Tracking efficiency through changes in Kinetic energy

 

Setting up the car for the race requires that both lap time and Watt-hours per lap, be considered.

At ProEV, we use changes in kinetic energy to quantify changes in efficiency, to give a more nuanced view of the effects of set up changes on efficiency.

We know the weight of the car and the speed it is traveling, so at any instant, we can calculate the kinetic energy.

We are also tracking current to the motor and voltage which gives us the energy (Watt-hours) going into the motor.

In a perfect frictionless world, all the energy going into the motor would become kinetic energy and the car would go that much faster.

Energy lost would be 0%.

 

Graph: Changes in kinetic energy -Energy loss vs. MPH

Graph of Energy loss vs. MPH

This is a graph from our world.

The orange line is speed.
The purple line is the percentage of energy that does not get changed into kinetic energy.

This energy is lost overcoming friction and drag.
 
The least amount of energy needed to overcome friction and drag is around 30% and this tends to occur at the lowest speeds (see point L).

At the highest speeds (over 100 MPH), as much as 70% of the energy is required to overcome friction and drag (see point H).

The jumps in energy lost to over 100% occur during braking.
Some energy is recovered through regenerative braking but the rest is being wasted through friction braking.

The 'Changes in kinetic energy' graph above is a useful tool in tracking changes in efficiency.

The biggest limitation is that it ignores what is happening in the batteries themselves.

The higher current from the batteries, the less total power (see Peurket's equation) which impacts efficiency.

Batteries also are not 100% efficient in recharge, so some efficiency will be lost there too.

This will be current dependant as well.

 
 

 

Consider efficiency
back to Lessons Learned menu
Regenerative braking efficiency

 


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