GB2347970A - Motor vehicle exhaust system insulated downstream of catalytic converter - Google Patents

Abstract

A motor vehicle driven by an internal combustion engine has an exhaust system with at least one catalytic converter 8 disposed along the system. A layer of thermal insulation 16 is provided along a length of the exhaust system downstream of the catalytic converter 8. The insulated section includes an exhaust pipe 10, a muffler 12 and a discharge pipe 14 and the insulation is such that the temperature drop along the insulated section is less than 25{C per metre. This is sufficient to maintain as high as possible the temperature of the exhaust gases before they emerge from the exhaust system into the ambient atmosphere, so as to reduce the formation of nano-particles after the exhaust gases are discharged into the ambient atmosphere.

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 whenever the 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 incorporating at least one catalytic converter, wherein insulating means are provided for reducing heat loss from at least part of the exhaust system downstream of the catalytic converter to retain heat in the exhaust gases after leaving the catalytic converter and before emerging from the exhaust system into the ambient atmosphere, the resulting higher exhaust gas temperature serving to reduce the formation of nano-particles after the exhaust gases are discharged into the ambient atmosphere.

It has previously been proposed to provide a heat shield between a section of an exhaust system and a nearby component in order to protect the component from being overheated. Such a heat shield however is not effective in thermally insulating the exhaust system since air can still circulate around the exhaust system and cool the exhaust gases flowing inside.

Thermal insulation is sometimes used in the exhaust manifold and the downpipe upstream of a catalytic converter, for example by means of a double skin, to keep the exhaust gas temperature arriving at the catalytic converter as hot as possible especially during a cold start. It is however not desirable to insulate the catalytic converter because it internally generates heat during operation and there would be a risk of overheating of the catalytic materials inside the catalytic converter during high load operation.

Downstream of the catalytic converter, the exhaust system is not normally insulated simply because there has been no reason to do it until the present.

The invention is predicated upon the realisation that by ensuring that the exhaust gases are as hot as possible when they are discharged into the atmosphere and undergo dilution with cold ambient air, the saturation pressure of the volatile materials contained in the exhaust gases at the start of the dilution process would be higher, such that the initial saturation ratio would be lower and more favourable than would be the case had the exhaust system not been thermally insulated and the exhaust gases were at a lower temperature.

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 as low as possible.

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 will be even lower when the gases are cooled along the exhaust system before eventually emerging into the ambient atmosphere. For example, it is found that in a passenger vehicle driven by a diesel engine in urban traffic conditions, without the thermal insulation of the invention, the temperature drop along the exhaust pipe is typically 100 C per metre and the exhaust gas temperature emerging from the discharge pipe is frequently less than 120 C. This temperature is below the saturation temperature of some of the volatile materials contained in the exhaust gases and nano-particles will be produced immediately.

With sufficient thermal insulation, on the other hand, the temperature drop between the exit end of the catalytic converter and the exit end of the exhaust discharge pipe may be kept as small as possible so that the temperature of the exhaust gases emerging from the discharge pipe will be as high as possible. Preferably the temperature drop along the insulated section should be less than 25 C per metre and the exhaust gas temperature emerging from the discharge pipe should be above 200 C.

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 perspective view of a part of an engine exhaust system of the invention for a motor vehicle.

Detailed description of the preferred embodiment In the single figure, there is shown a part of an engine exhaust system for a motor vehicle having a catalytic converter 8 leading to an exhaust pipe 10 having a silencer or muffler 12 followed by a discharge pipe 14.

Conventionally no thermal insulation is provided along this length of the exhaust system. Exhaust gases flowing along this length will be progressively cooled as the exhaust system is exposed to the air slipstream of the moving vehicle. The strong convective heat transfer of the air slipstream will result in a temperature drop of typically 100 C per metre along the length of the exhaust system.

By contrast, in the present invention, a layer of thermal insulation 16 is provided surrounding the exhaust pipe 10, the muffler 12 and the discharge pipe 14. The insulation 16 is such that the temperature drop between the exit end of the catalytic converter 8 and the exit end of the exhaust tailpipe 14 is as small as possible, preferably less than 25 C per metre, so that the overall temperature drop along the entire length the exhaust system downstream of the catalytic converter is less than 50 C. In this case, the temperature of the exhaust gases emerging from the discharge pipe 14 could reach 200 C more frequently even for a diesel engine vehicle driven in urban traffic conditions.

The thermal insulation may be formed of a blanket of insulating material surrounding the exhaust system or as an insulating jacket surrounding the exhaust system and trapping air between itself and the exhaust system.

Claims (2)

1. A motor vehicle driven by an internal combustion engine having an exhaust system incorporating at least one catalytic converter, wherein insulating means are provided for reducing heat loss from at least part of the exhaust system downstream of the catalytic converter to retain heat in the exhaust gases after leaving the catalytic converter and before emerging from the exhaust system into the ambient atmosphere, the resulting higher exhaust gas temperature serving to reduce the formation of nano-particles after the exhaust gases are discharged into the ambient atmosphere.

2. A motor vehicle as claimed in claim 1, wherein the insulating means is such that the temperature drop along the insulated section downstream of the catalytic converter is less than 25 C per metre.