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How Does My Car’s Cruise Control System Actually Work?

By Kimberlea Buczeke - March 27th 2017
3 comments

cruise control
Image Courtesy of Shutterstock

Cruise control is a great feature on modern cars – especially for long road trips. You can turn it on, set your preferred speed, (under the speed limit, right?) and just cruise. You don’t have to worry about maintaining your speed, and you can enjoy a smooth driving experience.

But many people still don’t understand exactly how cruise control systems work – how can my car accelerate if I don’t step on the gas? How can it maintain a constant speed? In this article, we’ll break down the basics about modern cruise control systems, and explain the system to you in detail.

How Does My Car Accelerate When Using Cruise Control?

 

Your car accelerates during cruise control in the exact same way it would when you press the gas pedal. Surprised? Here’s how it works.

 

Modern cars have multiple actuators connected to the throttle. One of these is connected to the gas pedal – and pressing the pedal opens up the throttle, increasing airflow to the engine and speeding up your car.

 

Cruise control systems use an electronic actuator to do this, rather than a pedal. When you hit “ACCEL” on your cruise control system, your car responds by opening up the throttle in the same way that it would if you pressed the gas pedal manually.

 

How Does A Car Using Cruise Control Maintain A Constant Speed?

 

The answer is – computers! Each car with cruise control has a basic computer under its dashboard, usually known as a PID (Proportional-Integral-Derivative Control) system. Don’t worry, we won’t get into any serious math here. Here’s how it works.

 

Proportional control is what keeps your speed close to the number that you set on your cruise control system. This is achieved by measuring the error – how fast your car is moving relative to the speed that’s been set on your cruise control system.

 

If your car is moving relatively slowly compared to the speed set on your cruise control system – say, 50 mph instead of 60 mph – your proportional control will begin accelerating quite a bit.

 

But as your vehicle moves more closely towards your intended cruise speed, the acceleration will slow down proportionally – the acceleration of your car at 55mph will only be about 50% as intense as it was at 50mph, and so on. As you approach your desired speed, your throttle will slow down, which helps keep your speed at the correct setting on your cruise control system.

 

The integral factor is a way in which the speed of a car is calculated over time. This system measures the actual speed of your car over a certain distance and then measures the intended speed of your car over the same distance.

 

Then, these two measurements are compared. If your car is still not traveling at the correct speed, the integral factor will augment proportional control to provide more acceleration. The integral factor is the main way that vehicle speed is kept steady over hilly terrain.

 

The derivative of these systems is acceleration, which is also closely monitored. Cruise control systems monitor current acceleration, reacting to changes in speed. This includes both speeding up and slowing down. Here’s an example.

 

If you begin going down a large hill, the cruise control system will adjust accordingly, reducing the throttle position to maintain speed. In this way, the cruise control system prevents your car from accelerating too quickly – and overshooting your intended speed.

 

Conversely, if you go up a hill, the reverse occurs – the level of acceleration that is detected by the PID system sinks, and the system responds by opening up the throttle.

 

This system – together with the integral factor of your PID system – ensures that you have all the acceleration you need to maintain the perfect speed. Together, these three factors are behind the entire cruise control process and form the basis for all modern cruise control systems.

 

Cruise Control Isn’t Magic – But It’s Sure Nice To Have!

 

Hopefully, this article has been helpful at revealing some of the mysteries of cruise control.  All it takes to maintain a steady speed is some basic mechanics, a small computer, and a bit of math – and it all leads to a spectacular, steady, and smooth ride.

 

So next time you get out on a long, open highway, pop on your cruise control system and relax – knowing that you understand exactly what your car is doing to maintain speed.


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3 User Comments

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Hello Vince, The fuel mileage can vary quite a bit when you're not using cruise control depending on how you drive. The key to lowering fuel mileage is to avoid rapid acceleration or braking. Cruise control avoids these rapid speed changes and will lower fuel mileage compared to a person who is heavily accelerating or braking.
thanks for the article I didnt know how cruise control worked until reading this