The natural gas mainly exists in oil and gas fields, the main components are alkanes, which accounted for the vast majority of methane, the boiling point of -161.5 C in 20MPa compression storage for compressed natural gas (CNG), at -162 DEG C insulation state was the liquid storage of liquefied natural gas (LNG). The natural gas produced in the combustion process can affect the health of human respiratory system is very small, no waste residue after combustion, waste water generated, with the use of safe, high calorific value, clean and other advantages. With the continuous improvement of emission standards and the rational use of energy allocation in China, LNG engine has been applied in large buses and heavy trucks. The national west east gas transmission project and the natural gas landing project in the East China Sea provide the energy guarantee for the popularization and application of the LNG engine. With the continuous entry of LNG engine into the market, combined with the structural characteristics and practical work experience of LNG engine, the reasonable use of LNG engine and several fault analysis are discussed.

Main components and functions of LNG engine fuel supply system

1. working principle

Turbocharged LNG engine for ECI EFR system. LNG from the liquid tank to the engine cylinder.

2. liquid storage tank

Storage of natural gas in liquid form is stored in low temperature and low pressure, the internal temperature is below -162 DEG C, the cylinder is filled with gas cylinders nominal working pressure is 1.6MPa, the minimum pressure of not less than 0.65MPa, otherwise it will cause the loss of power supply shortage of the engine, and lead to the sintering of catalytic converter. Low temperature gas tank adopts double layer vacuum insulation structure, and the tank consists of inner liner, outer shell, inner sandwich insulation and valve chamber. The inner liner is used for holding fuel, the shell supports the whole tank body and protects the inner liner, and the inner liner forms a closed vacuum space between the inner liner, and the two shell interlayer is filled with the glass wool and aluminum foil material with heat insulation property. The valve compartment is arranged at one end of the tank, including the storage valve, the safety valve, the manual vent valve, the pressure regulator, the pressure gauge and the liquid level display. Cab dashboard equipped with gas indicator and leak alarm device.

3. vaporizer

LNG required by the vaporizer temperature expansion after vaporization into gaseous natural gas, combustion can be supplied to the engine, LNG absorbs more heat in the vaporization process, the carburetor engine cooling fluid to provide heat to low temperature liquid natural gas into gaseous natural gas. Gaseous natural gas temperature depends on the temperature of the cooling fluid and flow, the actual work process, the temperature of the engine air-fuel mixture into the requirements are relatively relaxed, and the normal temperature of the engine coolant is generally 60 to 85 DEG C.

4. low voltage solenoid valve

The low pressure solenoid valve is installed at the entrance of the electronic regulator to control the supply of natural gas. Should pay attention to the cleanliness of the solenoid valve, depending on the cleanliness of the fuel supply, generally every 50 thousand km cleaning once. When cleaning, should remove the solenoid valve coil, take out the valve seat, and soak them in gasoline, and then dry compressed air drying after use.

5. electronic controlled voltage regulator (EPR valve)

Electronic control voltage regulator is electronic control of pressure regulator, the internal control chip to accept ECM instructions, according to the engine operating conditions to control the natural gas and pressure, so as to effectively control the air fuel ratio. Electronic control voltage regulator has a microprocessor controlled high-speed motor, through the CAN bus to receive ECM information, to ensure that the natural gas pressure between 0.55~1.35MPa. The pressure sensor is used to measure the pressure difference between the outlet of the regulator and the inlet of the mixed gas, and the gas temperature of the electronic regulator is controlled in the range of -40 centigrade ~105 centigrade.

6. mixer

The function of the mixer is to mix the cold air and the natural gas in the supercharger sufficiently. The air pressure of gaseous natural gas is greater than that of medium cooling air, which is sprayed into air in proportion to make the combustion more full and soft, which can effectively reduce the emission and exhaust temperature of NOx. The mixture is equivalent to a single point injection device installed in the throat inlet, in which there is a spring control diaphragm, according to the flow of air, decided to open its internal degree of the diaphragm and in the engine operating condition, the mixer can accurately control the air-fuel ratio.

7. electronic control throttle

The driver passes the accelerator pedal, transfers the power demand to the ECM, ECM receives the accelerator pedal signal, according to the engine operating condition controls the electronic throttle opening. By controlling the opening of the electronic throttle valve, the amount of air mixed into the cylinder is adjusted so as to control the speed of the engine and adapt to the change of the load. When installing the electronic throttle, the drive motor axis must remain horizontal.

Characteristics and reasonable use of two 、 LNG automobile engine

LNG engine has the following characteristics: first, the natural gas antiknock performance is good, the octane number can reach 130RON, the compression ratio of the engine can reach 11~12, between the gasoline engine and diesel engine. Two, the intake system is suitable for installation of turbocharger, LNG engine adopts electronic control high pressure ignition system, the same as gasoline engine. Three is the engine burning natural gas will not form oil film in the intake pipe wall, engine exhaust pollution is small when warming up. The above characteristics make the LNG engine and gasoline engine, diesel engine compared to the use of a certain difference, mainly in the following aspects.

