The fuel system has the job of supplying a combustible mixture of air and fuel to the engine. The fuel system consists of the fuel tank, fuel pump, fuel filter, carburetor and fuel lines. All automobile have some forms of fuel supply. The purpose of the supply system is to store and then to supply a clean, continuous, and adequate amount of fuel under sufficient pressure to the carburetor. Moreover, the system must perform these functions regardless of the outside temperature, altitude, and speed of the vehicle.

Components of the fuel supply System

A Typical fuel-supply system consists of a fuel tank assembly, fuel lines, or fuel pump, and outlet filter. The fuel tank stores a supply of fuel.

Fuel Tank

The actual capacity and design of a given fuel tank are a compromise between the available space, filler-well location, control of fuel movement within the tank, and space within the tank for fuel expansion.

Manufactures from the fuel tank itself in two half-sections, using a corrosion-resistant, thin-steer planting. The exposed half-section is usually constructed of a heavier steel than the unexposed section for protection against road damage and corrosion. In addition, each section has a series of formed ribs, which provide the tank with addition strength. Finally, before welding the two sections together to from the complete tank, the manufacturer usually inserts a vertical baffle plate. This plate has a series of fuel form one side of the tank to another. This desi-gn prevents the fuel form sloshing around in the tank as the vehicle moves.

Fuel Tubes

The main fuel tank must have all the following: a filler tube, a ventilation system, fuel pickup tube and filter, plus a gauge-sending unit. The filler tube is a tube that runs from the tank itself to a fuel service entrance on the outside of the vehicle.

Fuel-Tank Ventilation System

The fuel tank of any motor vehicle must have some form of ventilation system. This system vents the tank to the atmosphere to assist in the rapid filling of the tank, to compensate for the expansion and contraction of the fuel due to temperature changes, and to maintain a constant pressure in the tank over the fuel, regardless of the fuel level. This latter function is necessary to prevent one form of vapor lock: a stoppage of fuel flow caused by an insufficient amount of atmospheric pressure inside the tank. The atmospheric pressure must act in conjunction with the vacuum produced in order for it to deliver fuel to the carburetor.

Fuel Pickup Tube and Filter

A fuel pickup tube that is the same diameter as the main fuel line running to the fuel pump enters the tank to carry off the fuel nectssary for engine operation. Manufacturers generally locate the end of this tube about one-half inch from the inside bottom of the tank. This location permits room for some water and sediment to collect in the bottom of the tank without the fuel pump drawing either out with the fuel. On the end of the majority of all pickup tubes is a filter. No matter which design it has, this filter prevents sediment and water that settled to the bottom of the tank from entering the line to the fuel pump.

Tank Unit and Fuel Gauges

The pickup tube and filter are usually part of an assembly that also incorporates a fuel-tank unit. The tank unit electrically operates in conjunction with a dash gauge to indicate to the driver the level of the fuel in the tank. Two general types of fuel indicating systems, which consist of tank unit and dash gauge, are used: the thermostatic type and the balancing coil type.

Fuel Lines

Many components of the fuel system are connected together with lines and hoses. There lines and hoses carry the fuel from the tank to the fuel pump, from the pump to the carburetor, return excess fuel to the tank, and carry fuel vapors. Depending on the type of installation, these lines can be either rigid or flexible.

Fuel Pump-Function

Every fuel-supply system has some of pump. The function of this pump is to deliver fuel from the tank to the carburetor.

Mechanical-Type Pumps

A motor vehicle can have one of the two basic type of fuel pumps: mechanical or electrical. The most common type of fuel pump is the single-action, diaphragem-type mechanical pump. This pump design operates by means of eccentric (an off-center section) on the engine camshaft. The eccentric may be part of the camshaft itself or may bolt to it. In either case, the action of the eccentric produces one pump stroke for each revolution of the camshaft.

Electrical-Type Fuel Pumps

Electrical-type fuel pumps offer certain advantages over the mechanical fuel pump. For example, fuel is available at the carburetor as soon as the operator turns the ignition switch on. Also, the electrical type pump can deliver more fuel than the engine requires even under maximum operating conditions. Consequently, the engine never starves for fuel. Finally, electrical fuel pumps eliminate, in many cases, the problem of vapor locking that can be a problem for engins with mechanical fuel pumps.

Filter Types

Fuel-Pump Outlet Filter

Manufacturers may install an outlet filter in three locations: in the fuel pump, in the fuel line, or in the carburetor. Some vehicles have a filter in the outlet side of the pump. This filtering element may be in the form of a screen but usually is a throwaway paper element.

In-Line filter

In-line filters,used in some vehicles, are in the fuel line between the fuel pump and the carburetor.

Carburetor

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. In a liquid state, gasoline is of very little use to the engine. 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.

Basic Carburetor System

All carburetors perform a comparatively simple task but must do so under various conditions. Therefore, the carburetor must have many systems to alter its fuenctions to the various operating conditions of the engine. Although some carburetor only have five, most units contain six basic systems: the float system,idle system, main metering system, power system, pump system, and choke system.

