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Achates Power developing light-duty two-stroke opposed-piston diesel engine: the OP4

The OP4 light-duty opposed-piston diesel. Click to enlarge.

Achates Power, which is developing a family of two-stroke, compression-ignition (CI) opposed-piston (OP) engines, has designed and is developing a light-duty diesel concept engine, the OP4. The OP4 is a two-stroke, inline two-cylinder, four-piston diesel with a swept volume of 1.5 liters.

With nominal power of 96 kW (129 hp) @ 4000 rpm and maximum torque of 325 N·m (240 lb-ft) @ 1750-2250 rpm (achieved at 14 bar BMEP), the engine will meet Euro 6 and LEV 3 emissions requirements and shows modeled best point fuel consumption of 189 g/kWh. Benchmarked against the Mercedes-Benz 1.8-liter OM651 Euro 5 engine, said Fabien Redon VP, Technology Development at Achates Power, the OP4 design shows a modeled 13.5% cycle-average fuel consumption advantage. Redon presented the basics of the OP4 engine concept at the SAE High Efficiency IC Engine Symposium in Detroit.

Fuel consumption of the OP4 (left) and the Mercedes-Benz OM651 (right). In his presentation, Redon highlighted the “flat” fuel map of the OP4. Source: Achates Power. Click to enlarge.

Background. Founded in 2004, Achates Power is designing and developing engines based on a two-stroke, opposed-piston, compression-ignition technology. The company has demonstrated the engine and delivered validated performance results based on more than 4,000 test hours on several engine generations.

Fundamental advantages of the two-stroke opposed-piston engine compared to conventional four-stroke engines are a 30% lower surface area-to-volume ratio; leaner combustion; optimally phased and faster combustion at equivalent pressure rise rate; and no dedicated pumping stroke.

In 2011, studies found that the Achates two-stroke opposed-piston engine could support an indicated thermal efficiency of up to 53% (earlier post).

Compared to the Ford 6.7L V8 Powerstroke diesel, a medium-duty configuration of the Achates engine showed a 15% best point and a 22% cycle-average fuel consumption advantage, along with best point BTE of 48.5%, compared to 40.9% for the Powerstroke.

Achates started with a 1.06L/cylinder engine configuration, then moved up to a 1.6L/cylinder configuration as the primary development engine. In December 2012, the US Army Tank Automotive Research, Development and Engineering Center (TARDEC) awarded Achates Power and AVL Powertrain Engineering, Inc. a $4.9-million contract for design and construction of the Next-Generation Combat Engine. (Earlier post.)

The TARDEC engine, said Redon, will use a 1L/cylinder configuration. The new OP4 configuration features a 0.75L/cylinder configuration and leverages the work the company has been doing for several years on the 1.0L and 1.6L per cylinder engines (as does the TARDEC engine.)

Light-duty OP4. In developing an engine for the light-duty market, Redon said, Achates had to take into account the more stringent emissions requirements, stringent NVH requirements, and the need for a broad speed range that will support low-speed low-load with low-emissions as well as high-speed high-power requirements.

The company picked as its benchmark target 4-stroke diesel car engines in the displacement range of 1.6L – 1.8L; it settled specifically on the Mercedes-Benz OM651—a high-volume engine with a publicly available fuel consumption map—as its baseline.

Air handling system for the OP4. Click to enlarge.

The bore of the OP4 cylinder is 75.7mm , with a 166.6mm stroke and a stroke/bore ratio of 2.2. Compression ratio is 16.0. The engine is supercharged and turbocharged.

Compared to the OM651, the OP4 exhibits a 15% reduction in surface area; a 35% increase in volume; and a 37% reduction in the ratio of surface area to volume.

The OP4 cylinders use dual central injectors, 180° apart. The engine uses dual pilot injections and a staggered main injection. The proprietary combustion system eliminates direct spray impingement and provides high mixing.

At 1500 rpm and 25% load, the OP4 shows an indicated thermal efficiency of 52.0%, with indicated specific NOx of 0.36 g/ikWh. Indicated soot is 0.01 g/ikWh. Throughout emissions cycle load points, the OP4 demonstrated 0.4 g/kWh engine-out NOx.

