GB2344377A

GB2344377A - Motor vehicle exhaust system

Info

Publication number: GB2344377A
Application number: GB9826581A
Authority: GB
Inventors: Thomas Tsoi Hei Ma; Jonathan Austin Ma
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 1998-12-04
Filing date: 1998-12-04
Publication date: 2000-06-07
Also published as: GB9826581D0

Abstract

A motor vehicle 30 is described that is driven by an internal combustion engine and has an exhaust system 10. The discharge section 20 of the exhaust system 10 from which the engine exhaust gases are discharged into the ambient atmosphere is elongate, extends transversely to the direction of motion of the vehicle and comprises a plurality of discharge regions 24 distributed over the length of the discharge section 20. The size and spacing of the discharge regions 24 and their positions in relation to the wake region 32 of the vehicle 30 are such that, when the vehicle is in motion, the exhaust gases from the engine are dispersed to a sufficiently high dilution ratio of ambient air to exhaust gases within a sufficiently short period of time to suppress the formation of nano-particles.

Description

MOTOR VEHICLE EXHAUST SYSTEM Field of the invention The present invention relates to a motor vehicle having an engine exhaust system designed to minimise the formation of particulate matter after the exhaust gases are discharged into the ambient atmosphere.

Background of the invention It is found that particulate matter having particle sizes in the nanometre range, hereinafter termed nanoparticles, are formed in diesel and petrol engines, not inside the engine or inside the exhaust system, but outside the exhaust system when the exhaust plume is diluted in ambient air. After the hot exhaust gases have been discharged into the ambient air, they are diluted progressively and cooled, and in going from very concentrated exhaust gases to a very diluted mixture, the mixture must at some stage go through all the intermediate dilution ratios. The extent of formation of nano-particles is highly dependent upon the dilution ratio. The residence time of the gases in the regions of different dilution ratios is also an important factor affecting the formation of nano-particles. In practice, it is found that an air/exhaust dilution ratio in the range of 5: 1 to 50: 1, particularly at 20: 1, and a residence time in excess of 200 ms at those ratios, are the most favourable conditions for the volatile carbonaceous materials contained in the exhaust gases to be converted by condensation into nanoparticles. On the other hand, conversion into nanoparticles is suppressed if the residence time at those ratios is kept as short as possible, certainly no longer than 100 ms. Moreover the gas-particle conversion process is stopped completely when the dilution ratio exceeds 100: 1.

The air/exhaust dilution ratio may be measured by comparing the concentration of carbon dioxide in the diluted gases at a distance from the exhaust pipe with the concentration of carbon dioxide in the exhaust gases at the exit of the exhaust pipe after correcting for the background concentration of carbon dioxide in the ambient air.

Alternatively, other constituents, such as NOX, present in the exhaust gases may be used for the same purpose.

By following a moving vehicle having a conventional exhaust system and measuring the dispersion of the exhaust gases at different distances behind the vehicle, it is found that the exhaust plume is diluted substantially linearly with distance behind the vehicle, being diluted to a ratio of 50: 1 at approximately 5 metres, 100: 1 at 10 metres and 1000: 1 at 100 metres behind the vehicle. Thus the residence time during which the diluted gases have a dilution ratio below 50: 1 will be approximately the time taken by the vehicle to travel 5 metres, which is 360 ms at an urban speed of 50 kph. As explained above, such a long residence time at the low dilution ratios is most favourable for gasparticle conversion to form nano-particles.

Although the nano-particles formed will eventually grow during atmospheric ageing to larger particles of sizes up to 1 micron, particulate matter of this entire size range, irrespective of its chemical composition, has become the cause of concern of late because the particles are small enough to penetrate and deposit deep inside the lungs of humans and animals. Depending on air movement, the hazard can persist for minutes or even hours and will tend to be more noticeable in the vicinity of roadways and congested traffic areas.

Summary of the invention According to the present invention, there is provided a motor vehicle driven by an internal combustion engine having an exhaust system, wherein the discharge section of the exhaust system from which the engine exhaust gases are discharged into the ambient atmosphere is elongate, extends transversely to the direction of motion of the vehicle and comprises a plurality of discharge regions distributed over the length of the discharge section, the size and spacing of the discharge regions and their positions in relation to the wake region of the vehicle being such that, when the vehicle is in motion, the exhaust gases from the engine are dispersed to a sufficiently high dilution ratio of ambient air to exhaust gases within a sufficiently short period of time to suppress the formation of nano-particles.

