ADDITIVES TO HAVE KEY ROLE IN NEW GASOLINE ERA

R.J. Peyla
Chevron Research & Technology Co.
Richmond, Calif.

Fuel additives will be critical when meeting the challenges of reformulated fuels and cleaner engines in the decade ahead.

In the U.S., Europe, and Japan, valve deposit control additives will have a key role, particularly when modern engine designs are involved.

The Senate and House of Representatives recently passed the Clean Air Act Amendments of 1990. The Environmental Protection Agency, California Air Resources Board (CARB), Northeastern States Coordinated Air Use Management (Nescaum), and various carbon monoxide nonattainment cities and counties are active in changing the recipe for gasolines.

One outcome of the Clean Air Act is the requirement that deposit control additives be added to reformulated gasolines as one of the means to minimize exhaust emissions over the life of a vehicle. In addition, California will be requiring that effective dosages of deposit control additive be added to all gasoline statewide in 1992.

The same stringent regulations that affect the fuel industry also have contributed to dramatic changes in engine design as well. Today's modern small displacement/high output engines with port fuel injection, tuned manifolds, high swirl/fast burn, and multiple valves are a far cry from the relatively inefficient gas guzzlers of the past.

New car owners have grown accustomed to the nice snappy performance, smooth idle, and excellent warm-up/drive-away qualities designed into modern fuel-injected vehicles.

But these advances in engine technology have not left the driving public with trouble-free vehicles.

The relatively simple carbureted engines of the past could sometimes tolerate rather heavy levels of deposits. In contrast, some of today's most advanced engine designs need to be kept almost deposit free to avoid problems and provide acceptable performance.

REFORMULATED GASOLINES

A reformulated gasoline is generally defined as a gasoline for conventional cars made from conventional gasoline blend components-including oxygenates. The blend specifications are modified to reduce the emissions resulting from the use of the fuel.

Currently, eight U.S. oil companies have introduced reformulated gasolines, mainly in metropolitan areas having the poorest ambient air quality. ARCO was first, and so far appears to have had the biggest impact in the marketplace. Its reformulated gasoline, named Emission Control-1, or EC-1, was first introduced in September 1989 in the South Coast Air Basin in California. It is an unleaded product replacing a leaded regular grade and restricted to use in leaded fuel vehicles. ARCO has recently introduced a new EC premium that it claims reduces aromatic emissions, hydrocarbon emissions, carbon monoxide, and benzene.

Not long after EC-1, several other companies entered the marketplace with reformulated products. These are listed in Table 1. Each marketer claims a different amount of emissions reduction with the majority of the benefits attributed to the lowering of the vapor pressure and from the addition of 1.0 to 2.5 wt % oxygen to reduce CO. A few of the reformulated products have reduced amounts of aromatics including benzene. Olefin levels are reduced in the ARCO, Diamond Shamrock, and Conoco products by implementing a bromine number specification.

Controversy exists over the extent of the reformulation in some cases, but there is general agreement that these products are at least starting points on the road to cleaner air. Some of the products replace leaded regular gasoline with an unleaded product, thus claiming a lead reduction. Some have reformulated a premium unleaded, and some have introduced reformulated product into two or all three grades of their gasolines in limited geographical areas.

Chevron has taken this a step further with its reformulated Chevron Supreme unleaded gasoline. This gasoline has been introduced into the critical Southern California markets.

In addition to reducing vapor pressure and adding an oxygenate (methyl tertiary butyl ether), Chevron carefully tailored the distillation curve of the new premium unleaded to improve driveability and further reduce exhaust hydrocarbon emissions. The gasoline provides smoother performance during engine warm-up.

