In known systems, soot removal is usually achieved most effectively through the use of a filter. Regeneratable traps, such as CRTs (Continuously Regenerated Traps), work on the principle of retaining soot particles within a ceramic or silicone carbide filter which collects the particles within porous walls of the honeycomb structure of the filter. The accumulation of this soot within the surface of the filter, increases the back pressure of the filter, which then requires the filter to be regenerated.

Regeneration is achieved when the exhaust temperature reaches above around 400°C at which point the component of the exhaust gas stream reacts with the soot creating an exothermic reaction which increases the trap temperature to over 500°C and the soot is oxidised and burnt away.

The temperature of the exhaust gas and filter are critical to the regeneration process which has led to many problems with this technology. Experience has shown that for certain engine duty cycles it is not possible to achieve an exhaust gas temperature which enables unassisted regeneration. Major problems occur when the soot trap cannot be located local to the engine due to space limitation, resulting in long exhaust pipe runs requiring insulation lagging to retain as much heat as possible in the gas. Other cases where the engine duty cycle never

exceeds the 350°C regeneration target temperature requires additional heating local to the filter to increase the approach temperature to enable regeneration.

Auxiliary heating has further drawbacks as it requires more complex links to the vehicle's onboard power system, which in some cases will not be sufficiently sized to cope with the additional load; this also adds expense and maintenance difficulties.

One known solution is set forth in"Particulate Trap Technology for Light Duty Vehicles with a New Regeneration Strategy"Zikoridse et al, SAE Technical Papers Series No. 2000-01-1924, in which exhaust gas flows through a heating module having a convection section followed by a radiation section before entering a particulate trap to raise the trap temperature.

Before a soot filtration system can be fitted it is necessary to understand the engine's duty cycle to model the temperature profile of the exhaust gas to gain assurance that it will in fact regenerate the filter. This adds further problems because, for example, in a bus application the bus may have the temperature trending compiled on a motorway route where it is found to reach the correct regeneration temperatures. However, it may subsequently be assigned to an inner city route where exhaust temperatures are not sufficient for regeneration.

The invention is set out in the claims.

According to one aspect of the invention there is provided an engine exhaust treatment system comprising an engine exhaust treatment element having a temperature dependent treatment regime insulated by an insulator comprising inner and outer walls and an evacuated space therebetween.

The invention further relates to an engine system and vehicle including such a treatment system, and a method of forming an insulator for an engine exhaust treatment system comprising the steps of providing inner and outer walls, packing the space therebetween with insulating powder, mechanically evacuating the space therebetween and then getter evacuating the space therebetween. Further preferred aspects of the invention are set out in the claims.

As a result a simple, robust and extremely efficient engine exhaust treatment system is provided. The type of insulation reduces the bulk of the system significantly and its energy efficiency ensures that little or no auxiliary heating is required. This in turn means that, at most, a highly simplified auxiliary heating system is available allowing a product which can be applied to an engine which has variable duty cycles during its operating life, having minimum interface with the OEM product and requiring little or no auxiliary power for regeneration.

Embodiments of the invention will now be described, by way of example, with reference to the drawings of which: Fig. 1 is a schematic diagram of an engine system; Fig. 2 is a simplified soot trap model; Fig. 3 is a simplified soot trap model according to the invention; Fig. 4 shows a soot trap according to the invention; Fig. 5 is a graph of temperature against time showing regeneration in a conventional system; and

Fig. 6 is a graph of temperature against time showing regeneration in a conventional system according to the invention.

Referring firstly to Figs. 1 and 2, a simplified model of a regeneratable soot trap 10 in conjunction with an engine 20 is shown. Exhaust gases 22 pass from the engine 20 into the soot trap 10 at a temperature Tin, mass flow rate min and specific heat capacity Cpin. The exhaust gas exiting from the soot trap is at temperature Tout, mass flow rate mOut and specific heat capacity Cpout. As a result the heat energy loss Q, expressed as Q= th Cp AT (1) can be calculated as Q = m Cp [Tin-Tout] (2) These losses in heat energy will result from radiated heat from the soot trap 10 or the pipe work heat losses within the exhaust gas piping to the soot trap filter 10. Using conventional insulation techniques the energy retained is in proportion to the insulation thermal conductivity which can be expressed as the ratio of power to length and temperature, mW. m°K Referring now to the simplified pipe model shown in Fig. 2, where the pipe has an outer wall radius ro, an inner wall radius ri, length L and thermal conductivity k, using Fourier's Law the heat loss at radius r is given by: Qr = -kArdT dr

Where the temperature at radius ri is Ti and the temperature at point ro is To, we obtain: Qr = 2 ltk L (Ti-To) (4) In ro ri Given that thermal resistance R is given by: R- (ro/ri) (5) ln22zkL As can be seen from equations 3 to 5 above, therefore, the greater the thermal resistance, the less energy is lost from the system.

