Emission Control Systems

If not controlled, the automobile can give off pollutants from four places. Pollutants can come from the fuel tank, the carburetor, the crankcase, and the tail pipe. Pollutants from the fuel tank and carburetor consist of gasoline vapors. Pollutants from the crankcase consist of partly burned air-fuel mixture that has blown by the piston rings. Pollutants from the tail pipe consist of partly burned gasoline (HC), carbon monoxide (CO), nitrogen oxides (NOx), and if there is sulfur in the gasoline, sulfur oxides (SOx).

Emission Control Systems

A portion ò the air-fuel mixture remains incompletely burned during the combustion process. This incomplete combustion results in the formation of hydrocarbons (HC), and oxides of nitrogen (NOx). Excessive amounts of these emissions form a major contribution to atmospheric pollution and can constitute a serious health hazard.

Almost two-thirds of the emissions produced in the combustion process are carried away from the engine and expelled into the atmosphere through the automobile exhaust system. It is impossible to seal off the exhaust system because any attempt to stop the flow of exhaust gases would produce a backpressure that would stop the combustion process. Consequently, research has been directed toward reducing the harmful emission contained in the exhaust as to as low a level as possible.

Many common causes of excessive emissions can be attributed to a malfunctioning engine or engine auxiliary system. Some authorities estimate that it is possible to cut emissions by as much as 50 to 60 percent if the vehicle is properly tuned. A further reduction of emissions depends on the use of specially designed emission control systems.

Approximately 15 percent of the vehicle emissions that contribute to atmospheric pollution are in the form of vapors tat escape from the fuel tank and carburetor float bowl. Most evaporative fuel losses occur at the fuel tank and result from the expansion and contraction of air in the tank as the tank heats up or cools. The expanding air escapes through the tank vent tube or vented cap, carrying fuel vapors with it.

Several different evaporative emission control systems have been developed to prevent the loss of fuel vapors to the atmosphere. In general, these are closed systems which provide for the return of fuel vapors from the fuel tank and carburetor to the engine where they are burned. As such, these systems are sometimes called fuel-vapor recovery systems or canister storage systems. Depending on the type of system, the fuel vapors are first collected and stored either in a charcoal canister or in the engine crankcase.

Some evaporative emission –control systems use a charcoal canister as the storage receptacle for the fuel vapors. The General Motors fuel-vapor recovery system is an example of this type. This system consists of a special fuel tank, vacuum/pressure filler cap, vapor/liquid separator standpipe assembly, carbon canister, canister purge hoses, and a modified carburetor. Fuel vapors that would ordinarily escape to the atmosphere are directed into the canister by venting the fuel tank (or, in some applications, by venting the carburetor float bowl as well) where they are absorbed by the charcoal and stored until use. The fuel vapor is removed from the canister by manifold vacuum and burned in the engine.

A carbon (vapor) canister is an integral part of a gasoline engine fuel vapor recovery system. These canisters may be classified as open-bottom or closed-bottom.

A three-tube canister provides for venting both the fuel tank and the carburetor float bowl. Fuel vapors in the float bowl flow through a hose to a third tube and are absorbed by the carbon. The purging method is identical to the one for a two-tube canister

The three-tube canister has been improved to prevent venting of the carburetor float bowl during engine operation. A spring-biased diaphragm valve, which is normally open, controls the flow of fuel vapors from the carburetor to the canister. When the engine is not running, spring tension holds the valve open, allowing normal venting. When the engine is started, manifold vacuum pulls the diaphragm up to close the valve.

In the road-cirafuventilation system, openings at both the front and back of tube crankcase are used to provide a through-pas-sage for ventilation. The front opening con-nects to the oil filter cap and the opening at the back of the crankcase connects to a road-draft tube. Forward movement of the vehicle creates slight vacuum at the road-draft tube opening and a slight pressure of air around the oil filler pipe located under the hood. These pressure differences draw in fresh air to ventilate the crankcase and suck the fule and exhaust fumes into the air cleaner or intake manifold.

