Lesson: Understanding Electronic Control Vehicles

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Lesson: Understanding Electronic Control Vehicles

Postby Guest » Wed Nov 21, 2001 7:24 am

Introduction to Electronic Engine Management - by Paul Huizenga

As emissions and fuel economy standards got tougher during the late 80's, it became apparent that the days of the carburetor were numbered. Just as breaker-point ignitions went the way of the dinosaur before them, the increasingly complicated mechanical methods of mixing fuel and air weren't up to the task of providing squeaky-clean exhaust and decent power at the same time. The only way to meet the opposing demands of performance, economy, and emissions was to employ computer control of fuel, air, and spark.

The reason for this change is the ability of a properly designed electronic fuel injection and ignition system to very precisely monitor and control an engine, providing drivability, tuning and power improvements that previous systems couldn't hope to match.

In this article, we'll look at how the Engine Control Unit (ECU) in a typical vehicle does its job, and why understanding what's going on inside that box is critical for the person looking to improve the performance of their engine. Don't worry if I lost you in the previous paragraph with the phrase "breaker-point ignition" - you don't need an engineering degree or a trade school diploma to comprehend the tasks that your car's onboard electronics are performing.

The two main tasks an ECU has to perform are injecting the right amount of fuel and providing a spark at the correct time. In order to do this the system needs to know various information about the engine's current state. Depending on how sophisticated the ECU is, it can track dozens of different sensors, but every system needs to know three basic things: How much air is coming in, the position of the throttle, and how fast the engine is running. Engine speed is pretty simple - a sensor on the crankshaft reads the engine RPM and provides it to the ECU. Throttle position is also easily determined with a simple sensor. But there are two different ways to detect how much air is going into the engine: Mass Air Flow, or Speed /Density.

In a Mass Air Flow system, a device called, oddly enough, the Mass Air Flow (MAF) sensor in the intake tract directly measures how much air is coming in. A Speed / Density system takes a more roundabout way of determining the same thing. Using information on the current manifold air pressure and temperature, and comparing that to the known values of how efficiently the engine "breathes" at various RPM, a Speed / Density system calculates the mass of air entering the engine.

Both systems have advantages and disadvantages. Because a MAF system actually measures the amount of air entering the engine, it will automatically compensate for modifications that improve volumetric efficiency, up to a point. It's possible, however, to 'max out' a MAF sensor, pulling more air past it than it can register. When this happens, the ECU doesn't know about the extra air being drawn into the engine, and can only provide enough fuel for the maximum airflow the MAF can report. It's also important for all the air that the system meters to actually make it to the cylinders, so MAF systems are finicky about air leaks between the sensor and the intake port.

Speed / Density systems aren't as flexible when it comes to adapting to changes in the way an engine breathes. Improvements to the intake flow are unnoticed by a S/D system, which simply assumes a fixed amount of air has entered the engine for a particular combination of manifold pressure, temperature, and RPM. Changes like adding a turbocharger or supercharger, or turning up the boost on a factory turbo can exceed the pressure sensor's range and cause the engine to run extremely lean. But S/D systems lack the restriction of a MAF sensor in the intake tract, and don't particularly care about air leaks in the system ahead of the throttle body.

Using the information about how much air is flowing through the engine and how fast it's turning, the ECU uses a fuel map to determine how long each injector should stay open each cycle to squirt the right amount of fuel into the intake. During part-throttle operation, the injector pulse-width is also modified by the readings from the oxygen sensor. This is a device that sits in the exhaust collector and determines how much oxygen is left over in the exhaust. The oxygen sensors that the manufacturers use are really only accurate when the air/fuel ratio is close to the "perfect" 14 to 1 level, but fortunately that's just the thing for making mixture adjustments at part throttle. Although this ratio is best for economy and emissions, full throttle demands a much richer ratio for power and detonation resistance, closer to 11 or 12 to 1. Because normal oxygen sensors are hopelessly inaccurate in this range, and because emissions testing isn't done at wide open throttle, the ECU switches to "open loop" operation. So called because it no longer looks at the oxygen sensor to close the feedback loop, in open loop mode the ECU simply uses the values it's been programmed with, sans modification. While wideband oxygen sensors that can give meaningful values other than a simple rich or lean are available, they are breathtakingly expensive (think "I could buy a whole new engine for that!") so they are rarely used except for tuning on a dynamometer. Adding an A/F meter to read your stock oxygen sensor is pointless, considering that it won't be accurate except at part-throttle cruise, which is probably not what you're interested in.

