I've been reading some of the disadvantages of TFSI engines and the problem of carbon build up. Looks like we have to decoke our engines, which was a feature of pre 1980s cars ...
The carbon build up on the inlet tract and valves seems to cause poor running of the engine, loss of power & torque and poor fuel consumption.
The use of addictives, liquid cleaners such as BG44, Seafoam, injector cleaners, pure "fuel" treatments don't appear to work for our and similar engines. Looks like physical removal by scraping or media blasting is the only way.
What have been your experiences ? What have you guys done about it ?Found this article which is interesting reading.
Turbo Fuel Stratified Injection (TFSI) & Direct Port Injection Carbon Build-up Problem
by Andy Archer Published on Thursday, July 17th, 2014Inlet Valve Carbon DepositionThe problem with carbon build-up on the back face and stem area of intake valves in direct fuel injection petrol engines has been ongoing for some time now. It became prominent in 2007/2008 as Audi, VW, BMW Mini and other direct injection equipped vehicles started to cause engine warning codes and the Malfunction Indicator Lights (MIL) to become illuminated.
The presence of excessive carbon build-up has generally been attributed to the direct port injection design. This design enables a more complete and efficient combustion process as fuel is injected directly into the combustion chamber rather than behind the inlet valve as with conventional port injection designs. However, any cleaning capability of the fuel or more importantly, fuel additives is non-existent in the inlet tract as the liquid fuel never comes into contact with the back of the inlet valves. The cleaning effect on the front of the valves (combustion chamber side), combustion chamber surfaces and exhaust valves is easily achieved as a consequence of the clean burning characteristics of high quality fuel and/or additives but the downstream surfaces of the inlet valves are left to accumulate deposits. The volume of these deposits eventually alters the air flow dynamics within the inlet tract inhibiting airflow and thus volumetric efficiency considerably.
This result of this is more readily noticed on normally aspirated engines as they are less able to overcome air flow restriction whereas forced induction engines can overcome minor restrictions as air is “forced” into the combustion chamber under pressure.
The images below illustrate the direct fuel injection and port fuel injection design. You will notice on the port injection design that fuel is injected behind the inlet valve and the fuel and air mixture is then drawn into the combustion chamber as the valve opens. This is not as efficient as a direct injection design but it helps prevent deposit build-up on the intake valves.
Port Fuel Injection vs Direct Fuel Injection
Port Fuel Injection vs Direct Fuel Injection
So what has changed?A major breakthrough has occurred recently on the build-up issue and the exact cause has been isolated. The port injection design is actually not the cause but merely the reason why the issue cannot be controlled and managed through normal fuel derived cleaning processes.
It is now understood why even the most advanced post combustion cleaning fuel additives or solvent based cleaning through the fuel /air intake tract have little effect. Furthermore, it is also understood why rerouting crankcase breather by-product into segregated catch cans or using water/methanol injection are limited in their ability to reduce carbonaceous build-up in the inlet port and inlet valve surfaces.
Post combustion cleaning additives, solvent based intake cleaning and water/methanol injection are not effective because the carbon species responsible for the build-up are predominantly derived from lube oil; these produce active but very dense layers of carbons. In some cases even grit blasting techniques have failed to remove the build-up because of the integrity, toughness and adherence of deposits. In contrast to these lube oil derived deposits those resulting from decomposition of fuel tend to produce a satin black build-up that can be scraped off easily with a finger nail. This type of deposit can be removed with fuel borne additive technology. However, the deposits formed from the decomposition of lubricating oils during engine operation have been found far more difficult to remove. This deposition and growth of carbonaceous debris has been demonstrated on a test engine with inspection ports positioned in the inlet tract.
In the pictures below you will notice the solidity of the lube based build-up on the inlet valve of an Audi RS4 (4.2 V8 TFSI) versus the fuel-only carbon build-up as seen on an EGR valve on a different vehicle. The carbon on the latter is easily removed either manually or via fuel additive technology that is still active post combustion.
