Scr System

Throughout the world, regulations on gases emitted by internal combustion engines are increasingly stringent, with the aim of minimizing emissions to values close to zero, regardless of the type of fuel used. In order to do this, sophisticated technologies must be adopted to check combustion, together with post-treatment systems of the most complex exhaust gases.

The measures adopted for the combustion can very well reduce emissions, but at the risk of penalizing other engine performances: silent operation, specific power and fuel consumption.

Instead, post-treatment technologies of exhaust gases can make it possible to overcome this conflict.

Modern diesel engines, with fuel injection systems such as Common Rail have NOx emissions that are 95% lower than the engines produced in the early 1990s.

The post-treatment system called Selective Catalytic Reduction (SCR) allows a further reduction in NOx levels of up to 95 %. The SCR calls for the injection of a liquid reducing agent in the exhaust gases by using the FreeNOx system developed by Magneti Marelli. The reducing agent is a solution of 32.5% urea in water (generically called AdBlue or Diesel Exhaust Fluid, DEF), stored in a separate tank on the vehicle. In order to reduce the NOx, the reducing agent is pressurized and injected in suitable quantities in the engine exhaust gases, upstream the SCR catalytic converter. Through a process of thermolysis and hydrolysis, NH3 ammonia is released from the solution and reacts with the NOx in the SCR catalytic converter, turning them into nitrogen and water.

The urea solution dosing system was developed by Magneti Marelli Powertrain starting from 2007, for applications on both light and heavy vehicles, on and off-road.

The system is aimed at applications that have to satisfy Tier2 Bin5 (U.S.) and Euro 6 (EU) requirements for light and heavy vehicles, and STAGE4/TIER 4 Final (EU/U.S.) requirements for off-road applications, and it is suited for both original equipment and retrofit applications.

The Magneti Marelli FreeNOx system consists of the following devices:

Urea Delivery Module (UDM), system for the delivery of the reducing agent from the tank to the engine exhaust
Urea Dosing Valve (UDV), reducing agent injection and dosing valve assembled on the exhaust piping
Dosing Control Unit (DCU), electronic control unit of the SCR system with relevant software

The following components complete the management system of the SCR catalytic converter:

Reducing agent storage tank
Heated piping that connects the Delivery Module and the Dosing Valve
Sensors used to monitor the temperature and the composition of exhaust gases

Through its business lines, Magneti Marelli is able to supply a complete SCR system that includes, in addition to the reagent control and injection devices (Magneti Marelli - Powertrain), the exhaust line with the SCR catalytic converter (Magneti Marelli - Exhaust Systems), and the storage tank for the urea solution (Magneti Marelli - Plastics).

This way, Magneti Marelli can provide a technical solution optimized with respect to the customer’s needs, both under a technical and system cost standpoint.

Urea Delivery Module
Urea Dosing Valve
Dosing Control Unit

Urea Delivery Module

The Urea Delivery Module (UDM) is located inside the reducing agent tank, and groups together a set of devices whose purpose is to monitor the state of urea solution and to pressurize it, in order to allow it to be injected in the gas flow upstream the SCR catalytic converter.

The main functions carried out by the UDM are:

Monitoring of the top-up level of the tank, by means of a continuous sensor
Measurement of the fluid temperature
Measurement of the quality of the reducing agent, by means of a specific sensor
Measurement of the injection pressure, through an analogue sensor
Pressurization of the DEF, by means of a two-way volumetric pump, piloted by a brushless electric motor
Thawing out of the reducing agent at temperatures below -11 °C, through 1 or 2 electrical heaters

The flow rate values of the reducing agent can cover diesel engines for very wide applications, from automobiles to heavy vehicles, on and off-road industrial vehicles.

Urea Dosing Valve

The reducing agent dosing valve consists of an injector and a passive cooling heatsink, needed for the high temperatures that can be reached on the engine exhaust line.

The injector is derived from Magneti Marelli’s GDI technology, which makes it suited for working at high pressures, with the possibility to obtain high pulverization of the injected urea, and consequently an optimal use of the SCR catalytic converter.

The maximum working pressure of the developed system is 10 bar, however studies are currently being conducted to increase this value.

In order to optimize the exhaust layout of the SCR system, the dosing valve offers the flexibility of being mounted with different orientations towards the flow of exhaust gases and the possibility of different calibrations for the pulverizing device.

he heatsink is a finned metallic component that exchanges heat with the air in the surrounding environment, without resorting to liquid cooling used by other competitors and with obvious advantages in terms of cost.

Dosing Control Unit

The electronic control of the SCR system can be distributed between the engine electronic control unit (ECU) and a dedicated Dosing Control Unit (DCU).

The main control functions associated with the management of the SCR catalytic converter are:

Control of NOx reduction, which defines the amount of reducing agent to be injected based on a model of the engine, of the SCR catalytic converter and of the information on sensors on the engine exhaust line
Control of the SCR catalytic converter temperature
Management and acquisition of the NOx sensors
Acquisition of exhaust gases temperature sensors
Acquisition of ambient temperature and humidity sensor
Electrical diagnostics and EOBD/OBDII

The control functions associated with the management of the reducing agent injection system, Urea Dosing Module (UDM) and Urea Dosing Valve (UDV) are as follows:

Control of the reducing agent pressure, adjusting the pump speed based on the pressure measurement
Emptying out of the piping between the UDM and UDV when the engine turns off
Command of the UDV
Monitoring of reducing agent level and quality
Management of electrical heating used to thaw out the tank and the piping
Electrical diagnostics and EOBD/OBDII

The DCU of the Magneti Marelli SCR system was developed in order to be scalable from the standpoint of the hardware resources based on the control functions to be carried out.

For the newly-designed engine control systems, most of the control functions are incorporated in the ECU, for the purpose of maximizing system emissions. In this case, the DCU features a lower level of complexity, manages the command of the pump brushless motor and of the electrical heaters, and carries out the diagnostics of the handled devices.

In case of retrofit SCR systems, in which the ECU resources may be limited, the DCU features the highest complexity and is able to carry out all the control functions of the catalytic converter and of the reducing agent delivery system.

In all cases, communication between the DCU and the ECU takes place via CAN (Controller Area Network) bus.

The Software for the SCR control functions in ECU or DCU is developed through robust design based on:

Stratified software architecture, easily transportable on different-generation control units; this type of architecture is crucial in terms of hardware scalability
Model-based algorithms
SPICE 2-certified development process
AUTOSAR orientation

The Time-to-Market is reduced thanks to fast prototyping, which also allows customization according to the customer’s requests. The SCR software can be fully personalized with the customer according to predefined design lines, also thanks to easy prototyping and to computer-guides software code generation.

Magneti Marelli supplies “turnkey” systems, in other words systems that are completely optimized from the calibration standpoint thanks to full and specific skills developed over the last decades at different facilities located in Europe, China, United States, India and South America where, thanks to the necessary competences, systems are developed depending on the different needs of the markets.

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