After cleaning, injectors can be fitted to the rail of a flow bench for testing... cont. - http://www.motor.com/article.asp?article_ID=1317 ***** Motor Age Detecting And Servicing Carbon Issues By: Bernie Thompson Friday, August 29, 2014 http://www.searchautoparts.com/motorage/...g-carbon-issues 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. Additionally these bonds are that of allotropy. Allotropy means that these bonds can form in different structural arrangements such as sheets, spheres, ellipses, cylinders, and can be arranged in pentagon, heptagon, hexagonal, and tetrahedral. A large majority of all chemical compounds known contain carbon; carbon is known to form almost 10 million different 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. [..] 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. The carbon that is produced within the induction system is created under very different conditions than the combustion chamber deposits. The carbon in the intake is produced under low heat and low pressure. Due to the conditions within the induction system the carbon produced has high porosity; additionally the carbon thickness can be quite high. The intake carbon accumulation can be produced in different areas such as; the throttle valve, the intake plenum, intake runner, intake port, and the intake valve. These carbon deposits can disrupt the air flow into the cylinder causing performance and driveability issues. The more uneven the carbon accumulations are, the greater the air disruptions will be. [..] 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. Additionally if the engine is direct injected, the intake carbon accumulation will be very different. This is due to the carbon base having very little fuel in it. In a direct injected engine there is no fuel directly delivered to the intake port or valve. This means that the detergent that is added to the fuel base does not get applied to the intake valve or port. This fuel flowing across the intake valve and the detergent is critical in order to keep the carbon from accumulating on the intake valve and port area. The intake carbon with the lack of fuel will be formed mainly from the lubricating oil from the positive crankcase ventilation system and any exhaust gas recirculation system. The exhaust gas recirculation system can be one that operates internally with the camshaft phasing. This lack of both fuel hydrocarbons and detergents allows the carbon to bond differently producing a very different induction carbon. This type of carbon will need a different chemical in order to remove it. One of the biggest problems is determining whether the engine has carbon accumulation present or not. (babbittd note: this articles goes on to mention various scopes used in shops) [..] When cleaning heavy carbon deposits it is possible to cut through the carbon on the port floor, leaving the deposits on the port sides and top, this can increase the intake air disruptions thus lowering power, torque and fuel economy. This usually occurs when trying to clean high mileage heavy deposited engines. 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. - See more at: http://www.searchautoparts.com/motorage/...h.DTB2kUkm.dpuf So there's something for everyone in these articles...One part I found very interesting is this which basically explains the difference between highway driving and city / idling:
edit: in order to view the photographs in the first article (Motor Mag.), one must download the .pdf file (free) at the link
Rings. Many of today’s engines use aluminum pistons. Since aluminum pistons experience higher thermal expansion characteristics than cylinder bore walls, they must be designed to have sufficient clearance at the most extreme temperature conditions. Naturally, the expansion rates between the pistons and cylinder bore walls will be most extreme under full-load engine conditions, so under part-load operating conditions, the aluminum piston-to-bore clearance must be greater than ideal. This in turn increases the space between pistons and bore wall, increasing the likelihood of carbon buildup in the ring area.