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Old 11-13-2007, 07:30 PM
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Join Date: May 2002
Location: Blue Point, NY
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Quote:
Originally Posted by JimSmith View Post
I spot you the part throttle to full throttle discussion. When you are "engine braking" though, you most likely are at "no throttle" with your right foot ready to apply brakes. In that position on older Diesels, and I am not sure if this didn't exist up to the end of the 240D production, there is a throttle plate that blocks off the intake and produces a significant intake manifold vacuum. It made vacuum pump diaphragm failures at idle spectacular smoke events that were innocuous at any throttle position other than idle.

There is always blowby, from day one. For discussion purposes, lets restrict the blow by to the same leakage geometry since, for the most part the rings and cylinders and pistons are made with the same manufacturing methods. If both engines have an equivalent leak path from day one, the losses in the Diesel are than the gas engine because of the higher pressure differential.

What the figure is new, at 50k miles, 100k miles, 150k miles and so on is likely going to favor the gas engine as the miles build if for no other reason than they burn cleaner fuel. No soot and combustion by products preventing free movement of the rings or at least way, way fewer. So, you have to put more energy into the compression stroke and you get less back on the power stroke. That and a Diesel at idle barely tics over. In fact, at idle a gas car is significantly less efficient than a Diesel, one of the reasons they are so popular in city driving in Europe.

Anyway, this is another sum of many issues, not the least of which is the gearing of a Diesel. Jim
If you restrict your discussions to diesel engines with manual transmissions that have a throttle plate, you've got.........by far and away........the best possible machine to provide significant engine braking. Such a machine would have serious benefits in terms of brake torque as compared to any gasser. You've got the combination of pumping losses from the throttle plate, higher frictional losses from the greater mass of the rotating components on the diesel, and some additional pumping loss due to blowby.

In my previous analysis, I've encompassed the typical 617 engine with auto transmission...........a setup that provides significantly less brake torque than the aforementioned. The 617 has no throttle plate and there is no direct connection to the rear wheels. Yes, there is some increased friction and there is some benefit with regard to pumping losses.........but, these benefits won't stack up to an equivalent gasser with a throttle plate. The pumping losses due to a throttle plate are significantly greater than the pumping losses around the rings and the increased friction of the diesel due to its greater rotating mass.

Therefore, your experience with the m/t 616...........while perfectly valid..........is somewhat at the fringe of the discussion.

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