WO2002101208A1

WO2002101208A1 - Improvements in particulate filters

Info

Publication number: WO2002101208A1
Application number: PCT/GB2002/002625
Authority: WO
Inventors: Dominic Jonathan Burton; Craig Lucas Goodfellow; Julian Brownlow Wray; Peter Michael Fussey
Original assignee: Ricardo Consulting Engineers Limited
Priority date: 2001-06-12
Filing date: 2002-06-12
Publication date: 2002-12-19
Also published as: GB0114311D0

Abstract

A vehicle engine exhaust includes a particular filter regeneration control system including a controller (42) and first and second regeneration activators including an air intake throttle (50) and an electrical heater (46). As a result engine efficiency losses with higher back pressure levels can be traded off against improved self propagation of incineration at higher particulate loadings.

Description

IMPROVEMENTS IN PARTICIPATE FILTERS

The invention relates to particulate filters, in particular diesel particulate filters (DPF's). The use of DPF's, also known as diesel particulate traps, to collect particulate emissions in the exhaust systems of diesel vehicles is well known and a simplified schematic diagram is shown in Fig. 1. A vehicle 10 includes a diesel engine 12 having air intake and fuel intakes 14 and 16 respectively, an exhaust 18 and a DPF 20 in the exhaust line. In essence the DPF comprises a porous media through which the exhaust gases pass and which filters out particulates. Various filter materials are known including ceramic monoliths, wire mesh and so forth. Fig. 2 shows a simplified diagram of a known DPF arrangement in which particulate laden exhaust gas 24 is forced through a filter material 22 which provides the only route between closed passages 30,32. The cleaned exhaust emission 26 exits the outlet passage 32 leaving behind particulate 28 on or in the filter 22. Multiple passage arrangements are known and arrangements such as these have been found to reduce emissions of particulate by 90% or more.

A known problem with DPF systems is that as particulate matter builds up and the filter effectively becomes clogged, the back pressure on exhaust gases will also increase. The preferred scheme currently used for overcoming this problem is termed "regeneration" by which the particulates are effectively incinerated in the filter, regeneration takes place before a large amount of particulate builds up, by triggering regeneration at short time intervals. However diesel engine exhausts are relatively cool and as the thermal efficiency of engines improves the exhaust temperatures are decreasing further. Accordingly the diesel exhaust temperature alone may not be sufficient to raise the filter temperature to the self-oxidation temperature of the particulate at which a self-sustaining action can take place under the correct conditions of temperature, oxygen concentration and mass air flow.

Various schemes have been proposed in the art to initiate regeneration including using catalysts either coated on the DPF itself or carried in the fuel. International Patent Application No. WO 00/60228 proposes an alternative system of increasing the exhaust gas temperature by injecting fuel into cylinders of the engine during the expansion stage. US Patent No. 5,171,337 to Pollock relates to a filter unit further comprising an electrical heating element to raise the temperature of the filter sufficiently to initiate regeneration. In addition known proposals include throttling the air intake to the engine hence increasing exhaust gas temperature.

Various problems arise with the known arrangements including increased fuel/electricity consumption and reduced engine efficiency. A further problem with existing regeneration schemes is that the exhaust temperature is dependent upon driving conditions. In typical urban conditions where an engine runs on a low. load for a short period and is switched off frequently, exhaust temperatures can be too low to induce particulate burning without excessive additional heater work whereas long driving periods at high loads will create exhaust temperatures more conducive to regeneration without additional heating. As a result existing schemes may face particular problems during low load conditions of this kind.

According to the invention there is provided a particulate filter regeneration control system for an engine having an exhaust comprising a particulate filter for filtering an exhaust, and first and second regeneration activators in which the regeneration activators comprise a filter heater and an air inlet throttle to the engine. As a result regeneration can be achieved at optimum engine efficiency even in urban or other low exhaust temperature driving conditions.

According to the invention there is further provided a particulate filter regeneration control system comprising a particulate filter, at least one regeneration activator, a particulate load sensor and a controller arranged to trigger the regeneration activator when the particulate load in the filter falls in the upper part of the range 8-10g/l, more preferably approximately lOg/1. The invention hence identifies an optimum load value for trading off increased back pressure against improved regeneration conditions.

According to the invention there is yet further provided a particulate filter regeneration control system comprising a particulate filter, first and second regeneration activators, a regeneration condition sensor and a controller in which the controller selectively triggers one or both of the activators dependent on the sensed regeneration condition. As a result an efficient regeneration program can be tailored to current load conditions.

Further preferred features and methods according to the invention are set out in the appended claims.

