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Re: Plugs/DI Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Wed, 2 Feb 2005 07:28:51
In Reply to: Re: Plugs/DI, Paul Broeckx, Wed, 2 Feb 2005 06:42:36 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Once a coil starts leaking, two bad things happen-
The first is that the oil acts as a coolant. Without it, the coil can overheat and short. Second, oil is a better insulator than air. A spark can jump in air, but not as easily in oil. Once the coil starts to lose oil and 'gain' air, the chances are better for sparks to jump internal to the coil. That causes more overheating and more damage. And it creates carbon tracks for the current to follow. All this leads to no spark out of the spark plugs, which is why there are DIs.
Why is 70% worse than 90%? 90% is full - the coils always came with a little air gap at the top. Anything less than 90% means that the coil is leaking. Leaks don't heal, and since there is nothing replenishing the oil supply, the level will only drop. The speed of the drop depends on the crack size. I wouldn't say that 70% is OK - the coil will still work - it means that at some point, tomorrow or next month - it'll be 60%. If you're not bleeding, that's a good thing. If you are bleeding and the bleeding isn't stopping, do you not worry until you hit 60% blood remaining?
I have no hard facts, but I'll bet that leaks can come from both cracks and pinholes. The coil is sealed in plastic, but there are electrical connections coming in and out. Vibration and thermal cycling over time will make those joints open up - I'd call those cracks. Any time two things meet, something can leak out between them. I don't think you'd see a major crack in the plastic - it's more a very thin seam between the metal plate and the plastic, where a little oil can weep out. It can be very slow, it's got time. As to pinholes, internal arcing can punch a pinhole in the side.
The DI is designed to discharge a spark through the spark plug. If it's discharging inside the DI, that can come to no good. That can (will) cause internal damage to the coils and circuitry. The drive circuit is designed for certain voltages; internal arcs are usually due to higher-than-normal voltages. This stresses components. Why higher-than-normal? If the spark plug gap is too big, under boost it takes more voltage to form the spark. The coil voltage builds until the spark jumps - higher-than-normal. And if the spark doesn't jump at the plug, it'll jump inside, causing damage.
There has been a lot of talk about DIs - a nice one at last November's Tech Session at Saab with John Moss. Saab and the DI supplier has probably done some in-depth root cause analysis. That information isn't readily available. I'm going by near-30 years experience in the business of electronic system fault and root-cause analysis, but it is still speculation on my part. But I'll bet that the DI issue isn't a single problem - that would be fixed. It's a bunch of little problems, and you never know which one (or group) will get you. But the environment of the DI is about as bad as it gets - wide temperature swings, from a frigid morning to sitting on top of a hot engine. The thermal environment is about as bad as it gets. And vibration, the other killer of electronics, is as bad as it gets - it's mounted on top of a box filled with regular explosions. Oh yeah, add controlling high voltage - 30-50Kv, and you've got a receipe for disaster. Saab isn't the only one with DI issues - note the recent (1-2 years ago) recall by VW on all their DI coils. All it takes is one minor mis-step, one questionable design decision, and all hell breaks loose.
posted by 192.249....
, Wed, 2 Feb 2005 06:17:56, Wed, 2 Feb 2005 07:28:51 <-- Viewing This Message
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