This article was first published in 2001.
Almost without exception, commercial petrols would build up deposits on the components of an engine's intake system over time if they weren't additized. A number of additives have been developed which prevent and/or remove intake system deposits. Unfortunately, some of them increase combustion chamber deposits, solving one problem, but creating another. Since all base petrols are formulated to the same regulated specifications, the performance of the deposit control additive is one of the few features that differentiates one brand of petrol from another.
Engine Deposits and Performance
The diagram shows the critical areas of deposit formation and the associated performance impacts.
Fuel Injectors and Carburettors
Fuel injectors are designed to accurately meter fuel to the engine and to deliver it in a precise pattern of fine droplets. Because the fuel passages are small, injectors are highly sensitive to small amounts of deposits in the critical regions where the fuel is metered and atomised. These deposits can reduce fuel flow and alter the spray pattern, degrading driveability, decreasing power and fuel economy and increasing emissions. Deposits cause similar problems for carburetted engines because carburettors also use a number of small channels and orifices to meter fuel. Port fuel injector (PFI) deposits are believed to form during the hot soak period after the engine has been turned off. The stationary petrol trapped in the injector is exposed to a higher temperature for a longer time than the petrol which flowed through the injector when the engine was running. The heat degrades the petrol, initiating deposit formation.
Intake Valves and Ports
Intake valves and ports are subject to more deposit build up than fuel injectors because they operate at higher temperatures. Heavy valve and port deposits reduce maximum engine power because they restrict airflow. Intake valve deposits also have been shown to affect exhaust emissions. In some very sensitive modern fuel-injected engines, low levels of intake valve deposits can degrade cold start and warm-up driveability. Other valve deposit problems include valve sticking in very cold weather - because deposits interfere with the valve stem sliding in its guide, and burned valves - because severe deposits prevent the valve from seating properly.
When an engine is brand new, its octane number requirement is determined by its design and the quality of its manufacture. Generally, it will not knock when operated on petrol with the antiknock quality prescribed by the manufacturer. However, the engine's octane requirement increases as combustion chamber deposits form during the first several thousand kilometres of operation. If the increase is large enough, the recommended petrol octane rating may not prevent knocking or, if the vehicle is equipped with a knock sensor, the loss of power which accompanies knock suppression may occur.
Combustion chamber deposit interference (CCDI) is a new problem that has occurred in a few modern engines. It is the result of physical contact between deposits located on the piston top and cylinder head and is manifested as a very loud banging sound when the engine is cold. CCDI is limited to the engines which have been designed, primarily to reduce emissions, to have minimal clearance -1 millimetre or less - between some areas of the piston top and the cylinder head (squish areas) when the piston is at top dead centre. Deposits contribute to CCDI, but poor control of manufacturing tolerances in the susceptible engines also can be a factor. CCDI occurs primarily at cold-start. The interference and the telltale sound disappear as the engine warms up and the thermal expansion of the various engine components increases the clearance between the piston top and head at top dead centre.
Historical Development of Deposit Control Additives
This class of additives consists of relatively inexpensive low-molecular-weight surfactants used at low concentrations. When introduced in 1954, they were effective in preventing and, in many cases, removing deposits from carburettor throttle bodies. However, they could not handle deposits in other parts of the carburettor, like the air bleeds, or in the rest of the engine intake system. The introduction of PCV and EGR emission control systems in the 1960s and 1970s increased deposit levels in the whole intake system. As a result, carburettor detergents were not as effective as they were in the simpler 1950s vehicles.
This class of additives consists of polybutene succinimides. Additives with similar chemistry had been used widely as engine oil dispersants before the chemistry was applied to petrol in 1968. Detergent-dispersants are used at concentrations three to five times higher than carburettor detergents. Their performance is sometimes improved by using them in combination with a petroleum carrier oil. They provide keep-clean performance for the intake manifold and intake ports. But they don't control intake valve deposits and have poor carburettor and fuel injector clean-up performance.
Deposit Control (DC) Additives
The first additive of this class was introduced in 1970. It was based on polybutene amine chemistry and was used in combination with a carrier oil. While they have to be used at higher concentrations than detergent-dispersants, DC additives provide benefits throughout the engine intake system. They clean-up - and keep-clean - the throttle body and upper areas of the carburettor, fuel injectors, intake manifold, intake ports, and intake valves.
Lead salts are a combustion catalyst for carbon, so the shift to unleaded petrol changed the nature of combustion chamber deposits. When the first generation DC additives were used in unleaded petrol, they continued to control intake system deposits, but increased combustion chamber deposits. In response, a second generation DC additive designed specifically for use with unleaded petrol was developed and introduced in 1980. It was based on new polyether amine chemistry, which provides excellent deposit control performance throughout the intake system without contributing to combustion chamber deposits or causing any other adverse side effects.
No Harm and Compatibility
DC additives are used at concentrations which are twenty to fifty times higher than the concentrations of other petrol additives. At these higher concentrations, they have the potential to affect petrol properties, fuel system materials and engine oils. So DC additives are tested for the absence of negative attributes (no harm) as well as for the positive attribute of controlling deposits. The additized fuel must be fully compatible with the elastomers and metals it will contact. Also it must have good water tolerance and not contribute to spark plug fouling or crankcase sludge formation.
Required Additive Use
Because of the relationship between decreased deposits and decreased emissions, all motor petrol sold in the United States must contain an additive which provides a minimum level of deposit control performance. This requirement was established by the Clean Air Act Amendments of 1990 and became effective in January 1995. A similar requirement has been in effect in California since January 1992. Additive manufacturers are required to obtain EPA certification for their additives. The certification request must include documentation of the additive's effectiveness in specified fuel injector keep-clean and intake valve keep-clean tests and the additive concentration at which this performance is achieved. Certification may be obtained for use nationwide, for use in specific areas of the United States, or for use with particular types of petrol. Nationwide certification requires using a test petrol which meets ASTM D 4814, but has a greater tendency to form deposits than the average petrol.
Petrol formulators must add a certified deposit control additive to their petrol at the certification concentration level or higher. Over some range, higher concentrations often provide improved performance; for instance, changing keep-clean performance to clean-up performance. Historically, some petrol brands have provided much higher deposit control performance than the certification performance required by the EPA. For competitive reasons, they probably will continue to do so.
Aftermarket additives are additives intended to be added by the customer to a petrol (or oil) which is already in the customer's vehicle. Engine deposits are affected by engine design, driving conditions, petrol base fuel quality, and petrol additives. While all petrol must contain a deposit control additive, some additives are less effective than others or are used at concentrations which are less effective. In addition, some vehicle designs form heavier deposits than others, and some vehicle designs are extremely sensitive to deposits which do form. Aftermarket deposit control additives are available which can clean-up deposits which have formed due to these circumstances. Treating one tankful of petrol with the aftermarket additive is often sufficient. However, additive chemistry and dosage play large roles in determining the effectiveness of the product. Polyether amine-based aftermarket additives have been shown to be particularly effective at providing excellent intake system and combustion chamber deposit clean-up.