How much energy is lost in brakes

Braking in automobiles is very frequent, particularly in an urban built-up environment. Huge amount of energy is lost as heat as a result of braking. Even on a highway, braking event can mean loss of huge amounts of energy. Consider for example the sports car Regera, which has a combined horse power of more 1500 hp. It can reach speeds of over 300 km/hr. If the car is brought to a complete from 300 km/hr in the shortest possible time, than it has been estimated that during the braking process, an average of 1 MW power is dissipated. The process involves conversion of kinetic energy of the car into heat energy through friction on the brake plates. As a result the temperature on the brake disc can increase to several 100 degrees centigrade.

Imagine a normal size sedan car. The curb weight of such cars is around 1000 kg. Imagine the total weight with the occupants to be 1200 kg. Lets assume that the car is moving a 70 mph. From this data we can work out how much kinetic energy the car has.

We know the formula

Kinetic Energy = 1/2 mV^2

Velocity 70 mph = 31.29 m/s


Kinetic Energy = 0.5 x 1200 x (31.29)^2

Kinetic Energy = 587438.46 Joules = 587.438 KJ

Or in other words, when we bring a car running at 70 mph to a complete halt, we have wasted 587.43 KJ of energy. To put this into perspective, this energy is equivalent to 587.3/3600 = 0.16 kWh. Or in other word a 160 W TV could have run for an hour on the energy lost in the braking process.

Brake Disc Heat Loss

Brake Disc Heat Loss

From this value we can also work out how much quantity of diesel is equal to energy lost. The calorific value of Diesel is approximately 44800 kJ/kg. This means 587/44800 = 0.013 kg or 13 gms of Diesel equivalent energy was lost as heat.

While for some, these might be insignificant values, however they become huge numbers over the lifetime of the car. Therefore regenerative braking feature is an important to evolve for having sustainable transport. Most modern electric cars have this feature. Not only can regenerative braking be utilized when trying to bring the vehicle to a halt, but it can also be used when descending downhill. Modern cars are being made out of more and more composite material. This has reduced their weight to a significant degree making cars more efficient. A lighter car would lose much less energy during braking process.

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