Comparing the United States Government to a Car?

I'm using a car as a metaphor to the Unites States Government for my history project. But i'm not so sure about it. So i thought the Legislative branch would be the engine of a car. The Executive branch would be the brakes of a car. And the Judicial branch would be the steering wheel of a car. Is there any better comparisons? By the way, i need to draw each part and I kinda don't wanna draw a car's engine..... it's too hard... xD