Electrical & Ignition

Race ECUs

A special category of ECUs are those used by automotive racers. These units do not have a predefined fixed behavior, but are programmable, or mappable. Since a race car often has a modified engine, the behavior of the ECU must be modified as well to adapt it to its new working environment. The ECU can often be programmed/mapped while the engine is running by connecting a laptop to it using a serial or USB cable. A map, consisting of a large number of configuration parameters, tells the ECU how it should control the engine given a number of specific sensor inputs.

An example of this is the amount of fuel to be injected into each cylinder, which varies depending on the engine's RPM and the position of the gas 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 a gas pedal position (or the throttle position, as it is called). In this cell the number of milliseconds that each injector should fire fuel is entered.

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 dyno, giving the tuner a controlled environment to work in.

Other parameters that are often mappable are:

Ignition: Defines when the spark plug should fire for a cylinder

Rev limit: Defines the max RPM that the engine is allowed to rev to. After this fuel and/or ignition is cut.

Water temperature correction: Allows for additional fuel to be added when the engine is cold (choke).

Transient fueling: Tells the ECU to add a specific amount of fuel when throttle is applied.

Low fuel pressure modifier: Tells the ECU to increase the injector fire time to compensate for a loss of fuel pressure.

Closed loop lambda: Lets the ECU monitor a permanently installed lambda probe and modify the fueling to achieve stoichiometric (ideal) combustion.

Some of the more advanced race ECUs include functionality such as launch control, limiting the power of the engine in first gear to avoid burnouts. Other examples of advanced functions are:

Waste gate control: Sets up the behavior of a turbo waste gate, controlling boost.

Banked injection: Sets up the behavior of double injectors per cylinder, used to get a finer fuel injection control and atomization over a wide RPM range.

Variable cam timing: Tells the CPU how to control variable intake and exhaust cams.

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 look for anomalies after the event. The data logger usually has a capacity between 0.5 and 16 Mbytes.

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, CANbus or ethernet.

Ignition Basics

A GOOD SPARK MEANS MAXIMUM HORSEPOWER!

By Phil White

The ignition system of an internal-combustion engine is an important part of the overall engine system. It provides for the timely burning of the fuel mixture within the engine. Not all engine types need an ignition system - for example, a diesel engine relies on compression-ignition, that is, the rise in temperature that accompanies the rise in pressure within the cylinder is sufficient to ignite the fuel spontaneously. All conventional gasoline engines, by contrast, require an ignition system.

The earliest gasoline engines used a very crude ignition system. This often took the form of a copper or brass rod which protruded into the cylinder, which was heated using an external source. The fuel would ignite when it came into contact with the rod. Naturally this was very inefficient as the fuel would not be ignited in a controlled manner. This type of arrangement was quickly superseded by spark ignition, a system which is generally used to this day, albeit with sparks generated by more sophisticated circuitry.

The simplest form of spark ignition is that using a magneto. The engine spins a magnet inside a coil, and also operates a contact breaker or similar device, interrupting the current and causing the voltage to be increased sufficiently to jump a small gap. The spark plugs are connected directly from the magneto output. Magnetos are not used in modern cars, but they are often found on drag cars, mopeds with 2-stroke engines and also in aircraft piston engines, where their simplicity and self-contained nature confers a generally greater reliability as well as lighter weight. Another good thing about a magneto is that the spark output increases proportionally with engine rpm, this is an important factor for a drag race motor. Aircraft engines usually have multiple magnetos to provide redundancy in the event of a failure.

The disadvantage of the mechanical system is that it requires regular adjustment to compensate for wear, and the opening of the contact breakers, which is responsible for spark timing, is subject to mechanical variations. In addition, the spark voltage is also dependent on contact effectiveness, and poor sparking can lead to lower engine efficiency. Electronic ignition (EI) solves these problems. In an EI system, the contact breaker points are replaced by an angular sensor of some kind - either optical, where a vaned rotor breaks a light beam, or more commonly using a Hall effect sensor, which responds to a rotating magnet mounted on a suitable shaft. The sensor output is shaped and processed by suitable circuitry, then used to trigger a switching device such as a thyristor, which switches a large flow of current through the coil. The rest of the system (distributor and spark plugs) remains as for the mechanical system. The lack of moving parts compared with the mechanical system leads to greater reliability and longer service intervals. For older cars, it is usually possible to retrofit an EI system in place of the mechanical one.

During the 1980s, EI systems were developed alongside other improvements such as fuel injection systems. After a while it became logical to combine the functions of fuel control and ignition into one electronic system known as an engine management system.

In an Engine Management System (EMS), electronics control fuel delivery, ignition timing and firing order. Primary sensors on the system are engine angle (crank or Top Dead Center (TDC) position), airflow into the engine and throttle demand position. The circuitry determines which cylinder needs fuel and how much, opens the requisite injector to deliver it, then causes a spark at the right moment to burn it. Early EMS systems used analog computer circuit designs to accomplish this, but as embedded systems became fast enough to keep up with the changing inputs at high revolutions, digital systems started to appear.

Some designs using EMS retain the original coil, distributor and spark plugs found on cars throughout history. Other systems dispense with the distributor and coil and use special spark plugs which each contain their own coil (Direct Ignition). This means high voltages are not routed all over the engine, they are created at the point at which they are needed. Such designs offer potentially much greater reliability than conventional arrangements.

Modern EMS systems usually monitor other engine parameters such as temperature and the amount of pollution in the exhaust. This allows them to control the engine to minimize unburnt fuel and other noxious gases, leading to much cleaner and more efficient engines

Copyright © 2007-2008 DragTech.com. All Rights Reserved.