Misfire detection stroke. The gap in the torque output of the engine

5.4.8 Misfire detection stroke. The gap in the torque output of the engine

and a consequential momentary deceleration of

A small engine misfire can raise emissions signif- the crankshaft can be detected using the crank- icantly. If engine misfire occurs in excess of

shaft position sensor (Figure 5.25). By closely about 17% of the time, permanent catalyst dam-

monitoring the speed and acceleration of the age will occur. This clearly shows the importance

crankshaft misfiring cylinders can be detected. of identifying misfire to keep engine emissions

This technology is very commonly used in OBD low (both short and medium term). The OBD sys-

systems to detect non-firing cylinders that can tem monitors individual cylinder misfires, counts

cause harmful emissions and catalyst damage.

82 Advanced automotive fault diagnosis There are a number of technical challenges that

mounted directly above the spark plug (Figure have to be overcome with this technique; the

5.26). Eliminating the distributor and high-voltage accuracy achieved and reliability of the system is

leads helps promote maximum energy transfer very dependent on the algorithms used for signal

to the spark plug to ignite the mixture. In this processing and analysis. Under certain condi-

system the spark plug is not only used as a device tions, misfire detection can be difficult; particu-

to ignite the air/fuel mixture, but is also used as larly at light load with high engine speed. Under

an in-cylinder sensor to monitor the combustion these conditions the damping of firing pulses is

process. The operating principle used in this low due to the light engine load and this creates

technology is that an electrical current flow in an high momentary accelerations and decelerations

ionised gas is proportional to the flame electrical of the crankshaft. This causes speed variation

conductivity. By placing a direct current bias across which can be mistakenly taken by the OBD sys-

the spark plug electrodes, the conductivity can be tem as a misfire. With this method of misfire

measured (Figure 5.27). The spark current is used detection, careful calibration of the OBD system

to create this bias voltage and this eliminates the is necessary to avoid false detection. Another

requirement for any additional voltage source. vehicle operation mode that can cause problems is

The ion current is monitored and if no ion gen- operation of the vehicle on rough or poorly made

erating flame is produced by the spark, no current roads. This also causes rapid crankshaft oscilla-

flows through the measurement circuit during the tion that could activate false triggers – under these

working part of the cycle. The ion current vs. time conditions the misfire detection must be disabled.

trace is very different from that of a cycle when normal combustion occurs and this information can be used as a differentiator to detect misfire

Ionising current monitoring

from normal combustion. This method has proven An ionisation current sensing ignition system

to be very effective at monitoring for misfires consists of one ignition coil per cylinder, normally

under test conditions and also in practice.

Spark Event – Spark Current Flow Measurement Period – Ion Current Flow

BAT

BAT

Charged to – 80 volts

Discharging 80 volts Spark

D1 C1 Ion Flow

D2 ION SIGNAL

D2 ION SIGNAL

R1

R1

ISIM components added

ISIM components added

to secondary circuit Figure 5.26 Ion sensing circuit in direct ignition system

to secondary circuit

Ion Current Waveforms

Tek Stop: 50.0kS/s T 8 Acqs

Tek Stop: 50.0kS/s

16 Acqs

Tek Stop: Single Seq T 100kS/s

Ch 500m V *- M1.00m*

2.8 V

Ch 500m V*-

M1.00ms

2.8 V

Ch3 200m V -w Ch2 200mV-w M500ms 2.8 V

Normal combustion

Misfire in one cylinder

Knock

Figure 5.27 Resulting waveforms from the ion sensing system

On-board diagnostics 83

The signal the system produces contains mis-

Exhaust pressure analysis

fire information and in addition, can provide This solution involves using a pressure sensor in objective knock or detonation information. This

exhaust manifold combined with a Fourier analy- can be used for engine control systems where

sis as the first stage of the signal processing. Using knowledge of the actual combustion process is

a sensor to analyse the gas pulses in the exhaust required (as mentioned above).

manifold, it is possible to detect single misfires. It is also possible to identify which cylinder is

Cylinder pressure sensing

misfiring. This method is less intrusive than the This technology has great potential not just for

above and could potentially be retrofitted at the OBD applications but also for additional feedback

production stage. A sensor in the exhaust can to the engine management system about the com-

detect misfiring cylinders but cannot give use- bustion process due to the direct measurement

ful, qualitative information about the combustion technique. This additional control dimension can

process. This technique has been demonstrated as

be utilised to improve engine performance and capable of detecting all misfires at engine speeds reduce emissions further. With respect to misfire

up to 6000 rpm, for all engine configurations, detection, this method provides reliable detection

loads, and fuels. Generally, a ceramic capacitive of a positive combustion event and can easily

type sensor is been employed which has a short detect misfire with utmost reliability (Figure 5.28).

response time and good durability. The major drawback is the availability of suitable sensors that could be installed into the engine at production and would be durable

5.4.9 Testing vehicles for

enough to last the life of the engine and provide

compliance

the required performance expected of sensors in an OBD system. For certain engine applications

The manufacturer must demonstrate the correct sensors are available, and currently combustion

function of the system to the appropriate authority. sensor technology is under rapid development

For EOBD compliance this requires three com- such that this technical hurdle will soon be

plete emission cycle runs (NEDC). This is known overcome.

as a demonstration test.

