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 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.

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.

What are fossil fuels and how do they differ from other types of fuel? Which fossil fuels are used in diesel engines?

Fossil fuels are the remains of prehistoric animals and plants and are found below the surface of the earth; they may be solid, liquid or gaseous.
S Solid fuels. Coal is the most important solid fuel used commercially.
Liquid fuels of a wide variety are obtained from distillation and other processes carried out on crude oil. The products obtained are essentially engine fuels, j boiler burner fuels, and lubricants.
Note—The oiHndustry is also a large supplier of chemicals used in other industries such as plastics, paints and compositions, synthetic rubbers and the | like.
1 Gaseous fuels may exist naturally in the ground or be produced from coal or
crude oil. Liquefied petroleum gases (LPG) are increasingly used.
! The fossil fuels are essentially carbon-hydrogen compounds. The energy is
derived from them by the exothermic action of converting the carbon to carbon dioxide and the hydrogen to water, which will be in the form of steam at the end – of the combustion process.
! The other types of fuel used are nuclear, which are fissile materials used in a
| reactor. One of the isotopes of uranium is commonly used. „
The fuels used in diesel engines are the gas oils and diesel oils which boil off from crude at temperatures between approximately 200°C and 400°C, or blends of diesel oil and residual fuel which have higher boiling points.
Note Liquefied petroleum gases must be stored under pressure or in refrigerated conditions, since their boiling points are low.