GB2258168A

GB2258168A - Engine exhaust system

Info

Publication number: GB2258168A
Application number: GB9116581A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1991-08-01
Filing date: 1991-08-01
Publication date: 1993-02-03
Also published as: GB9116581D0; WO1993003263A1

Abstract

An exhaust system for an internal combustion engine 10 having an air pump 22 for pumping secondary air into the exhaust gases to permit secondary combustion or afterburning, comprises means 18 for regulating the air delivery rate by the air pump 22 in proportion and in synchronism with the exhaust flow rate in order to maintain a more constant exhaust/air ratio at the point of introduction of the secondary air into the exhaust stream. The air pump 22 of the preferred embodiment of the invention is a steady speed pump and flow regulation is effected by obstructing inlet flow area of the air pump by means of a rotating disc 18 driven by the engine. <IMAGE>

Description

Title Engine Exhaust System.

Field of the invention The present invention relates to the injection of secondary air into an engine exhaust pipe.

Background of the invention Injecting secondary air into the exhaust stream of an internal combustion engine under rich combustion conditions has been a common practice for oxidising the partially burnt combustion products. This oxidisation process can take place by secondary reaction purely from the relatively high temperature of the exhaust gases, or at a higher rate by catalytic action of an exhaust catalytic converter, or according to GB patent applications recently filed by the applicant (Nos. 9112601.2 & 9113949.3) by afterburning as a flame when the concentration of the partially burnt combustion products is sufficiently high for the exhaust/air mixture to be ignitable.

In all the above cases, it is desirable for the exhaust/air ratio to be accurately controlled within narrow limits to ensure rapid and complete reaction. A problem in realising this objective lies in the unsteady flow rate of the exhaust gases as each engine cylinder goes through it exhaust stroke. In practice, the exhaust flow pulsates, accelerates, slows down or even stops and reverses in between the exhaust strokes of consecutive cylinders. As secondary air is usually introduced at a steady rate by an air pump running at constant speed, the exhaust/air ratio is not constant and can vary widely during each exhaust stroke. Consequently, the mixture strength may be at its optimum setting at some instants but not at others.

Object of the invention The invention seeks to enable the exhaust/secondary air mixture strength to remain more constant and to reduce its deviation from its optimum setting.

Summarv of the invention The present invention provides an exhaust system for an internal combustion engine having an air pump for pumping secondary air into the exhaust gases to permit secondary combustion or afterburning, wherein means are provided for regulating the air delivery rate by the air pump in proportion and in synchronism with the exhaust flow rate in order to maintain a more constant exhaust/air ratio at the point of introduction of the secondary air into the exhaust stream.

In a preferred embodiment of the invention, this is achieved by running the air pump at a steady speed driven either electrically or mechanically by the engine and regulating the inlet flow area of the air pump dynamically. This is preferred to the regulation of the outlet flow area as it can be effected under low pressure and the sealing requirement are less onerous.

Advantageously, the means for regulating the inlet flow area may comprise a rotating disc driven by the engine and the intake mouth of the air pump is arranged in close proximity to the disc, the disc presenting an obstruction to the intake air flow which varies with the angularpo,s.iti6n'bf? the disc relative to the intake mouth.

Conveniently, the rotating disc may have radially or axially extending cutouts or recesses to expose and- obstruct,, - the' entry flow area cyclically.

By driving the rotating disc via the engine, from either the crankshaft or the camshaft, in proper phasing and with the correct duration and degree of exposure and obstruction of the entry flow area which is best determined by calibration for a specific engine running condition, for example idle, it is possible to match the air delivery precisely with the instantaneous dynamic variation of the exhaust flow at the point of air injection to keep the exhaust/air ratio within narrow limits.

In a system intended for an afterburner in the exhaust pipe to heat a catalytic converter immediately after start up to raise its temperature to its light off point, it is only necessary for the phase synchronisation to take place at one engine speed (idle). However, in a system in which secondary air is required throughout the engine operating range, a phase change mechanism may be used to vary the phase of the modulation of the pumped air in relation to the crank angle as a function of engine speed so as always to maintain the secondary air pulses in synchronism with the exhaust pulses.

