Fuel system monitoring

5.3.8 Fuel system monitoring

be drawn through a carbon canister in which the hydrocarbon vapours are collected and stored.

As vehicles accumulate mileage then so also During certain closed loop fuel control condi-

do the components, sensors and actuators of the tions the microprocessor activates a solenoid con-

emissions control systems. Mass airflow sensors trolled ‘vapour management valve’. This allows

become dirty and their response slows with age. the manifold vacuum to draw vapour from the car-

Exhaust gas oxygen sensors also respond slower bon canister along vapour lines, which terminate

as they are subject to the in-field failure modes in the intake manifold. The fuel vapour is then

such as oil and fuel contamination, thermal stress combined and combusted with the standard air

and general ageing. Fuel pressure regulators per- /fuel charge, the closed loop fuel control system

form outside of their optimum capacity, fuel injec- caters for the additional air fuel ratio enrichment

tors become slower in their response, and partial to ensure that stoichiometric fuelling continues.

blockages mean that they deliver less and some- The evaporative system monitor is responsible

times more fuel than requested. for determining the serviceability of the EVAP sys-

If this component ageing were not compen- tem components and to detect leaks in the vapour

sated for it would mean that the fuel system would lines. Most manufacturers check for fuel vapour

not be able to maintain normal fuelling around leaks by employing a diagnostic that utilises a

stoichiometric air fuel ratio as shown in Figure

On-board diagnostics 69

Ps ∆p

Pu 3

1 – Line from fuel tank to carbon canister.

4 – Canister-purge valve.

2 – Carbon canister.

5 – Line to intake manifold.

3 – Fresh air.

6 – Throttle valve.

Ps–Intake-manifold pressure. ∆p–Difference between intake-manifold Pu–Atmospheric pressure.

pressure and atmospheric pressure. Figure 5.9 Evaporative emissions control system (Source: Bosch)

exceed emission limits. A lambda value of one is Referring to Figures 5.10 and 5.11, it can be seen required in order for the three-way catalyst to

that when there is a component malfunction, which work. In addition to this, more severe fuelling

causes the AFR in the exhaust stream to be rich, errors would cause noticeable effects in the per-

then there is a need to adapt to this to bring the formance of the vehicle leading to customer

AFR back into the region of stoichiometry. The complaints and potential to damage the manufac-

value of the long-term fuel trim correction must turer’s brand image.

decrease because less fuel is required. Should the This compensation strategy is known as adap-

situation continue and the problem causing rich tive learning. A dedicated piece of software con-

AFR becomes slowly worse, then the error adap- tained within the ECU learns these deviations

tion will continue with an ever-decreasing value away from stoichiometry, while the fuel system is

for the long-term fuel trim being applied, learned in closed loop control. They are stored in a mem-

and stored in memory.

ory that is only reset when commanded by a tech- The purpose of the fuel monitor is to deter- nician and which is also robust to battery changes.

mine when the amount of long-term adaptive cor- These memory-stored corrections are often

rection has reached the point where the system termed ‘long-term’ fuel corrections. They are often

can no longer cope. This is also where long-term stored in memory as a function of air mass, engine

fuel trim values reach a pre-defined or ‘calibrated’ speed or engine load.

limit at which no further adaption to error is An exhaust gas oxygen sensor detects the

allowed. This limit is calibrated to coincide with amount of oxygen in the catalyst feed gas and

exhaust tailpipe emissions exceeding legislated the sensor produces a voltage, which is fed back to

levels. At this point, and when a short-term fuelling the microprocessor. This is then processed to deter-

error exceeds another ‘calibrated’ limit, a DTC is mine the instantaneous or ‘short-term’ fuel correc-

stored and after consecutive drives the MI is illu- tion to be applied. This is done in order to vary the

minated. The opposite occurs, with extra fuel being fuel around stoichiometry and allow three-way

added, via the long-term fuel trim parameter, catalysis to occur.

should an error occur that causes the AFR at the The microprocessor then calculates the amount

exhaust gas oxygen sensor to be lean. of fuel required using an equation, which is shown in its most basic form below.