Heat is heat, whether generated by burning lumps of coal or nuclear fission, and either way a given amount will boil a kettle of water for your tea just as efficiently.
And power is power, whether generated by the natural aspiration of fuel and air gulped in by the greedy cylinders of a great big V-8 or by turbocharged forced induction into a lightweight four-cylinder engine of perhaps less than half the displacement. Either way, a given amount will accelerate your vehicle just as efficiently.
So why do we care whether the engine under the hood has eight, six or four cylinders, like the neat little turbocharged 2.0-litre four-pot unit tucked under the hood of BMW's new-for-2012 528i xDrive sedan.
The logical answer is, we shouldn't. But buying a car is often anything but a logical process and North Americans were brainwashed during the last half of the previous century into subscribing to the car company marketing notion that V-8s offered the ultimate in power and prestige, sixes were only for frugally minded family-sedan-owning drones and fours fit only for the disadvantaged class's tinny econo-boxes.
That wasn't quite the case in Europe, where pretty much everybody would likely still have agreed a V-8 was indeed a wonderful thing if you could afford to put gas in it, but that smaller-displacement, higher-revving sixes and feisty turbocharged fours generally make more sense, even in higher-end models.
But turbocharged fours are the next perceptual hurdle that must be cleared by North American luxury brand buyers as auto makers, facing increasing pressure from governments to increase the fuel economy and lower emissions across the range of products, are increasingly turning to turbo-boost to keep performance at acceptable levels while reducing weight and increasing economy.
Chevrolet uses a turbo to boost its Cruze's output, Ford is EcoBoost-ing its F-150 to record V-6 sales, putting a turbo four in its new Fusion and working on a turbo-three. Volvo has announced it will trim cylinders from its fives and sixes and create three- and four-cylinder turbo-boosted models, turbo-fours power hot Hyundais and Buicks, VWs and Audis, the new award-winning Range Rover Evoque comes with one and BMW has brought back fours to North America for the first time in a decade equipping its X1 crossover and the 528i with the same 2.0-litre unit.
And this is just early days, according to U.S.-based turbo-technology company Honeywell, which predicts turbocharger usage will grow to 23 per cent of vehicles sold in North America by 2016 and to 80 per cent by 2025. Half of Europe's diesels are already turbocharged and 27 per cent of its gasoline engines.
Okay, but how does a four work as the prime mover for one of the most sporting and well-respected mid-size luxury cars on the planet, BMW's redesigned in 2010 sixth-generation 5-Series sedan? Surprisingly well, it turns out.
The first-generation 5-Series of 1972 actually came with a four-cylinder engine, but it was introduced to North America in 1975 with a six and until now has been powered by sixes and eights on this side of the Atlantic.
The engine in the sedans, the $54,500 528i rear-drive and $56,900 528i xDrive (the all-wheel-drive version, is the same 2.0-litre, twin-cam, four-valve turbocharged unit employed in the new X1 crossover. It makes 241 hp and 258 lb-ft of torque, the latter all available between just 1,250 rpm and 4,500 rpm and has an eight-speed automatic to deliver this to the wheels through the all-wheel-drive system.
Comparing the 528i xDrive to the 3.0-litre, 300 hp/300 lb -ft of torque turbocharged inline six-cylinder engined – and pricier at $64,900 – 535i xDrive we find the former weighs 105 kg less and is more efficient. Its fuel economy ratings are 8.8 litres/100 km city and 5.9 litres/100 km highway versus the 535i xDrive's 10.4 city/6.7 highway. My mostly highway and rural roads average was a bit more than 9.0 litres/100 km, and 7.4 litres/100 km at a highway pace in rolling hilly country.
But it's not a lot slower, charging up to 100 km/h in a quick 6.6 seconds while the 535i gets there in 5.9 seconds, a difference you'll never notice unless you're using a stopwatch. Despite my power-is-power comment above, however, there's a little bit more to the engine's contribution to driving pleasure than pure numbers.
Turbocharged four cylinders of the past weren't always the most civilized of engines, with power that often came in with a bang, but BMW's behavioural specialists have polished virtually all the rough edges off this one. No four could be quite as silky as BMW's wonderful six, but this one's rowdier revolutionary impulses have been balanced to the point you'd have to be paying very close attention to notice.
With all that turbo-torque available from just above idle and eight speeds available in the transmission's repertoire, drivability around town is… well, you'll never really notice what's happening most of the time, unless you want to squirt into a moving hole in traffic, which it will be more than happy to facilitate. If you want to step out around someone on a two-lane it will accomplish that safely too. And it's not "busy" at highway speeds, the ample torque enabling it to pull the top taller gears without undue downshifting.
Okay, the six would still be the engine of choice, but a 5-Series buyer definitely wouldn't be short-changing him or her self by opting for the 528i xDrive's four, and with the money saved could afford most of the options that upped the test car's price to $70,250.
2012 BMW 528i xDrive
Type: Sports sedan
Base Price: $56,900; as tested, $70,250
Engine: 2.0-litre, turbocharged, DOHC, inline-four
Horsepower/torque: 241 hp/ 258 lb-ft
Transmission: Eight-speed automatic
Fuel economy (litres/100 km): 8.8 city/5.9 highway; premium gas
Alternatives: Audi A6 Quattro, Cadillac CTS AWD, Acura TL SH-AWD, Lexus IS350 AWD, Mercedes-Benz E-350 4Matic