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 schematic side view of an engine fitted with an exhaust system of the invention, and Figure 2 is a front view of part of the engine shown in Figure 1.

Detailed descriPtion of the preferred embodiment An engine 10 has an exhaust manifold 12 with a downpipe 14 leading to a catalytic converter (not shown). Secondary air is pumped into the downpipe 14 by means of an electrically driven pump 22. The outlet side of the pump 22 is connected to the downpipe 14 by way of a delivery pipe 24 containing a shut-off valve 26.

As so far described, such a system for introducing secondary air into the exhaust stream is known. Known system use secondary air in a variety of way mostly connected with reducing carbon monoxide and hydrocarbon emissions, the additional oxygen being used to complete the combustion.

The reaction between the secondary air and such combustible products is initiated by the high temperature in the exhaust downpipe and may be assisted by a catalytic converter.

If the exhaust gases contain a sufficiently high content of combustible products, as caused by intentional overfuelling, the secondary air may mix with the exhaust gases to form an ignitable mixture which can be ignited to burn as a flame and act as a heat generator to assist in lighting off the catalytic converter during cold start and prolonged idling. Such an afterburner may also be used for other purposes, such as to warm up the passenger compartment.

In all the above applications, but especially in the case of an exhaust afterburner, the strength of the secondary air / exhaust gas mixture is important to proper operation.

Because the electric pump 22 delivers air at a constant flow rate whereas the exhaust gas flow rate will vary cyclically, further steps must be taken if the mixture strength is to be maintained at least approximately constant.

To modulate the delivery rate of the pump 22 a disc 18 having radially extending vanes is mounted on the crankshaft pulley 16. The mouth of the inlet pipe 20 of the pump 22 is arranged in close proximity to the disc 18 so that as the crankshaft rotates, the pipe 20 will be cyclically obstructed by the vanes thereby modulating the delivery rate. By suitable dimensions and positioning of the vanes, it is possible to ensure that the secondary air flow rate will vary cyclically in synchronism with the exhaust stream at the point of mixing and thereby maintain a constant mixture strength.

The synchronisation will not occur at all engine speeds because the propagation paths and speeds of the secondary air and the exhaust gases are not identical. However, in some applications, such as the afterburner, synchronisation at idle speed is adequate as it is mostly under such conditions that the afterburner is ignited. However, it possible to vary the phase of the disc relative to the crankshaft to allow the mixture strength to be maintain constant over a wider range of engine speeds.

It should be clear that various modifications can be made within the scope of the invention as set forth in the appended claims. In particular, the modulation of the secondary air may be carried out on the outlet rather than the inlet side of the pump 22. Furthermore, in place of the disc 18 one may use a rotary valve driven by the engine either directly or indirectly through a phase change mechanism.

Claims

1. An exhaust system for an internal combustion engine having an air pump for pumping secondary air into the exhaust gases to permit secondary combustion or afterburning, wherein means are provided for regulating the air delivery rate by the air pump in proportion and in synchronism with the exhaust flow rate in order to maintain a more constant exhaust/air ratio at the point of introduction of the secondary air into the exhaust stream.

2. An exhaust system as claimed in claim 1, wherein the air pump is a steady speed pump and means are provided for dynamically regulating the inlet flow area of the air pump.

3. An exhaust system as claimed in claim 2, wherein the means for regulating the inlet flow area comprises a rotating disc driven by the engine, the intake mouth of the air pump being arranged in close proximity to the disc, and the disc presenting an obstruction to the intake air flow which varies with the angular position of the disc relative to the intake mouth.

4. An exhaust system as claimed in claim 3, wherein the rotating disc may have radially or axially extending cutouts or recesses to expose and obstruct the entry flow area cyclically.

5. An exhaust system as claimed in claim 3 or 4, wherein, in use, the disc is mounted on the engine crankshaft.

6. An exhaust system as claimed in claim 2, 3 or 4, the means for dynamically regulating the inlet flow area of the air pump are driven by the engine by way of a phase change mechanism.

7. An exhaust system constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.