Theres all manner of considerations in engine construction... metallurgy is
a great part but techniques in "building" our alloys, machining techniques,
casting and forging techniques, internal tolerances and oil
control/chemistry are a few of the other considerations.
In the case of aluminum heads. Thermal stability is much different than
other metals used in the engine. Aluminum heads are a PITA to seal to a cast
iron block... enter the torque to yield bolt and multilayer steel shim
headgaskets (not to mention some pretty tight tolerances in regards to
machining finish and surface flatness.
Aluminum blocks are almost a conumdrum.... currently, technology is still
having us use iron sleeves. An aluminum piston just wont live in an aluminum
bore. Speaking of pistons... the modular engine pistons are hyper-eutectic
but they are still aluminum. Gone is the deeper skirt found on old
technology engines (hence the prevalence of piston slap on cold engines) but
there is a (I forget the high-tech term) super slippery insert on the major
and minor thrust surfaces of the lower portion of the piston.
Crankshafts... cast iron is still a bonafide material.... relatively cheap
to cast or forge close to finished dimensions.... machining and balancing
required. Forged steel, much stronger, same weight... much more expensive to
build.... interesting note.... they can be machined from a great big steel
billet to finished shape... or they can be machined from a half sized steel
billet (looking like a flattened out crank) and twisted to shape. Dome
properly, the latter yields a much stronger crank but they are difficult to
produce and spendy, spendy, spendy.
Todays engines are more "purpose built".... Grandads old Buick Roadmaster
had this great, huge lump of cast iron in the engine bay.... You could abuse
this thing all weekend, take it back and Grandad would never know the
difference. Todays motors are very good, but only if we don't exceed design
intentions.... overheating will kill a modern motor very quickly..... ditto
some of the other abuse we seem to be able to hepa on these machines.
For fuel economy, the gasoline Ottocycle engine has gone about as far as it
can.... fuel management is the big concern but emissions reductions
sometimes means we can't expect less gasoline to pass through the motor. The
Escape hybrid uses an (IIRC) Atcheson or Atkinson cycle engine... same
design as the Ottocycle but the valve timing has been drastically altered to
reduce fuel consumption.... However, it also reduces the usable power band
of the motor. As the CVT transmission gains acceptance, I think we will also
see even more changes to the engines powering these vehicles.
My own outlook sees the diesel engine as being the "next big thing"... new
management systems that have them running quieter, sulfur content in fuel is
being mandated at lower levels, reduce particulate emissions and easier fuel
computations being some of the driving factors.
For the most part, it is a complex dance between mileage, power output and
FWIW.... in our harsh northern climate, we see more new technology motors
lasting 300,000, 400,000 and even 500,000 kilometers than we ever saw with
old technology motors.