Page 286 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

9.2 Internal Combustion Engines 263

reaches the top of the stroke, the spark plug is fired to ignite the air-fuel mixture and
starts the combustion at the right instant. As the combustion proceeds, the burning
mixture expands at increasing temperature and drives the piton downward to pro-
duce mechanical energy. Combustion is expected to complete when the piton
reaches BDC, when the exhaust valves opens. The exhaust is pushed out of the
cylinder. A new cycle starts when the rising piston reaches top of the stroke.

9.2.1.1 Carburetor

The air and fuel are premixed in the carburetor, a device that controls the flow of air
and fuel. The mixture is usually at an equivalence ratio of 0:7\U\1:4 to match
the residence time for air emission control. A cold engine is started with a fuel rich
mixture to secure successful ignition. This results in highly incomplete combustion
and high CO and HC emissions. Evaporation of the liquid fuel results in evapo-
rative air emissions from the carburetor after the engine is turned off. Vapor
recovery system and the adsorption-desorption system can be employed to reduce
the evaporative air emissions. Activated carbon is a common adsorbent for the
vapor recovery. Recycled gasoline is purged by air and the mixture is delivered to
the engine for combustion without reentering the tank.

9.2.1.2 Flame in Engine

The local equivalence ratio in flame depends on the spot in the cylinder. After the
ignition of the air-fuel mixture, the gas mixture is not burned instantaneously. The
turbulent flame moves downward as the cylinder volume expands. The speed of
the flame depends on the engine design and the operating conditions such as the
equivalence ratio and the speed of the piston. As combustion continues in the
cylinder, the temperature and pressure of the gas rise. This is different from
the combustion in stationary systems, where pressure seems to be stable.

9.2.2 Diesel Engines

Diesel engines do not have carburetors for air-fuel premixing. Instead, air is drawn
into the cylinder through the intake valve and diesel is injected into the cylinder of
the engine. Diesel injection starts when it approaches the end of the compression
stroke. At the moment, the high-temperature high-pressure compressed air ignites
the vapor of the diesel droplets. Combustion in a diesel engine is unsteady and it
varies with the fuel injection mode and mixing with the air.
In a direct injection (DI) engine, both air and fuel are turbulent and they are not
homogeneously mixed. A prechamber can be used to improve the mixing effect and
as a result, this type of engine is called prechamber diesel engine, or indirect