Calculating the Efficiency of a Car Engine: A Scientific Approach

AI Thread Summary
A user is calculating the efficiency of a car engine by analyzing the energy used to accelerate a vehicle from 0 to 50 km/h in 9 seconds, with the engine producing 57 kW of power. Initially, they miscalculated the percentage of power used for acceleration, incorrectly comparing total energy to kinetic energy. After guidance, they correctly determined the increase in kinetic energy (KE) and calculated the power used to achieve this acceleration. The final efficiency of the engine is found to be approximately 21%, demonstrating the importance of comparing power output to the actual work done over time. The discussion highlights the need for precise calculations in understanding engine efficiency.
Izekid
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Here I have a hard one... A car with the mass 1100kg accelerates from 0km/h to 50km/h in 9,0s. In this time intervall the engine exhausts an power of 57kW. How many procent of the power the engine exhauts is used to increase the moveable energy...

For this I take :

Wres = Fres = Wk

Wk= 1/2 M * v^2

First I take Km/h and transform it to m/s 50km/h = 13,8 m/s

then

550kg *13,8^2 = 104742 J Totalty Cars Moveable power

Then to get Fres I use

S=1/2(v-v0)t

S=13,8/2 * 9 = 62,1m

To get Fres I use Wres = F*S

62,1 * 104742 = 6504478,2J = 6504kJ

57kJ/6504kJ=0,0008 = 0,8 procent

Which Is TOTALY WRONG please help me what shall I do!? It shall be around 21%
 
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You started out OK, but then went off track. How much power actually goes to increasing the speed of the car? Hint: What's the increase in KE? How much time does that take?
 
The KE is 104742J = 104kJ then shall I take 57/104 which is... 0,54 which is 54% which is also totally wrong ...
 
Of course it's wrong. You need to compare power (total) with power (used to accelerate car), not with the KE of the car.

You found the KE; now find the power. (That's why I asked you "How much time does that take?")
 
Thx man forgot that effect is P = W/T! You should have sade Effect and it should have popped up directly ... well anyway you helped me thanks man
So The answer is 104742/9 = 11638W = 11,6 kW

11,6kW/57 = 0,209= 0,21 :)
 

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