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...

@HarveyD,
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.

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.

@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.

@manousos
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.

@manousos
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 ….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

@Petter_XX

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

@manousos
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?

@manousos
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!

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.

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

@ 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