How to circulate the engine oil —— Talking about the engine lubrication system

Everyone knows that engine oil is used to lubricate the engine, and the engine oil should be changed every once in a while. But do you know how oil flows in the engine? How is the oil delivered to all friction surfaces? Today, we will talk in detail about how the engine lubrication system works and the process and route of oil circulating in the engine.

First, let me tell you why the engine needs lubrication.

Everyone knows that the engine must be oiled. The main function of engine oil is lubrication. Without oil, the engine will be damaged, but why it will be damaged may not be clear. Automobile engine is a complex energy conversion machine, which converts the chemical energy of fuel into mechanical energy and outputs it to the outside world. In the process of energy conversion, a large number of components move according to a certain trajectory. In the process of movement, a part acts on another part with a certain force, resulting in high-speed relative motion, which is called friction pair. There are dozens of friction pairs in automobile engines, and the fit clearance between friction pairs is also different, ranging from several microns to tens of millimeters.

The surface of any part is not absolutely smooth, but has a certain degree of surface roughness. Even if we look bright and clean, the surface of the parts is uneven under the microscope. Therefore, friction and wear will inevitably occur on the surfaces of these friction pairs. Friction will produce resistance, hinder the movement of parts and consume the power of the car. Experiments show that the friction resistance of the engine will consume about 25% of the power of the engine. We can also take a more vivid look at the magnitude of this friction resistance: we park the car on a slope of 30%, shift into first gear, and release the brakes, handbrake and clutch, so that the car can stop in place stably, which is mainly due to the internal friction resistance of the engine.

In addition, the relative motion between friction pairs generates heat through friction, which makes the parts heat. Without lubrication, two metals will rub at high speed, and the heat generated can even melt the metals and make them sinter together. This is the so-called engine "cylinder pulling" and "tile melting". Friction pairs are sintered together due to lack of lubrication and heat dissipation, which is the most serious engine mechanical failure.

The types of automobile engine friction can be divided into sliding friction and rolling friction. Rolling friction resistance is small, so try to use rolling friction in cars, such as various bearings are typical rolling friction; However, the sliding friction resistance is relatively large, such as between piston ring and cylinder wall, between crankshaft and cylinder block, and between connecting rods. In addition, according to the friction medium between friction pairs, it can be divided into dry friction, boundary friction, mixed friction and liquid friction. These kinds of friction exist simultaneously in the automobile, and most parts of the engine are liquid friction, such as crankshaft and cylinder block, connecting rod, camshaft and valve, etc. And there is boundary friction between the cylinder wall and the piston ring.

In order to reduce engine power consumption and parts wear caused by friction, it is necessary to lubricate the friction surfaces of parts, that is, add a layer of lubricating oil between the working surfaces of two parts to form an oil film, so that the parts are completely separated and in a state of complete liquid friction. In the automobile engine, the device that can accomplish this task is the lubrication system, which can continuously deliver enough clean engine oil with appropriate temperature to the friction surfaces of transmission parts when the engine is working, and form an oil film between the friction surfaces to realize liquid friction, thus achieving the purposes of reducing friction resistance, reducing power consumption, reducing parts wear and improving engine reliability and durability.

The engine lubrication system is mainly composed of engine oil, filter, oil pump, oil filter, oil cooler, oil pipeline, oil pressure sensor, oil pressure gauge, oil pressure alarm lamp, oil thermometer, pressure limiting valve, one-way valve and bypass valve. Among them, engine oil is the working medium of lubrication system, which has the functions of lubrication, cleaning, cooling, sealing, rust prevention, buffering and vibration reduction, and is also the working medium of hydraulic valve tappets, variable valve timing mechanisms, timing chain tensioners and other devices. It is the most important liquid in the engine and is called "the blood of the engine". There are many knowledge points about it, which have been discussed in detail in many articles before, so I won't go into details here.

The oil pump is the power device in the lubrication system, and the pressure of the lubrication system is established by the oil pump, which is generally driven by the crankshaft timing gear. Its function is to ensure the circulating flow of oil in the lubrication system, and at any engine speed, it can deliver enough oil to the lubrication part with high enough pressure. Oil pump can be divided into gear type and rotor type according to its structural form, and gear type can be divided into internal gear type and external gear type. The advantages of gear oil pump are high efficiency, low power loss and reliable operation. The advantages of rotary oil pump are compact structure, large oil supply, uniform oil supply, low noise and high oil suction vacuum. Generally, the rotary oil pump is used in gasoline engines for automobiles, and the gear oil pump is used in diesel engines for trucks. In addition, due to the different lubrication requirements of engines at different speeds, more and more engines begin to use variable displacement oil pumps, which can output oil with different pressures and flows at different speeds to meet the lubrication requirements of engines at various speeds.

