Field of the invention
[0001] The present invention relates to the field of methods and systems for managing the
activation of an engine brake in a supercharged Diesel engine by means of a turbocharger
equipped with a variable geometry turbine and a throttling valve arranged on the exhaust
manifold.
[0002] More specifically, the invention concerns a diesel engine equipped with an engine
brake operated by varying the opening times of the exhaust valves.
State of the art
[0003] In internal combustion engines intended for commercial and industrial vehicles, the
functionality of the engine brake is foreseen, named with the English expression.
In this operating condition, the engine is driven by the inertia of the vehicle, fuel
injection is inhibited and at the same time a strategy is implemented that dissipates
energy through the compression and expulsion of gas in the engine cylinders. Various
strategies are known for operating engine brake.
[0004] In heavy road vehicles equipped with a Diesel engine, when the accelerator pedal
is released while the vehicle is moving, the vehicle inertia continues to drive the
engine crankshaft, so that a considerable mass of air is sucked in and compressed
in the cylinders.
[0005] The compressed air during the compression stroke acts as an air spring, meaning that
the energy accumulated during compression is at least partially returned to the crankshaft
by pushing the pistons back.
[0006] When the engine brake function is activated, the profile of the cams that control
the opening of the exhaust valves is varied to release the compressed air in the exhaust
manifold before it can return the previously accumulated compression energy.
[0007] When used correctly, engine brake functionality can help a multi-ton vehicle to maintain
or even reduce speed with minimal use of the service brakes. This fact is crucial
especially on prolonged descents, where the vehicle brakes can be stressed to the
point of losing efficiency due to overheating.
[0008] In the engine brake function, the braking power depends on the mass of fresh, and
possibly exhausted, air, which is compressed and decompressed during engine brake
activation. Supercharged engines can exploit the presence of the turbocharger to further
increase the braking torque generated by the engine under engine brake conditions.
[0009] Indeed, the braking power of the engine brake depends both on the back pressure at
the exhaust manifold and on the boost pressure at the intake manifold.
[0010] In particular, to increase braking power it is necessary to increase the boost pressure,
and therefore the back pressure at the exhaust manifold. It is immediate to understand
that the increase in supercharging pressure allows the introduction of a greater mass
of air to be compressed into the cylinders.
[0011] The regulation of the back pressure at the exhaust manifold is carried out by means
of a throttling valve located on the exhaust manifold, generally upstream of the turbine.
[0012] This throttling valve allows to indirectly regulate the mass of air circulating in
the intake manifold and exhaust manifold.
[0013] In other words, braking power is a function of back pressure at the exhaust manifold.
[0014] The back paressure is influenced by the position of the turbine mobile blades and
the position of the throttling valve.
[0015] The moving blades of the variable turbine geometry affect the mass of gas driving
the turbine wheel, which, in turn, increases or decreases the boost pressure at the
intake manifold.
[0016] The throttling valve, mostly used in Diesel engines, allows the pumping work of the
engine to be increased, worsening its efficiency. When the engine is active, in the
sense that it burns fuel, the throttling valve allows to adjust the enthalpy of the
exhaust gases to regulate the temperature of the exhaust gas post-treatment system.
[0017] When the engine brake function is activated and a predetermined braking torque is
requested, the variable geometry, commonly appealed as "VGT", and the throttling valve,
commonly named "Exhaust Flap", assume predetermined positions in a look up table according
to the amount of braking torque request.
[0018] However, while the position of the VGT is designed to meet the required braking power,
the position of the flap is designed to prevent the turbocharger from exceeding a
predetermined rotational speed value.
[0019] During engine brake operation, the Flap has the task of limiting the enthalpy of
the gas flow that passes through the turbine and therefore the rotation speed of the
turbocharger.
[0020] During the calibration process the operator has the task of determining which the
positions of the VGT and the flap are to produce the required braking torque, maintaining
a rotation speed of the turbocharger lower than a limit rotation speed.
[0021] Unfortunately, the rotation speed of the engine and the temperature of the gases
at the exhaust manifold have a significant impact and therefore, the calibration of
the operation of the VGT and flaps is very complex.
[0022] Due to this complexity, the VGT and flap control tables are conservative with a negative
impact on engine brake response. In other words, stability conditions can be reached
with a several seconds delay due to a calibration that aims at preserving the integrity
of the components.
Summary of the invention
[0023] The purpose of the present invention is to indicate a method of managing the activation
of the engine brake, in particular which includes the control of the VGT and the flap.
