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Latest test results for Achates Power two-stroke opposed-piston engine show 20% improvement over four-stroke diesel; BTE of 45.1% at best operating point

Single-cylinder Achates research engine installed in test cell. Source: Achates Power. Click to enlarge.

Achates Power, the developer of a two-stroke, compression-ignition (CI) opposed-piston (OP) engine announced that the latest test results for a 3-cylinder Achates engine configuration indicate a 20% improvement in cycle-averaged brake-specific fuel consumption (BSFC) when compared to a recently introduced advanced medium-duty four-stroke diesel engine: 192.6 g/kWh BSFC for the Achates engine, versus 239.9 g/kWh for the reference engine.

This represents a 7% improvement since September 2010. Achates Power has demonstrated newly increased engine fuel efficiency results in every quarter of 2011, beginning with a 13% efficiency advantage presented at the Symposium on International Automotive Technology (SIAT) in January, a 15.5% efficiency advantage in June, and a 19% efficiency advantage presented at the SAE Commercial Vehicle Engineering Congress (ComVEC) in September. (Earlier post.)

Specs for medium-duty Achates engine
Max power 47 kW/cylinder @ 2400 rpm
Max torque 240 N·m/cylinder @ 1600 rpm
Number of cylinders 3
Displaced volume 1.06 L/cylinder
Stroke 212.8 mm
Bore 80 mm
Max BMEP 13.6 bar
Trapped compression ratio 16.7:1

Achates Power measures single-cylinder combustion results and then uses the interface model to predict multi-cylinder engine performance for an operating range typical of an engine in a medium-duty commercial vehicle.

Dr. Paul Miles, a Distinguished Member of the Technical Staff at Sandia National Laboratories and co-chair of SAE powertrain activities, approves of this method Achates Power employs to determine engine performance, fuel consumption and emissions characteristics.

Achates Power employs industry-standard instrumentation and methods to benchmark its engines at multiple engine load and speed points representative of regulatory test cycles. The single-cylinder results are then carefully extrapolated to expected multi-cylinder results using a rigorous interface model. These are real-world numbers. The outstanding fuel economy and emissions it reports are a testimony to the effectiveness of the fundamental research and development work I have seen at its San Diego facility.

—Paul Miles

Brake thermal efficiency of 45.1% was also demonstrated at the best engine operating point with an overall calibration that meets the US EPA 2010 emissions standard. An earlier closed-cycle simulation study with Dr. David Foster from the University of Wisconsin found that the opposed-piston 2-stroke Achates engine could show an indicated thermal efficiency of 53%. Achieving the indicated closed-cycle efficiency will require further improvements in combustion optimization, pumping work, mechanical friction, and the power consumption of engine accessories.

As confirmed by dynamometer testing, the Achates Power opposed-piston architecture with the two-stroke cycle has improved brake thermal efficiency resulting from a combination of the following four effects:

  • Reduced heat transfer due to the more favorable combustion chamber area/volume ratio of the opposed-piston architecture;

  • Increased ratio of specific heats due to the leaner operating conditions of the two-stroke cycle;

  • Decreased combustion duration achievable within maximum pressure rise rate limits due to the more rapid expansion of the in-cylinder volume-per-crank degree angle; and

  • Reduced pumping work as only a portion of the residual gases in the cylinder need to be scavenged at each cycle.

The thermodynamic rationale for these results is documented in the SAE International Paper 2011-01-2216, presented at SAE ComVEC on 14 September. The most recent 20% fuel efficiency improvement is from ongoing enhancements, including the latest hardware upgrades and calibration improvements and more than 2,500 hours of testing at the company’s San Diego facility.

When compared to the published performance data of one of the best medium-duty clean diesel engines in the world, the Achates Power engine demonstrates:

  • 20% lower cycle average brake-specific fuel consumption;

  • Similar engine-out emissions levels;

  • Less than 0.1% fuel-specific oil consumption; and

  • Reduced cost, weight and complexity

Through the application of rigorous science and engineering methods, Achates Power has overcome historical two-stroke engine challenges. Achates Power uses sophisticated models and powerful computers to analytically solve the complex combustion processes of the opposed-piston, two-stroke cycle.

