Opposing Piston Engines 10

[Painterman]

Hello to all,

I keep on noticing favorable comments about opposing piston engines, i.e., Junkers and Napier Deltics. If they are so good, then why aren't they used more widely? That is to say, these engines must have a problem or a shortfall. Can anybody say what the problem is? Also, what about their efficiencies?

Any other would be appreciated.
Kind thanks & best regards
Painterman (DFD)

 

[unclematt]

I have suggested a hypocycloidal crank system to greatly reduce the friction created by side forces on the piston. If you water jacket the area of the cylinder around the exhaust ports at the same time, the oil won't flash off, and the oil that does make it above the ports should be enough for lubrication and prevent the scuffing from occurring.

 

[unclematt]

I have a design I have been working on for a few months that would give the OPOC engine a hypocycloidal "cardan gear" crank system. The more I develop it, the more reluctant I am to post it in public. You know how that is...

It uses silicon nitride bearings to save weight, to increase rpm and load limits, and to reduce the need for bearing lubrication dramatically. It can use either helical or spur gearing, roller or ball configuration.

I will decide whether to post the illustrations or not as soon as I have a couple of improvements worked out. I know how the wolves are on this forum that just can't wait for fresh meat to pounce on, so I want to get some wrinkles ironed out first.

Does anyone know the smallest tolerance that silicon nitride bearings can be produced to, both ball and roller (needle)? I haven't been able to find that online, and I am curious how expensive the very accurate bearings are, and what is the closest tolerance that is available.

 

[unclematt]

I am curious, could the use of biodiesel in this kind of engine increase lubrication in the cylinder and perhaps help prevent piston ring scuffing? Though there seem to be some compatibility problems with some direct injection sytems, I have been reading that bidiesel has more lubrication ability than petrodiesel. Does this translate to improvements in cylinder wall lubrication, or does it flash off? I could see the combustion chamber area of the OPOC cylinder getting a nice coating of lube when the biodiesel is injected prior to TDC.

 

[BigVlad]

I think people are getting away from the original question which was - do OP engines have any advantages/disadvantages?
I personally can't see that they would have any thermodynamic advantages over a modern diesel, probably they would not be anywhere near as good as a modern 4-stroke diesel. I don't think anybody else who answered this question has come up with any real thermodynamic or effiency advantages either. Mechanically I think OP engines are a joke.
My first reaction on seeing the cross-section drawing of the Achates engine in the USPTO online files (7360511) was "Dear oh dear!". There would have to be enormous efficiency or power gains to make an apparent mess like this worthwhile. The real question is why do companies like Achates get so much high powered financial and staff support?

 

[TDIMeister]

Same reason as companies like Scuderi - they're much better salesmen than engineers.

 

[unclematt]

I think if you work out a couple of refinements that the OPOC engine DOES have a lot to offer in the way of increased efficiency. The desireable characteristics of the engine have been spelled out in detail. No valvetrain, increased power density, etc. I too would like to see independent testing done on the motor, but that isn't available yet. But instead of just ridiculing or dismissing the concept "just because", I choose to wait until all the facts are in before I come to a conclusion.

 

[SomptingGuy]

I thought the packaging and power density of these OP engines was primarily aimed at creating efficient powerplants for UAVs. These may not need long service intervals.

- Steve

 

[patprimmer]

The lack of a cylinder head should increase thermal efficiency due to reduced surface area at the same bore and stroke.

Regards
Pat
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[BigVlad]

I think Pat has a good point about reduced surface area etc. - but good enough to justify all the complexity?
Is lack of valve gear such a huge advantage? At diesel speeds the valve gear would last forever anyhow.
All the comments about power density - I am always amazed to see just how small an early model Subaru engine is when it is out of the car and lying on the floor. Another impressive one is the 4.3 litre Chev V-6 out of the car etc. and with all its ancilliaries taken off - hard to believe that it is 4.3l. The LC (water-cooled) 250/350 Yamahas - ridiculously tiny for the power they produce.
I will agree with Unclematt in that you shouldn't be too critical of of people trying something different until definitely proven otherwise.

 

[tbuelna]

BigVlad,

I agree with your statement about not being unduly critical of individual creative efforts. But unfortunately, this is an engineering forum. And engineers are trained to think critically and logically, not emotionally. So if the responses here seem somewhat blunt, that's just the nature of the engineering personality.

As for the opposed piston two-stroke engine, the concept has been thoroughly vetted over the past 60 years, by very bright engineers working for very competent companies, like Junkers, Rolls-Royce, Napier, GE, Detroit Diesel, Fairbanks-Morse, etc.

