GB2257642A

GB2257642A - Engine exhaust system

Info

Publication number: GB2257642A
Application number: GB9115628A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1991-07-19
Filing date: 1991-07-19
Publication date: 1993-01-20
Also published as: WO1993002279A1; GB9115628D0

Abstract

An exhaust system for an internal combustion engine comprises an exhaust pipe 10 for the gases emitted from the combustion chambers of the engine, an air supply 14 for introducing additional air 28 under pressure into the exhaust pipe 10, and an igniter 18 for igniting the gaseous mixture consisting of the additional air and the gases from the engine combustion chambers. The air supply 14 comprises at least one air injection orifice 16 injecting a stream of air into the flow of exhaust gases 12 and the igniter 18 is arranged close to an air injection orifice 16 in a region of the exhaust pipe in which the gaseous mixture is in homogeneous. In an alternative construction the air through a central nozzle 16, but supplied to an annular gallery (30) surrounding the exhaust pipe 10 and communicating with it through holes (32, Fig 2, not shown). <IMAGE>

Description

ENGINE EXHAUST SYSTEM Field of the invention The present invention relates to an afterburner for heating the exhaust gases of an internal combustion engine.

Backqround of the invention Prior to the advent of catalytic converters, it was proposed to reduce the emissions of an internal combustion engine by burning the exhaust gases in the exhaust pipe to allow the combustion to be completed. This proposal was described in an Internal Report No. 1967/5, circulated in 1967 to its members in England by the Motor Industry Research Association (MIRA). The report by C.D. Haynes was entitled Atmospheric Pollution from Petrol Engine - Simple Tuning and Manifold Air Oxidation. In the reported work, air was introduced into the exhaust stream and was passed over a heated filament. When the engine was idling with a rich mixture, the exhaust gases contained some 8% of carbon monoxide which was burned in the additional air and thereby removed from the gases discharged to the atmosphere.

The mixture of air and carbon monoxide proved particularly difficult to ignite reliably. The proposal in the MIRA paper requires thorough mixing of the fuel and air before it reaches the heated filament. The latter is placed in a compartment at the front end of a silencer into which the air and exhaust gas mixture enters through a perforated plate. The plate does not have any perforations in line with the filament to prevent the flame from being blown out.

The difficulty with such a construction is that there is only a narrow range of mixture strengths in which the mixture is ignitable and the air must be accurately matched to the carbon monoxide content if the mixture is to be ignited.

As admitted in the MIRA report discussed above, the experimental afterburner described by Haynes was not able to cope with emissions throughout the drive cycle, especially during acceleration and work on it was discontinued.

Furthermore, the system was not even assessed during cold starting, when the emissions are at their highest levels in the drive cycle.

More recently, the present Applicant proposed in co-pending GB Patent Application No. 9112601.1, filed on 12 June, 1991 and Application No. 9113949.3, filed on 26 June, 1991, to use an afterburner as a source of heat, for example to reduce the light-off time of a catalytic converter. Unlike the MIRA proposal, the afterburner is used predominantly during cold starting and the primary purpose of the afterburner is not to reduce emissions (though this is a highly desirable side effect) but to generate heat to allow the catalytic converter to reach its light off temperature as quickly as possible and enable it to cope with the emissions.

This proposal requires air and combustible gases emanating from the engine to be ignited within the exhaust pipe and faces the same problem of how the additional air can be accurately metered to create a readily ignitable mixture.

Object of the invention The present invention seeks to provide an afterburner which renders the overall ratio of oxygen to combustible gases less critical so as to enable the mixture within the engine exhaust system to be ignited reliably.

Summarv of the invention According to the present invention, there is provided an exhaust system for an internal combustion engine comprising an exhaust pipe for the gases emitted from the combustion chambers of the engine, air supply means for introducing additional air under pressure into the exhaust pipe, and an igniter for igniting the gaseous mixture consisting of the additional air and the gases from the engine combustion chambers, characterised in that the air supply means comprises at least one air injection orifice injecting a stream of air into the flow of exhaust gases and in that the igniter is arranged close to an air injection orifice in a region of the exhaust pipe in which the gaseous mixture is inhomogeneous.

