Description:
Vehicle speed limiting sets the maximum speed of the truck. Vehicle Speed Limiting cuts off the fuel to the injectors when the truck exceeds the programmed speed. With the release of NOV95 Personality Module Software, VSL can be exceeded using cruise control if the High Cruise Control (HCC) Set Limit parameter is programmed higher than the Vehicle Speed Limit parameter (See page 32).
Available:
All electronically controlled on-highway engines covered in this handbook. The ability to program HCC Set Limit above VSL is available on heavy duty engines only.
Range:
Advantages:
Using the VSL feature allows you to “gear fast – run slow” to optimize the balance between fuel economy and performance or simply specify for the best fuel economy. Setting VSL allows the truck operator to consistently operate the engine and truck at fuel efficient and safe vehicle speeds. Without VSL, most trucks can be capable of very high vehicle speeds, due to GFRS/GFRSS spec’ing methods.
Disadvantages:
VSL can be a hard limit that allows no fuel until the vehicle slows to a speed below the programmed limit. This makes the common practice of “running at hills” very difficult. Soft VSL (See page 26) may alleviate this problem if the driver expects to operate at or above VSL on downhill runs.
Recommendations:
Gearing for best fuel economy.
If engine RPM at Vehicle Speed Limit is less than or equal to Peak Toque RPM + 100, select another axle ratio or transmission to increase engine RPM. Otherwise driveability may be adversely affected. Use Design Pro to check for gradeability at cruise speed to ensure it is 1.0% or greater. Also, use Design Pro to check for gradeability at peak torque to ensure it is 1.8% (1.5% minimum).