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The use of lead-free gasoline in conventional passenger car engines poses some problems that are discussed in this paper. Under heavy-duty operation, severe exhaust valve seat wear may occur. This will eventually result in one or more valves remaining open with extremely high exhaust emissions. The combustion chamber deposits formed in the absence of lead are typically more carbonaceous. These deposits have a higher heat capacity than lead deposits and the result, after extended mileage, is higher octane number requirements for the engines operated on nonleaded gasoline. The use of aromatic blending stocks to increase the octane number of nonleaded fuels to approach the octane quality of today's leaded gasolines increases undesirable exhaust emissions. The amounts of phenol, benzaldehyde, and total aromatic aldehydes in the exhaust gas are directly proportional to the aromaticity of the fuel.

Single-cylinder engine studies show that severity of the test cycle used for deposit accumulation markedly affects the level of exhaust emissions obtained with stabilized combustion chamber deposits. These studies also show that the relative stabilized emission levels with nonleaded and leaded fuels vary significantly with the aromatic content of the base fuel. An extensive evaluation in three groups of passenger cars operated by their owners in normal service showed no significant difference between the stabilized emission levels obtained with commercial nonleaded and leaded fuels. A dynamometer engine test procedure has been developed that simulates short-trip, city-type operation. The accelerated cooldown procedure allows for rapid accumulation of test mileage. Using this dynamometer procedure, the stabilized deposit emission levels of a commercial leaded fuel and a prototype nonleaded fuel are compared.

RUMBLE is a type of abnormal combustion which may impose a limit on usable compression ratios if proper attention is not paid to fuel and lubricant factors. It is characterized by a low-frequency noise. This noise is much more likely to be present in engines which have been operated on light-duty schedules than in those which have been used in heavy-duty operation. Once deposits are present, rumble generally occurs at wide-open throttle and high engine speeds. Rumble does not appear to be confined to any particular type of combustion chamber, and increasing engine rigidity does not show promise of reducing the problem. Fuels with low deposit-forming tendencies help alleviate the problem. While not a complete cure, phosphorus fuel additives do a good job in reducing the incidence of rumble. This paper presents the results of work aimed at defining the problem, together with data illustrating the influence of various fuel factors.

The development of a successful fuel additive requires considerable effort, a large share of which must be expended on the effect of the additive on engine durability. Durability may be affected as soon as the fuel enters the fuel tank and the possibility of such good or bad effects continues until the exhaust gases have cleared the rear bumper. Some of the durability aspects which can be either improved or made more severe are listed, these include fuel system corrosion, carburetor and manifold deposits, combustion chamber and spark plug deposits, engine wear and general cleanliness and bearing, exhaust system and rear bumper corrosion. It is shown that of seven experimental additives tested in a fleet of passenger cars of one make operated under severe duty conditions, four additives decreased exhaust valve life while three increased exhaust valve life by from twenty-five to fifty per cent.