Arrangement for treatment of exhaust gases for a compression-ignition internal combustion engine

An arrangement for the treatment of exhaust gases from a compression-ignition internal combustion engine, in which, fuel is metered via a metering valve and introduced into the exhaust system via an evaporator to promote the action of a reduction catalyst arranged on the downstream side of the engine.

PRIOR ART 
Due to the fact that exhaust gases are operated with a large excess of air, 
the exhaust gases of compression-ignition internal combustion engines tend 
toward a high NO.sub.x emission. This occurs to a greater degree in the 
case of internal combustion engines with direct injection into the 
combustion space. In order to reduce this emission, one possibility is to 
perform NO.sub.x reduction with the aid of a corresponding reduction 
catalyst. Suitable catalysts for this purpose are, for example, those 
based on zeolites. Another problem of compression-ignition internal 
combustion engines lies in the relatively low exhaust-gas temperature, 
which makes light-off of the reduction function of such a catalyst more 
difficult. To promote this reduction process, it has also already been 
proposed to connect the exhaust system to a burner which heats up the 
exhaust gases. To promote the reduction process, a proposal has already 
been made for an arrangement to treat exhaust gases from a 
compression-ignition internal combustion engines. 
In such an arrangement, known from the publications by F. Schafer and R. 
Van Basshuysen entitled "Schadstoffreduzierung und Kraftstoffverbrauch von 
PKW-Verbrennungsmotoren" [Pollutant reduction and fuel consumption in 
passenger-vehicle engines], page 115, published by Springer-Verlag, urea 
in aqueous solution is provided as the reducing agent and this is fed to 
the exhaust system upstream of the catalyst. This urea is metered in a 
complicated manner by means of a solenoid valve which is subject to the 
high operating temperatures in the region of the exhaust system and 
therefore tends to malfunction. The provision and control of this solenoid 
valve for metering in small amounts--in the range of 1.5 milligrams per 
working cycle of the internal combustion engine--is very expensive. 
Particularly for metering the urea dispensed by the solenoid valve, the 
provision of compressed air is required, said compressed air on the one 
hand transporting to the exhaust system the urea metered and on the other 
hand being used to generate pressure in order to raise the urea storage 
tank to the pressure required for injection at the solenoid valve. To 
ensure accuracy of metering, this pressure must be regulated. The pressure 
drop across the solenoid valve must furthermore guarantee the finely 
distributed preparation of the urea, such that the NH.sub.3 required for 
the desired reduction of the NO.sub.x components in the exhaust gas will 
be formed at the latest in the catalyst by decomposition of the urea 
compound in conjunction with the action of heat. 
This arrangement is very complex and requires a high exhaust-gas 
temperature to ensure that the reduction process is reliably carried out. 
There is the risk that, given an oversupply of urea or the absence of the 
operating requirements at the catalyst, urea or ammonia will not be 
completely converted and will thus pollute the environment as a component 
of the emissions. 
EP-A-503 882 has furthermore disclosed the use of HC, i.e. fuel, as a 
reducing agent, this being introduced into the exhaust system of the 
internal combustion engine upstream of a NO.sub.x reduction catalyst of 
the zeolite type in a manner controlled by the temperature of the 
catalyst. Here, the metered addition is intermittent, the intention being 
that HC should be temporarily stored in the porous structure of the 
catalyst so that this HC is available for the conversion of NO.sub.x as 
the temperature of the catalyst rises. In addition to the disadvantageous 
use, already described above, of a solenoid valve and the associated 
expense, this arrangement has the disadvantage that the quantity of HC 
introduced cannot immediately bring about conversion of the NO.sub.x 
components but must first of all be conditioned in the catalyst. This may 
well be achievable in the case of the envisaged application of the known 
arrangements to an applied-ignition internal combustion engine, which, as 
is known, has high exhaust-gas temperatures. In the case of the relatively 
cooler exhaust gases of a compression-ignition internal combustion engine, 
this measure is inadequate. 
ADVANTAGES OF THE INVENTION 
In contrast, the arrangement according to the invention has the advantage 
that, despite the use of an intermittently operated electrically 
controlled valve, a reducing agent is introduced into the exhaust gas 
continuously and, what is more, advantageously already in vapor form, 
reliably guaranteeing thorough mixing with the exhaust gas and good 
distribution. This gives optimum reactions in the downstream reduction 
catalyst. A particularly advantageous embodiment of such an evaporator is 
obtained if a porous body is used. Such porous bodies are already widely 
used in the form of mass-produced sintered components in the region of 
high temperatures, making the implementation of the solution according to 
the invention unproblematic. The demands on the electrically controlled 
valve are very small, so that valves which are already commercially 
available, such as injection valves for petrol injection, can be used at 
an operating pressure of a few bar. Care must merely be taken to ensure 
that the outlet cross section is small enough for the required quantities 
to be metered within the cycle of the valve. 
