one-third of a revolution. In the remaining portions of the two revolutions, the engine rotates due to the inertia of the flywheel. This process leads to very strong engine speed oscillations and rough engine behavior. Having more cylinders and distributing their torque peaks evenly over the two revolutions results in a much smoother behavior. Accordingly, in a six-cylinder engine, a cylinder fires every one-third of a revolution, resulting in a rather constant engine torque over one complete engine cycle.
-100 0 100 200 300 400 500 600
0 5 10 15 20 25
-1000 0 100 200 300 400 500 600
0.5 1
compression stroke
power stroke
exhaust stroke
intake stroke
compression stroke
exhaust valve intake valve
crank angle [°]
scaled valve lift [-]
pc[bar]
Fig. 3.1.Timing diagram of a four-stroke engine.
At a certain crank angle (for port-fuel injected Otto engines this is the crank angle where the ignition occurs) all boundary conditions for the com- bustion are fixed and can no longer be changed. Hence, the torque production acts as a discrete system with a sampling period of
φseg = 2π·N ncyl
(3.1)
τseg = 2π·N
ωe·ncyl (3.2)
whereN indicates a two-stroke (N = 1) or a four-stroke engine (N = 2),ncyl
is the number of cylinders of the engine,φseg is the sampling angle in radians, and τseg is the sampling time in seconds, usually referred to as the segment time.
The sampling interval is constant in the crank-angle domain and depends upon the engine speed in the time domain. In discrete-time control theory, the sampling of the signals is usually assumed to take place with constant sampling times. However, this assumption does not hold for crank-angle-based sampling. As shown in Sect. 2.5.2, changes in engine speed normally are at least a factor of ten slower than the changes in manifold pressure, which set the boundary conditions for the combustion in the cylinder. Thus, crank- angle-based sampling can be assumed to take place at evenly spaced points in time.
In four-stroke engine applications, the synchronization of any device (con- trol system, measurement equipment, or actuator) with the sampling behav- ior of the engine requires information on the crank angle as well as on the camshaft position. Usually, this is accomplished by using two sensors that detect the variations in a magnetic field caused by the teeth on the outer rim of the flywheel and by the camshaft.
3.1.3 Discrete Action of the ECU
Before the advent of electronic control units, the ignition was changed con- tinuously by a centrifugal advance mechanism (for speed adjustment) and a vacuum unit (connected to the manifold for load adaptation). Therefore, the influences of the engine speed and of the manifold pressure on the ignition angle were separate, and the characteristics of that control action were given by the shape of the mechanical cam mechanisms. The time during which the ignition current was on, thus building up energy in the ignition coil, was given by the setting of the ignition contact breaker and was constant in the crank- angle domain, yielding a constant dwell angle, but a varying dwell time. No correction for low battery voltage was possible and the coils had to be short- circuit resistant, because at start-up, whenever the ignition was on, the full current was flowing in the coil for an extended period of time.
Fuel was brought into the cylinder by means of carburetors that continu- ously evaporated fuel into the air stream. The resulting air/fuel ratio was not constant over the entire operating region, which caused drivability problems, high pollutant emissions, and fuel economy problems. Mechanical devices, i.e., accelerator pumps, various adjustable nozzles, etc., were added in an attempt to deal with these problems. However, they just made carburetors costly, dif- ficult to adjust, and unreliable.
The introduction of emission legislation required the introduction of three- way catalytic converters. As shown in Sect. 1.3.2, for these devices to work properly, a precise air/fuel ratio control system is mandatory. For that reason, carburetors with some electronically controllable elements were introduced
that were able to utilize the signal of the air/fuel ratio sensor to compensate for non-stoichiometric mixtures. However, those systems turned out to be more expensive than the new injection systems, which injected the fuel using electronically controlled solenoid valves. The first devices of that type were central fuel injection systems that injected fuel at an upstream position in the manifold from where it was distributed to several cylinders. Such a system is simple to realize but difficult to control, due to the large wall-wetting effects and the long transport time delays. A port fuel injection system proved to be the better solution, i.e., a configuration in which the fuel is injected just upstream of the intake valve.
Since that approach required electronic control units with electronic power amplifiers, the next step was the integration of a solid state ignition system, which made it possible to control injection and ignition timing simultaneously.
Around the year 1980, the first of these SI engine control units with micropro- cessors appeared on the market [75]. They combined the control of ignition and injection in one control unit, and were able to implement more complex control algorithms.
