In this article we look at the feature of cruise control in cars and how they work.
What Is Cruise Control?
Cruise control is a control device located on or around the steering wheel of the driver. Its core function is to either increase or decrease the speed of the car while allowing the driver to remove his foot off the accelerator. There are also slight differentiations in the physical characteristics and appearance of the buttons or levers in various cars. However, the following functionality is common on many cars with the cruise control feature.
On/Off – These two buttons are not quite common in many cruise control cars. The obvious reason for this is because of their somewhat redundant functionality. Pressing the On button only prepares the car for another button which the driver should press soon thereafter. The off button disengages the cruise control which is ordinarily disengaged by a simple press of the brakes.
Set/Accelerate Button – Once again, this functionality can also be specific to the vehicle make. Once the required vehicle speed is reached, pressing this button will engage the engine to maintain this speed…hence the term ‘Cruise Control’. Depending on the type of cruise control in your car, holding down or tapping this button will cause your car to accelerate without the need of having to press the gas.
Resume button – By hitting the brake pedal, the cruise control is immediately disengaged. The resume button will cause the car to revert back to the previous speed setting. For instance, if your cruise controls speed setting was set for 55 miles, after hitting the brake pedal and pressing the resume button thereafter, the resume button will accelerate your car until it reaches 55 miles. This function does not apply for the ‘Off’ button as the ‘Off’ button will completely disengage the cruise control.
Coast button – This causes the car to decelerate almost as if you took your foot off the accelerator. Easy does it. However, for quick deceleration, it’s best to press the brake pedal.
Remember: The cruise control can be disengaged by either pressing the clutch or brake.
How Does It Work?
It’s obvious that the speed of the car is controlled by the throttle position on the accelerator. The deeper down the accelerator is pressed, the more fuel being burnt and the faster the car moves. Cruise control allows for this to be achieved via an actuator. In internal combustion engines, the throttle valve limits the amount of air into the engine and effectively controls the power and speed of the car. With Cruise Control, the actuator is connected by a cable to the throttle valve and the speed of the car can be controlled instead of pressing a pedal.
The synchronous functionality of the gas pedal and the actuator is achieved through a pivot that controls the throttle valve. In Cruise Control, the actuator serves a dual purpose. It adjusts the throttle through the cable connected to the pivot and also pulls on the gas pedal causing your car to accelerate. This is perhaps why you may notice your gas pedal move up and down when your cruise control is engaged.
Two Common Cruise Control Systems
Proportional Control: This type of cruise control adjusts the throttle position in accordance to the ‘Error”. The Error is the difference between the required speed and the actual speed of the car. The noticeable characteristic of this function is that the acceleration speed of the car may decrease as you approach your required speed. For instance, if you were travelling at 45 miles and your required speed was 60 miles, your Error would be 15miles (i.e. 60 -45). You may notice that when the car initially accelerates from 45 to say 55, the speed of the car would be a lot faster than what it would be in comparison to the trip from 55 to 60.
Proportional-Integral-Derivative-Control (PID control): To make understanding this concept easy, it’s perhaps best if we analyze the word structure of the control. Proportional control has already been defined above.
Integral – The Integral factor works on actual and intended distance. This is known as the ‘Distance Error’. The cruise control computer box is able to determine the distance the car would have achieved had it been traveling at the desired speed over a set period of time. Therefore, although the proportional control of the system may tend to slow the car down after a while, the integral control will open the throttle more and accelerate the car so as to avoid an increase in the distance error.
Derivative – Always remember that speed is derived from acceleration. This control responds to engine strain as a result of change to the driving terrain. A hill, for instance, will cause the car to slow down given its current throttle position and internal combustion demands. The derivative factor will increase the throttle position to work in conjunction with the proportional and integral controls in overcoming the demands of the engine.