Engine management

7.15 Engine management

7.14.4 Glow plug circuit

7.15.1 Introduction

Figure 7.35 shows a typical glow plug circuit con- As the requirement for lower and lower emis- trolled by an ECU. Most timer circuits put the

sions continues together with the need for better glow plugs on for a few seconds prior to cranking.

performance, other areas of engine control are

A warning light may be used to indicate the ‘ready’ constantly being investigated. This is becoming condition to the driver. Take care to note the type

even more important as the possibility of carbon

Engine systems 133

Engine does not start

Check battery voltage Check for fuel supply

Check vehicle security

Check glow plug

system is disarmed

operation

Check lift pump

operation Check condition of

air filter

Check glow plug

Check fuel filter is clean and for fuel at

resistance

Check valve clearances

injection pump

Check compression

Check injectors for

Check engine stop

pressures

spray pattern and

solenoid is not activated

buildup of deposits

Check valve timing is correct Check for air in

Check starter turns

fuel system

above 150 rev/min

Check injection

pump timing

Figure 7.36 General diagnosis pattern

dioxide emissions being included in the regula- valves. The length of the inlet tract has an effect tions increases. Some of the current and potential

on the frequency of these waves. areas for further control of engine operation are

With the widespread use of twin cam engines, included in this section. Most of the common

one cam for the inlet valves and one for the exhaust areas of ‘control’ have been covered under either

valves, it is possible to vary the valve overlap while Ignition (Section 7.7) or Fuel injection (Section

the engine is running. Honda has a system that 7.11). The main areas of control are as follows:

improves the power and torque range by opening

both of the inlet valves only at higher speed. ignition timing;

A BMW system uses oil pressure controlled dwell angle;

by valves to turn the cam with respect to its drive fuel quantity;

gear. This alters the cam phasing or relative pos- idle speed. ition. The position of the cams is determined from

Further areas of engine control may include:

a suitable map held in ROM in the control unit. ● EGR;

● canister purge;

7.15.2 Closed loop lambda

● valve timing;

control

● inlet manifold length;

Current regulations have almost made closed closed loop lambda control. loop control of air fuel mixture in conjunction

It is not possible for an engine to operate at its with a three way catalytic converter mandatory. best volumetric efficiency with fixed manifolds.

It is under discussion that a lambda value of one This is because the length of the inlet tract deter-

may become compulsory for all operating condi- mines the velocity of the intake air and in particu-

tions, but this is yet to be agreed. lar the propagation of the pressure waves set up

Lambda control is a closed loop feedback sys- by the pumping action of the cylinders. These

tem in that the signal from a lambda sensor in standing waves can be used to improve the ram

the exhaust can directly affect the fuel quantity effect of the charge as it enters the cylinder but

injected. The lambda sensor is described in more only if they coincide with the opening of the inlet

detail in Chapter 6.

134 Advanced automotive fault diagnosis

Figure 7.37 Catalytic convertor conversion rate – effect of lambda control

A graph to show the effect of lambda control in the lambda window (0.97 to 1.03), which is the conjunction with a catalytic converter is shown in

region in which the TWC is at its most efficient. Figure 7.37. The principle of operation is as fol- lows; the lambda sensor produces a voltage which

7.15.3 Engine management

is proportional to the oxygen content of the exhaust

operation

which is in turn proportional to the air fuel ratio. At the ideal setting this voltage is about 450 mV.

The combination of ignition and injection control If the voltage received by the ECU is below this

has several advantages. The information received value (weak mixture) the quantity of fuel injected

from various sensors is used for computing both is increased slightly. If the signal voltage is above

fuelling and ignition requirements. Perhaps more the threshold (rich mixture) the fuel quantity is

importantly ignition and injection are closely reduced. This alteration in air fuel ratio must not be

linked. The influence they have on each other too sudden as it could cause the engine to buck. To

can be easily taken into account to ensure that the prevent this the ECU contains an integrator, which

engine is working at its optimum, under all oper- changes the mixture over a period of time.

ating conditions. Figure 7.38 shows the layout

A delay also exists between the mixture for- and components of a common system. mation in the manifold and the measurement of

Overall this type of system is less complicated the exhaust gas oxygen. This is due to the engine’s

than separate fuel and ignition systems and in working cycle and the speed of the inlet mixture,

many cases the ECU is able to work in an emer- the time for the exhaust to reach the sensor and

gency mode by substituting missing information the sensor’s response time. This is sometimes

from sensors with pre-programmed values. This known as dead time and can be as much as one

will allow limited but continued operation in the second at idle speed but only a few hundred milli-

event of certain system failures. seconds at higher engine speeds.

