A Textbook of Internal Combustion Engines

Thermodynamics is an axiomatic science which deals with the relations among heat, work and properties of systems which are in equilibrium. It basically entails four laws or axioms known as Zeroth, First, Second and Third law of thermodynamics.

Any type of engine or machine which derives heat energy from the combustion of fuel or any other source and converts this energy into mechanical work is termed as a heat engine. The function of a carburettor is to atomise and meter the liquid fuel and mix it with air as it enters the injection system of the engine maintaining under all conditions of operation fuel air proportion approximate to those conditions.

A cycle is defined as a repeated series of operations occurring in a certain order. It may be repeated by repeating the processes in the same order. The cycle may be of imaginary perfect engine or actual engine. The former is called ideal cycle and the latter actual cycle. In ideal cycle all accidental heat losses are prevented and the working substance is assumed to behave like a perfect working substance.

In air standard cycles analysis highly simplified approximations are made. The air standard theory gives an estimate of engine performance which is much greater than the actual performance. This large variation is partly due to the non-instantaneous burning and valve operation incomplete combustion etc. ; the major reason being oversimplication in using the values of the properties of the working fluid for cycle analysis.

Combustion may be defined as a relatively rapid chemical combination of hydrogen and carbon in the fuel with the oxygen in the air, resulting in liberation of energy in the form of heat. Following conditions are necessary for combustion to take place : 1. A combustible mixture. 2. Some means to initiate combustion. 3. Stabilization and propagation of flame in the combustion chamber. In spark ignition (S.I.) engines, a carburettor generally supplies a combustible mixture and the electric spark from a spark plug initiates the combustion.

C.I. engine fuels (diesel oils) are less expensive than S.I. engine fuels (petrol or gasoline). Furthermore, since C.I. engines fuels have a higher specific gravity than petrol, and since fuel is sold on the volume basis (litres) and not on mass basis (kg), more kg of fuel per litre are obtained in purchasing C.I. engine fuel.

Air capacity (actual) is defined as the mass flow of fresh air through the engine per unit time. The engine output depends on this parameter.

In 1878, Dugald-clerk, a British engineer introduced a cycle which could be completed in two strokes of piston rather than four strokes as is the case with the four stroke cycle engines. The engines using this cycle were called two stroke cycle engines. In this engine suction and exhaust strokes are eliminated. Here instead of valves, ports are used. The exhaust gases are driven out from engine cylinder by the fresh charge of fuel entering the cylinder nearly at the end of the working stroke.

In chemical thermodynamics the study of systems involving chemical reactions is an important topic. A chemical reaction may be defined as the rearrangement of atoms due to redistribution of electrons. In a chemical reaction the terms, reactants and the products are frequently used. ‘Reactants’ comprise of initial constituents which start the reaction while ‘products’ comprise of final constituents which are formed by the chemical reaction. Although the basic principles which will be discussed in this chapter apply to any chemical reaction, here main attention will be focused on an important type of chemical reaction—“combustion”.

Steady running is defined as mean continuous operation at a required speed and power output with normal temperatures. N Transient operation includes starting, warming up, and changing from one speed or load to another, specially for automotive vehicle engines during acceleration and decelerations, and also idling.

The process of preparing in the S.I. engine, a combustible fuel air mixture outside the engine cylinder is called ‘‘carburetion’’. This complicated process is achieved in the induction system. A carburettor is a device which atomises the fuel and mixes it with air. It is the most important part of the induction system.

Fuel is injected into the cylinder at the end of the compression stroke ; the pressure of fuel injected lies between 100 to 200 bar. During the process of injection the fuel is broken into very fine droplets. The droplets vaporise taking the heat from the hot air and form a combustible mixture and start burning. As the burning starts, the vaporisation of fuel is accelerated as more heat is available. As the combustion progresses, the amount of oxygen available for burning reduces and therefore heat release is reduced.

