Last Friday, the US Environmental Protection Agency (EPA) and California Air Resources Board (ARB) charged that Volkswagen (Jetta, Beetle, Golf, Passat) and Audi (A3) passenger cars equipped with 2.0-liter diesels in the US have used a software defeat device to cheat on the results of NOx testing, and thus have violated the US Clean Air Act. According to the charges, the software, when it detected regulatory testing on a dyno, ran a different, more emissions-stringent engine calibration to meet test requirements than when it detected regular use. (Earlier post.) As a result, real-world NOx emissions increased by a factor of 10 to 40 times above the EPA compliant levels, depending on the drive cycle.
According to the charges, Volkswagen admitted to the software device. Both agencies have launched investigations; Volkswagen—which in addition to recall costs for the approximately 500,000 vehicles affected faces civil penalties and injunctive relief—says that it will fully cooperate and has launched its own external investigation; eager lawyers are ramping up for class action suits against the automaker; Volkswagen AG lost almost one-fifth of its market value on Monday; and some Volkswagen suppliers are also feeling a crunch.
The engines at the core of this evolving debacle began with a 2.0-liter unit introduced in the US in 2008 in the Jetta as Volkswagen’s first Tier 2 Bin 5/CA LEV II-compliant (i.e., able to be sold in all 50 states) diesel. (Earlier post). The new 2.0L TDI (EA189) featured a common rail injection system and was the first of a new generation of diesels from Volkswagen. The 2.0-liter with CR was based on the 1.9-liter TDI engine with the Unit Injector System (UIS) “pumpe düse”. The predecessor engine was one of the most frequently built diesel engines in the world and was widely used within the Volkswagen Group.
Volkswagen engineers redesigned a large number of the base engine components to improve acoustics, fuel consumption, and exhaust gas emissions. The conversion of the injection system to a common rail design was one of the major changes; the addition of a new lean NOx trap (LNT) to handle NOx emissions was another. (This is referred to as the Gen 1 aftertreatment design in the EPA/ARB documents.)
NOx (nitrogen oxides: nitric oxide and nitrogen dioxide) formation is a function of temperature. Nitrogen and oxygen gases in the air react during high temperature combustion and form NOx. Because diesels operate at higher temperatures than gasoline engines, NOx formation has always been an issue for diesels. While efforts continue to develop new combustion regimes to reduce engine-out NOxto acceptable levels, aftertreatment currently is still required.
Broadly, there have been two catalytic approaches used to reduce exhaust NOx: urea-based selective catalytic reduction (SCR) and lean NOx trap (LNT) catalysts.
The urea-SCR (urea=AdBlue) approach requires on-board storage of the reductant fluid which is introduced into the exhaust upstream of the SCR catalyst. It is then converted to ammonia which interacts with NOx on the SCR catalyst to form water and nitrogen.
LNT technology utilizes fuel from the vehicle and advanced engine controls to enable periodic operation of the engine at rich air-to-fuel ratios to produce oxygen-depleted exhaust suitable for reducing NOx stored on the LNT catalyst surface.
Volkswagen planned to use both. In 2007, as the automaker discussed introducing the Jetta with the new 2.0L diesel, it said that for car models of the Passat class and smaller, it was testing the new lean NOxOx trap catalytic converter (discontinuous). (Earlier post.) At λ of greater than 1, NOx was captured and stored; at λ less than 1, NOx was released and reduced. Ultra low-sulfur fuel was a necessity, and fuel consumption would increase as a result of catalytic converter regeneration. Larger and heavier models would feature Selective Catalytic Reduction (SCR) catalytic converter (continuous) with the use of AdBlue.
Combined EGR operation plays a role in NOxreduction as well. The new 2.0L engine featured high- and low-pressure exhaust gas recirculation (EGR) with a high-performance EGR cooler and a turbocharger with a low pressure EGR inlet nozzle. The 2.0L TDI continuously adjusted EGR operation depending on engine operating conditions and speed. No-load engine operation resulted in high amounts of High Pressure EGR application. With rising engine load and engine RPM, the recirculation of exhaust gases shifted to the Low Pressure EGR system to increase the recirculation rate to obtain optimal NOx reduction at middle and high engine loads. Particularly in the high engine loads, the cooled Low Pressure EGR provided an advantage over the High Pressure EGR system.
The emissions control subsystem included an oxidation catalytic converter; diesel particulate filter; the LNT and H2S catalytic converter.