In catalyst light-off mode, the OP4 showed a NOx flow rate of 0.9 mg/s, compared to 3.9 mg/s for the baseline. Exhaust temperature of the OP4 was 410 °C, compared to 246 °C for the baseline; the rate of exhaust enthalpy for the OP4 was 9.0 kW, compared to 4.1 kW for the baseline.

Given the emissions results, Redon said, the OP4 could meet Euro 6 standard when equipped with a diesel oxidation catalyst (DOC) and particulate filter, but would not require SCR. The OP4 could meet LEV 3 requirements (also presumably US EPA Tier 3) with DOC, DPF and 85% aftertreatment NOx conversion.

The engine, which features a lower part count due to the elimination of the valvetrain and cylinder head, packages in existing vehicles. Redon said that Achates Power is hoping to find some partners further to develop and commercialize the OP4.



The Dodge Slant-6 is reincarnated as the Achates Slant Two!


I wonder how much this beast weighs? Just thinking in terms of range extender possibilities.

Anyway, I think that 189g/kWh would yield ~43% efficiency (gasoline has ~12.4kWh/kg and .189kg/kWh=5.29 kWh/kg so 5.29/12.4=~43% efficiency).

That's good, but not over the moon for a diesel engine. If I'm screwing something up in my thinking/calculations, please correct me.



Right, I am little disappointed by their result , Delphi reported 175g/KWh with their gazoline compression ignition, as this engine doesn't have valve drivetrain so less frictions and also less thermal losses because of opposed piston geometry, they should do better ...



This unit uses diesel which has about 8% more energy than gasoline per volume.

That could lower the ICE efficiency to a bit under 40%?

However, a reduced size (to 1/3) could be enough as a range extender for small PHEVs?


Whoa !! Did I read that article right ? Or do my eyes deceive me ? Regardless.

Combat Engine = Me Want

You may have felt, as have I, that there was some automotive need not yet fulfilled - but for no longer it appears.

Finally for the aggressive driver, the one ingredient we've been missing all these years - the Combat Engine - may soon be available.

In your dreams Honda with your "Earth Dreams" powertrains, the Combat Engine is for me. I can see me now whizzing down the road with the C.E. on WOT.

Oops did I just nod off, sorry. But then this new engine is to be a two cylinder while an earlier report had recommended that a 3-cylinder configuration was optimal for exhaust gas flow when seeking to avoid unfavorable interactions with the discharge from the adjacent cylinders. (Or was this yet another technical article I clearly didn't comprehend).

But then would even a 3 cyl 3.0 litre engine be enough to propel a tank ? I sure don't know. They mention a possible hybrid spinoff application. Really ? In case someone from the project is reading this I can save you heap of dough.

Hybrids happen to need constant torque engines on account of the fact that +70Kw alternators have constant field excitation due to those expensive Neodymium magnets that you may have read about. But then diesel torque characteristically drops off above 2400 rpm. This infers that the allowable current generated will have to be proportionally reduced as the engine proceeds beyond that point towards top speed. And all just to reach that final 25% of max power. Not good. I would have thought that hybrid design - particularly the two machine variety - is difficult enough already without having to make that concession.
Just where is Rafael Siedl when you need him ?

Nick Lyons

I do miss Rafael...


Do not compare apples and oranges! Delphi says: "at 2000 rpm-11 bar IMEP showed ISFC and combustion noise were low at 175 g/kWh and 88 dB". ISFC is indicated fuel consumption, which does not take friction losses into account. Delphi does not mention if gas exchange losses are included in ISFC but normally they are. A BSFC of 175 g/kWh is not possible with a gasoline engine, not even for an advanced concept of this kind.

About the benchmark
Please note that the data for this Achates engine are “modelled”. We do not know what they will achieve in real measurements. Nevertheless, the figures are impressive, if they could be achieved in a practical engine. However, I do not like their benchmark. The Mercedes-Benz 1.8-liter OM651 Euro 5 engine is actually built on the block from the 2.15-liter engine. Thus, friction losses for this particular engine would be similar to those of an engine much larger than 1.8-liter. Why not benchmark against a 1.6-liter engine, or perhaps a 1.5-liter engine? A state-of-the-art 1.6-liter engine of this size can also give similar power. In fact, they are shooting at a moving target since at the time the Achates engine will be ready for production, competitor engines will also be further developed. BMW promise 180 PS from their new 1.5-liter 3-cylinder engine. Friction losses will most likely be very low from this engine as well. I realize that this engine is not available for benchmarking today but when Achates reaches production, they would be competing against it. My point is: There are publicly available fuel consumption maps for many other engines in the “right” size and power class. Why did they choose the only engine which has the “costume” from a much larger engine, making it less representative than any other engine on the market?

