26-11-2012, 05:10 PM
Cryogenics
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ABSTRACT
Cryogens are effective thermal storage media which, when used for automotive purposes, offer significant advantages over current and proposed electrochemical battery technologies, both in performance and economy.
An automotive propulsion concept is presented which utilizes liquid nitrogen as the working fluid for an open Rankine cycle. The principle of operation is like that of a steam engine, except there is no combustion involved. Liquid nitrogen is pressurized and then vaporized in a heat exchanger by the ambient temperature of the surrounding air. The resulting high – pressure nitrogen gas is fed to the engine converting pressure into mechanical power.
The usage of cryogenic fuels has significant advantage over other fuel. Also, factors such as production and storage of nitrogen and pollutants in the exhaust give advantage for the cryogenic fuels.
INTRODUCTION
The importance of cars in the present world is increasing day by day. There are various factors that influence the choice of the car. These include performance, fuel, pollution etc. As the prices for fuels are increasing and the availability is decreasing we have to go for alternative.
Here an automotive propulsion concept is presented which utilizes liquid nitrogen as the working fluid for an open Rankine cycle. When the only heat input to the engine is supplied by ambient heat exchangers, an automobile can readily be propelled while satisfying stringent tailpipe emission standards. Nitrogen propulsive systems can provide automotive ranges of nearly 400 kilometers in the zero emission mode, with lower operating costs than those of the electric vehicles currently being considered for mass production. In geographical regions that allow ultra low emission vehicles, the range and performance of the liquid nitrogen automobile can be significantly extended by the addition of a small efficient burner. Some of the advantages of a transportation infrastructure based on liquid nitrogen are that recharging the energy storage system only requires minutes and there are minimal environmental hazards associated with the manufacture and utilization of the cryogenic"fuel". The basic idea of nitrogen propulsion system is to utilize the atmosphere as the heat source. This is in contrast to the typical heat engine where the atmosphere is used as the heat sink.
Cryogenics
The use of extremely low temperatures (cryogenic temperatures, which scientists define as below -244°F.) to boost the performance and service life of critical components is now commonplace in the racing industry and is becoming more and more prevalent in the manufacture of high quality components. What was once considered by many to be a questionable science is becoming a bedrock solid means of insuring the greater performance of materials.
Why has it taken this long to gain acceptance? It is extremely hard to get across to people that changes can be made using cold. People easily grasp the fact that heat can be made to modify solid materials. Heat treating is all around us. Humans have been modifying metals with heat for over 7,000 years, and archeologists have found evidence that humans heated the rocks to make better tools with them over 140,000 years ago. So, the use of heat is second nature to most of us. We’ve only had extreme cold available to us in commercial quantities for about 100 years.
Racing Applications
Cryogenic processing can have a positive effect on virtually every engine, transmission, and drive line part, as well as many chassis parts. Increasing the durability of components in the vehicles is the main reason for using cryogenic processing. The great thing about cryogenic processing is that it allows an increase in durability without an increase in weight or major modifications to component design. In addition, the use of cryogenic processing has helped some racing teams reduce costs, enabling some expensive parts to survive the stresses of racing for use in subsequent races.
Performance Advantages
Brakes and Clutches. Brakes of a racing car take a real beating. It is not unusual for a racing vehicle to finish a race with the brakes totally worn out. This is especially true during road races and endurance racing, where brake rotors can get so hot they glow visibly at night. Cryogenic processing can be applied to both rotors and pads. The net result is two to three times the life of untreated components even under severe racing conditions. As a side benefit, the rotors are less prone to crack or warp. It is interesting that drivers report better braking action and feel. Some drivers are so sold on the concept that they have their street vehicle equipped with treated brakes. Clutches are a form of brake, and the results are very similar.
As an offshoot of racing development, cryogenically treated rotors and pads are making their way into fleet operations on the road. The U.S. Postal Service specifies cryogenic processing for their rotors and is experiencing up to five times as many miles as they were getting on the unprocessed rotors. Similarly, many police fleets are starting to adopt treated rotors and pads. They, too, are experiencing large maintenance savings on both parts and labor. What is metallurgically interesting is that the brakes are a gray cast iron that has a pearlitic structure. This rules-out the austenite to martensite transformation as the mechanism for increased life.
Springs
Springs fail in one of two modes. They either break or their spring constant starts to decline. Either way, it can have catastrophic effects on the performance of the vehicle. Most valve springs are made of specially made chrome silicon steel. The automotive valve spring is a fatigue failure waiting to happen. It typically can lose up to one third of its spring constant during a long race. In some forms of racing, it is just hoped that the valve springs will last through the race. Some drag racers routinely change the valve springs before every run down the drag strip to ensure consistent performance.
The Chassis
The chassis itself is basically a very large, complex spring, having numerous welds and using not very precise tubing. The metals used here vary, depending on the type of racing. NASCAR frames are made from 1020 steel; other forms of racing use 4140 steel. Of course, other high strength, lightweight materials are also used. As the chassis experiences vibration during the race, residual stresses in the welds and the tubing can start to relieve. This causes the chassis to change shape during the race, affecting the handling of the vehicle and therefore its speed.