This is only a preview of the August 1995 issue of Silicon Chip. You can view 31 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Vifa JV-60 2-Way Bass Reflex Loudspeaker System":
Items relevant to "A Gain-Controlled Microphone Preamp":
Articles in this series:
Items relevant to "Build The Mighty-Mite Powered Loudspeaker":
Items relevant to "Computer Bits":
|
By JULIAN EDGAR
Electronic diesel
engine management
Just as electronics has brought great strides in
the performance & reliability of petrol engines,
the application of similar techniques to diesel
engines has led to improvements in fuel economy
& a reduction in pollution. Here we examine the
electronic control system used by Detroit Diesel
on some truck engines.
Diesel engines are widely used in
railway locomotives, ships, heavy
industry and trucks. Although of a
similar age to the spark internal combustion engine, the diesel engine has
not been widely adopted for use in
passenger cars. However, just as elec4 Silicon Chip
tronic engine management has been
widely adopted to improve the performance of cars, similar techniques
have been applied in diesel engines
for trucks, particularly those used for
long distance haulage. As a result,
typical electronically controlled diesel
engines are as much as 20% more fuel
efficient than their predecessors.
Although the basic design of petrol
and diesel engines is similar (both are
two or four-stroke designs which use
reciprocating pistons driving a crankshaft), a diesel engine does not ignite
its fuel charge by the use of a spark
plug. Instead, only air is compressed
on the compression stroke. The fuel
charge for the power stroke is accurately metered and pressurised by a fuel
injection pump, of which there is one
for each cylinder. It then passes to the
high pressure injector and is sprayed
into the combustion chamber, where
it mixes with the hot compressed air
and self-ignites.
In a petrol engine (even one using
fuel injection), the fuel and air are
mixed prior to entry to the combustion
chamber. The fuel/air mix is drawn
into the cylinder on the intake stroke
and so the injectors or carburettor need
only mix the fuel and air at close to
ambient air pressures. On the other
hand, diesel injectors must operate
at pressures of over 20MPa (3000 psi)
and inject minute quantities of fuel at
a rate of 2.5-25Hz.
In order that the air in the diesel’s
cylinders becomes hot enough for
combustion to occur, the compression
ratio is much higher than in a petrol
engine. Compression ratios of 14:1
to 24:1 are commonly used, giving
a cylinder pressure when the air is
compressed of up to 3800kPa (560
psi). At a compression ratio of 16:1,
the air temperature will theoretically
be at 525°C, with actual temperatures
being around 425-550°C in working
diesel engines.
Table 1 gives a summary of some
of the differences between petrol and
diesel engines.
Because no throttling of the intake
air occurs in a diesel engine (load
changes are catered for by changing
the mass of fuel introduced), a diesel
engine needs to be governed to limit
maximum engine speed. In comparison to petrol engines, the maximum
engine speed of a diesel is quite low,
typically 2500 to 5000 RPM.
This is because of the inertial loadings created by the heavy internal
components which are required to
absorb the very high cylinder pressures created during the power stroke.
In order that the correct amounts of
fuel are introduced to the combustion
chamber at the right time, a complex
mechanical system is usually employed.
Mechanical fuel delivery
The mechanical delivery of fuel to a
diesel engine is complicated because
of the high fuel pressures and short
delivery times available. In fact, the
processes occurring during injection
are more akin to acoustic principles
than geometric laws of displacement.
In a mechanical system, an engine-driven camshaft drives an injection pump’s plunger which feeds
a high-pressure gallery. The delivery
valve opens and a pressure wave
proceeds toward the injection nozzle
at the speed of sound (approximately
8
10
7
9
11
4
6
5
2
3
1: FUEL TANK
2: GOVERNOR
3: FUEL SUPPLY PUMP
4: INJECTION PUMP
5: TIMING DEVICE
6: DRIVE FROM ENGINE
7: FUEL FILTER
8: VENT
9: NOZZLE AND HOLDER ASSEMBLY
10: FUEL RETURN LINE
11: OVERFLOW LINE
1
Fig.1: schematic of a mechanical diesel injection system: (1) Fuel tank; (2)
Governor; (3) Fuel-supply pump; (4) Injection pump; (5) Timing device; (6) Drive
from engine; (7) Fuel filter; (8) Vent; (9) Nozzle-and-holder assembly; (10) Fuel
return line; (11) Overflow line.
