Diesel Engine Sensors
Advanced c-r systems for motor vehicles include the following sensors:
• Temperature—ambient, coolant, intake air, fuel, and exhaust (upstream and/or downstream of turbo). Most of these sensors are negative temperature coefficient types, which lose internal resistance as temperature increases. Coolant and exhaust temperature sensor elements collect deposits that should be periodically removed.
• Pressure—atmospheric, fuel, turbo boost, crankcase, exhaust, and compressed air (for heavy trucks). Exhaust-gas pressure sensors monitor pressure drops across particulate traps.
• Position—rack, idle mode, variable-geometry turbocharging (VGT) control, turbo wastegate, back-pressure, and exhaust gas recirculation (EGR) valves. These sensors generally take the form of a switch or variable resistor.
• Mass air flow entering the manifold.
• Oxygen content in exhaust gas.
• Velocity—crankshaft rpm, road speed and, for some applications, turbo rpm. A coil responds to shifts in the magnetic field by generating a small signal voltage.
One-off EM systems, custom-built for existing installations, often retain the mechanical governor as a way to reduce costs. Production systems incorporate electronic governing, whose operation is controlled by the computer in concert with a magnetic engine-speed sensor. The ECM also uses speed-sensor data to calculate injector pulse width. Figure 6-9 illustrates sensor operation, which uses gear teeth as markers. Some speed sensors pick up the rpm signal from a single gear tooth, which has a distinctive shape and magnetic signature.
In addition, a Hall-effect sensor, triggered by slit on the camshaft gear, generates a timing reference and, in some applications, reports engine speed. Failure of the camshaft position sensor shuts the engine down, often without warning. Bosch systems have a fail-safe feature that enables the computer to calculate timing from the crankshaft speed sensor when the primary reference is lost. The engine will continue to run, but starting may be more difficult. For some Cat models, either sensor can stand in for the other.
As mentioned earlier, accelerator position is reported as changes in the band width (duration of voltage) generated by the throttle position sensor (TPS). These sensors are not without problems, and Bosch again comes to the rescue by providing a second, backup throttle position sensor.
A mass air flow (MAF) sensor measures the volume of air entering the engine as a function of its cooling effect on a heated film or a platinum wire (Fig. 6-10). Supporting electronics measure how much current is needed to maintain the film at target temperature, which is about 75° F above ambient. As engine speed and air flow increase, correspondingly more current is needed. Current draw appears as an analog output voltage of between 0 and 5V. MAF sensors lose accuracy when contaminated and can be ruined by rough handling.
One would imagine that the computer, capable of several hundreds of thousands operations a second, would exert absolute control over the engine. But things get out of hand in ways the computer cannot predict. For example, fuel quality varies with each fill-up, injector nozzles wear unevenly, humid air exerts a greater cooling effect on MAF sensors than dry air. These and a hundred other variables affect combustion efficiency. The O2, or lambda, sensor closes the control loop by monitoring the level of oxygen in the exhaust gases.
Unlike other sensors, the platinum-coated zirconium-oxide O2 sensor generates its own voltage. Once it reaches operating temperature (approximately 700 C), the sensor develops nearly 0.7V in fuel-rich environments. As less fuel is burned and the oxygen content of the exhaust approaches that of the atmosphere, sensor voltage drops off to nearly zero. Signal voltage exhibits a sawtooth pattern. Normally, the computer uses this data, in conjunction with air-flow data, to deliver no more fuel than needed for combustion. Under load, the computer relaxes its standards a bit, richening the mixture for more torque. The O2 sensor then functions as a smoke limiter. The sensor also influences the amount of exhaust gas recirculated into the combustion chambers.
Inline engines have an O2 sensor threaded into the exhaust manifold at Y, where temperatures are high and exhaust gases are representative of all cylinders. V-type engines replicate the arrangement for each bank. An electrically heated fourwire (two wires to ground, one carrying signal voltage and the other battery voltage to the heater) O2 sensor will also be installed downstream of the catalytic converter. This sensor monitors converter efficiency and has no effect upon mixture strength. OBD-2 sensors are good for 100,000 miles, although response slows with age and contamination.
In event of malfunction, the EMS can disregard the O2 sensors and enter into open-loop operation. Working without the benefit of feedback, the computer adjusts fuel delivery to programmed values. Open-loop operation is, with good reason, called the “limp-home” mode.