Background and state of the art In order to hasten the warming up of a combustion engine from a cold start, a known practice in the case of diesel engines in heavy vehicles is to increase the pressure in the exhaust system by blocking the exhaust line by means of an exhaust brake damper usually incorporated in the exhaust line of such vehicles.

The resulting increase in the load on the engine makes it reach its normal working temperature more quickly, thereby also reducing the discharge of emissions and of so- called white smoke.

Another known practice is to incorporate a line which connects the inlet and outlet systems of a combustion engine to one another in order to transfer exhaust gases from the outlet system to the inlet system. This line is usually valve-controlled in order to be able to modify according to the operating state of the engine the quantity of exhaust gases fed back to the inlet side of the engine. Exhaust gas feedback, also known as EGR (exhaust gas recirculation), is desirable during certain operating states in order to be able to hold down the engine's combustion temperature and thereby reduce the quantity of emissions from the engine.

Known technology has hitherto been unable, however, to indicate an effective method and device for appreciably reducing exhaust emissions during cold starting of combustion engines, particularly in the case of diesel-type combustion engines.

Object of the invention The object of the invention is to provide a method and a device which appreciably reduce exhaust emissions during cold starting of combustion engines, particularly in the case of diesel-type combustion engines.

The solution has to be simple, economic and operationally reliable.

Summarv of the invention The object of the invention is achieved by the method indicated in the introduction being distinguished by the features indicated in the characterising part of the main claim.

Controlling an exhaust brake valve and an EGR valve in the manner therein indicated makes it possible to shorten appreciably the time the engine takes to reach a steady operating state from a cold start. A corresponding decrease in the discharge of emissions follows therefrom.

A simple solution according to the invention is also achieved by using the sensor signals already available in the case of an electronically controlled engine which represent parameters referring to fuel input quantity, engine speed and the likewise monitored parameter which represents the vehicle's speed. Further reduction of emissions can be achieved by controlling in the manner indicated in the subclaims the opening and closing of valves in dependence of the number of turns during starting of the engine.

The device according to the invention is distinguished by the features indicated in the preamble to the independent patent claim pertaining to a combustion engine. A particularly simple and economic solution can be achieved with the valves, the EGR line layout and the control unit which are therein indicated. Increased stability and functional reliability of the method according to the invention can be achieved through what is also indicated concerning the layout of the EGR line and the valves in the subclaim to said independent claim.

Brief description of the drawings Figure 1 shows schematically a system according to the invention for exhaust gas feedback in a turbocharged diesel engine.

Description of an embodiment Figure 1 depicts a turbo-supercharged multi-cylinder combustion engine 1, preferably of diesel type. The supercharging of the engine is effected by a first turbo unit in which a turbine 4 driven by exhaust gases drives a compressor 5. The turbine 4 and the compressor 5 are coupled for joint rotation on a common drive shaft 28. In the diagram the inlet air flow is represented by unbroken flow arrows, whereas the exhaust gas flow is represented by discontinuous flow arrows.

The exhaust gases from the combustion engine are gathered in an exhaust manifold 3,3' which is here divided into two separate branches 3 and 3' respectively which connect to the inlet of the turbine 4. The turbine 4 is conventionally provided with a so-called divided inlet run so that exhaust pulses from one group of engine cylinders do not clash with pulses from the cylinders in the other group. Downstream from the turbine 4 there is also an exhaust brake 50, here of the damper valve type, which is acted upon by a control device 51 between a position which applies minimum throttling to the flow through the exhaust line (valve open) and a corresponding maximum throttling position (valve closed).

The inlet manifold 2 of the combustion engine conveys the air pressurised in the compressor 5 to the engine cylinders 8 in a conventional manner. In a manner likewise known per se, a charge air cooler 11 is arranged downstream from the compressor 5 but upstream from the inlet manifold 2. Also in a conventional manner, the inlet side of the compressor 5 is supplied with filtered air.

A pipeline 20, hereinafter called the EGR line, connects the outlet system upstream from the turbine 4 (advantageously directly from the exhaust manifold 3,3' or the turbine inlet) with the inlet system downstream from the compressor 5 (advantageously directly to the inlet manifold 2). The EGR line incorporates a valve 17 which is acted upon by a control

device 30 which controls the degree of opening (including closure) of the valve in a conventional manner on the basis of signals from a control unit 32 for an electronic control system for the engine.

