ASVAB Auto & Shop Information Subtest: The Engine - dummies

ASVAB Auto & Shop Information Subtest: The Engine

By Rod Powers

The military wants to know the extent of your mechanical knowledge and they discover this with the Auto & Shop Information Subtest of the ASVAB. How does an engine work? You turn the key, and if it doesn’t start, you call your mechanic or your folks, right? Well, not quite.

The internal combustion engine burns a mixture of gas and air. Burning the gas and air (the fuel mixture) makes it expand quickly (explode). The pressure from this explosion is transferred (via additional systems) to the wheels to make the car move.

The movement is brought about by a cycle, which your car’s engine repeats a zillion-and-one times. Here are the four strokes that make up a cycle:

  1. Intake: The intake valve opens as the connecting rod pulls the piston down, drawing the gas/air mix into the cylinder.

  2. Compression: The valves are closed. The connecting rod pushes the piston up, compressing the gas/air mix.

  3. Power: The spark plug ignites the gas/air mix, forcing the piston down. That pushes down on the connecting rod, turning the crankshaft; the crankshaft turns the flywheel, which keeps the engine going.

  4. Exhaust: The exhaust valve opens as the connecting rod moves the piston back up, pushing out the exploded gases. The valves are timed, of course, using push rods attached to the camshaft.

    A four-stroke engine.
    A four-stroke engine.

Generally cars have an even number of cylinders — four, six, or eight. These cylinders are arranged in a row or rows, which are called inline (one row) or V (two rows), depending on the arrangement.

Most people refer to their engines as four-cycle engines. This isn’t really true. It is a fourstroke, one-cycle engine. The intake stroke, compression stroke, power stroke, and exhaust stroke are one engine cycle. When the fourth stroke is completed, the cycle begins again. Automobile engines do this very fast. When the tachometer (an instrument measuring revolutions per minute [rpm]) on your dashboard shows 4,800 rpm, for example, that means the engine is performing 4,800 of these cycles every minute.

  • Carburetors: Carburetors are used on most older cars (pre-1990) to mix the fuel and air mechanically. As air moves quickly through the carburetor, it creates a vacuum, which draws more and more fuel into the mixture.

  • Fuel injectors: Fuel injectors have replaced carburetors on newer cars to perform the air/fuel mixture function. (Actually, fuel injectors have been around since the late 1950s, but they weren’t widely introduced until the late ‘80s and early ‘90s.) The fuel injector acts as the fuel-dispensing nozzle. It injects liquid fuel directly into the engine’s air stream. In almost all cases, this requires an external pump.

    A doodad called the EFI computer (electronic fuel injection computer) determines the amount of fuel entering the engine. The EFI computer receives information from the sensors in the fuel, air, and exhaust system, and from that information, it determines how much fuel the engine needs to operate at optimum levels.

A throttle is mechanically connected to the carburetor or electronically connected to the EFI computer. Advancing (opening) the throttle causes more fuel to be transferred to the carburetor or the fuel injectors. The accelerator (the gas pedal) is connected to the throttle by mechanical linkages. The harder you push on the gas pedal, the farther the throttle is advanced (opened). Thus, more fuel is transported to the carburetor or fuel injectors.