Toyota COMPUTER-CONTROLLED SYSTEM Training Manual page 91

ESA - Genera l
ESA (ELECTRONIC SPARK ADVANCE )
GENERAL
®
The ESA (electronic spark advance) system is a mechanical advancer) controls the ignition
system in which an ECU (rather than a timing of the ignition system .
Ignite r
Ignition coi l
OHP 59
BASIC CONSTRUCTION OF ES A
1 . IGNITION TIMING AND ENGINE RUNNING CONDITION S
In order to maximize engine output efficiency,
the air-fuel mixture must be ignited when the
maximum combustion pressure occurs ; that is, at
about 10° after TDC (top dead center) .
However, the time from ignition of the air-fuel
mixture to the development of maximum
combustion pressure varies depending on the
engine speed and the manifold pressure ; ignition
must occur earlier when the engine speed is
higher and later when it is lower . In con-
ventional EFI, the timing is advanced
retarded by a governor advancer in
distributor .
Furthermore, ignition must also be advanced
when the manifold pressure is low (i .e ., when
there is a strong vacuum) . In conventional EFI,
this is achieved by the vacuum advancer in the
distributor .
However, optimal ignition timing is also affected
by a number of other factors besides engine
speed and intake air volume, such as the shape
of the combustion chamber, the temperature
inside the combustion chamber, etc . For this
reason, governor and vacuum advancers cannot
provide ideal ignition timing for the engine . In
the ESA system, the engine is provided with
nearly ideal ignition timing characteristics .
The ESA works as follows : the ECU determines
ignition timing from its internal memory, which
contains the optimal ignition timing data for
each engine running condition, then sends the
appropriate ignition timing signal to the igniter .
Since the ESA always ensures optimal ignition
timing, both fuel efficiency and engine power
output are maintained at optimal levels .
and
the
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