(1) attention should be paid to the quality of LNG. Natural gas contains not only alkanes, but also hydrogen sulfide, carbon dioxide, nitrogen and water vapor. LNG engine with natural gas, each standard cubic meter (101.325kPa, 20 degrees Celsius) hydrogen sulfide content can not exceed 15mg/m3, vehicle length

Three, LNG automotive engine several fault analysis and Countermeasures

Failure analysis of 1. piston melting and cylinder liner breaking

In 2009, a company equipped with LNG engine passenger car, has several times the piston burning and cylinder broken serious fault, after the disintegration of the body for inspection and analysis. Through analysis, the main cause of the failure is the detonation combustion in the engine working process. The combustible mixture is ignited by spark plugs at the end of compression. In normal combustion, the flame travels from the spark plug to the far end of the spark plug. If the flame propagation process, at the end of the mixed gas by self ignition combustion, when the cylinder pressure increases sharply, and the occurrence of a strong shock, knocking sound of metal in the cylinder, the abnormal combustion phenomena, called deflagration. Severe deflagration will make the engine overheating, power down, engine wear intensifies, and even damage the engine parts, especially in the engine low speed, high load conditions deflagration caused by the most serious damage.

LNG engine deflagration causes are as follows:

(1) the quality of LNG fuel is poor, or the antiknock performance is decreased;

(2) the ignition timing is inaccurate and the ignition advance angle is too large;

(3) the mixture is too dense, more than 5% of the set concentration;

(4) turbocharger abnormal, can not limit the intake pressurization, resulting in over pressurization phenomenon;

(5) the inlet intercooler is seriously polluted and the inlet air temperature is too high;

(6) improper selection of lubricating oil quality, the use of high ash oil, resulting in excessive carbon content in the cylinder, compression ratio increased relatively.

The company's car is just put into use soon LNG passenger cars, you can temporarily exclude serious pollution, clogging, carbon deposition and other causes of deflagration. The deflagration reasons may be caused by excessive serious ignition angle, the gas mixture into the cylinder temperature is too high, or is stored for a long time in the tank LNG, methane gas in the gaseous component decreased, antiknock performance decline.

In the gas supply system, the temperature of the vaporized natural gas can not be too high. The temperature of the natural gas passing through the gas buffer tank and the heat exchanger should be controlled, and the excessive temperature will cause the engine to knock.

The LNG fuel in the storage tank has been stored for a long time after the transportation, transfer and inspection of the new car. When the storage tank absorbs the external heat and increases the internal pressure by more than 1.6MPa, the safety valve will automatically eliminate the overpressure of natural gas and accelerate the evaporation of natural gas in the tank. The methane content in liquid LNG decreases gradually due to the better evaporation of methane with better antiknock performance, which directly affects the antiknock performance of the engine. It is easy to cause knock on the LNG engine and cause damage to the engine in severe cases.

Generally, the knocking parts in the combustion chamber are easy to cause the burn damage of the upper part of the cylinder, but it will not cause the whole cylinder to be broken. From the overall crushing situation, the material of cylinder liner and quenching heat treatment are also directly related to the cylinder block.

The 2. generation cylinderscoring phenomenon analysis

There are several bus engines similar to knocking and channeling sound, after increasing the throttle engine trembling and power down, or even flameout, exhaust pipe smoke phenomenon.

The analysis may be the engine cylinderscoring phenomenon after the collapse of that judgment, there are varying shades in the longitudinal wall of vertical grooves and scratches galling. This is because when the piston or piston ring and cylinder sliding contact area is too small, high temperature friction, piston or piston ring and cylinder between the metal melt, after cooling hardening will pull cylinder grooves, namely cylinder.

Common causes are: cylinder ignition timing of engine overheating, serious misalignment, piston and cylinder with the gap is too small, vehicle maintenance technology or piston cooling enough to cause thermal expansion, the piston ring end gap is too small or broken, poor quality of lubricating oil or oil film pressure is not enough, the adverse effect of air filter impurities into the lead the cylinder, and the driver in the low temperature starting after Meng stepped on the accelerator pedal.

The LNG engine is the gas engine, the gasoline engine with high compression ratio, combustion temperature is high, the high temperature lubricant, selection of cylinder liner and piston material suitable, and the corresponding heat treatment, strict control of cylinder liner and piston assembly clearance, and the opening to the piston ring end gap.

Fault analysis of 3. spark plug fouling and crack damage

When the LNG engine is in normal working condition, the insulator of the spark plug appears slight yellow or brown sediment, and slight electrode corrosion occurs.

The spark plug made of alumina ceramic, the working environment is extremely bad, a sharp turn not only have to bear the pressure and temperature in cylinder of high frequency impact, but also to ensure the insulation performance in 40kV under high voltage and resistance to chemical corrosion. Generally speaking, the main cause of insulator damage is burst vibration. The LNG engine fuel because of difference, the air-fuel ratio and the cylinder compression pressure was higher than that of the gasoline engine, the ignition voltage is above 40kV, and the spark plug gap is much smaller than the gasoline engine, so the installation of spark plug and ignition coil, must ensure that the spark gap in the specified value, and should be in the ignition electrode coil and spark plugs with conductive adhesive, the spark plug ceramic body is coated with an insulating grease to prevent the rubber aging triggered spark plug and cylinder leakage.

Cause analysis of 4. engine insufficient power

LNG engines have less power than diesel engines, and sometimes it is difficult to speed up. The characteristics of LNG fuel and the structure of engine are the main reasons for the lack of power. The mixture of fuel and gas enters the fuel engine, while LNG does