GB2348243A

GB2348243A - Motor vehicle exhaust system

Info

Publication number: GB2348243A
Application number: GB9906616A
Authority: GB
Inventors: Thomas Tsoi Hei Ma; Jonathan Austin Ma
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 1999-03-23
Filing date: 1999-03-23
Publication date: 2000-09-27
Also published as: GB9906616D0

Abstract

A motor vehicle 30 driven by an internal combustion engine has an exhaust system and a warm air duct 32. The discharge section 14 of the exhaust system is positioned such that the exhaust gases are discharged into ambient air emerging from the warm air duct 32. The warm air duct 32 is connected to the engine compartment 36 of the vehicle to collect ambient air that has passed through the engine compartment 36 and has been warmed by the hot components, such as the radiator, engine, exhaust manifold, downpipe, catalytic converter etc, in the engine compartment 36. The exhaust system and the warm air duct 36 serve to reduce the formation of nano-particles by reducing the saturation ratio of the mixture at the start of the dilution process.

Description

MOTOR VEHICLE EXHAUST SYSTEM Field of the invention The present invention relates to a motor vehicle driven by an internal combustion engine having an 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 size 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 gases are diluted and cooled in ambient air.

Particles may be formed by nucleation or may grow by condensation wheneyerthe saturation ratio, which is the ratio of the partial pressure of a condensable material to its vapour pressure, exceeds unity. The saturation ratio depends on the concentrations of the condensable material in the exhaust gases, the exhaust gas temperature, the dilution air temperature and composition, and the air/exhaust dilution ratio. Because of the competition between partial pressure and temperature effects, for volatile materials like unburnt and partially burnt fuel, lubricating oil and sulphur compounds, the maximum saturation ratio exceeding unity is achieved during dilution and cooling of the exhaust gases, occurring at dilution ratios between 5: 1 and 30: 1 and producing a large number of nano-particles. At dilution ratios above 100: 1, the saturation ratio drops well below unity and the gas-particle conversion process is stopped completely.

After the hot exhaust gases have been discharged into the ambient air, they are progressively diluted and cooled, and while going from very concentrated exhaust gases to a very diluted mixture, the mixture must at some stage go through all the intermediate dilution ratios and achieve the maximum saturation ratio at some time.

The residence time of the gases in the regions of high saturation ratio is also an important factor affecting the formation of nano-particles. In practice, it is found that a dilution ratio in the range of 5: 1 to 30: 1 and a residence time in excess of 200 ms at those ratios are the most favourable conditions for the volatile materials to be converted into nano-particles. On the other hand, conversion into nano-particles is suppressed if the residence time at those ratios is kept as short as possible, certainly no longer than 100 ms.

Although the nano-particles formed will eventually grow by coagulation to larger particles of sizes up to 1000 nm, the number of particles in 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 vehicle is formed with a warm air duct shaped and positioned to collect and conduct ambient air that has passed through the engine compartment of the vehicle and has been warmed by the hot components in the engine compartment, and wherein the discharge section of the exhaust system is positioned to discharge the exhaust gases into the warmed ambient air emerging from the warm air duct, whereby the formation of nano-particles is reduced by lowering the saturation ratio of the mixture at the start of the dilution of the-exhaust gases in ambient air.

The present invention is predicated upon the realisation that by ensuring that the ambient air is warm so that its relative humidity is reduced and its capacity to hold vapour is increased, the saturation ratio of the volatile materials contained in the exhaust gases when they are mixed with the air will be reduced.

As the dilution of the exhaust gases in ambient air increases progressively from low to high dilution, the saturation ratio first increases to a maximum value and then decreases to a very low value and there could be a time period during this process in which the saturation ratio may exceed unity resulting in formation of nano-particles. This crucial time period will be significantly shortened in the present invention by ensuring the initial saturation ratio at the beginning of the dilution process is lower so that the margin before the saturation ratio reaches unity is extended and the residence time during which the saturation ratio is at or above unity is reduced.

The invention is particularly relevant for application in an exhaust system for a diesel engine or a stratified charge lean burn gasoline engine because the temperature of the exhaust gases coming out directly from such engine is typically low and the corresponding saturation pressure of the volatile materials contained in the exhaust gases will also be low, resulting in a high saturation ratio with less margin as it approaches unity. The warm ambient air of the invention will be beneficial in extending the margin before the saturation ratio reaches unity.

The amount of heat necessary to increase the temperature of the dilution air is substantial when considering the air mass flow is large and could be tens of times of the exhaust mass flow. On the other hand, a temperature rise of only a few degrees for the dilution air is sufficient to increase substantially its capacity to. hold vapour. A good source of low quality heat of ample quantity could be found by passing the dilution air through the engine compartment of the vehicle where it is warmed by the hot components, such as the radiator, engine block, exhaust manifold, downpipe, catalytic converter etc, in the engine compartment. This makes the invention an effective system for reducing the formation of nano-particles.

By extending the margin before the saturation ratio reaches unity and reducing the residence time during which the saturation ratio is at or above unity, the invention is effective in reducing nano-particles in two ways. If the residence time during which the saturation ratio is at or above unity is made shorter than 100 ms, nucleation of the volatile materials will be suppressed and few nano-particles will be produced. If the residence time is made shorter but exceeds 100 ms, though nano-particles are formed by nucleation, their growth in size by condensation will be reduced. Thus the overall presence of aerosols introduced into the ambient atmosphere by the exhaust gases is 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 the single figure shows a schematic view of a motor vehicle with an exhaust system and a warm air duct of the invention.

Detailed description of the preferred embodiment In the single figure, there is shown a motor vehicle 30 with a warm air duct 32 formed by a panel 34 mounted under the vehicle enclosing the engine compartment 36 and leading from the engine compartment 36 along the floor 38 of the vehicle 30 towards the rear end of the vehicle. Ambient air is drawn into the engine compartment 36 where it is heated by the hot components, such as the radiator, engine, exhaust manifold, downpipe, catalytic converter etc, in the engine compartment. A large flow of air will pass through the engine compartment 36 and will be warmed to a temperature at least several degrees centigrade higher than the ambient air temperature with a corresponding increase in its relative humidity and its capacity to hold vapour. This warm air then flows along the warm air duct 32 towards the rear end of the vehicle where it emerges near the discharge pipe 14 of the engine exhaust system. Exhaust gases discharging from the discharge pipe 14 will be immediately diluted with a large flow of warm air from the duct 32 and the saturation ratio of the mixture will be lower because of the presence of the warm air, thus extending the margin before it reaches unity and reducing the residence time during which it is at or above unity.

Claims

1. A motor vehicle driven by an internal combustion engine having an exhaust system, wherein the vehicle is formed with a warm air duct shaped and positioned to collect and conduct ambient air that has passed through the engine compartment of the vehicle and has been warmed by the hot components in the engine compartment, and wherein the discharge section of the exhaust system is positioned to discharge the exhaust gases into the warmed ambient air emerging from the warm air duct, whereby the formation of nano-particles is reduced by lowering the saturation ratio of the mixture at the start of the dilution of the exhaust gases in ambient air.

2. A motor vehicle as claimed in claim 1, wherein the warm air duct is formed by a panel mounted on the underside of the vehicle beneath the engine compartment and defining with the floor pan of the vehicle a duct leading to the rear end of the vehicle.