Exhaust Gas Recirculation - EGR
General Description
In order to achieve the emission targets for diesel vehicles it is necessary to reduce the emissions of nitrogen oxide (NOx). Diesel engines operate in lean-burn conditions. Therefore standard exhaust gas aftertreatment with a three-way-catalytic converter, as used in standard gasoline applications, will not operate properly. The target is to minimize the raw NOX emissions by optimizing the combustion process. Cooled exhaust gas, mixed with fresh intake air, is used to fulfil these requirements.
The exhaust gas recirculation (EGR) system consists of EGR valve, EGR cooling device, pipes and sensors. Activating and rating the EGR is controlled by the ECU and must consider several conditions such as ambient temperature, altitude, engine temperature, boost pressure, engine speed and injection quantity. The required quantity of recirculated exhaust gas is calculated using numerical flow models. The EGR application is vehicle specific and has to be restricted in terms of efficiency, overall emissions and engine protection.
Parameters Sensed And Controlled
Control Strategy
The exhaust gas recirculation is controlled via the EGR valve and it is predefined as a percentage of EGR related to fresh air. The set value for the EGR valve is a pulse duty factor in the range of 0-100%. The EGR ratio is modulated to an optimum level of NOX in relation to growing particulate emissions. The configuration of the EGR map reflects a typical diesel application considering this NOX -particulate trade-off with high rating on part load conditions and a reduced content of exhaust gas on high load condition.
Engine speed and injection quantity are the main parameters for setting the required EGR ratio. Of those, and integrated recorded parameters like back and boost pressure, gas and air temperature in different positions, coolant temperature and ambient conditions (altitude and temperature), the rate is defined from the map and converted into a set value for the valve. The proposed EGR strategy is to control the exhaust emissions over a wide range of environmental and driving conditions.
Engine start
After engine start the EGR will be turned on with a delay time which depends on the coolant temperature. To avoid a degradation of starting ability and misfire the lead time for EGR activation is related to the coolant temperature in a linear behavior. In any case a short initialization time is required before using EGR.
Engine warm up
The warm up strategy is dependent on the coolant temperature at engine start and the heating behavior. During cold conditions the internal heat transfer can delay ignition and as a consequence insufficient combustion will increase overall emissions. Due to this the EGR ratio is ramped in relation to the coolant temperature.
Ambient conditions
- Altitude:
With increasing altitude the ambient air density decreases and as a result the amount of oxygen in the cylinder is reduced. The NOX-PM trade-off is shifted to increased particulate emissions caused by incomplete combustion. On turbocharged engines the Turbo Charger is not able to deliver the desired boost pressure, this lack of boost will increase linear to the altitude, which leads to less compression and therefore to a increase of potential misfire. Both effects are avoided by rating the EGR towards lower ratios at lower barometric pressure.
- Intake air temperature:
To avoid deterioration of the combustion efficiency under cold ambient temperatures, it is necessary to restrict the EGR ratio in relation to the intake air temperature. To avoid icing inside the intake manifold at ambient temperature below 4°C and therefore the risk of engine damages the EGR rate has to be reduced and sometimes to shut off if the intake manifold temperature falls below 4°C.
At higher ambient temperature the behavior is the same that we have in altitude, less density which force the NOx-PM trade off.
Selective Catalytic Reduction (SCR)
General Description
The SCR system is used to reduce nitrogen oxides (NOx) by dosing urea into the exhaust gas. Ammonia is a product of a thermolysis and hydrolysis reaction after the injection of urea into the exhaust gas and reacts in the SCR catalyst with NOx to form water and nitrogen.
In order to cope with the dynamic effects the SCR catalyst is able to store ammonia. Dosing of urea (refilling of catalyst) is calculated considering raw NOx emissions, modeled SCR catalyst efficiency and the amount of stored ammonia. Additionally an adaptive function guarantees high efficiency in the long term.
Control scheme of the SCR system
Parameters Sensed And Controlled
Control Strategy
Dosing amount
The demand of ammonia in the SCR catalyst is computed continuously in the engine control unit depending on measured raw NOx emission and exhaust gas mass flow. Used ammonia for reduction of NOx to water and nitrogen is calculated with the SCR catalyst efficiency model. Balancing demanded and used ammonia determines the amount of stored ammonia in the SCR catalyst and the amount of dosed urea.
Catalyst efficiency model
Dependent on the measured values of raw NOx, temperature upstream of the SCR catalyst, NO2/NOx ratio into the SCR and age of the SCR catalyst, the calculated amount of stored ammonia and exhaust gas mass flow, the catalytic efficiency is determined.
Urea vaporization
The amount of urea which can be vaporized is limited by two main factors, the temperature of the exhaust gas and the exhaust gas mass flow. If the temperature is low the amount of urea which can be vaporized is limited. At high exhaust gas mass flow the amount of useful urea is also limited due to insufficient vaporization time.
Dosing system release conditions
To release SCR pumppressure build up, the engine must be running (minimum Idle speed), the temperature upstream of the SCR catalyst has to be above a threshold temperature, which is determined by the urea vaporization temperature and urea may not be frozen. Determination of whether or not the urea is frozen depends on measured values of ambient air temperature and urea tank temperature. After pressure build up dosing can be released.
