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L850 Compression Sense Ignition By Jim Garrido Copyright © 2002 James Garrido |
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The L850 is an aluminum 4 cylinder engine with 4 valves per cylinder. This engine uses a Speed Density based fuel injection system along with a “wasted spark” based electronic ignition system. The focus of this course will be to introduce to the automotive technician population the unique “Compression Sense Ignition” system this engine employs. Compression Sense Technology enables the Powertrain Control Module to determine proper engine phasing (Cam Position) without the use of a separate Camshaft position sensor mounted near a rotating engine member. Part 1 will be an overview of how Compression Sense Ignition operates as a system. Part 2 will cover some new and exciting misfire diagnostic techniques this new system enables by virtue of its unique design. The Compression Sense Ignition (CSI) system uses a modular style design to house nearly all the major ignition system components in a single cassette
The ignition cassette is mounted directly over the spark plugs with only a connector spring and insulating boot to transfer the ignition energy to the spark plugs. The cassette houses 2 ignition coils. Each coil sends ignition energy to two paired cylinders at the same time, one cylinder while on its exhaust stroke and the other cylinder on its compression stroke. Cylinders 1 and 4 are paired on one coil as running mates and cylinders 2 and 3 are paired on the other. Above the ignition coils is mounted the lead framed CSI signal plate which serves as a structural member of the 4 CSI input signal capacitors. The CSI signal plate output node is connected through the cassette cover to the above mounted Electronic Ignition (EI) module by a dual female connector called an inter-connect. The EI module houses the CSI signal processing electronics called the “Compression Sense Time Out” (CSTO) chip as well as the usual primary drivers and current limiting electronics.Before explaining how the CSI signal is acquired and processed to form the CAMOUT (Engine phase) signal to the PCM we should first cover a few of the more in depth characteristics of how “Wasted Spark” style ignition systems operate in general. These operating characteristics are important to understand because these characteristics are what make up the “information” contained within the CSI signal. SPARK POLARITIES
The polarity characteristics of the spark events are one part of the information reflected in the CSI signal. COMPRESSION SENSE? As a DIS ignition coil releases its stored energy an electric field is created in both secondary circuits of the paired cylinders. This growing electric field creates a voltage potential across both of the spark plugs electrode sets simultaneously. Over a period of around 10 microseconds this voltage level becomes high enough to reach the “Breakdown” voltage level. Each spark plug gap breakdown level is determined by the individual spark plug gaps impedance value. At the breakdown point the air gap between the spark plugs electrodes will ionize and quickly conduct current establishing a spark. One of the important characteristics of this event is the voltage level at which any spark plug gap impedance will reach Breakdown. The greater the spark plug gap impedance the greater the voltage level that must be reached in order for the plug gap to breakdown. The greater the voltage level required in order to reach breakdown, the greater the required time to reach this voltage level. Spark plug gap
impedance’s are dependent upon several factors one of which is in-cylinder pressure at the time of spark generation by the coil. A
cylinder on its exhaust stroke has less in-cylinder pressure than does a
cylinder during its compression stroke. A DIS coil releases its energy to
two cylinders at the same time, one cylinder while on it’s exhaust
stroke and the other on its compression stroke. However due to these
uneven pressure related impedances their respective spark plug gaps will
not breakdown at the same instant. The spark plug gap in the cylinder on
its exhaust stroke will breakdown first before the spark plug gap in the
cylinder on the compression stroke under most engine operating conditions.
The order of the spark plug gap breakdown events for the running mate
cylinders is yet another characteristic that is reflected in the CSI
signal. THE CSI SENSOR The L850 utilizes the unique “Compression Sense Ignition” sensor to detect the two above stated secondary ignition event characteristics. The way this is accomplished is by creating virtual capacitors between the secondary coils and the EI modules electronics.
These capacitors act as a 'probe' of the ignition secondary circuits and are utilized to bring the secondary event characteristics into the ignition module to be processed.
