Dale J. Long
Department of Engineering Technology
New Mexico State University
Mr. Anthony Hyde
Manufacturing Program Coordinator
Department of Engineering Technology
New Mexico State University
Sensor controlled systems have become an integral part of todayís automobile. A multitude of electro-mechanical devices have become better refined and more efficient with their application. This report provides an overview of several areas which have advanced with the use of sensors. It should attract the attention of engineering technologist and make them more aware of how sensors can contribute to system control and monitoring.
Automobile sensors can be classified into three basic areas: drive-train and vehicle control, driver safety/comfort/information and emissions. They are used to monitor temperature, gases, voltages/currents, vacuum and torque to name a few. Twenty years ago, the typical automobile had approximately five sensors. Today, over fifty sensors are used to control everything from braking to the fuel delivery system. Increasing awareness concerning emissions, safety and the technology age have all played a role in the advancement of sensing systems. However, some of the most advanced sensors have been developed to control the harmful emissions, such as carbon monoxide (CO) and nitrogen oxide (NOx), which are byproducts of the combustion engine.
Sensors and Emissions
During the late 1950ís, it was determined that the brown haze clinging over Los Angeles was the result of automotive exhaust particles. In 1964, California adopted laws that would require minimal emission control functions on all 1966 model cars. By 1966, the United States Congress adopted the same requirement to take effect on all 1968 and later model cars. The minimal requirement was in the form of a Positive Crankcase Ventilation Valve. Commonly referred to as the PCV valve. The PCV valve routes crankcase gases back to the intake manifold and eventually into the combustion chamber where they can be burned. Although the PCV valve cannot be considered a sensing device. It was the first step leading to a complicated process to reduce auto emissions. Unfortunately, for some auto enthusiast, it was also the demise of the American muscle car. The fact is that, at the time, the only way auto makers could meet the stringent emission laws was to reduce horsepower which made the engine less efficient. The balance of horsepower and emissions would require a technology base that did not exist.
In 1970 the Congress adopted the first major Clean Air Act and also established the Environmental Protection Agency. The EPA sent forth standards on auto emissions requiring new cars to meet .41 grams of Hydrocarbons (HC), 3.4 grams of Carbon Monoxide (CO) and .4 grams of Nitrogen Oxide (NOx), per mile. To put this into perspective, the typical car in 1960 emitted nearly 13 grams of HC, 87 grams of CO, and 3.6 grams of NOx per mile.
The first step in attempting to meet these requirements was the addition of the air injection pump and Exhaust Gas Re-circulation (EGR) valve. Air injection is a system that introduces fresh air to the exhaust manifold. This helps raise the exhaust temperatures which promotes continued combustion of the exhaust gases. The hot (partially unburned) gases leaving the exhaust valves receive a fresh shot of air, causing them to 're- burn' as they leave the manifold. The air injection also helps promote the chemical reaction which takes place inside the catalytic converter. The catalyst system, located in the exhaust flow, needs to reach an operating temperature of approximately 300 deg. F before it is effective.
The EGR system was designed to reduce the NOx emissions. The valve routes unburned gases from the exhaust manifold back to the intake manifold where they could be reintroduced into the combustion chamber. NOx emissions are a direct result of extremely high temperatures (2500 deg. F) which occur during the combustion process. Exhaust gases are considered to be relatively inert and cannot support the combustion process. Therefore, introducing exhaust gases back into the air intake system will dilute the air/fuel mixture and, in turn, reduce the temperatures achieved during combustion. The problem with this theory is that it can also have a very negative effect on the performance of the engine if the gases are introduced at the wrong time.
The first EGR valves were controlled by a relatively simple process which utilized manifold vacuum to open and close the valve. Basically, the valve would open when manifold vacuum was at its peak, under load, and close during reduced manifold vacuum. There were no electronic or mechanical devices to determine the valves position. This resulted in performance problems due to valves not functioning in a timely manner. For instance, if they were to stick open during low rpms, the engine would operate extremely rough. This condition resulted in the research and design of a better valve that would have to be able to "sense" when it should be opened or closed.
Todayís EGR valves have evolved into electronic control systems which utilize solenoids and position sensors in order to function properly. During the evolution, the valves were hybrid which consisted of part mechanical and part electronic. The vacuum portion would control the opening and closing and the electronic position sensor would report itís position to the vacuum controlling device. The vacuum controlling device could then adjust the amount of vacuum applied to the valve which resulted in a much more efficient system.
