The Petrol Engine / A Text-book dealing with the Principles of Design and Construction, with a Special Chapter on the Two-stroke Engine

CONTENTS

LIST OF ILLUSTRATIONS

PREFACE

This book deals with principles. There are many books which give a descriptive account of existing types of engines, but my object in writing this volume has been to assist the reader to obtain thoroughly sound notions of the principles of design and construction which underlie all current practice. If a man understands, for example, the construction of the elements of a carburettor and how they ought to perform their several functions, he should have no difficulty in understanding any special type of carburettor placed upon the market. In dealing with the subject of ignition I have purposely avoided any detailed explanation of the manner in which the spark discharge is produced, because I felt that it introduces new ideas and probably causes the reader to lose sight of the fact that the magneto is only, after all, an accessory, although of course a most important one. I hope that the accounts of my experiments with the two-stroke will be of some service to inventors and others; the many extraordinary breakdowns, defects and adventures encountered during this period of my career have not been inserted because they would undoubtedly cause the reader to forget, for the time being, his fundamental principles.

My colleague, Mr. Oliver Mitchell, who lectures at the Polytechnic on “Motor Car Management and Inspection,” has read through the proofs for me and very kindly suggested several small additions to the text, which I have incorporated; he also suggested the insertion of the valve-setting diagram in the Appendix. My thanks are due to Mr. Mitchell for his services and also to my wife for her assistance in the preparation of the Index.

FRANCIS JOHN KEAN.

The Polytechnic School of Engineering,
Regent Street, London, W.
July, 1915.

THE PETROL ENGINE

CHAPTER I
GENERAL PRINCIPLES

The Meaning of Suction.—Imagine an iron cylinder A (Fig. 1) held down on a rigid base C and fitted with a gas-tight piston B. If we pull the piston down sharply to the position shown in Fig. 2 we will realize that there is apparently some force inside the cylinder which is trying to suck the piston up again. The fact that the piston is being withdrawn and no more air or gas admitted above it to fill up the volume it has displaced on its descent causes a partial vacuum in the cylinder. Now if by means of a tap or valve of some kind we could put the cylinder in communication with the atmosphere, air would rush in and fill up the cylinder until the pressure of the gases in it became equal to atmospheric pressure, when no more air could enter, because there would be no excess of pressure to force it in. In technical language we would say, “the piston has sucked in a charge of air” through the tap or valve.

The Meaning of Compression.—Close the tap or valve and push the piston up again sharply to its original position of Fig. 1. You will now encounter considerable resistance and experience a force pushing down against you because you are reducing the volume of the gas and thereby increasing its pressure; that is to say, you are compressing the gas, because you are now making an amount of gas that recently occupied the whole cylinder fit itself into the small space between the top of the cylinder and the crown of the piston. In technical language you would say, “the piston has now compressed the charge” of gas within the cylinder.

The Meaning, of a Stroke.—In an engine such as is shown diagrammatically in Figs. 3 and 4, when the piston P moves from its topmost position in the cylinder down to its very lowest position we say it has completed a downstroke, and when it moves upwards from its lowest to its highest position we say the piston has completed an upstroke. The length of the piston’s stroke is equal to twice the length of the crank radius R, and is measured by observing the distance moved by the piston in travelling from its highest position in the cylinder to its lowest or vice versa. The space existing above the piston between it and the cylinder head when the piston has reached its highest position in the cylinder is called the clearance space. It is also referred to as the combustion chamber, or chamber in which the petrol gas is exploded. When the piston is either at the top or bottom of its stroke the crank radius R and connecting rod T are in one and the same straight line; under these conditions we say the crank is on its inner or outer dead-centre.

The Otto Cycle.—Most petrol engines operate on what is known as the “Otto” cycle, in which the cycle of events is completed once in every four strokes (or two revolutions) made by the engine. The “Otto” cycle is therefore usually referred to as the four-stroke cycle. In the accompanying diagrams (Figs. 3, 4, 5, and 6) we show in diagrammatic form the interior of a petrol engine cylinder fitted with mushroom type valves.

(1) On the first downstroke made by the piston a suction effect or partial vacuum is produced in the cylinder; the air and petrol vapour in the induction pipe being at atmospheric pressure, which is in excess of that now existing in the cylinder, flow into the cylinder as soon as the inlet valve I is opened by the engine mechanism. At the end of this, the suction stroke, the inlet valve closes and traps the charge of explosive mixture in the engine cylinder. This is shown in Fig. 3.

(2) On the first upstroke made by the piston the charge of explosive mixture is compressed ready for firing. Both valves are shut. This is shown in Fig. 4.