In order to suppress the formation of nano-particles, the size and spacing of the discharge regions and their positions in relation to the wake region of the vehicle should be such that the exhaust gases from the engine are diluted with ambient air to a dilution ratio of at least 20: 1 in less than 40 ms and further diluted to a ratio of at least 100: 1 in less than 150 ms of leaving the discharge section of the exhaust system.

Because of recirculating flows within the wake region of a moving vehicle, exhaust gases introduced inside the wake region will be diluted less quickly with ambient air and will remain at lower dilution ratios for relatively longer residence times than exhaust gases introduced outside the wake region and directly into the slipstream of the vehicle. It is therefore important to position the discharge regions of the exhaust system in relation to the wake region of the vehicle such that as small a proportion of the exhaust gases as possible enters into the wake region.

Preferably, for a vehicle travelling at a steady speed of 50 kph on a level road powered solely by the said engine, a substantial proportion of the exhaust gases is introduced outside the wake region of the vehicle such that the exhaust gases are diluted with ambient air to a dilution ratio of at least 20: 1 within 0.5 metre of leaving the exhaust system and further diluted to at least 100: 1 within 2 metres of leaving the exhaust system.

In a co-pending British Patent Application 9822484.3 aimed at increasing the dilution of the exhaust gases in order to improve the cabin air quality drawn in by a following vehicle, there is described a motor vehicle driven by an internal combustion engine having an exhaust system, wherein the discharge section of the exhaust system from which the engine exhaust gases are discharged into the ambient atmosphere is elongate, extends transversely to the direction of motion of the vehicle and comprises a plurality of discharge regions distributed over the length of the discharge section, the size and spacing of the discharge regions being such that, when the vehicle is travelling at a steady speed of 50 kph on a level road powered solely by the engine, the exhaust gases are diluted with ambient air to an air/exhaust dilution ratio of at least 50: 1 within 1 metre of leaving the discharge section.

In the above disclosure, the aim is to improve the air quality drawn into the cabin of a following vehicle and this is done by rapidly increasing the dilution of the exhaust gases of the leading vehicle as a function of the distance behind the vehicle. This earlier patent application takes no account of the residence time of the exhaust gases at different dilution ratios as its only concern is to achieve a certain level of dilution before the air enters the cabin of the following vehicle. As recirculation of the exhaust gases in the wake region of the vehicle could significantly prolong the residence time, the conditions set out in the earlier application are necessary but not sufficient to guarantee suppression of the formation of nano-particles.

More specifically, if the recirculation of the exhaust gases into the wake region of the vehicle is minimised, the dilution ratio should increase as a substantially linear function of distance from the vehicle. At 50 kph, if all the exhaust gases are introduced into the slipstream of the vehicle, the dilution ratio should increase from 25: 1 at 0.5 metre, to 50: 1 at 1 metre, to 100: 1 at 2 metre behind the vehicle. On the other hand, if a large proportion of the exhaust gases is introduced into the wake region of the vehicle, the dilution ratio will remain low for some distance close behind the vehicle before taking off linearly further away from the vehicle. In the latter case, the dilution ratio would stay at significantly below 25: 1 at 0.5 metre and dwell for a much longer residence time than 40 ms even though it might subsequently approach 50: 1 at 1 metre and increase linearly to 100: 1 at 2 metre behind the vehicle. Thus to suppress the formation of nano-particles it is important not only to ensure that a dilution ratio of 100: 1 is reached within 2 metre of the leading vehicle but that a dilution ratio of at least 20: 1 is reached within 0.5 metre of the leading vehicle.

The discharge regions may either be discrete separate discharge orifices emitting separate mini-plumes of exhaust gases or they may be joined to form a long slit extending over the length of the discharge section. In either case, the exhaust gases are dispersed over a large area, being rapidly surrounded with an ample supply of air across a large surface in relation to the exhaust gas volume, thus mixing with a substantial proportion of the vehicle air slipstream to be diluted rapidly into the ambient atmosphere within a short period of time and within a short distance of leaving the discharge section sufficiently to suppress the formation of nano-particles. By contrast, exhaust gases are conventionally discharged as a concentrated plume that penetrates much further into the atmosphere and takes much longer time to disperse, thus prolonging the conditions most conducive to the formation of nano-particles.

