DESIGN AND SIMULATION STUDY ON HIGH COMPRESSION IGNITION ENGINE PISTON AND PERFORMANCE MINI PROJECT REPORT

INTRODUCTION

1.1 INTERNAL COMBUSTION ENGINE
An internal combustion (IC) engine is a heat engine where the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is applied typically to pistons, turbine blades, rotor or a nozzle. This force moves the component over a distance, transforming chemical energy into useful mechanical energy.

The term internal combustion engine usually refers to an engine in which combustion is intermittent, such as the more familiar four-stroke and two-stroke piston engines, along with variants, such as the six-stroke piston engine and the Wankel rotary engine. A second class of internal combustion engines use continuous combustion: gas turbines, jet engines and most rocket engines, each of which are internal combustion engines on the same principle as previously described.

Internal combustion engines are quite different from external combustion engines, such as steam or Stirling engines, in which the energy is delivered to a working fluid not consisting of, mixed with, or contaminated by combustion products. Working fluids can be air, hot water, pressurized water or even liquid sodium, heated in a boiler. ICEs are usually powered by energy-dense fuels such as gasoline or diesel, liquids derived from fossil fuels. While there are many stationary applications, most ICEs are used in mobile applications and are the dominant power supply for vehicles such as cars, aircraft, and boats.

Typically an ICE is fed with fossil fuels like natural gas or petroleum products such as gasoline, diesel fuel or fuel oil. There is a growing usage of renewable fuels like biodiesel for compression ignition engines and bioethanol or methanol for spark ignition engines. Hydrogen is sometimes used, and can be made from either fossil fuels or renewable energy

Multiple cylinder engines have their valve train and crankshaft configured so that pistons are at different parts of their cycle. It is desirable to have the piston's cycles uniformly spaced (this is called even firing) especially in forced induction engines; this reduces torque pulsations[22] and makes inline engines with more than 3 cylinders statically balanced in its primary forces. However, some engine configurations require odd firing to achieve better balance than what is possible with even firing. For instance, a 4-stroke I2 engine has better balance when the angle between the crankpins is 180° because the pistons move in opposite directions and inertial forces partially cancel, but this gives an odd firing pattern where one cylinder fires 180° of crankshaft rotation after the other, then no cylinder fires for 540°. With an even firing pattern the pistons would move in unison and the associated forces would add.

1.2 WORKING PRINCIPLE OF FOUR STROKE DIESEL ENGINE
In the true diesel engine, only air is initially introduced into the combustion
chamber. The air is then compressed with a compression ratio typically between 15:1
and 23:1. This high compression causes the temperature of the air to rise. At about the
top of the compression stroke, fuel is injected directly into the compressed air in the
combustion chamber. This may be into a (typically toroidal) void in the top of the
piston or a pre-chamber depending upon the design of the engine. The fuel injector
ensures that the fuel is broken down into small droplets, and that the fuel is distributed
evenly.

The heat of the compressed air vaporizes fuel from the surface of the droplets. The vapour is then ignited by the heat from the compressed air in the combustion chamber, the droplets continue to vaporize from their surfaces and burn, getting smaller, until all the fuel in the droplets has been burnt. Combustion occurs at a substantially constant pressure during the initial part of the power stroke. The start of vaporization causes a delay before ignition and the characteristic diesel knocking sound as the vapour reaches ignition temperature and causes an abrupt increase in pressure above the piston (not shown on the P-V indicator diagram).

When combustion is complete the combustion, gases expand as the piston descends further; the high pressure in the cylinder drives the piston downward, supplying power to the crankshaft. As well as the high level of compression allowing combustion to take place without a separate ignition system, a high compression ratio greatly increases the engine's efficiency. Increasing the compression ratio in a spark- ignition engine where fuel and air are mixed before entry to the cylinder is limited by the need to prevent damaging pre-ignition. Since only air is compressed in a diesel engine, and fuel is not introduced into the cylinder until shortly before top dead center (TDC), premature detonation is not a problem and compression ratios are much higher.

1.3 CLASSIFICATION OF INTERNAL COMBUSTION ENGINE
The shape of the combustion chamber and the fluid dynamics inside the chamber are of great importance in diesel combustion. Diesel engines are divided into two basic categories according to their combustion chamber design.

Direct-Injection (DI) engines: This type of combustion chamber is also
called an open combustion chamber. In this type, the entire volume of the combustion
chamber is located in the main cylinder and the fuel is injected into this volume.

Indirect-Injection (IDI) engines: This type of combustion chambers, the
combustion space is divided into two parts, one part in the main cylinder and the other
part in the cylinder head. The fuel injection is effected usually into that part of the
chamber located in the cylinder head.

1.4 TYPES OF COMBUSTION CHAMBER
1.Open combustion chamber
The open combustion chamber is one, in which all the air for combustion is confined in one space. These chambers mainly consist of space formed between the flat cylinder head and a cavity in the piston crown in different shapes. The fuel is injected directly into this space. The injector nozzles used for this type of chambers are generally of multi hole type working at a relatively high pressure (above 200 bars).

2.Shallow depth chamber
In the shallow depth chamber the depth of the cavity provided in the piston is quite small. This chamber is usually adopted for large engines running at low speeds. Since the cavity diameter is very large, the squish is negligible.

3.Hemispherical chamber
This chamber also gives small squish. However, the depth to diameter ratio can be varied to give any desired squish to give better performance.

4.Cylindrical chamber
This design was attempted in recent diesel engines. This is a modification of the cylindrical chamber in the form of a truncated cone with a base angle of 30o. The swirl was produced by masking the valve for nearly 180o of circumference. Squish can also be varied by varying the depth.

5.Toroidal chamber
The idea behind this shape is to provide a powerful squish along with the air movement, similar to that of the familiar smoke ring, within the toroidal chamber. Due to powerful squish the mask needed on inlet valve is small and there is better utilization of oxygen.