[0024] The basic idea of the present invention is to acquire a back pressure value at the
exhaust manifold, when the engine brake function is activated, and to operate two
controls in parallel, where
- a first process arranged to dynamically control the variable geometry as a function
of a second error given by the difference between the objective backpressure value
and the current backpressure value, according to a closed loop control,
- a second process arranged to control the throttling valve according to a function
inversely proportional to an error given by the difference between the current rotation
speed value and a limit rotation speed value of the turbine, according to a closed
loop control.
[0025] Preferably, the first process controls the VGT as a function of an error between
an acquired backpressure value and a measured backpressure value, thus realizing a
closed loop control.
[0026] Advantageously, the first process tries to pursue, in the shortest possible time,
the acquired back pressure value. Advantageously, the second process, being in closed
loop and a function inversely proportional to the Rpm error, in the first instants
of activation of the engine brake function, causes the flap to open completely, allowing
the gases exiting the engine to flow towards the turbine without obstacles.
[0027] Thanks to the present invention, two benefits are obtained:
- Engine brake response is quicker,
- It is no longer necessary to calibrate the operation of the VGT and the flap, but
it remains only necessary to calibrate the gains of the closed loop controls described
above of the VGT and of the flap.
[0028] In particular, the controls are performed in closed loop and, therefore, insensitive
to variations in the operating conditions of the internal combustion engine.
[0029] The invention includes the acquisition of the absolute pressure value at the exhaust
manifold and the rotation speed of the turbocharger using a sensor, which is generally
integrated into the VGT.
[0030] Preferably, the temperature of the gases at the exhaust manifold is also acquired
as it is useful for increasing the precision of the braking torque/back pressure conversion
during transients.
[0031] When the required braking torque is relatively low, the turbine wheel accelerates
to a rotation speed far from the limit value. In this case, the flap can remain fully
open. When the required braking torque is relatively high, the same first control
adjusts the VGT so as not to exceed the required back pressure.
[0032] Preferably, the inverse function of the second control is non-linear. More preferably,
the inverse function of the second control is parabolic, so as to obtain a flap closure
relationship that is more rapid the closer the turbine wheel approaches the relative
limit rotation speed.
[0033] When the flap intervenes to limit the rotation speed of the turbocharger wheel, the
back pressure is given by the sum of the two contributions, i.e. the VGT and the flap.
[0034] This means that the first control, by regulating the VGT according to the error between
the acquired backpressure value and the measured backpressure value, automatically
tends to open the blades, so as to compensate for the backpressure contribution given
by the flap.
[0035] To avoid triggering control instability phenomena in stationary conditions, it is
preferable for the first control to be faster than the second control.
[0036] Preferably, the second control is supported by a third control arranged to control
the closing of the flap when the flow rate of gas exiting the internal combustion
engine is such as to trigger pumping phenomena (surge) in the turbocharger. Indeed,
turbocharger manufacturers provide corresponding optimal operation tables with specified
pumping areas. When the operating conditions are such as to trigger pumping phenomena,
then the back pressure is completely entrusted to the closing of the flap rather than
to the adjustment of the blades of the turbocharger turbine. The present solution
allows the desired and acquired backpressure value to be obtained as quickly as possible
without the risk that the turbocharger wheel may exceed the limit rotation speed.
[0037] The dependent claims describe preferred variants of the invention, forming an integral
part of the present description.
Brief description of the figures
[0038] Further objects and advantages of the present invention will be clear from the following
detailed description of an example of its implementation (and its variants) and from
the attached drawings given purely for explanatory and non-limiting purposes, in which:
- Fig. 1 shows an example of a control scheme for implementing the method of the present
invention;
- Fig. 2a shows a time diagram in which a comparison is represented between a trend
of the objective braking torque (wide hatching), a trend of the braking torque obtained
according to a control scheme of the known art (fine hatching) and a trend of the
torque braking obtained through the control scheme (continuous section) of Fig. 1;
- Fig. 2b shows a time diagram in which a comparison is represented between a trend
of the objective back pressure (broad hatching), a trend of the back pressure obtained
according to a control scheme of the known art (fine hatching) and a trend of the
back pressure obtained by the control scheme (continuous line) of figure 1;
- Fig. 2c shows a time diagram in which a comparison is represented between a speed
trend of the turbocharger wheel according to a control scheme of the prior art (fine
hatching) according to the control scheme (continuous line) of figure 1;
- Fig. 2d shows a time diagram in which a comparison is represented between a trend
in the temperature of the gases at the exhaust manifold according to a control scheme
of the prior art (fine hatching) and according to the control scheme (continuous line)
of the figure 1;
- Fig. 2e shows a time diagram in which a comparison is represented between a trend
of the geometry of the movable blades of the turbine of the turbocharger according
to a control scheme of the known art (fine hatching) and according to the control
scheme (continuous line) of Figure 1,
- Fig. 2f shows a time diagram in which a comparison is represented between a trend
of the position of the flap arranged on the exhaust manifold according to a control
scheme of the known art (fine hatching) and according to the control scheme of (continuous
line) Figure 1;
- Fig 3 shows an internal combustion engine equipped with means to perform the engine
brake function, a variable geometry turbocharger and a flap arranged on the exhaust
manifold, in which the engine is configured to implement the control scheme of the
figure 1.
