Power of your Car

What is it that makes your car move when you step on the accelerator in your car? I know what your thinking, the engine, right! Well that’s a start I guess. But what is it about your engine that moves your car. It’s the output power that your engine creates that pushes your car. Well one might ask what is power. Power is defined as the amount of work (U) done per unit time (t). Power can be mathematically described as dU/dt. Since we know that work dU can be expressed as F*dr, then we can also write the power as

P=F*v

where, v is the velocity of the point, which is acted upon by the force F.

You can use the SI or FPS system to label the units. The units go as follows:

1 W = 1 J/s = 1 N*m/s

1 hp = 550 ft*lb/s

Power provides an engineer with a useful basis of the type of motor or machine, which is required to do a certain amount of work in a given amount of time. An example of this would the power that drives your car. Every machine or motor has an efficiency associated with it. Efficiency is the ratio of output power to input power. As you might already know we cannot have an efficiency that is greater than one in the universe. That would represent a perfect machine or motor that requires no addition input power to get the equal amount of output power. Let’s think about this for a minute! Why can’t efficiency greater than one be achievable? It’s simple, forces such as friction or numerous parts that act upon the machine causing it to take more input power.

I have developed a problem to illustrate the efficiency and power of a normal car that may be parked in your garage.

Analytical Representation of Power and Your Car

In addition to a calculation I have also added a Working Model Simulation of a car being accelerated along a surface. You can see in the model that forces are acting upon the car causing the motor to put in more power than it will get in return causing the efficiency to be less than one. The car is accelerated and then the acceleration is taken away and the car begins to slow. If there were no forces and/or the output power = input power the car would move constantly and forever. Please use the below link to view the simulation.