—David Foster

Foster is a professor of mechanical engineering at the University of Wisconsin-Madison, an Achates Power technical advisory board (TAB) member, and an expert in the field of internal combustion and fluid dynamics.

The Achates Power TAB includes National Academy of Engineering members and SAE Fellows with more than 200 years of combined experience. Achates Power is backed by Sequoia Capital Partners, RockPort Capital Partners, Madrone Capital Partners, InterWest Partners and Triangle Peak Partners. It was founded by Dr. James Lemke with investment from the late John Walton, son of Sam Walton, the founder of Wal-Mart.


  • Randy E. Herold, Michael H. Wahl, Gerhard Regner, James U. Lemke, David E. Foster (2011) Thermodynamic Benefits of Opposed-Piston Two-Stroke Engines (SAE 2011-01-2216)



According to Ferdinand Piech (former VW group CEO), the VW 3L engine (in Lupo 3L & Audi A2 3L) had an efficiency of 45.1% at the best operating point. Contemporary passenger car diesel engines are subject to more compromise against other objectives and seldom exceed 43 % efficiency. The efficiency of heavy-duty engines has gone up and down due to emission legislation for the last 20 years and had an all-time-high peak at ~46 %. Current US HD 2010-compliant engines do not fully match this number but I have not made any survey to determine the exact level.

The UK Rootes 3-cylinder opposed-piston 2-stroke engine had difficulties achieving the same BSFC as the class-leading Gardner and Leyland engines during the same time period.

None of the comments above are particularly valid for a new concept but let’s conclude that it is difficult to match current production engines and even more difficult to improve on those numbers. A 20% improvement would require that you compare with something you do not find on the road but rather in a museum somewhere. Four factors that have positive effect on BSFC were mentioned. There is no problem in finding four negative factors as well. Maybe someone on this forum is interested in identifying those factors…

Note that I find an opposed-piston engine very interesting but the results posted here do not impress me very much. I will take the time to read the SAE Paper later...


Are talking 45.1% versus 45.1%, which one is the best?

Roger Pham

Even though the peak efficiency of the Archates Power engine is the same as the most recent heavy-duty TDI, the cycle-averaged efficiency efficiency of the Archates is 20% better.
Translation: The Archates has better part-load to low-load efficiency than a modern comparable 4-stroke TDI. Reasons: Lower engine friction due to less piston movement required in a 2-stroker,no valve train friction, much lower crankshaft friction due to the inherently balanced simultaneous push and pull actions of the connecting rods to the opposing pistons on both sides of the crank bearings...

I don't know if Archates has provision for separating the 3-cylinder engine into 3 separate independent modules like in the Ecomotor OPOC design, wherein only 1-2 module(s) operate(s) at near optimal cylinder pressure when less torque is needed. A little more complication in the drive train, but can result in even better part-load efficiency in city driving when cruised at ~30-40 mph than current 6-cylinder HD TDI.
At 60 mph cruise, a 40-ton tractor-trailer needs ~150 kW to cruise, from a 300 kW engine, meaning that the trailer can cruise at the engine's peak efficiency. However, at 30-40 mph speed, the trailer needs only 50-70 kW to cruise, thus operates well below its peak efficiency. With the 3-module Ecomotor design, only 1 module is run at peak efficiency, while the other 2 modules are resting, thus the trailer operates most of the times at the engine's peak efficiency with optimal combustion and much reduced friction.

Finally, the Archates engine got 45.1% peak efficiency at a BMEP of only 13.6 bar and CR of 16.7 instead of modern TDI requiring 20 bar BMEP and CR of 19 for the same peak efficiency. That's quite an achievement because the engine will likely meet the latest NOx emission without expensive SCR, and without DPF, another costly item, or at least without much regeneration of DPF due to the partial-mix combustion regime from higher EGR rate.


