Clickable Fuel Cell Car
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MFA MAP EFI
Voltage Drops
Voltage drops occur when current and voltage meet some kind of resistance. Resistance can be there intentionally, such as a light bulb, or it can be unintentional, like a bad connection.
Resistance and voltage drops have a proportional relationship. As the resistance increases the voltage drop increases, or if it is easier to conceptualize--as the resistance ncreases the voltage decreases.
This can be shown mathematically using the equation:
IxR=E
I: Intensity (Amps), E: Electrical Potential (Volts), R: Resistance (Ohms)
I= 6A, E=12V, R= 2 ohms
6 amps x 2 ohms = 12 volt drop.
Reference Voltage
A reference voltage is simply preset voltage sent to some component by the computer. The computer reads the amount of drop in that voltage after it has gone to the component. A reference voltage can be a full 12 volts or it can be as little as 5 volts or less. This voltage drop is used to determine the state of a given resistor.
Variable Resistors
Cars use two basic types of variable resistors--thermistors and potentiometers. The two types have similar end results but achieve those results differently.
Thermistors change resistance based on temperature. This is accomplished chemically. Thermistors are constructed of a semi-conductive material--such as silicone--that has been engineered to give a specific resistance when cold. When heat is applied, the resistance goes down. In some cases, the resistor itself will produce a small voltage by releasing electrons from the compound. This small voltage produced by the resistor can also be used to indicate temperature--such as a simple automotive temperature gauge. Thermistors are sometimes referred to as-- Negative Temperature Coefficients (NTC) or Negative Coefficient Thermistors (NCT).
Alternately, a potentiometer changes resistance mechanically. It is a large resistor--such as a spring--and three electrical contacts: A positive, a negative and a variable return contact.
The computer sends a reference voltage that can be measured in its entirety by checking the end connections. Or, by checking the variable return, the actual resistance can be measured. With this type of resistor the computer can determine airflow, manifold vacuum and position of the throttle plate.
Watts
A simple definition of Watts is the ability to do work. For our discussion purposes we will be looking at heat as a property of work. Watts can be described in the equation:
W = I x E
(W: watts, I: amps, E: volts)
As the current or the voltage increases the Watts increase. For example, a 12-volt system with initial amperage of 2 will produce 24 watts. Increasing the amps flowing through a circuit will produce more Watts, thus more heat. A circuit with 12 volts and 4 amps will produce 48 watts etc.
Electromagnetism
When a current is sent through a wire a small magnetic field develops. This magnetic field can be increased by looping wires together in such a way that the fields combine. If these fields are surrounding an iron object--such as a bar--that object will also become magnetized.
The strength of the magnet can be increased two ways. 1. Increase in the number of turns in the wire (more fields to combine). 2. Increase the current flow through the wire.
Electromagnetism drives the operation of solenoids. Electricity magnetizes the core, which is anchored so that it does not move. When the magnet is energized, it pulls the plunger toward it, creating a mechanical motion that can be put to work.
Starter Solenoid
Fuel Injection Systems
Modern gasoline engines use two basic types of fuel injection. These two types of injection systems operate by the same principles, yet look very different. The two systems are Throttle Body Injection and Port Injection.
Port Injection systems send fuel directly to the individual cylinders. Often these systems have various sensors located throughout the engine compartment to detect airflow, vacuum, temperature and exhaust.
Throttle Body systems usually have two injectors mounted in an aluminum housing, which is then mounted to the intake manifold. Often Throttle body systems have nearly a ll the required sensors located in the same aluminum housing as the injectors.
There are two significant differences between these systems--cost and performance. Port injection systems are far superior to throttle body in terms of performance. Since each cylinder receives fuel individually, the computer is able to control fuel consumption as well as power much more efficiently. However, to manufacture, this system is much more expensive. The individual sensors must be manufactured so that they can be mounted independently. Throttle body systems on the other hand, offer lower performance, but are cheaper to manufacture.
Throttle Body Injection System
Port Injection System
Fuel Injection Components
The key to an efficient electronic fuel injection system is a computer. All modern injection systems (as well as some carburetor systems) utilize some type of computer. The automotive industry has many names for these computers. Depending on the manufacturer, the computer may be called:
- Electronic Control Module (ECM)
- Power Control Module (PCM)
- Electronic Control Unit (ECU)
- Brain Box (common in the used parts industry)
All of these names accurately reflect the computer's purpose. The computer receives information from the sensors. Then, based on the information received sends a signal in the form of voltage to the fuel injectors (or other mechanical components).