Unlike the road-draft system, the positive-crankcase-venti-lation system (PCV) does not rely on vehicle movement for its operation, but instead uses manifold vacuum to maintain a positive movement of air through the crankcase at all engine speeds this greatly reduces the accumulation of harmful deposits when driving in heavy, stop-and –go traffic. The PCV system also results in increased gas mileage because the fuel vapors from the crankcase are drawn into the engine and burned instead of being lost to the atmosphere.

Fresh air enters the system from the clean-air side of the air cleaner or through a separate breather filter. Because intake-manifold vacuum is used to operate the PCV system, the PCV flow into the intake manifold must be regulated so that it varies in proportion to the regular air-ruel ratio being drawn into the intake manifold through the car-buretor. A PCV valve is used to regulate the airflow through the system and into the intake manifold. It is designed to vary the amount of airflow according to the various modes of engine operation

A plunger-type PCV valve consists of a coil spring, plunger, and a valve body. The amount of PCV valve opening or restriction is governed by the amount of vacuum present in the intake manifold. High vacuum overcomes the force of the valve spring and causes the plunger to bottom in the manifold end of the valve housing, thereby restricting the airflow to the intake manifold. At low vacuum, the spring force is stronger than the vttcuum pull, and the valve plunger is forced toward the crankcase end of the valve housing, allowing greater airflow to the intake manifold.

Fuel System Troubles

Fuel System Troubles

Experience gained in operating automotive engines has shown that most engine troubles rest with the fuel system. The more common fuel system troubles are fuel leaks through badly sealed fuel line connections, air leaks past air cleaners and filters, and clogged fuel lines and filters. Besides, carburetor engines may suffer from carburetor troubles resulting in inadequate air-fuel mixture composition (too lean or too rich); diesel engines may develop troubles due to air finding its way into the fuel lines, poor atomization of fuel by the fuel injector, or wrong injection timing.

Such trouble may cause hard starting, lack of power, misfiring, smoky exhaust, or sudden stalling.

Fuel leaks can be detected by visual injection. The leaks, if revealed, are eliminated by tightening up the leaky connections or by replacing defective gaskets.

If the engine does not start, the first thing to do is to make sure that there is fuel in the tank and that the fuel shut-off cock is open. Then check the operation of the fuel (transfer) pump and make sure that the fuel lines and filters are not clogged. Use a tire pump to clear clogged fuel lines, if any. Next check the carburetor (if your engine uses one) for clogged jets. Use a tire pump to clear clogged carburetor jets, if found. Never use wire to do the job. For this may damage jet orifices.

A diesel engine will not start if air gets into the fuel system, for in this case, the fuel-injection pump will compress air bubbles in its barrels instead of delivering fuel into the engine cylinders. Should this prove to be the case, bleed the fuel system, using the hand primer pump. Expel air first from the primary fuel filter, then from the secondary fuel filter, and finally from the fuel-injection pump head.

Wrong injection timing may also be the cause of hard starting. Therefore, check for this condition as well and make corrections, if necessary.

Should all the above corrective measures prove to be no help, check the fuel injector for spray pattern and injection pressure, and replace any one found to be defective. Still better, replace the injectors outright, if spares are available.

If a diesel engine lacks power and, in addition, misses or has a smoky exhaust, one should first of all locate the cylinder that is most responsible for these troubles. To do this, disconnect each fuel injector in turn from the fuel-injection pump. This can be done by backing off 1. 5 to 2 turns the injection-line union nuts on the pumping elements. The disconnection of any injector, except for the one in the misfiring cylinder, will affect the operation of the engine.

Loss of power that is not coincidental with an increased exhaust density is most probably caused by clogged fuel filter or worn pump plunger-and-barrel or injector nozzle-and-needle-valve assemblies. Therefore, the first thing to do in this case is to check the primary and secondary fuel filters and wash their filter elements, if necessary.

A reduction in engine power not attended by a smoky exhaust may be caused by a faulty fuel injector, clogged air cleaner, or incorrect injection timing. So, check the air cleaner and, if necessary, wash the oil path and screen filters with diesel fuel. Fill the bath with fresh oil. Check and adjust (if necessary) the injection timing.

Never disassemble the fuel injectors or fuel-injection pump under field conditions. Any operations involving the disassembly of these units can only be done by skilled mechanicians at specialized shops.

