What are the usual causes of starting air valve leakage?

Leakage of a starting air valve is usually caused by sluggish valve action preventing fast closure of the valve, or by dirt or foreign particles from the starting air supply lodging on the valve seat and so preventing the valve from closing fully. Sluggish valve action can be caused by dirty pistons or valve spindle guides and the like. In newly overhauled valves sluggish valve action may be caused by parts fitted with inadequate clearances.

What is an internal combustion engine? Name the various types

An internal combustion engine is one in which the fuel is burnt within the engine. It is usually of the reciprocating type. Combustion of the fuel and the conversion of the heat energy from combustion to mechanical energy takes place within the cylinders. Internal combustion engines can also be of the rotary type, such as the gas turbine and the rotary engine developed by Dr Felix Wankel.
Reciprocating internal combustion engines may be of the spark-ignition or compression-ignition type. Spark-ignition engines use gaseous or volatile distillate fuels and work on a modified Otto cycle. They operate on the two- or four-stroke cycle. Compression-ignition engines may also be of either two- or four-stroke cycle type. They use distillate liquid fuels or, where conditions allow, a blend of distillate and residual fuels. This type of engine is usually designed to operate on the dual-combustion cycle or a modification of it. In some cases the cycle is such that the whole of combustion takes place at constant volume.
Some engines are designed for dual-fuel operation and may use either liquid or gaseous fuel. When gaseous fuel is used a small amount of liquid fuel is injected to initiate combustion. _ __
Note Different names are used for compression-ignition engines. Nomen¬clature was discussed by a committee of distinguished engineers in 1922 and is still a matter of discussion and argument today. The name Diesel is in common use and has reached the point where it is often spelt with a lowercase ‘d’. The modern oil engine bears little resemblance to the engine developed by Dr R. Diesel, but more closely resembles the engine developed by H. Akroyd Stuart at Bletchley, near London, in about 1890 – some few years before Dr Diesel took out patents for the engine he developed at Augsburg in Germany. In using the name Diesel we must not forget the work done by Akroyd Stuart.

What are the relative advantages of crosshead and trunk-piston type engines?

Crosshead type engines are able to develop much higher power at lower rotational spieeds than trunk-piston type engines, because the space available for the crosshead bearings is greater than the space within the piston for the gudgeon bearing assembly. Trunk-piston engines have the advantage of requiring less head room than crosshead engines. Their working parts are fewer in number and much less costly to produce because their design lends itself to mass production methods. The gudgeon bearing assembly is not particularly suited for highly rated two-stroke engines unless special arrangements are made for its lubrication. Cheaper quality fuels may be used in crosshead engines as it is possible to isolate the cylinder space from the crankcase, thus preventing acidic residues entering the crankcase. The total cost for lubricants is less with crosshead engines than with trunk-piston engines of equivalent power.

What is an opposed-piston engine and how are the cranks arranged? What advantages and disadvantages do these engines have?

An opposed-piston engine has two pistons working in the same cylinder, which is much longer than normal. The cranks are arranged so that movement of the pistons towards each other takes place at the same time, as does movement away from each other. The opposed-piston engine always works on the two- stroke cycle with the uniflow method of scavenging. The combustion chamber is formed in the space between the heads of the pistons and the small exposed section or belt of the cylinder left between the pistons. The fuel injection valves, air starting valve, cylinder pressure relief valve and pressure-indicating cock are fitted to the cylinder in way of the belt left between the two pistons when they are at their inner-dead-centre position.
Opposed-piston engines may have two crankshafts, one at the top of the engine for the upper pistons and one in the conventional place {or the lower pistons. Engines with two crankshafts are arranged as trunk-piston engines for both upper and lower pistons. The two crankshafts are connected through a train of gears.
Another form of opposed-piston engine has one crankshaft. For each cylinder there are three cranks: the centre crank is connected to the lower piston through a connecting-rod and crosshead, and the two outside cranks, which are in the same line and opposite to the centre crank, are connected to the upper piston through connecting-rods, crossheads and tie or side rods. Movement of the pistons uncovers and covers the exhaust ports which are in the top of the cylinder and the scavenge ports which are at the bottom of the cylinder.
A third variation of the opposed-piston engine uses eccentrics for the upper piston instead of the two side cranks.
The advantage of the opposed-piston engine over other types of engine is that no firing loads are transmitted from the cylinders to the bedplates holding the crankshaft bearings. In consequence of this they may be constructed to lighter scantlings and therefore have a good power to weight ratio. Another advantage
is that a high degree of balance may be more easily achieved with opposed- piston engines than with’ conventional types.
Their disadvantage is the amount of headroom they require in comparison with other engines of equivalent power and rotational speed.