The filter screen is installed in front of the oil pump to suck the oil in the oil pan. It is a filter with metal mesh, which can prevent large mechanical impurities from entering the oil pump. In fact, it is part of the oil filtration system. Its structure is very simple, consisting of a cover, a filter screen and an oil suction pipe. When the engine is working, the engine oil is sucked out from the gap between the cover and the filter screen, larger impurities are filtered out, and clean engine oil is sucked into the oil pump. When the filter screen is blocked, the vacuum formed by the oil suction of the oil pump forces the filter screen upward, so that the round hole of the filter screen leaves the cover, and at this time, the oil directly enters the oil suction pipe to avoid oil supply interruption.

Everyone is familiar with the oil filter, commonly known as the "oil filter element", which should be replaced at the same time every time the oil is changed. The oil filter is installed behind the oil pump and in front of the main oil passage. Its function is to filter metal chips, carbon deposits, colloid and other impurities in the oil, ensure the cleanliness of the spare oil, purify the circulating oil and then send it to the surface of the moving parts, ensure the good lubrication of the friction surface and prolong its service life. At present, the paper filter element is the most widely used in the engine, which is made of specially made microporous filter paper and can filter out impurities with a diameter of 0.05~0. 1mm in the engine oil.

According to the way of oil filtering, lubrication system can be divided into two categories: full-flow type and split-flow type. The full-flow oil filter is connected in series between the oil pump and the main oil circuit, so all the oil is filtered through it. At present, full-flow oil filters are widely used in cars and trucks. Split-flow type means that part of the engine oil comes out of the oil pump, enters the fine filter, and returns to the oil pan after more detailed filtration. Fine filters are generally centrifugal, which can remove fine impurities with a diameter of more than 0.00 1mm from engine oil. Generally, it does not need to be replaced, but it can be cleaned during maintenance. This shunt filtration? Generally used in large diesel engines, but in order to ensure the supply of engine oil, only 10% of the engine oil is filtered by a fine filter, and 90% of the engine oil is filtered by a coarse filter. In addition, in order to ensure the pressure of the main oil circuit, when the oil pressure is too low, the filtering channel of the fine filter will be closed.

I want to say a few more words about the oil filter. It is a consumable in the car, and some people don't pay much attention to it. They often use very expensive oil, but only use cheap oil filter elements. In fact, its role is very important. If the oil is compared to the blood of the engine, the oil filter can be called "the kidney of the engine", which shows its importance. A very important index of oil filter element is pollution ability, which refers to the ability of oil filter to filter and contain impurities in oil. It is directly related to the amount of filter paper in the oil filter. Generally, the larger the filter, the more filter paper, the greater the pollution capacity and the longer it can be used. In addition, there are two important valves in the oil filter-check valve and bypass valve, which can prevent the oil from flowing back and avoid the interruption of oil supply when the oil filter is blocked. In many fake engine oil mechanical filters, these parts are unqualified, which will affect the filtering effect and supply of engine oil, so we must choose high-quality genuine engine oil mechanical filters.

Oil cooler is generally used for heavy load, high reinforcement and high pressurization engines. Ordinary naturally aspirated engines are rarely used, also called oil radiators. Its function is to dissipate the heat of engine oil into the surrounding air to prevent the engine oil from overheating. Series connection in lubricating oil channel is generally divided into air cooling and water cooling, which are widely used in civil vehicles. The oil cooler is installed in the cooling water channel to cool the oil through the circulating flow of coolant. This arrangement is small in size and convenient to arrange, which will not cause excessive cooling of the engine oil and stabilize the engine oil temperature. Because the pressure of engine oil is higher than that of coolant, if engine oil is found in water, it can basically be judged that the engine oil cooler is damaged.

Oil pressure sensor, oil temperature sensor, oil pressure gauge, oil pressure warning light and oil thermometer belong to the signal and alarm devices of lubrication system, which can display the working state of lubrication system on the dashboard to remind drivers to pay attention. Usually, there are two oil pressure sensors. One is installed on the main oil gallery to monitor the total pressure of the lubrication system. The other is installed on the cylinder head to monitor the oil pressure on the cylinder head. Generally, when the oil pressure in the main oil gallery is low, the oil pressure warning light will light up and ring for a long time, while when the oil pressure in the cylinder head is low, the oil pressure warning light will blink and ring intermittently. Generally, the oil temperature is slightly higher than the water temperature, ranging from 100 ~ 1 10℃. If it exceeds a certain threshold, an alarm will be given. However, there is generally no oil temperature display in ordinary household cars.

