Ford Taurus, Руководство пользователя

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IGNITION TIMING
Introduction
Ignition timing is the measurement, in degrees of crankshaft rotation, of the point
at which the spark plugs fire in each of the cylinders. It is measured in degrees
before or after Top Dead Center (TDC) of the compression stroke.
Because it takes a fraction of a second for the spark plug to ignite the mixture in
the cylinder, the spark plug must fire a little before the piston reaches TDC.
Otherwise, the mixture will not be completely ignited as the piston passes TDC
and the full power of the explosion will not be used by the engine.
The timing measurement is given in degrees of crankshaft rotation BEFORE the
piston reaches TDC (BTDC). If the setting for the ignition timing is 10° BTDC, the
spark plug must fire 10° before each piston reaches TDC. This only holds true,
however, when the engine is at idle speed.
As the engine speed increases, the pistons go faster. The spark plugs have to
ignite the fuel even sooner if it is to be completely ignited when the piston
reaches TDC. To do this, distributors have various means of advancing the spark
timing as the engine speed increases. On some earlier model vehicles, this is
accomplished by centrifugal weights within the distributor along with a vacuum
diaphragm mounted on the side of the distributor. Models covered by this manual
use signals from various sensors, making all timing changes electronically, and no
vacuum or mechanical advance is used. The 3.0L and 3.2L SHO engines and the
3.0L Flexible Fuel engines use a distributorless electronic ignition system.
Operation of this system allows for full electronic control of the timing.
If the ignition is set too far advanced (BTDC), the ignition and expansion of the
fuel in the cylinder will occur too soon and tend to force the piston down while it
is still traveling up. This causes engine ping. If the ignition spark is set too far
retarded, After TDC (ATDC), the piston will have already passed TDC and started
on its way down when the fuel is ignited. This will cause the piston to be forced
down for only a portion of its travel, and will result in poor engine performance as
well as a lack of power.
The timing marks on the 2.5L engine are visible through a hole in the top of the
transaxle case. The 3.0L and 3.8L engines have the timing marks on the
crankshaft pulley and a timing marker near the pulley. A stroboscopic (dynamic)
timing light is used, which is hooked into the circuit of the No. 1 cylinder spark
plug. Every time the spark plug fires, the timing light flashes. By aiming the
timing light at the timing marks while the engine is running, the exact position of
the piston within the cylinder can be easily read since the stroboscopic flash
makes the mark on the pulley appear to be standing still. Proper timing is
indicated when the notch is aligned with the correct number on the scale.
Стр . 1 из 7 IGNITION TIMING
18.11.2005 http://www.chiltondiy.com/content/8687/8687_2_5.html