You will find the figures mentioned in this article in the German issue of
MTZ 09|2006 beginning on page 636.
Der Direktstart –
vom Modell zum Demonstrator
Direct Start
From Model to
Demo Vehicle
To start an engine with the use of nothing but combustion energy – the
realization of this notion comprised the main theme of a project at the
corporate sector research and advance engineering of Robert Bosch
GmbH. With thermodynamic investigations as a starting point, Etas
tools provided the power needed to demonstrate and analyse several
different starting modes within a short period of time in a vehicle.
1 Introduction
Authors:
André Kulzer, Jochen Laubender,
Ulrich Lauff, David Mößner and
Udo Sieber
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MTZ 09|2006 Volume 67
Without the need to turn the crankshaft, direct-injection gasoline engines are capable
in principle of injecting fuel into the combustion chamber, triggering ignition, and
performing a direct start. What are the prerequisites for this so-called direct start procedure? When does combustion occur? Does
the process generate sufficient energy to rotate the engine and possibly achieve a reliable direct start? At Robert Bosch GmbH,
these and other questions formed the basis
for a research project aimed at direct-starting a gasoline engine by means of strategically timed combustions.
Researchers began by looking at the starting sequence depicted in the diagram in
Figure 1: First, the camshaft position determines an engine cylinder suitable for the
initial combustion. This cylinder’s intake and
exhaust valves are closed, and its piston is
positioned past TDC, forming a fully enclosed combustion chamber, to which fuel is
introduced by means of the high-pressure
injector. After a brief air-fuel mixture formation phase, the spark plug provides ignition.
The subsequent expansion of combustion
gases drives the piston downward, and engine rotation commences.
To enable direct-starting, a number of
prerequisites must be met. These can be
accurately visualized with the use of a thermodynamic simulation model. In this way,
researchers gain an understanding of the
complex interaction of direct start processes
– findings that become the basis for dedicated and ongoing direct start development.
The simulations produced sufficient information for the development of direct start
specific engine control functions. Using the
Ascet development environment, these were
implemented on an experimentation system.
In normal operation, engine management
was handled by a Bosch Motronic MED7.5
production ECU. This approach facilitated the
successful in-car implementation of several
starting modes within a relatively short time.
2 Direct Start Thermodynamics
The direct-starting process comprises an innovative and low-emission method for starting direct-injection gasoline engines that is
both quick and quiet. In the ideal case, the
engine is started without the support of a
starter motor or any other kind of electric
motor, simply by means of strategically
placed combustions [1]. A differentiation is
made between three direct-starting modes.
The conventional direct start (the first ignition in the working stroke rotates the engine in its forward direction), the so-called
extended direct start (the first ignition of
the working stroke initially rotates the engine in reverse direction to ensure precompression, Figure 2), and the starter assisted
direct start (the starter replaces the initial
direct start combustion). Bosch markets the
third variant under the shortening designation “direct start”.
Successful direct-starting is subject to a
set of prerequisites: To enable the generation of the required combustion energy, the
combustion chambers, which are closed upon commencement of direct-starting, must
already contain fresh air. The air charge depends directly on piston position, residual
gas contents, and thermodynamic conditions. Therefore, one essential prerequisite
for a successful direct start is the piston position after the engine stall (end-of stall position). The other is flushing the cylinder with
fresh air during the engine shutdown – or
engine stall – phase.
An engine model provided the basis for
the systematic optimization of direct-starting procedures [2]. The expanding combustion gases in the combustion chamber exert
downward pressure on the piston. As it travels downward, the piston transfers a torque
value to the crankshaft drive. The torque value actually occurring at the crankshaft can
be established by means of a torque analysis,
which is essentially determined by the internal torque, i.e., the torque derived from the
combustion pressure of each cylinder, the
friction coefficient, and the overall engine
inertia. By twice integrating the acceleration
α derived from the torque analysis, the engine speed (RPM) ω is calculated, which in
turn allows the determination of the angle
of engine rotation ϕ that occurs as a result
of the start, as shown in Figure 3. In order to
fine-tune the direct start under the most varied circumstances, the underlying theory
Engine Management
DEVELOPMENT
governing the thermodynamic processes occurring in the combustion chamber during
the starting phase were investigated. This
was accomplished by means of a customdeveloped simulation model, which accounts
for the influences of A/F mixture quality,
residual gas, turbulence, combustion, cylinder wall temperature, leakage, and gas exchange process, as shown in Figure 4.
