Exhaust gas recirculation (EGR) is the most common technology to reduce nitrogen oxides (NOx) emissions on diesel internal combustion engines (ICE). EGR takes exhaust gases from the exhaust manifold and reintroduces them into the intake manifold, mixing them with fresh air. By doing this, the main ingredients of NOx emissions are reduced:
- oxygen: which is displaced by inert (exhaust) gases
- combustion temperature: which is reduced because the higher heat capacity of carbon dioxide (CO2) and water vapour (H2O) draws in part of the combustion heat
Most of the EGR system contains at least an:
- EGR valve
- EGR cooler (optional)
- EGR cooler bypass (optional)
- intake throttle valve
Some vehicles have both high and low pressure EGR systems, which means that the EGR components are doubled.
- engine block
- EGR valve (high pressure)
- EGR cooler bypass (high pressure)
- EGR cooler (high pressure)
- intake throttle (high prerssure)
- second-stage intercooler (intake air)
- second-stage turbocharger
- variable geometry actuation mechanism (turbine)
- first-stage turbocharger
- diesel particulate filter
- exhaust throttle (low pressure)
- EGR cooler (low pressure)
- EGR valve (low pressure)
- first-stage intercooler (intake air)
The main function of an EGR valve is to allow the exhaust gases to flow from the exhaust manifold into the intake manifold.
The vast majority of valves in production, up until Euro 3 emissions standards, had an inward opening poppet valve operated by a vacuum actuator. Early versions of pneumatic EGR valve does not use electrical energy to move the actual valve, but a shaft connected to a diaphragm in vacuum cylinder. Compared with an electrical actuated EGR valve, the main advantage of the pneumatic valve is the low cost, lack of mechanical transmission and simplicity.
The other advantages of the pneumatic EGR valve are: resistance to high temperatures, especially in the absence of a position sensor (no electrical connection), and the small size and low mass of the overall valve.
Depending on the expected response time and the reserve of available vacuum to move the valve, the size of the diaphragm and vacuum cylinder might be significant. The disadvantages of the pneumatic EGR valves are:
- low actuation force for opening
- the closing is ensured only by a return spring (in the event of soot deposits on the valve, it is not possible to remove the deposit by fast valve closing/smashing, which can result in a permanent open position)
Electric actuators have now become the standard due to faster, more precise control, which fits stricter emissions standards. Modern electric EGR valves, regardless if they are fitted in a low pressure or high pressure EGR system, consist of the following elements:
- actual valve, which by opening/closing varies the gas flow area
- actuator (electric), which provides the necessary force to open/close the valve
- valve body, which holds the valve, the actuator, as well as the return spring and other mechanical components
- position sensor, which transmits to the engine control module the position of the valve
- case, which contains the position sensor and electrical connections
Electric actuated EGR valves may have inward- or outward-opening poppet valves, depending on the type of actuator. Linear solenoids, stepper motors, torque motors and DC motors are the main types of electric actuators being used and developed by various EGR component suppliers. These offer the advantage of faster and more precise operation compared to a conventional vacuum (pneumatic) operated system.
The solenoid is an electromagnet consisting of a coil (electric circuit) and a soft iron core (magnetic circuit), which generates an oriented magnetic field. When electric current passes through the coil, the resulting magnetic field pulls an iron shaft which opens the valve. The response time of the solenoid is lower compared with an electric motor but it has good heat dissipation from the winding (stator) to the frame.
- exhaust gas inlet
- position sensor
- coil assembly
- valve body
- valve (exhaust gas)
The actuation force produced by the solenoid is proportional with the square of the magnetic induction and it’s only possible in one direction (opening). For the opposite direction (closing) the valve has a return mechanical spring. The difference in response time between the two directions of actuation (fast solenoid vs. slow spring) makes the solenoid valves quite difficult to control. Another disadvantage of the solenoid is the low actuation force, which makes them very sensitive to vibrations on the same axis with the actuation force.