The ignition system is integrated and is oper- Due to the dead time the mixture cannot be

ated without a high tension (HT) distributor. The controlled to an exact value of lambda equals one.

ignition process is controlled digitally by the If the integrator is adjusted to allow for engine

ECU. The data for the ideal characteristics are speed then it is possible to keep the mixture in

stored in ROM from information gathered during

Engine systems 135

Figure 7.38 Engine management – Motronic

both prototyping and development of the engine. 12° gap positioned 114° before top dead centre The main parameters for ignition advance are

(BTDC) for number 1 cylinder. The signal pro- engine speed and load but greater accuracy can be

duced by the flywheel sensor is essentially a sine achieved by taking further parameters into account

wave with a cycle missing corresponding to the gap such as engine temperature. This provides both

in the teeth of the reluctor plate. The information optimum output and close control of anti-pollution

provided to the ECU is engine speed from the fre- levels. Performance and pollution level control

quency of the signal and engine position from the means that the actual ignition point must be in

number of pulses before or after the missed pulses. many cases a trade off between the two.

The basic ignition advance angle is obtained The injection system is multipoint and, as is

from a memorised cartographic map. This is held the case for all fuel systems, the amount of fuel

in a ROM chip within the ECU. The parameters delivered is primarily determined by the amount

for this are:

of air ‘drawn’ into the engine. The method for ● engine rev/min given by the flywheel sensor;

measuring this data is indirect in the case of this ● inlet air pressure given by the MAP sensor.

system as a pressure sensor is used to determine the air quantity.

The above two parameters (speed and load) give Electromagnetic injectors control fuel supply

the basic setting but to ensure optimum advance into the engine. The injector open period is deter-

angle the timing is corrected by: mined by the ECU. This will obtain very accurate

● coolant temperature

control of the air fuel mixture under all operating

● air temperature

conditions of the engine. The data for this is stored ● throttle butterfly position

in ROM in the same way as for the ignition. The main source of reference for the ignition

The ignition is set to a predetermined advance system is from the crankshaft position sensor. This

during the starting phase. Figure 7.39 shows a is a magnetic inductive pick up sensor positioned

typical advance, fuelling and other maps used by next to a flywheel ring containing 58 teeth. Each

the Motronic system. This data is held in ROM. tooth takes up a 6° angle of the flywheel with one

For full ignition control, the ECU has to first

136 Advanced automotive fault diagnosis

Ignition angle

● Partial throttle is the main area of control and,

Part load

as already stated, the basic timing is set ini-

Full load

Trailing

tially by a programme as a function of engine

throttle

speed and manifold pressure. Corrections are the added according to:

Cranking

Load Idle Engine speed

● operational strategy ● knock protection

Inverse Lambda factor

Full load ● Part load phase correction

The ECU will also control ignition timing vari-

Overrun

fuel cut off

ation during overrun fuel cut off and reinstate- ment and also to ensure anti-jerk control. When

Load Idle Engine speed

starting the ignition timing plan is replaced by a specific starting strategy. Phase correction is when

the ECU adjusts the timing to take into account the time taken for the HT pulse to reach the spark plugs. To ensure good driveability the ECU can limit the variations between the two ignition sys-

EGR valve position

tems to a maximum value, which varies according

Load

Engine speed

to engine speed and the basic injection period.

Dwell angle

An anti jerk function operates when the basic injection period is less than 2.5 ms and the engine speed is between 720 and 3200 rev/min. This func- tion operates to correct the programmed ignition timing in relation to the instantaneous engine

speed and a set filtered speed. This is done to sta-

Battery voltage

Engine speed

bilise the engine rotational characteristics as much

as possible.

Opening ratio (air bypass)

In order to maintain constant energy HT the dwell period must increase in line with engine speed. So that the ignition primary current reaches

its maximum at the point of ignition the ECU con- trols the dwell by use of another memory map,

Engine temperatur

e Engine speed

which takes battery voltage into account.

Transition compensation

Fuel is collected from the tank by a pump either immersed in it or outside, but near the tank. The immersed type is quieter in operation and has better cooling and no internal leaks. The fuel is directed forwards to the fuel rail or mani- fold, via a paper filter.

Load

Fuel pressure is maintained at about 2.5 bar

Engine speed

above manifold pressure by a regulator mounted

Figure 7.39 Motronic control maps (samples)

on the fuel rail. Excess fuel is returned to the tank. The fuel is usually picked up via a swirl pot in the tank to prevent aeration of the fuel. Each of

determine the basic timing for three different the four inlet manifold tracts has its own injector. conditions. The fuel pump is a high pressure type and is a

● Under idling conditions ignition timing is often two stage device: a low pressure stage created by moved very quickly by the ECU in order to con-

a turbine drawing fuel from the tank; and a high trol idle speed. When timing is advanced engine

pressure stage created by a gear pump delivering speed will increase within certain limits.

fuel to the filter. It is powered by a 12 V supply ● Full load conditions require careful control of

from the fuel pump relay, which is controlled by ignition timing to prevent combustion knock.

the ECU as a safety measure. When a full load signal is sensed by the ECU

The rotation of the turbine draws fuel in via (high manifold pressure) the ignition advance

the inlet. The fuel passes through the turbine and

Engine systems 137

The purpose of the fuel pressure regulator is to maintain differential pressure across the injectors at a predetermined constant. This means the regu- lator must adjust the fuel pressure in response to changes in manifold pressure. It is made of two compressed cases containing a diaphragm, a spring and a valve.

