Is the automobile fan blowing or exhausting —— Talking about the working principle of the automobile cooling fan

Today, a netizen asked me whether the fan in the car water tank blows in or exhausts out. This problem depends on the installation position of the engine. For most cars, trucks, etc. The engine is installed in front of the car, in which case the fan blows air in; For buses with rear engines, as well as construction machinery such as loaders and excavators, fans exhaust air.

Fans have a great effect on cars. Let me tell you an interesting thing first: the engine of the loader is always hot after being overhauled, let alone working, and it will be hot after being idle for a long time. The repairman repeatedly checked many places, including changing the water pump, cleaning the water tank, changing the thermostat, and even removing the thermostat, which was still on the high side. I suspected that the cylinder gasket was cracked, so I replaced it, but it still didn't work. The replaced cylinder gasket was also good. The fault is in trouble. Later, I accidentally walked behind the car and immediately felt that something was wrong-the engine should have hot air under normal circumstances. Why isn't this car available? I immediately thought of a problem. I opened the engine compartment and saw that the fan was installed backwards!

Of course, this is a special case. Most models of fans have fixed installation positions, so it is difficult to install them incorrectly. But it also shows the importance of fans to cars. Without it, the car can't work normally. The early automobile fans were relatively simple and generally operated synchronously with the engine. Now the automobile fans are more complicated, especially in the control logic. Today, let's talk about the types, working principles and control logic of fans in detail.

The full name of automobile fan should be "automobile cooling fan". Its function is to let more air flow through the radiator, enhance the heat dissipation capacity of the radiator, accelerate the cooling speed of the coolant, and let more air flow through the engine to take away the heat emitted by the engine. In fact, when the car is driving normally, the oncoming wind can basically meet the cooling demand of the car, so in most cases, there is no need for fans to cool down. Only under some special working conditions, such as high load and low speed, or driving at low speed for a long time and idling for a long time, there is basically no oncoming wind, so fans are needed to force air to flow through the radiator to speed up the cooling of the coolant.

Automobile cooling fans are all axial-flow, that is, when the fan rotates, the air flows along the axis of the fan shaft, just like our household fans. Corresponding to it is the radial fan, whose airflow direction is distributed from the middle to the periphery, similar to centrifugal pump, such as the fan of automobile air conditioning evaporation box, the fan of household air conditioning, and so on. Relatively speaking, the axial flow fan has faster wind speed, higher wind pressure, larger air volume, better cooling effect, but greater noise and vibration. Axial flow fans are also widely used, such as tunnels, factory ventilation projects, wind tunnels and so on. But this kind of fan is directional, so it must not be installed backwards, otherwise the wind direction will change completely. Just like the example at the beginning, the fan is installed backwards. Instead of cooling the engine, it returns heat from the water tank to the engine.

In the past, automobile cooling fans were basically made of thin steel plates, which were relatively heavy, had large moment of inertia and consumed large engine power. Now they are mostly made of engineering plastics. Generally, the number of fan blades is 4~7, and the deflection angle between the blades and the rotation plane is 30 ~ 45, so that the air can flow along the axial direction with the help of the suction ability of the car body. Fan air volume is related to fan diameter, speed, blade shape, blade installation angle and blade number. In order to reduce the back pressure and vortex resistance of the blade, the blade is made into different shapes, such as narrow blade tip, wide blade root, forward bending of blade tip, gradual change of deflection angle and so on. In addition, in order to reduce the noise and vibration when the fan rotates at high speed, the interval angles between the blades are not equal, which can make the interval impact time of the blades on the air different when rotating.

It is also particularly important that the fan is a high-speed rotating part, and its dynamic balance is very important. If the fan is damaged in use, the engine shakes seriously, the air volume decreases, and the wind speed decreases, it must be replaced in time. I once met a case where the fan blade of a forklift fell off without the driver knowing it. It was still used for a long time, until one day the engine sounded abnormally. I took it apart and saw that the crankshaft tile had been ablated first. This is because the high-speed rotation of the fan is unbalanced, resulting in centrifugal force acting on the first bearing bush of the crankshaft, which eventually leads to poor lubrication and ablation of the crankshaft.

There are two ways to drive the fan: direct drive and indirect drive.

Direct drive means that the fan is directly installed on the crankshaft of the engine, or the crankshaft drives the fan to rotate through a belt or gear. Most trucks and construction machinery adopt this driving mode. As long as the engine is running, the fan rotates synchronously with the crankshaft. It should be noted that this driving mode will greatly consume the power of the engine. The calculation shows that the fan will consume at most 10% of the engine power.

In order to reduce the power consumption of the fan to the engine, and at the same time avoid the engine from being too cold and overheated, the current engine generally adopts a fan clutch to control the working time and speed of the fan. The fan clutch consists of a front cover, a shell, a driving disc, a driven disc, a valve plate, a driving shaft, a bimetallic temperature sensor, a valve plate shaft, a bearing and a fan. Its working principle is that the bimetal senses the temperature of the water tank and controls the opening time and angle of the valve plate through the deformation of the bimetal. When the temperature of the water tank is low, the valve plate is closed, the silicone oil does not enter the working room, the fan does not rotate when separated from the driving shaft, and the cooling intensity is low; When the temperature of the water tank is high, the valve plate opens, and silicone oil enters the working chamber, so that the fan and the driving shaft are combined through the high viscosity of silicone oil, and the two rotate synchronously, so that the fan has higher rotating speed and greater cooling intensity. The greater the opening angle of the valve plate, the more silicone oil enters the working chamber, the closer the combination between the fan and the driving shaft, and the higher the rotation speed of the fan, thus realizing the adjustment of cooling intensity.

