.. ely 440 C (approximately 820 F) by this compression. At the end of the compression stroke vaporized fuel is injected into the combustion chamber Fig. 3, Four-Stroke Diesel Engines.
and burns instantly because of the high temperature of the air in the chamber.Some diesels have auxiliary electrical ignition systems to ignite the fuel when the engine starts, and until it warms up. This combustion drives the piston back on the third or power stroke of the cycle. The fourth stroke, as in the Otto-cycle engine, is an exhaust stroke. The efficiency of the diesel engine, which is in general governed by the same factors that control the efficiency of Otto-cycle engines, is inherently greater than that of any Otto-cycle engine and in actual engines today is slightly over 40 percent. Diesels are in general slow-speed engines with crankshaft speeds of 100 to 750 revolutions per minute (rpm) as compared to 2500 to 5000 rpm for typical Otto-cycle engines. Some types of diesel, however, have speeds up to 2000 rpm.Because diesels use compression ratios of 14 or more to 1, they are generally more heavily built than Otto-cycle engines, but this disadvantage is counterbalanced by their greater efficiency and the fact that they can be operated on less expensive fuel oils.
Two-Stroke Engines By suitable design it is possible to operate an Otto-cycle or diesel as a two-stroke or two-cycle engine with a power stroke every other stroke of the piston instead of once every four strokes. The efficiency of such engines is less than that of four-stroke engines, and therefore the power of a two-stroke engine is always less then half that of a four-stroke engine of comparable size. The general principle of the two-stroke engine is to shorten the periods in which fuel is introduced to the combustion chamber and in which the spent gases are exhausted to a small fraction of the duration of a stroke instead of allowing each of these operations to occupy a full stroke. In the simplest type of two-stroke engine, the poppet valves are replaced by sleeve valves or ports (openings in the cylinder wall that are uncovered by the piston at the end of its outward travel). In the two-stroke cycle the fuel mixture or air is introduced through the intake port when the piston is fully withdrawn from the cylinder.The compression stroke follows and the charge is ignited when the piston reaches the end of this stroke. The piston then moves outward on the power stroke, uncovering the exhaust port and permitting the gases to escape from the combustion chamber. Fig.
4, Two-Stroke Engines. Rotary Engine Wankel Engines Fig. 5 The Wankel Engine In the 1950s the German engineer Felix Wankel developed his concept of an internal-combustion engine of a radically new design, in which the piston and cylinder were replaced by a three-cornered rotor turning in a roughly oval chamber.The fuel-air mixture is drawn in through an intake port and trapped between one face of the turning rotor and the wall of the oval chamber. The turning of the rotor compresses the mixture, which is ignited by a spark plug. The exhaust gases are then expelled through an exhaust port through the action of the turning rotor. The cycle takes place alternately at each face of the rotor, giving three power strokes for each turn of the rotor.
The Wankel engine’s compact size and consequent lesser weight as compared with the piston engine gave it increasing value and importance with the rise in gasoline prices of the 1970s and ’80s.In addition, it offers practically vibration-free operation, and its mechanical simplicity provides low manufacturing cost! s. Cooling requirements are low, and its low center of gravity contributes to driving safety. Gas Turbine Also called as combustion turbine, engine that employs gas flow as the working medium by which heat energy is transformed into mechanical energy.
Gas is produced in the engine by the combustion of certain fuels. Stationary nozzles discharge jets of this gas against the blades of a turbine wheel. The impulse force of the jets causes the shaft to turn.A simple-cycle gas turbine includes a compressor that pumps compressed air into a combustion chamber. Fuel in gaseous or liquid-spray form is also injected into this chamber, and combustion takes place there. The combustion products pass from the chamber through the nozzles to the turbine wheel.
The spinning wheel drives the compressor and the external load, such as an electrical generator. In a turbine or compressor, a row of fixed blades and a corresponding row of moving blades attached to a rotor is called a stage.Large machines employ multistage axial-flow compressors and turbines. In multi-shaft arrangements, the initial turbine stage (or stages) powers the compressor on one shaft while the later turbine stage (or stages) powers the external load on a separate shaft.
