Tips and tricks
Diesel engine design
DIESEL ENGINE DESIGN
BY H. F. P. PURDAY
NEW YORK, D. VAN NOSTRAND COMPANY, 1919
DOWNLOAD FREE AUTOMOTIVE BOOK:
Diesel engine design
PREFACE
This book is based, on about twelve years' experience of Diesel Engines, mainly from the drawing-office point of view, and is intended to present an account of the main considerations which control the design of these engines.
The author ventures to hope that, in addition to designers and draughtsmen, to whom such a book as this is most naturally addressed, there may be other classes of readers - for example, Diesel Engine users and technical students -to whom the following pages may be of interest.
The text deals mainly with general principles as exemplified by examples of good modern practice, and it has not been possible to notice every constructional novelty. Apology is perhaps called for on account of the omission of any special treatment of the stepped piston and the opposed piston types of engine. These, however, are the specialties of a comparatively limited number of manufacturers, and have been very fully described and illustrated in the technical press.
The existence in its fourth edition of Chalkley's well-known book on The Diesel Engine for Land and Marine Purposes has enabled the present writer to proceed to details with a minimum of preliminary discussion. A number of references to other books and papers have been inserted in order to avoid, so far as possible, overlapping with other sources of information.
The author ventures to hope that, in addition to designers and draughtsmen, to whom such a book as this is most naturally addressed, there may be other classes of readers - for example, Diesel Engine users and technical students -to whom the following pages may be of interest.
The text deals mainly with general principles as exemplified by examples of good modern practice, and it has not been possible to notice every constructional novelty. Apology is perhaps called for on account of the omission of any special treatment of the stepped piston and the opposed piston types of engine. These, however, are the specialties of a comparatively limited number of manufacturers, and have been very fully described and illustrated in the technical press.
The existence in its fourth edition of Chalkley's well-known book on The Diesel Engine for Land and Marine Purposes has enabled the present writer to proceed to details with a minimum of preliminary discussion. A number of references to other books and papers have been inserted in order to avoid, so far as possible, overlapping with other sources of information.
CONTENTS
- FIRST PRINCIPLES
- THERMAL EFPICIENCY
- EXHAUST, SUCTION AND SCAVENGE
- THE PRINCIPLE OF SIMILITUDE
- CRANK-SHAFTS
- FLY WHEELS
- FRAMEWORK
- CYLINDERS AND COVERS
- RUNNING GEAR
- FUEL OIL SYSTEM
- AIR AND EXHAUST SYSTEM
- COMPRESSED AIR SYSTEM
- VALVE GEAR
FIRST PRINCIPLES
The Diesel Principle - The characteristic feature of the Diesel Engine is the injection of oil fuel into air which has been previously compressed by the rising of a piston to a pressure corresponding to a temperature sufficiently high to ensure immediate ignition of the fuel.
In the course of the pioneer experiments by which the commercial practicability of this engine was demonstrated, it was found advantageous to effect the injection of the fuel by a blast of air, and this feature was retained in all Diesel Engines until the lapse of the original patents.
At the present date there exists a class of high-compression oil engines operating on the Diesel principle, in which the injection of oil is effected by mechanical means without the assistance of an air blast. The design of these engines presents a variety of problems differing materially from those which arise in the design of Diesel Engines as defined below. Furthermore, the use for war purposes of one of the most conspicuous members of this class of engine prohibits anything like a satisfactory discussion of these so-called "solid injection" engines. The well-known "surface ignition," "hot bulb," or "hot plate" engines form a very numerous class by themselves and have in the past been misnamed "semi-Diesel" engines. The cycles on which they operate and the principles underlying their design differ so widely from those relating to Diesel Engines proper that they also fall outside the scope of this work.
The features which characterize the true Diesel Engine, in the correct use of the term, are now understood to be the following:
(1) Compression sufficient to produce the temperature requisite for spontaneous combustion of the fuel.
(2) Injection of fuel by a blast of compressed air.
(3) A maximum cycle pressure (attained during combustion) not greatly exceeding the compression pressure, i.e. absence of pronounced explosive effect.
Item 3 is deliberately worded somewhat broadly as the shape of a Diesel indicator card is subject to considerable variation under different conditions of load, blast air pressure, fuel valve adjustment, etc.
In the earlier days of Diesel Engine construction the square top indicator card, showing a period of combustion at constant pressure, was considered the ideal to aim at. It has since been found that a card having a more peaked top is usually associated with better fuel consumptions. When tar oil is used as fuel the square top card appears to be almost out of the question.
It should further be remembered that the existence of a period of combustion at constant pressure is no guarantee that all the combustion takes place at that pressure. This ideal is never realized. Combustion probably proceeds slowly well after half stroke, even under the most favorable conditions.
Compression Pressure. - The height to which compression is carried is governed by the following considerations:
(1) The attainment of the requisite temperature.
(2) The attainment of a desirable degree of efficiency.
(3) Mechanical considerations.
Considerations of temperature for ignition fix the lower limit of compression at somewhere in the neighborhood of 400 lb. per sq. in. The temperature actually attained depends on the initial temperature of the intaken air and the heat lost to the jacket during compression, so it is clear that the temperature attained on the first few strokes of the engine will be considerably lower than the value it assumes after the .engine has been firing consecutively for some time.
As regards efficiency, it is well known that increasing the degree of compression beyond certain limits does not very materially increase even the theoretical efficiency.
