Content: Carburettor »
Introduction………………………………………………… Introduction…………………… …………………………………………………………… ……………………………….... ....
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History & Development………………………… Development……………………………………………………… ………………………………………… ……………
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Operation……………………………………………………………………………………….. Main open-throttle circuit………………………………………………………………. Choke……………………………………………………………………………………………… Float chamber…………………………………………………………………………………. adjustment……………………………………………… …………………………………………….. ………………….. Carburettor adjustment…………………… Eliminating the disadvantages of a carburettor……………………………….. Manufacturers………………………………………………………………………………… Types of Carburetors……………………………………………………………………….
Spark plugs
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Introduction……………………………………………………………………………………. plug………………………………………………… What is the function of a spark plug…………………………………………………
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What is a resistor plug…………………… plug………………………………………………… ……………………………………………….. …………………..
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range"……………………………………….. What exactly is a spark plug "heat range"……………………………………….. What is the operating temperature of a spark plug tip……………………. What engine parameters affect the operating temperature of a spark plug……………………… plug……………………………………………………… …………………………………………………………… ……………………………….. …..
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important…………………………………………… ……………………………… Is the correct spark plug gap important…………… ignition……………………………………………… …………………………………………………… ………………………… What is pre-ignition…………………… What is detonation…………………………………………………………………………. Fouling……………………………………………………………………………………………. Hotter or Colder Plugs ……………………………………………………………………..
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Various racing spark plugs………………………………………………………………. plugs……………………………………………………………….
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Automobile spark plug…………………………………………………………………….
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References……………………………………………………………………………………….
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The Carburettor »
Introduction:
The carburettor is the control for the engine. It feeds the engine with a mixture of air and petrol in a controlled volume that determines the speed, acceleration and deceleration of the engine. The carburettor is controlled by a slide connected to the throttle cable from the handlebar twist grip which adjusts the volume of air drawn into the engine. A system of fixed holes (jets) and, adjustable needles control the volume of fuel (petrol) to match the volume of air. The ratio of fuel and air the carburettor delivers can be changed by selecting different jets and adjusting the position of the needles. Sometimes but not very often, the manufacturers restrict the speed of the scooter for certain countries by fitting a reducing washer to the entrance of the carburettor. The carburettor controls the amount of fuel in relation to the amount of air that enters the engine. This is carried out by the deployment of a butterfly valve, which is linked in turn to the throttle pedal in the vehicle. »
History & Development:
The carburettor was invented by the Hungarian engineer in 1893 . Frederick William Lanchester of Birmingham, England experimented early on
with the wick carburettor in cars. In 1896, Frederick and his brother built the first petrol driven car in England, a single cylinder 5hp (4kW) internal combustion engine with chain drive. Unhappy with the performance and power, they re-built the engine the next year into a two cylinder horizontally opposed version using his new wick carburettor design. This version completed a 1,000 mile tour in 1900 incorporating successfully the carburettor as an important step forward in automotive engineering. »
Operation:
– The fundamental function of a carburettor is fairly simple, but the implementation is fairly complex, because a carburettor must provide the ideal fuel/air mixture under a wide variety of different circumstances and engine Rpm range. Most carburettors contain equipment to support several different operating modes, called circuits.
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– A carburettor basically consists of an open pipe, the carburettor's "throat" or "barrel", through which the air passes. The pipe is in the form of a venturi - it narrows in section and then widens again. Just after the narrowest point is a butterfly valve or throttle - a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the flow of air. This valve controls the flow of air through the carburettor throat and thus the quantity of air/fuel mixture the system will deliver. This in turn affects the engine power and speed. The throttle is connected with a bowden cable or a set of rods and ball joints, to the accelerator pedal on a car or the equivalent control on other vehicles or equipment.
– The carburettor (or carburettor, "carb" for short) is a device which mixes air and fuel for an internal-combustion engine. Carburettors are still found in small engines and in older or specialized automobiles such as those designed for stock car racing. However, most cars built since the early 1980s use fuel injection instead of carburetion. Most carburetted (as opposed to fuel-injected) engines have a single carburettor, though some, primarily higher performance engines, can have multiple carburettors. »
Main open-throttle circuit:
– As the throttle is progressively opened, the manifold vacuum reduces since there is less restriction on the airflow. This reduction in vacuum reduces the flow through the idle and off-idle circuits, so another method of introducing fuel into the airflow is needed.