The invention achieves significantly improved heat retention, therefore, using the arrangement shown in Figs. 3 and 4. Referring firstly to the simplified model shown in Fig. 3, a soot trap filter 10 is surrounded by an inner wall 24 and an outer wall 26 spaced from the inner wall. The annular volume 28 between the inner and outer walls 24,26 is evacuated providing high thermal resistivity and hence high heat retention in the soot trap filter 10. Referring to the more detailed view shown in Fig. 4, the soot filter 10 and insulation 24,26, 28 are terminated by inlet and outlet connecting flanges 30,32. Exhaust gas enters at temperature Tl and exits at temperature T2. In the preferred embodiment the insulating space 28 is evacuated by including a barium lithium getter alloy within the space between walls 24,26 which, following a mechanical vacuum approach chemically absorbs a wide range of atmosphere gases such as nitrogen, CO, CO2, hydrogen and water. The soot trap 10 can be of any appropriate type, for example a soot filter, diesel particulate filter (DPF), catalytic converter such as in Engelhard DPX, or an E. T. B. soot filter (EWR).

Appropriate products are available, for example, from Eminox Limited of North Warren Road, Gainsborough, Lincolnshire, United Kingdom. The insulation can be, for example, of the type manufactured by SAES Getters S. p. A of Viale Italia, 77,20020 Lainate (Milano), Italy under the trademark COMBOGETTER. The inner and outer walls 24,26 of the insulation can, especially in low vacuum applications, for example in the region of 10-3 mbar, be separated by packing the intermediate space 28 with a Perlite (TM) insulation powder to limit the amount of getter material required and, for example, incorporating stays or spacers of any appropriate type, and of minimal thermal conductivity, or using metal slings between the walls. For higher vacuum in the region of 10-5 to 10-6 mbar the space will preferably be evacuated and clear of additional material, although more getter material will then be required. The getter material compensates for temperature related effects such as outgassing, where the walls release gas at high temperatures, by getter absorption.

Using an arrangement of this type the thermal conductivity can be reduced to as low as 0.0013 to 0.72mw per meter °C, compared with the insulation of standard media such as urethene foam blocks or fibreglass which have a thermal conductivity range from 20 to 100mw per meter °C. As a result, for some engine sizes or usage types, the invention will remove the need altogether for auxiliary heating as regeneration temperatures will be automatically achieved in view of the extremely low level of heat loss within the insulated volume.

However in some instances auxiliary heat may be required in which case an element such as the heating element 34 shown in Fig. 4 can be provided. The element can be, for example, a standard electrically powered heating element 34 or an arrangement such as that shown in the above referenced SAE technical

paper. The heating element is provided within the insulated volume to direct all heat energy at the filter target and again reduce the cost as far as possible.

Referring to Figs. 5 and 6 the improved regeneration regime can be clearly seen. In each case a regeneration cycle is shown over a period of approximately 100 seconds during which the filter temperature (C) rises to a peak during the exothermic reaction and then decreases as the soot is burnt off. An exothermic reaction is initiated where the surface temperature of the filter exceeds a threshold D at around 350°C. The engine exhaust gas temperature is slightly below this threshold as shown by trace E (which in practice would in fact show more noise). Because of heat losses, however, the surface temperature of the soot filter is significantly below the engine exhaust gas temperature as shown by trace F. As a result the energy required to initiate exothermic reaction (at which point auxiliary heating can be switched off) is a function of the area bounded by reaction curve C and filter temperature F as far as the point where the reaction temperature curve intercepts the threshold curve D. Because of the significant drop in temperature, the input energy required is significant. In fact, as can be shown, the energy can be represented by two areas, area A between the reaction curve C and the engine exhaust temperature D, and area B between the engine exhaust temperature D and the filter temperature F and is represented by: Qr = QA + QB (6) Fig. 6 shows the same set of curves but using the insulation according to the present invention. In this case, because there is minimal heat loss, the surface temperature curve F'is only negligibly lower than the engine exhaust gas

temperature C as a result of which QB is minimised. As a result less energy is required from the auxiliary heater to trigger the exothermic reaction.

Control of the auxiliary heater can be according to any appropriate scheme. For example by measuring temperature, both upstream and downstream of the soot filter, control of the auxiliary heater can be effected in any appropriate manner as will be known to the skilled person. In addition power redistribution schemes can be adopted. For example in a bus duty cycle, alternator load accommodates auxiliary equipment such as Air conditioning units, heaters, lights and windscreen wipers and when these are not in use, power is used to charge the battery. However this can also be used to activate a regeneration through an appropriate control system. The heater may also be insulated in the same manner as the soot trap filter.

