GB2252363A

GB2252363A - Smoothing pulsating gas flow

Info

Publication number: GB2252363A
Application number: GB9200234A
Authority: GB
Inventors: Richard Barfoot
Original assignee: GUNSON Ltd
Current assignee: GUNSON Ltd
Priority date: 1991-01-08
Filing date: 1992-01-07
Publication date: 1992-08-05
Anticipated expiration: 2012-01-07
Also published as: GB9100327D0; GB9200234D0; GB2252363B

Abstract

Apparatus for providing a positive gas flow from a pulsating flow, such as that in a vehicle exhaust gas system, comprising a body (1, 2) which defines a chamber and a flexible diaphragm (3) extending across the chamber, an inlet and an outlet port (5, 6) providing communication with the chamber on one side of the diaphragm (3), the other side of the diaphragm being open to atmospheric pressure (9), the inlet port (5) being positioned adjacent a central portion of the diaphragm (3) so that when there is a positive gas flow through the inlet port (5) into the chamber gas can flow into the chamber and out of the outlet port (6) and the diaphragm (3) distends away from the inlet port (5), when the pressure in the inlet port (5) is no longer positive, the diaphragm (3) moves back to its rest position and in doing so expels gas from the chamber through the outlet port (6), and when the gas flow in the inlet port (5) is negative the diaphragm (3) distends towards and occludes the inlet port (5) to prevent negative gas flow through the apparatus. A second outlet port (8) may also be provided to allow any water collecting in the bottom of the chamber to be expelled. <IMAGE>

Description

VEHICLE EXHAUST GAS PUMP This invention relates to apparatus for providing a substantially positive gas flow from a pulsating flow, such as that in a vehicle exhaust gas system, which may be of negligible or very low pressure.

Exhaust gas analysis equipment is conventionally provided with a powered pump in order to achieve a gas flow to the sensing device within the analyser. A positive and consistent gas flow is required in order to obtain an accurate measurement as well as a reasonable response time (e.g. less than 15 seconds). In order to maintain the analyser sensing device in a clean and dry condition, a second pump, or alternatively a second pump head, is often provided to draw moisture from the system before feeding gas to the sensing device.

Pulsation in the exhaust will normally occur due to the opening and closing of valves, as found on an internal combustion engine for example. Although the mean flow in such an exhaust system is in one direction, there are pulsations and actual reverses to the direction of this flow particularly at low engine rotational speeds. Various arrangements (without a powered pump) of pipes, chambers and ports have been used in an attempt to obtain a consistent positive gas flow from an engine exhaust but without success.

On a particular engine exhuast at a particular engine speed, positive gas flow will not be obtained and in extreme circumstances a reversal in direction will occur leading to air being drawn into the gas analyser.

According to the invention there is provided apparatus for providing a substantially positive gas flow from a pulsating flow, such as that in a vehicle exhaust gas system, the apparatus comprising a body which defines a pump chamber and a flexible diaphragm extending across the chamber, an inlet and an outlet port providing communication with the chamber on one side of the diaphragm and the other side of the diaphragm being open to atmospheric pressure, the inlet port being positioned adjacent a central portion of the diaphragm so that when there is a positive gas flow through the inlet port into the chamber gas can flow into the chamber and out of the outlet port and the diaphragm distends away from the inlet port, when the pressure in the inlet port is no longer positive, the diaphragm moves back to its rest position and in doing so expels gas from the chamber through the outlet port, and when the gas flow in the inlet port is negative the diaphragm distends towards and occludes the inlet port to prevent negative gas flow through the apparatus.

There is thus provided a diaphragm operated pump which requires no external power and acts only on the pulsation for instance in an exhaust system.

Preferred features of the invention will be apparent from the following description and from the subsidiary claims of the specification.

The invention will now be further described, merely by way of exampie, with reference to the accompanying drawing Figure 1 which is a cross section of one embodiment of the invention.

The apparatus comprises a pump body 1 and a pump cover 2 which support a thin flexible diaphgram 3. The pump body 1 is provided with an inlet pipe 4, which terminates at a sealing annulus or seat 5 within the body 1.

The diaphragm 3 is supported so that in its static position it lies just clear of the seat 5. The pump inlet pipe 4 is disposed adjacent the centre of the diaphragm and in use, would be connected to a flexible hose and probe (not shown) loosely inserted into the open end of a vehicle exhaust pipe. The pump body 1 has two outlets 6 and 7. Outlet 7 is arranged at the lowest point in the pump body 1 as it is provided for the purpose of draining surplus water from the system. Outlet 6 is arranged at the highest point in the pump body 1 in order to collect the exhaust gas in a reasonably moisture free condition, droplets of water having already fallen towards the lower parts of the pump.

