Page 39 - Mechatronics with Experiments

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JWST499-c01
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INTRODUCTION 25
In its simplest form, the engine control algorithm controls the fuel injector in order to
maintain a desired speed set by the accelerator pedal position sensor.
The actively controlled variables by ECM are the injectors (when and how much fuel
to inject – an analog signal per injector), and the RPCV valve which is used to regulate the
pressure of the amplification oil line. As a result, for a six-cylinder, four-stroke cycle diesel
engine, the engine controller has seven control outputs: six outputs (one for each injector
solenoid) and one output for the RPCV valve (Figure 1.19). Notice that at 3000 rpm engine
◦
speed, 36 of crankshaft rotation takes only about 2.0 ms, which is about the window
of opportunity to complete the fuel injection. Controlling the injection start time with an
◦
accuracy of 1 of crankshaft position requires about 55.5 microsecond repeatability in the
fuel-injection control system timing. Therefore, accuracy in controlling the injection start
time and duration at different engine speeds is clearly very important. Since we know that
the combustion and injection processes have their own inherent delay due to natural physics,
we can anticipate these delays in a real-time control algorithm, and advance or retard the
injection timing as a function of the engine speed. This is called variable injection timing
in engine control.
Solenoid actuated fuel injectors are digitally controlled, thereby making the injection
start-time and duration changeable in real-time based on various sensory and command data.
The injection start-time and duration are controlled by the signal sent to the solenoid. The
solenoid motion is amplified to high pressure injection levels via high pressure hydraulic
lines (i.e., in the case of HEUI injectors by Caterpillar Inc.) or by cam-driven push rod arms
(i.e., in the case of EUI injectors by Caterpillar Inc.).
The intake manifold absolute pressure is closely related to the load on the engine –
as the load increases, this pressure increases. The engine control algorithm uses this sensor
to estimate the load. Some engines also include a high bandwidth acceleration sensor (i.e.,
piezoelectric accelerometer) on the engine cylinder head to detect the “knock” condition in
the engine. Knock condition is the result of excessive combustion pressures in the cylinders
(usually under loaded conditions of engine) as a result of premature and unusually fast
propagation of ignition of the air–fuel mixture. The higher the compression ratio is, the more
likely the knock condition is. The accelerometer signal is digitally filtered and evaluated
for knock condition by the control algorithm. Once the control algorithm has determined
which cylinders have knock condition, the fuel injection timing is retarded until the knock
is eliminated in the cyclinders in which it has been detected.
In diesel engines with electronic governors, the operator sets the desired speed with
the pedal which defines the desired speed as a percentage of maximum speed. Then the
electronic controller modulates the fuel rate up to the maximum rate in order to maintain
that speed. The engine operates along the vertical line between the desired speed and the
lug curve (Figure 1.22). If the load at that speed happens to be larger than the maximum
torque the engine can provide at that speed, the engine speed drops and torque increases
until the balance between load torque and engine torque is achieved. In most gasoline
engines, the operator pedal command is a desired engine torque. The driver closes the loop
on the engine speed by observing and reacting to the vehicle speed. When “cruise control”
is activated, than the electronic controller regulates the engine fuel rate in order to maintain
the desired vehicle speed.

1.1.3 Engine Modeling with Lug Curve
If we neglect the transient response delays in the engine performance and the oscillations
of engine torque within one cycle (two revolutions of crank angle), the steady-state per-
formance of an engine can be described in terms of its mean (average) torque per cycle,
power, and fuel efficiency as a function of engine speed (Figure 1.22). The most important