Study on Development of a Urea-SCR System of Diesel Engine

Qiu Tao1,a, Li Xuchu1,b, Peng Jing1,c, Lei Yan1,d , Yue Guangzhao2,e 1 College of Environmental and Energy Engineering, Beijing University of Technology, Beijing

100124, China

2 Institute of Engineering (Baotou), College of Engineering Peking University, Baotou 014030, China

a qiutao@bjut.edu.cn, b lixuchu@emails.bjut.edu.cn, c pj65@emails.bjut.edu.cn, d leiyan@bjut.edu.cn, e yuegz@ie.pku.edu.cn

Keywords: Diesel engine; Aftertreatment; SCR; Control strategy; Urea pump.

Abstract. Aimed at the diesel engine, a selective catalytic reduction (SCR) system was developed. In

this system, the urea pump is integrated with a urea tank, air and urea mix in the injector, catalyst

convertor is based on vanadium. Combining with a self-developed control unit, the urea-SCR system

was tested on the engine bench. The ESC experiment results indicate that the NOx emission can be

reduced effectively which meets the China stage IV regulation.

Introduction

An after treatment system becomes necessary for a diesel engine to meet China stage IV even higher

emission standard [1]. Because the good fuel economy and sulphur tolerance [2-3], the selective

catalytic reduction (SCR) is the main technic to reduce NOx emission of diesel engine [4-8].

Generally, there are two kinds of SCR systems: the air-assisted system and the non-air-assisted

system, according to the operation mechanism of the urea pump. Compared to the non-air-assisted

system, the air-assisted system has no strict requirements for the urea nozzle and the pump.

Additionally, the injection pressure is lower and the atomizing efficiency is better.

At present, the air-assisted SCR system mainly used in China. Some Chinese companies have

been doing researches on the key technology of SCR system. This paper presents a SCR system

consisted of self-developed Dosing Control Unit (DCU), the urea supply and injection system and

catalyst converter. Besides, experiment study has been done on engine bench, and the results indicate

the WP5 diesel engine can meet China stage IV regulation with this system.

SCR System

The SCR system is shown in Fig. 1. An SCR system mainly consists of a DCU, a urea pump, a nozzle,

a urea tank, a SCR catalyst converter, the sensors, the corresponding hydraulic pipes and the electric

harnesses.

The DCU obtains the engine operating parameters by communicating with the engine ECU via

CAN bus, such as the engine speed and torque, the temperature of catalytic converter outlet and the

original NOx emission and so on. Then the urea injection quantity is calculated, and sent to urea

pump. According to the demand urea quantity decided by the DCU, the urea pump will extract certain

amount urea from urea tank and build to 2 bar pressure. In the nozzle, air which from air pump will

mix with the urea and inject into exhaust pipe.

Applied Mechanics and Materials Vols. 541-542 (2014) pp 747-751Online available since 2014/Mar/12 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMM.541-542.747

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Urea Pump

The urea pump is shown in Fig.2, which is integrated with tank. Engine cycling water can heat the

urea pump and the tank together, so that the whole system can work under low ambient temperature

condition.

The urea pump contains a diaphragm pump, check valves, air solenoid valves, a drive control unit,

pressure sensors and related wire harnesses. The pump is plunger diaphragm pump, certain quantity

urea are launched through the reciprocal motion of the plunger. The parameters of urea supply and

injection system are presented in Table 1.

Table 1 Parameters of urea supply and injection system

Name Value

Shape size of pump（length×width×highth） 175×175×105mm

Shape size of intergrated system（length×width×highth） 550×200×650mm

Weight（contain 35L urea tank） 7.2kg

Operating voltage 24V（12V）

Nominal power 30W

Range of urea injection 50-7500ml/h

Control presicion of injectino ±5%

Fig. 1 Structure of SCR system

Fig. 2 Urea Pump

748 Engineering and Manufacturing Technologies

Urea Nozzle

The urea nozzle is shown in Fig. 3. The nozzle has two lines, respectively, the urea circuit and the air

pipe. The air circuit around the urea pipe as concentric circle. Energy of urea spry is from the pressure

of the compressed air, which make the air and urea injection pressure lower, the air consumption

lower and the atomization better. Due to urea solution and air is not mixed inside the nozzle or

injection system, it can effectively avoid nozzle blocking, as well as crystallization and precipitation

of urea.

SCR Catalyst Converter

The SCR catalyst converter is an integral catalyst and muffler device, installed in a sealed steel shell.

