GB2342122A - Engine braking method for i.c. engine with variable geometry turbocharger - Google Patents

Abstract

During engine braking the turbine 11 is usually set to minimum inlet cross-section to increase exhaust gas back-pressure and thus increase engine braking. For a rapid build-up of engine braking power, (a) the engine braking valves 20 (which may be the gas-exchange exhaust valves or dedicated braking exhaust valves) are opened, (b) the turbine geometry is set to minimum cross-section and (c) fuel is injected via nozzles 19 and ignited before TDC so that the pistons must work against the expanding mixture and exhaust gas back-pressure is further increased. To maintain a minimum internal cylinder pressure, and thus prevent poor combustion, the stroke or cross-section of the braking valves 20 are reduced if cylinder pressure or engine speed falls to a threshold level. For emergency braking, the blow-off device 25 is also closed to build up maximum exhaust gas back-pressure.

Description

1 2342122 Ensine braking method for a supercharged internal combustion

eneine The invention relates to an engine braking method for a supercharged internal combustion engine having a turbocharger, the exhaust-gas turbine of which has a variable turbine geometry, which geometry is adjustable for the variable setting of the effective turbine cross section between a stemming position and an open position, the internal combustion engine being at least partly fired during engine braking operation.

DE 195 43 190 AI discloses an engine braking system for a supercharged internal combustion engine which is provided with an exhaust-gas turbocharger having a turbine geometry which is variable via an adjustable guide cascade. The guide cascade comprises guide vanes, which can be set by means of an actuator in such a way that the effective cross section of the turbine is changed. In this way, depending on the operating state of the internal combustion engine, exhaust-gas back pressures of different magnitude can be realized in the section between the cylinders and the exhaust-gas turbocharger, as a result of which the output of the turbine and the output of the compressor can be set according to demand.

In order to achieve an engine braking effect during braking operation of the internal combustion engine, the guide cascade is shifted into a stemming position, in which the turbine cross section is markedly reduced. A high exhaust-gas back pressure builds up in the line section between the cylinders and the exhaust-gas turbine, with the result that exhaust gas flows at high velocity through the passages between the guide vanes and a high impulse is applied to the turbine wheel. The turbine output is transmitted to the compressor, whereupon the combustion air fed to the engine is put under increased boost pressure by the compressor.

As a result, increased boost pressure is applied to the cylinder on the inlet side, and there is an increased exhaust-gas back pressure on the outlet side between the cylinder outlet and the exhaust-gas turbocharger, this increased exhaustgas back pressure counteracting the blow-off of air compressed in the cylinder via open braking valves into the exhaust-gas line. During engine braking operation, the piston, during the compression and exhaust stroke, must perform compression work against the high excess pressure in the exhaust-gas line, as a result of which a powerful braking effect is achieved.

2 Such engine braking systems are preferably used in heavy utility vehicles, in which engine braking power in the order of magnitude of several hundred kW can be produced.

Furthermore, it is known from publication DE 44 25 956 A1 to temporarily inject fuel for the firing of the internal combustion engine during engine braking operation in order to increase the braking power. The fuel/air mixture is ignited before reaching top dead centre, whereupon a pressure, against which the piston moving upwards has to perform work, builds up in the combustion chamber. This compression work is extracted from the kinetic energy of the motor vehicle, as a result of which effective braking is achieved.

The present invention seeks to specify an engine braking method with which a rapid build-up of braking power is achieved.

According to the present invention there is provided an engine braking method for a supercharged internal combustion engine having a turbocharger, the exhaust-gas turbine of which has a variable turbine geometry, which geometry is adjustable for the variable setting of the effective turbine cross section between a stemming position and an open position, the internal combustion engine being at least partly fired during engine braking operation, wherein braking valves at the cylinder outlet of the internal combustion engine are in the open position above a limit value of an engine state variable, and the braking valves for reducing the exhaust-gas mass flow diverted from the cylinders are put into a closed position when the engine state variable drops to the region of the limit value.

According to the novel method, the internal combustion engine is also at least partly fired during engine braking operation in order to increase the braking power. The firing results in the generation of an exhaust-gas mass flow, which is blown off via open braking valves into the exhaustgas line. In addition to the effect that the piston has to perform compression work against the fuel/air mixture ignited before top dead centre, the exhaust-gas back pressure is also increased in a short time, so that the piston must exhaust the combustion chamber volume against the high exhaust-gas back pressure. In addition, the turbine speed is rapidly increased and consequently the supercharger output is increased in a short time, which in turn results in an increased mass flow rate through the engine with corresponding increase in output.