All of this complexity was managed by software run by the electronic diesel control EDC17 from Bosch. The successor to EDC16, EDC17 had greater processing capability and a larger storage capacity than EDC16. It also offered the option of integrating control functions for future technologies.
|Vehicles affected by the EPA NOV and CARB In-Compliance letter. Source: EPA. Click to enlarge.|
Volkswagen vehicles affected by the agencies’ charges span model years 2009 to 2015, with three different aftertreatment system configurations (Gen 1, Gen 2 and Gen 3). Below, we’ll go into some detail outlining the complexity of NOx control for the Gen 1 system, and then go on to outline the broader hardware changes in Gens 2 and 3, without replicating the control complexity. The principles are the same, the details differ.
|Gen 1 exhaust system. Source: Volkswagen. Click to enlarge.|
Gen 1 exhaust treatment. The diesel particulate filter and the oxidation catalyst are installed separately in a shared housing. The oxidation catalyst is located before the particulate filter in the direction of flow. The carrier material of the oxidation catalyst is metal, so the light-off temperature is reached quickly. This metal body has an aluminum oxide carrier coating, onto which platinum and palladium are vapor-deposited as catalyst for the hydrocarbons (HC) and the carbon monoxide (CO). The oxidation catalyst converts a large portion of the HC and CO into water vapor and carbon dioxide.
The diesel particulate filter consists of a honeycomb-shaped ceramic body made of aluminum titanide. The ceramic body is partitioned into a large number of small channels, which are alternately open and closed at the ends, resulting in inlet and outlet channels that are separated by filter walls. The filter walls are porous and coated with a carrier coating of aluminum oxide. Platinum, which acts as the catalyst, is vapor-deposited onto this carrier layer. As the soot-containing exhaust gas flows through the porous filter walls of the inlet channels, the soot particles are captured in the inlet channels.
The particulate filter must be regenerated regularly so that it does not become clogged with soot particles and its function impaired. During regeneration, the soot particles collected in the particulate filter are burned off (oxidized). The Engine Control Module (ECM) has several ways to control the increase of exhaust gas temperatures during active regeneration:
The intake air supply is regulated;
The exhaust gas return is deactivated to increase the combustion temperature and the oxygen content in the combustion chamber;
Shortly after a delayed “late” main injection, the first post-injection is initiated to increase the combustion temperature;
Late after the main injection an additional post-injection is initiated. This fuel does not combust in the cylinder, but instead vaporizes in the combustion chamber;
The unburned hydrocarbons of this fuel vapor are oxidized in the oxidation catalyst. This ensures an increase in the exhaust gas temperature to approximately 650 °C (1202 °F) as it reaches the particulate filter;
The boost pressure is adjusted so that the torque during the regeneration operation does not change noticeably for the driver.
To attain the BIN5/LEV2 emission level, Volkswagen used the LNT for exhaust gas after-treatment. This NOx storage catalyst supplements the particulate filter system. In this application, Volkswagen placed the NOx storage catalytic converter away from the engine in the vehicle underbody, thereby reducing thermal aging. This also takes advantage that the CO and HC that have already been oxidized by the particulate filter, allowing an optimum NOx conversion in the NOx catalytic converter.
The exhaust system has two lambda sensors.
The lambda sensor upstream of the oxidation catalytic converter regulates the air-reduced operating modes for the NOx catalytic converter. It is also used for the initial value for the air model stored in the engine control unit. This air model helps determine the model-based NOx and soot emissions of the engine.
The second lambda sensor, which is placed downstream of the NOx catalytic converter, detects an excess of reduction medium in the regeneration phase. This is used to determine loading and the aging condition of the NOx catalytic converter.
Three temperature sensors integrated into the exhaust system enable the OBD functions for the catalytic components and are used as initial values in the regulation of the regeneration operating modes ad the exhaust temperature model.
Use of the LNT requires new regeneration modes. Unlike particulate filter regeneration, a sub-stoichiometric exhaust gas composition is necessary for the regeneration of the NOx storage catalytic converter. In sub-stoichiometric operation, the nitrogen oxides stored during the lean operation are reduced by the exhaust enriched reduction media consisting of HC, CO and H2.
A further regeneration mode is provided by the sulfur removal of the NOx storage catalytic converter (DeSOx Mode). This is necessary as the sulfur contained in the fuel causes sulfate formation which slowly deactivates the NOx storage catalytic converter. The de-sulfurization procedure was designed for a sulfur content of 15 ppm parts per million (ppm).
