24-01-2013, 12:19 PM
INTERNAL COMBUSTION ENGINES
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Combustion engines
Chemical energy in fuel converted to thermal energy by
combustion or oxidation
Heat engine converts chemical energy into mechanical
energy
Thermal energy raises temperature and pressure of gases
within engine, and gas expands against mechanical
mechanisms of engine
Combustion
Internal: fuel is burned within the engine proper (including
e.g. rocket engines, jet Engines, firearms)
External: combustion is external to the engine (e.g. steam,
Stirling engine, gas turbine)
Ignition
Spark ignition (SI): high-voltage electrical discharge
between two electrodes ignites air-fuel mixture in
combustion chamber surrounding spark plug
Compression ignition (CI): air-fuel mixture self-ignites due
to high temperature in combustion chamber caused by high
compression, Diesel engine
Number of strokes
Four-stroke: four piston movements over two engine
revolutions for each engine cycle
Two-stroke: two piston movements over one revolution for
each engine cycle
Reciprocating engines
Engine has one or more cylinders in which
pistons reciprocate back and forth
Combustion chamber in closed end of
cylinders
Power delivered to rotating output
crankshaft by mechanical linkage with
pistons
Position and number of cylinders
Single cylinder (e.g. lawnmowers)
In-line or straight: cylinders in straight line, one behind
the other in length of crankshaft
V: two banks of cylinders at an angle with each other along
a single crankshaft, angle typically 60-90◦
Flat or opposed cylinder (V with 180◦): two banks of
cylinders opposite each other on a single crankshaft (small
aircrafts)
W: three banks of cylinders on same crankshaft (not
common)
Opposed piston engine: two pistons in each cylinder,
combustion chamber between pistons
Radial engine: cylinders positioned radially around
crankshaft
4-stroke SI engine operation
First stroke: intake or induction
Piston travels from TDC (top dead center) to BDC
(bottom dead center) with intake valve open and exhaust
valve closed
Volume increases in combustion chamber and creates
vacuum
Air pushed through cylinder
As air passes through intake system, fuel is added