In carburetor engines, loss of power, resulting from defects in the fuel system, is due to an air-fuel mixture that is either too lean or too rich.

A lean mixture condition may result from either too little fuel or too much air in the combustible charge. The amount of fuel in the charge may be reduced because of clogged fuel tank air vent, clogged fuel lines, filters or carburetor fuel jets, defective fuel pump or too low fuel level in the carburetor float chamber. An increased amount of air in the charge may be due to carburetor body and intake manifold air leaks.

The clogged fuel tank air vent should be cleared from dirt or ice (in winter time). Clogged fuel filters should be disassembled, cleaned and washed in clean gasoline. Damaged fuel pump diaphragms should be replaced. Poorly fitting fuel pump valves should be cleaned and, if necessary, replaced. Air leaks should be eliminated by tightening the carburetor body and intake manifold fasteners or by replacing damaged sealing gaskets.

A rich mixture will also cause a loss of power. Excessive quantities of fuel will not vaporize and burn completely. Liquid fuel washes the lubricant from the cylinder walls, allowing the piston rings to make metal-to-metal contact. Scuffed rings and excessive oil and fuel consumption will result.

An excessively rich mixture may result from defects in the carburetor and also from too high a fuel pressure which forces the carburetor needle valve off its seat, causing flooding.

An engine may show a loss in power when unfiltered-air enters the cylinders, causing increased wear in the piston rings and cylinder liners. Pistonring wear results in poor compression, hard starting and increased exhaust smoke density. Therefore, the engine should be systematically checked for air-intake leaks.

A diesel engine will race (i. e., develop excessively high speed) if there is too much oil in the governor housing or in the oil bath of the air cleaner, or else if the fuel control rack or some governor components are frozen. In this case, the engine must be stopped and the condition causing the trouble must be remedied.

Engine Speed Governing

During operation, the load on the engine of an automobile frequently varies, depending on the ambient conditions (lay of the ground, soil condition, etc.). These load variations cause the engine speed to change accordingly, provided the position of the throttle valve or fuel control rack remains unchanged.

When the load on the engine is decreased, the engine speed may rise beyond safe limits, causing accelerated wear of the engine working parts and increased fuel consumption. A device for automatically controlling the speed of an engine by regulating the intake or injection of fuel, so that the engine speed is maintained at the desired level under all conditions of loading, is termed the governor.

Governor control may be effected by centrifugal (mechanical), hydraulic, pneumatic or combined (pneumatic-centrifugal) means. Speed governors may be classed as speed-limiting, constant-speed or all-speed. Speed-limiting and constant-speed governors are used on automobile engines and on auxiliary engines employed for starting tractor diesel engines. An automobile governor is in essence a maximum-speed governor.

The automobile maximum-speed governor is of pneumatic-centrifugal type. It comprises two mechanisms: centrifugal governor sender and diaphragm-operated governor actuator. The centrifugal governor sender includes a rotor whose shaft receives the rotation from the engine camshaft. The sender unit is mounted on the timing gear cover. The rotor houses the valve which is pulled away from its seat by a spring.

The governor actuator includes a diaphragm which is connected by a rod to one end of the bell-crank. The other end of the bell-crank is connected with the governor spring. The bell-crank is fixed to the throttle spindle. The throttle control mechanism incorporates a special claw coupling which enables the governor actuator to close and open the throttle valve irrespective of the position of the accelerator pedal. The spaces above and below the diaphragm are interconnected through the governor sender by pipes. On the other hand, the space above the diaphragm communicates with the carburetor chamber below the throttle valve and that below the diaphragm, with the space below the choke valve, via passages.

So long as the engine speed remains below the preset maximum (3200 rpm), the centrifugal force of the valve is insufficient to overcome the tension of the spring, and so the valve stays open. As a result, the space above and below the diaphragm communicate with each other through the governor sender. When the maximum speed is attained, the centrifugal force of the valve overcomes the resistance offered by the spring, and the valve closes ott to its seat, thus breaking communication between the spaces above and below the actuator diaphragm. This action causes the depression communicated from the carburetor chamber to the space above the diaphragm, via the lower passage, to grow higher. If the engine speed increases further, this depression becomes sufficiently high for the diaphragm to deflect upward against the governor spring force and close the throttle valve cy a certain amount, thus reducing the engine speed.