As for the efficiency benefits of eliminating cylinder heads and valve trains, the most thermally efficient IC engines in existence have both cylinder heads and valve trains:

Regards,
Terry

 

[BigVlad]

Tbuelna, Mate, - I'm on your side. I was just trying to appear "nice" by saying that one shouldn't be too critical. I agree entirely with what you are saying - it is some of the other writers (like unclematt)that seem to think that OP engines etc. do have potential virtues. I thnk Painterman's original question is a very good one. My interpretation of P'man's question is - " Clearly OP engines are, at best, verging on a mechanical nightmare compared to a "conventional" engine. There does not appear to be any overwhelming evidence that OP engines would have vastly better thermal efficiency, power density, reliability etc. or even some slight evidence that they may perform even slightly better than an engine of conventional layout. Why then did so many famous companies expend so much effort on OP engines? Surely these companies must have seen more advantages in OP engines than are apparent today."
I think something like the OP question can also be asked of the current rash of engines being promoted that replace the conventional crankshaft with a "scotch crank" or different types of cam and roller bearing mechanisms. Do any of the claims made have any truth in them? (I personally don't thik so). I like unconventional mechanisms and engines etc. - but just being different is not enough, it has to be better as well.

 

[GregLocock]

I suspect the original attraction is getting rid of the cylinder head. The thermal losses through the head are significant.

The possibly improved balance is also attractive, although of course there are solutions that are more straightforward.

So far as unusual engines with reasonable improvements in efficiency the Revetec engine is unusual in that they have released data from an outside lab showing real test results, with a thermodynamic efficincy of 38.6%, which so far as I know is the best seen for a real gasoline engine.



Cheers

Greg Locock

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[unclematt]

I think that the main attraction of the OPOC design is that it offers the efficiency of a diesel in a small package with a lot of power. Ecomotors estimate over 300 hp for their 2 cylinder, 4 piston version for automotive use. Even if that is off by a lot, and maintains diesel liek efficiency, that is still a nice engine considering how little room it takes up and how little it weighs. Have you seen the OPOC engine at advanced proplusion? 13 hp and easily sits in your hand. A 5 hp Briggs & Straton is bigger and heavier, and much less efficient. And with the advent of a variety of new injection technologies, further increases in efficiency are possible. I think this motor would make a great series hybrid generator, and just run it at its most efficient rpm all the time.

 

[GregLocock]

So, how big is this 300hp engine? When do they propose to get one running?








Cheers

Greg Locock

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[patprimmer]

I find it hard to imagine how adding the rocker arm and extra con rods does not more than completely offset the removal of a cylinder head.

Regards
Pat
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[GregLocock]

Depends on how you look at it. I'd say the thought process started from a single cylinder engine. Add another in mirror image and get rid of the cylinder head. No losses have increased, as a percentage of output power, and you've eliminated the cylinder head heat losses.






Cheers

Greg Locock

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[patprimmer]

Greg

I was thinking about engine bulk vs displacement, not thermal efficiency nor mechanical efficiency.

I am presuming horizontal cylinder axis with crank below centre.

I think as the exhaust and inlet ports have to be on opposite sides of the cylinder, they need to be between bores, thus spreading the bores, or on top and underneath, thus putting some in the crankcase or displacing the crank and crankcase downward.

The mechanical efficiency must be poor with the extra reciprocating weight of the rockers and the extra rods.

Regards
Pat
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[unclematt]

The extra connecting rods can be engineered from titanium and or take the form of thin wall tubes, adding little mass. If you change the crank to a cardan gear hypocycloidal system, the outer piston rockers, the inner piston rod connections, and the seperate crank journal pieces can all be eliminated - further reducing reciprocating mass. Also, the piston mass can be greatly reduced because the piston pin assembly would be removed as well. The pistons would all be rigidly connected to the rods.

And I don't buy in to the mindset that removing the entire valvetrain and head, while adding extra connecting rods, makes the engine less efficient. Yes, this design is weird. Yes, it is not like conventional motors. So what.

If you go to ecomotors website, they are in the process of setting up manufacturing facilities in Detroit. This engine has also gone through extensive testing by the military for their UAV programs. So this isn't just vaporware from a con artist. I too am waiting on the numbers and to see real world results. I too would like to see the over 300 hp version of this engine in reality, though advanced propulsion already has a 13hp version available for sale now. When that happens I will be eager to see what results, if anything. I don't buy into every new engine technology that comes along, and this technology is anything but new. But I do think there is enough evidence to warrant further interest in this motor design, especially with extra refinements.

 

[TDIMeister]

Mass does not come into a large play for mechanical efficiency for most engines.

 

[clasdauskas]

The original poster asked, amongst other things, about the efficiency of Opposed Piston designs. If you scroll to the bottom of this article

http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption

there is an interesting table.

It shows that the Jumo 204 was achieving very good figures back in 1931. Think how much more engineering effort has been applied to the engines that can beat it ...

Chris