Technical Information

– Optimization focus: emissions –

Lowering emissions and fuel consumption with flexible EGR systems

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Stuttgart, Germany, September 2010—Better fuel economy and lower pollution emissions are top priority on the development agenda—for passenger cars and commercial vehicles alike. To effectively lower emissions in commercial engines, MAHLE employs exhaust gas recirculation techniques.

In addition to reduced consumption, another important task is the prevention of pollution emissions, specifically nitrogen oxide emissions. Engine-internal processes such as exhaust gas recirculation (EGR) are aimed at preventing untreated emissions early on in the combustion stage. Engine-external solutions such as selective catalytic reduction (SCR) optimize emissions performance through the aftertreatment of exhaust gas in the exhaust gas system.

Ever stricter emissions regulations for commercial vehicles, such as EURO VI starting in 2013, along with new international test cycles such as WHSC (World Harmonized Steady State Cycle) and WHTC (World Harmonized Transient Cycle) are slated to heighten the requirements even further. From today's perspective, there are already various ways to meet the EURO VI limits. Three promising approaches are as follows:

  1. Engine without EGR, exhaust gas aftertreatment with oxidation catalytic converter (DOC), particulate filter (DPF), and SCR (NOx conversion rate > 90%)

The engine is operated with single-stage turbocharging and a maximum injection pressure of 2,500 bar. In order to achieve conformity of production (COP) and continue to meet emissions limits throughout the service life even as the system ages, a new SCR system should have an efficiency greater than 95% for EURO VI. Herein lies the problem at present. The service life and performance required of the SCR system necessitate higher system costs, and the additional urea used increases current operating costs.

  1. Engine with single-stage cooled EGR system (20–30% EGR rate), exhaust gas aftertreatment with DOC, DPF, and SCR (70–85% NOx conversion rate)

This engine application ensures EURO VI limits will be met. Owing to a smaller SCR exhaust gas aftertreatment system and lower conversion rates, system costs and operating costs are significantly lower throughout the service life. In the best case, this configuration does not negatively affect consumption. In some cases, however, fuel consumption might be slightly higher.

  1. Engine with dual-stage cooled high-EGR concept (EGR rate > 40%), exhaust gas aftertreatment with DOC and DPF

In high-EGR concepts, the required NOx limits can be achieved without cost-intensive nitrogen oxide exhaust gas aftertreatment. This configuration requires high-capacity, generally dual-stage, cooling of the recirculated exhaust gas and a high-performance particulate filter. Even so, this concept is the most cost-effective comparatively, and is the easiest to integrate into any vehicle or engine combination.

However, the volume of the EGR air mass is limited by the pressure gradient between the exhaust gas line and charge air line—a pressure gradient that is not always sufficient for high EGR rates, particularly at low engine loads and speeds. To achieve the required EGR rates of over 40%, the charge air mass flow must be throttled; this increases fuel consumption slightly due to greater charge exchange losses.

Precise control, less loss

MAHLE has therefore developed a technology aimed at achieving low-loss EGR rates of up to 50%, even when pressure gradients are small. The fast-switching charge air valve (SLV) replaces the conventional throttle valve. A reliable brushless DC motor is responsible for driving the continuously rotating flap in synchronism with the engine speed. Hence during one combustion cycle, the SLV briefly closes off the charge-air supply one time per cylinder in the valve-opening phase, temporarily lowering the level of the charge air pressure on the EGR admission side as a result. After each closing operation, the entire cross section of the charge air line is made available again; this allows the maximum charge air mass flow to stream in, thereby achieving an optimal cylinder fill. Small changes in the valve opening and closing times specifically affect additional combustion parameters in the diesel engine.

Not only does this ensure higher EGR rates, it also achieves a notable reduction in the total maximum pressure and temperature levels and hence also lower NOx emissions. In addition, by decreasing gas exchange losses by nearly 50%, the SLV provides significant consumption advantages compared to a conventional high-EGR solution with a throttle valve. Moreover, MAHLE has proven with a high-EGR system combined with SLV that the cost advantages (systems costs plus operating costs) are largely maintained compared to the other concepts.

SLV technology has already been successfully tested in a number of commercial vehicle and passenger car diesel engines with various turbocharging concepts—in terms of both the U.S. EPA and the European emissions legislations. Thanks to the SLV's flexible configuration in the charge air line, it was possible to implement the system in all engines with only minor adjustments. The results obtained show great potential for decreasing untreated NOx emissions with no negative impact on consumption. Carefully selected operating parameters also provide a good deal of application freedom in the conflicting areas of NOx/particulate emissions and fuel consumption.

The MAHLE Group is one of the 30 largest companies in the automotive supply industry worldwide. With its two business units Engine Systems and Components and Filtration and Engine Peripherals, MAHLE ranks among the top three systems suppliers worldwide for piston systems, cylinder components, as well as valve train, air management, and liquid management systems. The newly formed Industry business unit bundles the MAHLE Group's industrial activities. These include the areas of large engines, industrial filtration, as well as cooling and air-conditioning systems.

In 2009, the MAHLE Group generated sales of approximately EUR 3.9 billion; around 43,000 employees work at over 100 production plants and eight research and development centers.

Press contact

MAHLE GmbH

Birgit Albrecht

Corporate Communications/Public Relations

Pragstrasse 26–46

70376 Stuttgart

Germany

Phone: +49 (0) 711/501-12506

Fax: +49 (0) 711/501-13700

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