Float System

The float system is perhaps one of the most important systems in the carburetor. The float system consists of a bowl, float, needle and seat, along with a float-vent. The float system is responsible for storing and supplying an adequate amount of clean fuel to the various other operating systems of the doing so, the float system also regulates the fuel flow output from the fuel pump. While performing all of these tasks, the float system must maintain the fuel level in the float chamber at a precise, nearly constant level.

Idle and Low-Speed System

All automotive carburetors require some form of idle and low speed system. This system is necessary to proved the proper air/fuel ratios required to operate the engine at idle and during low-speed operation. During these phases of engine operation, airflow through the carburetor is very slight due to the nearly closed position of the throttle valve. The throttle valve is a round disc in the lower carburetor casting that when rotated admits more of less airflow through the carburetor throat, thereby controlling engine speed. With the throttle valve nearly closed, airflow through the venturi is insufficient to cause the main nozzle to deliver fuel.

Main-Metering System

The main-metering system is responsible for supplying an air/fuel mixture for high-speed engine operation. During this particular phrase of engine performance, the engine is not under heavy loads and therefore can operate on an economically lean air/fule mixture. The carburetor components of this system are the main jet, main nozzle, and the air bleed. The main jet fits between the fuel bowl and the main well. The main well forms the working fuel reservoir for the main-metering system. The main nozzle discharges an atomized air/fuel mixture into the throat of the carburetor. The air bleed adds air to the liquid fuel to atomize it partially.

Power System

The main-metering system provides the leanest air/fuel mixture of any of the other carburetor circuits; a richer mixture is necessary not only for extended high-speed operation but also for maximum engine power. For maximum engine power, a rich fuel mixture is mandatory in order for the combustion process to consume all the oxygen in the air entering the combustion chamber. To accomplish this action, the carburetor has some type of power system used to supplement the main-metering system. This power system provides an increase in fuel mixture flow from the nozzle tip, according to the amount of throttle operning and engine load. In other words this additional system functions to enrich the total air/fuel mixture during any phase of main-metering system operation, depending on throttle position and engine-load requirements.

Pump System

When the driver opens the throttle valve rapidly, from a closed or nearly closed position, in order to accelerate the vehicle, engine vacuum rapidly drops; but the air flow through the carburetor throat increase instantly. Due to the great difference in weight between air and fuel, the flow of fuel from the carburetor circuits lags behind the increase in air intake. As a result the engine experiences a momentary a momentary leanness, which causes a brief engine hesitation, stumble, or flat spot. The pump system provides the additional fuel flow necessary to overcome this leanness and maintains smooth engine operation during rapid low-speed acceleration.

Choke System

When starting a cold engine, the factors necessary for good fuel vaporization are missing or inadequate. For this reason, it is necessary to provide extremely rich mixture from the carburetor in order to provide sufficient combustible mixture to all cylinders for quick starting. The carburetor obtains this enrichment by the addition of a choke valve in the carburetor throat above the venturi and main nozzle. This choke valve, during cold engine starting, starts fuel to flow through the main system prematurely.

参考译文：

燃料供给系统的作用是为发动机提供被称为可燃混合气的汽车与空气混合物。燃料供给系统由油箱、燃油泵、汽油滤清器、化油器和油管组成。所有的汽车都有燃料供给系统。其目的就是存储燃油，为化油器提供清洁、连续且具有一定压力的汽油。此外，燃料供给系统在车辆处于不同环境温度、海拔和速度的条件下完成上述工作。

燃料供给系统的组成

典型的燃料供给系统由油箱总成、油管、燃油泵和汽油滤清器组成。油箱用于存储汽油。

油箱

油箱实际的容积与设计要考虑到汽车外形、安装位置、油箱内汽油流动的控制和汽油在油箱中的膨胀空间。

油箱体分成两部分，使用抗腐蚀的薄钢板。油箱暴露在外的部分采用比不暴露部分更厚实的钢板制成，用于保护油箱不受道路条件的损害与侵蚀。另外，两部分都有连续的加强筋用以提高油箱的强度。最后，在两部分焊接之前，会在油箱中加装夹板，用以减轻由于汽车行驶时汽油的振荡。

油管

油箱必须装备以下元件：加油管、油箱通风系统、吸入管、滤清器和油量传感装置。加油管用于与加油设备相连。

油箱通风系统

任何汽车的油箱都装有通风系统。当汽油受到温度影响产生膨胀或收缩时，无论油箱中的汽油有多少，与大气相通的安装在油箱上的通风系统阀门，都可以维持油箱内的压力。特别要防止汽油收缩时，由于油箱内的大气压力不足导致供油中断。为了汽油能输送到化油器中，燃油泵中的真空度必须与大气压力同步。

吸入管与滤清器

吸入管的直径与油箱主油管相同，并与燃油泵的进口相连，为发动机的运转输送必需的汽油。管子通常安装在高油箱底部约1.5英寸的位置。这样的安装好处就是可以避免燃油泵将油箱底部的水和沉淀物输送至发动机。在大多数的吸入管的尽头装有滤清器。这样就可以避免沉淀物与水被输送到燃油泵。

