I drive a petrol-powered Renault Grand Scenic III, and it’s not what you’d call overpowered in any way, shape or form. It has a handy display that lets you know when to change up and down, which I thought had to do with how many revs the engine was doing, but I noticed that at comparable speeds, but different inclines, the car wants me to change down when the going gets tough. Which makes sense, but how does it know that it’s struggling? It seems crazy to me that the combustion cycle could be monitored to such a degree.
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The position of the pedal tells it how hard the engine is trying to work. It compares this to the RPM and gets an idea of if you’re speeding up or trying to go uphill.
There are 3 basic input to the computer in the car (the ECU) that it can use to determine the appropriate time to shift gears. Engine speed (RPM), vehicle speed and throttle position.
There are systems with more complex inputs but generally the above 3 are the basics.
Older systems work on vacuum, the harder it pulls the more vacuum created. When the cars is struggling it has very little vacuum. The light can be a simple high or low vacuum.
Cars are controlled by things known as maps. A simple map is a 2D lookup table of precomputed values like a spreadsheet – with one parameter on the x and another on the y. An example would be the car knows how much fuel its currently using, and how fast the engine is going. With this it can plug that into two maps and work out engine power and engine efficiency. It can then look at another map and work out if for the given power requirements another engine speed gives better efficiency, this can be translated to a gear recommendation.
These maps are prepopulated by engineers in a test environment so the car never has to do complex combustion calculations. Choosing the correct maps to use is an art as well as a science – 5 dimensional maps are not uncommon, so it gets quite complicated for the engineer doing the calibration!
On a computer controlled car, the computer OS monitoring engine load, vehicle speed, and throttle position and tells the transmission when to shift.
On older cars they used mechanical versions, like governors to gauge speed & load, vacuum to measure load, and a kick down cable for throttle position.
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