Gasoline Four Cycle Engines (Four Stroke)

Four cycle engines, also known as four stroke, are the engines that most people are familiar with.  Four cycle engines are found in vehicles, tractors, and most lawn mowers.  Diesel engines can also be classified as four cycle engines, even though they are slightly different to the gasoline four cycle engines.  Unlike gas operated two cycle engines, four cycle engines have an oil reserve in the crankcase, providing constant lubrication of moving engine parts, as well as valves and a cam shaft.  Although there are more moving parts to these engines, their operation is still rather basic.  As your crankshaft turns (the part of the engine that converts the power of combustion to usable energy), it moves the piston up and down through the cylinder.  The crankshaft  also plays another vital key to the engine operation.  As it turns, the spline gear on the shaft engages your camshaft, which has lobes on the shaft itself with controls the opening and closing of your valves. As the camshaft turns the lobes come into contact with tapits, which push against your valve guide rod opening and closing the valves in sequence.

There are four stages in a four cycle engine operation, hence it being a four cycle engine.  The first is intake.  As your crankshaft and camshaft rotate, the camshaft opens the intake valve as the crankshaft pulls the piston away from the cylinder head, creating a vacuum which draws the atomized fuel-air mixture into the combustion chamber.  Once the piston reaches top dead bottom (TDB), it begins the second stage, compression.  [During this stage, the intake valve is open while the exhaust valve is closed, and the piston is fully retracted]

The camshaft closes the intake valve as the piston begins moving back towards the cylinder head.  As the cylinder approaches top dead center (TDC), it compresses and heats the fuel-air mixture, making for easier combustion  After reaching TDC, the mixture is ready for the next stage, combustion (also known as the power stroke).  [During this stage both valves are closed, and the piston is fully extended into the cylinder]

Now the the fuel-air mixture is ready to do its job, the ignition coil discharges, sending 20,000 volts through the ignition wire to the spark plug, igniting the fuel-air mixture. NOTE:  This is not an explosion.  As the spark reaches the diodes, it ignites the mixtures and a flame front spreads throughout the cylinder.  This build up in pressure forces the piston back down towards the crankcase, while rotating the crankshaft, which is what powers whatever you have attached to the engine.  As the piston reached TDB, the camshaft opens the exhaust valve, preparing the engine for the final stage, exhaust. [During this stage both valves remain closed until the end, and the piston is fully retracted]

Now that the cylinder has went through its power stroke, it must expel the waste from the cylinder in perpetration to fire again. At the end of the power stroke, the exhaust valve had opened, allowing gasses to escape.  During the exhaust stroke, the piston moves back towards the head, forcing the remaining gasses and carbon out of the cylinder.  Once reaching TDC, the exhaust valve closes, and the cylinder is ready for the intake stroke. [During this stage, the exhaust valve is open while the intake valve is closed, and the piton returns to TDC]

Here is a link to an animated version of a four stroke engine showing the crankshaft, camshaft, and valves during operation:    http://www.animatedengines.com/otto.html