The core of all ECUs is a microcontroller. This device runs in a discrete- event mode, i.e., it performs some of its tasks repetitively in constant crank angle intervals. In addition, the ECU runs several tasks using fixed sampling time intervals. A real-time operating systems coordinates these various tasks and resolves computation-time conflicts on the basis of predefined priorities (see below).
Two operating modes are possible for an ECU:
• Fully Synchronized Operating Mode: The ECU processor continuously counts pulses from a sensor that picks up the variations in the magnetic field caused by the teeth on the outer rim of the flywheel (where 60 teeth is a common number) and compares the demanded values for injection and ignition crank angle with the actual values of the crank angle. This oper- ation mode guarantees the additional delays to be minimal. On the other hand, a great deal of precious processor time is spent on pulse counting.
Therefore, modern ECUs avoid this fully synchronized operation mode.
• Asynchronous Operation Mode: The ECU processor has a dedicated slave processor, often referred to as a time-processing unit (TPU, see Sec. 1.2.2), which relieves the ECU from the synchronization tasks mentioned [78]. In fact, the microprocessor sends the new values for injection and ignition timing asynchronously to the TPU. The TPU obtains crank angle and cam shaft pulses from the engine and triggers the injection and ignition events at the correct crank angle values. This operation mode frees up more time for “intelligent” tasks performed by the processor. In production-type ECUs, these interface chips are directly mounted on the ECU board or, in more recent systems, are integrated on the same chip as the micro- controller. For research purposes, such TPUs with a very high degree of flexibility have been designed using field programmable gate arrays [69].
Even the processors of the most modern engine ECUs have clock rates that are much slower than those of modern desktop computers. This is due to the harsh environmental conditions in which they must work reliably, e.g., temperatures ranging from−40◦C to +100◦C, together with high humidity and electromagnetic fields (radio signals, GSM, ignition system, etc.). One consequence of these slow clock rates is that the computing resources must be carefully managed. This is achieved by utilizing several clock rates:
• 1 ms: very fast tasks, sensor signal post processing, etc.;
• 20 ms: normal time scale for control functions;
• 100 ms: slow (adaptive) functions, temperature effects, etc;
• 1 segment (3.1): control functions which have to be synchronized with the engine’s reciprocating behavior; and
• 1/2 segment or even faster: data processing for special tasks, i.e., cylinder- individual air/fuel control, interpretation of cylinder-pressure data, etc.
Computational power is limited such that every task must be run in ex- actly the time frame that is required by the physical constraints, but not faster. Every task is provided with a priority level, which enables the oper- ating system to resolve conflicts in case the computational resources are not sufficient. At high engine speeds, due to the large amount of calculation time spent in the high-priority segment tasks, the low-priority tasks are hardly ever executed.
This aspect can be illustrated by considering the available computation intervals at different engine speeds and for various crank-angle intervals. Ta- ble 3.1 lists the corresponding values for engine speeds ranging from 600rpm to 6000rpm. These values show that at low engine speeds there is sufficient time for many calculations, but the time delays associated with the crank- angle domain becoming very large render the feedback control problems hard to solve. On the other hand, while at high speeds these time delays are short, the time available for computations is so small that not many tasks can be run simultaneously.
Table 3.1.Time-domain values for engine events (4 stroke).
crank angle corresponding time-intervals at:
ne = 600rpm ne= 6000rpm
cycle 4π 200 ms 20.0 ms
revolution 2π 100 ms 10.0 ms
segment (6-cylinder) 2π3 33 ms 3.3 ms
segment (4-cylinder) π 50 ms 5.0 ms
3.1.4 DEM for Injection and Ignition
The primary task of the ECU is to guarantee the injection of the correct amount of fuel into the manifold and the ignition of the mixture in the cylinder at the desired crank-angle position with sufficient energy.
The fuel-injection module is analyzed in a first step. The timing of the injection is derived from the data shown in Fig. 3.2. The lift curves of the intake and the exhaust valves of an engine are assumed to be constant in the crank-angle domain (this is not the case with variable-valve timing, of course).
Both valves are open for approximately two-thirds of an engine crankshaft revolution. It is advantageous to inject the fuel on a closed valve1 such that sufficient time for fuel evaporation is available. Moreover, since the intake valve is very hot compared to the intake runners, the evaporation of the injected fuel is further improved.
