Energy and the Environment

Energy and the Environment Fall 2013 Instructor: Xiaodong Chu Email：chuxd@sdu.edu.cn Office Tel.: 81696127

Flashbacks of Last Lecture • Nuclear fuel cycle • Mining and refining • Gasification and enrichment • Spent fuel reprocessing and temporary waste storage • Permanent waste disposal

Flashbacks of Last Lecture • Nuclear fusion • Magnetic confinement • Laser fusion

Transportation: Introduction • The development of the internal combustion engine(ICE) (内燃机) and its supply of liquid fossil fuel transformed both urban (城市) and rural (农村) communities in the twentieth century • Passenger travel to and in central urban areas by automobiles (汽车), bus (公共汽车)and electric-powered rail (电动轨道车辆)(above ground, at ground level, or underground) • Most freight(货运) moved by truck(卡车) , but bulk commodities were shipped by rail (火车) and river barge(内河趸船) • The rapid development of commercial air travel, beginning at mid-20th century, expanded long-distance travel availability

Transportation: Introduction • The growth of transportation in the twentieth century paralleled that of electric power, with both becoming major factors in the economies of developed nations at the century’s end • The availability of both electric power and transportation in modern industrial societies is an important factor in human productivity, although the efficiency of their use may well be improvable

Transportation: Introduction • The growth in the number of highway vehicles (公路车辆), mostly privately owned, has been accompanied by a growth in highways and roads(公路与道路), publicly financed and maintained • The privately owned vehicles clearly dominate highway travel and energy use whereas public passenger travel(旅客出行) , by bus and transit on the ground and airplane above, accounts for only small part of total transportationvehicle miles(运输车辆里程) annually

Transportation: Introduction • The problem of air pollutant emissions (空气污染物排放)from transportation vehicles is primarily that associated with the private passenger vehicle(私人乘用车) • Their emissions, principally carbon monoxide (CO), oxides of nitrogen (NOx), hydrocarbons (HC), sulfur dioxide (SO2), and particulate matter (PM), are distributed geographically in proportion to vehicle usage, which is concentrated in urban regions • But the secondary pollutants photochemically (光化学)formed from the direct emissions of NOx and HC can reach elevated levels downwind from the vehicular sources and outside the urban region

Transportation: Introduction • All primary emissions are regulated to ensure that primary and secondary air pollutant levels do not exceed harmful levels, either locally or regionally • The manufacture, operation, and maintenance of highway vehicles and the refining and distribution (提炼和销售)of their fuels is regulated by governments • Two of the regulatory objectives (监管目标) are control of exhaust andevaporative emissions(废气排放控制) and vehicle fuel economy(车辆燃油经济性) • The regulation of emissions and fuel economy falls principally upon the vehicle manufacturer, to a lesser extent on the fuel supplier, and hardly at all upon the vehicle owner, whose principal responsibility is to maintain control equipment during the vehicle lifetime

Transportation: Internal Combustion Engines for Highway Vehicles • The most common engine in road vehicles is the gasoline-fueled spark ignition (SI) reciprocating engine(火花点火往复式汽油发动机), which is economical to manufacture and maintain, provides ample power per unit weight, and has a useful life (使用寿命)that equals that of the typical passenger vehicle • The less common alternative, the diesel-fueled compression ignition (CI) reciprocating engineengine(压缩点火往复式柴油发动机), is more durable(耐用) and more fuel efficient(省油) , but is more expensive to manufacture and provides less power per unit weight (unless turbocharged(涡轮增压))

Transportation: Internal Combustion Engines for Highway Vehicles • The SI and CI engines utilize the mechanism of a movablepiston(可移动的活塞) within a closed-end cylinder (封闭式气缸) linked by a connecting rod (连杆) to a rotating crankshaft(旋转曲轴) that converts the reciprocating motion of the piston to the rotary motion of the crankshaft • High-pressure gas in the cylinder exerts an outward force on the piston, doing mechanical work on the crankshaft as the piston recedes on the power stroke(作功行程(冲程)) • On the return inward compression stroke (压缩行程(冲程)), less work is done by the crankshaft on the piston than was done on the crankshaft during the outward stroke, so the crankshaft delivers net positive work

Transportation: Internal Combustion Engines for Highway Vehicles • Intake stroke (进气行程(冲程)): A fuel–air mixture or pure air is ingested prior to the compression stroke (压缩行程(冲程)) • In the SI engine, a fuel–air mixture flows into it prior to the inward compression stroke (压缩行程(冲程)), eventually being ignited by a spark when the piston reaches its innermost position (called top center(TC)(上止点)) at the end of compression • For the CI engine, pure air is ingested (注入) prior to the compression stroke(压缩行程(冲程)) and the fuel is sprayed into (喷入)the air at the end of compression, whereupon it ignites and burns quickly, without the necessity of a spark, to produce a pressure rise

Transportation: Internal Combustion Engines for Highway Vehicles • Exhaust stroke (排气行程(冲程)): It is necessary to replace the burned gases in the cylinder with a fresh charge (fuel–air mixture for the SI engine, pure air for the CI engine) • This is accomplished by opening ports in the cylinder connected to ducts (管道)that either conduct the fresh charge into the cylinder or collect the combustion gases leaving the cylinder at the end of the expansion stroke • There are two mechanical schemes for effecting this replacement of burned gas by a fresh charge • Two-stroke cycle(两冲程循环) • Four-stroke cycle(四冲程循环)

Transportation: Internal Combustion Engines for Highway Vehicles • Two-stroke cycle • The fresh charge displaces the combustion products during a short time interval when the piston is near its outermost position (called bottom center(BC)(下止点) ) and both an inlet port and an exhaust ports are open • Only two strokes of the piston, one inward to compress the fresh charge and the second outward to expand the burned gases, are needed to complete this two-stroke cycle, during which the crankshaft turns through one revolution

