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Edition 47 12-19
12
On-Highway Trucks
Engine Selection
ENGINE SELECTION
There are many factors that dictate the correct engine
size for a specific application. It’s possible to use a
mathematical approach to add up all the horsepower
demands to size the engine. However, it’s also impor-
tant to understand the application and driver habits. In
addition, understand how the customer’s other trucks
are configured. This critical customer knowledge is
used to verify or modify the results of the mathematical
approach to achieve the best engine selection for the
application.
Mathematical Approach Overview:
3 step process.
1) Add up all the HP loads at vehicle cruise speed.
Examples of HP demands are: Rolling Resistance,
Drag, Slopes, Road Surface, Driveline Losses,
Accessories, and Power Takeoffs.
2) Determine the Torque required to start fully loaded
on the customer’s maximum grade. This is called
(Startability).
3) Use your knowledge of the application to verify if
the Engine selected in steps 1 and 2 makes sense.
Step 1 Calculations
: Addup theHP required at the expected
vehicle cruise speed.
NOTE:
All the following formulas can be programmed
into a Spread Sheet for faster evaluation.
Rolling Resistance:
The HP required on a flat surface
to overcome Tire Squat. Since tires
are not perfectly round, (especially
when loaded) the Truck is climbing
a small slope just to move forward.
Pr =
(6.1 + (0.06 × MPH)) × Cp × GW × MPH
375,000
Cp = Tire Pavement Factors. See table below.
GW = Gross Vehicle Weight or Gross Combination
Weight required for the application.
6.1 = Constant.
0.06 = Constant.
375,000 = Constant.
MPH = Desired Cruise Speed (not maximum speed).
Tire Pavement Factors
Surface
Bias
Radial
LP Radial
Wide Base
Concrete
1.00
0.70
0.63
0.50
Cold Blacktop
1.20
0.85
0.70
0.60
Hot Blacktop
1.50
0.90
0.83
0.70
Hard Soil
1.75
1.00
0.98
0.85
Hard Gravel
2.00
1.20
1.13
1.00
Loose Gravel
7.50
1.70
1.63
1.50
Sand
12.00