(3) On the second downstroke made by the piston the sparking plug S passes a spark which explodes the charge at the very commencement of the downward movement of the piston. The force of the explosion drives the piston downwards, doing useful work. Both valves are shut. This is the power stroke, and sufficient power must be developed on this stroke not only to do the work required from the engine but also to tide it over the other three idle strokes. On this stroke the piston drives the crank by means of the connecting rod, but on the other three strokes of the cycle the crank has to drive the piston by means of the power or energy stored in the engine flywheel on the power stroke. Towards the end of the power stroke (or explosion stroke) the engine mechanism opens the exhaust valve E and allows part of the burnt gases to escape to the silencer along the exhaust pipe. This is shown in Fig. 5.

(4) On the second upstroke of the cycle the piston pushes the remaining burnt gases out of the cylinder through the exhaust valve. When the piston reaches the top of its stroke the exhaust valve closes. This is shown in Fig. 6. The cycle of operations then begins again, giving one power stroke and three idle strokes each time as already described.

CHAPTER II
DESCRIPTION OF A TYPICAL PETROL ENGINE

For the purpose of explaining the cycle of operations we have considered only a diagrammatic sketch of an imaginary motor-car engine, but in Fig. 7 we illustrate an up-to-date motor-car engine. In the first place we note the position and arrangement of the four water-cooled cylinders, A₁, A₂, A₃, A₄, containing their pistons and mushroom type valves. These are conveniently placed in a vertical position and mounted on top of the crankchamber C, to the bottom of which is attached the oil-base B. At the front of the engine are shown the timing wheels in their casing E, and at the rear end the flywheel F. The starting-handle connexion is at S, the fan pulley being shown at M. The high tension magneto which supplies the current to the sparking plugs is shown at H, and I is the induction pipe connected to the carburettor K. The water circulating pump is on the off side of the engine and does not appear in the illustration, but L₁ is the inlet water pipe leading from the radiator (not shown) to the water pump, and L₂ is the delivery pipe from the pump to the respective cylinder jackets, L₃ being the outlet water pipe. The exhaust pipe is shown at D, and the oil pump at P. The valve springs, valve tappets and guides can also be clearly seen. In examining the several parts of the engine in detail we must not lose sight of their respective positions in the general arrangement view of Fig. 7.

It is a good plan to extend this hollow chamber, containing the water in circulation, at least round the whole of the combustion chamber and all round the inlet and exhaust valve passages and down the barrel of the cylinder as far as the walls are likely to come into contact with the hot gases from the explosions. We refer to this hollow chamber, with its circulating water, as the water-jacket of the cylinder. It is not absolutely essential to have our cylinder water-jacketed, especially with small engines for motor-cycles and engines for aeroplanes which have revolving cylinders, but it is practically essential in nearly all other cases. Even in the special cases mentioned it is found necessary to form special heat radiating fins on the outside of the heated walls to assist in dissipating or getting rid of the surplus heat and preventing seizure of the piston within the cylinder. These fins are clearly seen on the cylinder of the motor-cycle engine shown in Fig. 13.

Thus we may say that motor-car engine cylinders are bound to be water-jacketed, i.e., to have a hollow space round them containing water in circulation. The cylinders themselves are nearly always made in the form of iron castings and the jacket spaces form part of the cylinder casting as a general rule, but occasionally the water-jacket space is formed by attaching plates or tubes to the cylinder casting by means of bolts or screws—not an easy thing to arrange successfully, as it requires water-tight joints.

The procedure for manufacturing a motor-car cylinder is first of all to design and calculate the proportions of the various parts and get out a set of working drawings. From these drawings we get patterns and core-boxes made in wood. The patterns are the exact shape of the finished cylinder on the outside, and the core-boxes are the exact shape of the inside of the finished cylinder (except in so far as allowance has to be made for parts which must afterwards be machined).

The patterns are pushed down into the moulding sand in the foundry, and when withdrawn leave their impression, thus forming moulds. The core-boxes are filled with sand, which when withdrawn furnishes us with masses of sand that are the counterpart of the interior of the cylinder in shape. These cores are supported centrally in the mould (which is usually in halves, or more than two parts), while the molten iron is poured into the intervening space to form the iron casting. When the casting has cooled down the sand can be cleaned off quite easily. One set of patterns and core-boxes will thus produce quite a number of cylinder castings, each being similar in every respect to the other, the process being a quick and fairly cheap method of reproduction. Later on the cylinder barrel has to be machined and bored out true to very fine limits by the use of boring tools and some kind of boring machine or lathe. The flanges or flat faces have to be planed true in a planing machine and the valve stem guides and valve seatings must be carefully and truly machined to correct size and shape.