The working principle of diesel engine

A machine that converts energy into mechanical energy is called an engine. According to different energy sources, various engines can be divided into: wind engine (wind turbine); Hydraulic engine (hydraulic engine for short); Heat engine (heat engine), etc. Engines that convert the heat energy generated by fuel combustion into mechanical energy are collectively called heat engines, such as steam engines and diesel engines. According to the location of fuel combustion process, the heat engine can be divided into external gas turbine and internal combustion engine.

An engine that burns fuel outside an engine is called an external gas turbine. Such as steam engine (reciprocating), steam turbine (rotary), etc.

A heat engine that burns fuel directly inside an engine is called an internal combustion engine. Such as diesel engine, gasoline engine, natural gas machine, etc.

An internal combustion engine USES the heat energy from burning fuel to do work. Diesel engine is an internal combustion engine, which is diesel fuel combustion in the engine cylinder, the production of high temperature and high pressure gas, through piston connecting rod and crankshaft mechanism into mechanical power.

I. working principle of piston internal combustion engine

Plunger is put inside a closed at one end of the cylinder, piston top face and the inner wall of the cylinder constitute a closed space, if there is a push rod connects plunger and a wheel, the plunger moves, then through the push rod driving wheel rotation, thus the heat of the air into the drive wheels rotating mechanical energy.

The working process of internal combustion engine is to continuously feed fuel and air into the cylinder according to certain rules, and burn in the cylinder to release heat energy. After absorbing heat energy, gas generates high temperature and high pressure, which pushes the piston to do work and converts heat energy into mechanical energy.

It is made up of a separate engine. When working, fuel and air are sent directly to the engine cylinder for combustion, giving off heat energy, forming high temperature and high pressure gas, and pushing the piston to move. Then the mechanical energy is output through the crank link mechanism.

1. Cylinder body 2. Fuel injector 3. Air inlet door

4. Exhaust valve 5. Piston 6. Connecting rod 7. Crankshaft

Mechanical drive mechanism of internal combustion engine

In a reciprocating internal combustion engine, the function of crank connecting rod mechanism is to change the reciprocating linear motion of piston into the rotary motion of crankshaft to realize the mutual transformation of thermal energy and mechanical energy.

It consists of piston 1, connecting rod 3 and crankshaft 4.

The piston can only reciprocate in a straight line along the cylinder. The crankshaft is made up of two axes whose center line is on the same line. One of the shafts is located in the body's central hole called the spindle. The spindle can only rotate around its center line in the body seat hole. The other shaft is connected to the main shaft by a crank, called the connecting rod shaft. It's rotating around the principal axis. The connecting rod is a straight rod with holes at both ends and one end is connected with the piston. The other end is connected to the connecting rod shaft, which oscillates as the piston moves and the crankshaft rotates.

When the piston reciprocates, the crankshaft is driven by a connecting rod to rotate around the center of the spindle. The piston motion is related to the crankshaft rotation. Therefore, the position of piston movement corresponds to the position of crankshaft rotation.
Working principle of single cylinder four-stroke diesel engine

Piston run continuously four stroke two weeks (crankshaft) in the process of completing a work cycle (inflation of intake, compression, combustion exhaust) of diesel engine, is called a four-stroke diesel engine.

In order to show the change of gas pressure in cylinder with volume more clearly, the power diagram of single cylinder four-stroke diesel engine is drawn in figure 1-6-5. In the figure, the horizontal coordinates represent the volume of the cylinder, and from the coordinates, the absolute pressure of the cylinder. The horizontal dotted line in the figure indicates that the absolute pressure is atmospheric pressure (i.e., 1 kg/cm 2). Vc and Vh respectively represent combustion chamber volume and cylinder working volume.

The working process of a single-cylinder four-stroke diesel engine is illustrated by comparing the schematic diagram and the power diagram to illustrate its working process.

First stroke - the piston moves from the top dead center to the bottom dead center during the intake process. The intake valve opens and the exhaust valve closes.

