Valve operation

Function

The two main types of internal combustion engine are: spark ignition (SI) engines (petrol, gasoline, or gas engines), where the fuel ignition is caused by a spark; and compression ignition (CI) engines (diesel engines), where the rise in pressure and temperature is high enough to ignite the fuel. Valves are used in these engines to control the induction and exhaust processes.

Both types of engine can be designed to operate in either two strokes of the piston or four strokes of the piston. The four-stroke operating cycle can be explained by reference to Fig. 2.1. This details the position of the piston and valves during each of the four strokes.

1. The induction stroke The inlet valve is open. The piston moves down the cylinder drawing in a charge of air.

2. The compression stroke Inlet and exhaust valves are closed. The piston moves up the cylinder. As the piston approaches the top of the cylinder (top dead centre – tdc) ignition occurs. In engines utilizing direct injection (DI) the fuel is injected towards the end of the stroke.

3. The expansion stroke Combustion occurs causing a pressure and temperature rise which pushes the piston down. At the end of the stroke the exhaust valve opens.

4. The exhaust stroke The exhaust valve is still open. The piston moves up forcing exhaust gases out of the cylinder.

READ MORE.......

N G I N E E R I N G R E S E A R C H S E R I E S

Automotive Engine Valve Recession

R Lewis and R S Dwyer-Joyce

Series Editor

Duncan Dowson

Professional Engineering Publishing Limited,

London and Bury St Edmunds, UK

for more details and updates about automotive-technology-guide please visit.........

www.automotive-technology-guide.com

---or---

www.automotive-technology-guide.com

Read More

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 precompression 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.

READ MORE.......

The science and technology of materials in automotive engines

Hiroshi Yamagata

Woodhead Publishing and Maney Publishing

on behalf of

The Institute of Materials, Minerals & Mining

CRC Press

Boca Raton Boston New York Washington, DC

WOODHEAD PUBLISHING LIMITED

Cambridge England

for more details and updates about automotive-technology-guide please visit.........

www.automotive-technology-guide.com

---or---

www.automotive-technology-guide.com

Read More