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post #4 of Old 02-25-2010
klem
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The information is not "out of date" but I there are a few points that I disagree on. The real advances recently have been in the fuel injection, not on the turbo side (it is somewhat effected). In the automotive world, variable vane or geometry turbos are becoming common and they are a different beast but most boats still use fixed blades. In my opinion, a moderately sized turbo makes a lot of sense on just about any diesel engine. Really large turbos will have too much lag and produce a lot of boost on the top end which increases cylinder pressures to undesirable levels. A moderately sized turbo will do a really good job of providing the necessary air to the cylinder.

The first place that I would disagree is on fuel consumption. If you look at a thermodynamic diagram, the biggest loss for turbodiesels is that they do not fully expand the gases in the combustion chamber (you would need infinite stroke). This means that the gases exiting the cylinder are still hot and can be used to power a thermodynamic device (turbo). By having a turbo, you ensure that you have enough air to fully burn the fuel in the combustion chamber and you can get more power out of a smaller displacement. Done properly, you will get better fuel economy out of a slightly smaller turbocharged engine versus a naturally aspirated one.

The other place that I would disagree is the carbon buildup. The exhaust needs to mix with something to lead to a really bad buildup and it is usually water or oil. A turbo will not introduce either of these to the exhaust stream (it has a problem if it is introducing oil). On an engine with a large laggy turbo, you can get a puff of smoke on throttling up but the engine tends to be less sooty in general than a naturally aspirated one due to improved airflow. I have never seen any exhaust tubing clogged due to having a turbo but I have seen plenty of piping clogged due to things like crankcase breathers being plumbed in.

In my opinion, turbos have made such large improvements for diesel engines (this is especially true for OTR engines). I have worked on a lot of both and the turbocharged engines do not give me any more problems than the naturally aspirated ones.
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