The innovative component of this model
consists of the extension of a quasi-dimensional combustion model to also include the
conditions of igniting a stratified charge at
zero engine speed, and those of the additional direct start specific combustion modes.
Because of the presence of two temperature
zones (two-zone model view) and two mixture zones, this extended quasi-dimensional
combustion model is called 2/2 zone model,
Figure 5. The extended quasi-dimensional
combustion model facilitates the simulation of the combustion of a stratified charge
by means of two charge zones. One is a homogeneous air-fuel mixture zone (e.g., with
λ ~ 1, where combustion can take place).
The other is an air zone that functions as
an inert gas buffer. In this way, the mixture
formation and combustion conditions – especially of the first working stroke at zero
engine speed – are captured with greater accuracy. Assuming the presence of a homogeneous mixture, the remaining direct start
combustion modes can be easily simulated
by means of the conventional quasi-dimensional combustion model.
The special properties of this simulation
tool comprise the realistic visualization of
the initial start phase – engine hesitation
upon direct start initialization and initial
crankshaft revolutions. The model’s outstanding predictive capacities are confirmed
by the highly correspondent data of measured and simulated combustion, pressure,
and engine speed characteristics. The entire
model was verified by means of suitable
analyses on a production test vehicle (1.4 l DI
engine). Figures 6 and Figure 7 depict engine
speed characteristics confirming that the
simulation is well capable of mapping behaviour patterns at the point of engine start.
As such, it provides for the dedicated development and assessment of direct start
modalities.
an ES1000 experimental system with an
Ethernet connection to the Ascet laptop
computer. Because the modular Etas
ES1000 VME system accepts a variety of VME
cards, it can be ideally adapted to the requirements of a given testing task, Figure 8.
3 Test Setup for Direct Start
Experimentation
With the aid of the Ascet development tool
for electronic control units (ECUs) by Etas,
the control functions governing direct-starting were modelled in the form of a state machine. In-vehicle testing of the model used
3.1 Experimental System
Part of the ES1000 system, the ES1130 Simulation/System Controller Board is programmed in Ascet with the state machine
code generated by Ascet. On the basis of the
camshaft angle, the model on the ES1130
board uses a 1-ms time frame to calculate
the ignition and injection signals for the initial four working strokes, effectively controlling the entire engine start-up procedure.
A 9-bit sensor directly coupled with the
camshaft reads the absolute camshaft in
one-degree increments. The camshaft angle
is presented at the parallel sensor output
in Gray encoded form. The signal is picked
up by the ES1320 Digital I/O Board of the
ES1000 experimental system. To accomplish
this, the camshaft signal level is adapted by
means of a specially developed conversion
board that shares a separate housing with
the control signal switch (see below).
The signals controlling injection and ignition are generated by the ES1330 PWM I/O
Counter Board of the ES1000 system. In order to enable the aforementioned controls
by means of individual pulses instead of
standard PWM signals, a suitable software
driver to enable the ES1330 board from
within Ascet was developed in-house.
The pulses are delivered to the output
modules of ignition coils and injection valves
by means of a specially developed control
signal switch. After the successful completion of the first four working strokes, the
switch causes the control of injection and ignition to be redirected to a Motronic MED7.5
engine control unit. The signal switchover is
triggered by the state machine by means of
an additional output signal that is generated by the ES1330 board.
3.2 Motronic with ETK Bypass
The MED7 control unit uses the initial four
working strokes to synchronize itself. It then
assumes control of injection and ignition.
Due to the fact that the MED7 control unit
already contains an injector output module,
the control signals for this output module
were deliberately applied to unused pins on
the ECU connector. To effect a comfortable
interruption of the existing cable harness
connection for injection and ignition signals, the ECU was connected by means of a
breakout adapter instead of connecting
directly to the cable harness. The MED7 conMTZ 09|2006 Volume 67
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DEVELOPMENT
Engine Management
trol unit was interfaced to the ES1232 ETK
Interface Board of the ES1000 system through
an Etas ETK memory emulator. The use of
the ETK connection enables the MED7 control unit to provide sensor signals as input
signals for the ES1000 system. The signals
thus transmitted comprise those that are
critical to the starting procedure – such as
engine temperature or rail pressure – on the
one hand. On the other hand, they convey
variables crucial to the control of the engine
stall, such as engine speed and intake manifold pressure.