It is advantageous for the valve 17 to be situated close to the point at which the EGR line is tapped from the exhaust manifold, with the result that no exhaust gas volume in the EGR line need be compressed when the EGR line is closed. The consequences include no impairment of response during conventional exhaust braking or conventional engine load increase.

In the solution depicted, the EGR line is directly connected to the inlet manifold 2 centrally or to the inlet air line connected thereto, in such a manner that the exhaust gases fed back are well mixed with the inlet air. It is possible with advantage, however, for the EGR line 20 alternatively to be connected to the inlet manifold 2 via a multiplicity of pipe orifices distributed so as to correspond to the connections of the inlet manifold 2 to the respective cylinder inlet ports.

The control unit 32 controls the control device 30 and hence the valve 17 on the basis of monitored engine and vehicle parameters such as engine speed, engine temperature and charge air pressure which together represent the operating state of the engine and the speed of the vehicle. These parameters are monitored by the control unit 32 via respective sensors 33,34,35 and 36 arranged on the engine and the vehicle. The vehicle speed sensor 36 appears in the diagram on a schematically depicted vehicle 19. Moreover, a flow sensor 42 which may for example incorporate a venturi meter in the line 20 provides the control unit 32 with a signal representing the EGR quantity delivered. This signal, like those representing the engine parameters, are received by the control unit 32 via signal input lines 39. On the basis thereof the control unit 32 controls the control devices 30 and 51 by means of signals via dotted control lines 38. Power supply to the control unit 32 and also to a conventional electric starter motor 21 intended for starting the engine is provided by a battery 43.

During engine starting, the starter motor 21 in a conventional manner makes the engine rotate at a speed of about 60 rev/min before any combustion takes place in any of the engine cylinders. At this stage both the exhaust brake valve 15 and the EGR valve 17 are closed in order to create maximum load on the engine and hence compression and heat build-up in the cylinders. The engine is supplied with fuel after the starter motor has made the engine rotate a few (approximately three) turns. At this stage, even when starting in very cold conditions (temperatures of the order of -20°C) the heat built up in the cylinders is usually sufficient for the injected fuel to ignite. After a few (approximately three) more turns, the EGR valve 17 opens so that exhaust gases from the combustion which has taken place can be fed back via the EGR line 20 to the engine inlet air manifold 2. Said few more turns may also be detected from the occurrence of a predetermined increase in the engine speed relative to the starter motor speed. The exhaust gases thus fed back via the EGR line are mixed with the cold inlet air and led thereafter into the engine cylinders.

The resulting inlet air thus has a higher temperature which appreciably facilitates the ignition of the fuel being injected into the cylinders. The greater the number of the engine cylinders in which this takes place, the greater will be the increase in exhaust gas feedback, resulting in the combustion in the cylinders reaching a normal state more quickly, thereby also achieving a desirable reduction in discharges of harmful emissions.

When the engine speed has for a certain predetermined time been held within certain limits which correspond to normal idling speed, the EGR gas flow is reduced by gradual closing of the EGR valve and likewise gradual opening of the exhaust brake valve. At this stage the starter motor also ceases to drive the combustion engine. In tests on a diesel engine for operating heavy vehicles, said predetermined time was about 10 seconds and the speed limits were 575 rev/min and 625 rev/min, i.e. a idling speed of 600 rev/min + 25 rev/min.

The proportion of EGR gases fed back should not exceed about 50% by weight of the engine's air requirement. It may with advantage be of the order of 33% by weight at the beginning of the starting process when the EGR valve has opened and may subsequently be lowered to the order of 25% by weight when the engine has maintained a steady idling speed for the aforesaid predetermined time. Thereafter the EGR valve begins to gradually

close the EGR line but full closure of the EGR line and full opening of the exhaust brake valve are only reached when the idling speed has for a predetermined time as described above been held within preselected limits with regard to variation and absolute level.

Alternatively, the fact that the engine has reached a certain predetermined working temperature such as that represented by the coolant temperature may be the criterion which decides when the EGR valve and the exhaust brake valve will close and open respectively