AUTO/OIL IMPROVEMENT PROGRAM

There are potentially many ways to change or reformulate gasoline to achieve lower emissions. Three major U.S. automobile manufacturers and 14 U.S. oil companies are conducting the most comprehensive fuels research program ever as part of the auto/oil air quality improvement program. Their objective is to determine the potential reduction in total vehicle emissions (exhaust, evaporative, and running losses) and estimate the resulting improvements in the air quality from gasoline reformulation and/or reformulated methanol fuels.1

The auto/oil program is expected to play a leading role in helping define the fuel or fuels of the future. In addition to having an impact on total vehicle emissions, reformulated fuels may contain fewer deposit precursors. With gasoline subject to continuing change, it has become increasingly important to consider the role of gasoline composition and of deposit control additives for achieving optimum engine cleanliness.

COMPOSITION AND DEPOSITS

Deposit levels have always been affected by fuel composition. In the critical fuel metering areas (carburetors in the past and now fuel injectors) olefins, diolefins, high boiling components, and heteroatom compounds (nitrogen containing, sulfur containing, oxygen containing) have been shown to be harmful. Research indicates that ethanol added at 10% to form gasohol increases deposits in carburetors but not in port fuel injectors.2 3

The same components have been linked to increased deposits in the rest of the induction system too, especially in the intake ports and valves. Many of these deposit precursors come from the fluid catalytic cracking (FCC) unit of a refinery. These blending streams can be especially severe if proper sweetening or after-treatment processes are lacking.4

It is routine industry practice to blend increased amounts of FCC fractions into laboratory test gasolines to increase deposit levels and test severity. Fig. 1 shows two examples.

In the combustion chamber, high molecular weight aromatic hydrocarbons are one of the base fuel factors affecting deposits and the resulting octane requirement increase (ORI). This correlation is graphically illustrated in Fig. 2.

Note that most of the gasoline compositional variables being studied in the auto/oil program have been shown to impact deposit levels. What level of engine cleanliness will be required in the future?

DEPOSIT CONTROL ADDITIVES

Fuel additives are a large group of chemicals that enhance performance or correct problems in fuel. Additives were first used in commercial gasoline in 1923, and many have been introduced since then. Typical types of fuel additives are oxidation inhibitors to control gum formation, corrosion inhibitors to prevent rust, and metal deactivators to inhibit gum formation that is catalyzed by some metals, primarily copper.

CLEANLINESS ADDITIVES

Cleanliness additives, generally categorized as either detergents or as deposit control additives, were developed to clean up and keep engines clean. Gasoline detergent, used to help keep clean and clean up carburetors and port fuel injectors, are generally low molecular weight amines and amine carboxylates.

These additives were first introduced in 1954 by Chevron. They are analogous to detergents in water. They are surfactants that successfully compete with deposit precursors for engine surfaces and also help disperse deposits, which allows the carbonaceous deposit to be carried away. These generally perform adequately in the carburetor and on fuel injectors, but most have a negative influence on intake valve deposits.

Deposit control additives not only remove carbon deposits from port fuel injectors (PFI) or carburetors, but also control deposits on intake valves, manifolds, and ports. High molecular weight polybutene amines, developed by Chevron Research & Technology Co. (CRTC) in 1970 as the first commercial deposit control additive, clearly demonstrated this ability. Polybutene amines were joined in 1980 by polyether amines which were developed specifically for unleaded gasolines.

DEPOSITS AND PERFORMANCE

The industry has a wealth of information about the effects of deposits in older designed carbureted engines. Deposits in the carburetor throttle body and air bleed areas cause driveability problems (such as hesitation, surge, and stalls), increased fuel consumption, and increased exhaust emissions, especially at low speeds and idle.5 Manifold hot spot deposits cause driveability problems.

Intake port and valve deposits can cause a loss of power by restricting air flow at full throttle and can increase HC and NOx emissions, especially at part throttle. Deposits on the valve seat/valve face can cause poor sealing, leading to rough idle and, in more extreme cases, to valve burning.6

Combustion chamber deposits cause ORI which has always been a direct constraint on engine performance. Light knock is not harmful but it is easily perceptible to the driver and a major source of dissatisfaction. Heavy knock can cause engine damage, especially at high engine speeds.

