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Fuel Induction and Ignition System Fuel Induction • In spark ignition engines the air and fuel are usually mixed prior to entry into the cylinder. • The ratio of mass flow of air to the mass flow of fuel must be held roughly constant at about 14.7 for proper combustion. • Initially a purely mechanical device known as a carburetor was used to mix the fuel and the air • Most modern cars use electronic fuel-injection systems Fuel Induction • Nozzles that inject a spray of fuel into the intake air. • Controlled electronically and mechanically. • Many modern SI engines have multipoint port fuel injectors. Spray fuel directly behind the intake valve. • Multipoint port injector systems are better than carburetors or throttle body injector system at giving consistent AF delivery. • Because of short duration after fuel injection for evaporation and mixing to occur, point injectors must spray very tiny droplets of fuel. Smaller droplet at high rpm. • Engines with pump systems and controls on each cylinder can be more finely adjusted than those with single-pump systems. • The duration of injection is determine by feed back from engine and exhaust sensors - Oxygen sensor in exhaust manifold. - RPM, T, air flow rate, throttle position Fuel Induction Sensors In order to provide the correct amount of fuel for every operating condition, the engine control unit (ECU) has to monitor a huge number of input sensors. Here are just a few: Mass airflow sensor - Tells the ECU the mass of air entering the engine Oxygen sensor(s) - Monitors the amount of oxygen in the exhaust so the ECU can determine how rich or lean the fuel mixture is and make adjustments accordingly Throttle position sensor - Monitors the throttle valve position (which determines how much air goes into the engine) so the ECU can respond quickly to changes, increasing or decreasing the fuel rate as necessary Coolant temperature sensor - Allows the ECU to determine when the engine has reached its proper operating temperature Voltage sensor - Monitors the system voltage in the car so the ECU can raise the idle speed if voltage is dropping (which would indicate a high electrical load) Manifold absolute pressure sensor - Monitors the pressure of the air in the intake manifold The amount of air being drawn into the engine is a good indication of how much power it is producing; and the more air that goes into the engine, the lower the manifold pressure, so this reading is used to gauge how much power is being produced. Engine speed sensor - Monitors engine speed, which is one of the factors used to calculate the pulse width Fuel Injector When the injector is energized, an electromagnet moves a plunger that opens the valve, allowing the pressurized fuel to squirt out through a tiny nozzle. The nozzle is designed to atomize the fuel -- to make as fine a mist as possible so that it can burn easily. The amount of fuel supplied to the engine is determined by the amount of time the fuel injector stays open. This is called the pulse width, and it is controlled by the ECU. ECU The engine control unit uses a formula and a large number of lookup tables to determine the pulse width for given operating conditions. The equation will be a series of many factors multiplied by each other. Many of these factors will come from lookup tables. We'll go through a simplified calculation of the fuel injector pulse width. In this example, our equation will only have three factors, whereas a real control system might have a hundred or more. Pulse width = (Base pulse width) x (Factor A) x (Factor B) In order to calculate the pulse width, the ECU first looks up the base pulse width in a lookup table. Base pulse width is a function of engine speed (RPM) and load (which can be calculated from manifold absolute pressure). Let's say the engine speed is 2,000 RPM and load is 4. We find the number at the intersection of 2,000 and 4, which is 8 milliseconds. ECU cont. Next examples, A and B are parameters that come from sensors. Let's say that A is coolant temperature and B is oxygen level. If coolant temperature equals 100 and oxygen level equals 3, the lookup tables tell us that Factor A = 0.8 and Factor B = 1.0. So, since we know that base pulse width is a function of load and RPM, and that pulse width = (base pulse width) x (factor A) x (factor B), the overall pulse width in our example equals: 8 x 0.8 x 1.0 = 6.4 milliseconds From this example, you can see how the control system makes adjustments. Real control systems may have more than 100 parameters, each with its own lookup table. Performance chips are made by aftermarket companies, and are used to boost engine power. The tables in the performance chip will contain values that result in higher fuel rates during certain driving conditions. Since