1400 metres per second under these
conditions). When the injection nozzle’s opening is reached, the needle
valve overcomes the force of the injection nozzle spring and lifts from its
seat so that fuel can be injected from
the spray orifices into the engine’s
combustion chamber.
Fig.1 shows a schematic diagram of
an in-line fuel injection pump system
with a mechanical governor.
The mechanical Detroit Diesel system differs slightly from this in that
a combined plunger-type injection
pump and hydrauli
cally-controlled
injector is used for each cylinder. The
amount of fuel injected is governed
by the period of injector flow. This is
Table 1: Diesel vs. Petrol Engines
Feature
High Speed Diesel
Petrol Engine
Admission of fuel
Direct from fuel injector
From carburettor via the
manifold, or injected into the
inlet port
Compression ratio
14:1 to 24:1
7:1 to 10:1
Ignition
Heat due to compression
Electric spark
Torque
Varies little throughout the
speed range
Varies greatly throughout the
speed range
Brake thermal efficiency
35-43%
25-30%
Compression pressure
3100-3800kPa
450-1400kPa
Compression temperature
425-550°C
August 1995 5
Up to 230°C
ENGINE SENSORS
BOOST PRESSURE
AIR TEMPERATURE
OIL TEMPERATURE
OIL PRESSURE
OIL LEVEL
FUEL PRESSURE
ELECTRONIC
CONTROL
MODULE
PWM
INJECTOR
DRIVERS
GMSCM
EPROM
OPERATOR INTERFACES
ELECTRONIC FOOT PEDAL
ELECTRONIC TACHOMETER
DIAGNOSTIC LIGHTS
OPTIONS
ENGINE BRAKE
CRUISE CONTROL
ROAD SPEED GOVERNOR
POWER TAKE-OFF
POWER CONTROL
STOP ENGINE OVERRIDE
(SRS)
EPROM
(TRS)
VEHICLE SENSORS
VEHICLE SPEED
COOLANT LEVEL
RAM
SERIAL
COMMUNICATION
LINKS
(3) PWM AUXILIARY
CONTROL PORTS
Fig.2: the DDEC-II
system uses a large
range of sensors
to set the engine
operating conditions.
In addition, there is
an engine protection
system which is
activated when the
ECM receives an outof-specification signal
from the oil or coolant
temperature sensors,
the oil pressure or
coolant level sensors,
or two additional
sensors which can be
specified by the truck
manufacturer.
to the cylinders by the electronic injectors which are cam-driven for pres
surisation of the fuel and controlled
by solenoid-operated valves to give
precise fuel delivery.
DDEC electronics
The electronic unit injector
incorporates a solenoid-operated
poppet valve which performs the
injection timing & metering functions.
Oil & fuel temperature sensors are
also used. The oil temperature is used
as part of the engine protection system
incorporated into DDEC-III, while fuel
temperature is monitored to aid in the
calculation of fuel economy.
dictated by the opening and closing
of ports within the injector body itself, which in turn are dependent on
mechanical linkages to the governor
mechanism and the throttle.
Electronic management
The air temperature sensor is used
to provide one of the ECM’s inputs.
White smoke suppression & improved
cold starting result from the use of
this sensor. Other sensors monitor the
intake manifold temperature & the oil
temperature
6 Silicon Chip
Detroit Diesel Electronic Control
(DDEC – pronounced ‘Dee-Deck’) was
introduced in September 1985. DDECII was released in 1986 and the system
currently in use is DDEC-III, released
in 1994. The major subsystems of
DDEC are the electronic injectors, the
electronic control module (ECM) and
the sensors. Fig.2 shows a schematic
diagram of DDEC-II. Fuel is delivered
The DDEC-II system uses a microprocessor designed by General Motors
(Detroit Diesel Allison is a subsidiary of General Motors Corporation).