Direct current (DC) motors are also used to actuate EGR valves. The DC motor therefore contains a rotor, consisting of a metal core with a copper winding, and a stator, consisting of permanent magnets whose magnetic flux passes through the rotor. The narrow space between the rotor and the stator is named air gap. The torque produced by the DC motors is converted into a linear actuation force through a system of mechanical gears and levers. Despite having significant dead-band and higher inertia of the mechanism, the DC motor offers the best compromise in terms of response time, stability and robustness against disturbances.
The position of an EGR valve actuated by a DC motor can only be achieved by using a position sensor. The position sensor of the valve is linear and provides a signal proportional to its supply voltage (usually 5V). The EGR valve position sensor signal is set to increases in the direction of the opening of the valve. The position sensor is used for three main reasons:
- allows a closed-loop control of the EGR valve: the mass of the exhaust gases is calculated function of the position of the EGR valve; function of the operating point of the engine (torque, speed and temperature), the electronic control module (ECM) of the engine sets a specific position of the EGR valve; this position is measured by the sensor and feed back to the control module; depending on the error between the set position and the actual position, the ECM controls the voltage applied to the valve in order to bring the EGR valve in the desired position
- allows the diagnostic of the EGR valve: the position of the EGR valve is used to detect a difference between the exhaust gas flow rate setpoint and the actual flow rate
- makes possible the calculation of the exhaust gas mass flow rate
Old types of EGR valves were using contact position sensors (resistive). Current generation of EGR valve use contactless position sensors (mainly Hall effect) which, compared with resistive sensors, have better precision and reliability.
The main problem of DC motors comes from the mechanical connection between the brushes and the rotary collector. The higher the speed of rotor, the more the brush pressure must increase to keep it in contact with the collector and higher friction is generated. Since there is electrical current flowing through the brushes and collector, arc discharge can occur, which wears the brushes quickly and generates interference in the supply circuit.
Lower operating speeds cause particles from the brushes to accumulate between the commutator segments, with the risk of a short circuit. The maximum speed of the motor is limited to around 10000 rpm. In order to operate with a useful speed of less than 1000 rpm and increase the output torque a gear mechanism is used. DC motors are therefore relatively bulky and need a gear to have the speed and torque in a suitable range.
Another variant of the DC motor is the torque motor. These are contactless rotary actuators (brushless DC motors) controlled by a standard H-bridge. The torque motor is capable of generating a constant torque indefinitely in a given fixed position without overheating or breaking.
Compared with the DC motor, the torque motor is more economical due to the reduced number of components: no commutator and brushes in contact. It is also more compact, because it is not necessary to have a mechanical gear transmission, the actuator being mounted directly on the shaft of the valve.
EGR valves are also actuated by stepper motors. The rotor contains a set of permanent magnets and the stator a set of electromagnets (coils with iron core) controlled by a power electronics circuit. The rotation comes from the interaction of the rotor and the magnetic field generated in the electromagents. The position of the rotor is controlled by a set of H-bridges, two for each pole. The stepper motor is capable of reverse rotation without the need of a gear mechanism.
The advantages and disadvantages of every type of electric actuation systems for EGR valves are summarised in the table below.
|DC motor (brushes)|
|Torque motor (brushless)|
|CPU – central processing unit|
In terms of pneumatic vs. electric actuation, the advantages and disadvatages of each technology is summarised in the table below
|Pneumatic||Solenoid (liniar)||DC motor|
|Actuation force [N]||100 … 120||25 … 35||350 … 450|
|Response time – opening [ms]||1000 … 2000||75 … 85||70 … 80|
|Response time – closing [ms]||60 .. 70||50 … 60||60 … 70|
|Resistance to blocking due deposits||medium||low||high|
|Sensitivity to exhaust pressure||low||high||very high|
|Position control acuracy||low||high||high|
Due to their overall advantages, DC motor actuated EGR valves are the most common used types of valves.
