THE ROLE OF THE

Power Train Applications 97
oxides are produced at higher temperatures in the presence of oxygen. Nitrogen molecules
will react and form NO and NO 2. For stoichiometric combustions,
( ) +
( )→ +( ) +
CH a b O N aCO b
a b + + / 4 3. 773
2
HO 3. 773
With y = b/a,
( ) +
( )
2 2 2 2 a b + N
( )
CH
y
O N CO
y
HO
y
y + 1+
( 2 4
3. 773 2)→ 2 +( 2 ) 2 + 3. 773 1+
4 2 (7.2)
4 2
N (7.3)
Starting with the stoichiometric air–fuel ratio or the ideal fuel ratio, burning
would consume all the fuel without leaving unburned constituents. If the air content
is higher than this ratio, then the mixture is said to be lean; if the air content is lower
than the stoichiometric ratio, then the mixture is rich. The air–fuel equivalence ratio
(Φ) shown in Equation 7.4 is considered rich if the value is greater than 1:
Φ=
( )
A F
A F
Stoich
Actual
(7.4)
Unburned fuel can contribute up to 1–2% of emissions on a modern engine due
to incomplete combustion or design issues such as cylinder-to-cylinder variation and
nonhomogenous air–fuel ratios. Combustion in oxygen is a radical chain reaction
that will contain intermediates. It is thought to be initiated by the abstraction of a
hydride from the fuel to oxygen. A hydroperoxide radical will react to give hydroperoxides
and hydroxyl radicals. The intermediates cannot be isolated, but the nonradical
intermediates are stable.
Temperature of the intake air will play a role in the combustion process. The temperature
affects the air density, which will in turn have an impact on the mass of the
air in the cylinder. For every 3°C increase in intake temperature there is an approximate
1% loss in density, which results in about a 1% loss in power. The effects of
intake temperature on combustion are not limited to the power loss due to density. It
also affects spark knock, spark retardation, elevated under-hood temperatures, and
elevated exhaust valve temperatures.
In a combustion reaction, the fuel and oxygen from the air will result in a high
amount of nitrogen. The nitrogen will be oxidized to various nitrogen oxides (NO x).
In addition, sulfur will produce sulfur dioxide and carbon will produce carbon dioxide.
Because it is practically impossible to have complete combustion, the automotive
chemist must devise ways of reducing the impact of the pollutants produced. This is
demonstrated by the use of urea injection in diesel engines to minimize the effects.
The design engineer must also devise ways of manufacturing and design to improve
turbulent flow and efficiency. For instance, in aluminum engine blocks, a steel liner
can be compressed by lowering its temperature in liquid nitrogen and then placed in
the cylinder and allowed to expand for a press fit.