A known electrically heated pencil-type heater plug, which projects into 
the hollow body and very largely fills the interior space of the hollow 
body, is advantageously used as a heating device. Such pencil-type heater 
plugs, the familiar glow plugs, can be obtained as a mass-produced product 
as a starting aid for compression-ignition internal combustion engines and 
permit an economical solution for the arrangement according to the 
invention. The pencil-type heater plug is advantageously exchangeable and, 
for this purpose, is, insertable into a connecting pedestal by means of 
which the hollow body configured the form of a sintered part, is connected 
to the wall of the exhaust-gas-carrying parts of the internal combustion 
engine. It is also particularly advantageous here to use fuel as a 
reducing agent, fuel which is already available in the case of the 
associated diesel internal combustion engine and, by burning, on the one 
hand increases the temperature of the catalyst and makes available ions 
with the aid of which the reduction of NO.sub.x components in the catalyst 
can be performed in an effective manner. An exhaust-gas heating device can 
advantageously be provided in addition and, by means of this device, the 
time up to the effective entry into service of the reduction catalyst can 
be considerably shortened, particularly when starting the internal 
combustion engine while it is still cold. According to the invention, an 
oxidation catalyst is arranged on the downstream side for complete 
conversion of all components of the exhaust gases which are still burnable 
in the catalytic reactor.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT 
The drawing shows, schematically, a section through part of a cylinder 1 of 
a compression-ignition internal combustion engine. In this exemplary 
embodiment, the engine is a compression-ignition internal combustion 
engine with indirect injection, i.e. the fuel is injected by means of an 
injection valve 2 not directly into the main combustion space 4 bounded 
directly by the piston 3 of the internal combustion engine but into a 
swirl chamber 5 which is arranged ahead of the said space and is connected 
to the main combustion space 4 by a transfer passage 6. A glow plug 7 
projecting into this swirl chamber is provided as a starting aid. 
Following completion of the expansion stroke, the components of the fuel 
injected and air which are burnt in the swirl chamber and the main 
combustion space are expelled via an exhaust valve 8 into an exhaust duct 
9 by the exhaust stroke of the piston 3 of the internal combustion engine. 
In general, the exhaust duct 9 is combined into an exhaust manifold which 
comprises a plurality of ducts, each leading off from one engine cylinder, 
and merges into a collecting pipe which leads into the open via one or 
more lines. In the example illustrated, a reduction catalyst 11 is 
arranged in the exhaust collecting pipe and, as indicated in dotted lines, 
has an oxidation catalyst and, if required, the customary muffler device 
of the exhaust system of an internal combustion engine connected to it on 
the outlet side. 
Fuel is supplied to the injection valve 2 of each cylinder by means of a 
fuel injection pump 14 which receives fuel from a fuel tank 15 and meters 
the fuel, which is raised to high pressure, via injection lines 16 and 
feeds it in turn to each of the injection valves 2. Excess fuel from the 
individual injection valves is returned to the fuel tank via leak-off 
lines 17. 
For the delivery of the fuel from the fuel tank to an internal inlet space, 
the fuel injection pump is connected to a presupply pump (not shown in 
detail), a fuel pressure controlled as a function of the engine speed 
generally being maintained in the inlet space in order to control 
functions dependent on engine speed. The overflow pressure for controlling 
this inlet-space pressure escapes, generally unpressurized, back to the 
fuel tank 15 via the overflow line 19. In the present example, however, a 
pressure regulator 20 is arranged in the overflow line 19 and this can, 
for example, be set to 0.3 bar, so that a supply pressure of 0.3 bar is 
available upstream of this pressure regulator 20. Via a fuel line 21, this 
pressure is fed to an electrically controlled metering valve 23 which, 
under the control of a control device 22, feeds fuel as a reducing agent 
to an evaporator 26 as a function of operating parameters such as load 
(Q.sub.x) and engine speed (n). This evaporator is located in the exhaust 
duct upstream of the reduction catalyst 11. The construction of this 
evaporator can be seen in detail from FIG. 2. Provided for this purpose in 
the wall of the fuel duct 9, is a screw-in sleeve 28 into which a 
connecting pedestal 29 can be screwed in leaktight fashion. Inserted into 
this connecting pedestal is a hollow body 30 which projects into the 
exhaust stream in the exhaust duct 9. This hollow body has a porous 
heat-resistant wall and can be composed, for example, of sintered 
material, sintered bronze or ceramic and, in the interior, has a blind 
hole 31 into which an elongated heater plug 32, which matches the blind 
hole in shape and fills it, projects as a heating device. This heater plug 
is screwed into a sleeve part 34 of the connecting pedestal 29 from 
outside the exhaust duct, coaxially with respect to the hollow body 30, 
which is of rotationally symmetrical design, such that the interior space 
remaining between the blind hole 31 and the heater plug 32 is sealed off 
in leaktight fashion from the outside, outside the exhaust duct 9. Opening 
into the blind hole 31 there is furthermore a connecting line 35 which 
leads from the metering valve 23 and carries the reducing agent, the 
diesel fuel, dispensed by the metering valve 23 into the remaining cavity 
in the blind hole 31. 
