Opposed Piston Engine

[iverson]

I'm surprised no-one has mentioned the Napier Deltic; three banks of six cylinders (each with two pistons) arranged in a triangle, giving 1750Hp. The triangular arrangement meant each bank of cylinders had a crankshaft at each end but saved weight as there were only three crankshafts for three banks. It did need a supercharger (and compressed air starting) to work, and gave good service in the Dark class FPBs and, of course, the Deltic class Locomotives.

SRJ.

It needed a scavenging pump, as do all two-stroke engines. Compressed air starting isn't that uncommon on large or very large diesels.

Two-strokes like the Junkers and Deltic need a scavenge blower or supercharger. But there are many two-strokes designs that don't.

Early engines used vertical loop scavenging. The inlet and exhaust ports were separated by a piston with a steep, wedge-like crown that was meant to deflect the incoming charge up and the exhaust down on the opposite side. It did not work very well and produced problems with combustion-chamber shape and piston heating.

Modern, horizontal loop- and cross-port-scavenging designs place and size ports to take advantage of the gas flow in the engine. Tuned inlet and exhaust pipes are also used to aid scavenging through resonance.

A supercharger or turbocharger probably makes for a cleaner and more fuel-efficient engine. But it is not essential.

Any text book on engine design can provide this information.

 

[iverson]

I recall reading that Fiat experimented with an opposed-piston, two-stroke, Otto-cycle, Grand Prix engine in the 1920s, before settling on the famous two-valve, eights and sixes used in the 802, 803, and 805 racers. The two-stroke was extremely powerful--and loud--but ran too hot.

 

[Nicknick]

Any two stroke engine needs a pumping device which forces the air into the cylinder, it can be the underside of the piston, a roots blower, a turbocharger (supported by a roots blower or electric power for low loads) or whatever, but none is doing it without.

Pile drivers are not engines (there is no definition of their mecanical power output, they produce impact impulses instead), but in fact they can work with a two-stroke cycle and without a scavenging device. This is possible, because about 75% of the piston stroke is used for gas exchange and only 25% for power production. They work with kind of "expansion, exhaust, intake, exhaust compression bounce" cycle, which has never been applied to a two-stroke engine (please mention a source if you think there is any). The piston is sucking fresh air in the cylinder on its long way up and will dilute the gases inside sufficiently for the next bang. Using this Princip in an engine would result in an extremely low power output with low efficiency, that’s why there aren’t any engines working this way.

 

[Nicknick]

I started to wonder, how the wonder two stroke engine of the Hayter lawn mover worked which Steelpillow mentioned, because, honestly, I don't know a single two stroke engine which has neither a rod ("The entire engine had just two moving parts, the piston and the crankshaft") nor any scavaging device. All Hayter lawn mover engines which I found in the internet had indeed no scavenging device, simply because they are all four strokes...

View: https://www.youtube.com/watch?v=X3A4e14LG_w


Flathead engines are often held for two stroke engines

 

[TomS]

GDLS AbramsX concept vehicle is currently being shown off at AUSA, and it's driving around on it's own power using an ACE powerpack, thus confirming that one scaled to MBT size currently exists.

Well, to be fair, it confirms that they were able to drop some existing version of the ACE engine in there, and hybrid electric gives them some more flexibility -- all they need is something that churns out enough electricity to charge a battery and move the vehicle. I doubt they are cranking it up to full-tilt at AUSA, so it could be the known 1000-hp model linked up to the hybrid drivetrain. But I expect you are right that they have the 6-cylinder 21.5-liter version running. It doesn't seem like a big stretch to add a pair of cylinders to the existing 4-banger.

There's a part of me wondering if the AbrahmsX even needs the full 1500-hp engine, though. In civilian vehicles, hybrid electric drivetrains use the battery to provide extra power for "launch" acceleration without needing a full-sized motor for "cruising." But battlefield use is different -- I don't know how often/long a tank actually need the full 1500 hp and how much time it could get by on less.

