Compression ignition engine is an internal combustion engine that does not rely on spark ignition, but relies on the high temperature and high pressure of the charge in the cylinder at the end of compression to cause the mixture to spontaneously ignite.Compression ignition engines or diesel engines are widely used in cars, freight cars, locomotives, ships and power generation.Most compression ignition engines often use exhaust turbocharging, which can reduce the size and mass of engine unit output power.As compression ignition is adopted, the compression ratio is larger than that of positive ignition engine, depending on the cylinder diameter and type of engine, as well as whether it is naturally aspirated or supercharged.Diesel engine is a typical compression ignition engine.
At present, the power devices used on the aviation model mainly include rubber band, piston engine, jet engine, electric engine and compressed gas engine.The piston engine can be divided into:Compression combustion type(compression ignition type)electrothermal (Hot hydrant type) andSpark ignitionThree.This book mainly introduces compression ignition engines widely used in China.Finally, the appendix briefly introduces the electrothermal and spark ignition engines.
The piston aero model engine is a small internal combustion engine, generally called a small engine.Its basic components and working principles are basically the same as those of internal combustion engines (including diesel engines and gasoline engines) introduced in middle school physics books, and are also similar to those commonly seenWalking tractor、motorcycleOr the engines used in cars are basically the same, but much simpler.Although the small engine is very small, there are only one or twentyspare partsBut it is already a kind of precision machine, so we must study it carefully and use it scientifically.
In the process of using small engines, model enthusiasts should pay attention to integrating theory with practice, and apply the basic knowledge about engines learned from books to specific practice.It is necessary to understand the working principle, fuel composition, starting steps and adjustment methods of small engines, learn how to remove faults, and pay attention to developing correct operation methods, so as to lay a good foundation for better learning and using various mechanical equipment in the future agricultural mechanization movement, or in industrial, mining and scientific experiments.[1]
Basic structure
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Gearbox
The gearbox is the housing of the engine, which is used to connect the cylinder, crankshaft, piston, carburetor and other parts to make it a whole.On the left and right sides of the gearbox, there are convex edges for installing the engine.The rear of the gearbox is provided with a threaded gearbox rear cover to ensure the internal cavity of the gearbox is sealed.The casing of the two-stroke engine is also a channel for fresh mixture. The fresh mixture enters the casing from the carburetor and enters the cylinder through the rotary port when the piston moves downward.
Cylinder head
The cylinder head is screwed on the cylinder through threads, and there are fins around it to increase the area of contact with air and help the cylinder cool.The top of the cylinder head is threaded to screw in the pressure regulating rod.
Carburetor
The function of the vaporizer is to change the fuel from liquid to mist, and then mix it with air in proper proportion to become combustible mixture.The carburetor of small engine is generally composed of intake pipe, fuel injection pipe and regulating oil needle, which can be regarded as the simplest carburetor.The fuel injection pipe crosses the intake pipe and has 1~2 small holes for spraying liquid fuel.The fuel flow is regulated by the oil needle with taper at the head.Tighten the oil needle clockwise, the injection hole is blocked, and the fuel cannot flow out;Unscrew the oil needle, and the fuel will flow out of the injection hole.Therefore, rotating the oil needle can adjust the fuel flow.
Cylinder and piston
The cylinder is the place where the mixed gas of fuel and air is burned, and also the place where the heat energy released after fuel combustion is converted into mechanical energy.The cylinder is cylindrical, and the internal surface is very smooth, similar to a mirror.When the mixed gas in the cylinder burns and expands, it produces high pressure, which acts on the piston crown and pushes the piston downward;Through the crankshaft connecting rod mechanism, the crankshaft rotates and drives the propeller to rotate, generating tension to make the aircraft move forward.When the engine rotates, the piston moves back and forth in the cylinder at a high speed.The cylinder wall is provided with air distribution holes such as exhaust port and air switch port.When the piston reciprocates in the cylinder, it controls the opening and closing of the air distribution holes such as the exhaust port and the rotary port at the same time.
The cylinder and piston are the most important and precise parts of the small engine, and their matching is very accurate to ensure the sealing and compression performance.If it is used improperly or dirt such as dust and sand enters the cylinder, it will wear the cylinder and piston quickly, affect the sealing performance, cause the engine speed to drop, or even fail to start.