A faulty component is installed or simulated which causes a violation of the emission limits; two preconditioning cycles are run and then one complete cycle to show that the error has been recorded and highlighted via illumination of the MIL. These phases are defined in EOBD legisla- tion as:

● simulation of malfunction of a component of the engine management or emission control

system; ● preconditioning of the vehicle with a simu-

lated malfunction; ● driving the vehicle with a simulated malfunc-

tion over the type 1 test cycle (NEDC) and measuring the emissions of the vehicle;

● determining whether the OBD system reacts to the simulated malfunction and indicates

malfunction in an appropriate manner to the vehicle driver.

Typical failure modes induced to be detected are: ● Petrol/Gasoline Engines

– Replacement of the catalyst with a deteri- orated or defective catalyst or electronic

Figure 5.28 Cylinder pressure sensor mounted in the engine

simulation of such a failure

84 Advanced automotive fault diagnosis – Engine misfire conditions according to the

5. Hold speed steady at cruise for 3 minutes. The conditions for misfire monitoring given in

OBD system monitors EGR, secondary air – Replacement of the oxygen sensor with a

system, oxygen sensors and EVAP system. deteriorated or defective oxygen sensor or

6. Overrun/coast down to low speed (i.e. 20 mph) electronic simulation of such a failure

without using the brake or clutch. The OBD – Electrical disconnection of any other emis-

systems check EGR and EVAP systems. sion-related component connected to a pow-

7. Accelerate back up to cruise for 5 minutes at ertrain management computer

three quarter throttle. OBD checks misfire, – Electrical disconnection of the electronic

fuel trim and EVAP.

evaporative purge control device (if

8. Hold steady speed of cruise for 5 minutes. equipped). For this specific failure mode,

OBD monitors catalytic converter efficiency, the type 1 test must not be performed

misfire, fuel trim, oxygen sensors and EVAP ● Diesel Engines

systems.

9. Slow down to a stop without braking, OBD – Where fitted, replacement of the catalyst checks EGR and EVAP. with a deteriorated or defective catalyst or

electronic simulation of this condition The system is now fully reset and ready for detec- – Where fitted, total removal of the particu-

tion of new faults. The necessary drive cycle to late trap or, where sensors are an integral

guarantee reset of the whole system is manufac- part of the trap, a defective trap assembly

turer specific and should be checked appropriately. – Electrical disconnection of any fuelling

system electronic fuel quantity and timing actuator

Roadside test

– Electrical disconnection of any other emis- An official in-service OBD2 emission test, as car- sion related component connected to a power-

ried out in the USA by inspectors from the regu- train management computer

latory authority, consists of the following three parts (a likely European development therefore).

Conditioning Run After Fault Rectification

1. Check MIL function at ignition switch on.

2. Plug in OBD scanner, check monitor readi- If an error has occurred with a component and

ness. If monitors are not all showing as ready, this error has been recorded by the OBD system,

the vehicle is rejected and further road testing then (after the problem has been rectified) it is

is to be done in order to activate all the readi- necessary to clear the fault code memory and test

ness flags. At this stage the scanner will also or condition the vehicle to ensure that:

download any fault codes that are present. ● the fault has really been fixed and does not

3. An additional test, scanner command illumin- reoccur;

ation of MIL via ECU to verify the correct ● the system is set up ready for correct future

function of the OBD system. detection of any faults.

This can be done by putting the vehicle through drive cycle. A typical manufacturer defined drive

5.5 Summary

cycle would consist of the following. Clearly OBD is here to stay – and be developed. It

1. A cold start (coolant temperature less than should be seen as a useful tool for the technician 50°C, coolant and air temp within 11°C of

as well as a key driver towards cleaner vehicles. each other).

The creating of generic standards has helped

2. Switch on ignition to allow oxygen sensor those of us at the ‘sharp end’ of diagnostics heating and diagnostics.

significantly.

3. Idle engine for 2 minutes with typical elect- OBD has a number of key emission related rical loads on (air conditioning and rear screen

systems to ‘monitor’. It saves faults in these sys- heater).

tems in a standard form that can be accessed

4. Turn off loads and accelerate to cruise at

using a scan tool.

half throttle. The OBD system will check for In the final chapter of this book there is a short misfire, fuel trim and EVAP (canister purge)

discussion on OBD3 and ways in which it may systems.

be implemented in the future.

On-board diagnostics 85

Knowledge check questions To use these questions, you should first try to answer them without help but if necessary, refer back to

the content of the chapter. Use notes, lists and sketches to answer them. It is not necessary to write pages and pages of text!

1. State the main reasons why OBD was developed.

2. Explain what is meant by OBD monitors and list the most common.

3. Describe how the P-codes are used to indicate faults.

4. Explain with the aid of a sketch, how the ‘before and after cat’ lambda sensor signals are used by the OBD system to monitor catalyst operation.

5. Explain what is meant by ‘healing of the fault memory’.