timing refers to the precise time spark occurs. Ignition is specified by
referring to the position of the #1 piston relation to crankshaft rotation.
Ignition timing reference timing marks can be located on engine parts and on a
pulley or flywheel to indicate the position of the #1 piston. Vehicle
manufacturers specify initial or basic ignition timing.
marks are aligned at TDC, or 0, the piston in cylinder #1 is at TDC of its
compression stroke. Additional numbers on a scale indicate the number of degrees
of crankshaft rotation before TDC (BTDC) or after TDC (ATDC). In a majority of
engines, the initial timing is specified at a point between TDC and 20 degrees
BTDC. A few manufacturers specified initial timing from 1 to 5 degrees ATDC for
vehicles built during the 1970s.
engine performance is to be maintained, the ignition timing of the engine must
change as the operating conditions of the engine change. Ignition systems allow
for these necessary changes in many ways; these are covered in greater detail
later in this chapter. All the different operating conditions affect the speed
of the engine and the load on the engine. All ignition timing changes are made
in response to these primary factors.
rpms, the crankshaft turns through more degrees in a given period of time. If
combustion is to be completed by 10 degrees ATDC, ignition timing must occur
sooner or be advanced.
air/fuel mixture turbulence increases with rpm. This causes the mixture inside
the cylinder to turn faster. Increased turbulence requires that ignition must
occur slightly later or slightly retarded.
factors must be balanced for best engine performance. Therefore, while the
ignition timing must be advanced as engine speed increases, the amount of
advance must be decreased some to compensate for the increased turbulence.
on a engine is related to the work it must do. Driving up hills or pulling extra
weight increases engine load. Under load, the pistons move slower and the engine
runs less efficiently. A good indication of engine load is the amount of vacuum
formed during the intake stroke.
light loads and with the throttle plate partially opened, a high vacuum exists
in the intake manifold. The amount of air/fuel mixture drawn into the manifold
and cylinders is small. On compression, this thin mixture produces less
combustion pressure and combustion time is slow. To complete combustion by 10
degrees ATDC, ignition timing must be advanced.
heavy loads, when the throttle is opened fully, a larger mass of air/fuel
mixture can be drawn in, and the vacuum in the manifold is low. High combustion
pressure and rapid burning results. In such a case, the ignition timing must be
retarded to prevent complete burning from occurring before 10 degrees ATDC.
Up to this
point, the primary focus of discussion has been ignition timing as it relates to
any one cylinder. However, the function of the ignition system extends beyond
timing the arrival of a spark to a single cylinder. It must perform this task
for each cylinder of the engine in a specific sequence.
cylinder of an engine must produce power once in every 720 degrees of crankshaft
rotation. Each cylinder must have a power stroke at its own appropriate time
during the rotation. To make this possible, the pistons and rods are arranged in
a precise fashion. This is called the engine¡¯s firing order. The firing order is
arranged to reduce rocking and imbalance problems. Because the potential for
this rocking is determined by the design and construction of the engine, the
firing order caries from engine to engine. Vehicle manufacturers simplify
cylinder identification by numbering each cylinder. Regardless of the particular
firing order used, the number 1 cylinder always starts the firing order, with
the rest of the cylinders following in a fixed sequence.
ignition system must be able to monitor the rotation of the crankshaft and the
relative position of each piston to determine which piston is on its compression
stroke. It must also be able to deliver a high-voltage surge to each cylinder at
the proper time during its compression stroke. How the ignition system does
these things depends on the design of the system.