When the piston moves back and forth in the cylinder, there are two limit positions due to the limit of the length of the crank arm.The highest position that the piston can reach, that is, the farthest position from the crankshaft rotation center, is called top dead center;The lowest position is called bottom dead center.The distance traveled by the piston from TDC to BDC (or from BDC to TDC), that is, the distance from TDC to BDC, is called piston stroke.When the piston is at TDC, the volume contained by the top surface of the piston, the lower surface of the counter piston and the side wall around the cylinder is called combustion chamber volume.When the piston is at BDC, the volume contained by the piston, counter piston and cylinder wall is called the full volume of the cylinder.The volume of the cylinder between TDC and BDC, that is, the volume of the piston in a stroke, is called the working volume of the cylinder.Usually we say that this is a 1.5 ml (c.c.) small engine, which means that the cylinder working volume of this small engine is 1.5 ml.Generally, 1.5~2.5ml small engines are suitable for universal use.Here is a comparison: when injecting an anti epidemic needle, one milliliter of medicine is often needed;It can be seen that the cylinder working volume of this engine is very small.Another example: the working volume of the engine cylinder of Qingqi dual-use motorcycle is 55ml;The engine working volume of Shanghai "Happiness" 250 two wheeled motorcycle is 250ml; the engine of "Jiefang" truck is six cylinders, and the total working volume is 5.55L, or 5550ml.Generally speaking, the greater the working volume of the cylinder, the greater the power.
The ratio of the total volume of the cylinder to the volume of the combustion chamber is called the compression ratio.In the example in Figure 2, the total volume of the cylinder is 12 times that of the combustion chamber.That is to say, when the piston is at the bottom dead center, the volume of mixed gas in the cylinder is 12 parts, and when it reaches the top dead center, it will be compressed into 1 part.Therefore, its compression ratio is 12.
Crankshaft connecting rod mechanism
The piston can only move in a reciprocating straight line in the cylinder.Through the crankshaft connecting rod mechanism, the reciprocating linear motion of the piston is transformed into the rotary motion of the crankshaft.Just like the sewing machine we see everyday, as long as you press the pedal up and down with your foot, the flywheel will rotate through the connecting rod and crank.
Crankshaft is a part with large force in the engine.Its front end is equipped with front and rear propeller pads and nuts, and the rear crank arm (crank) is connected with a crank pin.The connecting rod is used to connect the crankshaft and the piston. One end of the connecting rod is sleeved on the crank pin of the crankshaft, and the other end is sleeved on the piston pin and connected with the piston.These interconnected and movable parts are commonly called crankshaft connecting rod mechanism, or crank connecting rod mechanism and crank connecting rod mechanism.
Counter piston and pressure regulating rod
The counter piston is like a cylinder head that can move up and down.Tighten the pressure regulating rod (clockwise rotation), and the counter piston is pressed down;Unscrew the pressure regulating rod, and the counter piston can spring up again under the pressure generated when the gas in the cylinder is compressed.
The up and down movement of the counter piston can change the size of the combustion chamber, thus changing the compression ratio.The lower the position of the reverse piston in the cylinder, the smaller the volume of the combustion chamber and the larger the compression ratio. The smaller the volume of the combustible mixture in the cylinder after compression, the higher the pressure and temperature of the mixture, which is easy to ignite and burn.However, the compression ratio should be properly controlled.If the compression ratio is too large, the mixture will start burning too early, causing detonation and shutdown, and even breaking the connecting rod or crankshaft and other parts;If it is too small, the pressure and temperature generated by the compression of the mixture are not enough. If the temperature is lower than the ignition point of the mixture, the mixture cannot ignite and burn, and the engine cannot be started, or the engine cannot run stably and continuously.
For compression ignition engines, the compression ratio is particularly important for starting and running performance, and must be mastered.The compression ratio of compression ignition small engines is mostly adjustable, generally 15~25.The method of determining compression ratio will be introduced later.
Parts for fixing propeller
The propeller is installed between the front and rear propeller pads and is tightly fixed by the nut.The rear propeller pad usually has a tapered hole to press against the tapered part of the crankshaft.In order to prevent the propeller from slipping, there are concave and convex grooves on the surface of the rear propeller pad.