Rafael Seidl

Hi guys - been away from this site for quite a while now, but someone forwarded me this page. I'm surprised anyone still remembers me!

I've been a fan of opposed-piston concepts because of their inherent thermodynamic efficiency but they've always had two key drawbacks: short life expectancy due to hot exhaust ports and, poor emissions due to oil residue in those ports plus significant cycle-to-cycle fluctuations in scavenging efficiency. If this company really has solved these long-standing challenges, that would represent something of a quantum leap.

I'm not at all surprised they've decided to go with a diesel concept and not just because that's what you'd need for combat fuel logistics. Rather, diesel exhaust gas aftertreatment equipment does not require a stoichiometric air-fuel mixture so it doesn't matter that two-stroke scavenging cannot reliably deliver that.

@engineer-poet: This is a two-stroke design. Apples and oranges.

@treehugger: It is true that a modern supercharged/turbocharged autmotive gasoline engine can feature effective thermodynamic efficiency of about 35% in its best operating point (cp ~42% for a high speed diesel). As Peter XX points out, indicated and effective efficiency are not the same.

In part load operating points, which are far more typical for passenger car applications, diesels remain superior in this regard because they can run on very lean air-fuel mixtures. In addition, effective consumer fuel prices per unit of *energy* may vary drastically due to differences in taxation (e.g. in the EU).

@daveD: The design may be little bit lighter than a turbocharged four-stroke four-cylinder diesel engine with comparable nominal displacement, but probably not massively so. There's no cylinder head, but the long side connrods for the pistons on the inlet side are heavy and you need an external blower/supercharger for scavenging. Also, air handling requirements mean that only about 2/3 of the downward stroke is available for power delivery to the crankshaft. This is why two-stroke engines aren't rated at twice the specific power of a four-stroke engine with the same nominal displacement.

The lower heating value of diesel is around 45 MJ/kg, so 189g/kWh does indeed translate to 42% thermodynamic efficiency for the optimal operating point, about what you'd expect for a high-speed engine. The opposed piston arrangement does improve the surface/volume ratio and the stroke/bore ratio is high even if you factor in the portion of the stroke "lost" to air handling. However, the exhaust side pistons and ports still require very aggressive forced cooling, so you lose a fraction of these concept-related thermodynamic gains in the form of higher power requirements for water and oil pumps.

Fuel density is irrelevant in this context as they quote modeled(!) fuel mass consumption per kWh. On a per-kg basis, gasoline and diesel have similar lower heating values.

Also note that later on, the article mentions a value of 52% in a part load operating point, but that is for indicated (i.e. at the piston crown) not the lower effective (i.e. at the crankshaft) thermodynamic efficiency. The unstated difference reflects internal friction losses between piston rings and piston liner plus the parasitic loads of the pumps and blower/supercharger.

@T2: This type of engine would almost certainly be no more than an auxiliary power unit in a combat tank. Electric hybridization might make sense in situations in which there is a tactical advantage to eliminating engine noise while keeping electronic systems and the gun turret motor operational.

Note that the military version of this engine features higher displacement per cylinder and obviously wouldn't be engineered to meet EU or US on-road emissions requirements. Ergo, you'll never be able to buy a passenger car featuring the military version, no matter how much money you're willing to spend.

In a four-stroke engine, you need about 240 degrees crankshaft ignition separation between cylinders sharing an exhaust manifold to avoid crosstalk (i.e. spent gases from one cylinder at the beginning of its exhaust stroke reversing the flow of gases in another at the end of its own exhaust stroke). Since each cylinder fires every 720 degrees crankshaft, three pots is the limit. However, it is ok to combine the far ends of multiple manifolds into a single exhaust system provided they're distant enough from the engine and/or limit crosstalk by other means. One example of the latter is the twin scroll turbine volute commonly used in turbocharged I4 and V6 engines.

