Exhaust Emissions

The combustion process of a gasoline powered engine produces Carbon Monoxide (CO), Hydrocarbons (HC) and Oxides of Nitrogen (NOx).

Carbon Monoxide is a product of incomplete combustion under conditions of air deficiency. CO emissions are strongly dependent on the air/fuel ratio.
Hydrocarbon are also a product of incomplete combustion which results in unburned fuel. HC emissions are dependent on air/fuel ratio and the ignition of the mixture.
Oxides of Nitrogen are a product of peak combustion temperature (and temperature duration). NOx emissions are dependent on internal cylinder temperature affected by the air/fuel ratio and ignition of the mixture.

Control of exhaust emissions is accomplished by the engine and engine management design as well as after-treatment.

The ECM manages exhaust emissions by controlling the air/fuel ratio and ignition.
The ECM controlled Secondary Air Injection further dilutes exhaust emissions leaving the engine and reduces the catalyst warm up time.
The Catalytic Converter further reduces exhaust emissions leaving the engine.

Siemens Oxygen Sensor: The pre-catalyst oxygen sensors (single sensor for 323i and Z3) measure the residual oxygen content of the exhaust gas. The sensors vary in resistance proportional to the oxygen content that allows the ECM to monitor the air/fuel ratio. If necessary, the ECM will "correct" the air/fuel ratio by regulating the injection time. The sensors are mounted in the exhaust manifolds on the E36/E39 (1), up-stream of the catalytic converter. The 323i and Z3 sensor mounts in the down pipe.

The probe of the sensor which is exposed to the exhaust gas is made from titanium dioxide (semi-conductive material). When heated and maintained to an operating temperature of 600 to 700°C, the titanium dioxide becomes conductive and will allow current to flow, based on the amount of oxygen content.

Fig 1: Identifying Manifold

The ECM supplies the oxygen sensors with 5 volts.

The ECM then monitors the voltage drop through the tip of the sensors as a measure oxygen deviation.

Fig 2: Identifying Sensor

The resistance value of the sensor changes rapidly when the mixture deviates. If the oxygen content of the exhaust is high (lean mixture), the oxygen molecules will block the flow of electrons through the titanium dioxide. This creates high resistance and a small voltage drop across the sensor tip (4.6v monitored by the ECM). If the oxygen content in the exhaust is low (rich mixture), the resistance in the semiconductive tip decreases and allows electron flow, creating a large voltage drop (0.1v monitored by the ECM).

The voltage signal monitored by the ECM will vary between approximately 4.6 to 0.1 volts as the mixture changes from lean to rich. The ECM monitors the length of time the sensor is operating in the lean, rich (including time of rise and fall) and rest conditions. The evaluation period of the sensor is over a predefined number of oscillation cycles.

Fig 3: Display Of Lean And Rich Mixture

Direct Oxygen Sensor Heating: The oxygen sensor conductivity is efficient when it is hot (600° - 700°C). For this reason, the sensors contain heating elements. These "heated" sensors reduce warm up time and retain the heat during low engine speed when the exhaust temperature is cooler. OBD II requires monitoring of the oxygen sensor heating function and heating elements for operation.

The four oxygen sensor heating circuits (E36/E39 shown) receive operating voltage from the ECM Relay when KL15 is switched "ON". Each of the sensor heaters are controlled through separate final stage transistors.

The sensor heaters are controlled with a pulsed square wave voltage during a cold start. This allows the sensors to be brought up to operating temperature without the possibility of thermal shock. The duty cycle is then varied to maintain the heating of the sensors.

Fig 4: Identifying Direct Oxygen Sensor

When the engine is decelerating (closed throttle), the ECM increases the duty cycle of the heating elements to compensate for the decreased exhaust temperature.

Metal Monolith Catalytic Converter: The dual (single on the 323i and Z3) three-way catalysts on the E36/E39 (1) after-treats exhaust emissions leaving the engine. A properly operating catalyst consumes/stores most of the oxygen that is present in the exhaust gas which is a result of burning the remaining pollutants.

The oxygen sensor monitors the air/fuel mixture which allows the ECM to maintain the correct mixture for catalyst efficiency.

The gases that flow into the catalyst are converted from CO, HC and NOx to CO2, H2O and N2.