The calibration of the regulator valve is deter- mined by the spring tension. Changes in manifold pressure vary the basic setting. When the fuel pressure is sufficient to move the diaphragm, the valve opens and allows fuel to return to the tank. The decrease in pressure in the manifold, also act- ing on the diaphragm at say idle speed, will allow

the valve to open more easily, hence maintaining

Figure 7.40 Roller cell fuel pump

a constant differential pressure between the fuel rail and the inlet manifold. This is a constant across by rotation of the pump and the reduction of the

the injectors and hence the quantity of fuel injected volume in the gear chambers. This pressure opens

is determined only by the open time of the injec-

a residual valve and fuel passes to the filter. tors. The differential pressure is maintained at When the pump stops, pressure is maintained by

about 2.5 bar.

this valve, which prevents the fuel returning. If, The air supply circuit will vary considerably due to a faulty regulator or a blockage in the line,

between manufacturers but an individual mani- fuel pressure rises above 7 bar an over pressure

fold from a collector housing, into which the air valve will open releasing fuel back to the tank.

is fed via a simple butterfly, essentially supplies Figure 7.40 shows this type of pump.

each cylinder. The air is supplied from a suitable The fuel filter is placed between the fuel pump

filter. A supplementary air circuit is utilised dur- and the fuel rail. It is fitted one way only to ensure

ing the warm-up period after a cold start and to that the outlet screen traps any paper particles

control idle speed.

from the filter element. The filter will stop con- tamination down to between 8 and 10 microns.

7.15.4 Gasoline direct

Replacement varies between manufacturers but

injection (GDI)

80 000 km (50 000 miles) is often recommended. The fuel rail, in addition to providing a uniform

High-pressure injection systems for petrol/gas- supply to the injectors, acts as an accumulator.

oline engines is based on a pressure reservoir and Depending on the size of the fuel rail some sys-

a fuel rail, which a high-pressure pump charges tems also use an extra accumulator. The volume of

to a regulated pressure of up to 120 bar. The fuel the fuel rail is large enough to act as a pressure

can therefore be injected directly into the com- fluctuation damper ensuring that all injectors are

bustion chamber via electromagnetic injectors. supplied with fuel at a constant pressure.

The air mass drawn in can be adjusted through Multipoint systems use one injector for each

the electronically controlled throttle valve and is cylinder although very high performance vehicles

measured with the help of an air-mass meter. For may use two. The injectors are connected to the

mixture control, a wide-band oxygen sensor is fuel rail by a rubber seal. The injector is an elec-

used in the exhaust, before the catalytic converters. trically operated valve manufactured to a very

This sensor can measure a range between a lambda high precision. The injector comprises a body and

value of 0.8 and infinity. The engine electronic needle attached to a magnetic core. When the

control unit regulates the operating modes of the winding in the injector housing is energised the

engine with gasoline direct injection in three ways. core or armature is attracted and the valve opens

1. Stratified charge operation – with lambda val- compressing a return spring. The fuel is delivered

ues greater than 1.

in a fine spray to wait behind the closed inlet

2. Homogeneous operation – at lambda ⫽ 1. valve until the induction stroke begins. Providing

3. Rich homogeneous operation – with lambda the pressure across the injector remains constant

the quantity of fuel admitted is related to the open period, which in turn is determined by the time

Compared to the traditional manifold injection

138 Advanced automotive fault diagnosis

Figure 7.41 GDI system showing the main components (Source: Bosch Press)

operation, the lambda value in the combustion chamber is between about 1.5 and 3. In the part- load range, gasoline direct injection achieves the greatest fuel savings with up to 40% at idle com- pared to conventional petrol injection processes. With increasing engine load, and therefore increas- ing injection quantities, the stratified charge cloud becomes even richer and emission characteristics become worse.

Because soot may form under these conditions, the DI-Motronic engine control converts to a homo- geneous cylinder charge at a pre-defined engine load. The system injects very early during the

Figure 7.42 Atomization in a GDI engine (Source: Bosch Press)

intake process in order to achieve a good mixture of fuel and air at a ratio of lambda ⫽ 1. As is the case

entire fuel amount in full-load operation in a for conventional manifold injection systems, the quarter of the time. The available time is signifi-

amount of air drawn in for all operating modes, is cantly shorter during stratified charge operation

adjusted through the throttle valve according to in part-load. Especially at idle, injection times of

the desired torque specified by the driver. less than 0.5 ms are required due to the lower fuel

Diagnosing faults with a GDI system is little consumption (Figure 7.41). This is only one-fifth

different from the manifold injection types. Extra of the available time for manifold injection.

care is needed because of the higher fuel pressures The fuel must be atomised very finely in order to

of course. Injector waveforms can be checked as create an optimal mixture in the brief moment

can those associated with the other sensors and between injection and ignition (Figure 7.42). The

actuators.

fuel droplets for direct injection are on average smaller than 20 ␮m. This is only one-fifth of the droplet size reached with the traditional manifold

7.16 Diagnostics –

injection and one-third of the diameter of a single human hair. This improves efficiency considerably.

combined injection and

Direct injection engines operate according to

fuel control systems

the stratified charge concept in the part-load range and function with high excess air. In return, very