# Coasting to a stop vs regenerative braking



## bwilson4web (Mar 4, 2019)

One of the 'hyper miler' techniques is to coast to a stop. A manual transmission clutch makes this trivial. However, automatic transmission and electronically controlled shifters can be more challenging. But how does it work on a real world course?

Here is one of my favorite test loops:








There are four possible traffic stop locations:

peak - up slope approaching and down slope after stop. This works for coasting because the kinetic energy approaching brings the car to a stop with maximum potential energy. On the backside, the potential energy returns as kinetic energy.
halfway down - the down slope increases the car velocity requiring braking and/or regenerative energy capture. Once stopped, the continued down slope helps accelerate the car. Regardless, the down slope potential energy to a stop requires active energy capture.
bottom - the car potential energy requires energy storage or brake shoe heating. Then taking off, the car has to add energy for both kinetic and potential energy. This is the worst case.
halfway up - the slope allows coasting to a stop. But resuming speed require energy for both kinetic and potential energy.
So only one case, stopping at the top is energy efficient without storage or brake shoe heating. The other cases to a greater or lessor extent gain little by coasting to a stop.

I have easy access to this test loop and can easily run the benchmarks. Do we need to do this exercise when the physics and math indicate there is only one case, stopping at the peak, saves significant energy?

The easy solution is to use one-pedal driving with regenerative braking. Descending an 8% grade with the foot off the pedals, the Tesla will slow to a stop. The captured energy is available to accelerate the car.

Bob Wilson


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