the performance-chip makers are not as concerned with issues like reliability, mileage and emissions controls as the carmakers are, they use more aggressive settings in the fuel maps of their performance chips. Ignition System Ignition Timing The timing of the spark is critical to success. There is a small delay from the time of the spark to the time when the fuel/air mixture is all burning and the pressure in the cylinder reaches its maximum. If the spark occurs right when the piston reaches the top of the compression stroke, the piston will have already moved down part of the way into its power stroke before the gases in the cylinder have reached their highest pressures. To make the best use of the fuel, the spark should occur before the piston reaches the top of the compression stroke, so by the time the piston starts down into its power stroke the pressures are high enough to start producing useful work. The timing of the spark is important, and the timing can either be advanced or retarded depending on conditions. Spark advance: The faster the engine speed, the more advance is required Minimizing emissions, take priority when maximum power is not required. For instance, by retarding the spark timing (moving the spark closer to the top of the compression stroke), maximum cylinder pressures and temperatures can be reduced. Spark Plug The electricity must be at a very high voltage in order to travel across the gap and create a good spark. Voltage at the spark plug can be anywhere from 40,000 to 100,000 volts. Spark plugs use a ceramic insert to isolate the high voltage at the electrode, ensuring that the spark happens at the tip of the electrode and not anywhere else on the plug; this insert does double-duty by helping to burn off deposits. Ceramic is a fairly poor heat conductor, so the material gets quite hot during operation. This heat helps to burn off deposits from the electrode. Ignition Coil The coil is a high-voltage transformer made up of two coils of wire. One coil of wire is called the primary coil. Wrapped around it is the secondary coil. The secondary coil normally has hundreds of times more turns of wire than the primary coil. Current flows from the battery through the primary winding of the coil. The primary coil's current can be suddenly disrupted by the breaker points. The key to the coil's operation is what happens when the circuit is suddenly broken by the points. The magnetic field of the primary coil collapses rapidly. The secondary coil is engulfed by a powerful and changing magnetic field. This field induces a current in the coils -- a very high-voltage current (up to 100,000 volts) because of the number of coils in the secondary winding. The secondary coil feeds this voltage to the distributor via a very well insulated, highvoltage wire. Distributor The distributor handles several jobs. Its first job is to distribute the high voltage from the coil to the correct cylinder. This is done by the cap and rotor. The coil is connected to the rotor, which spins inside the cap. The rotor spins past a series of contacts, one contact per cylinder. As the tip of the rotor passes each contact, a high-voltage pulse comes from the coil. The pulse arcs across the small gap between the rotor and the contact (they don't actually touch) and then continues down the spark-plug wire to the spark plug on the appropriate cylinder. The points also control the timing of the spark. They may have a vacuum advance or a centrifugal advance. These mechanisms advance the timing in proportion to engine load or engine speed. Distributorless Ignition The coil in this type of system works the same way as the larger, centrally-located coils. The engine control unit controls the transistors that break the ground side of the circuit, which generates the spark. This gives the ECU total control over spark timing. Systems like these have some substantial advantages. First, there is no distributor, which is an item that eventually wears out. Also, there are no high-voltage spark-plug wires, which also wear out. And finally, they allow for more precise control of the spark timing, which can improve efficiency, emissions and increase the overall power of a car. Concept question 1. What is the main reason for a black smoke coming out from a car exhaust: • There is too much fuel for the air in the engine cylinder which results in rich mixture and incomplete combustion; • There is not enough fuel for the air in the engine cylinder that result in weak mixture that cause poor combustion and loss of power. 2. What is the main reason for a white smoke coming out from a car exhaust: ● Caused by water and or antifreeze entering the cylinder, and the engine trying to burn it with the fuel; ● Caused by engine oil entering the cylinder area and being burned along with the fuel air mixture.