If the fan clutch can't be combined with the drive shaft all the time because of some fault, then the fan can't rotate at high speed all the time, the cooling intensity is low, and the automobile may have a fault of too high temperature when running under high load. In order to avoid this situation, there is an emergency device on the fan clutch and a locking plate on the shell. Just insert the pin of the locking plate into the hole of the driving plate, and then screw on the screw, so that the housing and the driving shaft can be connected into a whole, and the fan will run synchronously with the driving shaft. But at this time, it can not be used for a long time only by pin drive, and the fan is always at the highest cooling intensity, which is not conducive to the preheating and heating of the engine. Judge the fan clutch fault? One method is: when the engine temperature is normal, turn the fan blades by hand, and if you can feel a large resistance, it means that the fan clutch is normal; If the resistance of the fan clutch is small at this time, it can be easily rotated, indicating that the fan clutch has been damaged.

There are two indirect driving modes for fans, one is electric and the other is hydraulic. Let's talk about electric first.

Most cars and passenger cars have electric cooling fans, that is, a motor directly drives the fan to rotate. The electric fan is simple in structure, convenient to install, does not consume engine power, and improves the fuel economy of the automobile. In addition, there is no need to check, adjust or replace the fan drive belt when using the electric fan, thus reducing the maintenance workload. Generally, there are two electric fans in the car, one is the same size, the other is big, and some cars have air-conditioning condenser fans. The starting and running speed of the fans are determined according to the difference of engine water temperature and whether the air conditioner is turned on or not.

The early electric fan control circuit and logic were simple, only controlled by temperature control switch and air conditioning pressure switch, which met the conditions of any switch and automatically turned on the fan. The temperature control switch is installed on the water tank and directly senses the temperature of the coolant. It is actually a two-stage resistance switch, and the internal resistance is divided into two stages to control the high and low speed operation of the fan respectively. When the water temperature exceeds 90℃, the first gear of the temperature control switch is turned on, the fan rotates at a low speed, and the heat dissipation capacity of the water tank is low; When the water temperature exceeds 105℃, the second gear of the temperature control switch is turned on, and the fan rotates at high speed, which increases the air flow through the water tank and the cooling intensity. If the air conditioner is turned on, the air conditioner pressure switch will directly give the electric fan? A signal that the electric fan runs directly has nothing to do with the water temperature.

In daily use, sometimes the temperature control switch fails. At this time, no matter how high the water temperature is, the electric fan will not start, causing the engine to heat up. In this case, we can unplug the temperature control switch, and then short two plugs with a wire, so that the electric fan will rotate. The speed is related to our shorted plug. Generally speaking, we can shorten a lower speed.

Nowadays, the automobile electronic control system is becoming more and more complicated, and the control logic of electric fans is becoming more and more complicated. Generally, the engine control unit is used to control the start and operation of the electric fan. Considering the parameters of the engine and its surrounding environment comprehensively, there is an emergency operation mode, which is more energy-saving and achieves the purpose of saving energy and reducing consumption. But it also brings the disadvantages of complex signal control and difficult maintenance. For example, the engine coolant temperature signal is lost, and the water tank outlet temperature signal is lost. In order to prevent the engine from overheating, the engine control unit will instruct the electric fan to run at high speed. If the air conditioning high pressure sensor signal is missing, it will indicate that the air conditioning system will stop working; There is also a very special situation, that is, when the speed signal is missing, the engine will mistakenly think that the car has been driving at a high speed, and at this time it will also command the electric fan to rotate at a high speed.

Another feature of electric fans is time delay control. We often encounter that after the car has been driven for a long time, even if the engine is turned off, the electric fan will run at a high speed for a period of time, even for more than ten minutes at the longest. This is mainly because the engine water temperature is high after the car has been driving for a long time. If we turn off the engine directly and the water pump stops running, the local temperature of the engine will rise rapidly, and in severe cases, metal will melt and stick. In order to avoid this situation, the electric fan still runs for a period of time after stopping to reduce the temperature of the engine and avoid the high temperature damage of the engine.

Another indirect fan drive mode is hydraulic drive, which is mainly used for excavators and some air-cooled engines. The fan is mounted on the hydraulic motor. When the engine starts, the temperature reaches a certain level, the oil circuit of the hydraulic motor is connected, and the motor runs to drive the fan to rotate, providing cooling air flow for the engine. The speed of the fan can be controlled by hydraulic motor, with low water temperature and low speed, high water temperature and high speed. The power of hydraulic motor on excavator comes from hydraulic pump, and the power of air-cooled engine comes from oil pump.

There is also a very important device on the fan, which is the wind guide cover, commonly known as the "wind ring". Its function is to avoid wind divergence in the direction of steering flow. If it is missing or damaged, the cooling capacity of the fan will be greatly reduced. The air deflector on an air-cooled engine has the same function. There was an example. After an air-cooled engine was overhauled, the air deflector was not installed properly and disappeared. The repairman didn't understand it and thought it had little impact. Results The engine pulled quickly at high temperature.

The above is the relevant knowledge of car fans. In daily use, the most common fault is that the fan does not turn. When this kind of fault occurs, firstly, directly power on the electric fan. If the fan rotates, it means that the fan itself is in good condition and the fault lies in the control line. Then, according to the control principle of the fan, we check step by step, and the failure rate of the general temperature control switch and water temperature sensor is high. For truck fans, the failure rate of fan clutch is the highest, and the inspection method has been mentioned above, so I won't go into details.

This article comes from car home, the author of the car manufacturer, and does not represent car home's position.