There are four modules inside, ammonia diffuser, catalytic converter, oxide layer to prevent leakage

of ammonia, and muffler device, respectively. The V2O5-WO3/TiO2 catalyst is adopted in the SCR

catalyst converter. It has a high NOx conversion efficiency within a wide temperature range. The

NOx conversion efficiency is up to 80% within the ESC 13 operating conditions.

Control Unit

The physical and schematic diagrams of control strategy in the control unit are shown in Fig4 and Fig.

5. First, according to the speed and torque signals of the engine, the NOx concentration and exhaust

gas of engine are obtained by the MAP of diesel engine. Then, the mass flow of NOx calculated on

basis of intake temperature and humidity of diesel engine, so that the basic injection amount of each

condition is determined. The actual injection is calculated after the correction of exhaust gas mass

flow and catalyst temperature.

This DCU can perform the following functions. (1) The communication and control of ECU and

CAN interface elements, such as the NOx sensor and the urea pump, based on SAE J1939. (2) The

ammonia storage model and the temperature calculation model. (3) The OBD diagnose and alarm. (4)

The integrated test with PC calibration software based on CCP protocol.

Experimental Setup

The urea-SCR system is matched by the self-developed DCU, the SCR catalyst converter, the

domestic supply and injection system and the BOSCH system. Test engine is Weichai WP5 diesel

engine, shown in Fig. 6. The parameters of engine are present in Table 2.

(a)Schematic Diagram (b) Physical Picture

Fig. 3 Urea Nozzle

Fig. 4 Control Strategy Fig. 5 Physical Picture

Applied Mechanics and Materials Vols. 541-542 749

Table 2 Parameters of Engine

Type Unit WP5

Engine type Inline, 4-cyliders, turbocharged intercooled

Rating power/rating speed kW(r/min) 132

Maximum torque/speed Nm(r/min) 650(1200-1600)

Idle speed r/min 600

Experimental Results Analysis

Compare domestic urea system with that of BOSCH

Operating points 4 and 5of ESC 13 are chosen in this experiment. At the same operating point,

different systems injected the same amount urea to compare the NOx conversion efficiency. The

results are shown in Fig. 7.

As shown in Fig. 7, as the injection amounts gradually increase, the NOx emission in the

downstream of the SCR catalyst converter decreases, so the NOx conversion efficiency improves. At

small injection amount condition, domestic SCR system has higher NOx conversion efficiency,

compare to BOSCH system. At large injection amount, BOSCH system has better performance, but

the difference is very small, both of them have very high NOx conversion efficiency.

ESC test result

The domestic SCR system consists of the self-developed DCU, the domestic urea supply and

injection system, the SCR catalyst converter. Fig. 8 is emission compare of each 13 operating point

and 3 random points test as reference [9] prescribed ESC test cycles. Table 3 is ESC test result.

Table 3 ESC Test Result

Standard

Requirement

Original Machin

Emission Emission with SCR

NOx Emission <3.5g/kwh 10.16 g/kwh 3.4 g/kwh

NH3 leakage <10ppm 0ppm <3ppm

Fig. 8 Result of ESC Test

Tem

pera

ture

Sen

sor

Tem

pera

ture

Sen

sor

Fig. 6 Structure of Engine Bench

Fig. 7 Comparation result

750 Engineering and Manufacturing Technologies

The first working point in 13 operating points is idle speed, the temperature of exhaust pipe is very

low, and the catalyst conversion efficiency is low as well, so there is no urea injection.

After using the domestic SCR system, the NOx emission has dynamic decrease, especially under

high load condition, the NOx conversion efficiency is much high. Under ESC test cycles, the final

NOx emission is 3.4g/kWh, the NH3 leakage is less than 3ppm. The ESC test results indicate the

diesel engine can reach China stage IV emission regulations by using this system.

Conclusion

This paper develops an SCR system, in which the urea tank and pump are integrated. The DCU fully

functional, and the entire system has high accuracy. The performance is comparable to the BOSCH

SCR system. Install this SCR system, diesel engine can effectively reduce the NOx emission, and can

reach China stage IV emission regulations.

Acknowledgement

We gratefully acknowledge financial support for this work by the Scientific Research Project of

Beijing Educational Committee (KM201310005033).

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Engineering and Manufacturing Technologies 10.4028/www.scientific.net/AMM.541-542 Study on Development of a Urea-SCR System of Diesel Engine 10.4028/www.scientific.net/AMM.541-542.747

DOI References

[6] D Chatterjee, P Kočí, V Schmeißer, et al. Modelling of a combined NOx storage and NH3-SCR catalytic

system for Diesel exhaust gas aftertreatment[J]. Catalysis Today, 2010, 151(3): 395-409.

http://dx.doi.org/10.1016/j.cattod.2010.01.014