Analysis of diesel internal purification methods: turbocharged, EGR

Preface

Diesel engine oil gas mixture uneven, fuel can not completely combustion, lead to the decomposition of carbon based particles. At the same time, compared with the gasoline engine, the excess air coefficient of diesel is very high, resulting in local high temperature and combustion, resulting in nitrogen oxide (NOx) generation, but the carbon monoxide (CO) and hydrocarbons (HC) emissions are much lower than gasoline engine, fuel economy is also very good. It can be seen that optimizing the emission performance of diesel engine mainly resolves how to reduce nitrogen oxides (NOx) and particulates.

The combustion temperature of the diesel engine reaches the maximum during the slow ignition period, which directly influences the formation of nitrogen oxides (NOx). At the same time, during the slow ignition period, if the engine is still in the oil injection, and sprayed into the high temperature exhaust zone, or the mixture is too dense, will lead to the formation of particles due to hypoxia.

Therefore, from the angle of internal purification, the formation of nitrogen oxides (NOx) can be reduced by adjusting the relationship between the maximum temperature and fuel concentration. Multi valve technology, turbocharged medium cooling technology, control injection rate and exhaust gas recirculation can be adopted. By regulating fuel injection or organizing air flow, fuel is rapidly and completely burned, and the formation of particles is reduced. Exhaust gas turbocharger technology can be adopted to increase injection pressure, improve combustion chamber structure, reduce oil consumption, use low sulfur fuel, control fuel injection process and adjust fuel quantity.

1 exhaust turbocharger technology

According to the different ways of pressurization, the supercharging of engine can be divided into mechanical supercharging, gas wave supercharging, exhaust turbo charging and compound supercharging. The turbocharger is the engine emissions from entering the turbine has a certain energy and expansion work, drive the compressor and turbine rotating coaxial full power for gas turbine, compressor, compressed air fresh in sending people to the cylinder, as shown in figure 1.

1.1 development status

The exhaust turbocharger can increase the intake density of the engine and increase the charge of the engine. The radial flow type is adopted in the general vehicle engines to meet the requirements of high speed and high response performance. The compressor part of turbocharger usually adopts single pole centrifugal structure, but the turbocharging system can be divided into constant pressure turbocharging system and pulse turbo charging system. Among them, the constant pressure turbocharging system has low exhaust utilization rate, low speed torque characteristics and low acceleration performance, and is suitable for low supercharging. The pulse turbocharging system has relatively high utilization rate of exhaust gas when low supercharging, obvious scavenging effect, small exhaust pipe volume, sensitive to load change, good dynamic response and complex structure. The speed and torque characteristics of automotive diesel engine are higher, so the pulse turbocharging system is adopted.

Compared with the supercharged engine, the exhaust turbocharger does not consume the power output by the engine crankshaft, does not affect the power of the engine, and does not increase fuel consumption. Compared with the gas wave supercharging, the supercharging pressure is up to 0.4MPa. When the power is used in the diesel engine, the power of single machine is larger than 35kW, and the technology is relatively mature, and the product has been realized. Compared with the compound supercharging, the utility model has the advantages of simple structure, easy control, and is more suitable for the use of a vehicle diesel engine. By using the exhaust turbocharger, the diesel engine can be improved by 30% to 50%, and the fuel consumption rate is reduced by about 5%, which is beneficial to improve the power, the economy and the emission performance of the engine.

But there are some defects in exhaust turbocharging technology. First of all, low speed performance is not good. When the diesel engine is at a lower speed, the power generated by the turbine will be reduced, which will lead to the reduction of the supercharging pressure of the compressor and the pressure reduction effect. Second, the acceleration response is slow. Because the exhaust gas is used to drive the supercharger, the engine passes through at least one cycle, and the exhaust gas will increase to reflect the supercharger, so the transient response is not good. Thirdly, the sensitivity to inlet and exhaust pressure is high. When the cylinder exhaust is too small, the supercharger will have a shock. When the cylinder exhaust is too large, the supercharger will block. The two abnormal conditions affect the supercharging pressure and efficiency of the turbocharger.

1.2 effect of exhaust turbocharging on emissions

The influence of 1.2.1 on CO emission

In diesel engine, CO is the product of incomplete combustion of fuel, which is mainly formed under partial hypoxia or low temperature. The diesel engine usually operates under lean combustion conditions, and the turbocharging technique increases the excess air factor, improves fuel atomization and mixing, enables fuel to burn more fully, and further reduces CO emissions.

The influence of 1.2.2 on HC emission

The HC in diesel engine is mainly composed of raw fuel molecule, decomposed fuel molecule and intermediate compound in combustion reaction. A small part is produced by lubricating oil through the human cylinder. With the increase of inlet density and excess air coefficient, the fuel atomization quality can be improved, the fuel oil deposited on the wall of combustion chamber can be reduced, and the HC emission will be reduced.

The influence of 1.2.3 on NOx emission

The generation of NOx depends mainly on the concentration, temperature, and reaction time during combustion. When the diesel engine is simply pressurized, the NOx emissions increase because of excessive air coefficient and combustion temperature. As a result, the emission of NOx will be reduced by reducing the compression ratio at the same time, delaying the injection, and recycling the exhaust gas. The intake air cooling technology can greatly reduce the intake temperature after pressurization, and effectively control the combustion temperature, which is beneficial to reduce the NOx.

Effects of 1.2.4 on particulate emissions

The reasons for the formation of particulates are complex, which are mainly affected by excess air coefficient, fuel atomization quality, fuel injection rate, combustion process and fuel quality. Generally, measures to reduce NOx are detrimental to particulate emissions. After pressurization, the inlet density increases and the charge increases