</font><font color="blue" class="small">( </font><font color="blueclass=small">( I've worked for the big 3 in the diesel engine business )</font>
Since you are involved in the design processes of engines (I've not) may I ask you that what type of stress is the most important stress, most important determining factor for the life span of the engines? Without thinking much, I'm assumin.g time-dependent thermal stresses, i.e. termal cycles as called in the public, will play the major role in the engine life. Right?
But I don't see the use of appropriate materials that will resist thermal stresses much in engine components like pistons, cranks, etc. For ex., as far as I remember, Nickel, a relatively expansive material, isn't more than 2-3% in some engine components like the crank which are under heavy thermal cycle stresses. Why is this so? I think, in the engine designs, the lower cost criteria comes first before the longer life criteria? Am I missing anything here? Is Nickel percentage low because its relatively higher ductility that won't be so good against the thermal cycles? Or, is it low just to reduce the cost of engines? )</font>
I'll try to answer this - I've given this presentation probably a hundred times and I think it's easier said than written out.
When designing an engine we are concerned about three things, cylinder temperature, cylinder pressure, and bearing load. To a certain degree temperature and pressure are related, bearing loads we can design for. Using the best design tools we have, and a lot of experience we manipulate injection pressure, nozzle patterns, injection timing, and piston design to give us an engine that is the best compromise between life, efficiency, and performance. We try to keep the cylinder temperature and pressure at the point of best flame front in the cylinder without causing the piston (or other components) to self destruct. It's a fine dance figured out by guys with really thick glasses and slide rule calculators on their belts.
Bearing loads are easy to figure out in comparison because we know the forces and the square inches of mating surface we have to work with and the wear rates of metals. Knowing that we design accordingly.
As far as use of appropriate materials, trust me, we try all kinds of things to increase engine performance and life. In the on-highway truck world fuel efficiency is measured down to the thousandth's of lbs fuel used per BHP. Yup, that's right, 3 decimal places. And orders for thousands of engines are based on those numbers combined with life to overhaul. Thermal cycling in engines isn't as big a deal as it used to be.
Most of the thermal problems in the past were coolant and oil leaks caused by the engine expansion and contraction and gasket failure. By switching to O rings and different sealing technology I think we are down to something like 3 gaskets in the engine now, and we don't have leaks.
I'm not a metalurgist, so I can't say why we don't use nickel in the engines, but my guess is that a forged and machined crank and connecting rod are the strongest, most cost effective components we can get for the job they have to do. Sure there is always a tradeoff between cost and performance, same reason I drive a Blazer and not a Hummer - do I want a Hummer, sure it'd tow like a bat out of hell, can I afford it and the additional gas cost? No. Actually in one instance we used one the best bearing technologies available (sputtered bearings-sorry I don't want to describe the process) and they caused more problems than a standard tin overlay bearing we had used for years.
Todays engines are worlds better than the ones I started with back in the mid 80's. They live longer and have much better performance.
