Qualifying Fuels to Avoid Intake Valve Deposits
Fuel-related driveability problems called for development of a new fleet test procedure
by Lee J. Grant and A. Doug Brownlow
For more than 40 years, staff members in the Automotive Fleet Laboratory in the SwRI Fuels and Lubricants Research Division have evaluated trucks, automobiles, buses, tractors, motor scooters -- vehicles of all types and sizes -- and their components, as well as fuels, lubricants, and other fluids for companies in the United States and abroad. It was this longstanding international reputation for excellent work that first brought the German automotive company BMW to the Institute in 1986.
At that time, the Institute was involved in a series of test programs to investigate intake system deposits and their effects on vehicle engine performance (driveability) during warm-up. The test series showed that while fuel injector deposits -- a problem that had previously plagued the automotive and fuels industries -- were now minimal, there were indeed wide variations in intake valve deposits.
BMW had begun similar investigations of their own in 1984 when their field analyses showed that, because of differences in fuel properties and driving styles, engine performance problems due to deposits on injectors and intake valves were more than 10 times higher in the U.S. than in Germany. These driveability problems occurred during startup and initial acceleration, and could produce a rough idle, hesitation, and backfiring at intermediate temperatures (50-79 degrees F). Laboratory and vehicle tests confirmed that fuel properties were the main cause of these deposits. However, when these vehicle results were compared with conventional measures of gasoline characteristics, no clear correlation was found. BMW, as a result, decided to continue their investigative work at SwRI.
Deposits and Additive Usage
Engine deposits result primarily from the combustion of fuel and air in vehicle cylinders. Most of these chemical reaction products exit through the exhaust system and the tailpipe. A small amount, however, remains in the engine to be picked up by the oil, which is fortified with additives to control excessive deposits and wear.
There are, in addition, some chemical reactions that occur in the fuel and air distribution system in the engine prior to combustion in the cylinder. These "fuel side" deposits are accelerated by heat and other combustion products that find their way back into the intake system. For this reason, it is advantageous if the gasoline contains deposit control additives for those areas that do not have direct access to the generally beneficial additives in the motor oil.
Deposit build-up in the fuel intake system is not a new phenomenon. The gas-guzzling engines of past years were designed to meet less-stringent standards of fuel economy and emissions control. Today, the conflicting demands of increased horsepower, fuel economy, and emissions control mean that fuel management systems must operate within close tolerances. Multiport fuel injection feedback systems control the operation of all cylinders much closer to the lean limit throughout most of the operating range. As a result, variables, such as small deposits, can have a pronounced effect, upsetting the system and causing driveability problems.
Conventional carbureted engines have shown a high tolerance for intake valve and port deposits, without generating driver complaints on engine performance. There are a number of reasons for this. Such engines are tuned to richer air/fuel ratios that allow for mixture malfunction. Their greater power-to-weight ratios mean that the driver is less apt to notice changes in peak power. The limited means of measuring small changes in fuel economy or emissions level masks the effects of heavy intake valve deposits from the driver. Finally, in the 1950s and 1960s it was found that using gasoline additives (detergents) at relatively low concentrations was effective in controlling such things as carburetor deposits that upset fuel-air mixtures.
In 1985, automakers began to introduce fuel-injected engines. The obvious advantages included improved fuel economy, cleaner exhaust, and excellent drivability. However, the tendency of fuel injectors to clog soon presented the automobile and fuel industries with a new set of problems. As the engine is turned off, fuel is dried by the heat, leaving deposits in the interior mechanisms of the fuel injector. The engine then becomes hard to start, and suffers from loss of power, hesitation, and rough acceleration. The problem is not caused by foreign matter that can be filtered and cannot be resolved by an engine tune-up.
Conventional amine detergent additives that had been effective in controlling carburetor deposits also proved to be effective for deposits in the fuel injector systems. However, it was necessary to use high levels of treatment and this, in turn, led to an increase in intake valve deposits. The detergents, together with other fuel components, accumulate on the intake valves when used in high concentrations and form carbonaceous deposits.
Polymeric dispersant systems with better thermal stabilities were then introduced. These systems, a combination of dispersants and fluidizers, were found to control deposits throughout the induction system. The dispersants have the capability of cleansing and dispersing particulate matter, holding it in suspension so that it can pass through the fuel system and burn with the fuel.
BMW Test -- Background
It was this series of events and circumstances that created the problem for BMW and other automobile manufacturers. Owners had cars with clean injectors but dirty intake valves that caused drivability problems. An expensive solution is a walnut shell blasting (similar to sand blasting, but with a less abrasive material) of the intake valves to remove the offending deposits. The prospect of doing this several times before the vehicle warranty expired was not appealing to either dealers or customers.
BMW did a significant amount of work on their own investigating the causes of valve deposit. One important part of their research effort included a contract with the Institute to conduct vehicular fleet and laboratory bench tests.
The initial vehicle program, begun in 1986, was an interdivisional effort. This project involved six 1985 BMW model 318i vehicles and used nine test fuels. A carefully designed test route was established, using trained drivers on a schedule consisting of 10 percent city, 20 percent suburban, and 70 percent highway driving. On the completion of 150,000 miles of road-test work, a protocol was established for a test route (cycle), vehicle, fuel-ranking methods, and drivability, supported by objective correlations in which intake valve deposit weights were compared with engine performance.
