Ignition Timing Accuracy : Causes, Impact, and Solutions

Ignition Timing Accuracy :
Causes, Impact, and
Solutions
Presented by: Fred Husher
12/9-10/2014
1
Assuming the engine is
capable of perfect
performance:
Ignition system performance is limited
by the cumulative errors:
 mechanical elements,
 sensors, and
 signal processing
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2
The Ignition Timing Shifts,
what does it do to Engine Performance?
If retarded:
 Easy start
 Acceleration is slowed because of HP drop
 Engine heat increases because combustion temp has
increased
 Higher exhaust temps
If advanced:





Hard start
Acceleration is aided because max HP possible
Engine runs cooler
Knock & pre-ignition can occur
If too advanced there wll be excessive stress on engine
parts
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As with a Crime Mystery:
Who Are the Possible Perpetrators?
Engine mechanical tolerances to driving the
trigger sensor
Trigger sensor behavior to its stimulus
Trigger detector edge detection accuracy &
repeatability
Signal processing delay through ECU/ICU
Managing the spark gap between the rotor
and cap on the distributor and the spark plug
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The focus of this discussion will be on:
trigger sensor,
sensor signal processing and
their mechanical supporting elements
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The mechanical causes are
often the easier to identify:
Distributor
 Shaft end play shifts timing
• CW: advance on acceleration
• CCW: retard on acceleration
 Rotational wobble causes timing bobble between
cylinders
 Backlash between cam & distributor gears
 Mismatch of advance springs & weights to cam, if
used
Cam Shaft
 End play imposes timing shift to the distributor
Crankshaft
 Torque can move crank timing disk and change
the trigger timing
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The Electrical Causes
Often the most insidious as they are not easily measured
with the common mechanics tools
 Poor head ground return: RF vs. DC
 Skin vs. bulk current conduction
 Insufficient current supply capacity to the ignition module
 MDI pulse to < 8A
 CDI can pulse to >200A
 Overdriven ignition coil
 Distributor rotor contact shape, metal alloy, and surface finish
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Trigger Sensor Behavior to
its Stimulus & Environment
Mechanical motion variations
Temperature: magnetic or optical
Induced EMI to signal lines
Ground loop currents such as spark return
Load dump sensitivity from solenoid release
Battery voltage sag and failure to/from
alternator, battery, & ignition module
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The input signal type has considerable
impact on how it can be processed
 Variable Reluctor coil
 Digital (open drain/collector)
 Hall Effect
 Opto-interrupter sensors
 Digital or analog variable reluctor mimics
 RS-232 output – Crane Pro-Race distributor
 Capacitor coupled outputs – Crane Race Billet
distributor
 Differential RC outputs – Crane Crank Sensor
 The exotics: magnetostrictive, piezoelectric, and Wiegand
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Variable Reluctor Coil
 Developed voltage by the VR is
Voltage developed = N(ΔΦ/Δt),
 Attributes:
 Unable to function at slow RPMs
From US6278496
 Noise immune with differential detector
 Noise sensitive with single ended detector
 Rise/falling trigger edge choice is by simply
exchanging the coil’s leads, MAG+ & MAG Fragile due to the coil’s wire winding breaking from
vibration
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Digital Attributes
Dominated by open drain/collector output
Types
 Hall Effect
 Opto-interrupter sensors
Output low is limited to Vsat ~ 0.2V
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Opto-interrupter Triggers
Crane Race Billet
Crane Pro-Race
Crane Points Replacement
 Pro-race supports individual cylinder adjustment for advance/retard,
developed initially for race use
 Etch lines on either side of the window provide 1° markers that the user
can file or mill to adjust the trigger timing advance/retard for each
