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Saab Direct Ignition Workings
Saab Direct Ignition
Part 1 of 2: Multi-Spark Firepower!
"We have ignition!" is an exciting phrase in this era of space exploration. Now exciting automotive ignition systems are evolving with that same space age technology which are sparking new levels of engine control and performance.
SAAB, the Swedish automaker, realized the limitations of conventional inductive-type ignitions were restricting further engine development. So they created the innovative Direct Ignition (DI) system to surpass present day requirements and allow enhancements well into the future. DI may have the most intelligent, powerful spark in the realm of ignition science. This first of a 2-part article takes a look at the power and tremendous reserve capacity of the Direct Ignition system.
DI is presented in a simple, powerful package. There is no ignition distributor, no cap and rotor, no visible coil. There are no secondary wires snaking over the engine, no fragile spark plug boots, no moving parts. The source of DI power is the Ignition Discharge Module (IDM) which is bolted between the overhead camshafts and is mounted directly on the spark plugs. The brain of the system is the Ignition Control Module (ICM) which interprets signals from the unique sensing circuitry in the IDM and responds with appropriate commands. The 1989-1991 ICM is located under the driver's seat. 1992 and later ICM's are located in the engine compartment under the plastic bulkhead cover. In Trionic systems, the DI is incorporated into the fuel/turbocharger management computer.
Within the IDM is a 400-volt transformer which charges a bank of primary circuit capacitors. When the ICM triggers the release of this charge, it races through the primary windings of the ignition coils mounted atop each spark plug developing 40,000 volts or more. The cast aluminum housing of the IDM shields the complete ignition process and prevents the powerful electrical circuitry from interfering with radio and computer operations on the car as well as protecting wayward humans.
The DI system is designed to enhance several engine operating parameters. Ignition system saturation time has been limiting engine performance for years. Exotic dual ignitions have been necessary to power high performance V6, V8 and V12 engines because a single inductive primary and secondary circuit couldn't respond fast enough to meet high RPM requirements. The speed of saturation is critical to the effective power delivered across the spark plug gap even at lower RPM. The slower inductive ignition takes much longer to build an adequate charge. This gives the slowly building voltage time to bleed off to ground through secondary wires, distributor caps, rotors and dirty spark plug insulators. The DI system's 400v capacitive primary circuit allows shorter length, heavier gauge primary windings to be used. The lower resistance to current flow and computerized switching allow a much quicker magnetic field build-up and collapse. The DI system's ability to generate the spark in 1/20th the time of a conventional system overcomes the possibility of voltage bleed-down while also providing enormous reserve capacity at high speed.
Spark plug heat range considerations are another major limitation to peak engine performance. If the spark plug runs too cold, it soots up and misfires. So, hotter spark plugs are used to prevent deposit build-up. The hotter plug brings the combustion chamber closer to detonation temperatures and possible internal engine damage. Therefore, the engine output must be restricted to provide a margin of safety. DI enables the use of a cooler spark plug which moves combustion chamber temperatures away from detonation and allows both higher compression and higher turbo boost to be used. DI also uses high technology to keep the spark plug clean. The DI system burn-off function creates a fire storm of more than 4000 ignition discharges to clean away contaminants from the spark plugs. A technician shocked by a conventional ignition at idle receives about seven 9000 volt shocks each second. While in burn-off function, DI creates 840 sparks per second at 40,000 volts each ... for a constant 5 seconds! And you thought ignition systems weren't exciting!
Most importantly, remember the DI burn-off function can occur while you are holding the IDM in your hands with the ignition key off! 1990 and earlier DI systems activate the burn-off function when the key is returned from the cranking position to the RUN position and the engine has not started. The 1991 and later IDM will trigger the burn-off about 5-10 seconds after the key is turned off and the engine has stopped running. For instance, if you are holding the IDM and someone cranks the car over for you, the burn-off will occur 5-10 seconds after the engine stops cranking.
The intense energy released can endanger the IDM itself. Don't trigger the burn-off function without having the prescribed resistor spark plugs installed and securely grounded. It is important that the IDM be positioned upright when testing to avoid overheating and electrical breakdown in the oil cooled and insulated ignition coils. To disarm the burn-off function, disconnect the IDM. The early style IDM has a short wiring harness which runs through two four-pin connectors mounted on a bracket near the battery. The later IDM has a 10-pin connector at the IDM.
In summary, the Direct Ignition System has the capacity to produce exceptionally quick, powerful spark energy. Its impressive reserve capacity assures a stable source of ignition and creates new opportunities for advanced engine performance.
Saab Direct Ignition
Part 2 of 2: Spark of Intelligence!