What do you understand by the following terms: swept volume, clearance volume, compression ratio, volumetric efficiency, scavenge efficiency, air charge ratio, natural aspiration, supercharging? What other names are used for the supercharging process?

Swept volume. This term refers to the volume swept by the piston during one stroke and is the product of the piston area and stroke.
Clearance volume is the volume remaining in the cylinder when the piston is in the top-centre position. The difference between the total cylinder volume and the Swept volume is equal to the clearance volume. The clearance volume space forms the combustion chamber.
Compression ratio. This is the value obtained from dividing the total cylinder volume by the clearance volume and will be from 12 to 18, depending on the engine design. If the compression ratio is below 12 the engine may be difficult to start. High speed engines with small cylinders usually have high compression ratios. Slow speed direct-propulsion engines have compression ratios of around 14.
Volumetric efficiency. This is the ratio of the volume of air drawn into the cylinder (at normal temperature and pressure) to the swept volume. In naturally aspirated four-stroke engines the volumetric efficiency will be from 0.8S to 0.95.
Scavenge efficiency. This is the ratio of the volume of air (at normal temperature and pressure) contained in the cylinder at the start of compression to the volume swept by the piston from the top edge of the ports to the top of its stroke.
Air charge ratio. This is the ratio of the volume of air (at normal temperature and pressure) contained in the cylinder at the start of compression to the swept volume of the piston. This term has now more or less replaced the previous two terms. It is spmetimes referred to as air mass ratio or air supply ratio. In four- strolfe engines the value will vary from 0.85 for naturally aspirated types up to 4 or more in highly supercharged engines. In two-stroke engines the value will.be from 0.85 for simple engines with ported scavenge and exhaust, up to 2.5 for supercharged engines. •
Natural aspiration is a term applied to four-stroke engines where the air charge is brought into the cylinder only by the downward movement of the piston without other aids.
Supercharging is a term used to indicate that the weight of air supplied to the engine has been considerably increased. This allows more fuel to be used per stroke with a consequent increase in engine output power. More power is developed by a supercharged engine than by a non-supercharged engine of the same bore, stroke and speed. Supercharging has had the effect of lowering the specific weight of diesel engines, i.e. more horsepower is obtained per ton of engine weight. The term pressure-charging is now used generally instead of supercharging. Where use is made of an exhaust-gas turbo-driven compressor, the term turbocharging is often used.

Name the factors an engine designer considers in the selection of the compression ratio for a compression ignition engine. Give some examples of compression ratio values.

The ratio between the total surface area of the cylinder space and the volume of the space is such that as the cylinder dimensions increase the ratio between the values decrease.
In a small engine this means more heat is lost to the cylinder space surface during compression than in a larger engine. For this reason smaller engines require a higher compression ratio than larger engines.
An engine started in low ambient temperatures without preheating requires a higher compression ratio than an engine started in higher ambient temperatures.
The factors considered by the designer are therefore the cylinder dimensions and the ambient starting temperature of the engine’s operating environment. Common values of diesel engine compression ratios are as follows.
Slow-speed two-stroke cycle engine used for ship propulsion 12:1. Medium-speed turbocharged four-stroke cycle engine used for propulsion purposes 12:1.
Emergency electrical generator set 14:1 to 16:1.
Small, high-speed, naturally aspirated four-stroke cycle automotive engine fitted with glow plugs up to 23:1.
Note Large engines are usually preheated by raising the temperature of the cooling water. This aids starting and reduces cylinder liner wear. The lubricating oil is also preheated to reduce its viscosity and to assist starting by reducing the friction in bearings.
For cylinders with identical proportions, the total area of the cylinder surfaces varies as the square of the linear dimensions, and the volumes vary as the cube of the linear dimensions .

