Engine control unit

An engine control unit (ECU), most commonly called the powertrain control module (PCM), is a type of electronic control unit that controls a series of actuators on an internal combustion engine to ensure the optimum running. It does this by reading values from a multitude of sensors within the engine bay, interpreting the data using multidimensional performance maps (called Look-up tables), and adjusting the engine actuators accordingly.

Before ECUs, air/fuel mixture, ignition timing, and idle speed were mechanically set and dynamically controlled by mechanical and pneumatic means. One of the earliest attempts to use such a unitized and automated device to manage multiple engine control functions simultaneously was the "Kommandogerät" created by BMW in 1939, for their 801 14-cylinder aviation radial engine.^{[citation needed]}

Working of ECU

Control of Air/Fuel ratio

For an engine with fuel injection, an engine control unit (ECU) will determine the quantity of fuel to inject based on a number of parameters. If the Throttle position sensor is showing the throttle pedal is pressed further down, the Mass flow sensor will measure the amount of additional air being sucked into the engine and the ECU will inject more fuel into the engine. If the Engine coolant temperature sensor is showing the engine has not warmed up yet, more fuel will be injected (causing the engine to run slightly 'rich' until the engine warms up). Mixture control on computer controlled carburetors works similarly but with a mixture control solenoid or stepper motor incorporated in the float bowl of the carburetor..

Control of ignition timing

A spark ignition engine requires a spark to initiate combustion in the combustion chamber. An ECU can adjust the exact timing of the spark (called ignition timing) to provide better power and economy. If the ECU detects knock, a condition which is potentially destructive to engines, and "judges" it to be the result of the ignition timing being too early in the compression stroke, it will delay (retard) the timing of the spark to prevent this. Since knock tends to occur more easily at lower rpm, the ECU controlling an automatic transmission will often downshift into a lower gear as a first attempt to alleviate knock.

Control of idle speed

Most engine systems have idle speed control built into the ECU. The engine RPM is monitored by the crankshaft position sensor which plays a primary role in the engine timing functions for fuel injection, spark events, and valve timing. Idle speed is controlled by a programmable throttle stop or an idle air bypass control stepper motor. Early carburetor-based systems used a programmable throttle stop using a bidirectional DC motor. Early TBI systems used an idle air control stepper motor. Effective idle speed control must anticipate the engine load at idle. Changes in this idle load may come from HVAC systems, power steering systems, power brake systems, and electrical charging and supply systems. Engine temperature and transmission status, and lift and duration of camshaft also may change the engine load and/or the idle speed value desired.

A full authority throttle control system may be used to control idle speed, provide cruise control functions and top speed limitation.

Control of variable valve timing

Some engines have Variable Valve Timing. In such an engine, the ECU controls the time in the engine cycle at which the valves open. The valves are usually opened sooner at higher speed than at lower speed. This can optimize the flow of air into the cylinder, increasing power and economy.

Electronic valve control

Experimental engines have been made and tested that have no camshaft, but have full electronic control of the intake and exhaust valve opening, valve closing and area of the valve opening.^[1] Such engines can be started and run without a starter motor for certain multi-cylinder engines equipped with precision timed electronic ignition and fuel injection. Such a static-start engine would provide the efficiency and pollution-reduction improvements of a mild hybrid-electric drive, but without the expense and complexity of an oversized starter motor.^[2]

The first production engine of this type was invented ( in 2002) and introduced (in 2009) by Italian automaker Fiat in the Alfa Romeo MiTo. Their Multiair engines use electronic valve control which drastically improve torque and horsepower, while reducing fuel consumption as much as 15%. Basically, the valves are opened by hydraulic pumps, which are operated by the ECU. The valves can open several times per intake stroke, based on engine load. The ECU then decides how much fuel should be injected to optimize combustion.

For instance, when driving at a steady speed, the valve will open and a bit of fuel will be injected, the valve then closes. But, when you suddenly stamp on the throttle, the valve will open again in that same intake stroke and much more fuel will be injected so that you start to accelerate immediately. The ECU then calculates engine load at that exact RPM and decides how to open the valve: early, or late, wide open, or just half open. The optimal opening and timing are always reached and combustion is as precise as possible. This, of course, is impossible with a normal camshaft, which opens the valve for the whole intake period, and always to full lift.

Programmable ECUs

A special category of ECUs are those which are programmable. These units do not have a fixed behaviour and can be reprogrammed by the user.

Programmable ECUs are required where significant aftermarket modifications have been made to a vehicle's engine. Examples include adding or changing of a turbocharger, adding or changing of an intercooler, changing of the exhaust system, and conversion to run on alternative fuel. As a consequence of these changes, the old ECU may not provide appropriate control for the new configuration. In these situations, a programmable ECU can be wired in. These can be programmed/mapped with a laptop connected using a serial or USB cable, while the engine is running.