Similar to the Motorola MC68HC11,
its features include 2Kb of EEPROM
and 256 bytes of static RAM. DDEC-III
has a much improved microprocessor
which runs eight times faster, has
seven times more memory and is 50%
smaller.
Electronic injectors operate on a
similar principle to DDA mechanical
injectors but a solenoid-operated
control valve performs the injection
timing and metering functions. Unlike
a petrol EFI system, it is the closing
(rather than opening) of the solenoid
valve which initiates the beginning
of injection. A bypass for the fuel is
blocked when the valve closes, forcing
the fuel to pass through the injector
nozzle and into the combustion chamber. Conversely, opening the valve
causes pressure decay and the end of
injection.
A pulse width modulated (PWM)
driver pulses the current to the injector solenoids at about 10kHz during
the injection phase. Fig.3 shows the
sequence of events during injection.
DDEC III Features
•
•
•
Watertight connectors are used to connect the sensors to the ECM. In engine
management systems, connectors & the wiring loom are responsible for most
of the faults which occur.
When voltage is applied to the injector solenoid, current begins to flow
through the coil. The current increases
until the magnetic field creates enough
force to move the armature and valve
assembly against the return spring
force. Once a preset current flow is
reached, the driver circuitry regulates
the voltage and monitors the current.
A detection circuit is used to monitor
the time at which the valve closes and
injection actually starts; this is critical
in maintaining injection timing and
fuel quantity control.
After the detection circuit signals
that the valve is closed, the current
passing through the coil is set to a level
sufficient to keep the valve closed. At
the end of the command pulse, the
low inductance coil design promotes
rapid current decay and fast valve
opening. During the first portion of
the command pulse, the time needed
for the valve to close is dependent on
the rate at which energy is supplied
to the magnetic field. This is highly
dependent on battery voltage and the
feedback signal supplied by the detection circuitry is used to compensate
for this variable. Fuel injector timing
of better than ±0.25 crankshaft degrees
is obtained with this system.
Engine control module
Unlike the electronic engine management systems employed in cars, the
DDEC system uses an engine-mounted ECM and the circuit boards are
designed to withstand the rigours of
vibration, heat and dust. Large components are hot-melt glued to the boards
and the board supports are designed
to prevent low frequency resonances.
Elastomer mounts are used between
the ECM and the engine, reducing
vibration levels to less than 1G RMS
for the majority of the frequency
spectrum. Fig.4 shows the measured
vibration of the ECM with and without
the isolation mounts.
Under-bonnet temperatures can run
as high as 150°C but the module is
protected from these extremes by being
mounted on a fuel-cooled plate. Using
the fuel to keep the ECM cool might
seem odd but water cooling would
not be suitable since the temperature
is too high, at above 120°C (due to
pressurisation of the cooling system).
The diesel fuel, on the other hand, is
normally at ambient temperature and
so the ECM is kept considerably cooler
than engine block temperatures. In
addition, all electronic components
in the ECM are rated for operation at
up to 125°C.
Sensors & engine protection
As with cars, lots of sensors are
used in the DDEC-III system. Some
are used to adjust the engine operating conditions while others can bring
the engine protection system into
play. The engine protection system is
activated when the ECM receives an
out-of-specification signal from the oil
or coolant temperature sensors, the oil
pressure or coolant level sensors, or
two additional sensors (which can be
specified by the truck manufacturer).
