essay on graduation

Maintenance and maintenance of automobile engines

Department of Automobile Inspection and Maintenance

Class 2, Grade 19 Auto Repair

Student's name Ye Guangyao

Instructor xxx

Professional leader x x x

Reply date May 11, 2112 < P > Yantai Automobile Engineering Vocational College

Contents

Abstract ........................................................................................................................................................ ........................................... 4

2.2 Engine fault diagnosis methods ....................... 5 < .................................................................................................................................................................................. If you don't know how to take good care of your car or have bad driving habits, once the car has to be overhauled and specially repaired, you will not only have to pay a considerable fee, but also waste time and mental torture, which is even more difficult to count. Therefore, the car should always pay attention to maintenance, from the first day you own the car, so as to avoid penny wise and pound foolish? Based on the theoretical knowledge of automobile, this paper explains the basic knowledge of automobile engine repair and maintenance for you.

Keywords engine diagnosis, overhaul and maintenance

I. Basic structure of engine

An engine is a machine that converts a certain type of energy into mechanical energy, and its function is to convert the chemical energy of liquid or gas into heat energy after combustion, and then convert the heat energy into mechanical energy through expansion and output power to the outside world.

automobile engine

A gasoline engine usually consists of two major mechanisms, namely crank and connecting rod, gas distribution mechanism and five major systems, namely fuel supply, lubrication, cooling, ignition and starting.

1. Crank-connecting rod mechanism

Crank-connecting rod mechanism is composed of cylinder block, cylinder head, piston, connecting rod, crankshaft and flywheel. This is the power generated by the engine, which converts the linear reciprocating motion of the piston into the rotary motion of the crankshaft and outputs the power to the outside.

2. Valve train

The valve train is composed of intake valve, exhaust valve, valve spring, tappet, camshaft and timing gear. Its function is to fill fresh gas into the cylinder in time, and exhaust the waste gas produced by combustion in time.

3. Fuel supply system

Due to the different fuels used, it can be divided into gasoline engine fuel supply system and diesel engine fuel supply system.

the gasoline fuel supply system can be divided into carburetor type and direct fuel injection type. The commonly used carburetor fuel supply system consists of fuel tank, gasoline pump, gasoline filter, carburetor, air filter, intake and exhaust manifold and exhaust muffler, etc. Its function is to supply the prepared combustible mixture into the cylinder, control the quantity of combustible mixture entering the cylinder, and adjust the output power and rotation speed of the engine. Finally, the combusted exhaust gas is discharged from the cylinder.

4. cooling system

water-cooled motor vehicles are generally used. Water-cooled system is composed of water pump, radiator, fan, thermostat and water jacket (in the machine body), and its function is to use the circulation of cooling water to dissipate the heat of high-temperature parts to the atmosphere through the radiator, so as to maintain the normal working temperature of the engine. 5. Lubrication system

Lubrication system is composed of oil pump, filter, oil passage and oil pan. Its function is to distribute lubricating oil to the friction surfaces of relative moving parts, so as to reduce friction, slow down the wear of parts, and clean and cool the friction surfaces.

6. Ignition system

The ignition system of a gasoline engine is composed of a power supply (battery and generator), an ignition coil, a distributor and a spark plug, and its function is to ignite the compressed combustible mixture in the cylinder at a specified time.

7. Starting system

The starting system consists of a starter and a starting relay, etc., which is used to start the stationary engine and turn it into a self-running state.

working principle of engine

the process of converting thermal energy into mechanical energy by an engine is realized through four continuous processes of air intake, compression, power work and exhaust, and each such process is called a working cycle. A four-stroke engine is one in which the crankshaft rotates twice and the piston reciprocates for four strokes to complete a working cycle.

1. Working principle of four-stroke gasoline engine:

(1) Intake stroke. The crankshaft drives the piston to move from the top dead center to the bottom dead center. At this time, the intake valve opens and the exhaust valve closes. During the movement of the piston, the internal volume of the cylinder gradually increases, forming a vacuum degree, so the combustible mixture is sucked into the cylinder through the intake valve until the piston reaches the bottom dead center and the intake valve closes.

(2) compression stroke. At the end of the intake stroke, driven by the crankshaft, the piston moves from the bottom dead center to the top dead center, and the internal volume of the cylinder gradually decreases. At this time, the intake and exhaust valves are closed, and the combustible mixture is compressed until the piston reaches the top dead center. During the compression process, the gas pressure and temperature increase at the same time, and the mixture is further mixed evenly. At the end of compression, the pressure in the cylinder is about 1.6 MPa ~ 1.2 MPa, and the temperature is about 611 K ~ 811 K..