FUEL INJECTOR DEPOSITS

In 1985, a major deposit problem arose in the U.S. when many car manufacturers discovered that minute quantities of deposit were forming in the fuel metering orifices of newly introduced port fuel injectors. These deposits upset the spray pattern and restricted fuel flow which seriously influenced driveability and performance. Fuel economy and exhaust emissions suffered too, although these were less noticeable to the driver.

To control PFI deposits, some gasoline producers introduced deposit control additives, but many others simply increased the less expensive carburetor/PFI detergent concentration in their products. Unfortunately, most of these detergents decompose and form carbon deposits on the hot intake valves. This may be contributing, in part, to the current intake valve deposit/driveability problem in the U.S.8

DEPOSITS AND DRIVEABILITY

Today's major deposit problem is related to relatively low levels of deposit on intake valves. BMW first reported this problem publicly in the U.S. in 1986. Studies by BMW showed that noticeable performance degradation occurred during warm-up when small levels of deposit formed on the intake valves. To help solve this problem, BMW took the lead in pushing for greater use of deposit control additives in gasoline.

BMW accomplished this by way of an extremely successful cooperative approach with the U.S. oil industry. BMW developed its 10,000 mile vehicle test to evaluate a gasoline's propensity to form intake valve deposits, set passing standards, and, starting 2 years ago, permitted the oil companies to use the BMW name and test results in their gasoline advertising.9 Oil companies worldwide recognized the need and accepted the test, thus obtaining another means to differentiate themselves from each other.

German vehicle manufacturers (including BMW) had taken a similar approach even earlier in Germany. Responding to a pervasive driveability problem that they linked to intake valve deposits in 1983, they developed and successfully positioned the M102E 60-hr engine test as an unofficial standard for measuring a gasoline's effect on intake valve deposits. The test continues to be better defined and has already become a widely accepted hurdle which many European gasolines are now required to pass.

French vehicle manufacturers are doing likewise. In 1989, they began issuing guidelines requiring the use of a battery of engine tests for finished gasolines containing additives. To gain "French Constructors" approval and to meet requirements for retail gasoline labeling, the following tests must be passed:

Renault R-5-carburetor deposits

Peugeot 205 GTI-PFI deposits

Mercedes Benz M102E-valve and crankcase deposits

Opel Kadett-intake valve deposits

Renault F2N/F3N-ORI

Petter W1-bearing wear

Similar problems have also occurred in Japan. In 1987, when super-high octane unleaded premium gasoline was produced and heavily marketed, the higher boiling mid-range of the gasoline caused driveability problems in some of the most advanced engines, especially when coupled with intake valve deposits. The oil companies responded by changing the distillation characteristics and by greater use of deposit control additives in the premium grade to control the valve deposits.

Both North America and Europe have widely adopted deposit control additives in all grades of gasoline. We would anticipate that use of deposit control additives will increase in Japan as well, because the need for cleaner engines, cleaner air, and better performance continues to grow.

CRTC PROGRAM

A large program was conducted at CRTC to determine the effects of intake valve deposits on warm-up driveability. Over 400 tests were completed covering a broad range of fuel volatility, ambient temperature, and valve deposit level with U.S., Japanese, and European vehicle types. Some vehicles were very sensitive to deposits, while others were not. A very strong negative interaction between deposit level and fuel volatility was found which indicates vehicles could be extremely sensitive to gasoline volatility if intake valve deposits are allowed to form. Many of the vehicles did not have acceptable driveability when deposits were present.

DEPOSITS AND EMISSIONS

The driving force behind the European requirements for additives has mainly been the need for improved performance. In Japan, it appears that performance needs have provided the main impetus so far. But as noted in the following, exhaust emissions concerns will also be a factor in increasing the need for deposit control additives. In the U.S., the need exists for improved performance, but regulatory pressures to achieve lower exhaust emissions are beginning to play a very large role as well.