Transportation: Internal Combustion Engines for Highway Vehicles • Four-stroke cycle • The outward power stroke is followed by a full inward exhaust stroke in which the piston displaces the combustion products, pushing them out through an open exhaust port located in the closed end of the cylinder • At the end of this inward stroke, the exhaust port is closed and an inlet port is opened, allowing the piston to suck in a fresh charge as it moves outward during the subsequent intake stroke • By adding these two extra strokes (an extra revolution of the crankshaft), a fresh charge is prepared in the cylinder, permitting the two subsequent strokes to compress, burn, and expand the charge, thereby producing work • These four strokes (two crank revolutions) comprise the four-stroke cycle

Transportation: Internal Combustion Engines for Highway Vehicles • Four-stroke cycle

Transportation: Internal Combustion Engines for Highway Vehicles • Almost all highway vehicles are powered by four-stroke cycle engines, predominantly gasoline fueled SI engines for passenger vehicles and diesel-fueled CI engines for trucks • Two-stroke-cycle SI engines are mostly used to power two-wheeled vehicles, such as motorcycles • Engines of two-stroke cycle have lower fuel efficiency and higher exhaust pollutant emissions, but are lighter and less expensive to manufacture

Transportation: Internal Combustion Engines for Highway Vehicles • Differences of SI and CI engines • In the SI engine the fuel and air are mixed together outside the cylinder to form a uniform fuel–air mixture that is ingested during the intake stroke; in the CI engine the fuel is sprayed into air in the cylinder, near TC, forming a non-uniform fuel–air mixture • In the SI engine, the combustible mixture is burned when ignited by a spark at the appropriate time in the cycle near TC, whereas the CI fuel–air mixture ignites spontaneously, shortly after the fuel is sprayed into the engine combustion chamber at the requisite time near TC • In the SI engine the method of controlling the power output is adjusting the air pressure in the intake ducts by means of a throttle valve (节油阀)that lowers that pressure below the atmospheric value; in the CI engine, the power is lowered by reducing the amount of fuel injected into the cylinder at each cycle

Transportation: Internal Combustion Engines for Highway Vehicles • To perform properly, any reciprocating internal combustion engine must burn a mixture of air and fuel in a very short time • The duration of combustion should not much exceed about 50 degrees of rotation of the crankshaft, which would equal about 2 ms if the crankshaft were rotating at 3600 revolutions per minute (rpm) • The fuel and air should be mixed in carefully controlled proportions prior to or while entering the engine cylinder • The conversion of hydrocarbon and oxygen molecules to carbon dioxide and water vapor molecules should be assured by the rapid temperature rise of the cylinder charge during the inward compression stroke

Transportation: Internal Combustion Engines for Highway Vehicles • At the molecular level, the combustion process is much more complex than might be inferred from the overall stoichiometry(化学计量比) of the reaction • The combustion process is not perfect, so that small amounts of un-reacted or imperfectly oxidized products may remain; • These molecules are dispersed among the principal combustion products and, unless removed, enter the atmosphere as air pollutants

Transportation: Internal Combustion Engines for Highway Vehicles • Nitric oxide (NO) is an important air pollutant that is a byproduct of the combustion process • It is formed from nitrogen and oxygen because it is thermochemically favored at the high temperature of the newly formed combustion products • It is produced rapidly by the two following reaction steps, facilitated by the presence of atomic oxygen (原子氧) in the combustion zone(燃烧带) N2 + O → NO + N N + O2 → NO + O

Transportation: Engine Power and Performance • The burning of fuel in the cylinder of a reciprocating internal combustion engine produces mechanical power by exerting a force on the moving piston • The total work done by the gas in one cycle of the engine is • The power input to the piston

Transportation: Engine Power and Performance Diagram of the pressure, as a function of volume, in a four-stroke-cycle SI engine cylinder

Transportation: Engine Power and Performance • Only a part of the work done by the gas on the moving pistons of an engine is delivered to the output shaft(输出轴) of the engine, where it can then be connected to the transmission (传动) and ultimately to the wheels to propel a vehicle • The net output, called the brake power(制动功率) , is determined while running the engine in a laboratory at various rotational speeds and throttle settings(节气设置) • Sometimes the power and torque are expressed in terms of an average cylinder pressure , called the brake mean effective pressure(制动平均有效压力)

Transportation: Engine Power and Performance • The size of engine installed in a vehicle is determined by the power required to propel the vehicle at the speed and acceleration needed to perform satisfactorily • An important characteristic of an engine is its power and torque when operated at maximum fuel input per cycle • For an SI engine, this occurs when the throttle is wide open

Transportation: Engine Power and Performance • The fuel economy of the engine is usually expressed as the brake specific fuel consumption (bsfc)(制动油耗率, 燃油消耗率, 制动单位油耗量), the ratio of the mass of fuel consumed per unit of mechanical work output by the engine shaft • The value of the brake specific fuel consumption depends upon the engine operating conditions • For SI engines, the most economical bsfc is about 0.27 kg/kWh, while for CI engines, it is lower, about 0.20 kg/kWh

Transportation: Engine Power and Performance • An alternative measure of engine performance is the thermal efficiency, the ratio of engine mechanical work to the heating value of the fuel mass consumed to produce that work

Transportation: Engine Power and Performance • The engine efficiency is a function of the engine speed and brake mean effective pressure • The efficiency declines rapidly with decreasing pbmep but less so with increasing speed • Because a road vehicle engine must provide the full range of its power over the speed range of the vehicle, it cannot operate at maximum efficiency all the time • By proper matching of the engine to the vehicle, it is possible to minimize the fuel consumption needed to meet a particular vehicle driving cycle

Energy and the Environment