Ignition is only a pre-requisite of combustion. It does not influence the gross combustion process. It is only a small scale phenomenon taking place within a specified small zone in the combustion chamber. N Ignition only ensures initiation of combustion process and has no degree intensively or extensively.

In an I.C. engine almost all machine parts have relative motion and rub against each other. To reduce this rubbing action lubrication is required, which increases the life of engine. The purpose of lubrication in I.C. engine is generally two-fold : (i) To reduce the rubbing action between different machine parts having relative motion with each other; (ii) To remove the heat generated inside the cylinder.

In an I.C. engine, the temperature of the gases inside the engine cylinder may vary from 35°C or less to as high as 2750°C during the cycle. If an engine is allowed to run without external cooling, the cylinder walls, cylinder and pistons will tend to assume the average temperature of the gases to which they are exposed, which may be of the order of 1000 to 1500°C. Obviously at such high temperature ; the metals will loose their characteristics and piston will expand considerably and sieze the liner. Of course theoretically thermal efficiency of the engine will improve without cooling but actually the engine will sieze to run. If the cylinder wall temperature is allowed to rise above a certain limit, about 65°C, the lubricating oil will begin to evaporate rapidly and both cylinder and piston may be damaged.

The purpose of supercharging is to raise the volumetric efficiency above that value which can be obtained by normal aspiration. The engine is an air pump. Increasing the air consumption permits greater quantities of fuel to be added, and results in a greater potential output. The indicated power produced is almost directly proportional to the engine air consumption. While brake power is not so closely related to air consumption, it is nevertheless, dependent upon the mass of air consumed. It is desirable, then, that the engine takes in greatest possible mass of air.

The primary task of the development engineer is to reduce the capital and running cost of the engine. This involves trial of various design concepts. The parameters are so enormous and different in nature that it is almost physically impossible to take care of all of them during the design of the engine. Therefore, it is necessary to conduct the test on the engine and determine the measures which should be taken to improve the engine performance. The nature and the type of the test to be conducted will depend upon a great number of factors such as, the degree of development of the particular design, the accuracy required, the funds available, the nature of the manufacturing company etc.

Air pollution can be defined as an addition to our atmosphere of any material which will have a deleterious effect on life upon our planet. Besides I.C. engines other sources such as electric power stations, industrial and domestic fuel consumers also add pollution. There has been a great concern, in recent years, that the internal combustion engines is responsible for too much atmospheric pollution, which is detrimental to human health and the environment. Thus concerted efforts are being made to reduce the responsible pollutants emitted from the exhaust system without sacrificing power and fuel consumption.

Owing to various technical and financial reasons, some engines are designed to operate, using a combination of two fuels. For instance, in some third-world countries dual-fuel engines are used because of high cost of diesel fuel. Large C.I. engines are run on a combination of methane and diesel oil. Methane is the main fuel because it is more cheaply available. However, methane is not a good C.I. fuel by itself because it does not readily self-ignite (due to its high octane number). A small amount of diesel oil is injected at the proper cycle time. This ignites in a normal manner and initiates combustion in the methane-air mixture filling the cylinder.

The compressed air finds application in the following fields : 1. It is widely employed for powering small engines, generally those of portable nature. Compressed air is used in such diversified fields as : (i) operating tools in factories ; (ii) operating drills and hammers in road building ; (iii) excavating ; (iv) tunneling and mining ; (v) starting diesel engines ; and (vi) operating brakes on buses, trucks and trains. 2. A large quantity of air at moderate pressure is used in smelting of various metals such as melting iron, in blowing converters, and cupola work.

Probably a wind-mill was the first turbine to produce useful work, wherein the there is no pre-compression and no combustion. The characteristic features of a gas turbine as we think of the name today include a compression process and a heat addition (or combustion) process. The gas turbine represents perhaps the most satisfactory way of producing very large quantities of power in a self-contained and compact unit. The gas turbine may have a future use in conjunction with the oil engine.

This chapter contains questions for university examinations along with the solutions for the same.

This chapter contains questions for "GATE" and "UPSC" examinations with solutions for the same.