Due to the high thermal stability of the sulfates, significant levels of sulfur reduction are only possible at temperatures above 620 ˚C (1150 ˚F). This sub-stoichiometric mode is very demanding in terms of engine management. To be able to set air mass and exhaust gas recirculation independently on each other, two separate control circuits are used. The air mass is set using the intake manifold throttle valve. The exhaust recirculation rate is set using a model-based regulation concept.
A suitable combination of high pressure and low pressure EGR, with corresponding compression temperatures, enable stable rich operation even in the low load range with the fuel qualities that are typical for the US. In addition to this, the injection strategy for the rich mode is changed. Up to six injections are used depending on characteristic values to attain a stable and low-soot combustion. This is particularly important in the sulfur reduction process to prevent soot accumulation in the particulate filter.
To attain the necessary exhaust gas temperatures in DeSOx operation, Volkswagen used very late, non-combustion post-injection. The fuel partially reacts at the oxidation catalytic converter with the residual oxygen contained in the exhaust gas and creates residual heat for the sulfur reduction of the NOx storage catalytic converter.
DeNOx-ing. The the engine control module prioritizes the NOxregeneration mode. The software takes into account the necessary engine operation and regeneration conditions as well as the catalytic converter properties. A loading and discharging model is stored in the engine control module for DeNOx regeneration. This maps the characteristics of the DeNOx storage catalytic converter. The load condition of the catalytic converter is modeled during engine operation that is dependent on the exhaust temperature and volume velocity as well as the calculated raw NOx emissions.
If the NOx load value exceeds a threshold value which represents the optimum conversion rate for the catalytic converter, the regeneration is conducted when the operating condition of the engine permits a regeneration mode to be activated. Two criteria, which relate to the lambda signal or a NOx discharge model, are available for determining the end of regeneration.
Gen 2: SCR. In addition to making modifications to the engine, Volkswagen subsequently introduced a Selective Catalyst Reduction (SCR) system in the Passat 2.0L TDI for NOx treatment. (Earlier post.) When the exhaust gases exit the engine, they pass through the oxidation catalyst and diesel particulate filter. This component converts gases and traps the soot in the oil. The soot is burned off through regeneration cycles.
|Gen 2 emissions system. Source: Volkswagen. Click to enlarge.|
After exiting the oxidation catalyst/diesel particulate filter assembly, the gases are sprayed with a reduction agent (urea) using the SCR injection valve. The gases then enter the SCR reduction catalysts. A NOx sensor downstream of the SCR reduction catalysts monitors the effectiveness of the reduction and is used to influence the amount of reduction agent used.
As with the LNT system, the parameters of all the elements that play into the aftertreatment are controlled by the ECM.
Gen 3: the EA288. In 2014, Volkswagen of America announced a major technology change. The strategically important new 2.0-liter EA288 diesel engine (earlier post) would replace the current generation 2.0L TDI, and would power the 2015 Golf, Beetle, Beetle Convertible, Passat, and Jetta. The EA288 is based on the Volkswagen MDB, its modular diesel engine toolkit (Modularen Diesel Baukasten) (earlier post).
The new EA288 engine was to replace all the 2.0-liter diesels then fitted in Audi and Volkswagen TDI Clean Diesel models. This turbocharged, common-rail, direct-injection four-cylinder engine produced 150 hp (112 kW)—an increase of 10 hp over the outgoing engine—and 236 lb-ft (320 N·m) of torque. This powerplant shares only the bore spacing with the previous diesel engine that had the same designation.
As part of this major redesign, the engine also features an entirely new exhaust aftertreatment system—a compact unit close-mounted to the engine, helping to lower heat and pressure losses. The system is modular. It can use an SCR system, or NOx storage, or oxy cat—essentially whatever the engine needs, without affecting the rest of the engine. At the time, Volkswagen said that the flexibility of the MDB would allow the engines to meet the coming EPA Tier 3, California LEV III emissions standards.
The EA288 TDI engine with the BIN 5 rating uses a low-pressure EGR system to reduce engine-out NOx emissions. The EA 288 aftertreatment system combines an oxidizing catalytic converter and a diesel particulate filter into a single module. This close arrangement allows the oxidizing catalytic converter and the diesel particulate filter to heat up quickly, and the operating temperatures of the catalytic converter can be reached faster.
In its Bin 5 design with SCR, the diesel particulate filter has an SCR coating.
|The aftertreatment module for the EA 288. Source: Volkswagen. Click to enlarge.|
In addition, the exhaust flap control unit has a throttle valve with an electric motor drive located downstream of the diesel particulate filter. The exhaust flap control unit can slow exhaust gas flow, helping to regulate EGR. The exhaust flap control unit is actuated by the ECM.