How does a push rod internal combustion engine work?
The heart of a push rod internal combustion engine (ICE) is the crankshaft. Rods connect the engines pistons to the journals of the crankshaft. The crankshaft is evenly balanced by  weights on the opposite side of each journal to ensure that at high revolutions per minute (RPM) the motor is stable. As the crankshaft rotates it pushes the rods up and down that are connected to pistons. At the front of the crankshaft is a gear, the gear is typically connected via a chain to the camshaft. If the crankshaft is the heart of the engine, a camshaft is the brain of the engine. The camshaft determines when to open and close valves that let in air and fuel into each cylinder and let out exhaust from the internal combustion. The camshaft is a shaft that runs the length of the engine and has lobes on it, small metal cylinders called lifters ride on these lobes and are connected to push rods, the push rods are connected to rockers which rock back and forth opening and closing valves in the cylinder heads. The valves are located in the cylinder heads that sit on top of the cylinders that the pistons are in. As the pistons rotate up, a valve (being driven by the camshaft) opens to bring fuel into the cylinder, when the valve closes a spark from a spark plug, ignites the fuel causing a mini explosion pushing the cylinder down, a second valve opens letting the exhaust from the mini explosion exit the engine. This in turn rotates the crankshaft, pushing another piston up where the process continues. An intake manifold is like the lungs of a motor, it breaths in air from the atmosphere and mixes it with just the right amount of fuel to create that mini explosion in the cylinder.  The timing of the air, fuel, spark delivery is critical if it is off just the smallest of margins the engine will not run at all.