GB2590486A

GB2590486A - Method to reduce urea crystal formation in an SCR doser

Info

Publication number: GB2590486A
Application number: GB1918899.4A
Authority: GB
Inventors: Lloyd James; Coster Simon; Cochet Thierry
Original assignee: Delphi Technologies IP Ltd
Current assignee: Delphi Technologies IP Ltd
Priority date: 2019-12-19
Filing date: 2019-12-19
Publication date: 2021-06-30
Also published as: GB201918899D0

Abstract

A method of reducing urea crystal formation in an injector of a Selective Catalytic Reduction (SCR) system. The system includes an injector to inject urea into a vehicle engine exhaust, the injector being supplied with urea from a tank via a supply line. The method comprises, after engine shut-down T1, controlling the injector to perform one or more injections, preferably within one hour of the engine shut-down. The injection(s) may take place only when a measured or estimated temperature of the injector or injector tip reaches a first temperature threshold, and may be terminated when the temperature falls to a second temperature threshold. The supply line may be pressurised at a time T2 between the engine shut-down and the first injection, and may be pressurised for a time after the last injection. The supply line may be purged following the injection(s).

Description

METHOD TO REDUCE UREA CRYSTAL FORMATION IN AN SCR

POSER

TECHNICAL FIELD

This relates to Selective Catalytic Reduction systems used in vehicle exhaust system and which include a urea injector (doser) adapted to inject urea into an exhaust system upstream of an SCR unit. It relates particularly of a method of reducing and dealing with urea crystal formation on such injectors.

BACKGROUND OF THE INVENTION

SCR systems used in vehicle exhaust systems typically include an SCR unit (catalytic unit) in the exhaust line which converts NOx, typically to nitrogen and water. The urea, provided by a doser, is converted to ammonia (NH3)which reduces the NOx in the exhaust output. So these SCR units are filled with ammonia (NH3) which reduces the NOx; the ammonia is supplied to the SCR unit by a urea injector often referred to as a doser, located upstream of the SCR unit.

The urea injector is adapted and controlled to inject urea during running of the vehicle. The term -injector" hereinafter can be interchangeable with the word -doser".

A problem is urea crystal formation within the SCR injector. Test work demonstrated the strong link between increasing doser peak temperature in interrupted Diesel Particulate Filter (DPF) regeneration,engine shut downs and severity of doser sticking. The link between increased doser temperature and doser (injector)sticking is believed to be due to greater urea crystal formation due to a higher proportion of water in the DEF (DEF = Diesel exhaust fluid) solution being boiled away. Customer applications are currently being developed which have a greater risk of doser sticking due to higher peak temperatures achieved in engine shut downs.

The issue of injector sticking is serious as the SCR process to control oxide of nitrogen emissions cannot occur while the injector is stuck.

SUMMARY OF THE INVENTION

In one aspect is provided a method of reducing urea crystal formation in a urea injector of a Selective Catalytic Reduction (SCR) system, said SCR system including said injector adapted to inject urea into a vehicle exhaust, and where said urea injector is provided with urea from a urea tank via a urea supply line, said method comprising, after engine shut-down, a) controlling said injector to perform one or more subsequent injections.

In step a) said injection(s) may be provided within a time period of up to an hour after shut-down The method may include the prior step of measuring or estimating the temperature indicative of the injector or injector tip, and implementing step a) when said temperature reaches a first threshold temperature.

Step a) may be terminated when the measured or estimated temperature of the injectors or injector tip falls to a second threshold temperature.

The method may include pressurizing the urea feedline prior to said injections Said pressurization may be initiated at a timepoint T2 between shut-down and the first of said injections.

Said pressurization may last for a period of time after said last injection The method may include a last step of subsequently purging the urea feed line.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 shows a schematic vie of an SCR system.; - Figure 2a shows a graph of doser stuck duration against estimated peak doser seat temperature and shows a clear correlation; - Figure 2b shows a chart of injections to unstuck against temperature.

- Figure 3 shows plots of temperature at the doser /seat of the doser after engine shut -down; Figure 4 shows a chart showing the strategy according to one example; - Figure 5 shows a further example similar to figure 4

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows a schematic vie of an SCR system. This shows a urea/SCR injector I adapted to inject urea into a vehicle exhaust line not shown) under the control of an ECU 3. Coolant may be supplied to the urea injector (shown by the arrows 2) and may be engine coolant. Urea is fed to the urea injector via a urea feed line 4 from a urea tank and pump system 5 which comprises a urea tank 6 and typically a supply and purge pumps denoted with reference numerals 7 and 8 respectively. The supply pump pumps urea to the injector and the purge pump pumps urea from the injector /urea feed line/ The pumps are controlled by the ECU 3 via urea injector/doser control module 9.

As mentioned, high temperatures are known to cause problems with urea crystal formation. This is particularly the cases in engine shut downs or during regeneration of DPFs. Often in Diesel engine there is a Diesel Particulate Filter which is regenerated by increasing the richness of the A/F ratio. This increases the temperature of the exhaust line, resulting in more chance of urea crystal formation in the injector.