The elongated heater plug 32 is heated by way of a control line 36 from the 
control device 24. 
The catalyst 11 is designed as a reduction catalyst and serves to reduce 
the NO.sub.x components in the exhaust gas from the internal combustion 
engine. The internal combustion engine under consideration is a 
compression-ignition internal combustion engine, which, as is known, 
operates with a considerable excess of oxygen and, owing to this 
combustion method, has a considerable proportion of NO.sub.x components in 
the exhaust gas. This proportion is already large with an internal 
combustion engine which operates with the swirl-chamber combustion method 
shown and this proportion is even larger in the case of internal 
combustion engines in which injection is directly into the main combustion 
space 4. Because of the large excess of air, which is still present in the 
exhaust gas and results in an extremely small proportion of CO in the 
exhaust gas, it is not possible to achieve to a satisfactory extent the 
reduction of the NO.sub.x components which could be carried out 
effectively with this CO. Another factor which creates difficulties is 
that the exhaust gases of compression-ignition internal combustion engines 
are at a considerably lower temperature than those in applied-ignition 
internal combustion engines and this temperature makes the light-off 
behavior of a downstream catalyst and a high efficiency of this catalyst 
considerably more difficult. These disadvantages are countered by the 
introduction of reducing agent. The fuel introduced promotes reduction in 
the catalyst in an effective manner. There is, at the same time, also a 
thermal conversion of fuel in the catalyst, increasing the working 
temperature of the catalyst and its efficiency. To this end, an increase 
in efficiency requires that the reducing agent introduced should enter the 
exhaust gas in finely divided and rapidly convertible form. It is also 
important that precisely the required amount of reducing agent for 
effective exhaust-gas detoxification is introduced. With the aid of the 
electrically controlled valve 23, piloted by means of the control device 
24, the respectively required quantity of reducing agent is introduced to 
match the volume of exhaust gas, determined from the load and engine 
speed, the amount also taking into account the temperature T of the 
exhaust gas and/or of the catalyst. 
The good preparation of the fuel used here as a reducing agent is 
accomplished by means of the evaporator 26, the quantity of fuel having 
been controlled with the aid of the abovementioned valve 23. The 
evaporator emits fuel only in vapor form and this passes into the exhaust 
gas through the porous wall of the hollow body. This hollow body is 
heated, on the one hand, by the exhaust gas and, on the other hand, also 
by the elongated heater plug 32, particularly in the starting phase of the 
internal combustion engine, as long as the temperature of the exhaust gas 
is too low to bring about an evaporation process. The heating is likewise 
controlled as a function of the parameters mentioned, and continuous 
addition of fuel in vapor form to the exhaust gab in the required amount 
is thus guaranteed. 
The metering valve is advantageously supplied by the fuel circuit of the 
fuel injection pump, which is necessary in any case to operate the 
internal combustion engine. With the aid of the pressure regulator 20, the 
required low pressure is available without any major additional 
expenditure. The metering valve can advantageously be a low-pressure 
injection valve which can easily be modified by reducing the outlet 
opening to a single hole and is available as an inexpensive mass-produced 
part. Elongated heater plugs are likewise mass-produced parts which can be 
used at low cost. By virtue of the control mentioned, which aids the 
efficiency of reduction but also avoids excess HC entering the environment 
as emissions, only a small additional consumption of fuel is required for 
the operation of the reduction catalyst. 
The foregoing relates to preferred exemplary embodiments of the invention, 
it being understood that other variants and embodiments thereof are 
possible within the spirit and scope of the invention, the latter being 
defined by the appended claims.