Exhaust gas recirculation

Proven Pierburg technology for controlling emissions

Exhaust gas recirculation - An indispensable way of controlling emissions

Exhaust gas recirculation (EGR) has not only proven to be an effective way of controlling emissions in petrol engines – practically all modern diesel engines must also be fitted with the technology to comply with increasingly stringent exhaust gas regulations. Compliance with even lower limit values will only be possible by using a cooled exhaust gas recirculation system.

Pierburg has made a substantial contribution to the current state of the art and, as a highly experienced system supplier, is able to offer a compact and efficient system for controlling emissions – for passenger cars and utility vehicles.

There’s a reason why Pierburg is represented as the OEM in a large number of modern vehicles with EGR valves and EGR coolers. The corrosion and temperature-resistant materials used in the Pierburg products guarantee lasting function under the harshest conditions, e.g. aggressive exhaust gas condensate, temperatures up to 700°C and pressures up to 3 bar.

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Components in exhaust gas recirculation

In the case of exhaust gas recirculation, a certain amount of exhaust gas is mixed with the intake air. This means that less oxygen reaches the cylinder. This leads to a lower combustion temperature. As a result, the amount of nitrogen oxides in the exhaust gas can be reduced by up to 50%. In petrol engines, this can also reduce carbon dioxide emissions and fuel consumption.

There are various positions for exhaust gas removal:

INTERNAL EGR

Valve overlapping means that some exhaust gas remains in the combustion chamber or is sucked back into the cylinder from the outlet duct.
The timing of the intake and exhaust valves is changed by means of adjustable cams.

EXTERNAL EGR

Exhaust gas is removed outside the cylinder head on the exhaust-gas side and is fed back via lines or ducts to the fresh air side through an external valve.
This provides the opportunity for additional exhaust gas cooling by an optional cooler with / without bypass flap.

There are two types of external EGR:

HIGH-PRESSURE EGR

The exhaust gas is

removed immediately behind the cylinders in front of the turbocharger turbine and
fed to the fresh air side behind the throttle valve.

LOW-PRESSURE EGR

The exhaust gas is

removed downstream of the turbocharger turbine or only downstream of the exhaust gas treatment systems and
supplied to the turbocharger compressor.

An exhaust gas flap provides the necessary exhaust gas back pressure if the pressure difference is not sufficient for the required EGR mass flow rates. The exhaust gas is also cooled by a special low-pressure EGR cooler.

Exhaust gas recirculation (schematic)

01 Exhaust gas flap
02 Low-pressure EGR valve
03 Low-pressure EGR cooler
04 Turbocharger (compressor)
05 Turbocharger (turbine)
06 Particulate filter
07 Charge air cooler
08 Throttle valve / regulating throttle
09 High-pressure EGR valve
10 High-pressure EGR cooler
11 Bypass flap

blue = High-pressure range
green = Low-pressure range

EGR cooler

The EGR cooler is a small but crucial component in vehicles. An EGR cooler is used for the targeted cooling of the recirculated exhaust gases and, in addition to the EGR valve, is one of the main elements of the exhaust gas recirculation system. It is integrated into the engine’s coolant circuit and uses the coolant to reduce the temperature of the exhaust gases. Pierburg EGR coolers are manufactured according to OE standards and quality management standards in the automotive sector. All main components meet the quality requirements for initial assembly products.

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EGR valves

A key component of exhaust gas recirculation is the EGR valve. It meters the quantity of exhaust gas that is fed back. EGR valves are available in many versions and models: electrically or pneumatically controlled for petrol or diesel applications or with connection to the coolant circuit. Electric EGR valves are predominantly used nowadays as they require neither a vacuum nor a solenoid valve for the actuation. Due to the higher return rates involved, EGR valves for diesel applications have relatively large opening cross-sections. The cross-sections for petrol engines are much smaller.

Cooled exhaust gas recirculation for even lower pollutant emissions

As a specialist with many years of experience in emission control, Pierburg is an expert OE supplier of systems for cooled exhaust gas recirculation. Motorservice is now bringing this technology to the aftermarket.

Ever more stringent emission regulations require a constant improvement in emission control methods. For diesel engines, this particularly applies to a further reduction in nitrogen oxides (NOx). This is why cooled exhaust gas recirculation is used here: It lowers the combustion chamber temperatures, thus reducing the formation of nitrogen oxides.