3 Since ignition and clean combustion of the fuel/air mixture is possible only at an adequately high cylinder internal pressure, but the combustion chamber volume is at least partly discharged on account of the open braking valves, measures are taken in order to maintain a minimum pressure in the cylinder interior space. These measures consist in the fact that a characteristic quantity characterizing the combustion is introduced, the characteristic quantity is monitored for falling below a limit value, and if need be the exhaust-gas mass flow discharged into the exhaust-gas line via cylinder outlets is reduced, in particular by reducing the valve stroke of braking valves or else by reducing the valve cross section. The result of the reduction in the exhaust-gas mass flow is that a further drop in the cylinder internal pressure is prevented, the cylinder internal pressure is possibly even increased and, accompanying this increase, the cylinder temperature is increased and the mixture formation is promoted, so that the combustion of the fuel/air mixture can be maintained.

The characteristic quantity need not necessarily be identical to the cylinder internal pressure. On the contrary, it is sufficient to use a variable correlating with the cylinder internal pressure and to form a limit value which corresponds to that minimum cylinder internal pressure at which ignition and combustion of the fuel/air mixture is just still possible.

As soon as the characteristic quantity drops to the region of the limit value, the braking valves are moved in the direction of their closed position, so that less combustion chamber volume can escape into the exhaust-gas line and a further drop in the cylinder internal pressure is prevented. The limit value of the characteristic quantity depends on the type of internal combustion engine, the fuel-to-air ratio, the geometry of the combustion chamber, etc., and may be stored in a characteristics map as a function of said parameters and state variables and can be updated when a parameter or state variable changes.

Taken into account as an characteristic quantity is an engine state variable, in particular the engine speed, which can be monitored without problem and is measured anyway as a rule and is present as a numerical value in an engine control unit for controlling the engine. It is normally easier to determine the engine speed than the cylinder internal pressure. Assigned to the engine state variable as limit value is a minimum engine speed which corresponds to the minimum cylinder internal pressure which is necessary for maintaining combustion. If the engine speed drops to 4 th egion of the limit value, the outflowing exhaust-gas mass flow is reduced, in particular by reducing the valve stroke of the braking valves.

It may be expedient, despite continuous engine braking operation, to completely close the braking valves in order to prevent the combustion from failing. Complete closing is advisable in the case of braking systems having braking valves of constant stroke for increasing the cylinder internal pressure. During a change in the parameters or state variables influencing the combustion, in particular during an increasing engine speed, closing of the braking valves may be counteracted again or the valve stroke may be increased again. In the case of continuously adjustable braking valves, the engine state variable to be observed is regulated to the limit value in order to maintain the combustion, the valve stroke being varied as a function of further parameters such as coasting, gradient, etc., at constant speed.

It may be expedient to provide a blow-off device in the internal combustion engine, the blow-off device forming an adjustable bypass to the exhaustgas turbine. To reduce the exhaust-gas back pressure and to reduce the boost pressure, the bypass may be opened in order to avoid component overloads.

In order to provide maximum engine braking power in emergency cases in the shortest possible time, the variable turbine geometry is advantageously shifted into its stemming position corresponding to the braking power maximum, the braking valves are opened to the maximum extent, and in addition the maximum possible fuel quantity is burned. The result of this is that a very high quantity of energy is produced in the cylinders and ejected into the exhaust-gas line, and this quantity of energy brings the turbine to a higher speed in order to increase the supercharger output. In this situation, the maximum permissible cylinder internal pressures are advantageously subordinate to the provision of maximum braking power. In particular, any blow-off device present, despite the risk of component overloads, is put into the closed position for the benefit of a rapid build-up of pressure.

Further advantages and expedient embodiments can be gathered from the further claims, the description of the figure, and the drawing, which shows a schematic representation of a supercharged internal combustion engine with variable turbine geometry.

The internal combustion engine 1 of a motor vehicle, for example a diesel internal combustion engine of a utility vehicle, has an exhaust-gas turbocharger 3, iich comprises a compressor 10, arranged in the intake duct 8 of the internal combustion engine and intended for producing an increased boost pressure at the cylinder inlet, and a turbine 11 in the exhaust-gas line 9. The turbine 11, which is driven by the exhaust-gas flow of the internal combustion engine, is provided with a variable turbine geometry, which is adjustable between an open position with maximum turbine inlet cross section and a stemming position with minimum turbine inlet cross section. In the fired drive operating mode, the variable turbine geometry, as a rule, is in the open position in order to permit as large a mass flow rate through the turbine as possible and to produce a high supercharger output. In engine braking operation, the variable turbine geometry, as a rule, is in the closed position, so that the exhaust-gas back pressure increases considerably and the combustion chamber volume, in order to produce engine braking power, has to be exhausted via opened braking valves 20 against the high exhaust-gas back pressure.

The variable turbine geometry may be realized by rotary vanes, by an axially displaceable guide cascade or by other vario-turbine types, for example by flap turbines having an entry which points towards the turbine wheel and can be shut off via a flap.

