Valve Operation

• The valves must open and close at precise times to allow an air-fuel mixture to be pulled in and to allow the exhaust gases to be pushed out. The valve is connected to a valve stem assembly, which includes a heavy spring. The spring is depressed to open the valve and pulls the valve closed when it is no longer being depressed. The valve is machined and ground to mate with the valve seat inside the combustion chamber, making a seal when closed.
  
  

Rocker Arm

• The rocker arm rocks back and forth on a pivot. When the valve is to be opened, the lifter or push rod pushes up on one side of the rocker arm. Since it sits on a pivot, the rocker arm pushes down on and depresses the valve stem, which opens the valve. The rocker arm remains depressed for a short period of time until the lifter rod is no longer pushed up. The spring on the valve stem pushes against the rocker arm which in turn pushes against the lifter rod, allowing the valve to close.
  
  

Camshaft

• The camshaft is the heart of the engine. It is responsible for timing the opening of the valves with rotation of the crankshaft and the pistons. It connects to the crankshaft via the timing belt. The cams, specially shaped pieces of machined metal, rotate with the camshaft and push the valve lifting rods up and down. Newer automobile engines sometimes employ two camshafts. In some engines, the camshafts are located on top of the engine, rather than in the engine block. Overhead camshafts make it possible to operate the valve without a rocker arm.
  
  

Lifting Rods

• Note the heavy springs on the valve stems on this four-cylinder engine.
  
  The lifting rods travel up and down through lifter tubes inside the engine block. They are in constant contact with the cams on the camshaft due to the pressure put on them by the valve stem and rocker arm springs. As the camshaft turns, the eccentric shape of the cams alternately pushes the lifter rods up or lets them down against the spring pressure. The valves remain open for only a moment before the cam turns enough for them to close. Some engines employ hydraulic lifters, which adjust the lifter rods for wear and temperature.
  
  

Friction and Speed

• A modern eight-cylinder car engine cruises on the highway at 1,500 to 2,500 RPMs, and each cylinder has four parts to its cycle, so it fires once every four turns of the engine. Each valve opens and closes once during that cycle, which means it opens and closes 625 times a minute--over 10 times per second. This creates considerable friction on the moving parts, including the cams and lifting rods. Oil reduces the friction and engine coolant circulating throughout the engine cools it; both actions reduce wear.