[0039] The same reference numbers and letters in the figures identify the same elements
or components or functions.
[0040] It should also be noted that the terms "first", "second", "third", "higher", "lower"
and the like may be used here to distinguish various elements. These terms do not
imply a spatial, sequential, or hierarchical order for the modified elements unless
specifically indicated or inferred from the text.
[0041] The elements and characteristics illustrated in the different preferred embodiments,
including the drawings, can be combined with each other without departing from the
scope of protection of the present application as described below.
Detailed descritpion
[0042] The present invention finds application in the field of internal combustion engines
equipped with
- engine brake functionality via a system for varying the actuation profile VA of at
least the exhaust valves V,
- a turbocharger TB with VGT variable geometry turbine,
- a throttling valve FP, hereinafter referred to as "flap", arranged on the exhaust
line EM, preferably upstream of the turbocharger turbine according to the normal flow
of gases expelled from the internal combustion engine.
[0043] Fig. 3 schematizes an internal combustion engine E, preferably diesel cycle, implementing
the present invention. The exhaust manifold EM of the engine E comprises a pressure
sensor PS and preferably also a temperature sensor TS.
[0044] The drive profile variation system VA, the flap and the VGT are controlled by a processing
unit ECU, which receives various engine operating parameters as input, including the
exhaust manifold pressure via the pressure sensor PS, the rotation speed of the turbocharger
TB via a sensor integrated into the turbocharger itself and possibly the temperature
via the temperature sensor TS.
[0045] Fig. 1 shows a control scheme according to the present invention.
[0046] The inputs identify
in1: braking torque request,
in2: rotation speed of the internal combustion engine,
in3: gas temperature measured at the exhaust manifold,
in4: back pressure measured at the exhaust manifold,
in5: rotation speed of the turbocharger wheel.
[0047] The outputs identify
out1: target position of the VGT mobile blades,
out2: flap target position.
[0048] The block
Torque_to_bkpress indicates a model suitable for converting a target braking torque value into a target
backpressure value
in4tg at the exhaust manifold EM as a function of the rotation speed
in2 of the internal combustion engine and preferably also of the gas temperature measured
at the exhaust manifold in3.
[0049] The block
TRB_SP_LIM indicates a model suitable for identifying a target rotation speed value
in5tg of the turbocharger turbine, at least as a function of the rotation speed of the
internal combustion engine.
[0050] The block
CTRL1 represents a first control arranged to dynamically control the VGT in order to reach
the target backpressure value acquired by the block
Trq_to_bkpress. Preferably, the control CTRL1 is of the closed loop type arranged to operate on an
error given by the difference between the target backpressure acquired by the block
Trq_to_bkpress and the backpressure measured at the exhaust manifold by means, for example, the
sensor PS.
[0051] It is worth highlighting that the backpressure at the exhaust manifold can also be
estimated using a model that takes into account the rotation speed of the engine E,
the position of the flap and the position of the VGT.
[0052] The block CTRL2 represents a second control, arranged to dynamically control the
flap according to a function inversely proportional to an error given by the difference
between the rotation speed of the turbocharger acquired through a speed sensor and
a predetermined limit value of the rotation speed of the turbine, realizing therefore
a closed loop control.
[0053] The second control acquires the limit value of the rotation speed of the turbocharger
wheel from the block
TRB_SP_LIM and the value of the current speed of the wheel. The more the two values differ,
the more the flap is open. Conversely, the closer the two speed values are to each
other, the more the flap is closed.
[0054] Therefore, the action of the flap control is an inverse function of the error between
the two speed values. Preferably, the flap control action is achieved through an inversely
proportional function, which can have a linear or parabolic trend.
[0055] Fig. 2a shows a braking torque request related to the activation of the engine brake
through a curve with wide hatching indicating a negative step. A first curve with
fine hatching is shown which represents the trend of the braking torque according
to the flap and VGT control scheme according to the prior art. Furthermore, a second
curve with a continuous line is shown, which represents the trend of the braking torque
according to the present invention. It can be seen immediately that thanks to the
present invention, the response of the system is much faster, guaranteeing the immediate
delivery of the braking torque.
