Engine Fundamentals

The Basics of the Internal Combustion Engine

An internal combustion engine is an often-misunderstood device. The best way to understand its operation is to see all the parts in front of you. Pictures and diagrams tend to give the reader a vague idea of how it works, but it is often difficult to understand how all these components come together to mow your grass or move you down the road. Lets begin with one of the most basic types of engines, and then progress to more modern designs.

The Briggs & Stratton© 5 hp motor

Most of us have used this motor at some point in our lives. Whether it was a pressure washer, a lawn mower, or a ride on a go-cart, you can see how common these motors are. The Briggs & Stratton 5 hp motor is a 4 cycle, flat head design. This makes for a compact, lightweight, inexpensive engine. It also is one of the simplest 4 cycle engines you can build. Refer to the Figure below, and you will see that the valves are actually next to the cylinder, as opposed to on top of it. Looking at the cross cut, you can see how the intake and exhaust gases flow in and out of the combustion chamber. The valves are opened and closed in this engine as they are in virtually all 4 stroke engines, by a camshaft that is coupled to the crankshaft. Take a look at the odd shape of the camshaft lobes. As the camshaft spins, a follower or lifter tracks the surface of the camshaft, pushing open the valves and then allowing them to close at the appropriate times.

When the intake valve is opened, the piston is moving down, or drawing an inlet charge (of fuel and air) into the cylinder. When the piston reaches the bottom, the intake valve closes, and the piston begins to move up to compress the air and fuel mixture. This compression increases the pressure and temperature of the fuel/air mixture, and just when the piston reaches the top, the spark plug fires. This increases the pressure and temperature extremely fast, and in turn begins to push the piston back down. Once the piston reaches the bottom, and all the available energy has been used, the exhaust valve opens, and the exhaust stroke begins. This is necessary to expel the burnt gases, and allow a fresh charge or air and fuel to come in on the next power stroke. During the exhaust stroke, the piston pushes the spent gases out the exhaust valve, and out the muffler. Once the piston reaches the top of its travel, the exhaust valve closes, the intake valve opens, and the power stroke starts again.

Ok, so the piston moves up and down, sometimes doing work (expel spent gases, sucking in a new charge) and sometimes receiving work (expansion of ignited fuel/air mixture). So what does all this mean? Well, the piston is tied to the crankshaft with a link known as the connecting rod. This ingenious device (also known as the slider-crank mechanism) transforms the linear (straight line) motion of the piston in the cylinder into rotary motion that is useful for turning tires or lawnmower blades.

Hopefully this gives you a good understanding of the 4 cycle internal combustion engine and allows us to dive further into some of the design aspects and compromises that are made to make the most power, yet maintain acceptable economy and emissions.