When it comes to the performance and efficiency of an opposed-piston, two-stroke engine, does the number of cylinders make a difference? If you guessed “yes”, you’re right.
Based on extensive analysis, Achates Power has determined that its three-cylinder, opposed-piston engine is the optimal design from a gas-exchange perspective, especially when compared to a two- or four-cylinder design. The reason: the gas exchange duration in a two-stroke engine is about 120 degrees crank angle. In a three-cylinder design, the scavenging events are aligned in a way that they have minimal interference with each other and still keep enough mass flow going over the cycle to provide adequate energy to the turbocharger so that it operates most efficiently to compress the intake air.
While two-, four- and five-cylinder options are all viable as part of a comprehensive family of engines, a three-cylinder, opposed-piston, two-stroke design is optimal. This three-cylinder design is just one of the many thermal efficiency benefits of the Achates Power A48 engine. Last month, we highlighted the importance of our powertrain’s stroke-to-bore ratio in a post called: Stroke-to-Bore Ratio: A Key to Engine Efficiency. And, in the coming months, we’ll cover other thermal efficiency strengths including the two-stroke architecture, the opposed-piston design, and the patented Achates Power combustion system.
Achates Power
Fundamentally Better Engines
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Oooh, the A48. It has a name! That’s a sign it is getting closer all by itself..
So excited for this design.
Hi,
in Your diagram it can be seen, that the exhaust port shuts somewhat _after_ the intake port. I was thinking, that one of the great benefits of OP design over conventional 2S is, that shutting the exhaust port _before_ the intake port can be achieved and so supercharging is at all possible. Here supercharging pressure seems to be limited to the intake pressure of the supercharger turbine (because it is always connected to the compressor exhaust via the cylinder) and its function is more or less reduced to drive scavenging. Is that correct?
Dominik:
First, the port diagrams we used in the blog post are illustrative only. The actual port timing of our engine designs varies.
Second, you are correct that if the exhaust closes before the intake, there is a supercharging effect if the intake manifold pressure is higher than the cylinder pressure after the exhaust port closes. To find the optimal port timing, there are a complex set of interactions including supercharger work, turbocharger work, scavenging effects, air mass, mean effective pressure, and heat transfer to pistons and cylinders that have to be considered. We have extensively studied various port timing configurations, and the optimal configuration varies based on cylinder count, stroke/bore ratio and other factors. As we design application-specific engines, we take all of these factors into account as we optimize the design for the application.
Gerhard Regner
Thank You for Your answer.
Are You considering variable port timing for some future version of the engine (e.g. by varying the angle at the crankshaft or by variation of the port lengths)? IMHO this would be useful to adapt to different speeds. Low speed needs approximately same time for scavenging but less crankshaft angle.
I know this is somewhat far off.
Dominik:
As I’m sure you can understand, we’re not able to comment about our future plans too much; however, the ability to vary port timing is interesting.
Gerhard Regner
Hello.
I am interested in your comment about gas exchange for 3 cylinders Vs 4 cylinders.
You will certainly get lower scavenging efficiency / power with port interference if the manifold design features short branches into a plenum or you have an exhaust design that produces back pressures. Rootes used long exhaust manifold branches with near atmospheric manifold pressures to great effect, as per P. Schweitzer guidelines. Interestingly, the power curves of the Rootes TS4 prototype (4 cyl opposed piston 2-stroke) show a marked improvement across the rev range over the Rootes TS3 (3 cyl OP 2-stroke) using the same stroke, bore and fuelling.
The exhaust manifold design with the TS4 is very clever, featuring not only long branches, but also expansion chamber characteristics. In driving an exhaust impeller, I would guess this is creating back pressure in your exhaust manifold which is interfering with scavenging. Rootes stayed away from turbochargers in their production OP engines because the back pressure interfered with (Kadenacy) scavenging efficiency and also created higher piston crown / fire-ring / exhaust port temps etc. They did build a turbocharged / scavenge blown prototype, which was road tested over long distances at heavy loads, but this was more to prove a point to Chrysler in the late 1960s than any other reason.
Is it possible to use a waste-gated scavenge blower (to bring in greater scavenge pressure at higher revs) on your prototype and modify your exhaust port timing and / or piston phasing to increase trapping efficiency so you can abandon the turbocharger? This would then let you move on to 4 plus cylinder prototypes / greater power density without the scavenging efficiency problems?? Or does the power required to drive the scavenge blower at higher rpms / pressures eat up any power gains from revised design?
Also – are you testing a twin rocker beam design (TS3) yet – I would be very interested to follow that development. Best Wishes.
Mark:
Thank you for the detailed comments. The effect of cylinder count on efficiency was specifically targeted around an air handling system that includes a turbocharger to increase the charge density while minimizing pumping loss and maximizing fuel efficiency. Our engine designs are primarily intended for applications that are highly sensitive to fuel consumption, so we have optimized the design for maximum fuel efficiency.
