Turbocharger Efficiency: An Underappreciated OP2S Advantage

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 losses

Compressor Map

A compressor map describes turbocharger efficiency. Two-stroke engine operation naturally aligns to the best efficiency points of the compressor.

are 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.
Volume Flow-to-Pressure Ratio

The relationship of the two-stroke engine volume flow-to-pressure ratio.

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.
Reduced Port Area

When a two-stroke engine is operated slowly, the intake and exhaust ports remain open longer. The reverse is true when the engine operates faster (upper graph). 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.

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.
Flow to Pressure Ratio

Four-stroke engines need a very broad compressor map to operate over a broad speed range.

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.

4 thoughts on “Turbocharger Efficiency: An Underappreciated OP2S Advantage

  1. Two questions:
    1. I still don’t quite understand the flow rate comparison of 2 and 4 stroke engines. Why can we say 2 stroke flow is independent to engine speed while 4 stroke is not. Using your logic, we can say at lower speed the valves open longer in 4 stroke so more air flows into the cylinder as same as 2 stroke engines.
    2. As a 2 stroke diesel (marine) engineer myself 20 years ago, I understood that turbocharger efficiency is very important for a 2 stroke engine. First, 2 stroke engines have lower exhaust temperature due to dilution of fresh air by scavenge process. Second, the difficulties of pure turbocharging at start and low ilde state because turbocharger can not supply enough air. How does Achates engine solve this issue?

    • Thank you for your questions.
      With regards to flow rate, we make two arguments:
      1.) In a four-stroke engine, the mass flow is dependent on the speed of the engine since at any cycle there is only one filling of each of the cylinders. The frequency of cylinder fillings changes with the engine speed. More flow occurs at higher speeds.
      2.) In a two-stroke engine, the exhaust flow is largely independent of the speed of the engine. Taylor’s book, “The Internal Combustion Engine in Theory and Practice: Thermodynamics, Fluid Flow, Performance”, has a good description of the scavenging characteristics of two-stroke engines, including a formula to calculate the reduced port area (page 244 of the 1985 publication).
      One major difference between comparing the exhaust mass flow of two-stroke engines and four-stroke engines is that because the four-stroke engines have a separate exhaust stroke, all the air that is trapped in the cylinder is sent out the exhaust after the chemical conversion caused by combustion. By contrast, in a two-stroke engine, the exhaust flow can be greater than the trapped mass (if the intake air that short circuits into the exhaust is greater than the exhaust gas retained in the cylinder) or less than the trapped mass (if exhaust gas retained in the cylinder is greater than the intake air that short circuits out the exhaust). Because of this difference, the argument we made in the blog about the effect of engine speed on exhaust flow does not apply to four-stroke engines, since the fluid flow dynamics are different.
      And, with regards to your second question, about two-stroke engine turbocharger efficiency, this is a good topic for another blog. On average, the exhaust temperatures of our engine are more balanced than a four-stroke engine. The high temperatures are lower (because at high loads we have a higher dilution of fresh air in order to manage piston and cylinder thermal loads) and the low temperatures are higher (because at low loads we can retain exhaust gases in the cylinder in order to inhibit NOx formation and reduce pumping work). This is a nice characteristic – catalyst formulations can be more efficient if they don’t have to serve a broad temperature range. We published our exhaust gas temperatures (both pre- and post-turbine) at different speed/load points in this paper: http://www.achatespower.com/pdf/modernizing_the_opposed-piston_two-stroke_diesel_engine_for_more_efficient_commercial_vehicle_applications.pdf.
      We add a supercharger to our engine, in addition to a fixed geometry turbocharger. The supercharger provides the positive pressure differential between intake and exhaust necessary to start the engine, and also provides good transient response. We also avoid the high cost of a variable geometry turbocharger.
      Gerhard Regner

  2. I understand that your engine design allows for highly efficient operation. 45-55% thermal efficiency is quite impressive. I am just wondering if other technologies like mild hybrids with an integrated starter generator and turbo compounding are technologies that could be mutually beneficial or if they would be exclusive technologies. I know you use a supercharger to moderate the airflow of the engine during operation which is why I am asking. If they aren’t exclusive though an electric turbo compounder could generate electricity to run the starter generator which could boost vehicle efficiency another 15-25% which would be amazing in combination with your engine.

    • Colby, Most technologies that can be used to improve efficiency of vehicles and engines can be additive to our OP Engine and would further increase the efficiency numbers.

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