At the beginning of the intake process, the piston is at the dead center. In the cylinder (combustion chamber), there are residual gases left over from the last cycle (represented by small cross symbol in the figure). Its pressure is slightly higher than atmospheric pressure, about 1.1 to 1.2 kg/cm.

When the crankshaft is rotating, the piston is driven down by the connecting rod and the intake valve is opened. As the piston moves down, the internal volume of the cylinder increases, and the pressure decreases. When the pressure is lower than the atmospheric pressure, fresh air from the outside begins to be sucked into the cylinder. The cylinder is filled with fresh air until the piston moves to the dead end.

In the process of fresh air into the cylinder, due to the air filter, air inlet pipe, the influence of the inlet valve and other resistance, make the gas inside the cylinder at the end of the inlet pressure is slightly lower than atmospheric pressure, which is about 0.8 ~ 0.9 kg/cm2, and because of the air to absorb heat from the high temperature of residual gas and combustion chamber wall, so the temperature can reach 35 ~ 50 ℃.

It should be pointed out that the inlet valve of diesel engine is always opened before the piston is at the dead point, and it is not closed until the bottom dead point. The reason is: if the inlet valve is opened at the beginning and cannot be fully opened immediately when the piston moves down at the beginning of the intake process, part of the vacuum in the cylinder will be generated, causing greater resistance when the piston goes down. So the inlet valve in advance before the top dead center opened, the pistons started by top dead center line, inlet valve is open to the largest position, ensure smooth air into the cylinder, so as to reduce the piston downward drag. Intake process, along the intake pipe inhaled air cylinder, air flow inertia effect, if to close the valve until after the bottom dead center, although the pistons have begun to uplink, still can make full use of the air flow inertia, the part of fresh air into the cylinder, to ensure that the suction more air. Due to the inlet valve open late close early, so the actual diesel engine intake process are greater than 180 ° crank Angle, generally for 220 ° ~ 240 °.

Second stroke - the piston moves from the lower dead point to the upper dead point during the compression process, during which the inlet and outlet doors are closed.

At the beginning of the compression process, the piston is at the bottom dead center. The crankshaft rotates under the action of inertia of the flywheel and pushes the piston up through the connecting rod. The content of the cylinder decreases gradually, the fresh air is compressed, and the pressure and temperature increase with it.

In order to achieve the purpose of high temperature gas ignition diesel, diesel engine has a larger compression ratio, made by the end of the compression, cylinder gas temperature higher than the diesel combustion temperature of 200 ~ 300 ℃, which is 500 ~ 500 ℃ (diesel combustion temperature of about 200 ~ 300 ℃), and the pressure about 30 ~ 50 kg/cm2.

To make full use of the heat produced by combustion, request combustion process in the piston to move to the top dead center position slightly after done quickly, so that the combustion gas after full expansion work more, make the diesel engine's efficiency. However, because the fuel is injected into the cylinder, it must go through certain fire preparation stages before it can be burned (see chapter 6, section 2). Therefore, the actual diesel engine work, before the end of the compression stroke (about 10 ° to 35 °) before TDC, began to spray fuel into the cylinder. In the indicator diagram, point m represents the starting time of the injection.

Third stroke - the piston moves from the top to the bottom during combustion and expansion. At this point, the inlet and outlet doors are still closed. The fuel injected into the cylinder will ignite and burn in the high temperature air, generating a lot of heat energy, which causes the temperature and pressure in the cylinder to rise sharply. High temperature and high pressure gas push the piston down and drive the crankshaft through the connecting rod. Because only this stroke can convert heat energy into mechanical energy, it is often called a working stroke.

In the process of combustion and expansion, the highest temperature of the gas in the cylinder can reach 1700 ~ 1700 ℃, the highest pressure is 60 ~ 90 kg/cm2. As the piston is pushed down, the cylinder volume increases and the gas decreases. The c - z - b line in the work diagram shows the change of cylinder volume and pressure in this process. On this curve, the nearly vertical c-z line segment indicates the degree to which the pressure increases when the fuel burns rapidly. The z point represents the combustion pressure Pz (also known as maximum explosion pressure).