Part One of this article (last issue) addressed the "Multi-Spark Firepower" of the SAAB Direct Ignition (DI) system. Part Two examines the innovative way SAAB uses the spark plug circuit to read combustion chamber activity. The usual task of a spark plug is to provide replaceable electrodes within the combustion chamber to enable ignition of the fuel/air mixture. Providing spark for a brief moment once every four strokes explains the "spark" part of the device's name. It spends the rest its time being a simple "plug." The DI system has upgraded the conventional spark plug's job description to that of a spark "sensor" by utilizing the spark plug as part of a combustion sensing and synchronization circuit. The spark plug is literally in the middle of everything happening in the combustion chamber, not located on the periphery as are knock sensors and camshaft sensors. The DI uses these spark "sensors" to determine which cylinder to fire when cranking as well as identifying which cylinder needs more fuel or less spark advance while running.
The brain of the system is the Electronic Control Module (ECM). This ECM is referred to as the Ignition Control Module (ICM) with LH Injection, as the DI-APC (Automatic Performance Control) ECM on earlier Turbos and as the Trionic system ECM in later vehicles both with and without turbos. The ECM works with the Ignition Discharge Module (IDM), often referred to as the "cassette," which is the coil-pack that bolts between the camshafts and connects directly to the spark plugs. A hall-effect crankshaft position sensor (CKP) is mounted behind the crank pulley and is connected to the ECM. So, how does it work?
When first cranking, the crankshaft position sensor indicates top dead center for each pair of partner cylinders but cannot tell which cylinder is in the compression stroke and which is in the exhaust stroke. The ECM has the ability to remember which was the last cylinder to fire as the engine was shut off and can use this information to determine which cylinders to fire when cranking. If this information is not available due to loss of battery power and memory, the DI will determine which cylinder to fire by using the spark plugs as combustion sensors. During the first few cranking revolutions of the engine the ignition will supply spark to both partner cylinders at Top Dead Center (TDC) until it can synchronize the spark to the proper cylinder. In addition to sending the actual spark, DI constantly applies a 70-volt reference signal across each spark gap. This 70-volt reference would not normally cause any current to bridge the resistance of the 1.1 mm spark plug gap. However, during the heat, pressure, and turbulence of combustion, the gases within the chamber become highly ionized. The ionized gases create a more conductive path across the spark plug gap allowing the sensing current of the synchronization circuit to flow. This current flow signal informs the ECM that combustion has occurred in this cylinder while its partner cylinder, on the exhaust stroke, does not transmit such a signal. The ECM will discontinue sparking both partner cylinders after the first 25 successful cylinder firings. The system eliminates the extra spark to prevent unnecessary spark plug erosion.
The innovation isn't over yet. As the engine cranks below 850 rpm, the DI will fire double sparks to the spark plug at 10° BTDC to enhance starting. If the cranking speed drops below 150 rpm, such as on a very cold day, the DI will fire multiple sparks from 60° BTDC through 10° ATDC! Once the engine is idling at 850 rpm the single spark returns ... unless the rpm drops below 420 rpm, which engages the multi-sparking again. The "burn-off function" described in Part One will clean and heat the spark plug electrodes if the engine fails to start on the first try.
When the engine is running, the spark plugs continue to send valuable information to the ECM. The circuit now determines the timing and frequency of combustion chamber events such as pre-ignition and detonation. As DI monitors the cylinder, it looks for combustion occurring outside the programmed parameters. If the cylinder initiates combustion BEFORE the spark is sent, DI senses this unprogrammed early combustion as pre-ignition. Pre-ignition is often caused by overheating of the air-fuel mixture by hot spots within the combustion chamber and can lead to melted pistons and spark plugs. Detonation is more commonly known as "knock" or "pinging."
The ECM will inject additional fuel into the pre-igniting cylinder to cool the affected chamber and stop the pre-ignition without decreasing power output.
Detonation occurs when a second flame front may spontaneously ignite AFTER the programmed ignition has occurred creating a hammering effect in the cylinder, which lowers performance and can damage the engine. The ECM will retard ignition timing only on the detonating cylinder while keeping the other cylinders at peak output. Turbo boost, which affects all cylinders, will only be decreased if individual cylinder management fails to limit the detonation.
The innovative Multi-Spark Firepower and Spark of Intelligence possessed by the SAAB Direct Ignition combine to give each engine cylinder the precise management needed to generate maximum performance and economy even in less- than-optimum conditions.
Written by Daniel Winterhalter, IDENTIFIX European
Team Leader. Daniel is certified Saab Master, ASE
Master and L1, with 25 years of diagnostic experience.
posted by 71.184.12...
http://www.ashill.org/saab/di_works.htm
, Sun, 9 Mar 2014 09:34:07
, Mon, 10 Mar 2014 12:47:42, Sun, 9 Mar 2014 12:24:06, Sun, 9 Mar 2014 11:21:51
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