Also provided is a blow-off device 25, via which exhaust gas, in order to reduce the exhaust-gas back pressure, can be diverted upstream of the turbine 11 and can be directed into the exhaust-gas line again downstream of the turbine 11. The blow-off device 25 is provided with a controllable valve. The variable turbine geometry and/or the blow-off device 25 are/is set via a regulating element 12 to the desired cross section of flow or to the quantity of exhaust gas to be diverted. The blow-off device 25 is able to ensure that maximum pressures, which may lead to destruction of components, are not exceeded. In particular, the boost pressure in the intake duct and the exhaust-gas back pressure in the exhaust-gas line upstream of the turbine 11 are monitored.

Via a clutch 13, the crankshaft 16 of the internal combustion engine 1 is connected to a manual change-speed transmission 2 connected on the output side. A first clutch disc 14 of the clutch 13 is connected in a rotationally locked manner to the crankshaft 16, and a second clutch disc 15 is coupled in a rotationally locked manner to a transmission drive shaft 17 of the transmission 2. The drive torque produced during forward drive operation is transmitted to the transmission output shaft 18 via the crankshaft 16 and the transmission drive shaft 17.

6 Furthermore, an engine control 4 is provided for controlling the driving and operating states of the internal combustion engine 1, the exhaust-gas turbocharger 3 and if need be the transmission 2. Assigned to the engine control 4 are a plurality of processing units 5, 6, 7, one each for controlling injection nozzles 19, for the braking valves 20, via which the cylinders of the internal combustion engine corrimunicate with the exhaust-gas line 9 in addition to the exhaust valves, and for the regulating element 12 of the exhaust-gas turbocharger 3. In the engine control 4 having the processing units 5, 6, 7, control signals are produced as a function of input signals which represent parameters or operating states of the vehicle, and these control signals are transmitted via signal lines 21 to 24 for applying to the respective vehicle components or their associated regulating elements.

The braking valves used may be decompression valves, which are desicmed to be separate from the exhaust valves and via which the cylinders communicate with the exhaust-gas line. The braking valves may be designed as valves having a constant stroke or as valves which are variable in stroke. However, it may also be expedient to use the exhaust valves themselves as braking valves by virtue of the fact that, unlike the opening cycle of the fired operation, the exhaust valves are opened continuously during the shift operation in engine braking operation.

For a rapid build-up of engine braking power, provision is made for the braking valves 20 to be put into the open position and for the variable turbine geometry of the turbine 11 to be put into the stemming position and at the same time for fuel to be injected via the injection nozzles 19 and ignited in the compression phase before top dead centre. The piston must perform compression work against the expanding mixture in the combustion chamber; in addition, the exhaust-gas back pressure is increased and in particular the turbine output is rapidly increased.

In the motor control 4, the engine speed is monitored, and the braking valves 20 are moved in the direction of their closed position if the engine speed drops below a minimum speed. In the case of braking valves having a variable stroke, the valve stroke is reduced until the cylinder internal pressure is above the limit required for ignition and combustion of the fuel/air mixture, whereupon the speed increases again above the critical speed limit. Braking valves which are designed as constant choke valves are closed. The monitoring or control of the speed is technically simpler to realize than the monitoring and control of the cylinder internal pressure. The setting of the blow-off device 25 may be included in the control.

7 In order to provide maximum braking power in emergency cases in the shortest time, the braking valves are opened to the maximum extent with variable turbine geometry in stemming position, and a maximum possible fuel quantity is injected and burned. At the same time, the blow-off device 25 is closed in order to build up as high an exhaust-gas back pressure as possible.

8 Clabw 1. An engine braking method for a supercharged internal combustion engine having a turbocharger, the exhaust-gas turbine of which has a variable turbine geometry, which geometry is adjustable for the variable setting of the effective turbine cross section between a stemming position and an open position, the internal combustion engine being at least partly fired during engine braking operation, wherein braking valves at the cylinder outlet of the internal combustion engine are in the open position above a limit value of an engine state variable, and the braking valves for reducing the exhaust-gas mass flow diverted firom the cylinders are put into a closed position when the engine state variable drops to the region of the limit value.

Claims (1)

1. 2. An engine braking method according to Claim 1, wherein the valve stroke

   of the braking valves is reduced when the engine state variable drops to the region of the limit value.

   3. An engine braking method according to Claim 1 or 2, wherein the braking valves are completely closed when the engine state variable drops to the region of the limit value.

   4. An engine braking method according to any one of Claims 1 to 3, wherein the engine state variable is the engine speed and the limit value is a minimum engine speed.

   5. An engine braking method according to any one of Claims 1 to 4, 0 wherein a blow-off device for the adjustable diverting of the exhaust gas is provided upstream of the turbine, and the blow-off device is moved from the open position into the closed position.

   6. An engine braking method according to any one of Claims 1 to 5, wherein, in the event of emergency braking, the variable turbine geometry is shifted into the stemming position, the braking valves are opened to the maximum extent, and a maximum possible fuel quantity is injected and burned.

   9 7 An engine braking method for a supercharged internal combustion engine, substantially as described herein with reference to, and as illustrated in, the accompanying drawings.