cylinder
 Points replacement for 4/6/8/12-cylinder distributors
 Accuracy of chemical etched plate & opto-interrupter is <0.01 °
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Digital or Analog
Variable Reluctor Mimics
RS-232 output –
Crane Pro-Race
distributor
Capacitor coupled
outputs – Crane
Race Billet
distributor
Differential RC
outputs – Crane
Crank Sensor
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Variable Reluctor Mimic:
RS-232 Output
+12V
+5V
V
U105
MAX3232
O
G
I
CON3
1
2
3
+IGN
MAG+
GND
Simplified circuit
Single ended output
+/- 7 to 10V output swing
Very high noise immunity to VR trigger detector
 Signal never at 0V
 Low output impedance
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Variable Reluctor Mimic:
Capacitor Coupled Output
CON3
2.2K
0.1
1
2
3
+IGN
MAG+
GND
2.2K
Simplified circuit
Single ended output
+/- 6 to 12V differentiated digital signal
Idles at ground
 Output is susceptible to transmission of ground impulse
noise
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Variable Reluctor Mimic
Differential RC Coupled Output
+5V
R28
4.7K
2
R26
4.7K
+
C3
1UF/16V
1
A
DI
B
6
4
R24
2.4K
MAG-
+
U6
HALL SENSOR
R27
1K
R25
2.4K
5
2
G
V+
3
DE
G
O
V
1
MAG+
3
U3
ISL3294EFHZ-T
C4
1UF/16V
SIMPLIFIED DIFFERENTIAL
OUTPUT HALL SENSOR
Differential output signaling
 MAG + & MAG- can be swapped just as with the VR sensor to change
trigger edge
Either output can be shorted to ground
Output signals can be floated to +2.5V to support use with
single supply differential input detectors
Very high noise immunity rejection into VR trigger detector
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Hall Sensor Attributes
The developed signal is independent of any
rate of change in the magnetic field acting
upon the Hall Effect bridge
 Uni-polar Hall sensor
 Operational to zero RPM
 Rugged and stable throughout -40 to
+125°C
 Not usable with VR detectors without
signal conversion
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Static Hall Sensor
Characterization Fixture
Motion adjustments are
 Rotation of trigger disk
 Translation to flux concentrator / magnet to Hall
Sensor
 Translation of Hall assembly to the trigger disk
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Crank Trigger Disks
STEEL TOOTH – BIASED HALL
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MAGNET TOOTH – UNBIASED HALL
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Crank Trigger Disks
 Magnetic Circuit
 Iron core flux concentrator for embedded magnet trigger disks
• Embedded magnets are matched for flux density
 Back biased sensor with integral magnet for steel tooth trigger disks
 Timing accuracy depends on all teeth being of
 Equal geometry & spacing
 Width, slope, and gap distance
 Tooth shape to optimize the magnetic field state change
 For embedded magnet disks
 The magnets are tilted relative to axis of rotation by 45-50° to
accelerate the field collapse in the variable reluctor sensor core
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Trigger Disk Eccentricity
Changes in gap distance = changes in flux density
Variable Reluctor Sensor
 Gap change = amplitude modulation of the developed coil voltage.
 Fixed threshold detectors are mostly unaffected
 Adaptive threshold detectors using (amplitude * rate-of-change)
are sensitive to gap modulation, but the jitter will be below
0.05°
Hall Effect Sensor
 Gap change = change in when flux threshold levels are passed
 Fixed threshold & adaptive threshold Hall sensors react
differently
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Trigger Disk Eccentricity
Hall Sensor with Fixed Threshold
 Sees a change in gap as a change in flux density >> timing shift
 Hall switch sensors incorporate their own threshold detectors
• Therefore, a change in gap will translate to a rotational shift
• This will be a linear relationship less than a 0.02°total impact.
 Tooth shape will play a role in how sensitive this will be.
Hall Sensor with Adaptive Threshold
 Uses each peak to set the next threshold level. Even with a 20% change in
gap distance the change in phase delay is insignificant.
