Why 0.5-Liter Cylinders Will Soon Dominate Automotive-Engine Design

A decade or so ago, a team of German university scientists settled on 500 cubic centimeters as the

ideal per-cylinder displacement for internal-combustion engines. A 500-cc cylinder with a stroke length larger than its bore diameter, these forgotten pioneers concluded, benefits the combustion process by minimizing the engine’s internal surface-to-volume ratio as the piston nears top dead center. Seeking to optimize power and fuel efficiency while reducing emissions, the three local powerhouses—BMW Group, Mercedes-Benz, and Volks­wagen Group—promptly joined the 500 club with boosted, direct-injected, long-stroke engines. Fiat Chrysler, Jaguar Land Rover, and Volvo are also applying for membership. Additional brands will surely follow, in part because 500-cc-displacement intervals align nicely with existing global taxation statutes. While hybrids and electrics will assist the move to better mileage with lower emissions, updated versions of Nikolaus Otto’s 139-year-old engine—many with 500-cc cylinders—will continue to do the heavy lifting.

Thus far, BMW is the most enthusiastic proponent of the 500-cc cylinder, with half-liter-based three-, four-, and six-pot gas and diesel engines either in production or about to be introduced. BMW’s newest family of TwinPower Turbo gas engines, which launched in the 2015 Mini Cooper Hardtop, shares its 82-mm (3.23-inch) bore and 94.6-mm (3.72-inch) stroke dimensions, plus direct fuel injection, variable intake- and exhaust-valve timing, and variable intake-valve lift systems. This modular approach enables 60-percent commonality of the component parts across three gas engines and 30 to 40 percent with the corresponding two diesel engines.

It’s not just the 500-cc displacement that matters, but also the dimensions that yield such volume. While a cylinder with a smaller bore than stroke (a so-called under-square design) contradicts the classic approach to maximum power at stratospheric rpm [see “Different Strokes”], there are compelling reasons for this move. A small bore shortens the flame travel needed to consume the fuel-air mix and diminishes quenching of the flame at the bore’s periphery. With smaller piston crowns and a more compact combustion chamber, there’s less heat lost to the cooling system. Smaller bores trade shorter overall block length for a modest increase in height, handy for jamming potent engines into tight confines.

One hardship enthusiasts must bear is slightly lower redlines. In exchange, we get better mileage (when we can resist the urge to pin the throttle) and enhanced flexibility attributable to the broader spread between torque and power peaks. Considering the global statutes in place mandating higher efficiency, we’re happy that Otto’s engine has more life left in it.

Different Strokes

Here’s a look at two different approaches to making power. Mercedes-Benz’s 6.2-liter AMG V-8 is the last gasp of the old way, but it’s unlikely to survive until 2025. Its replacement is the twin-turbo 4.0-liter new-wave V-8 spreading throughout AMG’s lineup.