Dr. Paul Miles of Sandia Lab / SAE and Dr. David Foster of Wisconsin know that the most impressive number about Achates is the “Less than 0.1% fuel-specific oil consumption”.

If this is true, this opposed-piston “crosshead” 2-sroke engine has less lubricating oil consumption than the best 4-strokes.

In this case the question to Dr. Miles and Dr. Forest is what is the advantage of this engine over the OPRE and PatOP engines at in terms of anything that makes an engine superior or inferior.

If the claim of “Less than 0.1% fuel-specific oil consumption” is a lie but the rest claims are true, then why not a 2-stroke with 4-stroke lubrication and 4-stroke specific lube consumption like the PatPortLess and PatMar?

Manousos Pattakos


We must now look at some basics! The definition of BMEP for 2-stroke and 4-stroke engines was established a long time ago. A BMEP level of 13.6 bar is equivalent to 27.2 bar for a 4-stroke engine. The top level for passenger car diesel engines is 29.3 bar (Merc 250 CDI). State-of-the-art single-turbo engines achieve ~25 bar. Similar levels could be cited for heavy-duty engines. It is obvious that we see similar specific torque and power for the Achates engine as contemporary 4-stroke engines. Having made this assessment, it is obvious that “cycle averaged” efficiency cannot be 20 % better than a conventional engine. In my former contribution, I asked other readers to come up with four negative factors to balance the positive factors mentioned in the article. I will mention one, which is of particular importance at light load. This is scavenging and pumping losses. Remember that the Detroit Diesel 2-stroke engines –as any other automotive 2-stroke engine – had to use a Roots compressor (not to be mixed up with the Rootes engine) for scavenging. The parasitic losses of this device are particularly important at low load. No other solution is presented in this article. Therefore, I would not expect better part load efficiency for the Achates engine than a conventional engine; on the contrary, the opposite might be expected (there are other negative factors not mentioned here).

It is a scam to compare with an engine that is said to have BSFC of 240 g/kWh at the best operating point. “Reference engine”? You can only find this reference in a museum. Heavy-duty engine manufacturers achieved this level when they moved from IDI to DI after WW II. Of course, you can find a point in the load and speed map of a modern engine where BSFC is 240 g/kWh. Maybe that is what the comparison is based on, i.e. apples and pears.

The CR of modern passenger car diesel engines is in the range of ~16:1. Heavy-duty engines usually have somewhat higher CR. For a 2-stroke engine, they refer to “trapped CR”, which is not exactly the same as the geometric CR for 4-stroke engines (I will not elaborate further on that here…) but it comes very close. Thus, we have no basic difference in this area.

Regarding oil consumption, it would be nice to know under what conditions this was measured. I would not say that 0.1 % is impossible but it would certainly be very good for a 2-stroke engine. The best 4-stroke engines can achieve a level down to 0.05 % but again, it is all about under what conditions we compare (e.g. test cycle).

If opposed piston 2-stroke engines will be of interest for automotive applications, all the negative issues would have to be addressed. Among them, the issue of supercharging and scavenging is very important. An automotive size of this kind of engine cannot rely on turbocharging alone, as a large marine engine can. Maybe electrically-assisted turbo(compounding) could finally find a niche of application here, i.e. where the advantages could outweigh the drawbacks.



Partial loads and transient conditions (acceleration, braking, warming up etc) are important in road engines, as you say.
And the turbocharger and supercharger are not as efficient at these partial and special conditions, as you also say.

But the fact that the Detroit Diesel, the Achates engine, the OPOC, the Rootes etc need a separate compressor does not mean that any 2-Stroke needs external/additional help/means like compressors etc.

Take a look at the PatOP, the OPRE, the PatPortLess and the PatMar engines. Does some of them needs an external help to work?
Of course a turbocharger is optional for higher torque/power, as it is the case for every four-stroke engine.

If the millions of the investors made possible the achievement of this incredible 0.1% oil consumption, then a 2-stroke is not inferior to a 4-stroke, any longer, in terms of oil consumption, oil-carbon pollution etc.
This is a gift from Achates to the OPRE and PatOP.