The latest EGR valves, General Motors in particular, are completely electronically controlled devices with no vacuum input required. The valves have three built-in solenoids that are connected to three different size openings for gases to flow. The valve can open any one, all three or any combination of the openings to control exhaust gas flow depending on the engine load. To determine engine load, the valve uses inputs from the Mass Air Flow (MAF) sensor, the throttle position sensor and engine RPM. Also, in order to prevent exhaust gas flow on a cold engine, the coolant temperature is monitored. Through the use of sensor controlled EGR valves, NOx emissions have been reduced dramatically.
By the late 1970ís the congress had to amend the Clean Air Act because the requirements could not be met. During the mid 1970ís the first catalytic converters were introduced in order to combat hydrocarbon (HC) and carbon monoxide (CO) emissions. However, it wasnít until 1981 that new cars were able to meet the Clean Air Act standards. This was accomplished through he use of an oxygen sensor and a computer controlled three-way catalyst system. The three way catalyst system converts carbon monoxide and hydrocarbons to carbon dioxide and water and also helps to reduce nitrogen oxides. The application of an oxygen sensor controlled by an on-board computer helped to optimize the system.
The oxygen sensor is used to maintain a chemically balanced (stoichiometric) 14.7:1 air/fuel ratio through monitoring the oxygen content of the exhaust gas. The sensor mounts directly on the exhaust system near the exhaust manifold. The sensor monitors the oxygen level in the outside air and compares it to the oxygen level in the exhaust gases. A small portion of the sensor, mainly itís zirconia tip, is in direct contact with exhaust gases. The zirconia reacts with the gases and develops a signal voltage that is sent to a computer that, in-turn, converts the data for use in the air/fuel delivery system. Depending on the level of oxygen, the fuel/air mixture is either increased or decreased.
Because the oxygen sensor does not perform properly at low temperatures, two modes of operation are employed. The modes are referred to as "closed-loop" and "open-loop." Open-loop mode is utilized during engine warm-up. During open-loop the fuel delivery system does not recognize or use the information from the oxygen sensor until a temperature of approximately 600 deg. F is obtained. Once the operating temperature is achieved, the system enters closed-loop mode were the oxygen sensor information is routed to the on-board computer to control the fuel and air delivery.
The latest oxygen sensor system utilizes a dual method that is becoming the industry standard in-order to meet more stringent emissions standards. The dual oxygen sensor method places a sensor before and directly after the catalytic converter. The oxygen storage capacity of the catylist is then compared by the on-board computer and the air/fuel ratio is adjusted to minimize HC emissions.
Another important component involved in the fuel/air delivery system is the Mass Air Flow (MAF) sensor. The MAF sensor is a thin wire or piece of film that is mounted in a cylindrical shaped hollow tube. The tube is then positioned between the air intake filter and the engines throttle body. The MAF is used to measure the amount of air that the engine is consuming by monitoring the air flow. Air flow, or volume, is dependent on the density of the incoming air which also changes with the temperature. Therefore, the MAF sensor has to be able to determine the volume of air intake with respect to outside temperature. This function is performed, on most cars, using the "hot-wire" principal.
The hot wire principal applies a constant voltage to the device while it is positioned in the air stream. The device is then naturally heated by the current flow produced from the voltage. When air flows across the device it cools down. The device acts like a resistor with a positive temperature coefficient. Therefore, the resistance of the device drops with respect to the temperature. Current flow then increases while the resistance drops until a preprogrammed temperature is maintained. The current is then converted to a signal which is routed to the onboard computer and interpreted as air flow. This information, combined with the oxygen sensor, is used to maintain the proper air/fuel mixture to ensure an efficiently operating engine.
Emissions control is going beyond the pollutants produced directly by combustion. In the near future, your gas tank will include a sensor to determine leaks. The EPA has known for years that leaking gas caps, or fuel systems in general, can and do produce a large amount of Volatile Organic Compounds (VOCs). Recent legislation has tightened VOC emissions and will require all cars to be equipped with a fuel system leak sensor. One such device has been developed by the Honeywell Corporation and is called a monolithic fuel tank pressure sensor. It is designed to mount either on the top or side of a fuel tank. The device will employ a single integrated circuit with a micro-machined silicon diaphragm containing piezo-resistors. Silicon was selected as a material because of itís superior strength and quick response. The sensor will be able to detect a leak as small as .5 mm in diameter. Once a leak is detected, the automobiles information system will be activated to warn the operator of the leak.
