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12-22 Edition 47
On-Highway Trucks
Engine Selection
Step 2A:
(Manual and Automated Manual Transmis-
sions) Verify the Engine selected in Step 1 can
get the Truck moving on the customer’s maxi-
mum grade.
Startability:
Is the ability of the truck to start moving on
the steepest grade the driver will face. The
equation below (S) calculates the
Theoret-
ical
Maximum Percent grade for the Engine
you selected in Step 1. To more accurately
predict Startability, ground conditions must
be considered. Subtract the effects of ground
conditions from (S) to determine the Maxi-
mum Predicted Startability.
S =
(Tc × R × M)
(10.7 × GW)
Tc = Fully Developed Torque (from Engine selected in
Step 1). See Engine Specification table.
R = The overall Driveline gear ratio.
Transmission (low gear) × Axle × Aux Trans ×Drop box
Omit Aux Trans and Drop Box ratios if not used
M = Tire Revolutions per mile. See Appendix I.
GW = Desired Gross Vehicle Weight or Gross Combi-
nation Weight.
10.7 = Constant.
Ground Conditions:
See Tire Pavement Factors table or
Subtract 6% for every inch of tire
penetration.
Maximum Predicted Startability
= S – Ground Conditions
Startability must be equal to or greater than the cus-
tomer’s maximum percent grade. If below Maximum,
Select an Engine of equal or greater HP with Higher Fully
Developed Torque from Engine Specifications table.
Rerun the Startability calculations to verify. Continue
to Step 3 for final Engine selection.
Step 2B:
(CX31 Automatic Transmission) Verify the
Engine selected in Step 1 can get the Truck mov-
ing on the customer’s maximum grade.
Startability:
Is the ability of the truck to Start moving on
the steepest grade the driver will face. For
an Automatic Transmission a simple equa-
tion cannot be used to predict Startability.
For the CX31, see the Startability chart
in Appendix G. It assumes starting on the
best concrete. To more accurately predict
Startability, ground conditions must be
considered. Subtract the effects of ground
conditions from (S) to determine the Max-
imum Predicted Startability.
S =
See Appendix G
Ground Conditions:
See Tire Pavement Factors table or
Subtract 6% for every inch of tire
penetration.
Maximum Predicted Startability
= S – Ground Conditions
Startability must be equal to or greater than the cus-
tomer’s maximum percent grade. If below Maximum,
Select an Engine with equal or greater HP and Higher
Fully Developed Torque from Engine Specifications
table. Rerun the Startability calculations to verify.
Step 3: Final Engine Selection.
In Steps 1 and 2 you
selected an Engine that had enough HP to main-
tain cruise speed and start on the maximum grade.
Before making the final Engine selection, factor
in application and driver habits. For example,
additional Torque may be required in mountain-
ous regions. Perhaps a smaller Engine can be used
to save fuel if the driveline gearing is modified.