It is possible to design variants of our engine without a turbocharger, particularly where we can take advantage of the Kadenacy effect. In these configurations, a four-cylinder engine with manifold branches may not incur the same efficiency penalty.
With regard to the rocker beam design, we are pretty open about releasing our test results when they are ready. And, the best way to find out about the latest Achates Power engine developments is to opt-in to our email newsletter.
Gerhard Regner
Thanks – yes I assumed that was the case.
The Rootes TS3 OP engines were very efficient and durable with astonishing fuel efficency and very low friction / pumping losses.
Even the early engines were getting .35 to .40 lbs of fuel per hour per HP with well made but antique, low pressure mechanical F.I.E. in the mid 1950’s.
With 10 years of pre-production development, and not even a calculator to use (they didn’t exist) Rootes made 54,000 X 3 cyl production engines between 1954 to 1968 with legendary performance and 14 X 4 cyl prototypes.
Were it not for Chrysler – makes you wonder where their OPE development would have been today.
Thanks for your suggestion – I’ll join the newsletter.
Regards, Mark.
Most new automatics have a lock-up touqre converter, so once cruising speed is achieved there really is no power loss. Look at it like this, either unit is going to have parts that wear if you have a automatic transmission and something goes wrong, a full re-build is typically $ 2000-$ 3000. You rarely see failures in the manual transmissions, usually the clutch wears out.. it’s $ 200 $ 300 in parts, and most shade tree mechanics can swap it out in a weekend easily. You can also gear down coming down this hill you live on which will use the motor as a brake and extend the life of your rotors and brake pads. I prefer a manual myself other than sometimes it’s harder to drive and eat at the same time.Good Luck.
I’m pleased to see substantially innovative efforts toward improving Brake Specific Fuel Consumption or BSFC rather than usual tweaking efforts toward trying to incrementally improve traditional mainstream engine design strategies. Your fine effort certainly is NOT just another tweaking effort on a traditional design strategy which my eventually be revealed to become as obsolete as steam locomotives were made by diesel-electric locomotives. You group deserves wide recognition and applause. But trust must be earned. Hundreds of engine development efforts based on their designer’s models and their assumptions have claimed the ability to outperform all or at least most other engine designs based on their specific design goal set. Since many designer goal sets exist, many may simultaneously “best” due to their differing goal sets. Your site repeatedly states or at least suggests that your design effort is to maximize shaft work output compared to fuel BTU input. At least that’s my reading’s interpretation of your loosely stated goal set. So how about publishing some prototype BSFC maps? Generally the lowest BSFC performances recorded to date have been produced by enormous engines with nearly 8-foot strokes and cylinder diameters large enough for human occupancy. If you can approach those low BSFC figures with a much smaller REAL TEST ENGINE through clever design, publish those test results and be prepared to demonstrate those tests to skeptics. If you can do that with a durable design that can pass Tier 4 emissions requirement, buyers will come to you. So PLEASE, publish some BSFC maps. Improve your proof to claims ratio.
John:
Thank you for the comment. As you note, there have been, and continue to be, many claims for improved engine performance that never quite materialize. That is why we have published our technical results for the last two years (http://www.achatespower.com/opposed-piston-engine-technical-papers.php), and commented on this almost a year ago in this blog (http://www.achatespower.com/diesel-engine-blog/2011/11/21/diesel-engine-data/). The most recent publication (http://www.achatespower.com/pdf/modernizing_the_opposed-piston_two-stroke_diesel_engine_for_more_efficient_commercial_vehicle_applications.pdf) provides detailed performance and emissions data across a wide range of speed/load points (please reference Table 3 on page 18). We have also published fuel maps (http://www.achatespower.com/diesel-engine-blog/wp-content/uploads/2012/11/2012-DEER-Poster_Achates-Power.pdf), but I think you’ll find this table to be more informative because, in addition to BSFC, we provide ISFC, friction loss, pumping loss, NOx, soot, CO, HC, air/fuel ratio, noise, peak cylinder pressure, burn duration, EGR rates, inlet and outlet temperatures, and even oil consumption. Try getting this amount of data from anyone else!
To be clear: this data is measured on a single-cylinder engine and the multi-cylinder results are modeled. The basis for the model is described in the paper. Because a model is involved, there is still some uncertainty but we are quite confident in the model. It has been reviewed by several different organizations and each has agreed that, if anything, it is conservative.
As you can see from the figures in the table, our engine is more efficient—by a wide margin—than anything else in its class and can meet the toughest emissions standards around—Euro 6, Tier 4 and U.S. EPA 2010—with conventional aftertreatment.
Larry Fromm
Vice President, Business and Strategy Development
Achates Power
how to improve the BSFC in 3 cylinder
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