In a two-stroke engine, the exhaust port of each cylinder is also open just under 1/3 of total cycle time (typically about 108 degrees crankshaft). Therefore, three cylinders per exhaust manifold is the crosstalk limit here as well.

@engineer-poet: This is a two-stroke design. Apples and oranges.

Rafael, please refer to the dictionary for the definition of "joke".  You appear to be unfamiliar with it.


What a waste of time, effort and money. If it has to be an investment in fossils, why not in something that makes at least a little bit of sense? E.g.:


As we may have noticed sarcasm and joking is difficult to do on chat-halls.

You almost have to label them.

(Caution. Joke follows.)


Thanks for a comprehensive contribution! I agree in most cases. I find only a tiny error. The energy content of diesel fuel is not as high as 45 MJ/kg; 42.7 MJ/kg is a more common value. Gasoline is very similar but mostly, slightly higher. Using 42.7 MJ/kg, 189 g/kWh equals an efficiency of 44.6%. This is, in fact, quite impressive compared to conventional automotive diesel engines. I know that former VW boss Piech claimed that the VW Lupo 3L (and Audi A2) engine could reach 45% but most diesel car engines typically get a few per cent lower numbers. The engine in my own car reaches almost 43%. The Atkinson cycle engine in Prius should give some 38%. You can find targets for much higher levels in various research programs but it is not only about showing such potentials in modelling or in concept engines. Achieving these levels in commercial and affordable engines is quite a different task. That might be difficult also for Achates.

I would like to mention the excellent scavenging process for an opposed 2-stroke engine due to the long cylinders. They are about 2 times longer than a conventional engine. If you make a conventional 2-stroke engine with very long stroke, normal limits for piston speed would reduce the engine speed and power by a factor of 2. This was recognized on aviation diesel engines (e.g. Junkers), which were of the opposed type. Very long stroke 2-stroke diesel engines are familiar in marine applications. These engines, at ~55% efficiency are the most efficient of all types of heat engines but part of this advantage can be attributed to their size. Conventional 2-stroke diesels, such as e.g. the former Detroit Diesel Co. 92-family engines, did not have the long stroke and could never compete with 2-stroke engines regarding efficiency. Eventually, the company gave up and introduced their first 4-stroke engine (Series 60) in the late 1980’s.

Let me challenge some of you with some questions. We recognize that the Achates 2-stroke engine has two times as many power strokes as a 4-stroke engine but the BMEP is only half of the latter (similar power and torque in both cases), yet the efficiency is fairly similar. Could that be just a co-incidence or is there some fundamental reason? How would you explain this? (Of course, I have my own explanations…). In addition, could this factor 2 provide any other specific advantage for the 2-stroke engine? If yes, please elaborate a little bit…

Roger Pham

Testing TP

Roger Pham

Ah, TP is working! Thanks to Rafael, Peter, et al for all the input.
OK, Peter XX, the problem with ported-valve 2-stroker is that with turbocharging, you don't get the kind of pressure boost that you really wanted. The reason is that the exhaust port opens up slightly before the intake stroke, and thus exhaust port will close AFTER the intake port, allowing for venting of the intake boost. Lower charging pressure means lower BMEP! Turbocharger converts the high volume exhaust gas into lower volume of intake air but at higher boost pressure. However in a 2-stroke ported engine, there is no way to keep in the boost. Perhaps that's why DD abandoned their 2 stroker, because they use Root supercharger which rob power from the engine.

The good thing about this engine is that even that, the exhaust is hotter than the referenced MB engine, meaning low heat transfer loss, and even at lower BMEP, the efficiency is equal, meaning low friction. How would you fix the ported valve timing though? Yamaha motorcycle engine did it by a valve behind the exhaust port to keep in the charge pressure.

Roger Pham

I see in the schematic diagram regarding the air charge system that there is both a supercharger and a turbocharger, yet the max BMEP is so low, only 14 bar? confirming the suspicion. They apparently put a back pressure valve behind the turbine of the turbocharger but why, if not to help retain the charge pressure in the cylinder, but then back pressure is bad because it impair scavenging. It seems that the Op-4 runs out of torque right after 2000 rpm, while the referenced MB engine kept a flat torque curve even after 3000 rpm, indicating scavenging problem in the Op-4. It seems that the efficiency of the Op-4 could improve quite a bit when the charge pressure problem is fixed or ameliorated.