Embodiments of the invention will now be described by way of example with reference to the drawings of which: Fig. 1 shows a schematic view of a known DPF scheme;

Fig. 2 shows a detail of a typical DPF;

Fig. 3 shows a DPF control scheme according to the present invention; and

Fig. 4 shows a plot of fuel consumption against DPF loading. Referring to Fig. 3 an improved DPF scheme according to the present invention includes a diesel engine 36 having an exhaust 38 and a DPF 40 mounted thereon, a controller 42 and a flywheel mounted electrical device (FMED) 44. The DPF includes an electrical heater 46 and the engine includes an air inlet throttle 50.

The various components individually will be well known to the skilled person. In the preferred embodiment the diesel engine is a 1.2 litre engine and forms part of a mild hybrid arrangement in which the FMED 44 operates at 42 volts and the maximum power of 6 kilowatts. The FMED 44 is used to power the heater 46 and in a typical mild hybrid implementation can also provide torque boast at low engine speeds, regenerative braking and powering of additional electrical ancillary devices and can be for example of the type known as a Continental ISAD - Integrated Starter, Alternator and Damper.

The DPF may be of any appropriate type for example a round geometry SiC monolith of dimensions length 6 inches (15.24cm) and diameter 5.66 inch (14.38cm) of the type sold by Corning and Ibiden. The electrical heater is a 42 volt metal foil element heater of the type provided by Emitec GmbH of

Germany primarily providing heating of the monolith by convection. In the embodiment shown it is mounted very closely upstream to the DPF and hence provides additional direct heating of the front face of the DFP monolith by radiation as the element approaches its upper temperature range. The heater is dimensioned to produce about 5.9kW of total heat at 42 volts.

The controller 42 can be an appropriately programmed micro processor which can be stand-alone or integrated with an engine management system. It includes control lines 48 and 52 to the air inlet throttle 50 and heater 46 The regenerative control scheme implemented by the controller 42 addresses low load conditions such as in during urban driving, in which exhaust temperatures may not rise high enough to initiate regeneration without significant additional electrical heating which in turn increases fuel consumption and potentially exceeds the power available from the FMED 44. In particular the complementary exhaust temperature raising mechanism of adjusting the air intake via throttle 50 is also implemented. The air throttle 50 may be of any appropriate type. In a preferred embodiment the throttle control comprises a combination of inlet throttling and variable nozzle turbocharger (VNT) control.

Yet further in conventional systems DPF is triggered sufficiently frequently that particulate loading remains at a low level, determined so as to avoid high levels of back pressure being fed back to the engine from a partial blockage of particulate. The present invention recognises, however, that the engine efficiency losses with higher back pressure levels can be traded off against improved self-propagation of incineration at higher particulate loadings. Accordingly the controller 42 further includes a particulate loading sensor 58 and sensor line 60 to determine when the particulate loading of DPF 40 reaches a predetermined, relatively high level - at that stage regeneration is triggered by activation of the air intake throttle 50 and electrical heater 46.

The loading sensor can be of any appropriate type as will be well known to the skilled person but in the preferred embodiment comprises a ΔP sensor coupled with a exhaust volumetric flow sensor. The ΔP sensor measures the pressure drop across the filter and this value allows the loading to be determined as will be well known to the skilled person.

According to this scheme oxygen based DPF regeneration occurs, without the need for fuel borne catalyst or active wash coat as the required temperature of around 550°C required to initiate the particulate burning reaction in a non-catalysed filter is attained. Furthermore the DPF is regenerated using the minimum amount of energy possible as the particulate burning reaction according the scheme becomes self-sustaining as the correct conditions of temperature, oxygen concentration and mass air flow are met.

In particular it is found that regeneration according to the scheme occurs effectively even during low load conditions such as experienced during urban driving. In the particular embodiment discussed herein it is found that at DPF particulate loadings of 25g (lOg/1), the temperature rise in the exhaust gas induced by inlet throttling allows an electrically heated regeneration powered by the FMED to take place even under low load conditions with 85% of the particulate burnt off within 300 seconds. Furthermore it is found that, over a typical drive cycle such as the new European drive cycle (NEDC) a fuel consumption penalty of only 0.91//100km is incurred when regeneration takes place during the cycle. More generally it is found that particulate loadings in the upper part of the range 8-10g/l provide the desired conditions for the invention to operate. As a result the invention identifies an optimum point in the fuel consumption against loading curve shown very roughly in Fig. 4, which is substantially flat in this region, such that the improved regeneration conditions at the upper end of the range combine with the minimal compromise on back pressure to provide optimum operation.

