Who makes the Smoothest & Quietest "I-4" (4-Cylinder) Engines?

Question

in other words, which automaker curently offers the Smoothest & Quietest "I-4" (4-cylinder) powertrain available? it also wouldn't hurt if it was pretty decently "peppy" off the line, "flexible" in everyday driving, fuel efficient (no doubt), and most importantly, reliable. i realize that these engines are the prime choice for fuel efficiency, but there are some good V-6 engines out there that are easy on fuel, too (Honda & Toyota come to mind). Nevertheless, i think V-6 engines are (sorry to say it) slowly dying off and losing their appeal to 4-cylinder engines when gas prices are this high. Generally speaking, V-6 engines USED to be quieter, smoother, more powerful, and "sportier" to drive while 4-cylinder engines were seen as being more "crude" with words like "noisy," "rough," "slow," and "boring" coming to mind. i really hope they aren't still like that.

Answer 1

The 2010 Camry 2.5 liter engine would be a candidate.  The smoothness of four cylinder engines relates in part to driver perceptions rather than solely to in-engine vibration.  Many things can create greater smoothness, both real and perceived.

Long connecting rods mitigate the inherent dynamic imbalance.  Dual counter-rotating balance shafts have become almost standard in 2.3 liter and higher capacity four cylinder engines.  Computer controlled hydraulic engine mounts further isolate vibration from the passenger compartment.  Large capacity, multiply-tuned intake and exhaust systems can nearly eliminate driver perception of intake and exhaust sounds. Dual crankshaft vibration dampers reduce crankshaft resonances.  An intermediate engine cradle between engine and vehicle frame adds another level of sound and vibration isolation.  Flexible joints between the exhaust system and the chassis reduce passenger compartment noise and vibration.  Seven and eight speed automatic transmissions allow the engine to run and moderate revolutions where vibration is less intents.  Constantly variable transmissions have thus far done less well, but have great potential. Turbo and super-charging permits high power at lower-vibration crankshaft speeds, and reduces overall engine noise.  Highly rigid engine blocks and crankshafts further reduce engine noise.  So does elaborate crankcase and oil sump webbing.  The oil sump can be elastomerically isolated from the crankcase.  Sandwich steel sheet steel between the passenger and engine compartments further dampen vibration and noise.  So does acoustic glass and premium door seals. Camshaft drives can be by belt or silent (inverted tooth) timing chain rather than gears or roller chain. Toyota features staggered valve opening and closing on some engines to reduce instantaneous noise from valve operation.  Isolated cam covers further reduce sound transmission.  Such things as tight control of stack-up tolerances can add refinement.  Some BMW four cylinder engines seem especially good in that respect.  Resonance tuning is hard to perfect, but very effective in stopping harmonic vibrations. Hydraulic valve lash adjusters keep the valve train quiet over many years, and under all driving conditions.  Such features as extended crankcases and main bearing cap supports can add both strength and smoothness to engines. Lighter weight valves, such as developed by Mahle in Germany lower valve and cam noise.  Those interested in any of the above topics will find many pertinent papers published by the American, German, and Japanese technical societies devoted to mobility engineering.

Overall, the biggest factor is probably the car maker's willingness to focus time, talent, and money on creating smooth, quiet, four cylinder engines.  I expect much progress in the next five years.