413

EVALUATION OF PARAMETERS

This section considers only the effect of fuel-air mixtures and ignition timing on exhaust emissions. The following section "Component Development" is concerned with the application of these results and describes how the engine components were modified for production release.

With the background emission data on the standard production engine it was possible to proceed with the evaluation of parameters. The approach taken was to examine each test mode for its contribution to the total emissions and to explore the factors which would reduce the emissions of each mode.

Table II shows that the two acceleration events in the cycle account for over 50 percent of the emission total. An examination of the manifold vacuum values showed these to be part load conditions. For this reason, it was possible to select lean mixtures for all cycle accelerations. Typical acceleration emission values with rich and lean mixtures are shown in Table III.

DRIVING MODE

TABLE III

ACCELERATION EMISSIONS AS AFFECTED BY

FUEL-AIR RATIO

During warm-up conditions, however, the choke system must operate to provide additional fuel for engine power and smoothness. Concentration on the conditions producing maximum emission contribution during warm-up showed that choke mixtures should be controlled near .075 F/A for the 15-30 acceleration mode of Cycles 1 and 2 in order to hold emissions for Cycles 1 through 4 at an acceptable level. A previous selection of choke mixtures from a "driveability" standpoint would have allowed a maximum choke acceleration mixture of 0.12 F/A during the first cycle with the following results:

ENGINE OPERATING PARAMETERS AT IDLE AND 40 MPH CRUISE ON STANDARD VEHICLE

CO

IDLE

CRUISE 40

RPM

500

1750

IGNITION TIMING

10° BTC

38° BTC

CO2

02

AIR FLOW
FUEL FLOW
MANIFOLD VACUUM

It seemed, for instance, if the engine could be made to carry a load at idle, similar to that existing at 40 mph, the emission level would also follow the same pattern. Accordingly, a small increase in friction load was provided by raising the engine speed at idle. Ignition timing at idle also was retarded to reduce the efficiency at which the friction loads were being carried. These modifications combined to increase the air and fuel quantities considerably at idle. This permitted firing and burning a leaner mixture at idle with an accompanying decrease in exhaust emission as shown in Table VIII.

TABLE VIII

FUEL-AIR RATIO

HC

CO

TABLE VI

To provide for the increase in idle airflow, a small hole was drilled in each throttle valve. One major advantage of this procedure was to retain the existing relationship of the transfer and spark vacuum ports to the throttle blade. During the development program, optimum hole locations were determined for best emission level and idle quality.

The major change related to the leaner idle mixture consisted of providing a slower rate of change of mixture with mixture screw turns. This was accomplished primarily by changes in the contour of the mixture screw.

Early development of CAP showed that operation of vehicles at an idle mixture level producing 1% CO was entirely feasible. (Current automotive engines controlled at idle by throttling alone operate at 5 to 9% CO.)

During this study of idle parameters it was of interest to determine the maximum manifold vacuum available with CAP idle timing and to compare the CO values obtained. The data obtained for three automatic transmission models at 600 rpm in neutral was as follows:

TABLE X

recommended for standard cars. This favorable result appears to be inherent with retarded timing.

Another important idle parameter was the allowable range of idle mixtures which could be tolerated with the assurance of meeting both emission and performance requirements. Production (non-CAP) carburetors were being accepted with a mixture range of ± 6% at idle with the intent that adjustments would be made when necessary in the production plant and in service. After a study of the various factors involved, the carburetor vendors supplying Chrysler were asked to produce with a mixture range of ± 3% at idle and to use reduced tolerances in the off idle and transfer flow rates. Standard mixture tolerances were satisfactory for all other conditions. The result, then, of maximum mixture limits at idle conditions is illustrated for the 318 cu. in. automatic transmission model in Table XI.

6 and 7 show the standard and CAP spark advance curves for two 1966 engines. In each case, the CAP curve employs a retard at idle. Maximum retard is employed in the low speed range for emission reduction consistent with the need for satisfactory driveability, power, and fuel economy. It will be noted that some small retard is employed in the mid speed range also. This was done to reduce the tendency for the engine to knock in a geographic region of low fuel anti-knock quality, especially regular grades.

The vacuum advance curve was likewise tailored to satisfy the emission and performance requirements of the engine.

FIGURE 6