ROAD RATING OF 100+ O.N. FUELS IN HIGH COMPRESSION RATIO CARS

Various investigators⁽¹⁾ have recognized that difficulties exist in obtaining octane ratings in the standardized single-cylinder laboratory CFR engine above 100 O.N. Efforts are being made by cooperative groups (such as ASTM) and individual laboratories to solve some of the problems. In the meantime, at the Sun Automotive laboratory, these developments have been paralleled by looking at the road rating problems using some “future design” experimental engines at a nominal 12:1 C.R. level. These engines were 1956 production engines of three different types modified to represent the high compression engine of a few years hence. The modified engines were installed in 1956 production cars.

Recognizing that the ability of a petroleum refinery laboratory group to anticipate future engine design features is subject to some criticism, it was still felt that the results might well be helpful in defining future problems. Actually previous experience has been encouraging along this line in that results obtained from engines modified to 10:1 C.R. in1954 have been substantiated closely by 10:1 C.R. 1957 production cars.

Questions have been raised as to whether cars at the 12:1 C.R. level will be knock-limited or whether other abnormal combustion phenomena such as surface ignition, rumble, etc., will confuse the knocking reaction. Experience at the Sun laboratory has led to the feeling that surface ignition and/or rumble can be a limiting factor at this level, depending on how deposits are accumulated. However, the three 12:1 C.R. cars used were all knock-limited under the suburban driving build-up procedure employed. Using a fuel-oil combination typical of commercial products anticipated for the next few years, there were no evidences of surface ignition or rumble either during the nominal 5000-mile build-up period or during the rating period.

While the limited amount of work to date does not justify drawing firm conclusions, results have indicated that:

1. (a)
   Fuels up to about 105 Research O.N. (or about 110 O.N. road rating) can be rated satisfactorily in 12:1 C.R. cars using either Modified Uniontown or Modified Borderline procedures.
2. (b)
   Reproducibility of road rating results on fuels in the 100 to 105 Research O.N. range is comparatively good; about as good as on fuels below 100 O.N.
3. (c)
   Relationships can be established between road and laboratory ratings which either compare with or extend previous correlations in the 90 to 100 O.N. bracket.
4. (d)
   The clarity of the knock, time consumed per rating, and individual operator's ability to rate are all about equal to those in the area below 100 O.N.

So much for the over-all relationships observed. The remainder of the paper will discuss in some detail the approach used and some of the more interesting fuel-engine relationships found.