Next, let's look at how the oil circulates in the engine. Before that, let's take a look at the lubrication mode of the engine. Because of the different working conditions of engine transmission parts, different lubrication methods are adopted for transmission parts with different loads and relative moving speeds, mainly including pressure lubrication, splash lubrication and grease lubrication.

Pressure lubrication refers to the lubrication method of supplying oil to the friction surface at a certain pressure, which is generally used for lubrication of friction surfaces with heavy loads such as main bearings, connecting rod bearings and cam bearings. Splash lubrication is a kind of lubrication method that uses oil droplets or oil mist splashed by moving parts when the engine is working to lubricate the friction surface. This method is mainly used to lubricate the working surfaces of the cylinder wall with light load and the cam, tappet, valve stem and rocker arm of the valve train. Grease lubrication refers to lubricating the working surfaces of water pumps, generator bearings and other components by adding grease regularly through grease nozzles, which is rare now.

Now let's look at the circulation route of oil in the engine. Engine oil is stored in the oil pan. When the engine starts running, it drives the oil pump to rotate. Because the oil pump has self-priming function, oil can be sucked up from the oil pan through the filter screen, and a certain pressure can be established and sent to the oil filter. In order to prevent the oil pressure from being too high and the oil viscosity from being too high when the cold car is started, a pressure limiting valve is usually installed on the oil pump when the engine is running at high speed. When the oil pressure exceeds a certain value (generally 0.8MPa), the pressure limiting valve is opened and the excess oil is directly discharged into the oil pan. The flow of oil pump is generally between 10-40L/min. For most engines, the oil can be circulated once in 30 seconds.

After the oil with a certain pressure comes out of the oil pump, if it is fully filtered, all the oil will enter the oil filter; In case of split-flow filtration, about 10% of the oil will enter the secondary oil filter. In order to ensure the normal oil pressure in the main oil circuit and prevent the oil from shunting when the oil pressure is too low, an oil inlet pressure limiting valve is set in front of the fine oil filter. Only when the oil pressure exceeds 0. 1MPa can the oil push the pressure limiting valve open and enter the secondary oil filter. After entering the fine oil filter, the engine oil with a certain pressure will be ejected from two tiny nozzles, and the generated reverse force will push the fine oil filter to rotate at a high speed (the rotation speed can reach 10000 rpm), which will drive the engine oil to rotate at a high speed, and the generated centrifugal force will throw impurities in the engine oil onto the inner wall of the fine oil filter housing for precipitation, and the filtered clean engine oil will directly flow back to the oil pan from the oil outlet. In order to prevent the abnormal oil pressure caused by the blockage of the secondary oil filter, a bypass valve is also set. When the pressure exceeds 0.6MPa, the bypass valve opens and the oil directly flows back to the oil pan from the bypass valve. The secondary oil filter will have a slight "buzzing" sound when working, which will last for several seconds after the engine is turned off. This is also one of the ways to judge whether it works normally.

Most automobile engines use full-flow filtration. After the oil with a certain pressure comes out of the oil pump, all the oil will enter the oil filter and enter through the small holes around the filter element. After being filtered by the filter paper, impurities and colloid will remain on the surface of the filter paper, and clean engine oil will enter the middle of the filter element, flow out from the oil outlet and enter the main oil circuit. There are two very important valves in the oil filter: bypass valve and check valve. When the oil filter element is blocked, the bypass valve is opened, and the oil directly enters the main oil circuit without being filtered by filter paper; The purpose of the check valve is to prevent the oil from flowing back to the oil pan after the engine is turned off, to ensure that the oil pressure is directly formed after the engine is started, and to minimize the dry friction time of the engine. The opening pressures of these two valves are accurately calculated. If inferior oil filter is used, the quality of these two valves is poor and the filtering effect of filter paper is poor, which will seriously affect the filtering effect of oil.

If equipped with water-cooled oil radiator, the oil will directly enter the oil radiator after coming out of the filter, and flow into the main oil circuit after being cooled by engine coolant. In order to avoid the shortage of oil supply caused by the blockage of the oil radiator, and to avoid the damage of the oil radiator caused by excessive oil pressure, a bypass valve is set on the oil radiator. When the oil pressure is higher than 0.4MPa, the bypass valve opens and the oil directly enters the main oil circuit.

