How an Engine Works

Engine Fundamentals and the Four-Stroke Cycle

An internal combustion engine converts heat energy from burning fuel into mechanical work. To understand this energy conversion, you must first become familiar with the geometry inside an engine cylinder. Cylinder bore refers to the internal diameter of an engine cylinder. Inside this cylinder, a piston travels between two fixed boundaries. Top Dead Center is the highest point that a piston reaches inside the cylinder, while Bottom Dead Center is the lowest point that a piston reaches inside the cylinder. Stroke refers to the linear distance a piston travels between Top Dead Center and Bottom Dead Center. When you consider all cylinders operating together, engine displacement is the total volume swept by all pistons during one complete stroke.

A standard four-stroke internal combustion engine repeats a specific sequence of four strokes: intake, compression, power, and exhaust. During the intake stroke, the piston travels downward from Top Dead Center toward Bottom Dead Center. For most of the intake stroke, the intake valve is open and the exhaust valve is closed. As the piston moves downward, it helps draw air into the cylinder, and in some gasoline engines, fuel enters with the air. Next comes the compression stroke, in which the piston travels upward from Bottom Dead Center toward Top Dead Center. Both valves are closed for most of the compression stroke, although the intake valve may close shortly after the piston starts rising. Compressing air or an air-fuel mixture increases its pressure and temperature.

In a conventional gasoline engine, a spark plug ignites the air-fuel mixture near the end of the compression stroke. The spark typically fires near the end of compression; after the piston reaches Top Dead Center, expanding gases drive it downward on the power stroke. Both valves are closed for most of the power stroke, although the exhaust valve often opens near its end. The power stroke is the stroke in which combustion supplies energy to the crankshaft; the other strokes use some of that energy. Finally, the exhaust stroke clears the cylinder. During the exhaust stroke, the piston travels upward from Bottom Dead Center toward Top Dead Center. For most of the exhaust stroke, the exhaust valve is open and the intake valve is closed, though the intake valve may begin to open near the end. This upward travel of the piston pushes spent combustion gases out of the cylinder.

Understanding this sequence helps build strong mechanical reasoning for your vocational testing. As you study, keep in mind that the Armed Forces Qualification Test score is derived solely from Arithmetic Reasoning, Mathematics Knowledge, Word Knowledge, and Paragraph Comprehension. Auto-related ASVAB questions assess technical knowledge and may contribute to certain composite scores, but they do not contribute to the AFQT. Keep in mind that service-specific minimum scores and job qualifications are set by each military branch and can change over time.

Crankshaft, Camshaft, and Mechanical Timing

Pistons move in a reciprocating, or back-and-forth linear, motion within the cylinder. However, the driven wheels and external components of a vehicle require smooth, rotating force. The crankshaft converts the reciprocating motion of the pistons into rotational motion. Connecting rods physically link each piston to a journal on the crankshaft. To establish this joint, a wrist pin, also called a piston pin, secures the piston to the small end of the connecting rod. As the piston pushes down, the rod transmits that force directly to the rotating crankshaft journal.

The violent pulses of combustion create significant mechanical stresses and uneven rotational forces. To manage these forces, crankshaft counterweights help balance moving parts and reduce engine vibration. A flywheel, when fitted, is attached to the crankshaft and stores rotational energy that helps smooth the engine’s intermittent power pulses; many automatic-transmission vehicles use a flexplate and torque converter instead. Because each cylinder delivers power during only one of its four strokes, stored rotational energy in the engine’s rotating parts helps carry it through the non-power strokes.

Engine valves must open and close in exact coordination with the movements of the pistons. A complete four-stroke cycle requires two full rotations, or 720 degrees, of the crankshaft. In contrast, a complete four-stroke cycle requires one full rotation, or 360 degrees, of the camshaft. This means that in a four-stroke engine, the camshaft rotates at half the speed of the crankshaft. A timing belt, timing chain, or set of timing gears synchronizes the rotation of the crankshaft and camshaft. Proper synchronization is vital, especially in an interference engine design, where a broken timing belt or chain can cause pistons to collide with open valves.

Pistons, Rings, and Cylinder Sealing

A piston must maintain a durable, tight seal against the cylinder wall while sliding rapidly up and down. Because a bare piston cannot easily provide this seal on its own, piston rings fit into machined grooves on the outside of the piston to seal the combustion chamber. These rings maintain contact with the cylinder bore to manage gases and lubricating fluids during engine operation.

Piston rings serve specialized functions depending on their placement along the piston body. Compression rings near the top of the piston reduce the amount of combustion gas that leaks into the crankcase. If these upper rings wear down, high-pressure gases escape past them during operation. Combustion gas that passes the piston rings into the crankcase is called blow-by; some blow-by is normal, but worn rings can increase it.

The lower section of the ring assembly handles lubrication control. The bottom piston ring is an oil control ring that scrapes excess lubricating oil from the cylinder wall. By continuously scraping away surplus oil, this ring prevents excess engine oil from entering the combustion chamber while ensuring that the cylinder walls remain adequately lubricated for smooth reciprocating travel.

Valve-Train Designs and Operation

Valves govern the intake of fresh charges and the exit of spent exhaust. Raised lobes on the camshaft open the valves, either directly or through other valve-train parts, against valve spring force. As a cam lobe rotates, it pushes on the valve or other valve-train parts to open the valve against its spring. Once the lobe rotates past the parts that open them, valve springs push the valves shut. Physical dimensions also differ between the valves: the intake valve head is usually larger in diameter than the exhaust valve head to maximize incoming airflow.

Engines differ in where the camshaft is mounted and how force is transferred to the valves. An overhead valve engine, often called a pushrod engine, mounts the camshaft inside the engine block. In an overhead valve engine, the camshaft acts on lifters, pushrods, and rocker arms to open the valves. This arrangement relies on these intermediary pushrods to transmit motion from the lower engine block all the way up to the cylinder head.

In contrast, an overhead camshaft engine positions the camshaft in or on the cylinder head, close to the valves; it may operate them directly or through other valve-train parts. Overhead camshaft configurations fall into two main varieties. A single overhead camshaft engine uses one camshaft per cylinder head to actuate both intake and exhaust valves. A double overhead camshaft engine uses two separate camshafts per cylinder head, one for intake and one for exhaust. Furthermore, in valve trains with adjustable mechanical lash, valve clearance is a small gap between valve-train parts that allows for thermal expansion as parts heat up during operation.

Cylinder Configurations and Ignition Methods

Engines are also categorized by the physical layout of their cylinders. In an inline engine, all cylinders are arranged in a single straight row along the crankcase. In a V-type engine, cylinders are arranged in two angled rows that share a single common crankshaft. In a boxer engine, cylinders extend horizontally from opposite sides of the crankshaft, and opposing pistons move inward and outward together. Distinct from these common cylinder layouts, a Wankel rotary engine uses a roughly triangular rotor instead of conventional reciprocating pistons.

Finally, internal combustion engines are distinguished by how they initiate the burning of fuel. Gasoline engines utilize spark plugs to initiate combustion near the end of the compression stroke. Diesel engines, by comparison, ignite injected fuel using the heat produced by compressing air, rather than a spark plug. By relying on high compression to elevate air temperature above the ignition threshold of the fuel, diesel engines achieve combustion without an electric spark.