BMW has spent decades refining the four-stroke combustion engine, but the quest to reduce emissions while maintaining performance is never-ending. A newly discovered patent at the EUIPO reveals BMW’s latest solution: using mismatched pistons within the same engine. Traditional methods, like cylinder deactivation, improve fuel efficiency by shutting down half the cylinders, but they come with trade-offs—higher nitrogen oxide emissions, increased stress on active cylinders, and potential reliability concerns. High compression ratios offer another efficiency boost but pose similar durability risks. BMW’s innovative approach seeks to optimize combustion without these drawbacks. Here’s how it works.
Cold Expansion for Greater Efficiency
Not every cylinder in an engine needs to perform ignition—meaning it doesn’t always require a dense air-fuel mixture and a spark (or, in the case of diesel engines, extreme heat for combustion). BMW’s patent introduces a system where an ignition cylinder, responsible for the primary combustion, is linked to an expansion cylinder via a two-way valve system. This expansion cylinder receives some of the exhaust gases, using the crankshaft for precise timing to optimize energy transfer.
In this setup, the ignition cylinder absorbs most of the explosive force needed to drive combustion, while the expansion cylinder endures less strain and requires less energy to keep moving. This redistribution of workload could lead to greater efficiency while minimizing wear on key components.
This concept, known as cold expansion, involves using expansion cylinders with longer connecting rods on the pistons, specialized materials for the ignition cylinders, and unique piston and valve designs. By carefully balancing the torques on the crankshaft and optimizing timing, the engine can function efficiently whether operating at partial or full capacity.
Thanks to the different torques on the crankshaft, and some very careful timing, hardier cylinders could do the work when half the engine is shut down, and would still be more efficient even when the entire engine is running. Interestingly, BMW notes that this would work best in engines with odd cylinder counts, although two-, four-, six-, eight-, and 12-cylinder engines were also mentioned.
Faster Warm-Ups for Lower Emissions
Another key advantage of this design comes from the aforementioned two-way valve system. During cold starts, the expansion cylinders can redirect exhaust gases back to the ignition cylinders. BMW plans to position catalytic converters and other emissions-control systems directly behind the ignition cylinders, allowing them to warm up faster and become effective sooner.
This setup offers multiple benefits, including improved efficiency, reduced emissions, and potential cost savings. With global hesitation toward fully electric vehicles, innovative solutions like this could extend the life of internal combustion engines—meaning you might see this technology in a BMW engine sooner than you think.
