Engine

Mechanism of internal combustion engine, gasoline engine and diesel engine

The engine is often compared to the human heart. It is an easy-to-understand metaphorical expression because the engine creates the power to drive the car forward. Do you know what the engine looks like?

 Engine looks like

How it works inside the engine?

Basically good fuel, good electricity, good compression

Simply put, the engine internally burns fuel to generate heat. The heat causes the gas inside to expand and generate force. Talking a little more mechanically, the engine uses the thermal expansion of gas to reciprocate the piston. However, since the wheel is an object that makes rotational motion, it is not possible to transmit force to the tire as it is, so it is necessary to convert reciprocating motion into rotational motion. Therefore, a part called the crankshaft converts the reciprocating motion of the piston into rotary motion. In other words, the energy generated by heat is converted from reciprocating motion to rotary motion, and is finally transmitted as the driving force for the tire. So how do you move the piston back and forth?

The operation of the engine is roughly divided into four processes. 

1. Intake process that takes in air (or a mixture of      gasoline and air)

2. Compressing process to compress the taken-in gas

3. Combustion process that burns and expands    compressed gas

4. Exhaust process to discharge combustion gas. Focusing on the movement of the piston, I will briefly explain it so that you can easily imagine the above process.

During the intake process, the piston is located on the upper side, and the piston moves downward and takes in air (or air-fuel mixture).

This is the same principle as pulling the piston of a syringe to let in air. In the subsequent compression process, the piston moves from bottom to top to compress the gas. This means that the piston has moved back and forth once (that is, it has moved twice).

In the combustion process, the piston is located on the upper side and burns the compressed gas. At this time, the combusted gas pushes the piston downward due to the expansion effect. The piston pushed down to the lower side moves upward while maintaining its momentum, and pushes out combustion gas (called exhaust gas in the exhaust process) to the outside. Now the piston has made one reciprocation. That is, the piston makes two reciprocations (that is, four movements) from the intake process to the exhaust process. From this, an engine that goes through the above process is called a 4-cycle engine (or 4-stroke engine). There is no big difference in the process flow between gasoline engine and diesel engine. So what is the difference between a gasoline engine and a diesel engine? It is a combustion process-strictly speaking, the combustion process changes, so the intake process also changes, but details will be described later-The gasoline engine ignites the compressed mixer with sparks, but the diesel engine Burns by injecting fuel into the compressed and hot air. The reason why ignition is required to ignite gasoline is that gasoline has the property of being less likely to self-ignite-this is called the octane number. On the other hand, diesel engines that use light oil with a low octane number do not require ignition because they can self-ignite at the temperature of compressed air. This difference exists in the combustion process between gasoline and diesel engines.

Next, I will explain in detail the flow from the intake process to the exhaust process of a gasoline engine.

 Intake process:

