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.

DaveD:

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?

I do miss Rafael...

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.

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.:
http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-6318//year-all/#gallery/8873

@Rafael
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…

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.

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
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.

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.

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.

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!