Calling on its many years of experience in the development and manufacture of EGR systems, Pierburg has designed a range of EGR cooler modules that permit precise cooling of exhaust gases. This is sophisticated echnology in the smallest of spaces.

What is special about Pierburg EGR coolers?

The lamella geometry developed by Pierburg reduces the danger of sooting in the cooler.
Special surface coating in the EGR cooler to prevent soot build-up
Greater integration: Compact EGR cooler modules made of aluminium with integrated EGR valve, bypass flap and other attachments such as oil coolers and oil filters

BYPASS FLAPS IN EGR COOLER

Many of today’s EGR coolers feature an electrical or pneumatic bypass flap. This allows the exhaust gases to be directed past the EGR cooler in the warm-up phase, to quickly bring the engine and catalytic converter up to operating temperature. This also reduces the amount of noise – what is known as “diesel knock” – as well as the level of raw hydrocarbon emissions in the warm-up phase. Bypassing is also possible if high exhaust gas temperatures are required, for example in the recovery of diesel particulate filters.

Why is cooled exhaust gas recirculation necessary?

Cooled exhaust gas reduces the peak combustion temperature. This significantly reduces the amount of nitrogen oxides. Additionally, cooled gases are denser than warmer ones. This means: at the same boost pressure, a larger volume of gas fits into one cylinder filling. This produces a “leaner” combustion which also improves fuel consumption and particulate emissions. EGR coolers are used for targeted cooling of the recirculated exhaust gases.

Cooled exhaust gas recirculation – advantages at a glance

Reduced peak combustion temperature

Up to 50% reduction in nitrogen oxides

Reduced particulate emissions

Lower fuel consumption

High-pressure / low-pressure exhaust gas recirculation

What is the difference?

Raw emissions from engines are continually being reduced with technical measures. But as the emission limit values keep getting tighter and tighter, the technologies in non-engine measures need to keep improving as well.

Exhaust gas recirculation has proven to be a successful emission control method. In the case of conventional high-pressure EGR, exhaust gas is removed immediately after the cylinder and mixed with the intake air. A low-pressure EGR system is also required to achieve the limit values from Euro 6 / Tier 2. With this system, the exhaust gas is taken after the particulate filter on the low-pressure side, and added again before the turbocharger compressor. An exhaust gas flap provides the exhaust gas back pressure required for this.

But what is the difference? The table below provides a brief overview.

	High-pressure EGR	Low-pressure EGR
Inlet pressure into the EGR section	High (up to approx. 3.5 bar)	Low (up to approx. 1.3 bar)
Inlet temperature into the EGR section	Very high (up to approx. 950°C)	High (up to approx. 800°C)
Pressure difference Δp over the EGR section	High (up to approx. 1.5 bar)	Low (up to approx. 0.3 bar)
Cyclical pressure fluctuations	High	low
Exhaust gas composition	Removal before exhaust gas after-treatment	Removal after exhaust gas after-treatment

Low-pressure EGR

Low-pressure EGR represents the state of the art with regard to diesel engines.
The advantages of adding low-pressure EGR to high-pressure EGR include:

Higher performance/efficiency of the turbine
Larger EGR map
More homogeneous mixing of exhaust gas with fresh air via the compressor
Resulting in lower NOx and particulate emissions
Improved EGR cooling (through EGR and charge air cooler)

The disadvantages compared to high-pressure EGR include:

Longer paths and additional components
Possible hazard due to soiling or damage to the turbocharger compressor, e.g. through droplet impact

The low-pressure EGR valves from Pierburg generally consist of a centrally positioned flap (“butterfly”) in an aluminium pressure die-cast housing. The integrated actuating drive usually consists of a DC electric motor and a two-stage spur gear. The low-pressure valves were designed using assembly groups from existing throttle valve and EGR valve product lines that have been in tried and tested series production for years.

The low-pressure EGR combi valve simultaneously assumes the tasks of the low-pressure EGR valve and an intake air throttle. Throttling causes a pressure gradient to the intake side. This causes the exhaust gas to flow in a controlled manner into the area in front of the compressor. As a combined component, the low-pressure combi valve is not only more cost-effective, but also offers the benefit of a lower weight.