Exhaust pressure analysis
This solution involves using a pressure sensor in
exhaust manifold combined with a Fourier analysis as the first stage of the signal processing. Using a sensor to analyse the gas pulses in the exhaustanifold, it is possible to detect single misfires.
It is also possible to identify which cylinder is misfiring. This method is less intrusive than the above and could potentially be retrofitted at the production stage. A sensor in the exhaust can detect misfiring cylinders but cannot give useful, qualitative information about the combustion process. This technique has been demonstrated as capable of detecting all misfires at engine speeds up to 6000 rpm, for all engine configurations, loads, and fuels. Generally, a ceramic capacitive type sensor is been employed which has a short response time and good durability.
5.4.9 Testing vehicles for compliance
The manufacturer must demonstrate the correct function of the system to the appropriate authority.
For EOBD compliance this requires three com-
plete emission cycle runs (NEDC). This is known
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
simulation of such a failure
– Engine misfire conditions according to the
conditions for misfire monitoring given in
– Replacement of the oxygen sensor with a
deteriorated or defective oxygen sensor or
electronic simulation of such a failure
– Electrical disconnection of any other emis-
sion-related component connected to a pow-
ertrain management computer
– Electrical disconnection of the electronic
evaporative purge control device (if
equipped). For this specific failure mode,
the type 1 test must not be performed
● Diesel Engines
– Where fitted, replacement of the catalyst
with a deteriorated or defective catalyst or
electronic simulation of this condition
– Where fitted, total removal of the particu-
late trap or, where sensors are an integral
part of the trap, a defective trap assembly
– Electrical disconnection of any fuelling
system electronic fuel quantity and timing
actuator
– Electrical disconnection of any other emis-
sion related component connected to a power-
train management computer
Conditioning Run After Fault
Rectification
If an error has occurred with a component and
this error has been recorded by the OBD system,
then (after the problem has been rectified) it is
necessary to clear the fault code memory and test
or condition the vehicle to ensure that:
● the fault has really been fixed and does not
reoccur;
● the system is set up ready for correct future
detection of any faults.
This can be done by putting the vehicle through
drive cycle. A typical manufacturer defined drive
cycle would consist of the following.
1. A cold start (coolant temperature less than
50°C, coolant and air temp within 11°C of
each other).
2. Switch on ignition to allow oxygen sensor
heating and diagnostics.
3. Idle engine for 2 minutes with typical elect-
rical loads on (air conditioning and rear screen
heater).
4. Turn off loads and accelerate to cruise at
half throttle. The OBD system will check for
misfire, fuel trim and EVAP (canister purge)
systems.
5. Hold speed steady at cruise for 3 minutes. The
OBD system monitors EGR, secondary air
system, oxygen sensors and EVAP system.
6. Overrun/coast down to low speed (i.e. 20 mph)
without using the brake or clutch. The OBD
systems check EGR and EVAP systems.
7. Accelerate back up to cruise for 5 minutes at
three quarter throttle. OBD checks misfire,
fuel trim and EVAP.
8. Hold steady speed of cruise for 5 minutes.
OBD monitors catalytic converter efficiency,
misfire, fuel trim, oxygen sensors and EVAP
systems.
9. Slow down to a stop without braking, OBD
checks EGR and EVAP.
The system is now fully reset and ready for detec-
tion of new faults. The necessary drive cycle to
guarantee reset of the whole system is manufac-
turer specific and should be checked appropriately.
Roadside test
An official in-service OBD2 emission test, as car-
ried out in the USA by inspectors from the regu-
latory authority, consists of the following three
parts (a likely European development therefore).
1. Check MIL function at ignition switch on.
2. Plug in OBD scanner, check monitor readi-
ness. If monitors are not all showing as ready,
the vehicle is rejected and further road testing
is to be done in order to activate all the readi-
ness flags. At this stage the scanner will also
download any fault codes that are present.
3. An additional test, scanner command illumin-
ation of MIL via ECU to verify the correct
function of the OBD system.