28-09-2012, 03:30 PM
ADVANCES IN TRANSPORTATION TECHNOLOGY
1TRANSPORTATION TECHNOLOGY.doc (Size: 190 KB / Downloads: 60)
Cars, planes, and trains, that's how we get around these days. (Oh, we take a boat now and then but that's mostly for fun.) What about the technologies involved in these means of transportation? Do we really know what makes them go?
Take cars, for instance. How many drivers understand the basic engineering principles governing engine operation or the means for transmitting power to the wheels? How important is aerodynamic drag to overall performance of our cars? Should we be considering alternative fuels or even alternative engines? What is the realistic potential for electric or hybrid cars? How is future technology likely to transform our cars?
Next, take planes. When the British-French Concorde made its first flight more than a decade ago, this sharp-nosed supersonic jet that could fly across the Atlantic in under four hours was the last word in high-tech passenger planes. That is about to change. On the drawing board are the high-speed civil transport aircraft with a capability of flying at speeds close to Mach 3, or three times the speed of sound (at sea level, the speed of sound is 740 miles per hour or 1184 km. per hour). The Mach 5.5 "Orient Express" is designed to fly 300 passengers across the Pacific Ocean in two hours. Finally, the hypersonic NASP (for National Aero-Space Plane) is designed to fly into orbit. Should we plan our future vacations around one of these concepts? A review of the technologies will help us decide.
The term high-tech train has, for many of us, been an oxymoron. Train technology, at least in [the U.S.A.], was stopped in its tracks many decades ago, and progress is not the word often used in connection with passenger train travel. There are those, however, who think this state of affairs is about to change. Those favoring a renewal in train transport anticipate a passenger-rail renaissance in the United States. They point to the growing congestion on the nation's roads and at its airports, and to the pollution caused by petroleum-burning engines. Can the success the Japanese, French, and Germans have with high-speed trains be duplicated in this country? Should we give trains another chance?
Fundamentals
Fundamental scientific laws govern the ability of an automobile engine to convert the energy in the fuel to a form that is able to propel the car down the road. These are referred to as the First and Second Laws of Thermodynamics. Thermodynamics is that branch of physics that deals with the transformation of heat into work and other forms of energy. The First Law simply states that "energy is conserved"; that is, it's undestructible--there is always the same total amount of energy in the universe. Energy is neither created nor destroyed, it just changes form, such as from chemical energy in fuel to heat or mechanical energy.
The Second Law is bit more complex. The Second Law states that "the entropy of the universe tends to a maximum." Entropy is a measure of the total disorder, randomness, or chaos in a system. The effect of increased entropy, then, is that things progress from a state of relative order to one of disorder. With this progressive disorder there is increasing complexity.
Everytime we convert energy from one form to another we lose on the deal. Some of the energy is wasted. It is not lost--that would be contrary to the First Law; but it is converted to heat that is dissipated in the environment. The portion of the energy that is unavoidably dissipated as nonuseful heat is reflected in the measurement of entropy. In our automobiles, this heat rejection occurs mostly through the cooling system's radiator and the exhaust pipe. In fact about 70 to 80 percent of the energy in gasoline flows out of the automobile in the form of rejected heat. Much of this is accounted for by the Second Law.
Diesels
The diesel engine is similar to the typical Otto engine in that there are pistons that carry out the same four strokes: intake, compression, power, and exhaust. The differences are that the inlet charge has no fuel mixed with it and there is no spark plug to initiate combustion. Instead, the fuel is injected into the cylinder near the top of the compression stroke, and a high compression ratio is used so that the charge becomes hot enough to ignite spontaneously. This is similar to the detonation phenomenon (or knock) that is considered objectionable in the Otto cycle engine and is the basic limit on that engine's compression ratio.
The diesel's main advantage is its superior fuel economy. There are some drawbacks, however. The high pressures in diesels are hard on engine components and generate more noise than does an Otto engine. Diesels must be built heavier and sturdier than their gasoline cousins to allow them to withstand the beating. Diesels also have a significant exhaust problem, although considerable research is underway to reduce diesel soot and other undesirable emissions. At this writing there are few diesel passenger cars being sold in the United States, but should future fuel prices increase, the diesel engine production lines may be opened again.
Wankel Engines
Wankel engines are Otto cycle engines with a major difference. The pistons are replaced with a rotor. The cycle is the same. The main advantage to the Wankel is that it can produce more horsepower per pound of engine than a conventional engine. However, the rotor configuration limits the compression ratio and requires many sliding seals, which sometimes leak, thus causing more emissions and lower efficiency.
Displacement
Before you leave the engine basics, consider the terminology used in automobile ads and brochures. These ads are confusing, if not misleading. Most automobile brochures today refer to engine displacement (engine size) in liters, but some use cubic centimeters or even cubic inches. They are all referring to the same thing: displacement, or the volume of space through which the piston travels during a single stroke in an engine. The numbers you see in the literature refer to the space traveled by all the pistons in a single stroke. That is, a six- or eight-cylinder engine has a larger displacement number than a four-cylinder engine with the same size pistons.
Because a liter is equal to 1000 cubic centimeters (expressed as 1000 cc), that conversion is an easy one. For example, a 1.9 liter displacement is the same as 1900 cc dis-placement. Manufacturers of subcompacts often advertise displacement figures in cubic inches (probably to make comparison difficult). Fool them. To convert displacement expressed in cubic inches to liters divide by 60. (Example: 173 cu. in. equals 2.8 liters).
Alternate Fuels
Because almost every city in the United States is now in violation of the Clean Air Act standards, Congress is considering new measures to reduce the air pollution caused by automobiles. One approach is to encourage the use of fuels other than gasoline. A bill introduced by Representative Henry Waxman requires those areas with the worst air pollution to establish extensive alternate fuel programs so that 30 percent of new cars in those areas would be operating on cleaner-burning ethanol, methanol, or natural gas by 1998.
Both Ford and General Motors plan to provide the state of California with test fleets of "flexible fuel" cars, able to run on methanol, ethanol, or gasoline, in the next few years. The flexible-fuel cars in the test fleet would emit 50 percent fewer smog-producing chemicals than comparable gas-only cars, according to the California Energy Commission, which is subsidizing the test fleet.
To provide fuel for the test fleet, both Chevron and Arco have agreed to install methanol pumps at 50 service stations throughout California by the end of 1989. None of the suggested alternative fuels is as efficient as gasoline nor are they cost competitive at current prices. It is only through the use of subsidies that alternative fuels can be made cost-effective at this time, but the growing problem of dirty air may make some sort of subsidy necessary eventually.
Aerodynamics
Because the industry has gone about as far as it can go in downsizing cars, improved aerodynamics will have to play the major role in reducing the resistance to motion in future passenger cars. The aerodynamic drag of a car at a given speed is determined by the frontal area and the drag coefficient. Typical frontal areas are between 17 and 20 square feet. With other factors being equal, the car with the smallest frontal area gets the best mileage. However, the need to seat passengers comfortably limits the ability to reduce the frontal area of cars. That brings us back to the coefficient of drag, usually expressed as CD.