Fourth stroke - the piston moves from the lower dead center to the upper dead center during the exhaust process. At this point, the exhaust valve opens and the intake valve closes.

At the beginning of the exhaust process, the piston is at the bottom dead center, and the cylinder is filled with fuel and the exhaust gas expands to work. When the exhaust valve opens, the exhaust gas moves up with the piston and is discharged out of the cylinder.

Combustion, at the end of the expansion of gas in the cylinder also has a larger pressure, if the exhaust valve in the dead point position just opened, not a sudden full influence exhaust timely discharge and the pressure inside the cylinder also cannot reduce rapidly, the piston upward movement is great resistance, consume more energy. , therefore, in the actual work of diesel engine, the exhaust valve in the piston open before it has moved to the bottom dead center (generally in the dead center before 40 ° ~ 60 °). In this way, the exhaust gas will flow out of the cylinder by itself under a large pressure difference, and the pressure in the cylinder will decrease rapidly. Greatly reduce the piston upward movement resistance, reduce the exhaust process of the consumption of work.

When the piston moves up to the upper dead center, the exhaust valve does not close immediately, but is delayed until the intake process begins. As mentioned earlier, because the inlet valve opens before the end of the exhaust process, a period of coincidence in which the inlet and outlet doors open at the same time is formed. In some cases (such as pressurization), fresh air can also be used to drain the remaining waste gas in the cylinder and fill the cylinder with more fresh air.

The crankshaft continues to rotate by the inertial action of full wheel rotation, and the above processes are repeated. So the cycle works, the realization of the diesel engine running continuously.

The working process of four-stroke gasoline engine is the same as that of four-stroke diesel engine. The main difference between gasoline engine and diesel engine is:

Fuel ignition compression ratio of the intake valve into the body structure
Gasoline engine gasoline ignites a set of ignition systems (including spark plugs, power dividers, high pressure ignition cable packs) and carburetors for the 5-10 gasoline/air mixture
Diesel fuel pressure combustion 15-22 air ignition system, no carburetor, there is fuel injector



The structure of diesel engine

The diesel engine consists of body, crankshaft connecting rod mechanism, gas distribution mechanism, fuel system, lubricating system, cooling system and starting system.

1. Body assembly: including body (cylinder - crankshaft cover), cylinder sleeve, cylinder cover and oil sump, etc. These parts form the diesel engine skeleton on which all the moving parts and auxiliary systems are supported.

2. Crankshaft connecting rod mechanism: the pressure of burning gas in the cylinder drives the crankshaft connecting rod mechanism, and changes the linear motion of the piston into the rotary power of the crankshaft. The main parts are: cylinder crankcase, cylinder cover, piston, connecting rod, crankshaft, flywheel, etc

3. Air distribution mechanism: provide fresh air to the cylinder at the right time and discharge the exhaust gas after fuel combustion in the cylinder at the right time. It is composed of inlet valve, exhaust valve, camshaft and drive parts.

4. Fuel system: the fuel supply system provides adequate fuel for the cylinder according to the time required for the internal combustion engine to operate. It consists of fuel tank, fuel filter, oil pump, fuel injector and so on.

5. Lubricating system: the lubricating system is to constantly provide appropriate amount of lubricating oil to the friction surface of various moving parts of the diesel engine. It consists of oil pump, oil filter, oil radiator and so on.

6. Cooling system: properly cool the parts working at high temperature, so that the diesel engine can keep the normal operating temperature. It consists of water pump, radiator, water jacket, thermostat, fan, etc.

7. Starting system: a device that rotates the crankshaft of internal combustion engine with external force to transfer the internal combustion engine from a static state to a working state. It consists of battery, starting motor and so on.