Sensors and Safety
Vehicle safety is another area were sensor use is expanding rapidly and Anti Lock Brakes (ABS) is one such system were they are a vital component. ABS is utilized to stop a car from skidding when heavy braking is required or when road conditions are slick from rain, ice or gravel. Basically, the system replaces the need for the driver to pump the break pedal in order to help maintain control of the car. Basic physics determines that when a car tire is skidding the coefficient of friction is reduced. The greatest amount of friction occurs just prior to skidding. Also, when a tire is skidding, lets say both front tires, steering the vehicle is virtually impossible. Pumping the brake pedal is how an operator can avoid this problem. However, it is next to impossible for an operator to determine exactly when to pump the pedal. This is where the ABS system can be extremely effective.
ABS employs sensors located at each wheel that monitor the rate at which the wheel is turning compared to the speed of the car. Most systems are not activated until the vehicle exceeds 8 miles per hour. One such ABS system utilizes a magnetoresistive gear tooth sensor which can accurately sense the movement of a ferrous type material. The ferrous material is a wheel that has notches milled into it and rotates along with the vehicles tire. The notched wheel is mounted directly behind the brake rotor or brake drum. The sensor detects the notches as the wheel rotates and develops a signal that is fed to the systems electronic control computer. The computer then calculates the vehicles acceleration, deceleration and skid factors. These factors are used to send control signals to a hydraulic valve which can control the amount of brake pressure at each wheel independently. When the system detects wheel lockup it can rapidly pulse the brake pressure up to 18 times per second. This will prevent skidding and allow the operator to steer the vehicle when rapid stopping conditions are encountered.
ABS is a fine example of how sensors can be employed to control a vehicles stopping power. Currently, there are systems being offered on 1999 model cars that can enhance a vehicles stability and control. Two of these systems are BMWís "Dynamic Stability Control" and General Motors "Active Handling." The systems are centered around the vehicles ABS which is used to control the vehicles path. Three inputs to the system are used: Operator (accelerator, brake pressure and steering), the capabilities of the vehicle and the current condition of the vehicles path. In order to do this, sensors are employed to monitor brake pressure, accelerator position, steering wheel position and speed, G-force, and axle sensors to monitor yaw rate. Yaw is the condition when the rear end of the vehicle wants to rotate to the front (fish tail). This anomaly most often occurs during sudden lane changes or when the operator over corrects. The vehicles stability control computer uses all of the sensor information to calculate the best yaw rate for the vehicles current speed. The system then uses this information to develop control signals for input to the ABS system. A technique similar to that of a tank track is employed where if you apply brakes to one side of the vehicle it makes it turn in that direction. When the system detects that the vehicle path is incorrect signals are sent to the ABS system which applies braking to the necessary wheel or wheels. The system can actually save a driver from a serious accident caused from over or under correcting the vehicles path. It is estimated that over 30 percent of all severe accidents are a direct result of this type of vehicle loss of control.
Sensors are being used to improve automobile driveabilty through suspension control. Cadillac is currently designing a system that will utilize electro-mechanical sensors to monitor and adjust shock stiffness levels. One sensor mounted to each corner of the vehicle between the control arm and body will determine suspension height and speed of movement. The information will be fed to a control system that directs signals to the shocks at each wheel. Even more advanced systems are on the drawing board that will use radar to look ahead of the car for irregularities in the road. The system will know ahead of time when to expect a bump and can then adjust the shocks stiffness to compensate for it.
Since emission standards are constantly changing, safety awareness is increasing and operator information is in high demand, there is little doubt that sensors will continue to evolve. The oxygen sensor is an excellent example. Their applications are continuously undergoing testing so improvements can be made to increase their reliability, life expectancy and efficiency. They have to be ruggedly manufactured in order to withstand the extreme heat and corrosive gases within the exhaust system. Testing, evaluating and integrating them involves almost every engineering discipline due to their application and function. With this information in mind, it appears that vehicle sensor systems will present an ever challenging area for engineers to study.
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