Over the years, a lot of research has been conducted to determine the effects of deposits on emissions. Many times, it has been conclusively shown in both laboratory engines and in vehicles that deposits increase emissions and that deposit control additives help prevent any increase in HC, CO, and NOx by preventing the formation of deposits. Results from one test are shown in Fig. 3. Vehicles using gasoline with deposit control additives had lower tailpipe emissions than cars using gasolines without cleanliness additives.

This study consisted of simulated EPA durability tests on 30 cars selected from five 1978 and 1979 models.10 But what about today's cars, and those of the future operating on reformulated gasoline? Data exist showing that PFI deposits increase exhaust emissions, but very little data are available showing the effect of intake valve deposits on emissions in late model vehicles.

CRC VALVE DEPOSIT WORKSHOP

At the Coordinating Research Council (CRC) workshop on intake valve deposits held in San Francisco in August 1989, two vehicle manufacturers, BMW and Toyota, shared recent data showing that induction system deposits can increase exhaust emissions and impair warmup driveability.

Their emissions results are presented in Figs. 4 and 5. BMW measured emissions on six vehicles before and after physically removing deposits throughout the induction system. Toyota used a gasoline additive to clean the deposits. Similar conclusions were drawn from both experiments-low levels of intake valve deposits can increase exhaust emissions compared to (essentially) clean valves. This conclusion was supported by other original equipment manufacturers in prepared statements."

At the same workshop, Texaco presented data from a 36-car field test using 1989 model vehicles. After 25,000 miles, no difference in exhaust emissions was measured between one half of the fleet which had a moderate level of intake valve deposits (average CRC deposit rating of 6.8) and the other half of the fleet which had a light level of deposits (average deposit rating of 8.2).

These results are consistent with the vehicle manufacturers' data and conclusions described in the preceding. It appears that some engine designs can be so sensitive that even a very low level of deposits on the intake valves is enough to increase emissions and impair driveability. Other designs are less sensitive. Comparing a moderate level of deposits to a light level of deposits may not be enough to observe an effect.

It is certainly possible to keep the intake valves of modern engines essentially deposit-free. Fig. 6 illustrates this fact using taxi cab field test results; while Table 2 makes the point by comparing exhaust emissions of used vs. new intake valves:

A BMW known to be sensitive to deposits was operated on marketplace unleaded regular gasoline containing a polyether amine (PEA) deposit control additive for 86,500 km in typical city/suburban driving service.

At the end of the test, the exhaust emissions were measured and the valve deposits rated and weighed. The valve deposits rated 9.5 on the CRC scale and weighed only 8 milligrams. Tests conducted in accordance with the 1978 Federal Test Procedure confirmed the obvious, that there was no effect on the exhaust emissions.

COMBUSTION CHAMBER

Combustion chamber deposits are also expected to be an issue in the future as we are required to achieve lower emissions levels and higher fuel economy. General Motors has recently challenged the oil industry to develop nonmetallic additives to prevent deposits in the combustion chamber. This would allow higher compression ratios to be used with a resulting boost in performance and fuel economy.12

Another incentive to reduce combustion chamber deposits is to achieve lower exhaust emissions. There are a few references in the literature which report that NOx is reduced when combustion chamber deposits are removed. This is attributed to the influence the deposits have on regenerating heat, thus increasing the charge temperature. More work is in progress. It is probable that clean combustion chambers will be helpful for achieving minimum NOx and HC emissions in the future.13

If that is the case, one way to achieve this is a clean fuel together with a clean deposit control additive which does not contribute to combustion chamber deposits. Combustion chamber deposit weights from three different laboratory test engines using a typical unleaded regular gasoline containing polyether amine additives are shown in Fig. 7. Cylinder head deposits ranged from 325 mg in one of the engines to 700 mg in another, as shown by the data points.

Also shown are results using an extremely clean alkylate fuel consisting almost exclusively of isoparaffins with the same amount of PEA additive. Note that the combustion chambers are almost deposit free with this combination in both the single-cylinder research engine and the 1.6 1. 1-4 carbureted engine.

