ENGINE FUNDAMENTALS 101
Definition of External Combustion Engines and Internal Combustion Engines
In the first years of engine designs, the steam engine was introduced. Steam engines are external-combustion engines because the fire, or combustion, takes place outside of the engine, in a boiler. Many of the early vehicles and fire trucks used steam engines. It did not take engineers of that era to realize that in order to allow vehicles to travel on highways, the combustion should occur within the engine. That is why it is called an internal combustion engine.
Engineers began to develop a different type of engine. They realized that if the explosive charge could be confined to a cylinder and then ignited, the force of the explosion could be carried through a connecting rod to turn the crank.
Many engineers worked diligently for years to perfect the internal combustion engine, it wasn’t until the last half of the nineteenth century when most of the work was actually complete. It was at that time when people began to operate the internal-combustion engine. The first of these engines actually came in 1860, by a French inventor by the name of Jean Joseph E. Lenoir. Then later in 1876, the Deutz Factory started production of the four-stroke gas engine, designed by German inventor Nickolaus A. Otto. Other inventors also built various types of self-propelled vehicles.
Another German inventor, Karl Benz, built the first automobile from 1885 and 1886, using an engine operated by the Otto cycle. This vehicle became the starting point for the automobile industry and the beginning of the specialized motor vehicle power plant, the automobile engine.
Internal combustion engines are of two types. The type used in almost all automobiles is the piston engine, either spark ignition or diesel. In this engine, the piston move up and down or reciprocate in the engine cylinders. The word reciprocate means to move back and forth or up and down in a line.
Since its pistons move up and down, the piston engine is also called the reciprocating engine.
The other type of engine is the rotary engine. The gas turbine and the Wankel are two kinds of rotary type engines. The rotary engine uses no pistons. Instead, one or more rotors are spun by the burning of the fuel in the engine.
The Engine Cylinder
I will now describe the piston style engine, so let’s see how it works. Most automobiles have engines with four, six or eight cylinders. The same action takes place in each cylinder. By examining just one cylinder, you can begin to learn about the entire engine. The display shown to the left is an example of a six cylinder engine and how it operates within the cylinders. As you can see in this photograph, it looks like two soda cans, one a little smaller than the other. The larger so called can, is open at the bottom so it is filled with air. The smaller one fits into the larger one. If you push the smaller one up into the larger one, you squeeze the air into a smaller volume. That is, you compress the air. Now let’s call these by their rightful names. The bigger one is the engine cylinder. The smaller one is the piston.
When the piston is pushed up into the cylinder, the air in the cylinder is compressed. Suppose there were some gasoline vapor in the compressed air. IF a spark occurred in the cylinder, the mixture of air and gasoline would explode. The explosive would blow the piston out of the cylinder.
The connecting rod and crankshaft
Blowing the piston out of a cylinder just once is not enough to make a car move. The piston must move up and down rapidly in the cylinder. Then this up and down, or reciprocating, motion must be changed into rotating motion to turn the car wheels. The connecting rod and crankshaft do the job of changing the reciprocation motion of the piston into rotary motion.
The piston is about 4 inches (101.6 mm millimeters) in diameter. The piston with the connecting rod detached. This image below shows a crankshaft. This illustration also shows how the connecting rod is attached to the crankshaft.
Also, in these illustrations, the piston rigs are not shown. The connecting rod is attached to the piston by the piston pin. The lower end of the connecting rod is attached to the crank on the crankshaft by the rod bearing cap. We will described the rod and crankshaft later.
The Crank Pin
As you can see, the crank pin swings in a circle around the crankshaft as the crankshaft rotates. Look at the cutaway view of an engine cylinder in figure ? Find the piston, the connecting rod, the crank pin, and the crankshaft. Compare these parts with the same parts shown.
When the piston moves up and down in the cylinder, the piston, the connecting rod and the crankpin go through the eight positions the crank pin moves in a circle. The connecting rod tilts first in one direction and then in the other. The lower end of the connecting rod moves in a circle with the crankpin. Study the previous video to see how the up and down, or reciprocating, motion is changed into rotary motion.
There are many cranks and connection rods around you. Look at a bicycle. The pedal and its support form a crank. Your lower leg is the connecting rod. As you pump the pedal, your knee acts as the piston pin and moves up and down. Your foot is the lower end of the connection rod. It moves in a circle as it follows the crank (the bicycle pedal and support).
When the piston moves from top to bottom or from bottom to top, it completes one stroke. The piston completes two strokes as it goes through the eight positions shown in fig. ? In position 1, the piston is at the top. It moves down through positions 2, 3, and 4 to arrive at the bottom, position 5. This is one piston stroke. Then the piston starts back up, moving through positions, 7,and 8 and back to 1. This is the second piston stroke.
When the piston is at the top position, it is said to be at top dead center or (TDC). When it is at the bottom position, it is said to be at bottom dead center or (BDC). You will see many mentions of TDC and BDC during this training and in shop manuals. It is referencing points for setting valve and ignition timing.
The Four Stroke
You may have heard someone call automobile engines, “four stroke engines”. What this means is that the engine is that the engines are four stroke cylinder engines. In other words, it takes each engine piston four strokes or two revolutions of the crankshaft to go through a complete cycle. I shall go into more detail in one of the following training sessions.
How Does The Engine Run
This is a good question. The piston moves up and down, and the crankshaft rotates (usually clockwise rotation as you face the front of the engine). But it is the burning of the fuel in the cylinder that makes the piston move.
In the spark-ignition engine, a mixture of fuel and vapor (usually gasoline) and air enters the cylinder. Then the piston is pushed up to compress the mixture. Next a spark occurs in the cylinder. This spark ignition the mixture and it burns rapidly, pushing the piston down. It is this push that makes the crankshaft turn and the car wheels rotate. Then the piston must be pushed up and the burned gases moved out of the cylinder.
The diesel engine operates differently. Only air enters the cylinder during the intake stroke. Then the fuel is injected and burned, pushing the piston down. Operation of the diesel engine will be covered in a future session.
There must be an opening in the top of the cylinder so that the air or air-fuel mixture can get into the cylinder. A second opening is needed to get rid of the burned gases. That means there must be two openings. However, these openings cannot remain open all the time. They must be open when they are needed, and closed the rest of the time. Valves are used to open and close the openings. The valves let air or air-fuel mixture into the cylinder and let the burned gases out.
An engine valve is a long metal stem on which there is a flat top. It looks so much like a mushroom that engine valves are sometimes called mushroom valves.
Each valve moves up and down in a valve guide, which is a round hole in the cylinder head. The guide keeps the valve moving up and down in a straight line. When the valve is up, the valve post is closed. The valve head is tight against the edge of the port, and the opening is closed.
End of Session One