– This is where the venturi shape of the carburettor throat comes into play. The Bernoulli effect shows that as the velocity of a gas increases, its pressure falls. The venturi (sometimes two venturi nested in the same barrel) makes the air reach a higher velocity at the middle than at the ends, and this high speed and thus low pressure in the middle sucks fuel into the air stream through a nozzle (a "jet") located in the centre of the throat.
– The main circuit requires a reasonable airspeed through the carburettor throat to function, and thus ceases to function during idle, where the idle circuit steps in.
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Choke:
– When the engine is cold, ignition and combustion happens less readily, and some of the fuel vapour condenses on the cold intake manifold and cylinder walls. Thus, a richer mixture - more fuel to air - is required.
– To achieve this, a "choke" is used. This is a device that restricts the flow of air at the entrance to the carburettor. This functions similarly to the throttle being closed, except for the fact that it is closed off before both idle and main circuits. Here, the low pressure caused by the restriction sucks fuel through all the f uel circuits - idle, off-idle, and main.
– The choke may be automatically controlled by a thermostat, or manually operated. »
Float chamber:
– To ensure a ready supply of fuel, the carburettor has a "float chamber" (or "bowl") that contains a quantity of fuel ready for use. It converts fuel from fuel pump pressure to atmospheric pressure.
– This works similarly to a toilet tank; a float controls an inlet valve. If the float drops, the inlet is opened allowing the fuel to flow under the fuel pump's pressure. Usually, special vent tubes allow air to escape from the chamber as it fills. »
Carburettor adjustment:
– The ratio of Fuel to Air is determined by the size of the holes in the "JETS" within the carburettor and is set by the engine designers. But other factors do come into play. The "choke" for example restricts the airflow in the carburettor and thus more fuel, in turn, is administered to the engine (necessary to compensate for the condensation of fuel inside a cold engine). A lso a blocked air filter will have the same effect of restricting the air and forcing more fuel into the engine causing fuel wastage through inefficiency and poor running of the engine.
– Blocked "Jets" on the other hand allow less fuel to enter the engine and weaken the mixture giving rise to overheating and also poor running. Although the carburettor is set by the manufacturer, it is sometimes necessary to make adjustments to match the characteristics of the engine. These are: 1- Tick-over, this controls the speed of the engine when the twist grip is closed to stop the engine from stalling. 2- Mixture screw, this controls the volume of fuel that enters the engine when accelerating from a low engine speed.
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3- The height of the float and float needle controls the amount of fuel available in the carburettor. If the float needle becomes worn it may cause the carburettor to leak and overflow. 4- Height of slide needle controls how quickly the needle is lifted from the main jet, to allow the fuel to pass through the jet. 5- Size of the main jet, selecting a main jet with a different size hole will affect the volume of fuel it will pass. For example when a new exhaust is installed or a custom air filter it is likely that a higher ratio of fuel to air will be necessary and therefore a main jet with a larger hole will be required.
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Eliminating the disadvantages of a carburettor:
– Carburettor ice occurs due to the effect of fuel vaporization and the decrease in air pressure in the venturi, which causes a sharp temperature drop in the carburettor. If water vapor in the air condenses when the carburettor temperature is at or below fre ezing, ice may form on internal surfaces of the carburettor, including the throttle valve. The reduced air pressure, as well as the vaporization of fuel, contributes to the temperature decrease in the carburettor. Ice generally forms in the vicinity of the throttle valve and in the venturi throat. This restricts the flow of the fuel/air mixture and reduces power. If enough ice builds up, the engine will suddenly stop with little or no warning.
– Carburettor ice is most likely to occur when temperatures are below 20°C (70°F) and the relative humidity is above 80 percent. However, due to the sudden cooling that takes place in the carburettor, icing can occur even with temperatures as high as 38°C (100°F) and humidity as low as 50 percent. This temperature drop can be as much as 20°C (70°F). As a preventive measure against carburettor icing, one can apply carburettor heat. It is an anti-icing system that preheats the incoming air before it reaches the carburettor. Carburettor heat is intended to keep the fuel/air mixture well above its freezing temperature to prevent the formation of carburettor ice.
– However, there is a disadvantage to its use: the use of carburettor heat causes a decrease in engine power (sometimes up to 15 percent!), because the heated air is less dense than the outside air that had been entering the
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engine. This also enriches the mixture as the carburettor does not compensate for air temperature. – In most carburettored engines, the air/fuel mixture going through the inlet system is also heated again to aid the vaporisation of the fuel droplets 'sucked' out of the nozzle in the carburettor. As a cold engine reaches normal operating temperatures, it loses horsepower due to the difference in air density.