[0056] Fig. 2b shows the time trend of the back pressure, in particular the trend of the
target back pressure (broad hatching), the trend of the back pressure according to
the prior art scheme (fine hatching) and the trend of the back pressure thanks to
the present invention (continuous line).
[0057] Figures 2c, 2d, 2e and 2f respectively show the comparison of time diagrams according
to the known art (fine hatching) and according to the present invention (continuous
line) respectively of the rotation speed of the wheel, of the temperature measured
at the exhaust manifold, of the VGT position and the flap position.
[0058] According to a preferred variant of the invention, the system for varying the actuation
profile VA of at least one of the exhaust valves V, during the activation of the engine
brake function, provides four lifts, which identify:
- during the intake phase, lifting of an intake valve to suck-in fresh air from the
intake manifold IM, and simultaneous lifting, at least partially, of an exhaust valve
to also suck gas from the exhaust manifold EM;
- near the top dead center at the end of the compression phase, the exhaust valve is
raised to prevent the energy accumulated inside the cylinder during the compression
phase from being returned to the piston during the subsequent expansion phase,
- during the expansion phase, lifting of the exhaust valve to suck gas from the exhaust
manifold EM,
- near the top dead center at the end of the exhaust phase, the exhaust valve is raised
to prevent the energy accumulated inside the cylinder during the exhaust phase from
being returned to the piston during the subsequent intake phase. The present invention
can advantageously be carried out by means of a computer program, which includes coding
means for carrying out one or more steps of the method, when this program is executed
on a computer. It is therefore understood that the scope of protection extends to
said computer program and further to computer readable means comprising a recorded
message, said computer readable means comprising program coding means for carrying
out one or more steps of the method, when said program is run on a computer. Constructive
variations to the non-limiting example described are possible, without departing from
the scope of protection of the present invention, including all the equivalent embodiments
for a person skilled in the art, to the content of the claims.
[0059] From the above description, the person skilled in the art is able to realize the
object of the invention without introducing further construction details.
1. Method of managing an engine brake procedure in a diesel cycle engine (E), comprising
an exhaust manifold (EM) to which a throttling valve (FP), a variable geometry turbine
(VGT) of a turbocharger (TB) and a pressure sensor (PS) are attached, the method comprising:
- acquisition of
+ a first signal (in4tg) representing a target back pressure value at the exhaust manifold,
+ a second signal (in4) representing a current backpressure value at the exhaust manifold,
+ a third signal (in5tg) representing a limit rotation speed value of the turbine,
+ a quarter (in5) signal representing a current turbine rotation speed value,
- a first process (CTRL1) arranged to dynamically control the variable geometry as a function of a second
error given by the difference between the target backpressure value and the current
backpressure value, according to a closed loop control,
- a second process (CTRL2) arranged to control the throttling valve according to a
function inversely proportional to a first error given by the difference between the
current speed value and the limit speed value, according to a closed loop control.
2. The method according to claim 1, further comprising
- acquisition of a fifth signal (in1) representing a target braking torque value e
- a procedure (Trq_to_bkpress) for converting said braking torque target value into said backpressure target value.
3. Method according to any one of claims 1 - 2, further comprising
- acquisition of a sixth signal (in2) representing a rotation speed of the internal
combustion engine and a seventh signal (in3) representing a gas temperature measured at the exhaust manifold (EM) and
- a procedure (TRB_SP_LIM) for generating said third signal representing the limit rotation speed value of
the turbine as a function of said sixth signal and said seventh signal.
4. Method according to any of the preceding claims, further comprising
a third control arranged to control the closure of the flap when the flow rate of
gas exiting the internal combustion engine is such as to trigger pumping phenomena
(surge) in the turbocharger.
5. Diesel cycle internal combustion (E) engine, comprising
- a system (VA) for varying an actuation profile of at least one exhaust valve (V)
to carry out an engine brake procedure,
- an exhaust manifold (EM) on which are connected
+ a throttling valve (FP),
+ a variable geometry turbine (VGT) of a turbocharger (TB) and
+ a pressure sensor (PS),
- processing means (ECU) configured to control said throttling valve and said variable
geometry turbine, configured to
. acquire a signal representative of a braking torque and calculate a corresponding
target value of back pressure at the exhaust manifold,
. control the variable geometry to achieve said target backpressure value according
to a closed loop control scheme,
. control the throttling valve according to a function inversely proportional to a
first error given by the difference between the current speed value and the limit
speed value, according to a closed loop control scheme.
6. Engine according to claim 5, wherein said processing means (ECU) are configured to
perform all the steps of any one of claims 1 - 4.
7. Industrial or commercial vehicle comprising a diesel cycle internal combustion (E)
engine according to claim 5 or 6.