The main component parts of the governor are a housing, a shaft with a drive disk, a spring, and ball-shaped weights (balls) (Fig. 9)

The Governor housing is held to the engine crankcase through the medium of an adaptor plate. The governor shaft notates together with the drive disk whose slots accommodate balls that are sandwiched between the thrust washer and the conical shaped movable disk.

The movable disk is pressed against the balls by the bell-crank loaded by a spring. Rigidly fixed on the pivot pin of the bell-crank is the external governor lever which is connected with the throttle rod. When the engine is not operating, the spring forces the movable disk to the extreme left-hand position and the throttle nod, to the extreme right-hand position. With the throttle rod in this position, the throttle valve is fully open. During engine operation, the centrifugal forces of the rotating balls cause the balls to move outward against the spring force and displace the movable disk to the right and the throttle rod to the left, thus closing the throttle valve.

With a ready load on the engine, an equilibrium exists between the centrifugal force of the rotating balls and the governor spring force. If the load on the engine is increased, the engine speed, hence the governor shaft speed, starts dropping. In this case, the centrifugal force of the balls decreases, and the governor spring, acting through the bell-crank, moves the throttle rod to the right to open the throttle valve. As a result, the engine speed rises until its nominal value is reached. As the engine load is reduced, the engine speed rises accordingly. The centrifugal force of the rotating balls then increases and, overcoming the governor spring force, moves the throttle rod to the left to close the throttle valve. This causes the engine speed to drop down to the nominal value.

The desired engine speed is set by varying the preload on the governor spring by means of adjusting the screw. This is done at the factory, after which the screw is locked in place by a lock nut and sealed.

Fig. 9 Constant-speed governor.
1-throttle rod; 2-external lever; 3-spring; 4-adjusting screw; 5-bell-crank; 6-housing; 7-pivot pin; 8-movabie disc; 9-governor shaft; 10-drive disc; 11-ball; 12-thrust washer.

Fuel Injectors and Fuel Line

The function of the fuel injector is to deliver finely atomized fuel under high pressure to the combustion chamber of the engine. All component parts of the injector are carried in nozzle holder. The main part of the injector is the nozzle comprising nozzle body and nozzle needle valve. The nozzle body and the needle valve are fabricated from alloy steel. They are thoroughly machined and have high surface hardness necessary for operation in conditions of high temperatures and elevated pressures. The bore in the nozzle body and the nozzle needle valve are lapped to a close tolerance and are a matched set, so that neither the nozzle body nor the needle valve may be replaced individually. The needle valve is pressed against a conical seat in the nozzle body by the spring acting through the intermediary of the stem. The spring pressure, hence injection pressure, is adjusted by the adjusting screw. The adjusting screw is screwed in the bottom of the injector spring cap nut which in turn is screwed in the nozzle holder. A lock nut is used to prevent the adjusting screw from unscrewing spontaneously. The screw is covered by the nozzle holder cap nut provided with a threaded hole to connect the leak-off pipe through which the leak-off fuel (used to lubricate the nozzle valve) filling the pressure spring and adjusting screw area is returned to the fuel tank or the secondary fuel filter.

In operation, fuel from the injection pump enters the pressure chamber (gallery) in the nozzle body through a supply passage and a high-pressure pipe. When the fuel pressure in the pressure chamber becomes so high that the force acting on the pressure taper of the needle valve from below exceeds the set spring force on the stem, the needle valve lifts off its seat and comes to rest with its upper shoulder against the face of the nozzle holder. Fuel is then forced out of the nozzle spray holes into the combustion chamber in a spray pattern which depends on the type of nozzle used.

After the injection of fuel has been ended, the fuel delivery from the injection pump ceases, the pressure in the pressure chamber of the nozzle drops instantly, and the pressure spring snaps the needle valve onto its seat, preventing unpressurized fuel from leaving the nozzle.

The fuel injector is installed in a brass injector tube, or sleeve, which is fitted in a hole in the cylinder head, and is held in place by a special clamp.

Low-pressure fuel lines use brass pipes or thin-walled steel pipes provided with a corrosion-resistant coating. Some engines make use of PVC tubing for the purpose.