Each of the above sensors can be
programmed to cause one of two
results. The first is complete engine
shutdown, 30 seconds after a dash
•
•
•
•
•
•
•
•
•
•
•
•
•
Excellent engine performance
Optimum fuel economy
Emission laws met without
exhaust gas after-treatment
Engine diagnostics
Simple programming
Engine protection system
Cruise control
Cooling fan control
Engine fan braking
Vehicle speed limiting
Vehicle over-speed diagnostics
Vehicle ID number
Idle speed adjustment
Idle timer shutdown
Warning lights for low DDEC
voltage, low coolant, low oil
pressure, high oil temperature,
high coolant temperature
Communication links – SAE
J1587, J1922, J1939
warning light comes on. This could
happen, for example, after loss of oil
pressure. The second possible result
is “Ramp Down” which illuminates
a yellow dash warning light and cuts
the engine power to 70%, then a red
dash light comes on and the power is
reduced to 40%. Alternatively, any of
the sensors can illuminate warning
lights on the dash.
Both the Ramp Down and Shut
Down modes can be overridden by the
driver if a switch is operated every 30
seconds, allowing a return to 70% of
operating power. This could allow a
driver to proceed safely to his destination, or at least to a safe position by
the roadside.
Engine fuel control
There are sensors for air temperature, intake mani
fold temperature,
and for oil temperature. Both are
used to adjust the idle speed and
fuel injection, to reduce white smoke
emissions and improve cold starting.
As well, there is a coolant temperature sensor which allows the ECM to
measure engine temperature and also
to trigger an over-temperature alarm.
The fuel temperature sensor does
not affect engine running but does
provide an input in the calculation of
fuel consumption. Similarly, the fuel
August 1995 7
The oil pressure sensor is used as an
input to the engine protection system.
This sensor is designed specifically
for fire truck operation & measures
fire pump water pressure. The engine
speed is then adjusted by the ECM to
give a constant water pressure.
pressure sensor provides an input for
consumption calculations.
Fire pump pressure sensor
As you might imagine, this facility
is only used on fire trucks, to monitor
water pressure for the Pressure Governor System. This signal causes the
ECM to set the engine speed to allow
the fire pump to maintain a constant
pumping pressure.
Perhaps the most crucial sensor is
that for throttle position and this is a
major area of difference to the control
of petrol engines. Whereas the accelerator pedal for a petrol engined car
directly controls the butterfly valve in
the throttle body, the accelerator pedal
for a DDEC-fitted diesel has no direct
connection to the engine. The driver
“demands” a certain amount of power
by depressing the accelerator pedal
but how much he gets is determined
by the ECM.
In effect, this is a “fly-by-wire”
system. One of its beneficial effects
is that it stops the emission of clouds
of smoke as a diesel truck accelerates
–because the fuel is always precisely
controlled, it can never be over-rich
and therefore, smoke is minimised.
Timing sensors
Two timing sensors are used to
control the fuel injection. The “SRS”
–synchronous reference sensor –
provides a ‘once per cam revolution’
signal, while the “TRS” – timing
reference sensor – provides 36 pulses
per crankshaft revolution. Both sense
the rotation of a toothed cog (called
a “pulse wheel”) on the crank shaft.
Working together, these sensors allow
the ECM to sense which cylinder is at
The Diagnostic Data Reader can be used to program factors such as the cruise
control settings, vehicle ID number, engine power rating & vehicle speed
limiting. It also can download engine faults logged in the ECM’s memory.
8 Silicon Chip
Fig.3: the relationship between the
electrical events & injection. BOI
indicates the Beginning of Injection.
Top Dead Centre. Precise monitoring
of piston position allows optimum
injection timing.
Other sensors include those for (1)
vehicle speed (for use with cruise control, vehicle speed limiting, and progressive engine braking); and (2) turbo
boost (for monitoring the compressor
discharge pressure, for smoke control
during engine acceleration).
Control functions
The fundamental variable controlled by the DDEC system is the engine fuel input. This is accomplished
by the fuel pulse width and timing
signals applied to the injectors. The
ECM calculates a desired torque level
based on the driver’s throttle position
and the engine RPM. The basic torque
request and injection timing are modified during transients to control smoke
and noise emissions.
