INTERNAL-COMBUSTION ENGINE

INTRODUCTION

Internal Combustion Engine, a heat engine in which the fuel is burned ( that is, united with oxygen ) within the confining space of the engine itself. This burning process releases large amounts of energy, which are transformed into work through the mechanism of the engine. This type of engine different from the steam engine, which process with an external combustion engine that fuel burned apart from the engine. The principal types of internal combustion engine are : reciprocating engine such as Otto-engine, and Diesel engines ; and rotary engines, such as the Wankel engine and the Gas-turbine engine.

In general, the internal combustion engine has become the means of propulsion in the transportation field, with the exception of large ships requiring over 4,000 shaft horsepower ( hp).

In stationary applications, size of unit and local factor often determine the choice between the use of steam and diesel engine. Diesel power plants have a distinct economic advantage over steam engine when size of the plant is under about 1,000 hp. However there are many diesel engine plants much large than this. Internal combustion engines are particularly appropriate for seasonal industries, because of the small standby losses with these engines during the shutdown period.

History

The first experimental internal combustion engine was made by a Dutch astronomer, Christian Huygens, who, in 1680, applied a principle advanced by Jean de Hautefeuille in 1678 for drawing water. This principle was based on the fact that the explosion of a small amount of gunpowder in a closed chamber provided with escape valves would create a vacuum when the gases of combustion cooled. Huygens, using a cylinder containing a piston, was able to move it in this manner by the external atmospheric pressure.

The first commercially practical internal combustion engine was built by a French engineer, ( Jean Joseph ) Etienne Lenoir, about 1859-1860. It used illuminating gas as fuel. Two years later, Alphonse Beau de Rochas enunciated the principles of the four-stroke cycle, but Nickolaus August Otto built the first successful engine ( 1876 ) operating on this principle.

Reciprocating Engine

Components of Engines

The essential parts of Otto-cycle and diesel engines are the same. The combustion chamber consists of a cylinder, usually fixed, which is closed at one end and in which a close-fitting piston slides. The in-and-out motion of the piston varies the volume of the chamber between the inner face of the piston and the closed end of the cylinder. The outer face of the piston is attached to a crankshaft by a connecting rod. The crankshaft transforms the reciprocating motion of the piston into rotary motion. In multi-cylindered engines the crankshaft has one offset portion, called a crankpin, for each connecting rod, so that the power from each cylinder is applied to the crankshaft at the appropriate point in its rotation. Crankshafts have heavy flywheels and counterweights, which by their inertia minimize irregularity in the motion of the shaft. An engine may have from 1 to as many as 28 cylinders.

Fig. 1, Component of Piston Engines.

The fuel supply system of an internal-combustion engine consists of a tank, a fuel pump, and a device for vaporizing or atomizing the liquid fuel. In Otto-cycle engines this device is a carburetor. The vaporized fuel in most multi-cylindered engines is conveyed to the cylinders through a branched pipe called the intake manifold and, in many engines, a similar exhaust manifold is provided to carry off the gases produced by combustion. The fuel is admitted to each cylinder and the waste gases exhausted through mechanically operated poppet valves or sleeve valves. The valves are normally held closed by the pressure of springs and are opened at the proper time during the operating cycle by cams on a rotating camshaft that is geared to the crankshaft . By the 1980s more sophisticated fuel-injection systems, also used in diesel engines, had largely replaced this traditional method of supplying the proper mix of air and fuel; computer-controlled monitoring systems improved fuel economy and reduced pollution.

Ignition

In all engines some means of igniting the fuel in the cylinder must be provided. For example, the ignition system of Otto-cycle engines , the mixture of air and gasoline vapor delivered to the cylinder from the carburetor and next operation is that of igniting the charge by causing a spark to jump the gap between the electrodes of a spark plug, which projects through the walls of the cylinder. One electrode is insulated by porcelain or mica; the other is grounded through the metal of the plug, and both form the part of the secondary circuit of an induction system.

