Exhaust Valve Recession With Low – Lead Gasolines
(This article was taken from the November 1987 Society of Automo tive Engineers journal “Automotive Engineering”).
Environmental Protection Agency regulations have drastically reduced the amount of tetraethyl lead that can be blended into gasoline, and it seems to be only a matter of time before it will be removed completely. This causes concern about the potential for serious wear problems in older vehicles and engines designed to run on leaded fuel. Due to the lack of data forjudging this concern’s validity, Phillips Petroleum Co. decided to conduct an experimen tal engine wear program to define the risks to such engines from low-lead and unleaded gasolines.
As of January l, 1986 the amount of alkyl lead antiknock additives which can be added to “leaded gasoline” is only 0.1 grams/ gal. Since this is a cal culated quarterly refinery average, some blends will be either above or below this value.
As part of the phasing-in of the EPA regulations, refiners were permitted to reduce their lead usage to levels lower than were mandated in 1985. By doing so, they could “bank” unused lead rights which could then be used in 1986 and 1987. This has tended to keep lead levels slightly above 0.1 g/ gal, until such rights are all used up. Beginning on January 1, 1988, such rights expired and gasoline can contain no more additive than the mandated maximum. The EPA requested comments on the feasibility of a total ban on lead use in automotive gasoline by that date. (Some off-road and aviation gasolines are still permitted to contain lead.) Whether accomplished in 1988 or not, EPA’s stated objective is the removal of all lead as a gasoline additive.
Its actions have been justified on the basis of the derived health benefits of reducing the amount of lead released into the environment via automobile emissions. Another benefit would be derived from the elimination of “fuel switching” and “mis-fueling”. Such actions, however, mandate another kind of fuel switching: use of unleaded or low-lead fuels in engines specifically designed for operation with leaded gasolines.
The engines of concern are those designed specifically to take advantage of the lubricating effects of lead oxide deposits formed in the engine when leaded fuel is burned . These deposits have provided needed protection against exhaust valve seat wear. With out adequate lead in the fuel, these engines are ex.posed to the risk of severe wear of exhaust valve seats, a phenomenon known as “valve reces sion.”
Prior to 1971, virtually all automobiles, gasoline-powered trucks and agricultural equipment, and 4-cycle marine engines were metallurgically designed to take advantage of the fuel’s lead content. Some manufacturers of the latter groups continued this until 1975 or beyond (for some heavy-duty applications). If these engines are still in operation, they face at least some risk of reduced life when operated on fuels with “insufficient” lead.
Though there is some variation, most industrial estimates agree that more than 30 million cars, trucks, trac tors, motorcycles and inboard power boats designed for leaded fuel remain in active use. An EPA estimate ac knowledges that over 20 million cars and light-duty trucks currently on the road may require lead-containing fuel to protect against abnormal valve seat wear . This excludes millions of engines in power generators, irrigation pumps, mowers, and other small equipment that could be at risk. One source esti mates the total number at risk as exceeding 70 million.
More and more engines will be at risk as “banked” lead credits are consumed during the remainder of 1987. Even more will be exposed in the event of a total EPA ban on lead-containing additives.
The need for catalytic converters mandated lower lead content to minimize catalyst poisoning. Many ways to provide the needed wear protection without lead were evaluated in the late l960’s and early 1970’s. One solution was to machine out the seat area in the head and replace it with a valve seat insert. This insert would normally be harder than the base metal due to a special heat-treatment or difference in alloy content. Another, more common method was that of induction hardening the (cast iron) cylinder heads in the seat areas. Since about 1971, most automobiles and light-duty trucks have had these integrally-hardened valve seats or inserts and can operate continuously on unleaded fuels.
Without sufficient wear protection from the fuel of valve hardware, the valve seat material will be abraded away, with the valve head actually sinking or receding into the seat mate rial. The seat is the more-susceptible wear surface in this metal-to-metal contact since the valve is metallurgically designed to have greater hardness.
As the exhaust valves recede into the cylinder head of an operating engine, its performance deteriorates progres sively. This normally results in only a minor performance loss, along with associated increased emissions, and is not likely to be detectable or objec tionable to the operator. However, as recession continues, it can reach a point at which one or more of the valves will no longer close. This will result in severe engine misfire and potentially severe valve and/ or engine damage.
A valve will “hang open” or no longer close when the operating range of its lifter is exceeded or when all the valve lash has been taken up by the recessed valve. A typical hydraulic lif ter may be able to compensate for up to 0.15″ of wear before such a condition is reached. With solid lifters, once the initial lash setting (typically 0.016″ to 0.020″) has been consumed by wear, readjustment would be needed to pre vent non-closure of the valves. Engine failures prior to “hang open” condi tions also are possible as the closing forces imposed by the valve springs are reduced with increasing valve reces sion. Additionally, if valve recession is allowed to continue beyond a cylinder head’s depth, catastrophic damage to the head and engine could occur. Valves have been known to penetrate the head completely and enter the sur rounding water jacket.
With the EPA’s aggressive schedule for removing from the market a signif icant amount of valve recession pro tection for older engines, there are concerns for the affected consumers. The major ones are:
- Will a gasoline containing 0.1 g/ gal lead provide sufficient protection?
- Are all consumers protected?
- Do engines using leaded gasoline need a minimum standard for their protection?
- What if regulation to a no-lead standard is put into place?
What are the consumer’s options? Prior investigations have determined mechanisms of valve seat wear. They are combined effects of two major mechanisms: material transfer (welding and tearing action), and sub sequent abrasion (lapping and grinding dry wear).
It is believed that iron oxide films or flakes are generated from the cast iron heads when exposed to hot combus tion gasses. These oxides adhere to and become embedded in the contacting valve face, producing nodules of harder material which are pounded into the base metal by the seating action and firing pressures. This causes plastic deformation of the valve face (referred to as warts or craters) producing a grinding-wheel type surface.
Once the first step of material trans fer has begun, subsequent axial and radial movement (scrubbing and rotation) of the valve face against its seat under higher than normal contact for ces aggravates abrasive wear. The abraded iron oxides additionally act as a lapping compound on the valve face, working with the harder compacted oxides to gouge into the valve seats.
The presence of lead in the combustion fuel has been proven to reduce drastically or totally eliminate this wear action. Lead compounds found in the cooler valve seats (halides) and on the hot valve faces (oxides) of engines run on leaded fuel are consi dered to have provided the extra wear protection. These compounds act as solid film lu bricants bet ween the metal-to-metal contact of the valve face and seat, inhibiting high tempera ture oxidation and abrasion while min imizing adhesion and material transfer. Engines not equipped with hardened exhaust valve seats are very susceptible to this localized wear. Many other independent hardware factors affect the rate of seat wear. These include valve rotation, spring tension, seat angle, seat width, operating temperature, valve geometry, and metallurgy. The actual risk faced by any given engine equipped with soft valve seats depends on several additional operat ing factors. The amount of lead in the fuel is one of the most important. Another major factor in determining risks of excessive seat wear is the severity of engine service. High-speed, high load operation, especially if the engine is under this condition for extended time periods, is more prone to cause valve seat wear than low-speed cyclic operation.
A test was developed to be sensitive to the key operating parameters of speed, load, and fuel content; with the many other factors affecting wear not intentionally varied . A comprehensive study was not intended.
Its results provide the following guidelines:
- A gasoline containing 0.1 g/ gal of lead provides sufficient protection against valve seat wear for engines requiring lead and operating under moderate conditions.
- As little as 0.05 g/ gal oflead in the fuel would be sufficient to protect these moderate-speed engines.
- Gasoline containing less than 0.05 g/ gal is not a satisfactory fuel for a large number of engines designed specifically to take advantage of the fuel’s lead content.
- Engine load is not a primary factor in valve seat wear consideration for moderate service operations.
- In high-speed service, unleaded gasoline can lead to catastrophic failure in a very short time for the affected engine population.
- A leaded gasoline containing the regulated maximum lead content provides only marginal protection to the high-speed engines in this affected group.
- A lead concentration of 0.2 g/ gal in the fuel would protect most high-speed engines requiring lead against abnormal valve seat wear.
Operators of high-speed engines already have significant concerns. Ifan outright ban on leaded gasolines is enacted, virtually all engines designed with leaded fuels in mind will exhibit shortened life spans. One option for continued operation would be use of an aftermarket additive to mix with the fuel at refueling times. It could con tain lead and protect in the original manner. At least one such additive has been registered with the EPA.
Other options ultimately lead to large capital outlays unacceptable to average consumers. These include: complete cylinder head rebuild and modification to hardened valve seat inserts; d riveline modification to achieve favorable tradeoffs between engine speed and load; engine replace ment after a shortened life span; or de-rating engine output by confining operation to lower speeds.
LA ACA, Whispering Bomb
This article was originally published in the June 1989 edition of Zundfolge