(3) power stroke. At the end of the compression stroke, the spark generated by the spark plug ignites the mixed gas and burns rapidly, so that the temperature and pressure of the gas rise rapidly, thus pushing the piston to move from the top dead center to the bottom dead center, and rotating the crankshaft through the connecting rod to do work until the piston reaches the bottom dead center.

at the beginning of work, the gas pressure and temperature in the cylinder rise sharply, and the instantaneous pressure can reach 3 MPa ~ 5 MPa, and the instantaneous temperature can reach 2211 k ~ 2811 K ..

(4) exhaust stroke. When the power stroke approaches the end, the exhaust valve opens and the intake valve closes, and the crankshaft pushes the piston to move from the bottom dead center to the top dead center through the connecting rod. The exhaust gas is discharged from the cylinder under its own residual pressure and under the push of the piston. When the piston reaches the top dead center, the exhaust valve is closed and the exhaust is finished. Because of the exhaust resistance of the exhaust system, at the end of the exhaust stroke, the pressure in the cylinder is slightly higher than the atmospheric pressure, about 1.115 MPa ~ 1.115 MPa, and the temperature is about 911 K ~ 1.211 K.

2. About engine failure and maintenance

2.1 Eight main factors of engine failure

Everyone has a heart, and if the heart stops beating, life will die with it. The car is no exception, the engine is the heart of the car, and the good or bad maintenance directly affects the performance and service life of the car. In order to keep our car away from "heart disease", we should love the engine of the car as much as our own heart. The following eight major causes of "heart disease" in vehicles may benefit you.

First, there is no maintenance on schedule

Usually, people always like to invest a lot of money in modification, but it is easy to neglect to maintain the engine on schedule. According to an experienced mechanic, "Among the cars they handle for maintenance, the failures caused by poor engine maintenance account for 51% of the total failures." It can be seen that engine maintenance can play a vital role in prolonging the service life of vehicles. Of course, it will also reduce unnecessary losses for you, otherwise there will be the term "nurturing instead of repairing".

The oil quality of different grades of lubricating oil will change during use. After a certain mileage, the performance of the vehicle will deteriorate, which may bring various problems to the engine. In order to avoid these failures, we should change the oil of the car regularly according to the service conditions, and make the oil quantity moderate, generally between the upper and lower limits of the oil gauge.

when the oil passes through the fine hole of the oil filter element, the solid particles and viscous substances in the oil are accumulated in the filter. If the filter is blocked and the oil cannot pass through the filter element smoothly, it will burst the filter element or open the safety valve, and pass through the bypass valve, still bringing the dirt back to the lubrication part, which will accelerate the engine wear and increase the internal pollution. Therefore, it is equally important to replace the oil filter regularly.

The air intake system of the engine is mainly composed of the air filter and the air inlet. According to different usage conditions, the air filter element should be cleaned regularly. The available methods include blowing high-pressure air from inside to outside to blow out the dust in the filter element. Because the air filter element is made of paper, it is necessary to pay attention to the air pressure when blowing, so as not to damage the filter element. Generally, the air filter element should be replaced after three times of cleaning, and the cleaning cycle can be determined by the air quality in the daily driving area.

Because the air intake pipe is too dirty

If the vehicle often runs in the road area with more dust and poor air quality, you should pay attention to cleaning the air intake pipe to ensure the smooth air intake. The intake pipe is very important for the normal operation of the engine. If the intake pipe is too dirty, it will lead to the decrease of charging efficiency, which will make the engine unable to operate within the normal output power range and aggravate the wear and aging of the engine.

There is too much sludge in the crankcase.