The programmable ECU may control the amount of fuel to be injected into each cylinder. This varies depending on the engine's RPM and the position of the accelerator pedal (or the manifold air pressure). The engine tuner can adjust this by bringing up a spreadsheet-like page on the laptop where each cell represents an intersection between a specific RPM value and an accelerator pedal position (or the throttle position, as it is called). In this cell a number corresponding to the amount of fuel to be injected is entered. This spreadsheet is often referred to as a fuel table or fuel map.

By modifying these values while monitoring the exhausts using a wide band lambda probe to see if the engine runs rich or lean, the tuner can find the optimal amount of fuel to inject to the engine at every different combination of RPM and throttle position. This process is often carried out at a dynamometer, giving the tuner a controlled environment to work in. An engine dynamometer gives a more precise calibration for racing applications. Tuners often utilize a chassis dynamometer for street and other high performance applications.

Other parameters that are often mappable are:

Ignition Timing: Defines at what point in the engine cycle the spark plug should fire for each cylinder. Modern systems allow for individual trim on each cylinder for per-cylinder optimization of the ignition timing.
Rev. limit: Defines the maximum RPM that the engine is allowed to reach. After this fuel and/or ignition is cut. Some vehicles have a "soft" cut-off before the "hard" cut-off. This "soft cut" generally functions by retarding ignition timing to reduce power output and thereby slow the acceleration rate just before the "hard cut" is hit.
Water temperature correction: Allows for additional fuel to be added when the engine is cold, such as in a winter cold-start scenario or when the engine is dangerously hot, to allow for additional cylinder cooling (though not in a very efficient manner, as an emergency only).
Transient fueling: Tells the ECU to add a specific amount of fuel when throttle is applied. The is referred to as "acceleration enrichment".
Low fuel pressure modifier: Tells the ECU to increase the injector fire time to compensate for an increase or loss of fuel pressure.
Closed loop lambda: Lets the ECU monitor a permanently installed lambda probe and modify the fueling to achieve the targeted air/fuel ratio desired. This is often the stoichiometric (ideal) air fuel ratio, which on traditional petrol (gasoline) powered vehicles this air:fuel ratio is 14.7:1. This can also be a much richer ratio for when the engine is under high load, or possibly a leaner ratio for when the engine is operating under low load cruise conditions for maximum fuel efficiency.

Some of the more advanced standalone/race ECUs include functionality such as launch control, operating as a rev limiter while the car is at the starting line to keep the engine revs in a 'sweet spot', waiting for the clutch to be released to launch the car as quickly and efficiently as possible. Other examples of advanced functions are:

Wastegate control: Controls the behavior of a turbocharger's wastegate, controlling boost. This can be mapped to command a specific duty cycle on the valve, or can use a PID based closed-loop control algorithm.
Staged injection: Allows for an additional injector per cylinder, used to get a finer fuel injection control and atomization over a wide RPM range. And example being the use of small injectors for smooth idle and low load conditions, and a second, larger set of injectors that are 'staged in' at higher loads, such as when the turbo boost climbs above a set point.
Variable cam timing: Allows for control variable intake and exhaust cams (VVT), mapping the exact advance/retard curve positioning the camshafts for maximum benefit at all load/rpm positions in the map. This functionality is often used to optimize power output at high load/rpms, and to maximize fuel efficiency and emissions as lower loads/rpms.
Gear control: Tells the ECU to cut ignition during (sequential gearbox) upshifts or blip the throttle during downshifts.

A race ECU is often equipped with a data logger recording all sensors for later analysis using special software in a PC. This can be useful to track down engine stalls, misfires or other undesired behaviors during a race by downloading the log data and looking for anomalies after the event. The data logger usually has a capacity between 0.5 and 16 megabytes.

In order to communicate with the driver, a race ECU can often be connected to a "data stack", which is a simple dash board presenting the driver with the current RPM, speed and other basic engine data. These race stacks, which are almost always digital, talk to the ECU using one of several proprietary protocols running over RS232 or CANbus, connecting to the DLC connector (Data Link Connector) usually located on the underside of the dash, inline with the steering wheel

History

Hybrid digital designs

Hybrid digital/analog designs were popular in the mid 1980s. This used analog techniques to measure and process input parameters from the engine, then used a look-up table stored in a digital ROM chip to yield precomputed output values. Later systems compute these outputs dynamically. The ROM type of system is amenable to tuning if one knows the system well. The disadvantage of such systems is that the precomputed values are only optimal for an idealised, new engine. As the engine wears, the system is less able to compensate than a CPU based system.^{[citation needed]}

Modern ECUs

Modern ECUs use a microprocessor which can process the inputs from the engine sensors in real time. An electronic control unit contains the hardware and software (firmware). The hardware consists of electronic components on a printed circuit board (PCB), ceramic substrate or a thin laminate substrate. The main component on this circuit board is a microcontroller chip (CPU). The software is stored in the microcontroller or other chips on the PCB, typically in EPROMs or flash memory so the CPU can be re-programmed by uploading updated code or replacing chips. This is also referred to as an (electronic) Engine Management System (EMS).

