Search

Email

A     B     C     D     E     F     G     H     I     J     K     L     M     N     O     P     Q     R     S     T     U     V     W     X     Y     Z



  Mobiles  

  F7     F9

 Bookstore

 Tests

 Formulary




Electric Cars and Emergency Braking



With our series 'chemistry' we are already slipped into physics, but that does not matter, we still gain insights. And when we have enough chemistry-related topics, we are storing this part to 'theory of thermodynamics'. So let's go on . . .

The new generation of medium-weight electric cars is said to have a consumption of about 12 kWh under favorable conditions, as with a careful gas foot and without air conditioning or heating. What would be the equivalent in gasoline?

For the fuel properties you will find the caloric value 42.3 MJ/kg, which corresponds to 42,300 kWs. Divided by 3,600, 11,75 kWh emerge, almost 12 kWh. A sparingly moved E-car thus consumes about 1 kg of gasoline which results with a density of 0.7 kg/cm3 in a little more than 1.33 liters per 100 km.

However, this is not yet an approximate 1-liter car because the fuel comes into the tank with considerably more efficiency than the electrical charge into the battery. Depending on the cold and speed, up to 30 percent can be lost here. But as 1.5 to 2-liter cars we can already designate this E-generation, however, calculated from tapping point or socket.

Another example: A braking system is said to have compared to the drive about six times the performance in a truck, four times in passenger cars and still double in super sports cars. For a medium-sized passenger car, we therefore take 400 kW of braking power, which would correspond to 400 kJ/s.

Did you know that a brake disc completely made of GG already weighs 10 kg in the compact class (B segment)? Combined with steel or aluminum in the middle it is slightly lighter, but for our calculation the fully GG disk is better. So together, a total of 40 kg, on which the heat input acts. The specific heat capacity of cast iron is 0.5 kJ/(kg•K).

Q400 kJ
ΔT =  =  = 20 K
m • c40 kg • 0,5 kJ/(kgK)

Admittedly a little theoretically, because omitting all the heat transfer to the environment, it is nevertheless interesting that the temperature of the brake disks increases on average by 20°C per second (!). And why they do not reach the approx. 700°C for red heat after 35 seconds of full braking? Because no braking takes such a long time, from 200 km/h, e.g. 7 to 9 seconds.

However, if the cooling between full braking e.g. Is not enough for racing cars or you are braking constantly for a long trip downhill, the more heavy the car, the more this is possible. If your accelerator pedal is clamping at full load, like happened in the USA, then you should definitely brake the vehicle with the engine to zero and not always hesitate. 02/17






Imprint