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Adaptive Threshold Hall Sensor
 Prediction threshold level correction on tooth-by-tooth basis
From Melexis data sheet
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Helical Gear End Play
(Cam and Distributor)
 If the end play is restricted to the distributor alone, the
system appears as a transverse helical rack. The relationship of
end play to timing error is:
Δ angle per 0.001” of end play = (0.001 * 360)/πPD,
where PD = Pitch Diameter
 Example: Chevy SB, 14 Pitch, 13 Tooth, PD=1.107194”
distributor gear the timing shift is:
 0.103549° per 0.001” of distributor shaft axial movement
 0.067288° per 0.001” of the mating cam end play
 Performance engines
• 0.005” of cam + 0.010” distributor end play = 1.37°
 Street engines
• 0.010” of cam + 0.025” of distributor = 3.26°
 Crane Pro Race & Race Billet distributors with near zero end
play means error is all cam so, 0.005” cam = 0.336°
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VR Replacement
Hall based Crank Sensor
 Flux concentrator used for embedded magnet trigger disk
 Biasing magnet used for steel tooth trigger disk
 Built-in LED to indicate trigger tooth present
 Operational from 6 to 24VDC with reverse polarity protection
 Load dump protected to +-200V
 Does not suffer from accumulated overload damage
 Available in 4-configurations: steel/magnet trigger disk, digital/analog
output
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Trigger Detector
Edge Detection Accuracy & Repeatability
Every trigger detector consists of:
 Input signal conditioning
 Protection from excessive input signals
 Translation between the sensor signal and the digital
needs within the ignition/fuel injection control modules
 Analog comparator which introduces delay in
recognizing a state change: <1usec
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Digital – Points Detector
+IGN
ISO1
OPTO IINTERRUPTER
Digital
trigger
+IGN
+5V
OUT#
+15V
220K
100
D9
47K
DIGITAL INPUT
+3.3V
15K
8
D8
3
2
+
4.7K
LM2903
1
TRIG
-
4
390
OUT#
Points
S1
POINTS
Typical points/digital input detector
Protected against shorts to ground, +Vbatt, or load
dump signals
Sensor Vsat up to +4V is acceptable
 Very high ground noise rejection
 Insignificant timing delay over all RPM
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Variable Reluctor Detector
Circuits
 Fixed threshold
 Single ended
 Differential
 Adaptive threshold
 Single ended
 Differential
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Single-ended VR detector
True zero-crossing discrete fixed threshold VR detector
+5V
R9
9.82K
R10
301K
R11
4.7K
8
R12
24.3K
3
L1
VR COIL
+0.226V
2
1
TRIGGER OUT
-
4
R15
10K
+
R8
301
R38
93.1K
THRESHOLDS:
+0.200V / -0.200V
Simplified circuit
 Negligible timing shift over all RPM
 Sensitive to ground impulse noise & impressed
EMI
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Adaptive Threshold VR
Detector
Example of TI LM1815 implementation
R36
20K
SIMPLIFIED LM1815 IN ADAPTIVE TRIGGER MODE
3
L2
VR COIL
2
R37
3.16K
C13
0.01
1
+
-
PEAK
DETECTOR
+
1
-
3
2
5
6
+
7
-
ONE-SHOT
TRIG
Q
TRIGGER OUT
RC
R34
1M
C11
0.1
R35
182K
C12
0.001
+5V
 Threshold level changes with signal amplitude
and RPM
 Unable to cope with constant amplitude input
without serious timing retard error vs. RPM
 Sensitive to ground impulse noise and
impressed EMI
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Differential VR Trigger Detector
Maxim MAX9924-9927 family are the only integrated
differential input detectors
 Inputs are floated to +2.5V
 True zero-crossing detection largely independent of
signal amplitude
 CMRR ensures all impressed EMI is rejected
 Not affected by ground noise
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MAX9924
 With external support detector can be
configured to support:
 VR
 Digital/points
 All VR mimics
 Both adaptive and fixed threshold modes are
supported
 With suitable stimulus, measurement, and
control the sensor can be identified causing
the MAX9924 to be configured for optimal
support and processing of the sensor’s signal
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How does trigger detection time affect
timing?
