Jim Smart
November 1, 2002
Photos By: Mustang Monthly Archives

When we turn the key, what happens under the hood and why? How does an automobile engine work? What is that sound we hear and the propulsion we feel when the accelerator is depressed?

We know gasoline is flammable. It burns. And we know gasoline creates heat energy when it ignites. Heat created by the igniting gasoline and air mixture is what gives us the power necessary to propel our Mustangs. With all of this in mind, let's talk about internal combustion engines. "Internal combustion" means we burn fuel and air inside the engine to create heat and generate the power necessary to propel a car.

"External combustion," for example, is similar to that found with a steam engine. Fuel is burnt outside of the engine in a boiler to heat water that becomes steam and heat energy. With both internal and external combustion engines, heat energy (expansion) exerts force on a piston and connecting rod tied to a crankshaft to make rotary motion. In physics class, we call this turning linear motion into rotary motion.

Okay, so what does all of this "linear" and "rotary" stuff mean? Think about what you see when you're watching an old episode of Petticoat Junction. The Cannonball steam locomotive to nearby Pixley has a series of arms tied to pistons and wheels. Steam pressure moves the piston, which moves the arm, which turns the wheels to propel the locomotive. Pistons and arms make linear (back and forth) motion. Wheels make rotary (around and around) motion. The wheels are counterweighted to help momentum. "Chuga-chuga-chuga-chuga" motion that comes from steam pressure gets us down the track. Your Mustang's engine works on the same principle, only the "chuga-chuga-chuga" motion is inside the engine, invisible from the outside.

The crankshaft, like locomotive wheels, is counterweighted for balance and momentum. Around and around it goes-channeling energy to your Mustang's transmission, driveshaft, and rear axle.

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Burn, Turn And Roll

Mustangs are equipped with four-cycle gasoline engines. The four cycles are intake, compression, power, and exhaust. How do we make power from the four cycles? Air and fuel must first be mixed into a vapor or mist before entering the combustion chamber above the piston. Liquid gasoline, as a rule, doesn't burn. If you were to throw a lighted match into a bucket of gasoline (do not do this!), the liquid wouldn't burn; the fumes above the fuel ignite and burn. When gasoline becomes vaporized (mixed with air), it ignites with fury, which makes heat and pressure to do our work.

Fuel becomes a vapor two basic ways. Carburetors, common in Mustangs prior to 1986, mix fuel and air to create the vapor needed to support combustion. Electronic fuel injection atomizes (vaporizes) fuel under pressure either at a single entry point (central fuel injection) or at each intake port (sequential electronic fuel injection). Central and sequential fuel injection are both computer-controlled systems.

Vaporized fuel is drawn into the combustion chamber by the moving piston in the cylinder bore. An open intake valve provides the entry point. This is called intake stroke. When the piston reaches the bottom of the bore, the intake valve closes, terminating the entry of fuel and air. When the piston begins its journey back to the top of the cylinder bore, it squeezes the fuel and air against the closed valves. This is called compression stroke. As the piston nears the top of the cylinder bore, the spark plug fires, igniting the fuel/air mixture. The heat and pressure created during ignition exerts force on the piston, pushing it downward in the cylinder bore, applying pressure on the connecting rod and crankshaft. We call this the power stroke. This linear (straight line) force turns the crankshaft, becoming rotary motion. As the piston nears the bottom of the cylinder in the power stroke, the exhaust valve opens. The piston begins its journey back to the top of the bore, forcing exhaust gasses out through the open exhaust valve. This is called the exhaust stroke. Our engine's four power cycles are complete.