The engine oil entering the main oil gallery has a certain pressure, that is, the so-called "engine oil pressure", which is generally between 0.2~0.5Mpa and 0.5Mpa. An oil pressure sensor is installed here, which converts the oil pressure value into an electrical signal and transmits it to the dashboard for the driver's reference. Generally, on trucks or construction machinery, the oil pressure gauge is used to display the oil pressure; Oil pressure warning lights are widely used in passenger cars. If the oil pressure is too low, it will cause poor lubrication and aggravate the wear of parts; If the oil pressure is too high, it will lead to engine sealing difficulties, oil seal leakage, increased fuel consumption and so on.

In the main oil gallery, the oil will flow in different directions according to different engine structures, such as crankshaft main bearing, connecting rod bearing, piston nozzle, camshaft, valve rocker arm, hydraulic valve tappet, timing gear, timing chain, supercharger, air pump, fuel injection pump, variable valve timing mechanism, timing chain tensioner, etc. Let's talk separately.

1. Main oil gallery→ crankshaft main bearing and connecting rod bearing: oil enters the crankshaft main bearing from the branch on the main oil gallery. When the crankshaft has multiple supports, there are usually multiple branch oil passages. For example, a four-cylinder engine usually has five branch oil passages, and a six-cylinder engine usually has seven branch oil passages. There are corresponding holes in the crankshaft main bearing, from which oil enters the lubrication friction pair between the main bearing and the crankshaft, and the excess oil flows out from the gap between the main bearing and the crankshaft and directly returns to the oil pan. Part of the oil flows from the crankshaft main bearing into the connecting rod bearing through the oil passage inside the crankshaft to lubricate the connecting rod bearing and the connecting rod journal, and the excess oil also flows out from the gap between them and returns to the oil pan. For the lower camshaft, part of the oil flows into the camshaft bearing from the crankshaft main bearing to lubricate the friction pair between the camshaft and the cylinder block, and the excess oil flows out from the overflow hole on the camshaft journal and returns to the oil pan under the action of gravity. These parts belong to the category of pressure lubrication, and most of the engine oil is used in these parts. When the clearance between friction pairs in these parts increases, the oil leakage will increase and the oil pressure will decrease, which generally requires overhaul.

2. Main oil gallery → piston nozzle: At present, the combustion chamber temperature of many high-strength engines is high, and the temperature of the piston is also high, so special devices are needed to cool the piston, which is done by the piston nozzle. The nozzle is installed at the position corresponding to the main oil passage of each cylinder. When the pressure of the main oil passage exceeds 0. 1MPa, the valve on the nozzle is opened, and oil with a certain pressure is directly sprayed to the bottom of the piston to cool the piston. A small part of the spilled oil is used to lubricate the piston pin and cylinder wall, and the rest falls back to the oil pan.

3. Main oil gallery → overhead camshaft, valve rocker arm, valve, etc. The oil branches upward from the main oil passage, directly reaches the cylinder head, and then enters between the camshaft journal and the cylinder head, and between the valve rocker arm and the rocker arm shaft to lubricate these parts under pressure. Excess oil flows out from the gap between these friction pairs and is splashed around by the high-speed rotating camshaft to lubricate the valve rocker arm, valve stem, valve spring and so on. Finally, all this oil flows out of the oil passage on the cylinder head. In order to avoid the oil backflow after the engine is turned off, there is a one-way valve at the position of the oil port, which can keep a certain oil pressure on the cylinder head.

4. Main oil gallery→ timing gear, timing chain, supercharger, air pump, fuel injection pump, etc. These parts need special lubrication, so a special lubricating gallery or nozzle is generally set up to introduce the oil, and the lubricated oil flows back to the oil pan under the action of gravity. What is particularly important here is the lubrication of supercharger. Due to the high rotor speed and working temperature of the supercharger, the required oil flow is large, the pressure is sufficient, and the corresponding oil pipe is relatively thick.

5. Main oil circuit → hydraulic valve tappet, variable valve timing mechanism, timing chain tensioner: These devices belong to the hydraulic servo mechanism of the engine, and the oil will keep a certain pressure in them, often in a closed state, and there is little circulation communication with the lubrication system. When adjusting the internal pressure, the excess oil flows out directly and returns to the oil pan along the fixed oil passage.

The above is the structure and working principle of the main components of the engine lubrication system, and the circulating flow path of the engine oil in the lubrication system. It can be seen that the oil in almost all parts flows directly into the oil pan after lubrication, and then is sucked into the oil pump again to participate in the next cycle. Generally, the oil circulates in the engine every half minute, which should be said to be very efficient.

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