Preferably, the end of the discharge section of the exhaust pipe is closed and the discharge regions are provided in a lateral wall of the exhaust pipe transverse to the direction of motion of the vehicle.

Preferably, the discharge regions are positioned to direct the exhaust gases away from the wake region of the vehicle in order to avoid recirculation of the exhaust gases within the wake region.

Alternatively, the discharge regions may be positioned upstream of the wake of the vehicle such that the exhaust gases are diluted rapidly beyond the nano-particles formation domain before entering the wake region of the vehicle.

The minimum contemplated dilution ratio in the present invention is significantly higher than the dilution ratio proposed in known systems in which the exhaust gases are intentionally diluted with air for the purpose of reducing the gas temperature.

An exhaust gas diffuser for reducing the exhaust gas temperature by air induction into the diffuser has been proposed in US-A-4,198,817 in which the ratio of the reduced temperature to the inlet temperature to the diffuser is of the order of 0.6 as the exhaust gases pass through the diffuser. From the exit end of the diffuser, the air/exhaust mixture is discharged as a plume into the ambient atmosphere and is further cooled by atmospheric dilution to an overall temperature reduction ratio of 0.3 at 0.3 metre downstream of the diffuser. In this case, the air/exhaust dilution ratio is approximately 3.5: 1 at 0.3 metre or 12: 1 at 1 metre based on linear extrapolation. These dilution ratios are significantly lower than the dilution ratios contemplated in the present invention and would not be effective in suppressing the formation of nano-particles.

An exhaust discharge manifold for a gas turbine has also been proposed in US-A-5, 699,965 for use in a helicopter where the exhaust gases are forced to discharge laterally as a flat ribbon from the side of an elongate exhaust duct extending along the length of the helicopter. In this proposal, the ribbon of exhaust gases is mixed with air from the down wash of the helicopter rotor blades to a sufficiently low temperature to suppress infrared radiation in order to avoid detection. There has been no reference to the dilution ratio or residence time affecting the formation of nano-particles. This proposal does not therefore provide any teaching for the design of a motor vehicle exhaust system for minimising the formation of nano-particles.

In the present invention, by suppressing the formation of nano-particles, the agglomeration of a wide spectrum of larger particles of sizes up to 1 micron (that would have been formed from the nano-particles through atmospheric ageing) is also suppressed. Thus, a whole range of particulate pollutants associated with atmospheric dilution and ageing of exhaust gases may be reduced.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of an exhaust system of a vehicle of the invention, Figure 2 is a view of a vehicle showing the positioning of the exhaust system in relation to the wake of the vehicle, and Figure 3 is a logarithmic plot of dilution ratio versus distance from the vehicle.

Detailed description of the preferred embodiments In Figure 1 an engine exhaust system 10 has a silencer 12 and a tailpipe 14. The discharge section 20 at the end of the tailpipe 14 is elongate and extends transversely to the direction of motion of the vehicle and to the slipstream as represented by the long arrows. The end of the discharge section 20 is blanked off by means of a plug 22 and the exhaust gases are discharged from openings 24 that are distributed over the length of the discharge section 20. It is possible as an alternative to form the discharge section 20 with a single elongate discharge slit. In either case, instead of the exhaust gases being discharged as a concentrated plume, they are more evenly distributed over the width of the vehicle to mix with the slipstream and to be diluted to a high air/exhaust dilution ratio in as short a time as possible.

The invention recognises that the suppression of the formation of nano-particles requires the exhaust gases not to be resident in a region of high concentration for any length of time. Such a region can form in the wake 32 of a vehicle 30 as shown schematically in Figure 2 because of the gas recirculation in that region and it is therefore important to direct the discharge of exhaust gases away from the wake as shown by the small arrow.