The third engine tested, a 2.2 1. 1-4 PFI turbo-charged design, showed slightly higher deposit levels, but still well below that obtained with typical unleaded regular gasoline. Because the deposit response can differ depending on engine design, the fuel industry, additive suppliers, and the engine designers need to work together to achieve optimum engine cleanliness.

ADDITIVE DEVELOPMENT REQUIREMENTS

Many important criteria need to be considered when developing a new deposit control gasoline additive. To ensure that additives achieve their goals without any harmful side-effects, several lengthy test phases are necessary.

After testing during chemistry development, potential additives must undergo exhaustive laboratory engine performance studies to verify that an additive performs as designed and creates no unexpected problems in other engine areas.

The final and most important test is actual field use. For the past 12 years, San Diego Yellow Cab has been an integral part of CRTC's ongoing field test program to study the performance of gasoline additives. An average of 200 cabs using a variety of chemistries continually run 80,000 km tests. Researchers then carefully examine the cabs for any impact on engine condition. This type of effort is time consuming and expensive but thorough testing is the only realistic choice. We have logged over 125 million km of controlled field testing to date.

OUTLOOK FOR ADDITIVES

Ongoing regulatory, engineering, and marketing needs guarantee that the use of additives will have global growth in coming years. Many of the changes affecting oil refiners are international in scope. Removing lead from fuel is a fact. Japan has converted to unleaded gasoline and Europe has begun a similar switch.

Improved fuel economy and emissions control are major goals in many countries, and high-precision engines are increasingly used world-wide.

After detergents first appeared, the industry considered cleanliness additives helpful but not absolutely necessary. Now deposit control additives are vital to help the oil industry enhance gasoline quality.

Undoubtedly, continuing changes in engine design will create continuing changes in the role for additives. In this situation, the additive developer's ability to provide effective solutions is more crucial than ever.

REFERENCES

1. SAE Government/industry Meeting, May 1-4, 1990, Washington, D.C.

2. Gibbs, L.M., and Richardson, C.E., "Carburetor deposits and their control," SAE Paper No. 790202.

3. Benson, J.D., and Vaccarino, P., "Fuel and additive effects on multiport fuel injector deposits," SAE Paper N. 861533.

4. Dimitroff. E., and Johnston, A., "Mechanism of induction system deposit formation," SAE Paper No. 66074.

5. Hall, D.W., and Gibbs, L.M., "Carburetor deposits-are clean throttle bodies enough?" SAE Paper No. 760752.

6. Gething, J.A., "Performance robbing aspects of intake valve and port deposits," SAE Paper No. 8721156.

7. Dumont, L.F., "Performance mechanisms by which combustion chamber deposits accumulate and influence knock," SAE Transactions, Vol. 5. 1951.

8. Taniguchi, B.Y., Peyla, R.J., Parsons, G.M., Hoekman, S.K., and Voss, D.A.. "Injector deposits the tip of intake system deposit problems," SAE Paper No. 861534.

9. Bitting, W., Gschwendtner, F., Kohlhepp, W., Kothe, M., Testvoet, C., and Ziwica, K.H., "Intake valve deposits - fuel detergency requirements revisited," SAE Paper No. 872117.

10. Lewis, R.A., Newhall, H.K., Peyla, R.J., Voss, D.A., and Welstand, J.S., "A new concept in engine deposit control additives for unleaded gasolines," SAE of Japan. Paper No. 830938.

11. Proceedings of the CRC Workshop on Intake Valve Deposits, Aug. 22-24, 1989, San Francisco.

12. Colucci, J.M., "Automotive fuels for the 1990's - challenges and opportunities," Mar. 9, 1989.

13. Liekkannen, H.E., and Bachman, E.W., "The effect of leaded and unleaded gasolines on exhaust emission as influenced by combustion chamber deposits," SAE Paper No. 710843.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.