In operation, the fluctuations in pressure in the vehicle exhaust are passed through the flexible hose (not shown) to the pump gas inlet 4. A positive pressure causes gas to flow into the pump and out through the outlets 6 and 7 and the flexible diaphragm 3 is caused to distend away from the seat 5. When the fluctuating pressure is no longer positive, the diaphragm 3 returns to its rest position as shown, and in doing so continues to expel gas through the outlets 6 and 7.

When the fluctuating pressure in the gas inlet 4 is negative, the diaphragm is drawn onto the seat 5 as the other side of the diaphragm 3 is open to atmospheric pressure through holes 9 in the pump cover 2. This ensures that the flow direction in the pump does not reverse.

In operation, the diaphragm position fluctuates rapidly and its flexibility and low inertia allow expansion of the pump cavity unaer the action of pressure of the exhaust gas and this has the effect of smoothing out peaks in the flow giving a more consistent flow to the gas analyser (not shown) from outlet 6. The large area of the diaphragm 3 relative to the inlet 4 and seat 5, ensures that capiliary action does not draw any moisture in the system into the gap between the diaphragm 3 and the seat 5 which mignt stop the pump from operating.

Water which collects in the bottom of the pump is expelled through an aperture 8 which leads to outlet 7. The size of this aperture and the size of the pipe (not shown) fitted to outlet 7 are carefully chosen to ensure that any minor flow fluctuations do not draw ambient air into the pump.

The pump 1 and cover 2 are most conveniently made from plastics mouldings.

Typical dimensions might be as follows: pump body diameter 50 mm, inlet 4 and seat 5 diameter 5 mm, outlet 6 diameter 2 mm, aperture 8 diameter i.5 mm, clearance between seat 5 and diaphragm 3 in rest position about 0.5 mm and movement of centre of diaphragm from rest position about 1.0 to 1.5 mm.

The diaphragm 3 is preferably manufactured from a relatively rigid or stiff elastic material which springs back to the rest position shown when not under the influence of any pressure. A thin-flat polycarbonate film having a thickness of approximately 0.10 mm to 0.015 mm has been found to be suitable.

The size of the cavity in the pump body 1, the sizes of the inlet 4 and outlets 6 and 7, the behaviour of the diaphragm 3 and the lengths of the inlet and outlet pipes (not shown) attached to inlet 4 and outlet 6 influence the efficiency of the pump and are preferably chosen so that the system resonates at the frequency of operation of an internal combustion engine exhaust at typical idle speeds (eg about 5 - 10 cycies/sec). It is particularly important to obtain effective operation at engine idle speeds as this is the condition most widely used for exhuast emission testing. At higher engine speeds it is not difficult to obtain a positive gas sample flow so the pump efficiency can be lower.

The pump described above has an extremely low manufactured cost and this is an important consideration as the conventional electrical pump system can form a large proportion of the total analyser cost.

Tests have shown that the apparatus described above provides a sufficiently consistent flow to an exhaust gas analyser to enable an analysis to be produced with around 10 seconds which is well within the normal 15 second requirement.

Claims

1. Apparatus for providing a substantially positive gas flow from a pulsating flow, such as that in a vehicle exhaust gas system, the apparatus comprising a body which defines a pump chamber and a flexible diaphragm extending across the chamber, an inlet and an outlet port providing communication with the chamber on one side of the diaphragm and the other side of the diaphragm being open to atmospheric pressure, the inlet port being positioned adjacent a central portion of the diaphragm so that when there is a positive gas flow through the inlet port into the chamber gas can flow into the chamber and out of the outlet port and the diaphragm distends away from the inlet port, when the pressure in the inlet port is no longer positive, the diaphragm moves back to its rest position and in doing so expeis gas from the chamber through the outlet port, and when the gas flow in the inlet port is negative the diaphragm distends towards and occludes the inlet port to prevent negative gas flow through the apparatus.

2. Apparatus as claimed in claim 1 in which the body comprises a body member and a cover member which are secured together with the periphery of the diaphragm held therebetween.

3. Apparatus as claimed in claim 1 or 2 in which the inlet port is provided at the end of an inlet pipe about 5 mm in diameter and is positioned atound 0.5 mm from th diaphragm.

4. Apparatus as claimed in any preceding claim having a second outlet port in communication with the chamber which positioned on the opposite side of the apparatus to the first outlet port so that, in use, the apparatus can be positioned with the second outlet port at the lowest point of the body so any water in the chamber drains towards the second outlet port and the first outlat port is at the highest point of the body so as to receive gas in a substantially moisture free condition.

5. Apparatus as claimed in claim 4 in which the second oulet port is around 1.5 mm in diameter so that, in use, water collecting in the chamber adjacent thereto is able to fill the second outlet port whereby is is expelled through the second outlet port by positive pressure within the chamber.

6. Apparatus for providing a substantially positive gas flow from a pulsating flow substantially as hereinbefore described with reference to the accompanying drawing.

7. An exhaust gas analyser provided with apparatus as claimed in any preceding claim.