The principal type of high-tension ignition now commonly used is the battery-and-coil system. The current from the battery flows through the low-tension coil and magnetizes the iron core. When this circuit is opened at the distributor points by the interrupter cam, a transient high-frequency current is produced in the primary coil with the assistance of the condenser. This induces a transient, high-frequency, high-voltage current in the secondary winding. This secondary high voltage is needed to cause the spark to jump the gap in the spark plug. The spark is directed to the proper cylinder to be fired by the distributor, which connects the secondary coil to the spark plugs in the several cylinders in their proper firing sequence. The interrupter cam and distributor are driven from the same shaft, the number of breaking points on the interrupter cam being the same as the number of cylinders.

Cooling System

Because of the heat of combustion, all engines must be equipped with some type of cooling system. Some aircraft and automobile engines, small stationary engines, and outboard motors for boats are cooled by air. In this system the outside surfaces of the cylinder are shaped in a series of radiating fins with a large area of metal to radiate heat from the cylinder. Other engines are water-cooled and have their cylinders enclosed in an external water jacket. In automobiles, water is circulated through the jacket by means of a water pump and cooled by passing through the finned coils of a radiator. Some automobile engines are also air-cooled, and in marine engines sea water is used for cooling.

Starter

Unlike steam engines and turbines, internal-combustion engines develop no torque when starting, and therefore provision must be made for turning the crankshaft so that the cycle of operation can begin. Automobile engines are normally started by means of an electric motor or starter that is geared to the crankshaft with a clutch that automatically disengages the motor after the engine has started. Small engines are sometimes started manually by turning the crankshaft with a crank or by pulling a rope wound several times around the flywheel. Methods of starting large engines include the inertia starter, which consists of a flywheel that is rotated by hand or by means of an electric motor until its kinetic energy is sufficient to turn the crankshaft, and the explosive starter, which employs the explosion of a blank cartridge to drive a

turbine wheel that is coupled to the engine. The inertia and explosive starters are chiefly used to start airplane engines.

Otto-Cycle Engines

 The ordinary Otto-cycle engine is a four-stroke engine; that is, its pistons make four strokes, two toward the head (closed head) of the cylinder and two away from the head, in a complete power cycle. During the first stroke of the cycle, the piston moves away from the cylinder head while simultaneously the intake valve is opened. The motion of the piston during this stroke sucks a quantity of a fuel and air mixture into the combustion chamber. During the next stroke the piston moves toward the cylinder head and compresses the fuel mixture in the combustion chamber. At the moment when the piston reaches the end of this stroke and the volume of the combustion chamber is at a minimum, the fuel mixture is ignited by the spark plug and burns, expanding and exerting a

pressure on the piston, which is then driven away from the cylinder head in the third stroke. At the end of the power stroke the pressure of the burned gases in the cylinder is 2.8 to 3.5 kg/sq. cm (40 to 50 lb./sq. in). During the final stroke, the exhaust valve is opened and the piston moves toward the cylinder head, driving the exhaust gases out of the combustion chamber and leaving the cylinder ready to repeat the cycle.

Fig. 2, Otto-Cycle Engines.                                                             

The efficiency of a modern Otto-cycle engine is limited by a number of factors, including losses by cooling and by friction. In general the efficiency of such engines is determined by the compression ratio of the engine. The compression ratio (the ratio between the maximum and minimum volumes of the combustion chamber) is usually about 8 to 1 or 10 to 1 in most modern Otto-cycle engines. Higher compression ratios, up to about 12 to 1, with a resulting increase of efficiency, are possible with the use of high-octane antiknock fuels. The efficiencies of good modern Otto-cycle engines range between 20 and 25 percent (in other words, only this percentage of the heat energy of the fuel is transformed into mechanical energy).

Diesel Engines

Theoretically the diesel cycle differs from the Otto cycle in that combustion takes place at constant volume rather than at constant pressure. Most diesels are also four-stroke engines, but operate differently than the four-stroke Otto-cycle engines. The first or suction stroke draws air, but no fuel, into the combustion chamber through an intake valve. On the second or compression stroke the air is compressed to a small fraction of its former volume and is heated to approximately 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 costs. 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.

Fig. 6,  Texaco Combustion process (TCP).

Fig. 7,  Honda CVCC combustion process.