The Ascet model on the ES1130 Simulation/System Controller Board makes the signals controlling the engine stall – such as
the required throttle valve angle or the engine speed required at the point of engine
shutdown – available to the MED7 controller
via the ETK connection.
The so-called ETK bypass enables the Ascet
model to read and write control unit signals in real time. In the bypass test setup under discussion, a 10-ms and an ignition-synchronous time frame were used.
conventional direct start using a single lowpressure injection does not offer 100-% starting reliability, and that the engine is no
longer direct start capable at moderate engine temperatures of Tmot>40 °C. By contrast, the introduction of multiple highpressure injections significantly improves
the starting reliability, and the temperature range for successful engine starts can
be extended upward from 20 °C up to 60 °C.
The main reason for the sharp drop of starting probability is the insufficient air mass
in the cylinders, this being a consequence of
the reduction of air density in the presence
of rising engine temperatures. The extended direct start produced decidedly better results. In this case, the engine started with
unfailing reliability after a single low-pressure injection at engine temperatures up to
60 °C. At higher temperatures however, successful direct-starting is impeded for the
same reasons as with the conventional direct start. A significantly improved mixture
formation is achieved through precompression of the cylinder charge during the working stroke, along with the attendant temperature increase of the captive air mass in
the combustion chamber. This produces a
greater combustion force and decidedly
greater starting reliability. On this particular
test vehicle however, engine start remained
largely unsuccessful with both injection
variants at engine temperatures upward of
Tmot>90 °C. These failures notwithstanding, successful direct starts were sporadically achieved at engine temperatures of
Tmot>100 °C, albeit with little reproducibility. In short, test engineers had good cause to
widen their investigations to include other
combustion methods and different injector
types. At this point, as some of the findings
thus obtained have already been published,
making reference to the bibliography [4]
shall suffice.
Quick and reliable engine direct starts
are ensured through a minimum of starter
motor assistance. Initial tests in this investigation have shown that this starting mode
facilitates dependable engine starts, even at
engine temperatures above 100 °C. However,
the trade-off for the starting stability comes
at a price – the decidedly louder starting
noises represent a significant loss in terms
of operator comfort, Figure 9. An analysis of
the inherent opportunities and limitations
of this starting mode was the subject of a
separate study [3].
3.3 Inca
The Inca measuring and calibration system
by Etas was used in the calibration of engine
control parameters. For precisely these
tasks, Inca also supports the ES1000 system,
with the Ethernet connection to the Inca
host PC interfaced through the ES1120 System Controller Card. The use of the Inca-EIP
add-on enables not only the simultaneous
precalibration of both the Ascet model on
the ES1130 board and the engine control
unit but also the synchronized acquisition
of variables from both model and control
unit. Whenever required, Inca was also run
concurrent with Ascet on the same laptop
computer.
4 Test Results
The following sections describe the findings
obtained in the investigation of both conventional and extended direct-starting modes.
4.1 Starting Modes
The fuel supply and injection system of the
test vehicle corresponded to the first-generation production version of gasoline direct injection featuring high-pressure swirl injectors. A variety of starting probability test
series investigated direct-starting characteristics based on an optimum 100-degree crankshaft position. This included single injections
at low pressure as well as multiple high-pressure injections within an engine temperature
range between 20 °C and 100 °C. The studies
have shown that, on this test vehicle, the
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MTZ 09|2006 Volume 67
4.2 Manipulated Engine Stall
The mandatory prerequisite for a successful
direct start is a defined engine stall. The objective is to position the engine, upon com-
pletion of its stall, at a crank angle position
that favours direct-starting, and to flush the
cylinders with a sufficient volume of fresh
air. The framework of the investigation also
included the development of a control algorithm that enables the engine to reach the
desired end-of-stall position in a manner both
reliable and repeatable, while taking into
account all possible interference variables.