Preferably, the igniter is arranged in the region of turbulence at the interface between the injected air stream and the flow of exhaust gases.

The invention takes advantage of the fact that because of turbulence at the interface between the additional air and the exhaust gases, the mixture strength is constantly fluctuating within wide limits and if a rapid fire spark plug is used as an igniter there is a high probability of a spark finding an ignitable mixture within a short time of the afterburner being turned on.

Advantageously, in use, the igniter is operated prior to commencement of the additional air injection whereby the turbulent interface at the front of the additional air flow traverses the region of the igniter while the latter is in operation.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a section through part of an exhaust system having an afterburner with a single injection orifice, and Figure 2 is a section similar to that of Figure 1 showing an exhaust system with several orifices distributed about the circumference of the exhaust pipe.

In Figure 1, a section of exhaust pipe 10 is connected at one end to the engine to receive a flow of exhaust gases as represented by the arrows 12. The gases leaving the engine, either naturally or because of deliberate over fuelling, have a surplus of combustible gases, in particular carbon monoxide and hydrogen. The section of the exhaust pipe 10 includes an afterburner which uses the surplus of combustible gases as fuel to generate heat. The heated gases then leave the section of the exhaust pipe 10 along the direction of the arrows 28 and flow towards an item to be heated such as a catalytic converter of a petrol engine, the filter trap of a diesel engine or a heat exchanger for heating the passenger compartment.

The afterburner comprises an air supply line 14 connected to a single injection nozzle 16. The supply line 14 receives ambient air, represented by the arrow 28, from an air pump (not shown), which may be electrically driven or mechanically driven by the engine. A spark igniter 18 is arranged near the injection nozzle 16 and is formed of a high tension electrode 18a shielded from an earth electrode 18c and the exhaust pipe 10 by means of a ceramic insulator 18b. The igniter 18 is connected to an oscillator circuit which rapidly fires a spark across the spark gap between the electrodes 18a and 18c, for example at the rate of fifty sparks per second.

The air from the injection nozzle 16 forms an expanding jet which fans out from the nozzle 16. At the boundary 20 of the air jet with the exhaust gases, there is considerable turbulence represented by the vortex arrows 22 which causes the air to mix with, or diffuse into, the exhaust gases.

This region of highly inhomogeneous mixture will have a combustible gases to air ratio which fluctuates constantly within wide limits. Provided the combustible gases content of the exhaust gases is sufficiently high, there is bound to be at some time in this inhomogeneous region a readily ignitable mixture. If a spark is correctly timed and positioned to intercept this mixture, it will succeed in igniting it and the resulting flame will rapidly spread to all the ignitable regions-of the mixture and will sustain itself for as long as the flows of air and combustible gases are maintained.

In effect, this jet of air injected into a stream containing combustible gases is analogous but complimentary to a conventional gas burner, in which a jet of combustible gas is injected into ambient air or an air stream and ignited. In a conventional burner, combustion cannot take place within the fuel jet as there is no oxygen, nor can it take place in the ambient air stream where there is no fuel.

Instead, the burning takes place at the interface between the two gas streams, where there is intense mixing. By analogy, in the present invention, all the burning takes places at the boundary of the injected air stream giving the classic shape of a flame. The inner cone emanating from the nozzle 16 contains air only and is cold. In the flame shaped region 24, there is intense mixing and heat transfer, and at the boundary 20 combustion takes place generating heat and light to raise the temperature of the exhaust stream.

The analogy to a conventional burner also helps to explain the reason that the air flow rate is not critical in the invention. Just as the gas from a burner can be turned up or down without the flame being extinguished so can the air flow rate be raised and lowered in the present invention in relation to the exhaust flow rate. For this reason, it is possible to inject a surplus of air to ensure complete combustion of the combustible content of the exhaust gases, without adversely affecting the ignitability of the mixture and the stability of the flame. By contrast, in the homogeneous mixture used in the MIRA article, the addition of surplus air would quench the flame and careful control is required over the air flow rate, for the flame to be ignitable and stable.