It will further be appreciated that enhanced control schemes can be implemented in which the air intake throttle 50 is activated selectively dependent upon engine load such that it only supports the electrical heater when the regeneration temperature would otherwise not be attained. To implement this the controller can, for example, have a simple exhaust temperature sensor and an engine mass flow rate sensor. From the readings taken from these the controller can determine whether the electrical heater alone can provide the required ΔT to initiate regeneration and if not can institute inlet throttling as well. It will be appreciated that the controller can take measurements directly or can access appropriate data from an engine management system dependent on the type of load sensor employed. Yet further the system can provide model-based predictive regeneration. Where a model of loading level is constructed against engine conditions and/or time then the regeneration point can be derived in advance in real time based on engine mapping. This can embrace more complex schemes which determine whether throttling will be required and can be integrated into a general hybrid predictive control scheme whereby optimum load sharing between power sources is derived based on predicted loads.

It will also be appreciated that appropriate particulate loading values triggering regeneration can be determined for other systems empirically by monitoring fuel consumption/exhaust back pressure and filter/heater temperature (to avoid exceeding safe temperature limits in the filter when the particulates are burnt at very high loadings) against FMED drain. Alternatively the system can be calibrated using appropriate modelling techniques, for example modelling the filter temperature, and the exhaust pressure drop against particulate loading and exhaust volume flow rate providing a measure of increased fuel consumption to define the optimum DPF particulate loading at which to trigger regeneration. An appropriate pressure drop sub-model is characterised by the following equation:

where μ: exhaust dynamic viscosity Q: exhaust volumetric flow V_trap: DPF volume : DPF honeycomb filter cell size w_s: filter wall thickness ko: clean wall permeability k_part: particulate permeability

F: constant L: effective channel length.

It will be appreciated that the invention can be applied to other particulate filters where the particulate bearing gas temperatures are insufficiently high to trigger self-sustaining regeneration or vary dependent upon system loadings. It will further be appreciated that any type of DPF and specific regeneration triggering loading value within the ranges discussed above can be adopted as desired.

Claims

1. A particulate filter regeneration control system for an engine having an exhaust comprising a particulate filter for filtering an exhaust, and first and second regeneration activators in which the regeneration activators comprise a filter heater and an air inlet throttle to the engine.

2. A system as claimed in claim 1 further comprising a controller to trigger the regeneration activator when the particulate load in the filter falls in the range 4-20g/l.

3. A system as claimed in claim 2 in which the controller is arranged to trigger the regeneration activator when the particulate load in the filter falls in the range 4-10g/l, more preferably 8-10g/l, most preferably approximately lOg/1.

4. A system as claimed in any preceding claim further including a regeneration condition sensor in which the controller selectively triggers one or both of the activators dependent on the sensed regeneration condition.

5. A system as claimed in any of claims 2 to 4 further including a sensor for sensing engine load directly or indirectly in which the controller selectively triggers one or both of the regeneration activators dependent upon sensed engine load.

6. An engine having an exhaust and a system as claimed in any preceding claim in which the exhaust is filtered by the particulate filter.

7. An engine as claimed in claim 6 comprising a diesel engine and in which the particulate filter is a diesel particulate filter.

8. An engine as claimed in claim 7 further comprising an electrical machine in hybrid or mild-hybrid configuration arranged to power a regeneration activator.

9. An engine as claimed in any of claims 5 to 8 when dependent on claim 4 in which the regeneration condition sensor comprises at least one of an exhaust temperature sensor and an engine mass flow rate sensor.

10. An engine as claimed in any of claims 5 to 9 when dependent on any of claims 2 to 4 in which the controller triggers regeneration based on a predicted particulate load derived from sensed engine conditions.

11. A vehicle including a system or engine as claimed in any preceding claim.

12. A particulate filter regeneration control system comprising a particulate filter, at least one regeneration activator, a particulate load sensor and a controller arranged to trigger the regeneration activator when the particulate load in the filter falls in the upper part of the range 8-10g/l, more preferably approximately lOg/1.

13. A particulate filter regeneration control system comprising a particulate filter, first and second regeneration activators, a regeneration condition sensor and a controller in which the controller selectively triggers one or both of the activators dependent on the sensed regeneration condition.

14. A method of regenerating a particulate filter comprising the steps of sensing a particulate load and triggering at least one regeneration activator when the sensed load exceeds a threshold in the upper part of the range 8-10g/l.

15. A method of regenerating a particulate filter have first and second regeneration activators in which one or both activators are selectively triggered dependent on a sensed regeneration condition.

16. A system, engine, vehicle and method substantially as herein described with reference to Fig. 3.