 In a perfect world the sensor’s trigger edge would be
processed with no advance or retard error
 Reality, however includes: signal filtering, adaptive
threshold predictive algorithms, and comparator delay
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Dynamic Characterization
of Hall & VR Sensors
 Crank sensor to trigger wheel gap can be adjusted as in vehicle
 Spindle can be set from static to 10,000 RPM
 Sensor gap can be adjusted to centerline of trigger disk with X & Y
adjustments
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Detector – Sensor Results
For a 0.080” sensor – disk gap
RPM vs. ZERO-CROSSING to DETECTOR OUTPUT TIME
10000
Crane HI6 - CRANK SENSOR
9000
8000
CRANE HI6 - VR SENSOR
7000
RPM
LM1815 - CRANE CRANK SENSOR
6000
LM1815 - VR SENSOR
5000
4000
MAX9924 - CRANE CRANK SENSOR
3000
MAX9924 - VR SENSOR
2000
1000
-80 -70 -60 -50 -40 -30 -20 -10 0
10 20 30 40 50 60 70 80
TRIGGER DETECT in usec
ADVANCE -- RETARD
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COMPETITOR - CRANE CRANK
SENSOR
COMPETITOR - VR SENSOR
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Detector – Sensor Results
For a 0.080” sensor – disk gap
RPM vs. ZERO-CROSSING to TRIGGER DETECT OUTPUT
PHASE ERROR
10000
CRANE HI6 - CRANK SENSOR
9000
CRANE HI6 -- VR SENSOR
8000
7000
LM1815 - CRANE CRANK SENSOR
6000
RPM
LM1815 - VR SENSOR
5000
MAX9924 - CRANE CRANK SENSOR
4000
3000
MAX9924 - VR SENSOR
2000
-0.3
-0.2
1000
-0.1
0
COMPETITOR - CRANE CRANK
SENSOR
0.1
0.2
TRIGGER DETECT in DEGREES
ADVANCE -- RETARD
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0.3
0.4
0.5
COMPETITOR - VR SENSOR
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Managing Rotor to
Distributor Gap
 Sparks always jump from a point to a surface, which may not
be the shortest distance
 Electrons are emitted from the negative potential, the rotor,
where the electrical field strength is greatest: a point
 The rotor blade geometry dictates how much of the ignition
energy is lost in the cap via
 Shape
 Edge roughness & sharp corners
 Metal alloy & surface chemistry
From http://tesladownunder.com/tesla_coil_sparks.htm
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Managing Rotor to
Distributor Gap
 Increased spark length = energy wasted as heat to maintain
plasma
 The stator contact should be curved to reduce the
spark gap distance throughout the spark duration(s)
 Cutting back the trailing edge of the rotor blade
ensures the next is the shortest path
 If the spark is unable to slide along the edge of the rotor
blade then the spark duration will be cut short
 If the preferred launch point to stator is too far, timing will
be retarded
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Ignition Wiring
 Spark event in cylinder is really a RF
transmitter within a metal container
 RF energy only flows on metal surfaces
 Braided straps provide the low RF impedance
path for spark current between
 Head(s) to block
 Block to ignition ground. The head bolts
do not conduct the spark current even
though they measure as a DC short circuit.
 CDI recharge demands are at low RF
frequencies that demand both DC & RF wiring
considerations
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Sensor Wiring
 Any connection of a sensor ground to the
engine provides a pathway for ground loop
currents to the measurement circuit
 EMI can be impressed upon any single-ended
sensor signal
 EMI cannot be impressed upon a differential
sensor signal as it will be common mode
rejected on receipt
 Twisted pair or triad ensures that all leads get
the same EMI exposure
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Sensor Wiring
LOAD-2
LOAD-1
BT1
12V BATTERY
ENGINE HEAD
R
R
+
M2
VOLTMETER
R
R
R
+
A
A
-
-
M1
VOLTMETER
 If load 1 = load 2 then Voltmeter M1 is >
Voltmeter M2 because of the addition of 2R
more sheet resistance in the head
 Thus, spark currents will change the ground
potential spatially depending upon the position
and path to ground
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Closing
 Ignition triggering accuracy can be well within 0.1
degree of accuracy over all RPM by choice of trigger
sensor, measurement site, trigger signal detector
circuit, and distributor
 Differential signal processing rejects EMI and
ground loop currents since only the difference is
considered
 Very valuable in noisy engine environments
 Treatment of grounding from a RF basis will greatly
reduce the radiation of EMI signals
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