Further embodiments of the invention are shown are Figures 7 and 8 in which pipe work upstream of the soot trap is insulated by means of an evacuated double outer wall including getter material. This prevents heat loss along the length of the exhaust and hence achieves similar benefits in maintaining the trap temperature at or close to the regeneration temperature.

Referring in more detail to Fig. 7 it will be seen that the exhaust run designated generally 40 includes an inner wall 42 comprising a cylindrical exhaust conduit and a concentric outer wall 44. The exhaust conduit runs between an engine manifold designated generally 46 and a soot trap or other exhaust treatment system designated generally 48. The space 50 between the inner and outer walls 42, 44 is evacuated to a pressure below 10-3mbar. The evacuated space 50 includes getter material 52. In the embodiment shown the geter material 52 is

in tablet form and is designated generally 52. The getter material 52 is held at or near the inner wall 42 by means of a perforated sleeve 54. The sleeve 54 is held in place at axially separated locations by respective wires 55 wrapped around the sleeve and holding it against the inner wall. This further ensures that getter remains evenly distributed and does not settle as it is held in place in the cylindrical pouch 57 formed between respective wires 55. For the purposes of clarity, only one such pouch is shown in the Figures and the getter is shown spaced from the inner wall whereas in practice it will preferably be held in engagement with it. Because of the insulating effect of the evacuated space 50 a significant temperature difference can build up between the inner and outer walls 42,44. Accordingly a flexible coupling in the form of a mechanical metallic expansion bellows 58 couples one end of the inner wall 42 to a mounting flange of the exhaust treatment system 48 allowing expansion/contraction of the wall relative to the outer wall 44. In addition the bellows compensate for vibration effects.

The inner wall 42, outer wall 44 and wire 55 are preferably formed of stainless steel more preferably electro-polished stainless steel, and assembled where appropriate using TIG welding. This minimises oxidisation and unclean surfaces which in turn reduces the risk of outgassing. The getter material can be any appropriate getter material and in any appropriate form although in the present embodiment tablet form is used. Preferably the getter material is selected so as to have a suitably low hydrogen equilibrium pressure that there is no hydrogen release at high temperature. One such suitable material is St 787 alloy available from SAES Getters S. p. A.

The sleeve 54 holding the getter material in place is preferably formed of aluminimum or other reflective foil. The sleeve 54 is perforated to allow free circulation of the remaining gas in the evacuated chamber 50 such that the absorption capability of the getter material is maximised. Its reflective nature provides increased thermal efficiency by reflection of radiated heat back towards the exhaust conduit. As a result of this arrangement the getter material 52 is maintained in close proximity with the inner wall 42. As this is the highest temperature region of the evacuated chamber 50 the performance of the getter material, which is temperature dependent, is maximised.

Preferably the system is manufactured by welding the inner and outer sleeves and bellows and evacuating them by mechanical pumping. The vacuum is sealed using an appropriate technique. Once such technique comprises applying the vacuum via a copper pipe 62 brazed to a stainless steel cynindrical stub 64 itself TIG fillet welded to the outer wall 44. Once the mechanical pumping is finished the copper pipe 62 is pinched and brazed to form a seal as can be seen in Figs 7 and 8. The arrangement is then"baked"with the foil and getter material in place. The baking process will be well known to the skilled person but in summary comprises repeatedly heating and cooling the assembly.

This reduces the level of outgassing in subsequent cycles. By using the materials discussed, ensuring that all surfaces are as clean as possible of moisture and hydrocarbons and baking the system, highly efficient vacuum pressures are available to levels as low as 10-4 to 10-5 mbar even at high temperatures. As a result thermal efficiency of the system is very high improving regeneration efficiency.

Fig. 8 shows a low vacuum application corresponding to that shown in Fig. 7 where the inner and outer walls are spaced by an insulating powder, 60 allowing vacuum pressure in the region of 0. lmbar It will be appreciated that various components and materials can be altered as appropriate without departing from the invention. In addition components, elements or features from different embodiments can be interchanged or juxtaposed as appropriate; for example the expandable bellows can be incorporated into either the inner or outer wall around the exhaust treatment system/conduit or both. The system could be any appropriate exhaust treatment system which requires high temperature to operate and hence retaining exhaust gas temperature. Insulation can be added to one or other or all of the treatment element and the exhaust piping leading from the engine to the treatment element, and the auxiliary heater. This is particularly advantageous because of reductions in bulk; as the insulation relies on a vacuum, the spacing between the inner and outer walls can be minimised and can be as little as 4mm in the best mode presently contemplated. Of course multiple treatment elements can be included in the system, one or more of them being appropriately insulated and/or the pipework between them similarly insulated. Elements such as the getter material and bellows can be located in any appropriate place in the assembly.

It will also be appreciated that the system can be used in conjunction with any appropriate type of engine for example a petrol (gasoline), diesel or LPG driven engine, where exhaust treatment is required.