How does cruise control function?

C

camtwister

Member
Howdy,

I have a question about an observation I've made when using cruise control in my manual E86.
It's not a problem or fault. I'm asking because my observation suggests that I misunderstand how cruise control works and I'd like to rectify that.

Here's an overview of how I think drive by wire functions.

- the accelerator setting is measured by a position sensor
- the ECU reads the position sensor
- the ECU uses this data to open the throttle butterflies accordingly, by motor
- a throttle position sensor feeds back to the ECU

When cruise control is activated:
- the ECU uses data from a road speed sensor & crank speed sensor and positions the throttle butterflies accordingly

When I activate cruise control, my car maintains a constant road speed, regardless of gradient.
All good.
However, the tachometer (and engine sound) indicate that the crank speed is also constant.

When travelling from no gradient to steep gradient for example, how does the vehicle stay at a set road speed without changing engine speed, according to my rev counter?

What am I missing here?
Thanks.
 
camtwister said:
When travelling from no gradient to steep gradient for example, how does the vehicle stay at a set road speed without changing engine speed, according to my rev counter?

What am I missing here?
Thanks.
Click to expand...
Because the only time engine speed would change is if you (or an automatic gearbox) changed gear. i.e. The engine is working harder up hill (burning more fuel) but does not rev faster for a fixed road speed in a given gear.
 
Correct, the gear ratio and final drive ratio are what links engine speed to road speed. These are constant for any gear, so speed is directly related to revs.

There is a bit of “slush” in an autobox, where going up hills (or accelerating) the revs will rise slightly ahead of the car picking up speed.

With cruise control on, you should notice a hardening of the engine note going up hills even though the revs don’t rise.
 
I press the button and it stays at the speed I was going.

I can then press a button to go faster, another button for slower and a third to stop it from going at a constant speed.

That's my basic understanding of it. Personally I can't stand it now that I have used it. I'd rather press the pedal and yes i tried it out on a journey from Scotland to England and back in both the z4 and the Lexus.
 
I use cruise control on motorways and in speed restricted 50,s etc.
its a useful thing, and great for giving your right foot a rest on a long motorway stretch...
 
Zedebee said:
Correct, the gear ratio and final drive ratio are what links engine speed to road speed. These are constant for any gear, so speed is directly related to revs.

There is a bit of “slush” in an autobox, where going up hills (or accelerating) the revs will rise slightly ahead of the car picking up speed.

With cruise control on, you should notice a hardening of the engine note going up hills even though the revs don’t rise.
Click to expand...

Of course. Thank you Zulu4 and Zedebee.
A constant road speed is maintained, defined by the relationship of engine speed and gearing.
Is the following correct?
When climbing a gradient and maintaining a consistent road and engine speed, the increase in energy required to overcome the gravitational potential, is generated by changing the fuel/air charge, which in turn changes the combustion pressure in the chambers.
 
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