What are the advantages and disadvantages of cross-scavenged and uniflow-scavenged engines?

Cross-scavenged engines do not require exhaust valves or scavenge valves so some simplicity is obtained Over other engine types.
The cylinder liners of cross-scavenged engines require a complicated pattern
of scavenge and exhaust ports in the lower part of the cylinder. The surfaces left by a core in the casting process of the liner are inadequate in their profile and surface finish. In order to be acceptable the ports must be milled out to give a correct shape and a smooth surface finish. The height of the ports extends relatively high in the cylinder liner and the effective stroke for expansion of the gases is reduced. The cross-sectional area of the ports is relatively large compared with the area of the port bars. This often leads to an excess of liner wear in way of the port bars.
Piston ring breakage is more common in cross-scavenged engines than in uniflow-scavenged engines.
Because they are so complicated the cost of a cylinder liner for a cross- scavenged engine is considerably more than for a uniflow-scavenged engine of similar dimensions.
Uniflow-scavenged engines require an exhaust valve or valves, the number depending on engine speed and cylinder size.
In slow-speed engines only one exhaust valve is required. When one exhaust valve is required two or more fuel injection valves must be fitted whereas in the cross-scavenged engine only one centrally located fuel injection valve is required.
The cylinder liner for a uniflow-scavenged engine has the scavenge ports fitted around the whole of the circumference of the liner. The full circum¬ferential spacoavailable allows the ports to be made circular. This arrangement of ports does not extend as far up the cylinder liner so the effective length of the piston stroke is considerably more in a uniflow-scavenged engine than in a cross-scavenged engine of similar dimensions.
Cylinder liner wear in way of scavenge port bars in uniflow-scavenged engines shows no increase over those parts above and below the ports.
The cylinder liners of uniflow-scavenged engines cost considerably less than those for equivalent cross-scavenged engines.
The arrangements for sealing the bottom of the cooling water space are much simpler in uniflow-scavenged engines.

Why has the cross-scavenged engine been superseded by the uniflow-scavenged engine?

The cross-scavenged engine cannot take advantage of an increase in thermal efficiency by increasing the stroke-bore ratio. The stroke-bore ratio of modern uniflow-scavenged engines may be between 2.4 and 2.95. This allows for a greater ratio of expansion; the increase in thermal efficiency reduces the specific fuel consumption and so reduces fuel costs. As fuel costs make up a large part of the daily running cost of a ship, engines, if they are to be commercially attractive, must have the lowest possible specific fuel consumption.
Note The ratio of expansion is governed by the compression ratio, the bore- stroke ratio and the timing of the opening of the exhaust valve. The opening point of the exhaust valve is related to the power demand of the turbocharger. An increase in the efficiency of the turbocharger allows the exhaust valve to be opened later. Opening the exhaust valve later increases the thermal efficiency of the engine and lowers the specific fuel consumption.
Note By 1981, only one of the three principal slow-speed engine builders was still building cross-scavenged engines. The other two builders had always built uniflow-scavenged engines. Today ail slow-speed engine builders and their licensees build uniflow-scavenged engines only, but large numbers of loop- and cross-scavenged engines will remain in service for some years to come.

Why is it necessary to cool the cylinder heads or covers, cylinder liners and pistons of diesel engines? What is used as the cooling medium?