Long term adaptation
The adaptation is a function to guarantee long term efficiency. Therefore the NOx-sensor downstream of the SCR catalyst is compared to the calculated value downstream of the SCR catalyst. If deviations occur, the dosing amount is corrected temporarily. The systematics of these corrections are evaluated and an adaptation factor is applied on the dosing amount.
Engine warm up
At engine cold start the temperature of the SCR catalyst is too low for a reaction of the reductant with NOx (catalyst light off). Injection of urea into the exhaust system is limited by the temperature at which the urea can decompose into NH3. If urea is injected at a temperature below this threshold, all urea dosed will crystallize on the exhaust pipe wall and will not be useful for NOx conversion. For that reason urea is not injected until the exhaust temperature exceeds the temperature required for vaporization of the urea solution.
Cold ambient temperature (below -7°C)
Depending on the urea temperature in the urea tank a defrost time is set for the heating prior to activating the urea pump. Low ambient temperatures are leading to decrease of exhaust gas temperature which leads than to a reduced area where the surface temperature of the SCR catalyst is above the light-Off temperature.
Lean NOx Trap (LNT)
General Description
The Lean NOx Trap (LNT) is an aftertreatment device used to attain a reduction in nitrogen oxide emissions for Diesel and lean burn engines through a trap system.
During lean operation mode, the LNT provides to efficiently convert carbon monoxide (CO), hydrocarbons (HC) acting as typical Diesel Oxidation Catalyst (DOC) and nitrogen oxides (NOx), produced during engine operation, into more desirable gases. Thanks to the active catalyst layer containing palladium, platinum and/or rhodium. The reaction of nitrogen oxides is necessary because most NOx trapping materials more effectively absorbs NO2 compared to NO. When the LNT storage capacity reaches the limit or the NOx storage efficiency drops below a threshold, the purging process, called regeneration, is triggered. This last phase needs energy, provided in form of reductant, as HC, CO and H2 that substitute oxygen in the mixture.
The rich operation mode provides the necessary enrichment mixture. The first purging step regards the NOx release from the nitrate sites caused by excessive fuel conditions or elevated temperatures. In this engine conditions the nitrate species become thermodynamically unstable and decompose, producing NO or NO2. Under rich conditions there is the possibility to have the reductant species to reduce the nitrogen oxides over the catalyst, in a conventional three-way catalyst process.
This process needs a control strategy to manage the two different engine combustion modes (lean and rich) and the transition between them
Control Strategy
NOx Storage evaluation.
To defined the right time for regeneration a NOx storage Model is needed.
This model takes in to account the actual NOx engine out emission and the storage capacity. The storage capacity strongly depends on the LNT temperature
Typically the storage efficiency starts (>50%) at about 200°C-250°C and ends (<50%) at about 400°C-450°C.
Switching between lean / rich operating modes
A challenge for the application of an LNT in the diesel motor is represented by the rich operation mode (Lambda < 1) for the regular regeneration of the LNT (DeNOx phase).To achieve this, the fresh air mass is severely throttled and the exhaust gas enriched accordingly by further injection of fuel.The periodic switchovers should take place regardless of the engine speed and without being acoustically noticeable.This should not cause any appreciable rise in fuel consumption.
NOx Regeneration (deNOx)
Regeneration of the LNT (deNOx phase) is initiated by a Lambda controller integrated into the engine electronics.The rich operating range is only possible in a window of engine speeds an load which are frequently used by diesel engines, the limited value is mainly the exhaust gas temperature which have to be below a certain limit. Also the rich mode can only be used during low dynamic driving to guarantee a stable combustion
Desulphurization (deSOx)
Over the life-span of the vehicle, the sulphur contained in the diesel fuel results in sulphur contamination of the LNT. Sulphur is stored as barium sulphate in the LNT, but at the temperatures normally used in lean operation, however, can no longer be removed by means of rich phases.A discharge of the sulphur is only possible at high temperatures (over 600°C).As a loss of efficiency is essentially only detectable after the engine has covered several thousand kilometers, it is sufficient to combine a desulphurization with the cyclically occurring particle filter regeneration.The duration of the rich pulse is set in such a way that the desulphurization process does not produce the foul smelling H2S, nor will the temperature in the component during and after the rich phase result in thermal aging or damage.In the engine control unit therefore, a sulphur charging and discharging model is implemented on the one hand and a predicted temperature calculation model is implemented on the other.
Lambda control
Lambda control is needed to guarantee the right mixture enrichment to manage the regeneration requests (during deNOx and deSOx). It acts on after injection quantity.
It is important to keep information about gas composition and condition (space velocity and temperature) The Lambda control can only carry out with an high quality during non dynamic driving maneuver.
Engine warm up
At engine cold start the temperature of the LNT is too low for a storage of NOx (catalyst light off, capacity curve). Therefore neither storage nor regeneration is possible below the light off temperature,
Hot ambient temperature (over 30°C)
During deNOx, deSOx, events the fresh Air is strongly reduced and the fuel is burned with low efficiency. This leads to high exhaust temperature. If the intake Air temperature is on top very high an effective rich operation is impossible.
High speed driving
During high speed driving, trailer towing or high dynamic acceleration the exhaust temperature is rising and the LNT capacity decreases. At such high load and engine speed conditions a rich operation mode is not possible and thermal desorption will take place.