Undergrad Car, gas and total energy calculation

[jeanmarc69]

Hi,

What I understood about the principle of conservation of energy: Et = Ep + Ec = constant.
For example: Et = 1/2mv^2 + mgh (h = height).

Consider a car moving at speed v.

For example: Et = 1/2mv^2 + E(gas + exhausts). Indeed, I include the exhausts, otherwise with the drop in the quantity of gas necessary to drive the car, there is an imbalance, therefore no conservation.

My question: What would be the algebra for E(gas + exhaust) in the context of Et?

Thank you for your answers.

 

[berkeman]

Wow, it's hard for me to decode your post. What are $E_p$ and $E_c$? What are you trying to calculate? Are you trying to add the chemical potential energy of gasoline into a total energy calculation for a car? If so, the chemical potential energy of the exhaust can be taken as zero. If you have a turbofan driving the car, then I suppose you could include the exit momentum of the exhaust in the energy equations, but otherwise it makes little sense to worry about the exhaust gasses from a car, IMO.

 

[Baluncore]

My question: What would be the algebra for E(gas + exhaust) in the context of Et?

The air, taken in through the air filter of a car, must be first accelerated to the speed of the car. The energy required would normally appear as part of the aerodynamic drag of the vehicle. The fuel in the tank of the car would appear to be part of the mass of the vehicle.

The nitrogen in the air would be heated before being released as exhaust. The oxygen would be combusted with fuel, to be exhausted as hot H₂O and CO₂.

The exhaust gasses of piston-engined aircraft were directed backwards to gain a few HP of forward thrust. That 'jet' effect would also be true for cars, but is not as important as it once was in military aircraft.

 

[russ_watters]

Your math makes little sense to me too, but please note a car moving at constant speed on flat ground is not a situation where energy is conserved: 100% of it is lost.