High-pressure fuel lines connecting the fuel-injection pump with the injectors are steel pipes 2mm inside diameter and 7mm in outside diameter. The external surfaces of the pipes are oxidized for corrosion protection. The ends of the pipes are upset by a special device to form tapered male seats for connecting the pipes to the fuel-injection pump discharge and injector inlet fittings. The pipes should be fitted well to their respective fittings, so that the union nuts may be screwed on the fittings by hand. The nuts are finally tightened with a wrench.

Before installing them on the engine, all fuel pipes should be thoroughly washed in diesel fuel and then blown through with compressed air.

The fuel-injection pump serves the purpose of delivering, under high pressure, accurately timed and metered quantities of fuel to each cylinder of an engine, in accordance with the engine speed and load.

The pumping element consists of a plunger-and-barrel as-sembly, a spring, a roller tappet, the cam of the injection pump camshaft. And the delivery valve with its body.

The plunger and barrel assembly is the heart of the pumping element. It comprises a plunger reciprocating in the close-fitting barrel. The plunger and the barrel are tab ricated from alloy steel and heat treated to have a high hardness, for they must withstand high fuel pressures during operation. On the delivery stroke of the plunger, the fuel must lubricate the rubbing surfaces of the pumping element, but at the same time, it must not leak past the plunger from the upper to the lower side of the barrel. Therefore, these ports are lapped together in pairs to a very close tolerance (0. 001 to 0. 002 mm) and must not be replaced individually.

The fuel injection pump barrel is a center and a cylinder with somewhat thickened upper portion. In the thickened part of the barrel, there are two opposite side openings located at different heights. The upper opening, the inlet port, serves to fill the barrel space above the plunger with fuel. The lower opening the spill port, is used to bypass fuel to the inlet side of the pump. When the barrel is mounted in the pump housing, both ports are open to the U-shaped fuel manifold of the pump.

The upper part of the plunger has a center-aged a cross-drilled hole and a fuel cut-off (spill) spiral groove, called the helix, the center-drilled hole and the helix communicating with each other via the cross-drilled hole. The helix makes it possible to vary the delivery of the fuel-injection pump without changing the actual stroke of the pump plunger. An annular groove in the center part of the plunger serves for uniform distribution of fuel over the barrel face, the fuel in this case playing the part of lubricating oil.

On the lower part of the plunger, there are two flanges and a circular recess. The flanges slide in the slots of the control sleeve which carries a gear segment meshing with the control (fuel) rack of the pump. The circular recess is used for hooking the lower seat of the plunger spring to the plunger. The spring serves to return the plunger-its lower position on the intake stroke.

Diesel Engine Fuel System

The type of fuel available for use in diesel engine varies from highly volatile jet fuels and kerosene, to the heavier furnace oil. Automotive engines are capable of burning a wide range of fuel between these two extremes. How well a diesel engine can operate with different types of fuel is dependent upon engine operation conditions, as well as fuel characteristics.

Diesel fuel is a mixture of kerosene, gas oil and solar oil fractions obtained after distillation of gasoline fraction from petroleum. The main characteristics of diesel fuel are ignitability estimated in certain numbers, viscosity, pour point, purity, etc. Diesel fuel is produced in different grades, such as summer fuel, winter fuel, arctic fuel, which differ mainly in the pour points, flashing points and viscosity values.

A diesel engine fuel system consists of a fuel tank, a primary filter, a secondary filter, a fuel supply pump with a hand primer, an injector pump with a speed governor and automatic injection timing clutch, nozzle holders with nozzles, low-and high-pressure fuel lines (Fig. 8)

During operation of the engine, the fuel supply pump draw fuel from the tank, forces it through the primary filter and delivers through the secondary filter to the injector pump. From the injector pump the fuel is fed through the high-pressure lines to the nozzles; the fuel atomized by the nozzles is injected into the cylinders according to the engine firing order. Surplus fuel is returned from the injector pump and nozzles to the fuel tank. The air is supplied to the cylinders through the air cleaner.

Fig. 8 Diesel engine fuel sysem.