Audi RS4 Inlet Valve Carbon Buildup
Audi RS4 Inlet Valve Carbon Buildup
EGR Valve Part Cleaned
EGR Valve Part Cleaned
Oil on valve stems – It should be noted that the presence of lubricating oil in this location is normal as there should be a controlled amount of oil to keep valve stems lubricated. One reason why NA engines tend to suffer more from inlet valve deposits is simply that under vacuum the oil from the valve stems is more difficult to “control” as it is sucked through by the pressure differential existing between inlet manifold and atmosphere. In comparison forced induction engines (turbo or supercharged) generally operate with the intake manifold under positive pressure so less oil is pulled through the seals.
So if the small amount of oil bypassing the valve stem seals is normal, and indeed required, then why the excessive build-up on the valves? One hypothesis is that;
The oil is being broken down by the catalytic (reacting) action of the materials used to manufacture or coat the valve stems. In particular, nickel and chrome alloys. This pyrolytic decomposition is widely recognised in the industrial power generation sector where hydrocarbons are in contact with superalloys used in the construction of combustors, nozzle guide vanes and exhaust components.
In layman’s terms the materials used to manufacture and harden the valves is reacting with the lubricating oil creating an aggressive bond between the lube and the valves!
Although this hypothesis seeks to explain the mechanism behind the formation of these carbonaceous deposits there are still many challenges ahead. As carbon is the constituent part of all lubricating oils and fuels and each of these is fundamentally required by engines in their present form a method of reducing or eliminating carbon build up must be sought.
Once oil has initially decomposed and formed a carbon deposit bonded with the valves it remains chemically active and further carbons whether from engine oil or recirculating fuel emissions adhere to the existing mass with ease.
Some manufactures have incorporated a more complicated fuel system with a combined port/direct port engine design to retain the benefits of direct port injection whilst injecting some fuel behind the valves to help keep them clean. However, for existing direct port engine designs there are few viable options: change valve material and/or use a coating that doesn’t catalyse with carbons or enable the adherence of carbon or, introduce an additive pack that can inhibit carbon formation.
Valves have to work very hard and current valve materials are chosen for their toughness and durability. Any replacement material and/or coating would have to at least share or improve upon these properties. There are proposals in the area of material and surface coating choice but we are not at liberty to share them at this stage.
Other theories consider that at certain engine operating conditions there is a small amount of backwash as the early injection of fuel occurs whilst the inlet valve is still open; the contribution of EGR also needs to be considered. For compression ignition engines – diesels – the heavy contamination of inlet tracts with a dense but greasy carbon based deposit is well known. There are many EGR deletion methods focussed on the prevention or re-occurrence of this deposit build up which as in the case of their petrol fuelled counterparts can seriously impede the flow of inlet air to the combustion chamber.
Operating temperatures of engines have tended to increase with commensurate increases in combustion chamber parts, heat soaking on shutdown and extensive idling periods have been shown to affect the degree of build-up on upper cylinder parts and valve gear.
Regardless, the issue of cleaning existing deposits does not go away. The use of more advanced polar solvents will be investigated but this process is still constrained by the hardness of the carbon build-up as well as the risk of unmanageable chunks of carbon being dislodged and damaging valves or cylinder bores during engine operation. Managing the gradual fluidising of deposits enabling them to be safely consumed during combustion is a significant challenge.
There is some data to suggest that the use of certain oil additives or group IV and above base stock oils (pure PAO, esters etc.) reduce the speed of build-up. However, this is not fully substantiated as back to back tests were not conducted on the exact same vehicle. The tests show visual build-up compared to other similar vehicles of similar mileage that are not using additives or group IV and above engine oils. Furthermore, some of the PAO derived oils are more readily broken down by catalytic action and tend to have better high temperature resistance to degradation, thus presenting a fluid film onto the valve stems where decomposition may occur. One area of interest is the use of mineral oils containing carbon fluidising additives as found in many two stroke engine oils; however these compositions generally, do not meet the lubrication specifications required by modern engines.
Archoil® has been using proprietary esters and fluidising technology for some time and we have initiated further tests relating to this technology and direct port engines. We will keep you posted as soon as we have more information.
Andy Archer
Source :
http://www.oilem.com/turbo-fuel-stratified-injection-tfsi-direct-port-injection-carbon-build-up-problem/