Figure 2a shows a graph of doser stuck duration against estimated peak doser seat temperature and shows a clear correlation. Figure 2b shows a chart of injections to unstuck against temperature.

After engine shut down, typically the urea feed line is purged using the purge pump, i.e. to suck out urea of the feed line and from the doser. This is so that under cold conditions urea does not freeze and apply pressure to internal surfaces of the closer.

In addition, after shut down, if the measured or estimated temperature at the doser, or within the exhaust system is high, then the doser coolant pump is run for a period of time after shut-down to keep the doser relatively cool and this helps to prevent DEF/AUS32 distilled water evaporation and hence crystal formation.

Figure 3 shows plots of temperature at the doser /seat of the doser after engine shut -down. Generally after shut down at time Ti the doser and portions of the exhaust tends to absorb heat. Thus generally temperature rises. This rise in temperature is tempered i.e. reduced by continuing to run the coolant pump for a period of time after shut-down. The engine is shut down at time point TI, between phase A and phase B. Plots 11, 12, 13 shows the temperature development with time when the coolant pumps is run for a period of time of 150, 450 and 780 seconds respectively after shut down.

According to one example urea crystal formation is reduced by performing one or more injections of the doser after engine shut down, the injection(s) may be performed anything up to an hour after shut down. Hence the feed line is not purged during this time and the feed line may be pressurised, Figure 4 shows a chart showing the strategy according to one example. The figure shows a number of plots of parameters against the same timeline. Plot 18 shows the engine state. The engine is initially running (engine state is ON) and at time Ti the engine is switched off-engine state Off. Plot 15 shows the temperature of the doser tip. Plot 19 shows the urea pressure in the urea feed line /supplied to the doser. Plot 20 shows the volume/quantity of urea injected with time.

After time point Ti the temperature of the doser tip rises. In order to counter this a series of urea injections 16 is performed over a time period 17 show also as Dl. In the examples shows a total of 1.5 g of urea is injected in dosing shots of 100mg. The timeslot between shots is 100ms The strategy of post injection may not be started for a period of time after shut-down and/or until the doser (tip) temperature rises to a certain first threshold level.

Here the temperature threshold level Thl is 140 degrees which is shown as the base line of the urea injection plot. The post injection process of 17 may be finished when the measured or estimated temperature of the injectors falls to a second threshold temperature T3. As in the example the first and second threshold temperatures may be the same.

In a further embodiment after shut down, there is a period of time D2 until T2, which is a monitoring time where it is determined if at T2 the temperature is still rising, or where it is decided there may be a need to provide the post shut-down urea injection. So at time T2 the urea pump may be activated to pressurise the urea feed line to a higher level The plot 19 shows the pressure in the urea feedline. After engine shut down the urea pump is turned off and so the pressure is reduced from operating pressure (e.g. 5.8 bar) to a lower value e.g. 3bar. At time T3 it is decided that there will possibly be a need to provide post shutdown urea injection according to methodology of the invention. Thus the urea line is pressurising by activation of the urea supply pump and subsequent to time T3 the pressure rises. Subsequent to the injections the pressure falls. However the urea line is kept pressurised albeit at a lower level until time T4. The pressurisation additional assists in reducing the risk of urea crystal formation.

Figure 5 shows a further example similar to figure 4 and like reference numerals show the same parameters. There is shown an alternative threshold temperature Th2 at 150 degrees. The coling injection doses have an on time Ton of 120ms and at 5.8 bar this is approximately 100mg/dose.

Subsequent to shutdown the total mass of aqueous urea solution (e.g.AdBlue) injected is 2.7g or 0.98 of urea. This can be accounted for determined by a NH3 stored model.

The term urea should be interpreted in the claims as urea or an aqueous solution thereof After the post injection process, the urea feed line may be purged; i.e. sucking urea out of the feed line/doser back to the urea tank. This may be done by the purge pump. Reference to purging should be interpreted as such.

Claims

CLAIMS1. A method of reducing urea crystal formation in a urea injector of a Selective Catalytic Reduction (SCR) system, said SCR system including said injector adapted to inject urea into a vehicle exhaust, and where said urea injector is provided with urea from a urea tank via a urea supply line, said method comprising, after engine shut-down, a) controlling said injector to perform one or more subsequent injections.
2. A method as claimed in claim I wherein in step a) said injection(s) are provided within a time period of up to an hour after shut-down.
3. A method as claimed in claims 1 to 2 including the prior step of measuring or estimating the temperature indicative of the injector or injector tip, and implementing step a) when said temperature reaches a first threshold temperature.
4. A method as claimed in claims 1 to 3 wherein step a) is terminated when the measured or estimated temperature of the injectors or injector tip falls to a second threshold temperature.
5. A method as claimed in claim 1 to 4 wherein including pressurizing the urea feedline prior to said injections.
6. A method as claimed in claim 5 wherein said pressurization is initiated at a timepoint T2 between shut-down and the first of said injections.
7. A method as claimed in claim 6 wherein said pressurization lasts for a period of time after said last injection.
8. A method as claimed in claim s 1 to 7 including a last step of subsequently purging the urea feed line.