If extra back pressure is required to increase the air charge, one obvious option is to add an energy-recovery turbine to the exhaust to help restrict the flow.  Efficiency would increase too.

Dumping excess air through the exhaust might be a method for managing the exhaust-side piston and port heat load.

Roger Pham

Good point, E-P. With more enthalpy out of the exhaust of the Op-4 engine perhaps due to lower heat transfer loss, then a power turbine would be highly desirable, like TIGERS. Still, the intake charge cannot achieve higher pressure than the exhaust back pressure. A charge-retaining valve at the exhaust would be more complicated and would defeat the simplicity of port valving.

Marine 2-stroke engine has a poppet exhaust valve in which the exhaust closure can be before the intake port cover, thus retaining intake charge to build up boost pressure. Piston ported exhaust valve could be the limitation of opposed piston engine.


The back pressure valve is most likely there to create a positive pressure drop over the EGR route. This is a common feature on many automotive diesel engines. It is only used at very low speeds and loads where there is no pressure to drive the EGR flow. Thus, it has a very small impact on the pumping losses and fuel consumption. Throttling in the intake manifold is another option to get the desired EGR flow. VGT also helps (common on HD engines) but sometimes it is not sufficient (LD engines).

2-stroke engines are not the best candidates for using a power turbine in a turbocompound system, since pressures in the exhaust manifold (and consequently also the inlet manifold) are relatively low. On 4-stroke engines, turbocompounding gives excellent conditions for high-pressure (short route) EGR. A general remark is that turbocompounding improves engine efficiency at higher loads but can be detrimental at low loads. An automotive engine is frequently run at low loads, so achieving benefits from turbocompounding in common driving cycles is very difficult. This does not exclude that improvements might be done to overcome those problems but with the additional drawback of the low pressures in a 2-stroke engine, it is not very likely that we will see this in the near future. This is in contrast to HD engines, where this technology is already used to some extent – and with success.


Another possibility is to advance the phase of the exhaust-side pistons relative to the intake.  This allows the cylinder pressure to blow down before the intake ports open, allowing the overpressure to be converted into turbine work.  The exhaust ports close while the intake is still open, allowing a tuned intake runner to over-charge the cylinder above the compressor outlet pressure.

This would sacrifice the inherent self-balancing of the engine, and require extra vibration-control measures.

Roger Pham

Thanks, Peter, for the feedback.
@E-P, the problem with ported piston valve is that earlier exhaust port opening also means later exhaust port closing, and longer exhaust duration relative to intake duration. There is no option of timing variation with piston port valve.


Roger... phasing.  Realize that the crank throws for the intake and exhaust sides do not have to be symmetrical.

Roger Pham

Even if the crank throws are not symmetrical but the phasing or timing are the same, it won't make any difference in the timing. The timing have to be almost symmetrical in order for the opposed piston concept to work, otherwise there won't be enough compression. It means that the pistons have got to reach TDC at about almost the same time. Piston movement is much more sensitive to crank angle on the piston upstroke than down stroke. If you are gonna have enough timing difference on the down stroke to close the exhaust port before the intake port, then on the upstroke, the pistons will be too far apart to produce enough compression.


Sure you can have high compression with out-of-phase crank throws.  It's just that the point of maximum compression isn't at the exact TDC of either piston, and part of the cylinder volume is swept by both pistons.

Roger Pham

If as you stated above, it would be difficult for the injector to inject fuel, since the dead-space area will be shifting from one side to another. There will fuel wetting of the piston crown, because the both piston crowns will get to very close to the mid point of the cylinder. Even that, you may not get sufficient phase shift at BDC, because piston position at BDC is very insensitive to crank angle. If it is so easy, Archates would have done it already!


Diesels typically have bowls in the piston crowns, you'd just put them on the sides where the injectors are.

you may not get sufficient phase shift at BDC

Listen to yourself.  If you shift the angle of the crank throw by 15°, you'll shift the port opening and closing by the exact same 15°.

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