Sophisticated engine management systems receive inputs from other sources, and control other parts of the engine; for instance, some variable valve timing systems are electronically controlled, and turbocharger wastegates can also be managed. They also may communicate with transmission control units or directly interface electronically-controlled automatic transmissions, traction control systems, and the like. The Controller Area Network or CAN bus automotive network is often used to achieve communication between these devices.

Modern ECUs sometimes include features such as cruise control, transmission control, anti-skid brake control, and anti-theft control, etc.

General Motors' first ECUs had a small application of hybrid digital ECUs as a pilot program in 1979, but by 1980, all active programs were using microprocessor based systems. Due to the large ramp up of volume of ECUs that were produced to meet the US Clean Air Act requirements for 1981, only one ECU model could be built for the 1981 model year.^[3] The high volume ECU that was installed in GM vehicles from the first high volume year, 1981, onward was a modern microprocessor based system. GM moved rapidly to replace carburetor based systems to fuel injection type systems starting in 1980/1981 Cadillac engines, following in 1982 with the Pontiac 2.5L "GM Iron Duke engine" and the Corvette Chevrolet L83 "Cross-Fire" engine. In just a few years all GM carburetor based engines had been replaced by throttle body injection (TBI) or intake manifold injection systems of various types. In 1988 Delco Electronics, Subsidiary of GM Hughes Electronics, produced more than 28,000 ECUs per day, the world's largest producer of on-board digital control computers at the time.^[4]

Other applications

Such systems are used for many internal combustion engines in other applications. In aeronautical applications, the systems are known as "FADECs" (Full Authority Digital Engine Controls). This kind of electronic control is less common in piston-engined aeroplanes than in automobiles, because of the large costs of certifying parts for aviation use, relatively small demand, and the consequent stagnation of technological innovation in this market.^{[citation needed]} Also, a carbureted engine with magneto ignition and a gravity feed fuel system does not require electrical power generated by an alternator to run, which is considered a safety advantage.^[5]

References

^ Ian Austen (2003-08-21). "WHAT'S NEXT; A Chip-Based Challenge to a Car's Spinning Camshaft". New York Times. Retrieved 2009-01-16. {{cite news}}: Italic or bold markup not allowed in: |publisher= (help)CS1 maint: date and year (link)
^ Kassakian, J.G (1996). "Automotive electrical systems circa 2005 - IEEE Spectrum". IEEE Spectrum. 33 (8). IEEE: 22. doi:10.1109/6.511737. {{cite journal}}: Italic or bold markup not allowed in: |publisher= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ http://history.gmheritagecenter.com/wiki/index.php/GM_Emission_Control_Project_Center_-_I_Was_There GM Emission Control Project Center - I Was There - GMnext]
^ Delco Electronics Electron Magazine, The Atwood Legacy, Spring '89, page 25
^ Pilot's Encyclopedia of Aeronautical Knowledge. Federal Aviation Administration.
^ "ACCEL-DFI Engine Management Unit". Retrieved 17 July 2011.ACCEL-DFI

External links

[1] Ian Austen (2003-08-21). "WHAT'S NEXT; A Chip-Based Challenge to a Car's Spinning Camshaft". New York Times. Retrieved 2009-01-16. {{cite news}}: Italic or bold markup not allowed in: |publisher= (help)CS1 maint: date and year (link)

[2] Kassakian, J.G (1996). "Automotive electrical systems circa 2005 - IEEE Spectrum". IEEE Spectrum. 33 (8). IEEE: 22. doi:10.1109/6.511737. {{cite journal}}: Italic or bold markup not allowed in: |publisher= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

[3] ttp://history.gmheritagecenter.com/wiki/index.php/GM_Emission_Control_Project_Center_-_I_Was_There GM Emission Control Project Center - I Was There - GMnext]

[4] Delco Electronics Electron Magazine, The Atwood Legacy, Spring '89, page 25

[5] Pilot's Encyclopedia of Aeronautical Knowledge. Federal Aviation Administration.

[DFI_EMU-6] "ACCEL-DFI Engine Management Unit". Retrieved 17 July 2011.ACCEL-DFI

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