Wide band oxygen sensors
An oxygen sensor is positioned in the exhaust pipe, and provides the engine management ECU with an electrical signal that relates to the amount of oxygen in the exhaust gas. The ECU uses this and other information to determine the correct amount of fuel to be injected into the engine.
Originally automotive oxygen sensors were stoichiometric sensors. Although still in use today, they indicate only if the air/fuel ratio is rich (deficient oxygen) or lean (excess oxygen). They do not indicate how rich or how lean.
Their output signal changes almost vertically either side of an air fuel ratio of stoichiometric 14.7 :1.
It is the “Nernst Cell” inside the sensor that produces this signal voltage.
The Nernst cell operates by comparing the amount of oxygen in the exhaust gas to oxygen levels in the outside air. To operate this cell needs to be hot. Exhaust gas can heat the sensor, however when an internal heater is added, the sensor becomes operational quicker. This is most relevant when the sensor is fitted in cooler parts of the exhaust system away from the exhaust manifold.
As emission standards become tighter for both petrol and diesel engines, a more precise signal is required. In these systems the broad band oxygen sensor informs the ECU of a range of air fuel ratios from 9:1 to air.
These systems are ideal for optimum emissions for car petrol engines diesel engines, particularly where the incoming air is unthrottled and not restricted by a throttle butterfly lean running direct injection petrol engines.
The wide band oxygen sensor is far more sophisticated in operation than the earlier type sensors, however it does include some similar parts.
The Nernst cell is still used, however the exhaust gas oxygen levels are referenced to a sealed chamber of air within the sensor and not outside air. These sensors have an electrical heating element that heats the sensor quickly from a cold start, typically in less than 10 seconds. This is a shorter time than that taken by the older sensors. This is achieved because they have far less material within the sensing element.
Current through the heater is controlled by the ECU.This allows the correct operating temperature to be continuously maintained.
A minute chamber within the sensor has access to the exhaust gas, it is this chamber that the Nernst cell samples exhaust gas from.This sensor works by using a solid state pump to add or remove oxygen from the exhaust gas chamber.
The computer controls the current flowing through the pump so that the Nernst cell output is at stoichiometric. Current flowing in one direction through the pump adds oxygen whilst current in the opposite direction removes oxygen.
The value and direction of current required to do this represents the level of oxygen in the exhaust gas. This allows the ECU to control the amount of fuel delivered and maintain correct emission levels.
Twin oxygen sensors
Some manufacturers fit an oxygen sensor before and after the catalytic converter to test for correct operation. For a catalytic converter to change exhaust gases correctly it needs to be capable of storing and releasing oxygen from the catalyst.
When this happens the amount of oxygen entering the converter will differ from that leaving. The ECU compares these two signals to determine if the catalytic converter is functioning. If a malfunction is detected for a predetermined period, the Malfunction Indicator Lamp or MIL will be illuminated.
Engine misfire can be caused by a variety of faults, however the result is always the same. There is the potential for unacceptable levels of pollution to be produced. Manufacturers can incorporate misfire monitoring to address this.
As an engine rotates through the 4-stroke cycle, the speed of the crankshaft varies. This is due to the change in forces placed on the piston and crankshaft. These changes are gradual and predictable when the engine is under load. By using a high definition (many teeth) crankshaft position sensor/reluctor, the ECU can be programmed to monitor and compare crankshaft position to set predictions.
Misfire monitoring software identifies unacceptable changes in crankshaft speed when a piston is at the relevant position. Depending on the severity of the misfire the ECU may immediately illuminate the MIL or after a delay.
Engine knock occurs in the combustion chamber when two high-pressure waves collide. This unwanted and damaging event can be caused in different ways.
Two examples are excessive load on the engine and engine overheated
Excessive load on the engine:
Overheated engine – faulty thermostat:
One way of overcoming engine knock is to allow the fuel to be ignited later in the compression stroke, this means having less ignition advance. If the fuel is ignited later, then the pressures above the piston will be less during the early stages of the power stroke. This results in engine knock being removed.
The function of the knock sensor is to produce an electrical signal that the ECU can use to determine if knock has occurred. The ECU will then provide less ignition advance until knock is removed.
The sensor is screwed into the engine block where it is influenced by all engine vibrations. Using a piezo crystal the sensor produces a signal voltage proportional to the vibrations applied to it. The ECU interprets the strength, frequency and timing of the signal to determine if knock has occurred.
Once engine knock has been registered, the ECU will gradually reduce ignition advance until Knock is removed.
Twin knock sensors
V configuration engines often have two knock sensors fitted, this allows the ECU to have control over knock on separate banks.
Oil deterioration sensor
The quality of engine oil deteriorates as it is used over time. Additives are used up and contaminants from engine use and wear are added. The net result can be little or no change in oil level but a large reduction in oil quality.
The function of the oil deterioration sensor is to produce an electrical signal that the ECU can use to determine the quality of the oil.
The electrical capacitance value of oil varies with various oil properties, these include Viscosity anti-foaming cleaning.
Electronic circuitry within the sensor converts the capacitance value of the oil to a voltage signal.The ECU monitors voltage signal from sensor and uses this data to determine service requirements.
Exhaust gas recirculation sensors
During lean cruising conditions, combustion temperatures are high due to the slow burning of the lean air/fuel mixture. Under these conditions the pollutant Oxide of Nitrogen or Nox is produced as a by-product of combustion.
To reduce combustion chamber temperatures and resultant Nox, a small amount of the inert exhaust gas can be returned to the combustion chamber via the EGR valve. As emission standards become more stringent it is necessary to have greater control over EGR valve operation. The ECU now has control over this valve and monitors several inputs to determine the amount and timing of operation.
The variables measured can include:
While at idle, engine speed can be adversely affected by fairly minor changes in load. Loads from transmissions, power steering pumps and AC compressors are examples.
For optimum engine control it is necessary for the ECU to be informed of these loads before the engine speed drops.
Switches are used to inform the ECU when the power steering pump pressure is high, automatic transmission drive gear is selected and the air-conditioning compressor is engaged.
The result is excellent ECU control of idle speed as minor loads vary on the engine.