Misfire monitor

5.3.9 Misfire monitor

Air mass ⫻ Long-term fuel trim Fuel mass ⫽

When an engine endures a period of misfire, at

70 Advanced automotive fault diagnosis

Voltage characteristic of the Lambda-sensor signal

Air-fuel mixture

lean rich lean

lean rich

lean

Sensor- voltage characteristic

Lambda- correction factor

Richer Neutral value 1.0

Leaner Figure 5.10 Rich AFR lambda sensor signal fuelling error (Source:

Time t

Bosch)

Cyclic change between mixture adaptation and adaptation of the cylinder-charge factor Lambda

correction factor

Richer Neutral value 1.0

Leaner

Adaption variable

Figure 5.11 Adaptive fuel strategy

Time t

in operation (Source: Bosch)

catalyst damage and even destruction can occur. When misfire occurs, the unburned fuel and air is

A ⫽ Load at Neutral Idle

discharged direct to the exhaust system where it

B ⫽ Load at Neutral 3000 rpm C ⫽ Load at Neutral Redline

passes directly through the catalyst.

D ⫽ Load at 4" Hg Less than Vacuum

Subsequent normal combustion events can

combust this air/fuel charge in something akin to

a a bellows effect, which causes catalyst tempera- D

tures to rise considerably. Catalyst damage fail- ure thresholds are package specific but are in the B

region of 1000°C. The catalyst itself is a very

A Negative Torque Disablement

expensive service item whether replaced by the customer or the manufacturer under warranty.

Idle

3000 RPM Readline

The misfire monitor is responsible for deter- mining when misfire has occurred, calculating the Figure 5.12 Misfire enablement window (Ford Motor Company)

rate of engine misfire and then initiating some kind of protective action in order to prevent cata- lyst damage. The misfire monitor is in operation

throughout the revs range but European legislation continuously within a ‘calibrateable’ engine speed/

requires monitoring only up to 4500 rev/min. load window defined by the legislation (Figure

The crankshaft sensor generates a signal as the 5.12). The USA requires misfire monitoring

wheel rotates and the microprocessor processes

On-board diagnostics 71

EEEC Deviant Acceleration

VRS (Crank Signal)

Low Data Rate

(Calculated by SW)

Delta Times (PIP_DWN_DEL)

SW EDIS

Figure 5.13 Crankshaft 36-1 Tooth Wheel

(Conditioned VRS) (Synthesised by SW)

Acceleration

mounted wheel and sensor

(Calculated by SW)

source of angular acceleration (Source: Ford Motor Company)

this signal to determine the angular acceleration The rate of misfire that will cause catalyst dam- of the crankshaft produced by each engine cylin-

age varies as a function of engine speed and load. der when a firing event occurs. When a misfire

Misfire rates in the region of 45% are required to occurs the crankshaft decelerates and a cam pos-

damage a catalyst at neutral idle whilst at 80% ition sensor identifies the cylinder that misfired.

engine load and 4000 rev/min misfire rates in the Processing of the signal from the crank pos-

region of only 5% are needed (Figure 5.14). ition sensor is not straightforward. A considerable

A type B misfire is defined as that rate of mis- amount of post-processing takes place to filter the

fire that will cause the tailpipe emissions to exceed signal and disable monitoring in unfavourable

legislated levels. This varies from vehicle to ve- conditions. The misfire monitor must learn and

hicle and is dependent upon catalyst package. MI cater for the differences in manufacturing toler-

operation is the same as for standard MI DTCs. ances of the crankshaft wheel and so has a spe- cific sub-algorithm for learning these differences and allowing for them when calculating the angu-

5.3.10 Exhaust gas recirculation

lar acceleration of the crankshaft (Figure 5.13).

monitor

These correction factors are calculated during deceleration, with the injectors switched off. They

As combustion takes place within the engine should be re-learned following driveline compo-

cylinders, nitrogen and oxygen combine to form nent changes such as flywheel, torque converter,

various oxides of nitrogen; collectively these are crankshaft sensor, etc.

termed NOx. NOx emissions can be reduced up The misfire monitor must be able to detect

to a certain point by enriching the air/fuel ratio two types of misfire:

beyond the point at which hydrocarbon (HC)

and carbon monoxide (CO) emissions begin to Type A misfire;

increase. NOx emissions are generated as a func- Type B misfire. tion of combustion temperature, so another way

A type A misfire is defined as that rate of misfire, to reduce these is to decrease the compression which causes catalyst damage. When this occurs

ratio which leads to other inefficiencies like poor the MI will flash at a rate of 1 Hz and is allowed to

fuel economy.

stop flashing should the misfire disappear. The MI Most manufacturers employ an emissions con- will stay on steady state should the misfire re-occur

trol sub-system known as exhaust gas recircula- on a subsequent drive and the engine operating

tion (EGR). This by definition recirculates some conditions are ‘similar’ i.e. engine speed is within

of the exhaust gases back into the normal intake 375 rev/min, engine load is within 20%, and the

charge. These ‘combusted’ gases cannot be burnt engine’s warm-up status is the same as that under

again so they act to dilute the intake charge. As a which the malfunction was first detected (and no

result, in-cylinder temperatures are reduced along new malfunctions have been detected).

with NOx emissions (Figure 5.15).

72 Advanced automotive fault diagnosis

Figure 5.14 System development screen showing type A misfire rates normalised by engine speed and load (Source: Ford Motor Company)

Electronic

PCM

vacuum regulator

Fresh air inlet

SIG RTN VPWR

EGR

DPFE SIG

EGR Tube

DPFE Intake Manifold Side (IMS) Signal

Orifice

DPFE Exhaust Manifold Side (EMS) Signal

Exhaust

Figure 5.15 EGR system using differential pressure monitoring (Source: Ford Motor Company)

The exhaust gas recirculation system monitor non-intrusive, such as a change in manifold pres- is responsible for determining the serviceability

sure as EGR is flowed and then shut off. of the sensors, hoses, valves and actuators that

One method monitors AFR excursions after belong to the EGR system. Manufacturers

the EGR valve is opened and then closed as the employ systems that can verify that the requested

AFR becomes lean. Another system employs a amount of exhaust gas is flowing back into the

differential pressure scheme that determines the engine intake. Methods can be both intrusive and

pressure both upstream and downstream of the

On-board diagnostics 73

Secondary Air

Secondary Air

Injection System

Injection System

OK

Not OK

Figure 5.16 Secondary airflow diagnostic monitoring

exhaust to determine whether the requested flow Older systems support a belt driven mechan- rate is in effect. Yet another system employs a

ical pump with a bypass valve for when second- temperature sensor, which reports the change in

ary airflow is not required. Modern vehicles temperature as EGR gases flow past the sensor.

employ an electric air pump operated by the engine The temperature change will be mapped against

management ECU (powertrain control module the amount of EGR flowing so when an amount

[PCM]) via relays.

of EGR is requested then the flow rate is inferred The secondary air monitor is responsible for by measuring the change in temperature.

determining the serviceability of the secondary air system components (Figure 5.16). Most strategies