The concept of a computer is quite simple. However, actual implementation of those concepts is not so simple. The computer distributes battery voltage through a complex system of resistors and computer chips. The "input" is merely voltage and current that the computer interprets based on where that "signal" is coming from. For example, when a component sends voltage (or does not send voltage) that is outside acceptable range the computer will store a trouble code, which in turn often triggers t he "check engine" light.
Modern computers can actually compensate for faulty components by extrapolating needed input from other sources. For example, if the air flow meter malfunctions, the computer will extract he information it needs to keep the vehicle operating from the throttle position sensor and the oxygen sensor. The change in performance may be slight enough that the driver will no notice the difference, but will trigger a trouble code.
A trouble code is a computer's internal system for recording errors. Scanners, used by mechanics, look for these trouble in order to diagnose a specific problem.
Air Flow Metering
Multiple airflow devices are available. The first is called a Mass Air Flow Sensor (MAF). An MAF measures airflow with heat. Two types of heating elements are used in MAF systems. One type uses a titanium wire. The other type uses a conductive heating film. Both types work the same way. The MAF generally has a built-in thermistor to measure the actual air temperature in relation to the heating element. The heating element is heated to a temperature 70 degrees Celsius above the ambient air temperature. For example, if the ambient temperature is 10 degrees Celsius the computer--using Watt's law W=IxE--will send sufficient current through t he heating element to maintain the 70 degree difference; in this case the temperature of the element would be 80 degrees Celsius. As air flow increases the element is cooled. Accordingly the module inside of the MAF housing will increase the amount of current going through the heating element. The module converts those current fluctuations into a voltage signal that is sent to the computer in waveforms. The computer uses this information to extrapolate the volume of air going through the system, then adjusts the duty cycle of the fuel injectors to match.
Another type of airflow sensor is called an Air Flow Meter (AFM). An air flow meter also has a built thermistor to measure ambient air temperature. However, an AFM measures airflow mechanically using a potentiometer.
An AFM uses a movable vane to indicate where the contact is on the resistor in relation to the amount of air passing through it.
The Throttle Position Sensor (TPS) is another device used to measure airflow. However, rather than measuring the actual airflow, the TPS measures the position of the throttle plate. This is vital for the computer to know for two reasons. First, this is how the computer knows when the vehicle is at idle and when the operator is trying to accelerate. Second, this is how the computer can determine if enough airflow will be possible in order to maintain the proper stoichiometric ratio. Again a TPS is nothing more than a potentiometer. The computer sends a reference voltage then reads the amount of voltage drop--which is determined by the position of the throttle plate.
The final air-sensing device that will be discussed here is called the Manifold Absolute Pressure Sensor (MAP). The MAP sensor is a combination of a vacuum diaphragm and a potentiometer. The MAP is attached to the intake manifold or plenum in some way, either directly mounted to the manifold or via a vacuum hose. Engine vacuum pulls on the diaphragm, which is connected, to the potentiometer. Again, via the voltage drop the computer can determine the vacuum at any given moment.
Fuel Injectors
With information from various sensors the computer can control fuel delivery to meet the needs at a given instant. A Fuel Injector is used to deliver that fuel. A fuel injector is a solenoid that opens and closes allowing fuel to pass through it. The computer controls how long the injector stays open or in some cases how quickly it opens and closes.
A basic understanding of how components work will allow testing of nearly all parts of a fuel injection system. Before any testing, a good service manual should be obtained. Many components work on the same principles, however they do not operate within the same tolerances. For example, one system may use a 5-volt reference voltage for the AFM with a resistance range of .05 ohms to 2 ohms. Where as another system may use an 8-volt reference voltage with a resistance range of 3 ohms to 5 ohms.
Below is a typical example of what would be found in a service manual.
To test the various sensors one must have the ability to recognize what type of sensor is being worked on. Once that is determined, testing is quite simple.
Resistor--Use an ohmmeter to determine if the resistor is within specifications.
Thermistor--Test resistance with an ohmmeter, apply heat to test range.
Potentiometer-- First, check resistance across the entire resistor, make sure it has continuity as well as being within specifications. Then test across the variable connection. Watch carefully for any "dead" spots. Again, determine if it is within specifications.
