A modern engine makes use of a wide variety of technology, but it's not necessarily well-understood by the majority of the motoring public. Considering that this stuff isn't rocket science (for the most part), we figured that it's time to lift the hood on one of the cars in our garage and walk through its engine management system. Getting through it all will take a while, but your patience will be rewarded with a significantly improved understanding of what makes your car's powertrain tick.
The vehicle that's serving as the model for this photo shoot is a '96 Buick Roadmaster Limited, equipped with the LT1 5.7L V8. While this engine is a decade old, rest assured that the fundamental practice engine management of hasn't changed all that much in the past ten years. Where there are differences, we'll point them out.
First of all, we need to define what exactly engine management is all about. In the case of the typical gasoline engine, the driver controls the throttle opening via the accelerator pedal. Depending on the engine speed, this will determine the amount of air that's drawn into the engine. This airflow must be measured in some manner, and the appropriate amount of fuel needs to be injected. Once the air/fuel mixture enters the cylinder, a spark event needs to be initiated in order to light off the air/fuel mixture. After the combusted mixture leaves the cylinder, a measurement of the remaining oxygen or fuel has to be performed, and this information needs to be interpreted and appropriate adjustments made to the air/fuel ratio. Finally, the exhaust stream requires post-combustion treatment to remove any remaining hydrocarbons, carbon monoxide, and oxides of nitrogen.
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Everything is coordinated via the engine control module (ECM - also called the engine control unit, or ECU). In this particular case, transmission control functions are incorporated into the ECM to form a powertrain control module (PCM). Here, it's shown on my living room floor, and not in its natural environment (buried underneath the airbox on the left inner fender).
The PCM in the LT1 uses a pair of 8-bit microprocessors that based on the venerable Motorola 6800, but contain custom features specific to engine management. Several application-specific integrated circuits (ASICs) are provided to interface the microprocessors with the various inputs and outputs of the PCM, while 160kB of Intel Flash non-volatile memory and a small amount of RAM serves to store the application code and calibration constants for the system. Newer ECMs and PCMs make use of significantly more powerful 16- and 32-bit processors (such as Motorola's PowerPC) and larger amounts of storage and memory (several megabytes in newer powertrain modules).
Speed is essential when controlling an engine, as an engine spinning at 6000 RPM rotates one degree every 20 microseconds. Extremely "lean" software - working in conjunction with free-running peripheral modules (such as timers) - makes sure that information is processed and commands are issued in a timely fashion. This application code may consist of less than 10% of the total ECM software, with the remainder dedicated to diagnostics and communication. In the case of this PCM, communication to other modules and to the outside world is provided via GM's proprietary ALDL (Assembly Line Diagnostic Link) interface, and through the SAE J1850 protocol. Modern vehicles typically employ CAN (Controller Area Network), and upcoming by-wire systems will likely employ the lightning-fast (by vehicle standards) FlexRay. These communication links provide a means for technicians to retrieve diagnostic information and to reprogram the module with updated code.
While many older EFI systems housed the control modules in the passenger compartment, away from the brutal underhood environment, the increased system cost of the necessarily long wiring harness has resulted in underhood mounting. Many modern vehicles take this one step further by placing the module on the engine and inside the transmission. With underhood temperatures ranging from -40 to 150C, intense vibration and shock, and exposure to the elements (water, fuel, oil, antifreeze, and solvents), it's necessary to protect the electronics with a layer of silicone conformal coating and a robust housing that is capable of shielding the device not only from mechanical and chemical effects but also EMI (electromagnetic interference). Consider that each spark event results in the generation of a 50kV electrical pulse, where as many underhood sensors generate an output of less than 1V. Keeping those signals separated from the noise is an extremely difficult task that borders on being a black art.
On-board diagnostics (OBD) has been required by federal law since 1988. The first systems provided only a minimal amount of self-diagnostics, so OBDII was introduced in 1996 to greatly increase the ability of ECMs to determine if a condition has occurred that could increase emissions beyond federal limits. Problems causing an immediate emissions problem are indicated to the drive via the service engine soon (SES) lamp (also called the malfunction indicator lamp, or MIL) and the appropriate diagnostic trouble code (DTC) is stored, while problems not directly affecting emissions will typically be stored as a DTC without illuminating the SES lamp.
Now that we've covered the "brains" of the system, we'll next move on to take a look at the various sensors that feed information to the powertrain control modules. Stay tuned...