Transcript IGNITION TIMING

IGNITION TIMING
AUTO 2
2001
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Ignition Timing
• Timing wasn’t adjustable on your Briggs
& Stratton's
• Engines were designed to run at a steady
governed speed so timing was set at factory
to work best at that speed
• An Automobile needs good power and
fuel efficiency at all driving speeds so
timing needs to be changed to meet all
these demands
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Ignition Timing
• Time it takes to burn an A/F mixture
depends on:
• Compression ratio
• Mixture (ratio & mixed up)
• Combustion chamber shape and size
• Placement of spark plug in chamber
• Throttle opening & RPM
• Other small factors
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Ignition Timing
• Engineers generally agree that we need
the biggest push down on the average
piston around 23o ATDC
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Overadvanced Timing (too soon)
• Causes
• Detonation
• Second explosion of A/F after plug lights
• Hard cranking (balking)
• More of one type of emission
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Retarded Timing (too late)
• Causes
•
•
•
•
•
•
Low Power
Less efficiency (mpg)
More emissions
Backfiring out the throttle plate
Lower engine vacuum (manifold)
Higher cylinder temperature
• Hotter running engine
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Ignition Timing
Lets say this engine is running at 1650 RPM
(write it down by the picture)
Lets say this engine takes
4 mS to burn the A/F to get
good pressure and we want
the big push at 23o ATDC
Look at this picture
In your notebook
(top left picture)
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23o
Ignition Timing
• At 1650 RPM, how far is the crankshaft
going to move during 4 mS (.004 seconds)?
Think think think thinkkkkkk, how am I going to do this?
• Lets get RPM to RPS
• 1650 divided by
60 = it,
27.5
I got
AtRPS
1650
the crank
• There are 360oRPM
in a circle
so
o per second of crank
• 360 times 27.5will
= about
10,000
move
about
• There are 1000 mS
40oper
in second
4 mS so
• 10,000 divided by 1000 = 10o per mS
• 10o per mS times 4 mS = 40o
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Ignition Timing
Calculate how many
degrees before TDC
the spark will need start
to get the big push at
23o ATDC
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40o
23o
ATDC
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Ignition Timing
• Now take the same engine and rev it up to
2500 RPM and say it still takes 4 mS to get
the good burn and push on the piston.
• If we leave the starting point at 17o BTDC
the BIG PUSH will happen too late
• Lets calculate
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Ignition Timing
17o
BTDC
TOO
LATE
43o
ATDC
2500 divided by 60 = 41.66 RPS
41.66 times 360 = 15,000 degrees
per second
15,000 divided by 1000 = 15 degrees
per mS
15 times 4 mS = 60 degrees
2,500 RPM
Calculate when the big push will
Happen and fill in on drawing
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Ignition Timing
• So what we need to do is to advance
the starting point when we rev up the
engine
• Copy the numbers from your top engine
over to the top engine on the next page
in your notebook
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Ignition Timing
17o
At 1650 RPM
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40o
23o
BTDC
ATDC
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Ignition Timing
If we are going to
maintain the big push
at 23o ATDC at 2500
RPM, Calculate where we
are going to have to
advance the timing too?
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23o
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Ignition Timing
• So when we speed the engine up, we will
need to have the spark take place sooner
• We are not going to change the dwell
• Specifications needed to maintain proper
saturation time and point opening
• So how are we going to change the timing
according to engine load and speed?
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Vacuum Advance
Vacuum advance rotates points on breaker plate around
The distributor cam to advance when the points open and close
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VACUUM APPLIED
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Vacuum Sources
Find page in
notebook
#1 Venturi
#2 Ported
#3 Manifold
600-800 RPM
1200-1500 RPM
1
O-1”
2
3
15-20”
16-21”
1400-2200 RPM
15-20”
3-5”
1-3”
Venturi not strong
Enough to use for
Vacuum advance
13-20”
0-2”
13-20”
0-2”
0”
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0”
19-26”
Vacuum Advance Notes
• Vacuum comes from either:
• Manifold
• Ported
• Dwell variation is the amount the dwell
changes as the vacuum advance arks
around the distributor cam or as the
distributor cam wobbles in it’s bushing.
• Maximum acceptable variation is 3o of dwell
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Mechanical Advance
Mechanical (centrifugal) advance advances the distributor
cam to the distributor shaft to open points sooner
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Mechanical Advance Notes
• Weights over-come springs to turn the
distributor cam as engine speed increases
• As engine speed decreases, springs pull
back weights and retard distributor cam back
to where we started
• Advance affected by:
• RPM
• Spring tension
• Weight of weights
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Manual Scale Meter
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MILLIONS
THOUSAND
SIX
WHOLE NUMBER
THREE
WHOLE NUMBER
THOUSANDTHS (mili) THREE
DECIMAL
MILLIONTHS (micro)
DECIMAL
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No “M”,
.251
“K”,
amps
or “m”
674 ohms
6,740 ohms
6,740,000 ohms
.109 volts
.816 volts
900 ohms
970 ohms
972 ohms
Infinite or immeasurable
.001173 amps
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1,173 ohms
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Resistor By-Pass
• Ballast resistor is used to cut voltage and
amperage
• Point type ignition first was used on 6 volt systems
• When switched to 12 v systems points couldn’t
handle added amperage
• Resistor was added to prevent burning points
• During cranking, battery voltage may drop to 10v
leaving around 4 v pushing at “+” of coil
• Makes spark weaker when we need it the strongest
• Resistor by-pass by-passes resistor while
cranking to give full battery voltage to “+” of coil
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Resistor By-Pass
Run
Chrysler’s
Start
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Resistor By-Pass
Run
Ford’s
Start
S
I
Starter
Relay
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Resistor By-Pass
Run
GM’s
Start
R S
Starter
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Resistor By-Pass Questions
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Ignition Timing
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Ignition Timing
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Ignition Timing
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