If the 0.1% oil consumption is not correct, then the 2-stroke PatPortLess and PatMar have the advantages of the 4-stroke in terms of the lubrication and the emissions.

If the aforementioned do not agree with your last paragraph, I can further explain.

Manousos Pattakos


@Peter XX
According to the article, the 240 g/kW-hr was not a peak efficiency point, but a cycle averaged value. I don't know what cycle they used, but that sounds like a reasonable number for a high efficiency modern (i.e. meets modern emissions specs) engine running one of the standard steady-state or transient cycles.

"the latest test results for a 3-cylinder Achates engine configuration indicate a 20% improvement in cycle-averaged brake-specific fuel consumption (BSFC) when compared to a recently introduced advanced medium-duty four-stroke diesel engine: 192.6 g/kWh BSFC for the Achates engine, versus 239.9 g/kWh for the reference engine."

I personally wouldn't call this a scam just yet. They will probably have to make some concessions for emissions, production cost, and durability, which will bring the efficiency numbers down. Assuming they make it to production we'll see how they end up comparing to what is the state-of-the-art production engine at that time.

Disclaimer: I was acquainted with Professor Foster during grad school. Also, browsing the Achates web site, I found that one of their employees is someone I worked with several years ago.


Well, in your interpretation, it is even worse than I thought, i.e. definitely a comparison of apples and pears. They compare their peak efficiency with a cycle-averaged efficiency of a “reference” engine. Scam, in other words. Or else, there is some misunderstanding either by us or those who wrote the article. I have been around long enough to understand that you cannot market an engine that has BSFC at 240 g/kWh, so of course, the Ford engine is better than this in the maximum efficiency point. Perhaps I should have explained this initially and we could have avoided any speculation of this kind.


Oh, my dear Watson, you need a positive displacement compressor of some kind for a 2-stroke engine if you are going to use it in an automotive application. All the engines mentioned use a positive displacement compressor for scavenging. These examples definitely prove my point to the extent that I do not have to explain this any further.

One exception where a positive displacement compressor is not needed is large marine 2-stroke engines. These engines are started by using compressed air and they operate at fairly high specific power, where the turbochargers have high enough efficiency to create a positive pressure drop over the engine (BTW, turbochargers become more efficient, the bigger they are). This strategy is not possible for automotive engines, so for the moment, you need some kind of a compressor. I do not rule out piston compressors but they do not tend to be more efficient than Roots type of compressors. It is often a very bulky solution, as well. Lysholm compressors are the most efficient positive displacement compressors known but they use internal compression, provide little option for variable geometry and thus, are not well-suited for low load operation. Common for all these positive displacement compressors is still that you do not utilize the exhaust work (hyperexpansion, as the Atkinson cycle, would help but I will not go into detail here). I find one “outlier” candidate very interesting in this case. This would be the free-piston supercharger. It might be possible to start-up such a device using stored compressed air and to maintain operation even at low load. It utilizes the “free” exhaust energy. Camless valve systems (hydraulic, pneumatic or electric) that are necessary for the operation are now more or less fully developed. The problem is that very little work has been done in this area.

While waiting for the “perfect” supercharging system for 2-stroke automotive engines, we can only conclude that the well-known solution that Detroit Diesel used, i.e. a Roots-type compressor and a turbocharger, is still one of the best options available. Although the turbocharger does not operate at ideal pressure ratios, it utilizes some of the exhaust energy. The Roots compressor is necessary for start-up and low load but cause parasitic losses although some of the “pressure” on this device is “lifted” at higher loads by the turbocharger. Achates apparently intends to use this solution.


@Peter XX

If you take a look at the PatOP engine at
or at the OPRE engine at
you will see positive compressors.
If you take a look at the PatPortLess or at the PatMar engines, you will see 2-stroke uniflow engines having true 4-stroke lubrication and scavenging as in the 2-stroke marine engines.

