There are a number of factors that contribute to the inherent thermal efficiency of the opposed-piston engine. Often, however, turbocharger efficiency is an overlooked and underappreciated advantage of opposed-piston, two-stroke engines (OP2S) like ours. Due to the two-stroke cycle, the OP2S has a natural fit to the high efficiency points of a turbocharger’s compressor map.
Turbochargers are used in all clean diesel engines—and, increasingly, in gasoline engines—to improve engine performance and powertrain efficiency. They are comprised of two main parts—a turbine, which is spun by the exhaust gasses from the engine (almost free energy since it would otherwise be sent out the tailpipe), and a compressor, which compresses the incoming air. By compressing the intake air, more air can be put into the combustion chamber. With more air in the chamber, more fuel can be added. So a turbocharged engine delivers more power than a similarly sized, naturally aspirated engine. Because of the power boost provided by a turbocharger, engines can be downsized and still deliver excellent performance. Downsized engines usually deliver improved vehicle fuel economy because the engine operates at higher efficiency regions during the normal drive-cycle, and also because smaller engines tend to have lower frictional losses.
But, like everything, turbochargers are less than 100% efficient. Turbocharger lossesare subtracted from ideal engine efficiency. The efficiency of a turbocharger is described with a compressor map, like the one to the right. The x-axis is the air flow rate—how much air is entering the compressor—and the y-axis is the pressure ratio—the ratio of the air pressure after the compressor compared to the air pressure before the compressor. The contours in the compressor map describe the compressor efficiency. The middle island is the most efficient, at 77%, with efficiency falling in each successive contour.
The characteristic air flow in two-stroke engines naturally aligns with the best efficiency points of the compressor. However, this isn’t the case for four-stroke engines.
Let’s look at a four-stroke engine first. Four-stroke engines have a dedicated compression stroke. The same volume of air is compressed during each stroke. The volume of air flowing through the engine increases with the density of the air (or pressure ratio) and with the engine speed. This relationship is depicted in the chart to the left, with separate lines for different engine speeds (n).
The practical consequences of this are that four-stroke engines need a very broad compressor map in order to operate over a broad speed range. This can be achieved, but at the cost of lower turbocharger efficiency and greater turbocharger cost. Another consequence is that four-stroke engines are often operating at low-efficiency points on the compressor map, rather than at the peak efficiency points.
By contrast, two-stroke engines are flow engines—the air volume flow rate through the engine increases with the density of the air, but it’s independent of the speed of the engine. When the engine is operated slowly, the intake and exhaust ports of the engine are open longer so more air flows during each engine revolution. When the engine operates faster, the ports are not open as long, so less air flows during each engine revolution. This relationship is depicted for two engine speeds (in the upper graph on the right). The lower graph shows the integral over time of these two curves, demonstrating that the reduced port areas stay the same regardless of the engine speed. The reduced port areas combine the port areas of intake and exhaust together and replace them with a single orifice.
The relationship of the two-stroke volume flow-to-pressure ratio is depicted in the graph to the left. By comparing the two-stroke compressor map to the turbocharger compressor map above, two favorable features are evident:
- The compressor map can be very narrow for a two-stroke engine because it doesn’t need to accommodate increases in volume flow with engine speed. A narrow compressor map designed for a two-stroke engine can have a higher peak efficiency than the broad compressor map required for a four-stroke engine.
- The two-stroke engine operating points are aligned with the peak efficiency curve of the compressor maps, resulting in overall engine efficiency improvements in actual, real-world operation.
Improved turbocharger efficiency is just one more benefit of the Achates Power opposed-piston, two-stroke architecture that—when combined with the engine’s inherent thermodynamic advantages—gives the OP2S engine a significant edge over its four-stroke counterparts.