This document discusses an opposed-piston, two-stroke diesel engine developed by Achates Power that can meet future fuel efficiency and emissions regulations. Achates Power was founded in 2004 to design cleaner, more efficient engines. Testing of their 1.6L single-cylinder engine showed indicated thermal efficiency of 53.2% and emissions performance within regulations. Modeling predicts their multi-cylinder heavy-duty engine can achieve up to 15% better fuel efficiency than conventional engines through design features like reduced heat transfer and optimized combustion. The document addresses how Achates has overcome historical challenges for this engine type through innovations in fuel injection and piston design.
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Hinweis der Redaktion
So here’s our game changer.Like so many other “new ideas”, ours is an old and proven idea that needed to be dusted off, re-invented, modernized. Problems and challenges that we, we the industry, thought were insurmountable, had to be addressed. That’s what we’ve been up to since 2004. The authors of this 2010 book had a similar good idea. We appreciate their affirmation of the quest our founder initiated in 1998 and which became Achates Power in 2004.
Opposed piston engines are not new. The first two stroke opposed engine in fact dates back to the late 1800’s. In fact, the many design iterations of opposed piston engines have been captured in a recent SAE publication by JP Pirault and Martin Flint, from which I borrowed this illustration.The first thing you’ll immediately notice about opposed piston engines is that there is no cylinder head. Instead, they essentially look like two bottom halves of a conventional engine somehow “stuck together”. So let me explain quickly how they operate because it is not obvious.-- When the pistons come together, fuel is directly injected and as the case with every compression ignition engine, it autoignites.-- This then creates the expansion event during which time force on each piston is translated by the crank-slider mechanism into torque. A geartrain connecting together both cranks then transmits this torque to the engine’s output shaft.-- As the distance between the pistons increase, intake and exhaust ports (essentially holes in the cylinder liner) are exposed… and part of the combustion gas is expelled. A crank drive supercharger provides the boost pressure or “muscle” to drive the exhaust gases out and replace it with fresh air. This process is called scavenging.-- When the ports close (with the exhaust closing a bit earlier than the intake), there is a supercharging or pressure building event in the cylinder and this then begins the compression stroke which brings the pistons together, thereby increasing cylinder pressure.One of the most well known compression ignition two-stroke opposed piston engines was the Junker’s Jumo, which set world records for fuel efficiency.Unfortunately, it suffered from durability and poor oil consumption… so much so that when women in Germany would hear the distinctive sound of a Jumo Junkers plane overhead, they would run outside to remove their clean clothes from their clotheslines.
A quick re-cap of how this works:Reducing the combustion chambers surface area to volume ratio reduces heat losses, increases efficiency.Leaner is better. We run leaner than the best diesels.Shorter and better timed combustion is more efficient. Ours is. And we match pressure rise rates of benchmark engines, as well as engine out emissions.Efficient breathing… no throttling, ports that breath more freely than poppet valves, efficient uni-flow scavenging, especially at part loads…
So the first questions you may ask are:-- What virtues do all opposed piston engines have in common.. . And why?, and-- What special features on the Achates engine set it apart from other opposed piston engines?First, let’s discuss fuel efficiency.As discussed earlier, all opposed piston engines have no cylinder head.-- On non-opposed-piston engines, you need a cylinder head with poppet valves for the gas exchange. An opposed piston engine simply moves gases in and out of the combustion chamber through ports in the cylinder liner.-- Each unit of diesel fuel has a given energy value.-- When combusted, that energy goes into either work or heat. That heat can go into coolant system or out the exhaust.-- When you have a cylinder head, you add a heat path… more heat to coolant… and that reduces the work transmitted into the engine output shaft.-- Without that heat path on an opposed piston engine, an increased percentage of fuel energy goes to the output shaft and that means increased fuel economy.Secondly, in a conventional four-stroke, there is a pumping loop… essentially you have half the strokes (an exhaust followed by an intake stroke) used to evacuate the combustion gas from the cylinder.-- To achieve low NOx in a conventional four-stroke engine, this exhaust gas is then cooled and recirculated back into the intake.-- In our case, when we want more external recirculated exhaust gases, we can simply reduce the boost pressure from the supercharger. That is, leave the combustion gases only partially scavenged.-- And yes, we do have an external recirculated exhaust gas loop on our engine as well, but it is there only to manage trapped temperatures in the cylinder.-- This is an advantage for all two stroke engines.Thirdly, our Achates opposed piston engine also has a relatively long stroke-to-bore ratio… notably between 2.2 and 2.6 depending upon the application… which reduces its surface area to volume relationship versus other engines.-- This also reduces the percentage energy from the fuel that finds a heat transfer path to coolant.-- Unfortunately, you can’t do this with a non-opposed-piston or cylinder head engine, because with such a stroke-to-bore ratio, you would end up with a high mean piston speed, small bore diameter, small valves… and therefore the engine can’t breath well enough.Also unique to the Achates Power two-stroke, we have done much work using the latest computed aided engineering tools to get the combustion right. By this, we mean getting the right combination of cylinder liner ports, injector spray patterns, piston bowl shapes, and calibration to get a very rapid combustion with the right timing while maintaining an unacceptable rate of pressure rise.-- Some not familiar with our work have made statements about not being able to “make good combustion take place in a pancake shaped combustion chamber”.-- To this, I would just say “we don’t do it in a pancake”… and with the freedom we have with two pistons coming together and two opposed injectors, we actually have shown opposed piston engine architectures to be a combustion advantage.-- More precisely, we have been able to get large λ (or air/fuel ratio) = 1 isosurfaces at the point of autoignition… with the right combination of tumble and swirl charge motion to ensure rapid propagation of the diffusion flame.-- That plus the fact that we expand more rapidly (1/V dV/dT) than with a single piston… so we don’t have quite the same concern with maximum rate of pressure rise… which is a limiting factor due to noise.Our engine also offers the advantage of lower NOx. -- Torque is a function of BMEP (or brake mean effective pressure) times displacement. -- On the lower left, the x-axis is engine displacement volume and the y-axis is BMEP or (brake mean effective pressure).-- Keep in mind that BMEP is related to peak cylinder pressure and incylinder temperatures… and that nitrous oxide generation is a function of high pressure and high temperature.-- The blue line shows possible combinations of BMEP and displacement that result in equivalent torque output to a medium duty four-stroke diesel of roughly 6-1/2 liters of displacement. -- Since a 2 stroke fires on every crank revolution (compared with a 4 stroke that fires every other crank revolution), we can achieve the same power at either 1.) the same 4-stroke BMEP and half the displacement or 2 2.) the same 4-stroke displacement and half the BMEP.-- In execution, we have chosen a “sweet spot” for fuel efficiency and package somewhere in the middle. -- The result? Lower BMEP, lower peak cylinder pressures, lower temperatures less NOx-- And yes, we see some increased friction from going in at a larger displacement than running with the 4 stroke BMEP, but we more than regain the friction penalty by having to pump less exhaust gas during our scavenging.Lastly, why do have lower cost and weight? Well the biggest contributor is the lack of cylinder head with all its valvetrain, camshaft, lubrication, and cooling components.
And our advantages are demonstrated, documented and highly scrutinized…About 10% better at the best pointAbout 20% better on a cycle average basisWe are approaching 50% BTE on a very small engine and without any exotic or expensive technologies, such as waste heat recovery.And that’s versus the very best diesel engines with comparable performance on all other attributes, torque, hp, emissions, noise…
Considering therefore our definition of game changing technology, the OP2S fits the definition:Big improvementLess costNo back sliding on other attributesCompatible with conventional engine improvement roadmap