– Another disadvantage of most carburettors is that the fuel bowl can flood. As a result fuel can overflow outside of the carburettor body into the engine compartment. This is a significant hazard as there is a danger of petrol leaking onto a hot engine or exhaust and potentially start a fire or cause an explosion. Often a drip tray is used to collect the spilt fuel and eventually route it outside of the aircraft. »
Manufacturers:
Some manufacturers of carburettors: -Solex: The Solex Company was founded by Marcel Mennesson and Maurice Goudard to manufacture vehicle radiators. These were fitted to several makes of early cars including Delauney-Belleville and buses of the Paris General Omnibus company. -SU: The SU carburetters featured a variable venturi controlled by a piston. This piston has a tapered, conical metering rod (usually referred to as a "needle") that fits inside an orifice ("jet") which admits fuel into the airstream passing through the carburettor. Since the needle is tapered, as it rises and falls it opens and closes the opening in the jet, regulating the passage of fuel, so the movement of the piston controls the amount of fuel delivered, depending on engine demand. -Zenith: Zenith Carburetters was a British company making carburettors. In 1955 they joined with their major pre-war rival Solex Carburettors and over time the Zenith brand name fell into disuse. -Autolite, a division of the Ford Motor Company. -Carter. -Holley. -Pier burg. -Rochester (A General Motors subsidiary; also sold Weber/Magneti-Marelli carburettors under license).
-Walbro and Tillotson for small engines. -Weber owned by Magneti-Marelli. -Briggs and Stratton . »
Types of Carburetors: 6
-Gravity Feed called a “Flo-Jet” by Briggs & Stratton. -Vacuum Feed called “Vacu-Jet”. -Pulse Feed called “Pulsa-Jet”. Flo-Jet Carburetors:
Gravity feed system - Fuel flows by gravity to the carburetor. - Gas tank must have a vent hole to provide atmospheric pressure to “push” fuel to carburetor. -The float is found in the bowl, (As fuel flows into the bowl, it raises and lowers the float). -The float is attached to a needle valve, (The needle, along with its seat, work together to turn on and off fuel flow to the main discharge nozzle). -Float level, (should be high enough to allow an ample supply of fuel at full throttle, yet remain low enough to prevent flooding and/or leaking).
Vacu-Jet Carburetors:
Fuel tank is below the carburetor - Again, atmospheric pressure is employed to help get the fuel to the carburetor. - Vacu-Jet carburetors require a richer mixture setting because the fuel system “lags” behind the fuel requirements of the engine at the high speeds that most small engines run at. - As the throttle closes to idle, the leading edge takes a position between 2 different sized discharge holes that are found on the main discharge nozzle. The larger of the holes now becomes an area of high air pressure and the flow of fuel ceases. - The smaller hole now becomes an area of lower pressure and fuel continues to flow--just enough for the engine to idle. -Choke valve is also different: (There are many problems with this choke system including sticking open and closed, especially when remotely controlled).
Pulsa-Jet Carburetors:
- Only “true” fuel system contains a diaphragm type fuel pump and a “constant level” fuel chamber . -Newest design of carburetors for small engines. - The venturi can be made larger, allowing more air/fuel mixture into the engine allowing an increase in horsepower within the same sized engine. 7
Spark plugs »
Introduction:
Spark plugs are one of the most misunderstood components of an engine. Numerous questions have surfaced over the years, leaving many people confused. This guide is designed to assist the technician, hobbyist, or race mechanics in understanding, using, and troubleshooting spark plugs. The information contained in this guide applies to all types of internal combustion engines. Spark plugs are the "window" into the engine, and can be used as a valuable diagnostic tool. Like a patient's thermometer, the spark plug displays symptoms and conditions of the engine. The experienced tuner can analyze these symptoms to track down the root cause of many problems, or determine air/fuel ratios. A spark plug is an electrical device that fits into the cylinder head of internal combustion engines and ignites compressed aerosol gasoline by means of an electric spark. Spark plugs have an insulated wire which is connected with an induction coil or magneto circuit on the outside, and forms, with another terminal on the base of the plug, a spark gap inside the cylinder. Spark plugs are typically designed to have a variable spark gap. The spark gap may be adjusted by the technician installing the spark plug. A spark plug gap gauge is used to set the gap. The simplest gauges are a collection of keys of various thicknesses which match the desired gaps and the gap is adjusted until the key fits snugly. Internal combustion engines can be divided into spark-ignition engines, which require spark plugs to begin combustion, and compression-ignition engines (diesel engines), which compress the fuel/air mixture until it spontaneously ignites. Compression-ignition engines may use glow plugs to improve cold start characteristics. »
What is the function of a spark plug?