The temperature inside the cylinders of diesel engines rises to approximately 2000°C during combustion of the fuel and drops to approximately 600°C at the end of expansion. With temperatures in this range the metal of the cylinder covers, cylinder liners and pistons would quickly heat up to the point where its strength would be insufficient to withstand the cylinder pressures; also, no oil film would be able to exist on the cylinder walls, and lubrication of the cylinder and piston rings would break down. Cooling is necessary to maintain sufficient strength in the parts and to preserve the oil film on the cylinder.
The cooling medium for cylinder liners and covers is a flow of distilled or fresh water: the medium for cooling pistons is also distilled or fresh water, or oil from the crankcase system. The amount of heat extracted from the various parts /must be such that they operate at temperatures well within the strength limits of the materials used. The coolant flow patterns must also be arranged so that the surfaces of all parts are as near uniform temperature as possible to prevent large thermal stresses being set up.
With modem highly rated engines the temperatures of the parts subjected to combustion temperature are much lower than in earlier engines. This has been made possible by the availability of better temperature measuring devices and the research carried out by engine builders. The temperature of the combustion chamber surfaces of cylinder covers, piston crowns and cylinder liners varies between 200°C and 350°C in modern highly rated engines. The variation in temperature of the different parts of the surface of cylinder covers will be within about 50°C to 100°C, and for piston crowns the temperature variation will be 75°C to 100°C. Cylinder liners show greater temperature variation throughout their length, but in the highly critical area at the top of the liner the variation is kept to within approximately 100°C.
Small diesel engines with pistons less than about ISO mm (6 in) diameter have ■ only the cylinders and covers cooled by water. The piston crown will be cooled by excess lubricant from the gudgeon bearing and by the heat transfer to the walls of the piston which are then cooled by the cylinder liner. Small high-speed diesel engines may also be cooled by forced air flow passing over fins fitted on the outside of cylinders and cover. It should be noted that air-cooled diesel engines have very low cylinder wear.
Note With pressure-charged engines the air flow during the scavenge period (in two-stroke and four-stroke engines) over the hot internal surfaces of the cylinders, covers and piston crowns helps to maintain low surface temperatures. It also reduces the temperature gradient across the material section and in turn lowers the thermal stresses.

How is the combustion chamber formed in diesel engines? What governs its shape?

In normal engines the combustion chamber is formed in the space between the cylinder cover and the piston crown. The upper part of the cylinder liner usually forms the periphery to the space.
The shape of a combustion chamber may vary between that of a spheroid which will be formed from a concave piston crown and cylinder cover, to that of an inverted saucer, formed from a concave cylinder cover and a slightly convex piston crown. In opposed-piston engines the combustion chamber will be spheroidal. The piston crowns on the upper and lower pistons are usually identical in form. Combustion chambers of the shapes mentioned are referred to as open types.
The shape of a combustion chamber must be such that all parts of the space are accessible to the fuel sprays. If any part is not accessible, the space is wasted and combustion has to take place in a reduced space, which causes further difficulties due to less air being available in the region of the fuel spray. The wasted space is sometimes referred to as parasitic volume. The shape of the various parts must also be satisfactory in respect of their strength as they must be able to withstand the pressures in the cylinder without flexing.
With high-speed engines, open combustion chambers can create problems with very high rates of pressure rise due to the shortness of time available for injection and combustion. To overcome this problem the fuel is injected into a separate chamber which is connected to the main combustion chamber by a restricted passage. The restricted passage is at a high temperature, the fuel spray is long and narrow. Following injection the fuel commences to burn in the separate chamber and issues from the restricted passage at a high velocity due to the pressure rise in the chamber. The fuel enters the main combustion chamber as burning vaporized particles and combustion is then completed. The small chamber is about one-third of the clearance volume and is called a pre¬combustion chamber or antechamber. Its use allows high-speed engines to operate over wide speed ranges without combustion difficulties, and is a necessity in automotive engines. It is met in the marine field when automotive engines are used for electrical generation or other auxiliary purposes.