Page 284 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

9.1 Stationary Combustion Devices 261

lower and particles are larger, but mechanical stresses are stronger due to strong
turbulence and the impact between particles. Although ash-forming material
remains in the bed, fine particles are produced due to attrition and abrasion.

9.2 Internal Combustion Engines

An internal combustion engine is a device where atomized liquid fuels are burned to
produce thermal energy (heat), which is converted to mechanical energy to drive the
transportation vehicles. Combustion in engines takes place at high pressure with a
variable volume, most commonly a piston for liquid fuels. The most common
internal combustion engines are spark ignition engines used in automobile industry
and diesel engines primarily for trucking industry. Large diesel engines are also
used in off road power generation and trains. Another type of internal combustion
engine is gas turbines for air craft or power generation. It is excluded from this
book.

9.2.1 Spark Ignition Engines

A schematic diagram of a basic piston engine (or reciprocating engine) is shown in
(Fig. 9.2). It is a metal block containing a series of chambers. The core of the engine
is a cylinder that houses a piston. The inner diameter of the cylinder is also referred
to as bore. The upper part of the block consists of outer walls that form hollow
jackets around the cylinder walls. The hollow jackets contain the coolants to pre-
vent the engine from being overheated. The lower part is called the crankcase,
which provides rigid mounting points for the bearings to fix the crank.
The crank drives the piston up and down through the connecting rod. The angle
between the crank and the centerline of the cylinder is defined as crank angle, θ.
When the piston is at the top of the cylinder, it reaches its top dead center (TDC)
and θ = 0; when the piston is at the lowest position, bottom dead center (BDC),
θ = π. The distance between TDC and BDC is referred to as stroke.
A typical operating cycle of this type of engine involves four stokes (Fig. 9.3):
1. intake
2. compression
3. combustion/expansion
4. exhaust

When the piston moves downward, the intake camshaft opens the intake duct
and fuel and air mixture is drawn into the cylinder. When the piston starts moving
upward, both the intake valve and the exhaust valve are closed forming an
enclosure. The rising piston compresses the air-fuel mixture. When the piston