In practice the compression most usually adopted is about 500-550 lb. per sq. in. for four stroke engines. For two stroke engines the compression is frequently in the neighborhood of 600 lb. per sq. in. or over, owing to the fact that the charge of air delivered by the scavenge pump may itself be at a pressure slightly above atmospheric. At first sight it might be thought that this initial compression might be considered as a first stage in the temperature rise of the charge of air; but apparently compression in the scavenge pump is not so effective in raising the temperature as compression in the main cylinder. The mechanical considerations which limit the compression are numerous, and some are mentioned below. Higher compression involves:
(1) Heavier load per sq. in. of the piston and necessitates massive construction of all the main parts.
(2) More highly compressed air for injection and consequently increased trouble with the air compressor, and its valves particularly.
(3) Increased wear of cylinder liners due to increased pressure behind the piston rings.
Compression Temperature - With a compression of 500 lb. per sq. in. in a fair-sized four cycle cylinder working under full load conditions the compression temperature is about 1200° F. On starting the engine from a cold state the compression pressure and temperature are considerably lower owing to the cold state of the cylinder walls and the piston crown.
In addition to this the injection of cold blast air with the fuel in the proportion of about 1 lb. of blast air to 12 lb. of suction air still further reduces the temperature apart from the probability that the blast air has momentarily a local cooling effect in the zone of combustion.
DOWNLOAD FREE AUTOMOTIVE BOOK:
Diesel engine design
In the course of the pioneer experiments by which the commercial practicability of this engine was demonstrated, it was found advantageous to effect the injection of the fuel by a blast of air, and this feature was retained in all Diesel Engines until the lapse of the original patents.
At the present date there exists a class of high-compression oil engines operating on the Diesel principle, in which the injection of oil is effected by mechanical means without the assistance of an air blast. The design of these engines presents a variety of problems differing materially from those which arise in the design of Diesel Engines as defined below. Furthermore, the use for war purposes of one of the most conspicuous members of this class of engine prohibits anything like a satisfactory discussion of these so-called "solid injection" engines. The well-known "surface ignition," "hot bulb," or "hot plate" engines form a very numerous class by themselves and have in the past been misnamed "semi-Diesel" engines. The cycles on which they operate and the principles underlying their design differ so widely from those relating to Diesel Engines proper that they also fall outside the scope of this work.
The features which characterize the true Diesel Engine, in the correct use of the term, are now understood to be the following:
(1) Compression sufficient to produce the temperature requisite for spontaneous combustion of the fuel.
(2) Injection of fuel by a blast of compressed air.
(3) A maximum cycle pressure (attained during combustion) not greatly exceeding the compression pressure, i.e. absence of pronounced explosive effect.
Item 3 is deliberately worded somewhat broadly as the shape of a Diesel indicator card is subject to considerable variation under different conditions of load, blast air pressure, fuel valve adjustment, etc.
In the earlier days of Diesel Engine construction the square top indicator card, showing a period of combustion at constant pressure, was considered the ideal to aim at. It has since been found that a card having a more peaked top is usually associated with better fuel consumptions. When tar oil is used as fuel the square top card appears to be almost out of the question.
It should further be remembered that the existence of a period of combustion at constant pressure is no guarantee that all the combustion takes place at that pressure. This ideal is never realized. Combustion probably proceeds slowly well after half stroke, even under the most favorable conditions.
Compression Pressure. - The height to which compression is carried is governed by the following considerations:
(1) The attainment of the requisite temperature.
(2) The attainment of a desirable degree of efficiency.
(3) Mechanical considerations.
Considerations of temperature for ignition fix the lower limit of compression at somewhere in the neighborhood of 400 lb. per sq. in. The temperature actually attained depends on the initial temperature of the intaken air and the heat lost to the jacket during compression, so it is clear that the temperature attained on the first few strokes of the engine will be considerably lower than the value it assumes after the .engine has been firing consecutively for some time.
As regards efficiency, it is well known that increasing the degree of compression beyond certain limits does not very materially increase even the theoretical efficiency.
In practice the compression most usually adopted is about 500-550 lb. per sq. in. for four stroke engines. For two stroke engines the compression is frequently in the neighborhood of 600 lb. per sq. in. or over, owing to the fact that the charge of air delivered by the scavenge pump may itself be at a pressure slightly above atmospheric. At first sight it might be thought that this initial compression might be considered as a first stage in the temperature rise of the charge of air; but apparently compression in the scavenge pump is not so effective in raising the temperature as compression in the main cylinder. The mechanical considerations which limit the compression are numerous, and some are mentioned below. Higher compression involves:
(1) Heavier load per sq. in. of the piston and necessitates massive construction of all the main parts.
(2) More highly compressed air for injection and consequently increased trouble with the air compressor, and its valves particularly.
(3) Increased wear of cylinder liners due to increased pressure behind the piston rings.
Compression Temperature - With a compression of 500 lb. per sq. in. in a fair-sized four cycle cylinder working under full load conditions the compression temperature is about 1200° F. On starting the engine from a cold state the compression pressure and temperature are considerably lower owing to the cold state of the cylinder walls and the piston crown.
In addition to this the injection of cold blast air with the fuel in the proportion of about 1 lb. of blast air to 12 lb. of suction air still further reduces the temperature apart from the probability that the blast air has momentarily a local cooling effect in the zone of combustion.
DOWNLOAD FREE AUTOMOTIVE BOOK:
Diesel engine design
Free books category: