Engine management and emission controls (Part 2)

In Part 1 of this series, we took at look at the engine control module (ECM), the "brains" of a modern vehicle's engine management and emission control systems. Next, we're going to dig a bit deeper and investigate the various sensors that feed information about the vehicle's operating conditions to the ECM.

As the throttle opening and engine speeds varies, so to does the amount of airflow drawn by the engine. This can be measured directly via a mass airflow (MAF) sensor, (as shown above circled in green). This particular style of sensor uses a pair of heated wires, one of which is placed in the airflow, and one is shrouded. The sensor's electronics attempts to heat both wires to the same temperature; the one placed in the airstream will require more power to maintain that temperature, and the difference in current required can be related to the mass of the incoming airstream. MAF data is now typically transmitted via a 5V digital pulse that varies in frequency as the airflow changes. Doing so provides a signal that's less susceptible to interference than the previous method of encoding the sensor output as an analog signal.

The MAF sensor provides a very accurate measurement, especially at lower operating speeds, which means that it's well-suited to larger engines (federal emissions standards do not discriminate according to displacement). The trade-off comes in the form of increased cost and potentially decreased reliability, although significant strides have been made in improving the sensor's robustness.

Airflow can also be indirectly measured by determining the manifold absolute pressure (MAP). Modern MAP sensors (circled above in red) comprise a MEMS (microelectromechanical system) element to determine the deflection of a spring-loaded diaphragm, and some additional signal processing to convert this movement to a signal (usually analog) proportional to manifold pressure. The relationship between MAP, engine speed, intake air temperature, and airflow can be determined by experimentation or modeling (usually a combination of both); this technique of measuring airflow is referred to as "speed-density" (SD). Older ECMs store this data in the form of lookup tables, where as modern ECMs take advantage of increased computing power and utilize algorithm-based calculations.

Speed-density systems are less expensive, as no MAF sensor is required. The trade-off is a potential decrease in airflow measurement accuracy at low speeds, although the significance of this can be debated, as SD has been used on hundreds of millions of clean-running engines. SD can also provide better measurement accuracy at large rates of airflow, as MAF sensors usually give up some resolution on the high end in order to provide better accuracy at low rates of airflow.

MAF systems utilize a MAP sensor to provide diagnostics and a backup means of airflow measurement, as well as to provide an indication of engine load to determine shift points and ignition timing.

An intake air temperature (IAT) sensor (circled above in green) is used to measure the temperature of the incoming airflow. Cooler air is denser and less prone to detonation, and therefore requires additional fuel and can allow more ignition advance. With extremely high intake air temperatures, additional fuel may be added to provide a cooling effect and decrease the chances of premature ignition.

IAT sensors are usually with a simple thermistor (a device that provides a varying resistance with temperature). The devices aren't extremely accurate, but this is not a measurement that requires extreme precision.  

The device circled in yellow above is the throttle position sensor, or TPS. As its name implies, it provides an output that varies with the angle of the throttle blades. While it can provide an indication of airflow via a calculation process referred to as "alpha-n," this is not typically used on production vehicles, due to its low accuracy at low engine speeds (competition engines and some motorcycles use alpha-n at low engine speeds if an accurate MAP signal cannot be maintained due to wild camshaft timing). The information from the TPS can be used to validate the airflow measurements obtained via MAF or SD techniques, and the TPS is also used to determine when the throttle blades are at or near the idle position. In vehicles with automatic transmissions, the throttle position can also be used to determine shift points. A malfunction of the TPS normally manifests itself in the form of inconsistent idle speed and poorly-timed shifts.

The object circled in red is the IAC motor, which we'll cover in part three of this series.

The coolant temperature sensor (CTS) determines the temperature of the engine in order to provide for cold-start enrichment of the air/fuel mixture. Many engines will also retard the ignition timing at low temperatures in order to allow the engine to warm up more rapidly. At extremely high coolant temperatures, additional fuel may be added in an attempt to cool the combustion charge, and some engines will go so far as to shut down cylinders to alleviate the condition. Like the IAT sensor, the CTS sensor utilizes a thermistor.

This particular engine uses GM's short-lived Optispark distributor, which was mounted at the front of the engine and driven directly off the front of the camshaft. Two optical sensing systems are employed to determine the exact position of the camshaft, with the low-resolution system providing eight pulses of varying width during each rotation to indicate which cylinder is on the compression stroke, and a high-resolution channel providing a total of 720 pulses per revolution for determining the exact position of the cam. By improving the sensing accuracy, the spark advance can be run closer to the limit of detonation, resulting in increased performance and decreased emissions. The system was probably the most sophisticated distributor offered on a mass-production vehicle, but was soon rendered obsolete by the far superior individual coils systems that are now used nearly everywhere.

At the front of the engine, mounted in the timing cover, is a crankshaft position (CKP) sensor. Hall-effect devices with back-biasing magnets are used to detect a toothed metal wheel that is mounted to the crankshaft. In this particular case, the CKP sensor is simply used to detect misfires at idles (as these have been identified as a significant cause of increased emissions), and as such has only four teeth (one for each pair of pistons). Contemporary ignition systems use the CKP sensor to accurately reference spark events, and typically use a far greater number of teeth to increase measurement precision.

Sensing the crank position for the purposes of establishing ignition timing provides far greater accuracy than the detection of cam position, as the lash in a worn cam drive system can add several degrees of uncertainty to a measurement.

A pair of oxygen sensors are used ahead of the catalytic converters (AKA "cats" or "cat-cons") to measure the quality of the exhaust. A voltage below 0.55V indicates a lean condition (not enough fuel), while a voltage above 0.55V indicates richness (potentially too much fuel). The ECM can use this information to adjust the air/fuel ratio (this is referred to as "closed-loop" operation), with the output of the sensor "dithering" about the expected average voltage when all is working properly. During the rich portion of the varying cycle, the cat can reduce oxides of nitrogen, and the lean portion of the cycle is used to oxidize carbon monoxide and hydrocarbons.

This particular type of sensor can only measure whether the exhaust stream is rich or lean; it does not provide an accurate indication of how rich or lean the engine is operating. During large throttle openings, the engine must run rich to provide optimum power and prevent detonation, so additional fuel is added and the engine operates in an "open-loop" manner and ignores the oxygen sensors. To enable closed-loop operation under all conditions, several modern engines employ a wideband O2 sensor. Such sensors providesan output that is roughly linear over a wide range of air-fuel ratio, but at a greatly increased cost and complexity.

This type of sensor requires operating temperatures above 600 degrees Fahrenheit to provide the proper output, so a heater is used to accelerate the warm-up of the sensor. Thus, such sensors are properly called heated exhaust gas oxygen (HEGO) sensors, or HO2S.

Behind each cat is an additional HO2S that serves to monitor the "health" of each device. Ideally, the post-cat exhaust stream should be neither rich nor lean. If the post-cat sensor provides some other response, it is likely that the catalyst is worn or damaged and needs to replacement, and the ECM's diagnostic functions will respond accordingly.

In the final installment of this series, we're going to take a look at some of the other devices that contribute to a vehicle's engine management and emission controls functionality.