Solenoids--First, check resistance across the two terminals. Then ensure resistance is within specifications. If possible, apply power to see if it is actually functioning.
Conceptual Questions
- What are some of the factors that could cause an MAF to fail?
- What could cause an AFM to fail?
- What could cause a TPS to fail?
- What could cause a fuel injector to fail?
Something to ponder
Imagine that you just purchased a new CD player for your car. With your trusty test light in hand you poke around under the dash until you find a wire that has power when the key is on. You install your new stereo and it works fine. You put the dash back together and start your car. As you are pulling out of the garage you turn on the stereo, and the car dies. It turns over but will not start.
What could be the cause?
One possibility is that the wire spliced into was a reference voltage circuit. That circuit is designed to have a set current running through it. This same circuit is fed by the computer, which is designed to handle a certain amount of current. What happens when the load on the circuit is increased? There is a good chance that the computer has been damaged.
The question is how to avoid such mistakes. The solution is simple--wiring diagrams.
When diagnosing a problem it is necessary to perform the proper tests before buying new parts. Imagine that the computer indicates--through a trouble code--that the MAP is malfunctioning. The MAP is replaced at a cost of $150.00, yet the problem persists--now what?
Consider how a computer functions. If, for example, the resistor in the reference voltage circuit itself is defective and sends 8 volts to a 4-volt system. The voltage drop the computer reads will be incorrect. Generally speaking computers are not programmed to detect an internal malfunction. Another consideration is wiring. The wiring in a vehicle has been chosen based on a specific resistance. The wiring to the AFM must be of a certain resistance in order to maintain the proper voltage drop reading. If that wiring is damaged or has a bad connection the computer may interpret the increased voltage drop as a defective component.
New and used electrical parts are non-returnable in most cases. That means buying a new MAP for $150, "just to try it" becomes very expensive very quickly. Individual components should always be tested before replacing.
Conclusion
Modern fuel injection systems seem very complicated. However, with a little bit of knowledge and time spent analyzing, they are not as terrible as they look. Nearly all the components in an entire vehicle utilize the same principles. Understanding these principles is key to understanding how to diagnose and repair a car.
Some of these concepts are not easily understood the first time through. The key to working with electrical components is to take a little time and think about how they work, once that is clear then testing should not be a problem
نام قطعات
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1. Battery
2. Relay set - Operates electronic control unit 3 and motor driven fuel pump 17.
3. Electronic control unit - Receives information about oil quantity, coolant temperature and temperature of cylinder head, position of throttle valve, starting phase as well as engine rpm and injection point. It processes this information and transmits electric pulses to the solenoid injector. It is connected with the individual components by means of a multiple connecting plug and associated connecting cables.
4. Coil - In addition to its normal function, it transmits the number of engine rpm or the injection point to the control unit.
5. Air measuring instrument - This supplies information to electronic control unit of quantity of air drawn and activates fuel supply pump.
6. Supplementary air valve - Supplies extra air during engine heating stage, depending on temperature level.
7. Throttle switch - Signals idle and full load to electronic control unit.
8. Temperature sensor - Signals coolant temperature at the cylinder head outlet.
9. Cold starting solenoid injector - During starting in low temperatures, injects extra fuel into intake line.
10. Solenoid injectors - Inject fuel in intake port of cylinders.
11. Delay thermal switch - This automatically controls injection of cold starting solenoid injector.
12. Fuel pressure regulator - Keeps fuel pressure constant in fuel lines.
13. Fuel delivery line
14. Excess fuel exhaust line
15. Ignition switch
16. Fuel filter - Is fitted on fuel supply line for fuel filtering.
17. Motor driven fuel supply pump - Delivers a constant supply of fuel to solenoid injector.
18. Fuel tank
19. Oxygen sensor (Lambda probe) - Measures the oxygen content of exhaust gases and transmits any adjustment of air-fuel mixture to control unit.