Fig 5: Identifying Metal Monolith Catalytic Converter

The exhaust flow heats the catalyst and with the remaining oxygen, the exhaust pollutants are further reduced by burning. The temperature operating range for the highest efficiency is 400° - 800° C which is also influenced by the air/fuel mixture. This type of catalyst will store small amounts of excess oxygen which will aid in diluting the exhaust.

The metal monolith matrix consists of thin (0.04 mm) metal strips (flat and corrugated) that are wound together to form circular bodies. The complete wrapped assembly is inserted into a round sheet metal jacket (1.5 mm thick). The jacket and matrix are coated with chrome-nickel and chrome steel.

Fig 6: Identifying Compact Catalyst

This type of catalyst is compact and offers low back pressure with a large internal surface area. The metal monolith has a very rapid "light off" time and an even heat distribution.

Catalytic Converter Monitoring: In order to determine if the catalysts are working correctly, post catalyst oxygen sensors (2 E36/E39 and single on the 323i and Z3)) are installed to monitor exhaust gas content exiting the catalysts.

Fig 7: Identifying Catalytic Converter Monitoring

The signal of the post cat. O2 sensor is evaluated over the course of several pre cat. O2 sensor oscillations. During the evaluation period, the signal of the post cat. sensor must remain within a relatively constant voltage range (3.5 - 4.6v).

The post cat. O2 voltage remains high with a very slight fluctuation. This indicates a consistent amount of oxygen when compared to the pre cat. sensor.

Fig 8: Identifying Good Signal

If this signal decreased in voltage and/or increased in fluctuation, a fault code will be set for Catalyst Efficiency and the "CHECK ENGINE" Light will illuminate when the OBD II criteria is achieved.

Fig 9: Identifying Defective Signal

Secondary Air Injection: Injecting ambient air into the exhaust stream after a cold engine start reduces the warm up time of the catalyst and reduces HC and CO emissions. The ECM controls and monitors the Secondary Air Injection. The components of the system (E36 shown) include:

Secondary Air Injection Pump - Electric pump that draws in ambient air and supplies it to the Non-Return Valve. The pump motor is activated by Secondary Air Injection Pump Relay.
Secondary Air Injection Pump Relay - Supplies operating voltage to the Secondary Air Injection Pump when activated by the ECM.
Vacuum Vent Valve (Solenoid) - Switches vacuum to open the Non-Return Valve when activated by the ECM.
Non-Return Valve - Opened by the Vacuum Vent Valve allowing air to be pumped into the exhaust stream. Sprung closed to prevent exhaust (pressure) from reaching the Secondary Air Injection Components.
Secondary Air Injection Pipes - Distributes injected air to the exhaust manifold for bank 1 (cylinders 1, 2, 3) and bank 2 (cylinders 4, 5, 6).

Fig 10: Testing Air Injection

Misfire Detection: As part of the OBD II regulations the ECM must determine misfire and also identify the specific cylinder(s), the severity of the misfire and whether it is emissions relevant or catalyst damaging based on monitoring crankshaft acceleration.

In order to accomplish these tasks the ECM monitors the crankshaft for acceleration by the impulse wheel segments of cylinder specific firing order. The misfire/engine roughness calculation is derived from the differences in the period duration of individual increment gear segments.

Each segment period consist of an angular range of 90° crank angle that starts 54° before Top Dead Center.

If the expected period duration is greater than the permissible value, a misfire fault for the particular cylinder is stored in the fault memory of the ECM.

Fig 11: Identifying Crankshaft Position Sensor

Depending on the level of misfire rate measured, the ECM will illuminate the "CHECK ENGINE" Light, deactivate the specific fuel injector to the particular cylinder and switch oxygen sensor control to open-loop.

In order to eliminate misfire faults that can occur as a result of varying flywheel tolerances (manufacturing process) an internal adaptation of the flywheel is made. The adaptation is made during periods of decel fuel cut-off in order to avoid any rotational irregularities which the engine can cause during combustion. This adaptation is used to correct segment duration periods prior to evaluation for a misfire event.

If the sensor wheel adaptation has not been completed the misfire thresholds are limited to engine speed dependent values only and misfire detection is less sensitive. The crankshaft sensor adaptation is stored internally and is not displayed via DIS or MoDIC. If the adaptation limit is exceeded, a fault will be set.