One side of these capacitor plates is connected to the ignition
secondary outputs. The other side is connected to one end of a resistor
network. The other end of this resistor is connected to ground inside the
EI module. As current flows on the capacitor plates a voltage is created
on this resistor. The
voltage pattern measured across this resistor is what makes up the
information in the CSI signal. This measuring resistor, since it is combined with a capacitor, also serves to create a first order “high-pass filter” network. This high-pass filter tends to reject the signal created by the rise of the secondary firing voltages and only allows the higher frequency edge of each plug gap breakdown voltage to pass through it for measurement. Refer back to Fig 3, for when either cylinders #1 or #3 are firing on their respective compression strokes. As either of cylinders #4 or #2 fires first on their respective waste stroke, there will be a high-speed transient voltage change from the positive peak toward zero volts which is passed across the CSI measuring resistor in the form of a rapid voltage change in the negative direction. Then as either cylinders #1 or #3 fires on compression there will be a high-speed transient voltage change from the negative peak toward zero volts which is passed across the CSI measuring resistor in the form of a rapid voltage change in the positive direction Refer back to Fig 4. Conversely, for when either cylinders #2 and #4 are firing on their respective compression strokes. As either of cylinders #1 or #3 fires first on their respective waste stroke, there will be a high-speed transient voltage change from the negative peak toward zero volts which is passed across the CSI measuring resistor in the form of a rapid voltage change in the positive direction. Then as either cylinders #2 or #4 fires on compression there will be a high-speed transient voltage change from the positive peak toward zero volts which is passed across the CSI measuring resistor in the form of a rapid voltage change in the negative direction. The CSI signal will reflect the polarity and timing (thus amplitude) of each the cylinder’s spark plug breakdown voltage event. Now that we have seen all the various pieces of information contained in the CSI signal, as well as the method in which it is acquired lets pull it all together and see how it is processed.
The EI module houses the CSI input signal logic electronics called the “Compression Sense Time Out” chip (CSTO). The CSTO electronics are responsible for interpreting the CSI input signal and creating a 5V square wave output called a CAMOUT signal. The CSTO chip logic works as follows, the first EST rise of either cylinder pair awakens the CSTO circuitry at which time the CSTO chip looks for the CSI input signal. The CSTO chip recognizes unique characteristics of the CSI input signal, then decides whether to output a CAMOUT high or CAMOUT low signal to the PCM When the 1/4 coil fires, as cylinder #1 is on compression, the event will generate a negative going, then positive going CSI signal. This negative then positive CSI signal will cause the CSTO chip inside the EI module to output a CAMOUT high signal. When the 1/4 coil
fires, as cylinder #4 is on compression, the event will generate a
positive going, then negative going CSI signal. This
positive to negative CSI signal excursion will cause the CSTO to output a
CAMOUT low signal In the same manner as above, as the 2/3 coil fires, while cylinder #3 is on compression a negative to positive CSI signal will generated which will cause the CAMOUT to be switched high. As the 2/3 coil fires, while cylinder #2 is on compression a positive to negative CSI signal will be generated which will cause the CAMOUT to be switched low. PCM LOGIC This system utilizes a variable reluctance CKP sensor mounted in the engine block near the crankshaft. The crankshaft has seven machined target notches, six of which are evenly spaced apart. The 7th crank notch is positioned about 50 degrees ATDC of cylinders 1&4 and is used by the PCM as a sync pulse. The PCM uses the CKP sync pulse notch to identify when the 2/3 coil is due to be charged. The engine always starts firing the 2/3 coil first during cranking. Charging of the 2/3 coil always begins near the 2nd crank notch. Charging of the 1/4 coil always begins near the 5th crank notch. Once the ignition process has started with the 2/3 coil the PCM will look for the sequence of CAMOUT signals from the EI module in order to determine engine phasing. The PCM will accomplish this by sampling the CAMOUT state (high or low) near the 3rd crank notch, coincident after the 2/3 coil fired near the 2nd notch, and sample again on the 6th notch coincident after the 1/4 coil fired near the 5th crank notch. After 2 crankshaft revolutions all 4 cylinders will have fired and generated the 4 CAM ID bits. These sequences of high and low CAMOUT signals are read by the PCM as a series of 4 data bits a, 1 bit if CAM is high and a 0 bit if CAM is low. The bits will be in the order of 1001 if cylinder #3 came up on compression first, or 0110 if cylinder #2 came up on compression first, upon initiation of ignition during cranking. The Tech 2 scan tool can display this series of bits as a PID called “Calculated Compression Output” (CCO). The Tech 2 will actually read 8 bits but only the lower 4 bits are used. The first 4 bits always remain as zeros. Considering the fact that in cylinder pressures effect spark plug breakdown voltage levels and therefore the timing of the breakdown events, the PCM needs to take into account engine operating conditions during which the in cylinder pressures for the running mates can be nearly equal. During deceleration the pressure of the compressing cylinder can be as low or lower than that of the cylinder on its waste stroke. This condition would of course render the CSI signal information invalid. For this reason the PCM will only consider the CSI signal valid during certain MAP ranges. We have now seen the how the Compression Sense Ignition system works as a whole as well as the inherent information the CSI signal provides. Next time we will explore some time saving misfire diagnostic techniques this new system can provide for us by reading the CCO bit patterns in conjunction with the individual cylinder misfire PIDS. In the mean time feel free email me any questions you might have regarding this article or better yet post your questions on the International Automotive Technicians Network (www.iatn.net) and I will see you there!
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