The fuel injector pump is intended to inject fuel under high pressure to the engine cylinders in a particular sequence. The injector pump a disposed between the cylinder banks and is driven from the camshaft by means of a gear train. The pump comprises a body, a camshaft, eight sections (according to the number of cylinders), and a plunger control mechanism. The front part of the injector pump carries a positive speed governor which meters the fuel in accordance with the load thus maintaining the engine speed preset by the driver.

The rear end of the pump camshaft mounts an injection timing clutch which is used to change the instant of fuel injection depending on the engine speed.

A section of the injector pump consists of a plunger and a barrel, a roller tappet and a delivery valve.

The barrel has two ports located at different levels; the plunger top is also provided with two ports and a helix. The plunger is lapped to the barrel.

When the plunger moves down forced by the spring, the fuel under a slight pressure created by the fuel supply pump flows, through the longitudinal inlet passage in the body filling the space above the plunger. As the plunger is moved upward by the cam and the tappet, the fuel is by-passed to the fuel passage till the plunger edge seals off the barrel port. As the plunger continues to move upward the pressure in the space over it will rise. When the pressure reaches the delivery valve limit, the plunger lifts slightly and the fuel is discharged through the high-pressure line to the nozzle. The plunger keeping on moving builds up a pressure that overcomes the nozzle stem spring load. The stem lifts up and the fuel injection begins. The injection continues until the edge of the plunger helix opens the port in the barrel; now the fuel pressure drops the relief band of the delivery valve is lowered to the seat by the spring thus jncreasiflg the volume in the fuel line between the nozzle and the valve, hence insuring positive shutoff of the fuel. When the rack is moved, the plunger rotates and the helix edge opens the barrel port in advance or with delay so that the time of port opening and the quantity of fuel injected into the cylinder are changed.

The fuel metering is controlled from the pedal in the driver’s cab through a system of rods and levers acting up-on the positive speed governor.

The nozzle is used to inject metered quantities of finely atomized fuel under pressure into the cylinders. The closed-type nozzle consists of a steel nozzle holder, a cap nut, a spray nozzle, a stem (or needle), a spindle and.a filter. The fuel passes through the filter, a vertical passage, annular slot to the, fuel space of the spray nozzle. When the pressure in the fuel space overcomes the spring, the stem is lined from the seat and the fuel in injected into the combustion chamber. As the pressure in the fuel line drops, the stem is shut off. Surplus fuel is by-passed via a return tine to the tank. The nozzle is adjusted for the injection pressure of 115-185 kgf/cm2. 5-18. 5MPa)

All the units of the diesel fuel system are inter connected by high-and low-pressure lines, the low pressure lines are made from transparent oil-gasoline resistant plastics tubing, while the high-pressure lines are made from thick-walled steel pipes.

The automatic injection timing clutch serves to change the instance of fuel injection depending on the engine speed for improving engine starting conditions and economy at acceleration.

The injection timing clutch consists of a driving half and a driven half. The driven half is secured to the shank of the injector pump camshaft. The driving half runs freely on a bub of the driven half and is rotated from the timing gear through flexible couplings. Hinged on the pivots of the driven half are the weights urged to the initial position by two springs. When the engine speed increases, the centrifugal forces move the weights apart, the shaped lugs on the weights turn the driven half, hence the camshaft for-ward (along the direction of its rotation) thus advancing timing. When the engine speed decreases, the springs return the cams to the initial position so that the driven half turns in the opposite direction to slow timing.

The Carburetor of Gasoline Engine

Practically speaking, there are tremendous differences between carburetors designed for various automobile models. In addition, the linkages, assist devices, and various controls on all carburetors can vary widely, even between two automobiles of the same make and model but with different engines. Yet all carburetors work on the same basic priciples.

A carburetor is a metering device that mixes fuel with air in the correct proportion and delivers this mixture to the intake manifold, where it delivers the air/fuel mixture to the various combustion chambers. Metering, in this situation, means that components within the carburetor accurately measure and control the flow of fuel and air passing through the various carburetor systems (Fig. 10).

The engine must have some form of metering device when its source of fuel for power is gasoline. In a liquid state, gasoline is of very little use to the engine. Contrary to popular belief, gasoline in a liquid is not combustible only gasoline vapor burns. Therefore, the carburetor or another metering device must combine the gasoline properly with the correct amount of air in order for the combustion process to release the energy in the gasoline.