A second program, testing seven cars, followed in 1987. This program provided additional data for the establishment of a fuel evaluation procedure, added to the database of commercial fuel performance, and allowed refiners to investigate new additive technology. An additional 130,000 miles were accumulated by the BMWs during this program.
BMW presented results of the 1986 project at the 1987 Society of Automotive Engineers International Fuels and Lubricants Meeting in Toronto, Canada.* The presentation, coupled with the 1987 follow-on program, served to increase the interest of some refiners and additive companies. Others, however, considered it a "non-problem" or one isolated to a small importer of foreign cars. For the most part, BMW continued its early efforts to increase interest in raising fuel quality in the U.S. without much direct support from other automobile companies.
Gradually, however, other vehicle manufacturers acknowledged that they, too, had problems or acknowledged the importance of the problem BMW had brought out of the closet. Furthermore, BMW developed a mechanism by which a fuel marketer at the service-station level could notify BMW owners that a particular gasoline could be depended on to provide good performance in their cars. Established performance criteria, SwRI experience and capabilities in fuels evaluation, and availability of the fleet of appropriate vehicles combined to make the Institute a logical choice to offer to refiners and marketers seeking a test by which they could evaluate their gasolines. Successful qualification using Institute procedures would allow manufacturers to market a fuel as having met BMW requirements.
Starting with the original six 1985 model 318i vehicles, the SwRI/BMW North America (NA) fuels evaluation program was initiated in January 1988. There has been a continual increase since then in the number of fuels tested and cars used as more marketers have realized that fuel quality is an important issue. As part of the overall developmental process, there have been frequent additive/fuel response and product development studies conducted in these vehicles.
The magnitude of the program and its rapid and continuing growth were not anticipated. There are currently 30 vehicles assigned to the program. Six of the original seven vehicles are still in service, each approaching 300,000 miles (the only dropout being one that was irreparably damaged in an accident). These vehicles may have had an engine and transmission change or two and other running gear repairs, but they have proven to be durable. The 5,000,000 total test miles in the project have required 125,000 driver hours (60 person-years). The test procedure has received widespread attention as fuel marketers have used it as a subject in television commercials and national media campaigns. Sponsors include companies not only in North America, but in Asia and Europe as well. Many national magazines have featured SwRI and the test procedure; most recently it was covered in Consumer Reports in January 1990.
The 10,000-mile Test
The fuel evaluation procedure, as currently structured, is based on 10,000 miles of driving in the BMW model 318i. These vehicles are equipped with 1.8L 4-cylinder engines and automatic transmissions. The testing is initiated with new, carefully weighed intake valves. This is followed by 10,000 miles of operation with the candidate fuel, and then disassembly of the cylinder head to reweigh the intake valves. An optional inspection at 5,000 miles is frequently conducted.
The primary data consists of intake valve deposit ratings and weights and photographs of the intake valves. The significant data, however, is the actual deposit weight on the intake valves at 10,000 miles. Fuels are then classified in one of the three categories based on the following criteria established for the average of the four intake valves:
1) 100 milligrams maximum: meets BMW-NA standards of intake valve cleanliness for unlimited mileage.
2) 250 milligrams maximum: meets BMW-NA standards of intake valve cleanliness up to 50,000 miles.
3) More than 250 milligrams: does not meet BMW-NA standards of intake valve cleanliness.
Although the engine is the most important piece of test hardware, there are many other vehicle systems that play an important role in the test. A test fleet that has accumulated five million miles requires constant attention to maintenance. Therefore, a quality assurance program was initiated to ensure test quality, maintain test repeatability, ensure vehicle integrity, and reduce or eliminate costly repairs and road calls.
Such systems are mechanical and electrical and include the engine cooling system, engine electronics, and emission control components. The QA program currently requires that a number of parts be replaced every 20,000 miles. A partial list of components included in this changeout program are:
Cooling system -- Radiator and radiator cap, water pump, thermostat, antifreeze, belts and hoses
Electrical system -- Engine wiring harness, distributor cap and rotor, spark plugs and wires, fuel injectors
Emissions system -- Mass air flow sensor, barometric pressure sensor, idle control valve, air cleaner, fuel filter, oxygen sensor
The results of such a program were immediately obvious. The reduction in road calls and failed components have increased our testing capacity and are responsible for a 20-25 percent reduction in test variability.
In conclusion, as engines continue to become smaller and more complex, incorporating multiple intake valves, and demands on fuel economy and emissions control continue, engine performance problems caused by intake valve deposits are likely to increase. If the intake valve deposits and drivability problems were limited to BMWs, it is unlikely that we would have logged more than five million miles in the procedure. While there is little doubt that the SwRI/BMW procedure has contributed greatly to improvement of U.S. fuels quality and decrease in driveability problems, there is a desire by engine and fuel producers for a quicker, less expensive test procedure. Development of such a procedure for evaluation of the impact of fuels and lubricants on formation of deposits on intake valves, correlated with engine performance, will certainly continue at the Institute and elsewhere.Kevin Brunner, Principal Engineer, Fuels and Lubricants Research Department, Fuels and Lubricants Research Division, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas 78228-0510, Phone (210) 522-3579, Fax (210) 681-5344.