1. The lid inside the cylinder (this is called the intake     valve) opens 

2. The piston at top dead center in the cylinder   descends and air is drawn in. 

3. Fuel is injected from the injector (that is, it becomes   a mixture)

4. Close the intake valve when the piston reaches   bottom dead center Compression process

5. The piston at bottom dead center rises and     compresses air Combustion process

6. When the piston comes near the top dead center,   sparks are emitted from the spark plug to ignite.

7. The air-fuel mixture burns to generate combustion     gas and expand, pushing down the piston Exhaust   process 

8. When the piston reaches bottom dead center,   another  lid (exhaust valve) inside the cylinder opens.

9. The piston rises and exhausts exhaust gas

In the intake process, first the intake valve opens and the piston lowers. When the piston goes down, the pressure inside the cylinder also goes down, so air flows into the cylinder through the intake port (the hole that was blocked by the intake valve). At the same time, fuel is injected from the injector into a mixture. In this way, when the piston reaches bottom dead center, the intake valve is closed and the intake process ends. In the compression process, the movement of the piston from bottom dead center to top dead center is used to compress the air-fuel mixture. The temperature of the compressed mixture rises as the pressure rises. When the piston passes the top dead center, the spark plug ignites the air-fuel mixture and moves to the combustion process. The heat generated by the combustion of the air-fuel mixture expands the combustion gas, pushing the piston down to bottom dead center. When the piston reaches the bottom dead center, the exhaust process starts, so the exhaust valve opens, and the piston rises and releases the exhaust gas to the exhaust valve. This is a series of gasoline engine cycles. As an aside, I would like to touch on the direct injection gasoline engine that has become popular recently. In the conventional port injection type, the injector injects fuel into the intake port, whereas in the direct injection type, the injector injects fuel directly into the cylinder. In the case of port injection type, the injected fuel may adhere to the wall surface inside the port or the back side of the intake valve. Since it may later enter the cylinder later, it affects the change of the air-fuel ratio (fuel-to-air weight ratio) and deteriorates the response. On the other hand, in the case of the direct injection type that directly injects fuel into the cylinder, the response of changing the air-fuel ratio is excellent, and more precise control is possible. In addition, the temperature in the cylinder can be lowered by the heat of vaporization of gasoline, which also helps prevent knocking (knocking is abnormal combustion after ignition caused by abnormally high temperature in the cylinder).

Appearance of diesel engine 

Next, let's look at diesel engines.

Intake process

1. The lid inside the cylinder (this is called the intake   valve) opens

2. The piston at top dead center in the cylinder   descends and air is drawn in.

3. Close the intake valve when the piston reaches     bottom dead center

      Compression process

4. The piston at bottom dead center rises and   compresses air

       Combustion process

5. When the piston comes near the top dead center,   fuel  is injected from the injector

6. Combustion gas generated by combustion expands   and pushes down the piston

        Exhaust process

7. When the piston reaches bottom dead center,   another  lid (exhaust valve) inside the cylinder opens.

8. The piston rises and exhausts exhaust gas.

As mentioned earlier, the big flow is no different from a gasoline engine. First, in the intake process, the intake valve opens and the piston lowers, sucking in air. However, in a gasoline engine, fuel is also injected in this intake process and taken in as a mixture, but in a diesel engine, injection is performed in the combustion process, so only air is taken in. This is the difference between the two engines in the intake process. The subsequent compression process is exactly the same as a gasoline engine. The movement of the piston from bottom dead center to top dead center is used to compress the air in the cylinder. Then, when the piston moves near the top dead center, fuel is injected from the injector and mixes with the air, creating an air-fuel mixture. Since light oil has a low octane number, the air-fuel mixture spontaneously ignites and burns at the temperature of the compressed air.

This is a big difference from a gasoline engine that requires a mechanism for ignition because it uses gasoline with a high octane number. The flow after that is the same as in a gasoline engine. When the combustion gas pushes down the piston and reaches the bottom dead center, the exhaust valve opens and the process goes to the exhaust process. The above is the operation process of the diesel engine.
Summary:
            In summary, the biggest difference between gasoline engine and diesel engine is the injection timing by the injector. In addition, the gas used in the compression process also changes, which leads to differences in the characteristics of gasoline engines and diesel engines.

Finally, I will briefly explain the characteristics of each engine. The gasoline engine can be turned up to a high speed but has a small torque, while the diesel engine has a large torque but a low speed. Generally, torque refers to the force that moves the wheel forward, but if traced back, it will be determined by the force with which the combustion gas pushes the piston. This force is stronger when the air is compressed. Due to the structure, the diesel engine compresses only air in the compression process, so unlike a gasoline engine that compresses an air-fuel mixture, knocking is less likely to occur, so more air can be compressed. Therefore, diesel engines have higher torque than gasoline engines. Regarding high rotation, a large amount of very fine air vortices are generated in the cylinder when the vertical movement of the piston becomes high speed, but in a gasoline engine it helps prevent combustion because it helps spread the combustion, so high rotation is required. It is possible. However, in a diesel engine, a vortex in the air creates a mixture before the air and fuel are mixed together, causing a sudden combustion and vibration-this is called diesel knock-and therefore gasoline The engine has the advantage of high rotation compared to the diesel engine.

I try to avoid technical terms as much as possible, or to add annotations, and try to write sentences that are easy for people who do not understand cars to understand, but I am sorry if there are points that cannot be reached.

We hope that the reader will become interested in the engine.

Finally, another type of engine is the rotary engine. A phantom engine that Mazda has put into practical use.