At you can see a different twin charger:
When the turbocharger pressure is low (cranking, low revs, light loads etc), air enters through the one way (reed) valve into the scavenge cylinder and is trapped there for the scavenging.
When the turbocharger pressure increases, the one way valve remains constantly closed and the scavenging is made by exploiting the energy of the exhaust gas.
If you take a closer look, you will see that this arrangement is like a “free-piston supercharger” that utilizes the “free” exhaust energy (without the free-piston problems).
Can an electric turbo-charger have better efficiency?

@Roger Pham

Regarding the variable capacity of EcoMotors / OPOC: the cylinders are deactivated two-by-two; this is because each basic module of the OPOC comprises two cylinders.
Special clutches are used for the disengagement; and the power of the distant basic module passes to the load indirectly, through the crankshaft of the next to the load basic module. I.e. the OPOC basic module the nearest to the load, works overtime.
In case of failure (reasonably, the overworking module will fail first) the complete system halts.
Take a look at for a more reliable and way simpler variable capacity approach.


It would be nice if your Professor Foster and/or the employee of Achates you mentioned make a comparison of the Achates engine with the PatOP engine or the OPRE engine or the PatMar engine or the PatPortLess engine, in terms of everything Achates engine is proud of.

Manousos Pattakos


Well, it is easy to prove that electric turbocompounding can be better than positive displacement compressors. All the different versions of positive displacement compressors in the concepts you refer to must be driven by the engine, i.e. you always (under any operating condition) get parasitic losses. It does not matter where the air goes in and out; the power must come from the engine. Electric turbocompounding can produce both the positive pressure drop over the engine that is needed and provide a positive contribution via the “surplus” electric energy delivered to the battery. Of course, it is inevitable that electric power must be supplied at idle and very low load but it would still be better than a Roots compressor or any of the positive displacement compressors you refer to. How can I know this? Well, Detroit Diesel tried a concept in the past, where they used a hydraulic impeller (similar to a Pelton wheel water turbine) on the turbocharger shaft instead of the Roots compressor. This concept was more efficient although the power provided came from an oil pump and eventually from the crankshaft. This was in spite of the aerodynamic drag from the Pelton wheel when not used. Electric turbocompounding would be at least as efficient as this concept. I recommend you to seriously consider this option in your concepts.


The Achates concept engine was presented at the DEER Conference. I looked at the other concept engines also presented at DEER. The most comparable one had demonstrated an efficiency of 46.5 %. Maybe this puts the data published on the Achates engine into perspective…


“The Achates concept ….presented at the DEER conference …Maybe it puts the data published on the Achates engine into perspective…”

Congratulations to Achates on proving an unconventional innovation better and greener than the state-of-the-art 4-stroke.

But, does it mean that this Achates 2-Stroke (being already greener than the modern 4-strole) cannot get greener ?

“Base your knowledge on facts and analysis, and not on what everyone knows, when everybody knows that something is so, nobody knows nothing” is the best advise Andy Grove, of Intel, ever got.

Maybe the data published on the Achates engine at the DEER conference put the other 2-stroke engines, like the OPOC of Bill Gates or the PatOP , the OPRE, the PatPortLess, the PatMar etc engines into perspective, too.

Manousos Pattakos



The Electric turbocompounding is always an option for all engine concepts.

On the other hand, the fact that

“Detroit Diesel tried a concept in the past, where they used a hydraulic impeller (similar to a Pelton wheel water turbine) on the turbocharger shaft instead of the Roots compressor. This concept was more efficient although the power provided came from an oil pump and eventually from the crankshaft. This was in spite of the aerodynamic drag from the Pelton wheel when not used.”

shows the poor efficiency of the Roots compressor outside a narrow band of revs / pressures. At small revs the sealing of the Roots is questionable, while at high revs its aerodynamic suffers.

In comparison, a piston-type volumetric pump like those in the 2-stroke pattakon engines is quite different than a Roots compressor.
It needs not transmission.
The power used comes not from the crankshaft but directly from the working piston: the connecting rod passes to the crankshaft the rest power.
The sealing is guaranteed at all revs / pressures.
The volumetric efficiency is so good that it enables these 2-stroke engines to operate in a range as wide as the 4-stroke engines.

