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Old 04-28-2003, 07:36 PM
JimSmith JimSmith is offline
Join Date: Jul 2001
Location: Woolwich, Maine
Posts: 3,598

Diesel fuel ignition and combustion is not the same for all Diesel engine designs. In broad terms there are two main kinds used in automotive applications, the units with prechambers like your car and the bulk of the other Diesel cars in the USA (called indirect injection because the injector squirts into the prechamber and not the volume on top of the piston), and the new, direct injection styles where the fuel is injected into the volume bounded by the piston top, the head surfaces and a small measure of the cylinder bore in the block.

In your engine the glow plug heating element is inside the prechamber as well. The prechamber is "connected" to the volume above the piston by an array of small holes. As the piston moves up in the compression stroke, the volume inside the prechamber is pressurized by air rushing in through these small holes. At the point of injection the compression of the total volume of the air charge is at a level high enough to provide a temperature that will self ignite a thoroughly atomized stream of Diesel fuel. This stream of burning, hot gasses shoots back out the little holes and continues the burning process in the compressed air volume above the piston and outside the prechamber.

This concepts provides some control of the explosive forces of the ignition event, sort of spreading it out over time a little bit. It reduces noise and vibration, but adds particulate emissions, and reduces efficiency.

If you add to this event some blobs of engine oil, not well atomized and arriving with the charge of air coming through the intake manifold, you can imagine they will not be easily passed through the prechamber vent holes in the compression stroke. As a result they kind of "lie in wait" for the flaming hot gasses shoot out after ignition in the prechamber. If there is enough of a volume, of sufficiently large droplets, they snuff out the flame front rather than keep it going. This will lead to those misses and a burp of blue or even white smoke. As the engine heats up, the engine oil being drawn into the combustion chambers gets better prepared to burn and the bluish color begins to diminish. At load the turbocharger pressurizes the intake manifold and the path to suck liquid oil into the intake manifold is converted to a path to have air from the intake, pressurized by the turbo, squirt into the valve cover. This air adds to the engine blow by volume and can lead to oil being dragged into the intake air filter.

So the simple answer to your original question is, yes, anything other than what was intended to be in there can cause misses and smoking. Engine oil is not intended to be in there in a measureable quantity.

As for the glow plugs not having oil on them, they are shielded by the prechamber vent holes, which do not promote admission of stuff of a density higher than air, and, once the engine warms up the temperatures inside the prechamber will be high enough to burn off most anything combustible. At those temperatures engine oil would burn off quickly.

The fact that the oil arriving thereafter, with engine under load, comes through another path where it is much better prepared to be burned (it is intended to travel through a couple of paths to remove droplets of significant mass in the valve cover and the cyclone separator in the path to the intake manifold so only vaporized oil and other gasses make it into the intake manifold) adds to the probability the glow plugs will not get wet like a fouled spark plug.

Modern Diesels with direct injection achieve control over the ignition/combustion event by electromagnetically activating the injectors, which draw fuel from a highly pressurized header (called a common rail), and go through a series of openning and closing events per power stroke. The quantity of fuel, and its injection sequence is "designed" to achieve the best possible compromise of emmisions, noise and vibration, power output and efficiency for every given load and transient condition. Thus the "CDI" designation for Common rail, Direct injection and Intercooled on the new MB units.

Hope this helps, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Last edited by JimSmith; 04-28-2003 at 07:57 PM.
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