Following this approach, the time available for fuel injection at 6000rpm amounts to approximately 13 ms. This value should not be exceeded except for very harsh transients. At idling, which is the other extreme, a minimal amount of fuel must be injected. Opening and closing the fuel injector takes approximately 1 ms, a quantity that strongly depends upon manufacturing tolerances, wear, bouncing, and fuel quality. To prevent large variations in the amount of fuel injected, the minimum injection time should not be shorter than approximately 2 ms. These considerations determine the dimensions of the injection valve. Obviously, a trade-off has to be made between low amounts of hydrocarbons emitted at full load and fueling precision at idle, i.e., the more fuel that needs to be injected, the more critical the manufacturing tolerances become at idle.
Fig. 3.2. Timing diagram for the injection.
1 Injecting fuel directly into the cylinder through the open valve would increase hydrocarbon emissions.
The timing diagram for the injection depicted in Fig. 3.2 will now be discussed in detail.
• In normal operation, the injection is completed at point a. In the ECU, this event is fixed. The start of injection is set at the necessary crank angle, which is computed using the air mass flow information obtained at an earlier crank angle.
• Between the points aandb, the air mass and the fuel mass flow into the cylinder. Changes in these mass flows into the cylinder during that period can no longer be corrected with a regular injection.
• In normal operation, pointb is the earliest possible start of injection for the next cycle. For high loads, the resulting injection duration must be sufficient.
• At pointc, the measurements for the calculation of the duration of the in- jection are taken. This event is usually triggered at all crank-angle intervals φseg (3.1). This is the relevant event for the calculation of the injection-to- torque delay, because the measurement signals valid at care responsible for the combustion ath. Betweencandd, all calculations associated with the injection control algorithms are performed. This event will be referred to as the update injection event.
• Atd, new values for the duration of the injection duration (as well as for the ignition timing) are sent to the time processing unit (TPU).
• Untileis reached, no new information for the injection timing is available.
An average delay between d and e of one-half sample interval τsample
can be assumed, reflecting the fact that an even distribution of the time delays between zero (the calculations finished just in time and the injection starts immediately after the update event) and one sample interval (the calculations required too much time such that the result is transferred to the TPU only in the next update injection event) may be expected.
• At pointe, the injection with the most recent air mass information starts.
• Depending on the type of ECU, between eand f, the injection pulse can be extended if necessary, but only if critical air/fuel ratios are to be ex- pected (i.e., misfire). Most ECUs typically allow an immediate closing of the injection valve.
• At pointf, the regular injection pulse is finished. During very heavy tran- sients, some ECUs offer the possibility of injecting a small amount of fuel onto the open intake valve. Of course, fuel can only be added, thus if the air mass is reduced, very rich conditions must be accepted.
• After the point g, the amounts of air and fuel in the cylinder are fixed, and the combustion starts at the pointh.
Summarizing these considerations, the time delay between the start of the calculation (update injection event) and the injection start event can be approximated by
τc)−e)=τcalculation+1
2 ·τsample (3.3)
Fig. 3.3.Timing diagram for the ignition.
For the ignition, similar considerations exist, as shown in Fig. 3.3. Modern ignition coils must be connected to the full battery voltage for approximately 4 ms to build up enough energy for a sufficient spark. However, if the coil is connected to the battery voltage for more than 10 ms, it may suffer irreversible damage.
• At point a, the measurements of the engine variables necessary for the determination of the ignition timing are triggered by the ECU. This point is now referred to as theupdate ignition event˙In general, this is not be the same crank angle as that at which the update injection event takes place.
• Calculations are performed up to point b, at which the calculated values are sent to the TPU.
• Again, until ignition starts, an additional time delay occurs. For this delay, a value of one-half of the sample time can be assumed. At c, the coil is connected to the battery terminals.
• Atd, the ignition (or spark advance) angle, the circuit breaker is opened causing the spark to start. The time between c) and d) is the dwell an- gle. Both variables are chosen such that the thermodynamic conditions of combustion are optimized.
Changing the spark advance between c and d is possible within small limits. The case of missing a spark event must be strictly avoided because such misfires can cause severe damage to the TWC. Accordingly, increasing the spark advance is always more critical.
The calculation of the spark advance must be done considering the avail- able information on engine speed, air mass in cylinder, knock onset during the last combustion event, etc. Since the dwell-time is rather short and the ignition takes place during the compression of the gases in the cylinder, the
ignition has a smaller inherent delay than the injection. The ignition is thus the fastest actuator for a port-fuel injected SI engine.