More fuel for the debate, so to speak.

When comparing turbo and non-turbo diesels, there is a variable that has not been included yet; valve timing.

With enough turbo 'boost' or airflow, you can leave the exhaust valve open longer, removing the products of combustion completely AND cooling the combustion chamber. Then, when the valve closes there's still enough air in the airstream to fill (overfill?) the chamber. Purge, squirt, boom, repeat.

Two-stroke diesels, such as the well-known Allison/Detroit Diesel line would be useless without tubos ramming a lot of air through for cooling, because with combustion and power on each stroke heat will build up that coolant and oil flow can't take away.

I look forward to more discussion.

BCingU, Jim

I too hope adding a little fuel to the fire does not cause excessive heat to build up on this thread....but I can not contain myself.

I believe some of the controversy in this discussion is the result of qualitative terms being mixed in with technical, mostly quantitative terms, which leaves open a lot of interpretation.

For example, the torque and horsepower vs. engine speed curves for the turbocharged 300D differs from the normally aspirated 300D by enough to suggest they are not the same engine, which is the point of adding a turbocharger. So, regardless of the details of combustion theory, the turbocharged machine can burn more fuel at optimum combustion conditions than the normally aspirated machine. Probably enough more by looking at the differences in the performance curves that you could estimate the displacement of a non-turbocharged machine based on the normally aspirated design of the 300D engine would be nearer 4.2 liters to produce the same results (a 7 cylinder engine? Imagine that!).

Given the basic efficiency of the whole operation inside the combustion chamber is about the same (combustion process, internal friction, windage and other losses) the turbocharged engine is making more heat, and the heat is what gets turned into power, but heat is produced first, in the same package as the non urbocharged engine. The process for converting thermal energy to mechanical energy is identical, since the mechanical parts are nearly identical, so the efficiency of that step is essentiallly the same.

The design challenge then is to use the same basic geometry of the 3 liter engine and still manage the thermal performance of the machine so it does not damage itself, as with the higher heat load left at the end of the cycle (combustion to torque) the cooling system will be challenged. It is kind of like a light bulb rated at 30 Watts reaches a surface temperature of some number of degrees (likely painful to touch), but a light bulb of 100 Watts using the same basic technology and materials to turn electricity into heat and then light, will produce more heat to produce more light. Given the same external surface to reject heat, but more heat input, the equillibrium temperature reached on its surface must be higher (not clear it will hurt more to touch, but it will likely do more damage).

Without doing something to manage the additional heat "left behind" either the head and components suffer or the pistons suffer or both. As long as there is no damage to anything I do not believe you can call the added heat load for the cooling system "exessive" since it has been accounted for in the design. To do this Mercedes elected to use the underside of the piston as another heat exchange surface. They may have changed the valve timing to blow some clean, relatively cool air through the combustion chamber as noted above, but I am not sure. They may have increased coolant flow in the head and block (I do not know this either, but comparing pump impellers and the pulley diameter that drives the pump might give an indication) or any number of other boring things. They advertised the cooling oil jets that spray the bottom of the pistons probably because someone thought they were kind of cool in the marketing department. In any case, by effectively changing the displacement of the engine without changing the geometry of the hot parts, Mercedes established conditions where damage from heat could occur and in that case I would call the heat excessive. They did some things to manage the thermal challenge this posed, some of which we know very well and other stuff we don't know so well. We do know it works great, so there is no excessive heat as a result of adding a turbo to a Mercedes-Benz 300D.

The nitrided crank is a result of higher loads on the bearings. Once again the geometry could not change much without changing the piston spacing, while the average load probably went up more than 25%. With the bearing design rules developed for their engines, this made Mercedes slightly uncomfortable. To make the expensive part last longer under these higher stresses a number of changes were made. Once again we all heard from marketing literature that the crank was nitrided. Nitriding makes the surface harder to better resist wear. Depending on how it is done, nitriding should have little effect on the bulk mechanical properties of the steel (It achieves its results by diffusing nitrogen into the metal matrix at high temperatures so nitrides are formed. These nitrides swell the volume of the surface layer, typically under around 30 mils, which puts the layer into compression in the bearing journal geometry, achieving a sort of preload against fatigue. It also usually raises the surface hardness substantially, which adds resistance to wear). In this case the higher load in the bearing Mercedes was addressing by nitriding had to do with the pounds per square inch acting on the surface of the bearing journal and the cyclic loading of the these surfaces. This higher load most likely occurs in the power stroke as the rest of the cycle should not be that much different. If the bulk properties of the crank were changed, or the diameter of the critical crankshaft dimensions changed to deal with the higher internal stresses in the crankshaft, we probably don't know about it because the marketing department did not think it was cool. But someone can dope it out by looking at drawings of the two crankshafts or the bearing caps and I would bet they are not interchangeable.

The idea behind tubocharging is to get more power out of a given displacement engine. The benefits are usually related to an improved overall thermal efficiency, lower weight, fewer parts, and lower cost for the added performance compared to adding cylinders. The cost benefit can be thwarted by the need for better materials and low production rates. But you almost always get the other benefits in a well executed design. If you buy a Mercedes Diesel that is turbocharged you also get excellent reliability and longevity because there is no excessive heat. This is not a given in all other machines that have been turbocharged.

Jim

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Own:
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

Owned:
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)

Hey guys,

What an interesting thread.Just think;it all started because some guy wanted to know if he could flip his air cleaner cover over.I must appologize to the group for thinking radicals last night;yes,the term is nitride.I have another question though.For quite some time I have been trying to find the difference between a turbo and non turbo OM617 cylinder head.I think I've measured everything and I can't find any difference.They look identical to me.If I could lay 2 heads in front of someone out there and that someone could tell which is which,I'd like to know how.Thanks.

Peter

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