By the way, take a look at the “Hydrid” OPRE, at the bottom of the page, wherein the hydraulic pistons are part of the working pistons; here the crankshafts take a small portion of the power: the arrangement is actually a free piston engine without the problems of the typical free-piston engine.

Back to the piston scavenge pump.
When the turbo-charger pressure is low (twin-charger at ), the scavenging-piston does consume a part of the energy provided to the working-piston by the fuel burned into the cylinder.
In an Electric turbocompounding, the compressor also consumes energy stored into the battery. This energy comes from the combustion of fuel several cycles earlier. This process also involves parasitic losses: the engine has to operate at slightly higher load to make the part of the power that passes – through a transmission - to the electric generator, then the mechanical power is transformed to electrical power at relatively good efficiency, then the electrical power is stored into the battery as chemical energy – at not so good efficiency – and, when necessary, the reverse energy transformation provides power to the turbocharger to make the scavenging. The Electric turbocompounding has its own drawbacks. In some experimental 2-strokes it seems to solve more problems than it introduces; but practice has always the last word.

What counts is the efficiency of the engine at partial loads, just like in the hybrids wherein with an engine having a nearly constant brake efficiency along a wide rev / load range (like the Diesels), the hybridization is not cost efficient.

Manousos Pattakos



Regarding the Detroit Diesel engine lubrication: in practice their lube consumption is / was nearly 1 gr/KWh.
With a fuel consumption of 250 gr/KWh, this means their lube consumption is nearly 0.4% of the fuel consumption, which is 4 or 5 times higher than Achates 2-stroke.
The PatOP engine, just like the Achates engine, has no thrust loads on the cylinder liner.
But the Detroit Diesel liner receives the thrust loads and needs plenty of oil on it to prevent scuffing. A part of the lubricant is inevitably lost into the intake ports and then into the combustion chamber.
For the rest, the combustion bowl and the injector location of the 2-stroke Detroit Diesels is as optimized as in the best 4-strokes. Also their friction MEP is not worse than the 4-strokes of similar power output.
It seems the excessive lube consumption (emissions, cost etc) is what phased out the Dietroit Diesel 2-stroke engines.

Does the PatMar and PatPortLess 2-stroke through-scavenging engines have true 4-stroke lubrication, 4-stroke lube consumption and 4-stroke scuffing resistance? The space at the opposite side of the piston is there and can be used, if desirable, as the volumetric scavenge pump of the engine. Achates architecture does not enable this.

Manousos Pattakos


You should know better. In a 4-stroke engine, the piston, ring pack, cylinder liner surface and roundness control the lubrication. In a 2-stroke engine, the problem is the transfer ports. Oil is blown into the cylinder, burns or escape with the exhaust. Detroit Diesel used similar piston ring pack as in their 2-stroke engine as other 4-stroke engines. Thus, oil control via the piston rings is not a problem. Modern 4-stroke engines have double the cylinder pressure and double the trust compared to a 2-stroke engine and they still can control scuffing with low oil consumption. All 2-stroke engines with transfer ports will lose lot of oil. This is an inherent disadvantage of the scavenging principle, nothing else. To me, it looks as the Achates engine has twice the oil consumption of a modern 4-stroke engine. Nice (if true), they have reduced it compared to older engines. However, so have the 4-stroke engines. A normal oil consumption of an engine introduced at the same time as the Detroit Diesel engine was 1 g/kWh. If you could achieve 0.5 g/kWh, it was considered very favorable. So, the difference to the 2-stroke engine was not so big after all. However, with development of the previously mentioned components, oil consumption of the 4-stroke engines could be reduced below 0.2 g/kWh and even below 0.1 g/kWh in some cases. In a 2-stroke engine these improvements do not pay off as well, since oil consumption tend to be dominated by the loss via the scavenging ports. I cannot imagine that this would improve if you also have exhaust ports, as in (most) opposed piston engines. How could a big disadvantage suddenly become an advantage? No way!

