The Science and Technology of Materials in Automotive Engines

Hiroshi Yamagata

• Editorial: Woodhead Publishing
• Año: 2005
• ISBN:
  9781845690854
• Categoría: Technology & Engineering
• Categoría: Materials Science

The science and technology of materials in automotive engines provides an introductory text on the nature of the materials used in automotive engines. It focuses on reciprocating engines, both four and two stroke, with particular emphasis on their characteristics and the types of materials used in their construction. The book considers the engine in terms of each specific part: the cylinder, piston, camshaft, valves, crankshaft, connecting rod and catalytic converter. The materials used in automotive engines are required to fulfil a multitude of functions. It is a subtle balance between material properties, essential design and high performance characteristics. The science and technology of materials in automotive engines describes the metallurgy, chemical composition, manufacturing, heat treatment and surface modification of these materials. It also includes supplementary notes that support the core text.The book is essential reading for engineers and designers of engines, as well as lecturers and graduate students in the fields of automotive engineering, machine design and materials science looking for a concise, expert analysis of automotive materials.

• Provides a detailed introduction to the nature of materials used in automotive engines
• Essential reading for engineers, designers, lecturers and students in automotive engineering
• Written by a renowned expert in the field

1

Engines

1.1 The reciprocating engine

The engine is the heart of a car although it is normally hidden under the bonnet. The engine is exposed in a motorcycle but the detailed mechanisms are not visible. This chapter looks at these mechanisms.

Figure 1.1 shows a four-stroke cycle petrol engine with the various parts indicated. In a reciprocating engine a mixture of petrol and air burns explosively in a narrow container when ignited. The piston then receives the combustion pressure, and the connecting rod and crankshaft mechanism converts this pressure into rotation. This is the basic mechanism of a reciprocating engine. The reciprocating mechanism was originally inherited from steam engines and has been used for more than 200 years. One of the earliest mechanisms using a piston and cylinder can be seen in a 1509 drawing by Leonardo da Vinci, the famous painter and scientist of the Renaissance period. There are two main types of reciprocating engine, the four-stroke and the two-stroke engine. Figure 1.2 illustrates the sequence of operation. The four-stroke-cycle engine rapidly repeats strokes 1 to 4.^{1, 2}

1.1 Cutaway of four-stroke cycle petrol engine. (courtesy of Volvo Car Corporation)

1.2 Basic operations of four-stroke cycle engine.

1.1.1 The four-stroke engine

The four-stroke engine is also referred to as the Otto cycle engine after its inventor N.A. Otto. Most cars use the four-stroke engine. An individual cycle comprises four strokes: 1, intake stroke; 2, compression stroke; 3, power stroke and 4, exhaust stroke. These four strokes repeat to generate the crankshaft revolution.

1. Intake stroke: the intake stroke draws air and fuel into the combustion chamber. The piston descends in the cylinder bore to evacuate the combustion chamber. When the inlet valve opens, atmospheric pressure forces the air-fuel charge into the evacuated chamber. As a result, the combustible mixture of fuel and air fills the chamber.

2. Compression stroke: at the end of the intake stroke, both inlet and exhaust valves are closed. The inertial action of the crankshaft in turn lifts the piston which compresses the mixture. The ratio of the combustion chamber volume before and after compression is called the compression ratio. Typically the value is approximately 9:1 in spark ignition engines and 15:1 in diesel engines.

3. Power stroke: when the piston ascends and reaches top dead center, an electric current ignites the spark plug and as the mixed gas burns, it expands and builds pressure in the combustion chamber. The resulting pressure pushes the piston down with several tons of force.

4. Exhaust stroke: during the exhaust stroke, the inlet valve remains closed whilst the exhaust valve opens. The moving piston pushes the burned fumes through the now open exhaust port and another intake stroke starts again.

During one cycle, the piston makes two round trips and the crankshaft revolves twice. The inlet and exhaust valves open and close only once. The ignition plug also sparks only once. A petrol engine, whether four-or two-stroke, is called a spark ignition (SI) engine because it fires with an ignition plug. The four-stroke-cycle engine contains the lubricating oil in the crankcase. The oil both lubricates the crankshaft bearings and cools the hot piston.

1.1.2 The two-stroke engine

The two-stroke engine is similar to that of the four-stroke-cycle engine in its reciprocating mechanism. It uses the piston-crankshaft mechanism, but requires only one revolution of the crankshaft for a complete power-producing cycle. The two-stroke engine does not use inlet and exhaust valves. The gas exchange is implemented by scavenging and exhaust porthole openings in the bore wall. The upward and downward motion of the piston simultaneously opens and closes these portholes. The air-fuel mixture then goes in or out of the combustion chamber through the portholes. Combustion takes place at every rotation of the crankshaft.

In the two-stroke engine, the space in the crankcase works as a pre-compression chamber for each successive fuel charge. The fuel and lubricating oil are premixed and introduced into the crankcase, so that the crankcase cannot be used for storing the lubricating oil. When combustion occurs in the cylinder, the combustion pressure compresses the new gas in the crankcase for the next combustion. The burnt gas then exhausts while drawing in new gas. The lubricating oil mixed into the air-fuel mixture also burns.

Since the two-stroke engine does not use a valve system, its mechanism is very simple. The power output is fairly high because it achieves one power stroke per two revolutions of the crankshaft. However, although the power output is high, it is used only for small motorcycle engines and some large diesel applications. Since the new gas pushes out the burnt gas, the intake and exhaust gases are not clearly separated. As a result, fuel consumption is relatively high and cleaning of the exhaust gas by a catalytic converter is difficult.

In the past, petrol engines almost universally used³ a carburetor. However, the requirements for improved fuel economy have led to an increasing use of fuel injection. In a petrol engine the fuel is normally injected into the inlet manifold behind the inlet valve. The atomized fuel mixes with air. When the inlet valve is opened, the combustible mixture is drawn into the cylinder. However, a recent development has occurred in direct injection petrol engines whereby fuel is injected directly into the combustion chamber, as with direct injection diesel engines.

1.1.3 The diesel engine

The name diesel comes from the inventor of the diesel engine, R. Diesel. There are both four-and two-stroke-cycle diesel engines. Most automotive diesels are four-stroke engines. The intake stroke on the diesel engine draws only air into the cylinder. The air is then compressed during the compression stroke. At near maximum compression, finely atomized diesel fuel (a gas oil having a high flashpoint) is sprayed into the hot air, initiating auto ignition of the mixture. During the subsequent power stroke, the expanding hot mixture works on the piston, then burnt gases are purged during the exhaust stroke. Since diesel engines do not use a spark plug, they are also referred to as compression ignition (CI) engines. In the case of petrol engines, too high a temperature in the combustion chamber ignites the petrol spontaneously. When this occurs, the plug cannot control the moment of ignition. This unwanted phenomenon is often referred to as ‘knocking’.

The diesel is an injection engine. A petrol engine normally needs a throttle valve to control airflow into the cylinder, but a diesel engine does not. Instead, the diesel uses a fuel injection pump and an injector nozzle sprays fuel right into the combustion chamber at high pressure. The amount of fuel injected into the cylinder controls the engine power and speed. There are two methods³ by which fuel is injected into a combustion chamber, direct or indirect injection. With direct injection engines (DI) the fuel is injected directly into the cylinder and initial combustion takes place within the bowl that is machined into the piston head itself. With indirect injection engines (IDI) the fuel is injected and initial combustion takes place in a small pre-combustion chamber formed in the cylinder head. The burning gases then expand into the cylinder where combustion continues. Pistons for IDI engines usually have shallow depressions in their heads to assist the combustion process. Although an ...