During the operation of the engine, high-pressure unburned gas, acid, moisture, sulfur and nitrogen oxides in the combustion chamber enter the crankcase through the gap between the piston ring and the cylinder wall, so that they are mixed with the metal powder produced by the wear of parts to form sludge. A small amount of sludge can be suspended in the oil, and when the equivalent is large, it will be separated from the oil, blocking the filter and oil hole, making it difficult to lubricate the engine, thus aggravating the wear of the engine. In addition, when the engine oil is oxidized at high temperature, it will produce paint film and carbon deposit and stick to the piston, which will increase the fuel consumption and reduce the power of the engine, and even make the piston ring jam and pull the cylinder.

six reasons: poor maintenance of the fuel system

The maintenance of the fuel system includes replacing the gasoline filter element, cleaning the carburetor or fuel nozzle and the oil supply pipeline. When the fuel is supplied to the combustion chamber through the oil passage, it will inevitably form colloid and carbon deposit, which will be deposited in the oil passage, carburetor, nozzle and combustion chamber, which will interfere with the flow of fuel, destroy the normal air-fuel ratio, make the fuel atomization poor, and cause performance problems such as engine chattering, knocking, unstable idle speed and poor acceleration. Cleaning the fuel system with fuel system cleaning agent can always keep the engine in the best condition.

Reasons VII. Rust and scaling of water tank

Rust and scaling of engine water tank are the most common problems. Rust and scale will limit the flow of coolant in the cooling system, reduce the heat dissipation effect, lead to overheating of the engine, and even cause engine damage. Oxidation of cooling liquid will also form acidic substances, which will corrode the metal parts in the water tank, resulting in damage and leakage of the water tank. Regular use of powerful and efficient cleaning agent for water tank to remove rust and scale can not only ensure the normal operation of engine, but also prolong the overall life of water tank and engine.

Eight, the cooling system is in poor condition

People pay special attention to the maintenance of automobile engines, especially the lubrication system, and seldom pay attention to the cooling system. I don't know that the most common faults of automobile engine, such as piston cylinder pulling, knocking, internal leakage of cylinder block punch, serious noise, acceleration power drop, etc., are all caused by abnormal working temperature, excessive pressure and poor cooling system of automobile engine. The poor condition of the cooling system will directly lead to the failure of the engine to work at normal temperature, which will lead to the above-mentioned serious failure phenomenon.

2.2 engine fault diagnosis methods

definition of fault diagnosis

fault diagnosis refers to determining the technical condition of the engine and finding out the fault location and causes without disassembling (or only dismantling individual small parts).

fault diagnosis methods

When diagnosing faults, we should follow the principles of "easy first, difficult later, simple first, complicated later, external first, internal later, sectional search and gradually narrowing the scope".

artificial intuition method

artificial intuition method is a method to determine the technical condition and fault of the machine by asking, seeing, smelling, touching, trying and listening directly, or with the help of simple tools.

(1) Asking

refers to asking the driver about the vehicle mileage, service life, maintenance and repair, fault omen, fault occurrence process, etc. Before and after the fault.

(2) Looking at

refers to observing the conditions of the mechanism, assembly and parts with suspected faults, such as the indicated values of various instruments, cracks and deformation of the body, the color of exhaust gas emitted by the muffler, and the dripping oil traces.

(3) sniffing

means judging the fault location according to the abnormal smell emitted by the engine during operation. If there is gasoline, it indicates oil leakage or poor combustion.

(4) Touch

That is, touch with hands to test the temperature and vibration of the possible fault parts, so as to judge the tightness of fit and the size of bearing clearance. The balance of parts' counterweight, the pulsation of diesel pipeline and the temperature of oil and water, etc.

(5) Try

By various test methods, the fault phenomena can be fully revealed, such as pressing the horn, turning on the ignition switch or light switch, "cutting off" the spark plug, pulling the throttle, rapidly increasing or decreasing the engine speed, etc. If necessary, the assembly or parts can be replaced for comparative test.

Such as whether there is knocking sound in the cylinder, whether there is "backfire" in the carburetor, and whether there is shooting sound or "sudden, sudden" sound in the exhaust muffler.

The above methods, not every fault diagnosis must follow this procedure, and different faults should be applied flexibly according to their specific conditions.

2.3 Simple maintenance of the engine

(1) First, check whether the liquid level of the automobile antifreeze is appropriate. The correct liquid level position should be no lower than the bottom line and no higher than the middle of the highest line, which is the most suitable liquid level position!

(2) The battery inspection of the vehicle is divided into three small steps. Step 1 is to check whether the electrolyte of the battery is insufficient (except the maintenance-free battery). Step 2 is to check whether the "positive electrode" and (negative electrode) of the battery are loose. Step 3 is to remove the oxide on the battery poles!

(3) Check and replace the oil. In this step, first pull out the oil dipstick to check the oil level, and then observe the oil color. If it is as sticky as sesame sauce and water, it must be replaced.

(4) Check or replace the air filter, which is very simple. Just remove the filter and check it. If it is not particularly dirty, it is not necessary.