Fig. 10 The carburettor of autombile

The function of any carburer found on a gasoline engine is to meter, automize, and distribute the fuel through the air flow passing into the engine. The carburetor is designed in such a way that it carries out all of these functions automatically over a wide range of operating conditions such as varying engine speeds, loads, and operating temperatures.

The carburetor also must regulate the amount of this air/fuel mixture that flows into the intake manifold. This regulation gives the driver the necessary control of the speed (rpm) of the engine.

Good combustion requires the correct mixture ratio between the air and fuel-commonly known as the air/fuel ratio. This ratio is necessary for the combustion process to release all the possible energy contained in the gasoline. An excessive proportion of fuel in the ratio results in a “rich” mixture; whereas, too little fuel bring about a ”lean” mixture. The metering task of any carburetor then is to furnish the correct air/fuel ratio for all operating conditions, so that the operation of the engine is not excessively lean to meet its power requirements or too rich for economy while still meeting the prime requirements of low emission.

The carburetor must not only meter the amounts of air and fuel entering the engine but also atomize the fuel. Atomization simply means the breakdown of the liquid into very small droplets of particles so that it can easily mix with air and vaporize. As the carburetor breaks the fuel into these small droplets, this action permits additional air contact with the liquid fuel. The greater the air contact, the easier the fuel turn into a vapor inside the intake manifold.

For excellent combustion and smooth engine operation, the carburetor must thoroughly mix the air and fuel together, and the intake manifold must uniformly distribute this mixture in equal quantities to all the engine’s combustion chambers. Adequate distribution of the mixture requires good vaporization. Vaporization is the act of changing a liquid, such as gasoline, into a gas, this change of state only occurs when the liquid absorbs sufficient heat to boil.

Because complete vaporization is the result of many factors such as outside air temperatures, fuel temperature, manifold vacuum, and intake manifold temperatures, it should be quite apparent that anything that reduces any one of these factors will adversely alter the vaporization process and therefore reduce engine power and fuel economy plus increase harmful exhaust emissions. Some of the conditions that interfere with proper vaporization are cold weather, inoperative heat-riser valve, high overlap camshaft, and heavy throttle demands.

The carburetor must provide an air/fuel mixture within the range of 8:1 and 18.5:1 in order for an engine to run. For the sake of efficiency, the engine should utilize a ratio that produces peak power output, minimum emission, and peak fuel economy. Unfortunately, no single air/fuel ratio permits an engine to meet all these conditions. Tests have proven that the best engine power output comes from using a 12.5 to 13.5:1 mixture; whereas, the best fuel economy results from using a 15, 16:1 mixture. Since no singe fuel ratio is satisfactory, the carburetor must quickly match the varying engine load requirements with the best possible air/fuel mixture in order to achieve the most efficient operating conditions. This simply means that the carburetor not only must provide a ratio to meet power demands, caused by such things as light-speed variations and changing engine load conditions, but also provide reasonable fuel economy and minimum exhaust emissions.

One of the main reason why the carburettor must vary the air/fuel ratios is the imperfect conditions within the combustion chambers. For example, exhaust gases remaining in the combustion chamber dilute the incoming fresh air/fuel charge. In addition, there are timed when the carburetor does not properly mix the air and fuel together. As result, tiny droplets of unvaporized fuel move into the combustion chamber, carrying along by the mixture of air and evaporated fuel. Finally, the intake manifold itself does not always deliver equal air/fuel mixture to all the cylinders.

Fuel Supply System of Gasoline Engine

All the gasoline engines have substantially identical fuel systems and run on a mixture consisting of fuel vapor and air. The fuel system comprises the units designed to store, clear and deliver fuel, the units intended to clean air and a unit for preparing a mixture from fuel vapor and air.

Fuel Supply System of Gasoline Engine

In a fuel system different components are used to supply fuel from the fuel tank into the engine cylinder. Some of the important components are fuel tank, fuel pump, fuel filter, carburetor, intake manifold and fuel-line or tubes connecting the tank, pump and the carburetor.