The efficiency of the gas-turbine cycle is limited by the need for continuous operation at high temperatures in the combustion chamber and early turbine stages. A small, simple-cycle gas turbine may have a relatively low thermodynamic efficiency, comparable to a conventional gasoline engine. Advances in heat-resistant materials, protective coatings, and cooling arrangements have made possible large units with simple-cycle efficiencies of 34 percent or higher.The efficiency of gas-turbine cycles can be enhanced by the use of auxiliary equipment such as inter-coolers, regenerators, and reheaters.
These devices are expensive, however, and economic considerations usually preclude their use. In a combined-cycle power plant, the considerable heat remaining in the gas turbine exhaust is directed to a boiler called a heat-recovery steam generator. The heat so recovered is used to raise steam for an associated steam turbine. The combined output is approximately 50 percent greater than that of the gas turbine alone. Combined cycles with thermal efficiency of 52 percent and higher are being put into service.Gas turbines have been applied to the propulsion of ships and railroad locomotives.
A modified form of gas turbine, the turbojet, is used for airplane propulsion. Heavy-duty gas turbines in both simple and combined cycles have become important for large-scale generation of electricity. Unit ratings in excess of 200 megawatts (MW) are available. The combined-cycle output can exceed 300 MW.The usual fuels used in gas turbines are natural gas and liquids such as kerosene and diesel oil. Coal can be used after conversion to gas in a separate gasifier.
Internal-Combustion Engines and Air Pollution Air pollution from automobile engines ( smog ) was first detected about 1942 in Los Angeles, CA. Smog arises from sunlight-induced photochemical reactions between nitrogen dioxide and the several hundred hydrocarbons in the atmosphere. Undesirable products of the reactions include ozone, aldehydes, and peroxyacylnitrates ( PAN ).These are highly oxidizing in nature and cause eye and throat irritation. Visibility-decreasing nitrogen dioxide and aerosols are also formed.
Five categories of air pollutants and percent contribution from all transportation source and the highway vehicle subset are show in Table -1. Virtually all of the transportation CO, about half the hydrocarbons, and about one-third of the nitrogen oxides come from gasoline engines. Diesel engines account for the particulate. Table-1. Estimated Total Annual US Emissions from Artificial Sources (1980) Carbonmonoxide Hydrocarbons Sulfuroxides Nitrogenoxides Particulate Total, teragram/yr.85.
4 21.8 23.7 20.7 7.8 All transportation, % 81 36 3.8 44 18 Highway vehicles, % 72 29 1.7 32 14 SOURCE: EPA Report 450/4-82-001, 1982.
Emissions from internal-combustion engines include those from blowby, evaporation, and exhaust. These can vary considerably in amount and composition depending on engine type, design, and condition, fuel-system type, fuel volatility, and engine operating point. For an automobile without emission control it is estimated that of the hydrocarbon emission, 20 to 25 percent arise from blowby, 60 percent from the exhaust, and the balance from evaporative losses primarily from the fuel tank and to a lesser extent from the carburetor.
All other non-hydrocarbon emissions emanate from the exhaust. At least 200 hydrocarbon (HC) compounds have been identified in exhaust.Some such as the olefin compounds react products. These are termed reactive hydrocarbons.
Others such as the paraffin are virtually unreactive. Special Developments The Stratified-Charge Engine a modification of the conventional spark-ignition piston engine, the stratified charge engine is designed to reduce emissions without the need for an exhaust-gas recirculation system or catalytic converter. Its key feature is a dual combustion chamber for each cylinder, with a prechamber that receives a rich fuel-air mixture while the main chamber is charged with a very lean mixture.The spark ignites the rich mixture that in turn ignites the lean main mixture. The resulting peak temperature is low enough to inhibit the formation of nitrogen oxides, and the mean temperature is sufficiently high to limit emissions of carbon monoxide and hydrocarbon. Two rather distinct means for accomplishing the stratified charge condition are under consideration : 1. A single combustion chamber with a well-controlled rotating air motion. This arrangement is illustrated (Fig.
6) by the Texaco Combustion Process (TCP), patented in 1949.2. A prechamber or two-chamber system. This is illustrated by Fig.7, which shows the general arrangement of the Honda Compound-vortex controlled-combustion (CVCC) system.
For both systems, very careful development has proved to be necessary to obtain complete combustion of the fuel under the wide range of speed and load conditions required of an automotive engine.