So as you can see, the fuel delivery to the engine is only as good as the information the ECU is receiving, and even then changes from stock can make the engine run rich or lean or both. If big enough changes are made to the engine, the ECU can't hope to keep up. Something will have to be done to either "fool" the ECU into adding more fuel to match the increase in air, make the injectors flow more fuel in the same amount of time, or completely reprogram or replace the computer.

There's a huge market right now for add-on electronics that alter the information your ECU is receiving. All of them trick the computer in one way or another, by modifying the signal sent from the MAF, manifold pressure, or some other sensor. Some are quite sophisticated, capable of varying the signal in a non-linear manner based on the input from several sensors or running an electronic boost control solenoid as well. It's even possible to change a MAF system to Speed / Density in order to remove the intake restriction the MAF sensor presents. But all of these electronics essentially drug the ECU, causing it to see the world in a way that differs from reality. By themselves, these electronics don't make any extra power. Instead, they support other improvements to the engine. Used within their limits, they can be quite successful, but the person doing the tuning needs a thorough understanding of how the add-on changes the input to the ECU and how it will react. One of the reasons this approach to modifying the ECU works at all is that the fuel maps for wide-open-throttle operation as they come from the factory are usually quite conservative- in other words, rich. But the built-in margin for error can be quickly used up by someone who doesn't understand what he's doing. Nothing will kill an engine faster than a big lie told to the computer at the wrong moment.

Another approach to upgrading the system is changing how much fuel passes through the injector for a given pulsewidth. The ECU is still telling the injectors to stay open for the same length of time, but more fuel gets through. This can be done by swapping the injectors for ones that have a higher flow rate at stock pressure, or by raising the fuel pressure using an adjustable regulator. Regulators designed for use with forced induction usually also have the ability to alter fuel pressure in response to boost or vacuum as well, but all this really does is compensate for the extra "push" needed to overcome higher-than-atmospheric pressure in the intake manifold. For some applications, matched MAF sensors and injectors are available, with the airflow sensor recalibrated to match the higher flow characteristics of the new fuel injectors. With a combination like this, the ECU needs no adjustment to operate properly.

Probably the best way to overcome the limitations of the stock ECU is to replace it altogether, either with a new unit, or a stock unit reprogrammed to take the engine's state of tune into account. Unfortunately, this can be an expensive undertaking, but when the cumulative costs of all the necessary add-ons are totaled up for a heavily modified motor, the price tag for a stand-alone system starts to seem pretty reasonable. Considering the tuning flexibility and accuracy a new system can provide, it may be a downright bargain for the serious racer. Just be sure that the person doing the tuning or burning the chip understands what they're doing, especially if that person is you.

Knowing how your engine's control electronics inputs and outputs relate will make it apparent when a proposed mod will require some recalibration or a specific piece of hardware will be needed. For instance, putting a blow-off valve on a MAF-type system that dumps to the atmosphere instead of recirculating that air will cause the engine to run very rich every time it's activated, because some of the air the engine counted on getting was tossed overboard. In the same way, doing a lot of intake porting on a speed / density equipped motor may cause it to lean out, because the computer doesn't know the engine is breathing better. Putting a supercharger or nitrous system on an engine that has its timing advanced for optimum normally-aspirated power can be the kiss of death due to detonation. Understanding how engine modifications will change the ECU's sensor readings and what can be done to overcome the limitations of the stock system is the key to tuning a modern fuel injected vehicle. Done haphazardly, it's expensive and frustrating. It's worth the time and effort to really get to know your engine control electronics, even if you don't intend to do the work yourself. At least then, you'll know when someone is trying to sell you something you don't need, or isn't right for your car.



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