The fueltank is a fuel container used for storing fuel. It is made of sheet metal. It is attached to the vehicle frame with metal traps and is located at the rear of the vehicle. They are mounted in a boot or boot-floor pan in case of front-engined cars and small commercial vehicles. In order to strengthen the tank as well as to prevent surging of fuel when the vehicle rounds a curve or suddenly stops, baffle plates are attached to the inside of the tank.

A cap is used to close the filler opening of the tank. The fuel line is attached at or near the bottom of the tank with a filtering element placed at the connection. The other components of the fuel tank are the fuel gauge sending unit, a vent pipe, receiving unit.

To prevent the dirt and water from entering the luggage compartment, a sealing strip is fitted between the fuel tank and boot floor pan. Moreover to limit the transmission of frame distortion to the tank giving rise to squeaking as the metal parts get rubbed together, rubber or felt pads are of-ten fitted between the mountings and the tank. Provision is also made against drumming of the tank by these mountings. The tank may be placed at the side of the chassis frame for convenience in case of large commercial vehicles. The length of the connecting lines or tubes from the tank to the carburetor is also restricted by this at the same time.

A porous filter is attached to the outlet lines. By drawing fttel from the tank through the filter, any water in the bottom of the tank as well as any dirt into the fuel gathers on the surface of the filter. To keep the fuel always under atmospheric pressure, the filter pipe or tanks is vented.

In order to prevent dirt in the fuel from entering the fuel pump or carburetor duel filters and screens are used in the fuel system. Lithe dirt is not removed from the fuel, the normal operation of these units will be prevented. The engine performance will also be reduced.

The filter is either fitfed inside the fuel tank and pump or operates as a separate unit connected between the fuel tank and pump or between pump and carburetor into the fuel lines. Carburetors are also provided filter screens while a filter element is provided in the fuel tank.

The fuel titter used is generally a sediment bowl made of glass or metal and a strainer screen. When the fuel drawn from the tank passes through the filter (through the bowl and strainer screen), particles of dirt and water settle in the bottom of the bowl. In certain vetiicles, a separate filter either of the disk or ceramic type is used. It is either located between the fuel pump and carburetor or in the fuel line.

For connecting the fuel tank to the fuel pump, metallic tubes or synthetic rubber hose used are called fuel lines. They are usually positioned with metallic clips along the frame side members. The tubing or fuel lines are also used to connect fuel pump to the carburetor. In order to absorb vibration as well as prevent breakage of the fuel lines, a short flexible line is used between the fuel pump and the tank.

In order to meter and caution the driver of the motor vehicle about the quantity of fuel consumed and left in the tank, a fuel gauge is used. It is generally fitted on dash board for easy reading of the fuel. It is uslly a balancing coil type having construction similar to that of an oil gauge. It is generally electrically operated.

It consists of a sending unit mounted on the fuel tank and a receiving unit having a caliberated gauge mounted on the instrument panel.

A sending unit consists of a float controlled thermostat or variable resistor. With a float and the float arm extending into the fuel tank, the whole unit is mounted on it. The level of fuel in the tank varies the position of the float. The amount of electrical resistance within the variable resistor for controlling the amount of current sent to receiving unit on the instrument panel is determined by the float position.

The receiving unit mounted on the dash board mdioates the amount of fuel in the tank on a caliberated gauge by the amount of current received from the sending unit.

On merdon automobiles, two types of fuel gauges; thermo-static type and an electromagnetic type are used.

In order to prevent the rapid wear and tear of engine operating components causing reduced performance air cleaner is fitted to the carburetor air intake, it is mounted on the carburetor air-horn for trapping dirt. Before entering the carburetor, the air must pass through it.

To reduce the noise produced by the air rushing into the carburetor, a silencing chamber is built into the air cleaner. In case the engine misfires back through the carburetor, it acts as the flame arrestor.

There are in general three types of the air cleaners used in modern automobiles. They are (a) oil bath cleaner (b) oil-wetted mesh air cleaner (c) dry type air cleaner. The first two are also known as heavy duty air cleaner while the third is known as light duty air cleaner.

Fuel pumps are the devices used to supply fuel from the fuel tank to the carburetor. There are in general two main types of fuel pumps used in automobiles. They are (a) mechanical fuel pump (b) electric fuel pump.