An opposed piston engine with ports has one “cold” and one “hot” piston. For the hot piston we are talking about serious problems with thermal stress and lubrication (=scuffing). This is much worse than for a conventional 4-stroke engine and a conventional 2-stroke engine that has exhaust valves.

The Detroit Diesel type of 2-stroke engines has much better combustion chambers than opposed piston engines. It is difficult to envision anything better than a multi-hole central injector. Large 2-stroke marine engines have to use 2-3 multihole injection nozzles, located on the side of the cylinder head. So many nozzles would be both impractical and too expensive for an automotive opposed piston engine. Well, maybe 2 nozzles would have to be used anyway to get close to acceptable combustion quality. However, these nozzles must be mounted on the cylinder liners, not in the cylinder head, which makes the problem even worse.


You must have serious problems with numbers. How could 45.1 % efficiency of the Achates engine be better that 46.5 % for a conventional engine?


Regarding efficiency, a piston compressor is no better than a Roots compressor. However, the main problem is that both have to take the power from the crankshaft. You fail to recognize this fact. A turbocharger on a 4-stroke engine utilizes exhaust energy otherwise wasted.


It seems as you have a serious problem in understanding electric turbocompounding. You could look at some DEED presentations this year and in the past for more details but in short, I could summarize: Under some driving conditions electricity must be supplied from the battery. Under other driving conditions, turbocompounding provides surplus electricity for battery charging. However, most important to note, is that the n e t energy contribution is positive. This is the whole idea of the concept. What a difference compared to the positive displacement compressor where power has to be supplied from the crankshaft for scavenging and supercharging all the time!


@ moderator
I try to reply, but my posts are rejected.
I hope it is not censorship.
If it is, the honest thing is to inform the rest members that "Manousos is suspended".


Correction: I cited 46.5 % as the best level for a conventional engine presented at DEER 2011 but after some more reading, I found one at 48 % as well.

Roger Pham

If your post got rejected, just edit out certain part and resubmit it. Always save your writing just before hit the "Post" button.

To sum up, modern OPOC engines have advantages and disadvantages.

The advantages are low friction, light weight and much lower cost, due to the lack of valve train, and much lower post combustion emission treatment. The low friction result in significantly higher part-load efficiency than conventional 4-stroke TDI Diesels. This is even more so when modular-type of OPOC's are used, to run only 1 module at low loads, while increase to 2-3 modules at high loads, without requiring gear downshifting or upshifting, hence simplifying transmission workload and reduce transmission wear, using fewer transmission gears ratios, and reduce oil consumption at part-loads as well, when only one module is running.

The disadvantages are somewhat higher oil consumption, which is totally manageable because it is still so low, and suboptimal combustion chamber and fuel injection geometry, with lower compression ratio used, resulting in lower peak Indicated efficiency than 4-Stroke TDI's.
However, the lower friction allows the modern OPOC's to match the peak Brake efficiency of the best of the 4-stroke TDI's.
At part-load, fuel quantity injected is much less than at high load, combustion is cooler, thus the suboptimal combustion chamber and injection geometry of the OPOC have almost no indicated efficiency penalty in comparison to the 4-stroke TDI with a bowl-type of combustion chamber, yet, the lower friction results in higher part-load efficiency than a 4-stroke TDI.

Bottom line: modern OPOC's are promising due to their low cost, low maintenance, and higher cycle-average efficiency than contemporary TDI diesels.