Because of the analogy to a burner, design features which can be used in such burners to improve ignition and flame stability are equally applicable to the present invention.

For example, one may shield the igniter from the exhaust gas flow by means of a baffle, one may introduce swirl to enhance mixing at the boundary of the air jet or one may include small obstructions to the gas flow within the flame in order to enhance turbulence and hold the flame.

The flame should preferably spread over the entire cross section of the exhaust pipe as illustrated and in the embodiment of Figure 1 this is achieved using single injection nozzle. If, owing to the diameter of the exhaust pipe or the maximum exhaust gas flow rate at which afterburning is required, a single injection nozzle does not suffice for the flame to spread over the entire cross section of the exhaust pipe then multiple injection orifices may be used as shown in Figure 2.

In Figure 2, the air represented by the arrow 28 is pumped into a gallery 30 which surrounds the exhaust pipe 10 and orifices 32 are distributed around the exhaust pipe for injection of air into the stream of exhaust gases. The interface 20 between the two gas streams, in this case, extends from the surface of the pipe inwards towards the centre and the igniter 18 is arranged in the vicinity of this interface. One igniter 18 still suffices in this case, located near one of the orifices 32 but more than one igniter may be provided if preferred.

The igniter illustrated is a spark igniter but it is alternatively possible to employ a heated filament or even a catalytic element.

It has been assumed that the gas flow from the engine is always in the direction of the arrows 12 but in practice this is not always the case, especially with highly tuned engines having a large valve overlap. In such a case, because of internal exhaust gas recirculation, the flow of gases may momentarily be reversed while the inlet valve is open at the end of the exhaust period. Such flow reversals and fluctuations in the velocity of forward flow make for flame instability in any system in which the gases are mixed homogeneously before they are ignited because it is difficult to control the amount of additional air in accurate and rapid response to the changing flow conditions.

In the present invention, surplus air may always be present and fluctuations in the flow rate of the exhaust gases will only change the flame size but will not affect its stability.

Another problem of homogeneous mixing is that a buffer chamber is required to allow thorough mixing prior to ignition. When ignition occurs, a flame is created which spreads throughout the chamber and having done so will be extinguished until the chamber is refilled with an ignitable mixture. Consequently, the igniter needs to be active at all times and in each and every such cycle the flame has to be reignited. In the present invention, the flame, once it has been ignited, will remain at the air injection orifice for as long as additional air is pumped into the exhaust gases and the latter contain a sufficient proportion of combustible products. Though the igniter is operated continuously, this is only done as a precaution in case the flame should be extinguished and under normal circumstances these sparks do not act to reignite the mixture as the flame is self sustaining.

Claims

1.

An exhaust system for an internal combustion engine comprising an exhaust pipe for the gases emitted from the combustion chambers of the engine, air supply means for introducing additional air under pressure into the exhaust pipe, and an igniter for igniting the gaseous mixture consisting of the additional air and the gases from the engine combustion chambers, characterised in that the air supply means comprises at least one air injection orifice injecting a stream of air into the flow of exhaust gases and in that the igniter is arranged close to an air injection orifice in a region of the exhaust pipe in which the gaseous mixture is inhomogeneous.

2. An exhaust system as claimed in claim 1, wherein the igniter is arranged in the region of turbulence at the interface between the injected air stream and the flow of exhaust gases.

3. An exhaust system as claimed in claim 1 or 2, wherein, in use, the igniter is operated prior to commencement of the additional air injection whereby the turbulent interface at the front of the additional air flow traverses the region of the igniter while the latter is in operation.

4. An exhaust system as claimed in any preceding claim, wherein the igniter is a rapid fire spark igniter.

5. An exhaust system as claimed in any of claims 1 to 3, wherein the igniter is a heated filament.

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