Theory of Operation
The Cruise Control on your ISM engine operates like Cruise Control on an automobile. The operator sets a vehicle speed and the engine provides the fueling commands to keep it constant. Several cab accessory switches are used to set, resume, coast, and accelerate. Droop settings are used to tailor the feature to provide the desired performance, fuel economy and speed control.
Cruise Control Maximum Vehicle Speed
This parameter can be set to limit road speed while the operator is using Cruise Control.
Setting the Cruise Control Maximum Vehicle Speed slightly higher than the Accelerator Maximum Vehicle Speed provides an incentive for drivers to operate in Cruise Control. This can have a positive effect on fuel consumption.
If Cruise Control Maximum Vehicle Speed is set 2-4 MPH below the Accelerator Maximum Vehicle Speed the operator will "feel" like he has a little extra power when the accelerator pedal is needed for passing situations.
Cruise Control Auto Resume
When this feature is active the engine will resume "cruising" after a clutched gear change is made. It should be setup with the operator's preference in mind. It is important to remember that when this feature is active, tapping the clutch pedal will still disengage Cruise Control. The control system can differentiate between tapping and a normal clutched gear change. Cruise will only resume if the engine is sure the operator used the clutch for shifting purposes. As always, tapping the brake or turning cruise off will also disengage the feature.
Optimal Droop Settings
Droop settings are used to control how Cruise Control performs on grades. Droop is defined as the rate of change when approaching the maximum control limit.
The Lower Droop setting is used to control how your Cruise Control operates on downhill grades. It controls fueling as you drive down a grade. It may be set be at 0, 1 ,2, or 3 MPH depending on desired performance and speed control. If you wish to limit fueling at your maximum vehicle speed it should be set to 0 MPH. Settings of 1, 2, or 3 MPH will allow the engine to give extra fueling 1, 2, or 3 MPH above the maximum vehicle speed. This lets you increase maximum vehicle speed in anticipation of the next grade, but reduces speed control. Higher settings result in better performance, but less speed control.
The Upper Droop setting is used to control how your Cruise Control operates on uphill grades. It controls fueling as you drive up a grade or accelerate to your maximum vehicle speed. It may be set be at 0, 1, 2, or 3 MPH depending on desired performance and economy. If you desire maximum performance it should be set to 0 MPH. This means your engine will receive maximum fueling until it reaches the maximum vehicle speed setting. Settings of 1, 2, or 3 MPH will limit fueling at 1,2, or 3 MPH below the maximum vehicle speed. This has positive impact on fuel economy, but will slightly reduce maximum vehicle speed during hill climbing. Higher settings result in better fuel economy, but reduced performance.
In summary, Upper Droop settings provide a tradeoff between performance and fuel economy and Lower Droop settings provide a tradeoff between performance and speed control. Effective tailoring of the Lower and Upper Droops should result in better fuel economy, especially for vehicles that travel over hilly terrain.
Upper Droop Performance vs. Fuel Economy
Lower Droop Performance vs. Fuel Economy
No Fuel Economy Benefit
Best Speed Control,
No Performance Benefit
Slight Fuel Economy Benefit
Good Speed Control,
Slight Performance Benefit
Moderate Fuel Economy Benefit
Fair Speed Control,
Moderate Performance Benefit
Best Fuel Economy Benefit
Reduced Speed Control,
Cruise Control Automatic Engine Brake
This feature allows the engine to automatically activate the Engine Brake. If the vehicle exceeds the Cruise Control Speed Delta parameter and the engine brake switch is on, the engine brake will automatically be activated to slow the vehicle. When this feature is active the engine has better vehicle speed control and can even reduce vehicle service brake wear. Â