EGR systems usually have intake throttle valves, upstream of the EGR valve. The purpose of the throttle is to create a pressure difference between the exhaust and intake manifold (when closed) and to allow the exhaust gases to flow into the cylinders. Most of the EGR throttle valves are traditional ‘butterfly’ valves, similar to petrol engine throttles, and are controlled electronically the the engine control module (ECM).
The EGR cooler (radiator) lowers the temperature of the exhaust gas before it is introduced into the air charge stream. The lower the temperature, the higher the density, the better the efficiency in reducing the NOx emissions. Cooling the exhaust gas prior to mixing with the intake air lowers the combustion temperatures and increases the oxygen to fuel ratio. The higher flow of CO2 and H2O into the engine with cooled exhaust gases increases the heat absorbing capacity of the inlet charge; the lower inlet charge temperature generally reduces the combustion temperatures. Cooled EGR was introduced in order to achieve Euro 4 and Euro 5 NOx limits.
The vast majority of EGR coolers are made up from stainless steel or aluminium tubes and plates.
Most EGR systems have a by-pass valve integrated with the EGR cooler. When the engine is cold, the exhaust gas is circulated directly into the engine. The EGR cooler has a very high heat exchange efficiency and, without by-pass, the recirculated gases would be very cold and delay the warm-up of the oxidation catalyst, which would lead to excessive emissions of HC and CO. The solution is therefore to bypass the EGR cooler until the oxidation catalyst reaches nominal working temperature.
- EGR cooler
- electrical connection
- EGR valve
- exhaust gas inlet (hot, connection to exhaust manifold)
- EGR cooler by-pass valve actuation mechanism (pneumatic)
- engine coolant inlet
- engine coolant outlet
- exhaust gas outlet (cold, connection to intake manifold)
- guidance through the cylinder head
The cooler can be I or U type, depending on the shape of the gas passage through the cooler. In the case of a U-shaped EGR cooler, the inlet and outlet flanges are combined into a single unit at one end of the cooler.
1. EGR cooler
3.a EGR valve (closed position)
3.b EGR valve (open position)
4. exhaust gas inlet (hot, connection to exhaust manifold)
5.a EGR cooler by-pass valve (closed position)
5.b EGR cooler by-pass valve (open position)
6. engine coolant inlet
7. engine coolant outlet
8. exhaust gas outlet (cold, connection to intake manifold)
Compared with high-pressure, low-pressure EGR systems are more efficient in reducing NOx emissions in state of the art for diesel engines . BorgWarner shows the potential of a so-called Inlet Swirl Throttle (IST) to make use of the exhaust gas losses and turn them into a pre-swirl motion of the intake air entering the turbocharger to improve the aerodynamics of the compressor. The IST replaces the conventional low-pressure EGR throttle in the intake manifold, usually a simple butterfly valve, before the turbocharger.
Throttling always means inducing losses. The approach of the IST is to make use of the losses and turn them into a pre-swirl motion of the intake air entering the turbocharger to improve the aerodynamics of the compressor. Obviously, pre-swirl will have a positive impact on the compressor also in are as where no throttling is required. So the IST can be used to improve engine efficiency and performance also in regions where no throttling or EGR is required.
With IST the throttling effect is achieved by adjustable inlet guide vanes in the fresh air duct. In other words, IST is an intake throttle designed as a compressor pre-swirl device. This approach is expected to have positive impact on the combustion engine, like:
- higher low-end torque
- reduced exhaust gas emissions
- lower fuel consumption
To gain most out of the IST it needs to be operated in different modes depending on the engine operating point. The angle of the inlet guide vanes is adjusted continuously with changing engine load and speed and the set point of the vanes is determined by a controls algorithm, also taking the position of the VGT (variable geometry turbocharger) and the EGR valves into account.
 Emission-based EGR strategies in diesel engines for RDE requirements, Thomas Körfer, dr. Thorsten Schnorbus, Michele Miccio, Joschka Schaub, ATZ.
 Advanced direct injection combustion engine technologies and development, Volume 2: Diesel engines, Edited by Hua Zhao, CRC Press, 2010
 Intake throttle and pre-swirl device for low-pressure EGR system