20. Catalytic converter - Reduces harmful residues contained in exhaust gases to a minimum.
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Gnome
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| The Gnome was one of several rotary engines popular on fighter planes during World War I. In this type of engine, the crankshaft is mounted on the airplane, while the crankcase and cylinders rotate with the propeller. | |||||||||||||
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| The Gnome was unique in that the intake valves were located within the pistons. Otherwise, this engine used the familiar Otto four stroke cycle. At any given point, each of the cylinders is in a different phase of the cycle. In the following discussion, follow the master cylinder with the green connecting rod. | |||||||||||||
| During this portion of the stroke, a vacuum forms in the cylinder, forcing the intake valve open and drawing the fuel-air mixture in from the crankcase. |  | ||||||||||||
| The mixture is compressed during this phase. The spark plug fires toward the end of the compression stroke, slightly before top dead center. |  | ||||||||||||
| The power stroke happens here. Note that the exhaust valve opens early -- well before bottom dead center. |  | ||||||||||||
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This engine has a fairly long exhaust stroke. In order to improve power or efficiency, engine valve timing often varies from what one might expect. |
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When I first learned how these engines worked, I thought the only person crazier than the engine designer was the one who paid money for it. At first glance it seems ridiculously backwards. Nonetheless, a number of engines were designed this way, including the Gnome, Gnome Monosoupape, LeRhone, Clerget, and Bentley to name a few. It turns out there were some good reasons for the configuration: | |||||||||||||
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Most of my information on the Gnome came out of Air board Technical Notes7 | |||||||||||||
Jet Propulsion
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| I've grudgingly included this section by popular request. Rocket and turbojet engines are fabulous technological achievements--But they're so simple the animations are boring!
...At least I think so. You be the judge! | |
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Rocket | |
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The rocket engine is the simplest of this family, so I'll start with it. | |
| In order to work in outer space, rocket engines must carry their own supply of oxygen as well as fuel. The mixture is injected into the combustion chamber where it burns continuously. The high-pressure gas escapes through the nozzle, causing thrust in the opposite direction. |  |
| To illustrate the principle yourself, inflate a toy balloon and release it (without tying it off!). ...rocket propulsion at its simplest. | |
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Turbojet | |
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The turbojet employs the same principle as the rocket. It burns oxygen from the atmosphere instead of carrying a supply along. | |
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Notice the similarities: Fuel continuously burns inside a combustion chamber just like the rocket. The expanding gasses escape out the nozzle generating thrust in the opposite direction. |
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| Now the differences: On its way out the nozzle, some of the gas pressure is used to drive a turbine. A turbine is a series of rotors or fans connected to a single shaft. Between each pair of rotors is a stator -- something like a stationary fan. The stators realign the gas flow to most effectively direct it toward the blades of the next rotor. |  |
| At the front of the engine, the turbine shaft drives a compressor. The compressor works a lot like the turbine only in reverse. Its purpose is to draw air into the engine and pressurize it. |  |
| Turbojet engines are most efficient at high altitudes, where the thin air renders propellers almost useless. | |
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Turboprop | |
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| The turboprop is similar to the turbojet, except that most of the nozzle gas pressure drives the turbine shaft -- by the time the gas gets past the turbine, there's very little pressure left to create thrust.
Instead, the shaft is geared to a propeller which creates the majority of the thrust. 'Jet' helicopters work the same way, except that their engines are connected to the main rotor shaft instead of a propeller. Turboprops are more fuel efficient than turbojets at low altitudes, where the thicker air gives a propeller a lot more 'traction.' This makes them popular on planes used for short flights, where the time spent at low altitudes represents a greater percentage of the overall flight time. | |
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Turbofan | |
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| The turbofan is something like a compromise between a pure turbojet and a turboprop. It works like the turbojet, except that the turbine shaft also drives an external fan, usually located at the front of the engine.
The fan has more blades than a propeller and spins much faster. It also features a shroud around its perimeter, which helps to capture and focus the air flowing through it. These features enable the fan to generate some thrust at high altitudes, where a propeller would be ineffective. Much of the thrust still comes from the exhaust jet, but the addition of the fan makes the engine more fuel efficient than a pure turbojet. Most modern jetliners now feature turbofan engines. | |
| As you can see all of these engines are conceptually very simple, and have very few moving parts, making them extremely reliable. They also have an excellent power-to-weight ratio, which is partly why they're so popular in aircraft.
Like most of my illustrations, these are extremely simplified. Turbine engines often employ more than one shaft and have other more complex features that I really don't understand and, frankly, don't care to investigate further. For some terrific illustrations and a lot more information on these engines, see the NASA web site: http://www.grc.nasa.gov/WWW/K-12/airplane/shortp.html ...Now, don't you think the other engine pages are a lot more fun? | |