Several governing modes which
modify the basic torque request are
available to control engine and vehicle
speeds. These are as follows:
(1) The idle governor – this provides
fixed speed control over the whole of
the torque capability of the engine. The
idle speed is set as a function of engine
temperature to provide optional cold
idle boost. This controls cold white
smoke suppression and provides
faster engine warm-up.
(2) The cruise control – this in-
Using the ‘ProDriver’ option, the DDEC system can also
be interrogated via its dash-mounted data link. Fuel
economy, trip distances & the number & status of logged
cludes set, resume and coast features,
as well as an acceleration mode which
provides a fixed speed increase for
each application.
(3) Road Speed Limiting – this enables the customer to determine the
maximum vehicle speed attainable,
independently from the engine-governed speed.
(4) Engine Speed Limiting – this
provides a programmable maximum
engine speed.
Once all of the modifications to
the base requested torque have been
calculated, a high precision torque to
injector pulse width output calibration
is performed to drive the injectors. The
sensing of the beginning of injection
interacts with the fuel rate algorithm to
control noise and exhaust emissions.
faults can all be sourced from the engine management
computer. The photo above shows the internal details of
the electronic control module of the DDEC-III system.
The fuel injection timing is carefully
controlled during starting to reduce
cranking times, allowing unaid
ed
starting down to ambient temperatures
of -12°C.
User definable programming
Because the DDEC system is used on
a variety of engines and trucks, the system must be programmed to suit each
application. An EEPROM is located
within the ECM and is programmed
via the serial communication link.
During engine assembly, and just prior
to the final test, the ECM is interfaced
to the factory scheduling computer to
program the EEPROM to the specific
sales order for the engine. Data such as
the engine’s horsepower rating, torque
curve and maximum engine speed is
downloaded.
Unlike any car engine management
system, some aspects of the DDEC
system can be reprogrammed by the
customer, using a DDEC Diagnostic
Data Reader, via a connector on the
dashboard. This allows the following
features to be customised: engine
power ratings, variable speed governor, engine protection, cruise control,
vehicle ID number, idle speed, engine
braking and vehicle speed limiting.
Data logging
Fig.4: engine vibration spectrum,
before & after the installation of
elastomer vibration-suppressing
mounts. The DDEC system is unusual
in that the ECM is mounted on the
engine.
Part of the EEPROM is used for
logging accumulated operating hours,
fuel consumption, diagnostic codes
and other cumula
tive information.
By the use of an additional electronic
module (dubbed ‘ProDriver’) and
dedicated software, the SAE diagnostic data link can be used to extract
information such as total dis
tance
travelled; average, shortest and longest
trips; total fuel use and fuel used during idling, driving and cruising; and
perhaps most importantly, the number
and status of engine alerts.
The self-diagnosis function of the
ECM can register intermittent and
continuous faults and then display
these via flashed codes on an in-cabin
‘check engine’ light. No less than 57
different fault codes can be registered,
including any of the sensors being either too high or low in output, engine
or vehicle overspeed, torque overload,
slow injector response time, data link
and EEPROM faults.
In short, while those large and imposing trucks and semi-trailers may
seem like ponderous beasts, as indeed
they are, underneath the bonnet they
are often every bit as advanced as the
best car engines. And, at least with
the DDEC system, electronic circuitry
plays an even greater role in providing
information and control.
References
(1). Asmus, A. & Wellington, B., Diesel
Engines and Fuel Systems, Longman
Cheshire, 1992.
(2). Bosch Automotive Handbook,
Third Edition, 1993.
(3). Detroit Diesel Series 60 – A Success Story, [pamphlet].
(4). Detroit Diesel Electronic Controls,
[pamphlet].
(5). Electronic Diesel Engine Controls,
SAE Collected Papers SP-819.
(6). Hames, R. (et al), DDEC II - Advanced Electronic Diesel Control, SAE
SC
Technical Paper 861110, 1986.
August 1995 9
|