Because the attendant development process
also focused on the factors of cost neutrality
and lowest tolerable loss of comfort, engineers
had only the existing engine-specific actors
and sensors to fall back on. The throttle valve
was therefore pressed into service as the main
actor controlling the engine stall. Manipulations of pressure build-up and pressure drops
in the intake manifold were introduced with
a view to achieving dedicated engine acceleration and deceleration, respectively.
Each direct start in a conventional startstop system is preceded by an engine stall
phase that is divided into three segments:
The initialization phase with stationary idle
is followed by an end-of-rotation phase and
oscillation phase, after which the engine finally comes to a standstill, Figure 10. As each
segment of the engine stall phase progresses, the number of possible variables decreases. Constituting the essential input variables
of the control plant, engine speed, and manifold pressure were identified at the end of
both initialization phase and end-of-rotation
phase (marked with red arrows in Figure 10).
The oscillation phase no longer supplies any
effective variables that would provide options for dedicated engine control. Simply
put, the final system standstill occurs as a
function of the degradation of rotational energy, the cushioning effect of the cylinder
charge, and due to friction and leakage losses on valve seats, valve guides, and piston
gaps. The friction component comprises both
sliding and static proportions. Because the
influence of both is subject to large fluctuations at low engine speeds, it is very difficult
to handle with a single control algorithm.
Figure 11 shows an example of the frequency
distribution of the resulting end-of-stall positions in a 159-event test series with controlled engine stalls. It is apparent that there
is a relatively wide distribution centred
around a mean position at a crank angle of
approx. 105 degrees. Throughout this test series, only 60 % of events are located within
the direct start suitable crank angle range of
100 to 105 degrees. This proportion can be
further increased to 85 % by improving the
control algorithm. However, this would
mean that, based only on the end-of-stall position, 15 % of all direct start attempts would
fail. It is also possible that the engine stand-
still occurs at the top or bottom dead centre.
An engine start from this position is not
possible without starter motor assistance.
The reason for the inadequate engine stall
repeatability thus demonstrated lies in a diversity of interference variables, such as
changes in engine oil viscosity and friction.
Beyond this, these interference factors,
whose reliable acquisition eludes even adaptive control algorithms, are also subject to
manufacturing tolerances in engine production. Accordingly, this constitutes yet another hurdle to migrate a control algorithm developed on a specific sample engine to series
production.
In the event of cylinder prepositioning or
postpositioning during the engine stall
phase and/or after engine standstill – e.g., by
means of an electric motor located in the engine’s belt drive system or drive train – the
system’s starting reliability depends exclusively on the thermodynamic conditions of
the direct start. In this case, the drop in air
density as a by-product of higher engine
temperatures and altitudes represents the
most formidable obstacle.
4.3 In-vehicle Application / Demonstrator
Once the investigations had progressed beyond simulation and testing, the development objective shifted to system operation
as an independent in-vehicle installation. As
a result, a variety of powertrain concepts
providing starter assisted direct starts in
combination with a start-stop system were
implemented on two vehicles.
With a view to ensuring independent driving operation, the first test vehicle equipped
with automated manual shift transmission
(auto shift gearbox, ASG) was optimized
while retaining all degrees of freedom
achieved up to that point. The variability of
the system facilitated the in-vehicle implementation of a more comprehensive means
of controlling a specially developed starter
motor, the “smart starter motor”. The main
focus of these starter applications were the
start-stop relevant issues of starter noise and
pinion engagement duration. Applying a
simplified start-stop strategy, the response
to a given driving situation was to stop and
restart the engine by means of the brake
pedal. As the measured starting intervals fell
far short of the ASG shift intervals required
to initiate vehicle motion, they did not impede driving comfort in any way.
The second test vehicle was equipped with
a manual transmission, serving to support
the declared premise of providing its driver
with a vehicle whose modifications were
anything but obvious. The focus was therefore not on system variability but on the im-
plementation of a start-stop demonstrator
engaged in real-world driving operations.