箱体与燃油表

通常吸入管与滤清器是安装在一起的总成并属于油箱单元。与油箱相连的燃油表通过电控的方式向驾驶员显示油箱中的油量。两种常见的油量显示系统是温控式和平衡线圈式。

燃油管

燃料供给系统大多数部件之产是通过金属管和软管连接的。通过这些管路，将燃油从油箱送至燃油泵，再从燃油泵输送到化油器，剩余的燃油送回油箱。根据安装的形式。这些管路可以是刚性连接也可以是柔性连接。

燃油泵的形式

每种燃油供给系统都有燃油泵。其作用就是将燃油送至化油器。

机械式燃油泵

汽车上安装的燃油泵有两种基本形式：机械式和电动式。大多数的燃油泵是单膜片机械式的。这种泵是由发动机凸轮轴上的偏心轮驱动。偏心轮可能与凸轮轴是一体的，也可能是通过螺钉连接的。当凸轮轴旋转时，偏心轮推动燃油泵工作。

电动式燃油泵

电动式燃油泵比机械式燃油泵更有优势。例如，当点火开关接通的同时，燃油就能送至化油器。当发动机满负荷工作时，电动式燃油泵可以提供比发动机需要的更多的燃油。因此发动机不会在这种工况下缺油。最后，在多数情况下，电动式燃油泵解决了机械式燃油泵水蒸气凝结（气阻）的问题。

滤清器的形式

燃油泵出口滤清器

滤清器一般安装在三个位置：燃油泵、油管或者化油器。有些汽车的滤清器安装于燃油泵的出口。这种滤清器中安装滤膜，但通常安装纸制滤芯。

管路滤清器

管路滤清器在汽车中是很常见的，通常安装在燃油泵与化油器之间的油管上。

化油器

化油器是将空气与燃油按一定比例混合的计量装置，形成的可燃混合气送至进气歧管最终进入燃烧室。作为计量装置的化油器要能精确地控制燃油与空气通过化油器。发动机无法充分利用液态状汽油，只有雾化后的汽油才能真正为发动机燃烧利用。因此，化油器或者其他的类似装置必须精确地按照比例将汽油与空气进行混合，使得汽油充分燃烧。

化油器的基本系统

化油器必须在各种工况下完成其功能。因此化油器有一些装置可以根据发动机的不同工况对化油器的工作进行调节。化油器一般有五个，或者六个基本装置：浮子装置、怠速装置、主供油装置、大负荷加浓装置、加速装置和启动装置。

浮子装置

浮子系统也许是化油器中最重要的装置。浮子系统包括浮子室、浮子、针阀和化油器基座。浮子装置的作用是为化油器其他各部件储存和提供清洁的燃油，控制燃油泵输出的燃油量，同时必须精确地保证浮子室中的燃油量不变。

怠速装置

所有汽车化油器都必须有怠速（低速）装置。这个必不可少的装置在发动机怠速（低速）时为发动机提供适当比例的可燃混合气。在这样的工况下，节气门几乎关闭，流过化油器的空气很少，从而控制发机的转速。由于节气门几乎关闭，因此流过喉管的空气量使得主喷管喷出的燃油量很少。

主供油装置

主供油装置为发动机高速运行时提供可燃混合气。在这个阶段，发动机的复合较小，因此此时可采用经济比例的可燃混合气。主供油装置包括主量孔、主腔、主喷管和渗气孔。主量孔在浮子室与主腔之间。主腔为主供油装置蓄油。主喷管喷出的燃油在化油器喉部与空气雾化。渗气孔增加的空气使雾化更充分。

大负荷加浓装置

主供油装置为化油器每个工作循环提供最稀的可燃混合气，在发动机急加速和大功率工况下，必须提供更浓的可燃混合气。在发动机最大功率的工况下，燃烧过程中浓的可燃混合气会将燃烧写中的所有空气消耗完。要完成这样的工况，化油器要求有大负荷加浓装置为主供油装置作补充。大负荷加浓装置根据节气门的张开度和发动机负荷，从加浓量孔增加燃油量。换句话说，就是这附加装置根据节气门位置和发动机负荷的需求，在主供油装置供油的基础上增加碰油量。

加速装置

当驾驶员为了加速迅速踩下油门时，节气门从关闭或接近关闭的位置迅速打开，发动机中真空迅速降低，此时流过化油器的空气量增加。由于燃油与空气的重量有很大的差别，因此燃油总是比空气进入化油器迟。使得发动机功率只要一降低，输出动力就减弱。加速装置为这种情况提供额外的燃油，从而克服这种问题，使发动机在加速过程中平顺工作。

启动装置                                               

当发动机冷启动时，燃油雾化的条件较差。由于这个原因，要使发动机迅速启动，必须通过化油器向每个汽缸提供浓的可燃混合气。所在在节气门和主喷管的上方增加了一个阻风门。在冷启动时，阻风门使燃油提早进入主供油装置。