The spark plug has two primary functions: 1) To ignite the air/fuel mixture. Electrical energy is transmitted through the spark plug, jumping the gap in the plugs firing end if the voltage supplied to the plug is high enough. This electrical spark ignites the gasoline/air mixture in the combustion chamber.
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2) To remove heat from the combustion chamber. Spark plugs cannot create heat, they can only remove heat. The temperature of the end of the plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. The spark plug works as a heat exchanger by pulling unwanted thermal energy from the combustion chamber and transferring heat to the engines cooling system. The heat range of a spark plug is defined as its ability dissipate heat from the tip. »
What is a resistor plug?
In a resistive plug, the center electrode is split inside the insulator body and a resistive element is inserted between the two pieces of the center electrode. Ignition voltage and current are conducted through this resistor to reduce radio interference from the electromagnetic field that is generated each time the spark jumps the electrode gap inside the cylinder.
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What exactly is a spark plug "heat range"?
The heat range has nothing to do with the actual voltage transferred through the spark plug. Rather, the heat range is a measure of the spark plug's ability to remove heat from the combustion chamber. The heat range is determined by the insulator nose length and its ability to absorb and transfer combustion heat, the gas volume around the insulator nose, and the materials/construction of the center electrode and porcelain insulator. In identical spark plug types, the difference from one heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber. A longer the nose on a spark plug forces the heat from the tip to travel farther before it is absorbed by the cylinder head, which reatins more of the heat in the plug tip - making the plug "hotter" than a similar plug with a shorter nose. Engine temperature will affect a spark plug's operating temperature, but not the plug's heat range. »
What is the operating temperature of a spark plug tip?
Whether a spark plug is installed in a lawn mower, boat, truck, or race car, the plug tip temperature must remain between 450° C and 850° C. If the tip temperature is below 450°C, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits. These deposits will accumulate and foul the plug gap, leading to a misfire. When the tip temperature exceeds 850°C, the ceramic around the tip will blister and the electrodes will melt. Preignition and detonation caused by the overheatede tip are likely to cause engine damage. 9
The borderline between the fouling and operating regions (450°C) is called the spark plug self-cleaning region. This is the temperature at which the accumulated deposits burn off of the center insulator. A function of temperature only, the self -cleaning region has no relationship to spark quality or intensity.
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What engine parameters affect the operating temperature of a spark plug?
Air/Fuel mixture: Rich mixtures reduce the tip temperature, causing fouling and poor drivability. Lean mixtures increase both tip temperature and combustion chamber pressure, resulting in pre-ignition. It is important to read the spark plugs frequently while tuning after engine modifications; the spark plugs will help you achieve an optimum air/fuel ratio. Compression ratio/forced induction: As effective compression ratio increases through engine modifications or forced induction, a colder plug using a narrower gap and higher ignition voltage is required. Ignition timing: Advancing the ignition timing by 10° increases the plug tip temperature by approx. 70°-100°C. Engine speed and load: Increases in firing-end temperature are proportional to engine speed and load, when consistanly traveling at high engine speeds or hauling, towing, or pushing loads - install a cooler plug. Weather: Temperature, humidity, and barometric pressure all affect air density and intake volume. Higher intake volumes result in leaner mixtures and hotter plug tips. »
Is the correct spark plug gap important? 10
Always use factory recomended gaps unless the engine has been modified. The manufacturer has designed the complete ignition system as a package; the voltage and timing of the spark from the ignition system is tailored for a specific plug gap. Changing the gap can lead to intermittent misfirng, which will foul the plug. When altering the gap size for high performance ignition modifications, never go more than +/- 0.008" from the stock setting because the gap surfaces between the ground and the center electrode may not be parallel for larger changes. Higher compression or boosted engines generally require a smaller plug gap and a higher ignition voltage. »
What is pre-ignition?