With this design objective, the system was
further developed to include the specified
combination of modified conventional
starter, starter assisted direct start, and a
supplied start-stop controller. To enable integration in the overall system, both Ascet
model and ES1000 system were enhanced by
means of a CAN connection. The engine’s instant re-ignition function during the engine
stall phase – e.g., as a result of a sudden start
demand (“change of mind“) during the endof-rotation phase – was integrated in the
Ascet model. The resulting engine output
during start-stop operation ensured in this
manner facilitates the immediate execution
of driver intent. In order to guarantee the
self-sufficient operation of the system, a
bootable code containing defined control
parameterization was stored on the ES1000
system. This code is enabled instantly upon
system start-up and provides for independent system operation. On both vehicles, it
was thus demonstrated that the starter assisted direct start in combination with optimized starter intervention and an intuitive
start-stop strategy represents a cost efficient
building block of a start-stop system.
5 Summary
The preceding article introduces the results
of a systematic analysis of the thermodynamic, mechatronic, and physical prerequisites
for direct starts on a 1.4 l gasoline engine
featuring first-generation direct injection
with the help of the software development
tool Ascet and the experimental system
ES1000. The investigation has demonstrated
the possibility of starting an internal combustion engine, from total standstill, without the use of a starter motor but only within a specific temperature range, merely by
means of injecting fuel and igniting the
same. The demonstrated algorithms provide
for quick and silent engine starts which increase the acceptance of a conventional
start-stop system in terms of comfort and
rapid starting significantly. Due to physical
and thermodynamic conditions, the demands imposed on starting reliability across
the entire range of engine temperatures
cannot be fulfilled without the use of additional motoric assistance. The investigation
identified the limited controllability of the
engine stall toward a defined end-of-rotation
position as one constraint factor. Another
limitation is imposed by the insufficient
combustion force caused by the insufficient
air charge density resulting from high engine temperatures. In this context, and be-
cause of improved mixture formation in the
limit range at high engine temperatures,
second-generation gasoline direct-injection
technology still holds significant potential
for improvement. This aspect is the subject
of current investigation.
6 Outlook
Bosch Gasoline Systems Division is currently developing a start-stop system based on
the direct start with minimal starter motor
assistance to the point of series production
readiness. Thanks to the brief starter intervention during the initial compression
phase, absolute starting reliability is guaranteed [3]. In this case, the starting procedure
is independent of the end-of-rotation position and the previously discussed interference variables. The overall system comprises
an engine speed sensor with rotational direction sensing, an extended-service electric
starter, and a battery sensor to monitor the
charging equilibrium in start-stop operation.
The starter was specially modified to reduce
the engage and starting noises, and thus delivers a high degree of starting comfort. The
deployment of the start-stop system facilitates fuel savings of 3 to 5 % over the New
European Driving Cycle (NEDC). The system
is characterized by an excellent cost-benefit
ratio, and supports automobile manufacturers with their efforts to comply with ACEA’s
Commitment on CO Emissions taking effect
in 2008.
Acknowledgements
The core team engaged in the Direct Start research project has received essential support
and valuable contributions from a number of
colleagues belonging to a variety of engineering teams within the Bosch Group. The authors
therefore wish to take this opportunity to express their heartfelt appreciation to all of their
colleagues.
References
[1] Gerhardt, J.; Kassner, U.; Kulzer, A.; Sieber, U.: Der
Ottomotor mit Direkteinspritzung und Direktstart –
Möglichkeiten und Grenzen. 24. Internationales
Wiener Motorensymposium 2003
[2] Kulzer, A.: BDE-Direktstart – Startoptimierung eines
Ottomotors mit Direkteinspritzung mittels eines
thermodynamischen Motorsimulationsmodells.
Universität Stuttgart, Dissertation, 2004
[3] Laubender, J.; Kassner, U.; Hartmann, S.; Heyers, K.;
Benninger, K.; Gerhardt, J.: Vom Direktstart zum
marktattraktiven Start-Stopp-System. 14. Aachener
Kolloquium Fahrzeug- und Motorentechnik 2005
[4] Alt, M.; Blattmann, T.; Bošnjak, J.; Laubender, J.;
Gerhardt, J.: Untersuchungen zum Direktstart eines
Ottomotors. 10. Tagung „Der Arbeitsprozess des
Verbrennungsmotors“, Graz, 2005
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