Page 37 - Mechatronics with Experiments

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INTRODUCTION 23
electronic control module (ECM) in real time. The control decisions are made relative to
the crankshaft angular position based on a number of sensory data. The timing relative
to the crankshaft position may be varied as a function of engine speed in order to opti-
mize the engine performance. The delay from the time current pulse sent to the injector
and the time that combustion is fully developed is in the order of 15 degrees of crankshaft
angle. The typical shape of the pressure in the cylinder during a four-stroke cycle are also
shown in the Figure 1.20. The maximum combustion pressure is in the range of 30 bar
(3 Mpa) to 160 bar (16 MPa). Notice that even though the pressure in the cylinder is positive,
the torque contribution of each cylinder as a result of this pressure is positive during the cycle
when the piston is moving down (the pressure is helping the motion and the net torque contri-
bution to the crankshaft is positive) and negative during the cycle when the piston if moving
up (the pressure is opposing the motion and the net torque contribution to the crankshaft
is negative). As result, the net torque generated by each cylinder oscillates as function of
crankshaft angle with a period of two revolutions. The mean value of that generated torque
by all cylinders is the value used for characterizing the performance of the engine.
In electronically controlled engines, the fuel injection timing relative to the crankshaft
position is varied as a function of engine speed in order to give enough time for the
combustion to develop. This is called the variable timing fuel injection control. As the
engine speed increases, the injection time is advanced, that is, fuel is injected earlier
relative to the TDC of the cylinder during the compression cycle. Injection timing has a
significant effect on the combustion efficiency, hence the torque produced, as well as the
emission content.
It is standard in the literature to look at the cylinder pressure versus the combustion
chamber volume during the four-stroke cycle. The so-called p-v diagram shape in general
looks like as is shown in Figure 1.20. The net energy developed by the combustion process
is proportional to the area enclosed by the p-v diagram. In order to understand the shape
of the torque generated by the engine, let us look at the pressure curve as a function of
the crankshaft (Figure 1.20) and superimpose the same pressure curve for other cylinders
with the appropriate crankshaft phase angle. The sum of the pressure contribution from
each cylinder is the total pressure curve generated by the engine (Figure 1.20). The pressure
multiplied by the piston top surface area is the net force generated. The effective moment arm
of the connecting rod multiplied by the force gives the torque generated at the crankshaft.
The net change in the acceleration is the net torque divided by the inertia. If the inertia
is large, the transient variations in the net torque will result in smaller acceleration changes,
and hence smaller speed changes. At the same time, it takes a longer time to accelerate or
decelerate the engine to a different speed. These are the advantages and disadvantages of
the flywheel used on the crankshaft.

1.1.2 Engine Control System Components
There are three groups of components of an engine control system: (1) sensors, (2) actuators,
(3) electronic control module (ECM) (Figure 1.21). The number and type of sensors used
in an electronic engine controller varies from manufacturer to manufacturer. The following
is a typical list of sensors used:
1. accelerator pedal position sensor,
2. throttle position sensor (if the engine has throttle),
3. engine speed sensor,
4. air mass flow rate sensor,
5. intake manifold (boost) absolute pressure sensor,