Thank you Roger Pham

I think you mean the OP engines in general, and not the OPOC engine of EcoMotors / Bill Gates / Khosla.
OPOC's basic module comprises two opposed cylinders and four pistons. It is actually a pair of Junkers-Doxford engines sharing the same crankshaft. The arrangement of the crankpins enables a very good balance, which is the main advantage. A disadvantage is the lube consumption: imagine the inner-exhaust piston (the one with the single - and short - connecting rod) thrusting onto the cylinder liner over the exhaust ports. This piston is some 10 degrees advanced relative to its opposite intake piston; this means that most of the torque / power produced by this cylinder is provided by the inner-exhaust piston, and that the skirt of this inner-exhaust piston bears significantly heavier thrust loads than a normal piston of a 4-stroke engine. A thick oil film is necessary on the cylinder liner surface to prevent the metal-to-metal contact between the liner and the piston skirt. A thick oil film around the exhaust ports and on the port bridges makes inevitable the loss of a lot of lubricant to the exhaust.
In the opposite cylinder of the OPOC basic module things are better: the exhaust piston connects to the crankshaft by a pair of long connecting rods that substantially reduce the trust loas.
To put in political use the OPOC, a sigificant reduction of the lube consumption is mandatory. Worse than the lubricant cost is the increase of the exhaust emissions (especially the particles).
Cosshead 2-stroke OP engines like the twin crankshaft Achates and OPRE, and like the single cylinder PatOP, combine in a single cylinder basic module the balance of the twin-cylinder basic module of the OPOC with way lower lube consumption: the cylinder liner needs a "dye" of lubricant to prevent the metal-to-metal contact between the piston rings and the cylinder liner (because the thrust loads are taken away from the cylinder liner and the ports).

Manousos Pattakos


To Roger Pham

The advantages of the modular type proposed by EcoMotors are also exaggerated.
In the EcoMotors’ variable-capacity-engine approach (modular type), the cylinders are deactivated two-by-two; this is because each basic module of the OPOC comprises two cylinders.
Special clutches are used for the disengagement; and the power of the distant basic module passes to the load indirectly, through the crankshaft of the next to the load basic module.
Note: each OPOC basic module is full balanced as regards its inertia forces, it is well balanced as regards its inertia moments, but it is not balanced as regards its inertia torques: a heavy 2nd order inertia torque loads the crankshaft (the four pistons are near their maximum speed -middle stroke - the same moment).
I.e. the OPOC basic module the nearest to the load, works overtime.
In case of failure (reasonably, the overworking module will fail first) the complete system halts.
In the pattakon approach ( ) the cylinders are deactivated one-by-one, enabling a closer to the optimum capacity.
With a twin PatPOC engine at the one side of the primary shaft of the gearbox, and a single PatPOC at the other side, the set can run as either a single cylinder, or as a two cylinder or as a three cylinder engine.
The power of each module arrives directly, and independently, to the load.
In case the one engine fails, the other continues normally for “ever”, improving the reliability/safety of the system.
And they are needed neither special clutches, nor high tech control systems (even a manual system works fine).

Manousos Pattakos


@ Peter_XX
You are right saying that the problem in the 2-stroke engines is the transfer ports (provided the 2-stroke engine has transfer ports).
The problem gets worse in case the cylinder liner receives the thrust loads from the connecting rod, because a thicker oil film is necessary. The thicker the film on the cylinder liner, the more lubricant “is blown into the cylinder, burns or escape with the exhaust”. This is the case for the Detroit Diesel with the intake ports on the lower side of the liner and the exhaust poppet valves on the cylinder head.
The problem softens a lot in case the cylinder liner receives no thrust loads (crosshead architecture), which is the case for the Achates engine (and for the PatOP and OPRE engines of pattakon). The oil film on the cylinder liner is now quite thinner and prevents the metal-to-metal contact between the piston rings and the cylinder liner. The oil film between the compression rings and the cylinder liner is some 5 times thinner than the oil film between the typical piston skirt and the cylinder liner. This “dye” of lubricant remains on the cylinder liner above the piston rings.

You are wrong thinking that all 2-stroke engines have transfer ports through which the lubricant is lost. Obviously you didn’t see the engines mentioned. Both the PatMar and the PatPortLess engines are 2-stroke uniflow engines rid of ports on the cylinder liner. So please take a better look and write again about the possibility of a 2-stroke to have lower lube consumption than the best 4-strokes.
Manousos Pattakos

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