When I met with David W. Anderson, manager, Bar & Rod Applications, American Iron & Steel Institute (AISI; Southfield, MI) about the use of his materials of interest for powertrain, I was somewhat mystified: After all, let's accept that blocks and heads are going aluminum for cars and that intake manifolds are being increasingly made with plastic. Magnesium is sometimes heard about. Titanium seems to be a material of interest—although let's face it: with titanium we are talking about comparatively exotic vehicles, not the run-of-the-road sorts of things that most of us roll in.
Given that the non-iron/steel blocks, heads and manifolds are comparatively recent developments, evolutionary results of recent time, chances are, they're off the table for consideration. But isn't just about every other structural component in an engine made of one of the AISI's preferred materials?
As Anderson explains, the “Iron” in AISI's name notwithstanding, they look at cast iron, a material with significant content in a typical North American engine (at least a North American engine not produced by, say, Honda) as a competitive material. As such, there are plenty of opportunities available for a switch from cast iron (and powder metal) to steel.
Which is, of course, of interest to people who are looking to sell bar stock. But what is the benefit for those who are in the business of producing engines?
Well, according to AISI, which has conducted sponsored research into this area (e.g., a case study titled “Steel's Technical and Economic Progress in the Production of Lighter and Smaller Engine Components”), there is a distinct advantage of forged steel vs. cast iron: steel is stronger. This means several things. One is that when it comes to small, high-performance engines, where strength is a requirement, steel is capable of meeting the requirements handily. Given the strength/weight ratio vis-à-vis cast iron, steel permits the downsizing of components, such as a camshafts and crankshafts.
Krupp Gerlach (Homburg/Saar, Germany) did a study of a forged steel crankshaft compared with a cast crankshaft for an actual engine. In the study they determined that the total length can be reduced by 9% and the weight by 8%
Obviously, if one is to reduce the size of one of those components, then it is sensible to reduce the size of associated components. This leads to an explanation of why it may take some time for a greater utilization of steel in engine applications: Engine programs tend to be somewhat infrequent and long-in-becoming.
There is a perceptible but invisible advantage of steel, too: it has a high ratio of yield point to tensile strength, a relationship that is germane to rigid-ity. Because of this characteristic, there is better intrinsic dampening in a steel crankshaft, which means less vibration and noise: a smoother-running engine.
In addition to which, static and dynamic stiffness are said to reduce the distortions and excitation of moving parts. Consequently, the engine is able to work at producing useful power, not to overcome those vibrations.
Given the reduction in weight and the improved operating performance, it is argued that forged steel crankshafts can be used to advantage from the standpoint of increasing the fuel efficiency of vehicles, no small manner nowadays when the price at the pump seems to be heading north all too frequently.
Powder metal (PM) had made its move into powertrain component manufact-uring by offering a marked advantage: the parts are produced with the process to near-net shape. This means, of course, that less machining is required. Which meant that the production of things like PM con rods were less expensive to produce than steel con rods. Interestingly, steel had been the dominant material in that area, but when faced with the opportunity to reduce the amount of required machining, PM took over.
Anderson says that there is now the opportunity to switch back to steel because of process (e.g., impact forg-ing) and material developments (vanadium microalloys and air-cooled forgng steels). Essentially, there is now near-net forging capability, which reduces the amount of required machining, and the materials have improved machinability, so whatever machining needs to be done can be done more efficiently than was previously the case. Also, Anderson says that there is re-peatable weight performance achieved through the improved forging process, which means less con rod balancing than can be the case with PM.
(While we talked primarily about engine applications, Anderson noted that transmission gear sets—especially in the next-generation transmissions that tend to be compact, with more forward gears, and which generate greater load—are a place where steel shines.)
Apparently, the relative steel-intensiveness of European and Asian engines tends to be greater than that of North American engines. Anderson and his colleagues at AISI think that if the advantages of steel are taken into account for things like camshafts, con rods, and crankshafts, there could be a considerable increase in the amount of steel employed in powertrain applications.
|Comparison of Material Use|
Con rod material:
Con rod weight:
Total steel weight:
150 hp @ 5,200 rpm
Nodular cast iron
Nodular cast iron
150 hp @ 5,700 rpm