In Figure 3 the line B represents the manner in which the dilution ratio varies with the distance behind the motor vehicle with a conventional exhaust system. The graph is plotted on a logarithmic scale because of the wide range that needs to be represented. In such a graph any linear variation of dilution ratio with distance will be represented by a straight line at 45 to the axes but the intercept on the y-axis will increase as the proportionality constant increases. Hence the line A representing the faster rate of dilution of the exhaust gases in a motor vehicle of the invention also lies at 45 to the axes, but is located entirely above the line B. At any distance behind the vehicle the dilution ratio on the line A is some six times as great as the dilution on the line B.

The line A is shown as represented by three sections Al, A2 and A3. If the exhaust gases do not enter the wake of the vehicle, then the dilution will follow the lines A1-A3.

On the other hand, if a large proportion of the gases find their way into the wake region, they will reduce the dilution ratio immediately behind the vehicle to the level represented by the line A2 before eventually merging with the slipstream and continuing along the line A3.

The effect of gases entering the wake region of the vehicle is to prolong the residence time at a low dilution thereby favouring the formation of nano-particles. It is therefore preferred to take steps to avoid following the line A2-A3 and instead to follow the lines A1-A3 as closely as possible. Hence it is preferred that the dilution ratio should not only be 100: 1 at 2 metres from the vehicle, but also that it should be at least 20: 1 at 0.5 metre from the vehicle, these being the value represented by the dotted lines in Figure 3. The second of these conditions can only be fulfilled if exhaust gases are prevented from dwelling in the wake region of the vehicle, or if exhaust gases are already diluted beyond the nano-particles formation domain before entering the wake region.

Claims

1. A motor vehicle driven by an internal combustion engine having an exhaust system, wherein the discharge section of the exhaust system from which the engine exhaust gases are discharged into the ambient atmosphere is elongate, extends transversely to the direction of motion of the vehicle and comprises a plurality of discharge regions distributed over the length of the discharge section, the size and spacing of the discharge regions and their positions in relation to the wake region of the vehicle being such that, when the vehicle is in motion, the exhaust gases from the engine are dispersed to a sufficiently high dilution ratio of ambient air to exhaust gases within a sufficiently short period of time to suppress the formation of nano-particles.

2. A motor vehicle as claimed in claim 1, wherein in order to suppress the formation of nano-particles, the size and spacing of the discharge regions and their positions in relation to the wake region of the vehicle are such that, when the vehicle is in motion, the exhaust gases from the engine are diluted with ambient air to a dilution ratio of at least 20: 1 in less than 40 ms and further diluted to a dilution ratio of at least 100: 1 in less than 150 ms of leaving the discharge section of the exhaust system.

3. A motor vehicle as claimed in claim 1 or claim 2, wherein the discharge regions of the exhaust system are positioned in relation to the wake region of the vehicle in such a manner that as small a proportion of the exhaust gases as possible enters into the wake region.

4. A motor vehicle as claimed in claim 1 or claim 2, wherein the discharge regions are positioned upstream of the wake of the vehicle such that the exhaust gases are diluted rapidly beyond the nano-particles formation domain before entering the wake region of the vehicle.

5. A motor vehicle as claimed in claim 3 or claim 4, wherein when the vehicle is travelling at a steady speed of 50 kph on a level road powered solely by the said engine, the exhaust gases are diluted with ambient air to a dilution ratio of at least 20: 1 within 0.5 metre of leaving the exhaust system and further diluted to a dilution ratio of at least 100: 1 within 2 metres of leaving the exhaust system.

6. A motor vehicle as claimed in any preceding claim, wherein the discharge regions are discrete separate discharge orifices emitting separate mini-plumes of exhaust gases.

7. A motor vehicle as claimed in any one of claims 1 to 5, wherein the discharge regions are contiguous and form a slit extending over the length of the discharge section.

8. A motor vehicle as claimed in any preceding claim, wherein the end of the discharge section of the exhaust pipe is closed and the discharge regions are provided in a lateral wall of the exhaust pipe transverse to the direction of motion of the vehicle.

9. A motor vehicle driven by an internal combustion engine having an exhaust system, wherein the discharge section of the exhaust system from which the engine exhaust gases are discharged into the ambient atmosphere is elongate, extends transversely to the direction of motion of the vehicle and comprises a plurality of discharge regions distributed over the length of the discharge section, the vehicle being substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 2 of the accompanying drawings.