The simplest single-cylinder internal combustion engine (ICE)

4.2.1 Description of the device of the simplest engine

In order not to immediately confuse with complex terms and bulky definitions, first consider the basic single-cylinder internal combustion engine (ICE) running on gasoline, the device of which is shown in Figure 4.1. This engine consists of a block with a cylindrical hole inside - a cylinder liner. The liner contains a piston, which is connected through a connecting rod to the crankshaft. The crankshaft, in turn, is connected to the camshaft through a chain (this connection is constant, and the gear ratio ( The term "gear ratio" will be discussed in Chapter 5 "Transmission") is 1 to 2, that is, the camshaft makes one revolution in two crankshaft revolutions).

Figure 4.1 Single-cylinder internal combustion engine

Figure 4.2 Sectional view of gasoline internal combustion engine.

Figure 4.3 Air-cooled internal combustion engine.

The camshaft together with the valves is located in the cylinder head, which is respectively mounted on the engine block. 

Now let's describe everything separately.

4.2.2 Cylinder block

Cylinder block is a cast iron or aluminum alloy part. The crankcase is part of the engine block, which is often casted with the cylinder block. In fact, this is a body, inside which are the main elements of the crank gear (which will be discussed below). This body is double-walled (referred to as a water jacket). Coolant flows in the cavities between the walls in case if it is a liquid-cooled engine. In case if the engine is air-cooled, then the block has one wall with multiple fins for heat dissipation, as shown in Figure 4.3. 

The block contains a liner and oil channels for supplying lubricant to rubbing parts. The working surface of the liner with which the piston contacts is called the cylinder face.

4.2.3 Piston

The piston looks like an inverted cup, usually cast from an aluminum alloy. The piston is installed in the cylinder with a very small clearance (usually hundredths of a millimeter). The piston is sealed with piston rings in order to prevent the gases that are created during fuel combustion from breaking into the crankcase of the engine block through this gap. Usually, two compression rings are installed (they perceive the main load when the piston moves) and one oil-scraper ring (it consists of several elements), which is necessary for removing engine oil from the cylinder walls. The piston, pivotally, that is, via the pin, is connected to the upper head of the connecting rod. And the connecting rod, in turn, is pivotally connected to the crankshaft. If joined together, these elements are called the connecting rod and piston group. And the connecting rod and piston group, which is connected to the crankshaft, creates the movable elements of the crank gear (further, speaking about this mechanism, we will mean precisely these elements). Thanks to the connecting rod, the upward and downward movement of the piston is converted into a rotational movement of the crankshaft.

Note
Dear reader, you may think that you missed a whole section, because in figure 4.1, both the stud and the upper connecting rod head are missing, but this is not so: the above description is provided for a general idea of the internal combustion engine, but the structure of each of the elements is discussed in detail in section 4.7 "Cylinder Block and Crank Gear".

4.2.4 Cylinder head

The cylinder head is essentially a body (usually made out of an aluminum alloy), which, depending on the design*, contains the camshaft (or shafts), as well as the intake and exhaust valves. The camshaft and valves are called the gas distribution mechanism (GDM). The camshaft is necessary for the timely opening of the intake and exhaust valves. The valves fit tightly against the cylinder head and are pressed down by valve springs. 

That's the complete description of four-stroke gasoline internal combustion engine. There is nothing complicated.

* The expression "depending on the design" means that the camshaft or camshafts are not always located in the block head. This will be discussed in more detail in chapter 4.6 "Cylinder Head".

4.2.5 Principle of Operation of Internal Combustion Engine 

A four-stroke engine is called so, because the entire workflow is divided into four intervals - strokes. Of these cycles, only one is operating. That is, the one during which the piston moves under the action of gases that are released during the combustion of the fuel-air mixture. Each stroke falls on (approximately) one half turn of the crankshaft.

Note
Top dead center (TDC) - the extreme position of the piston in the upper part of the cylinder. Bottom dead center (BDC) - the extreme position of the piston in the bottom of the cylinder. The distance from TDC to BDC is called the piston stroke.

Surely everyone had a bicycle in their childhood. And if any tire was flat, then it had to be pumped up using a pump. So, albeit remotely, this tire inflation pump reminds us of a single-cylinder engine. There are also valves inside the cylindrical pump casing and the piston moves in the same way. 

When you pull the piston handle towards you, air is sucked in through the valve in the casing. When you push the piston down - the valve at the intake closes and air goes out through it into the tube, getting into the tire of the bicycle wheel. Now let's mentally imagine an inverted pump, inside of which we began to move the piston downward, while drawing air inside the casing. Then we also mentally close the outlet, for example with a finger, and begin to move the pump piston upwards - the air will begin to compress, since it has nowhere to go. Upon bringing the pump piston to the stop, we took and set fire to the gunpowder in the housing, filled up before the start of this action. While burning, this gunpowder will emit a large amount of gas, which, in turn, will increase the pressure inside the body and begin to move the piston, only without our participation - on its own. Once the gunpowder is completely burnt out, and the piston reaches the lowest point, we will open the outlet and begin to move the piston up again, pushing the exhaust gases out of the pump housing. Having pushed the combustion products out, we again close the pump outlet with our finger and begin to repeat all of the above in the same sequence. Any four-stroke gasoline engine works in much the same way. Place the pump casing inside the block, install the valves inside the head, which is also mounted on the block, and connect the piston via the connecting rod to the crankshaft and get the simplest single-cylinder engine.

4.2.6 «Working cycle»

There is such a concept as "working cycle". This is a set of processes that occur sequentially in the engine cylinder when the crankshaft rotates two full revolutions (720˚). The working cycle consists of strokes.

Note
While reading about the description of the processes further, remember the pump that was described before.

Actually, nothing complicated. Almost all four-stroke internal combustion engines that use gasoline as fuel operate according to this principle (see table on the next page).

4.2.6.a Fuel-mixed air inlet

First stroke. Inlet of fuel-mixed air.

The crankshaft, while rotating, moves the piston down from TDC. At this moment, the inlet valve is open. The air is sucked through it into the cylinder. It is mixed with atomized fuel (in the form of very small droplets). Then the piston reaches BDC, the inlet valve closes. Direct injection diesel engines admit clean air.

4.2.6.b Compression

Second stroke. Compression

The crankshaft continues rotating. And the piston begins to move upward from BDC, compressing the fuel-air mixture (and air - for diesel engines), additionally more thoroughly mixing fuel with air so that the mixture is as homogeneous as possible. The both valves are closed.

4.2.6.c Working stroke

Third stroke. Operating stroke

When the piston is at TDC, and the mixture, which is compressed and heated to a high temperature, is in the combustion chamber, a discharge occurs between the electrodes of the spark plug, which ignites the fuel. The air-fuel mixture in the process of combustion releases gases that are heated to 800 °C. A high pressure is created, under the action of which the piston moves downward, pushing the crankshaft. The whole process continues until BDC. In diesel engines, fuel is injected into the cylinder with compressed air, heated to an extremely high temperature, which ignites spontaneously.

4.2.6.d Release

Fourth stroke. Exhaust

The gases have done their job. Now it is necessary to get rid of them in order to prepare the cylinder for the intake of next portion of the air-fuel mixture. After BDC, the exhaust valve opens, the piston rises under the action of inertia, pushing out the exhaust gases. Once the piston reaches TDC and all exhaust gases have been removed, the whole process will be repeated again.