This post is to correct misinformation presented
by PDGOOD in his post #4 regarding the air/fuel
mixture under acceleration as follows: "For acceleration
and high load conditions, a richer mixture (lower air-fuel
ratio) is used to produce cooler combustion products and
thereby prevent detonation and overheating of the cylinder
head.". The part of the statement that says that the A/F
is enriched during acceleration is correct. However, the
part that states "is used to produce cooler combustion
products and thereby prevent detonation and overheating of
the cylinder head." is incorrect.
Chemists, Mechanics, Mechanical Engineers, Physicists,
Automotive Companies, and so forth, all adher to the same
fundamental principal: Pressure equals Temperature times
Volume (P = T x V). Every Internal Combution Engine, be it
a Rotary, Piston, or Turbine, uses this principle for the
purposes of increasing power (the act of increasing pressure
applied to a moving surface). In the case of a 4 stroke
piston engine, as the piston moves upward in the cylinder
during the compression stroke, the fuel air mixture is
compressed until it reaches a point that is Before Top Dead
Center. While the reduction in volume in the cylinder
increases both the pressure and temperature of the
unburned fuel mixture, this increase is not enough to provide
the power the engine needs to make it useable. For non-diesel
engines, a spark is applied at an optimum point BTDC that
starts the burning of the fuel. As the piston moves into
the power stroke, the burning fuel causes a rapid and
explosive jump in temperature and pressure that is ultimately
converted into the mechanical energy that drives the wheels.
Diesel Engines do not use spark plugs, but rely upon glow
plugs or some other hot point that starts the burning of the
diesel Air/Fuel Mixture at the right pressure during the
compression stroke. Diesel engines typically operate at
higher compression ratios than regular gasoline powered
engines.
When an engine needs to provide more power to accomplish
acceleration, it does this by altering the Air/Fuel mixture,
that is, by increasing the amount of fuel relative to the
amount of air. This allows more energy to be produced during
the power stroke, along with an increase in pressure and
temperature. At no point does this create cooler combustion products.
Engines that require more than the normal power, such as when
towing, moving up hills, continuously accelerating and running
at maximum rpm, or hauling a heavy load, will build up a heat
surplus that can eventually lead to overheating. However,
overheating can also be related to engine maintenance problems
in which the engine cooling system is failing to operate
properly. Detonation is a condition that occurs when the
Air/Fuel mixture ignites before it is supposed to. This can
be caused by incorrect timing (due to a problem in the ECM)
or a buildup of residue in the cylinder that gets hot and
causes the preignition of the Fuel/Air Mixture (which in turn
causes the pinging or knocking that you can hear). All of
these conditions are serious maintenance problems that need
to be checked to prevent premature failure of the engine.
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