Defined as: The ignition of the fuel/air mixture before the pre-set ignition timing mark. This is caused by hot spots in the combustion chamber, which results from advanced ignition timing, a spark plug that is too hot, high compression, low octane fuel, lean air/fuel mix, insufficient engine cooling, or carbon in the combustion chamber. Pre-ignition and detonation are separate events, but pre-ignition frequently leads to detonation.
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What is detonation?
Detonation involves rapid, uncontrolled burning of the air/fuel mixture while the piston is still rising. During normal combustion, the flame front propagates across the cylinder at a controlled subsonic speed with the piston near TDC. During detonation, the flame front goes supersonic and/or collides from different directions. This rapid detonation strikes the top of the piston as it is still being pushed upward in the cylinder by the crankshaft. The shock wave resulting when the detonation flame front strikes the top of the piston causes the piston to rattle in the cylinder. The sound of this shock wave and the sound of the rattling piston is what people commonly refer to as ping. Detonation radically increases cylinder pressure and temperature, and can quickly burn the end of the spark plug. Excessive heat i s usually what causes engine damage when detonation is ignored. »
Fouling:
Causes of Carbon Fouling:
Continuous low speed driving and/or short trips Spark plug heat range too cold Air-fuel mixture too rich Reduced compression and oil usage due to worn piston rings / cylinder walls 11
Over-retarded ignition timing Ignition system deterioration
Pre-delivery fouling: Carbon fouling occurs when the spark plug firing end does not reach the self cleaning temperature of approximately 450°C (842°F). Carbon deposits will begin to burn off from the insulator nose when the self-cleaning temperature is reached. When the heat range is too cold for the engine speed, the firing end temperature will stay below 450°C and carbon deposits will accumulate on the insulator nose. This is called carbon fouling. When enough carbon accumulates, the spark will travel the path of least resistance over the insulator nose to the metal shell instead of jumping across the gap. This usually results in a misfire and further fouling. If the selected spark plug heat range is too cold, the spark plug may begin to foul when the engine speed is low or when operating in cold conditions with rich air-fuel mixtures. In some cases, the insulator nose can usually be cleaned by operating the engine at higher speeds in order to reach the self-cleaning temperature. If the spark plug has completely fouled, and the engine will not operate correctly, the spark plug may need to be cleaned / replaced and the fouling cause identified.
Causes of Overheating:
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Spark plug heat range too hot Insufficient tightening torque and/or no gasket Over-advanced ignition timing Fuel octane rating too low (knock is present) Excessively lean air-fuel mixture Excessive combustion chamber deposits Continuous driving under excessively heavy load Insufficient engine cooling or lubrication
Hotter or Colder Plugs:
There are three basic determinations for spark plugs: good, fouled or overheated. The borderline between fouling and optimum operational performance is 450 degrees Celsius which is the plug's self-cleaning temperature(accumulated carbon and combustions deposits are burned off.) Nose length determines heat range, the longer the insulator, the less heat absorbed and the further heat has to travel into the cylinder head water passages. This means the plug has a higher internal temperature and is determined to be a hot plug. A cold plug has a shorter insulator nose and absorbs more combustion chamber heat. Heat travels a shorter distance and allows the plug to operate at a lower temperature. A colder plug is required when an engine is modified for performance, 12
subject to heavy loads or run at high rpms for long pe riods. The colder plug removes heat more quickly, will reduce pre-ignition(ping) and reduces the chance of firing end burn-out. Engine temperature affects the spark plug's operating temperature but not its heat range.
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Various racing spark plugs:
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Automobile spark plug:
- Automobile spark plug : electric part generating sparks to ignite an internal combustion engine. - Ceramic insulator: pottery support for the parts that conduct electricity. - Terminal: place where a current-conducting wire is attached. - Spline: hollow channel. - Resistance: device that controls the strength of the current. - Ground electrode: current device that unites the electrodes. - Spark plug gap: space separating the current conductors. - Center electrode: central current conductor. - Gasket: spot where two part join together. - Spark plug body: metal part of the spark plug. - Hex nut: hexagonal piece of metal used to screw in a spark plug.
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References:
-http://www.\spark plug\Automobile spark plug.mht -http://www.\spark plug\NGK Spark Plugs USA.mht -http://www.\spark plug\Spark Plug.mht - http://www.\carburettor - definition of carburettor by the Free Online Dictionary, Thesaurus and Encyclopedia.htm -http://www.\carburettor\The Working Principle Of Simple Carburettor.htm -Racing_Spark_Plugs_Performance_Applications.pdf -Carburetors_Types.ppt
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