 

[Jemiba]

In this discussion here, a lot of expertise is shown, so probably a lot of experts are contributing to this theme.
Experts usually are highly qualified professionals, and a characteristic of real professionals is, that they
handle different opinions without getting personal ... and, if one of them get carried away, they are fully
able just to ignore this, without condescending to the same level.
For short "answer the argument, not the one, who came up with it !". If there's an irresistible urge to start a
dialogue, there's a perfect instrument here, called "PM". If one don't want to see the contributions of another
member, there's another instrument, called "ignore" button.
Those tools not only make the own life more comfortable, but maybe that of other people, too !
And in the interest of most of those others, this piece of advice should be considered a warning !
4 reports in a single thread within one day is more than enough !

 

[Nicknick]

I’m honestly interested if there is any two-stroke engine which can work without a savaging device. Despite the pile driver and the totally antiquated Lenoir engines (neither an Otto- nor a Diesel engine), nothing comes into my mind. There have been many exotic engines developed, so if you try hard, you might find some.

 

[steelpillow]

References? Any book on internal combustion engine design. You could start with the classic (and somewhat dated) The Internal Combustion Engine in Theory and Practice, by Charles Fayette Taylor, or Internal Combustion Engine Fundamentals, by John Heywood.

Can you quote a passage from that source which supports your use of the term "scavenging pump"?

I would use scavenge pump as a more relevant term for inverted four-strokes that need to suck oil out of inverted cylinder heads, to prevent hydrostatic locking.

After investigating various sources, I have found the term "scavenging pump" applied in different circumstances; on its own it is ambiguous. For example a large two-stroke engine might have pistons which in themselves scavenge the exhaust gases in a pumping action, and/or a supercharger to push air through, while also having a dedicated scavenging pump to remove oil mist from the crank case or cylinder head.

So it seems we are all correct as far as we go, but we need to acknowledge the other applications of the term too.

Opposed pistons do indeed act as a scavenging pump in the appropriate meaning, which is common in two-stroke parlance and easily findable in the sources cited above.

 

[Nicknick]

So up to now, nobody could find any two stroke engine which works without any kind of air pump which forces the air into the cylinders (I don't count the Lenoir engine or the Pile hammers as two stroke engines). There is a story of a mysterios lawn mover engine which could do that, but no evidence for that exists.

Opposed piston engines (Fairbanks Morse used Roots Blower, Jumo used gear driven Supercharger, Doxford used lever driven piston pumps) are depending on scavaging devices like any other two stroke engine.

So is there any example which could proove that two stroke engines could work without it? Or can anybody explain exactly how the gas exchange could work in this cases?

 

[Nicknick]

So I make an offering. I will donate 100 € for a good purpose if anyone can proof the claims of Steelpillow or Iverson, which is defined in my own words:

-Two stroke engines don’t require any pumping device for the gas exchange-

Knowen pumping devices are e.g. the piston underside, Roots blower , centrifugal or axial compressores/blowers, Turbocharger (usually supported by a blower or a electric motor), van rotors and so an. A rotating engine which uses the centrifugal effect to create a pressure for scavanging is also a pumping device.

What would define the definition and make me loose my bet, would be a an intake or exhaust system which is defined in a way that only with these systems the gas exchange can be done without any additional moving parts.

Please correct me, if you would like to change this definition

To verify that, I will not be convinced by a blurred picture and/or a text from a popular magazine. It needs to by a production engine with a type definition and a manufacturer. It can also be, a well-documented race engine or a disassembled serial production engine of unknown ancestry which clearly had a working life (must be visible).

I don’t accept Patents, text only references, drawn pictures except they are directly related to a well knowen company, Homemade engines (Youtube or elsewere), Lenoir engines, Pile drivers or other types which have little to do with standard two stroke engines.

My definition of a standard two stroke engine is, that it must have a compression before ignition and the gas exchange should not require more than 40 % of the piston stroke (so race engines should be included here). Atkinson engines are also excluded, because they work with four piston strokes in one rotation.

I will even double that, if anybody can find the lawn mower engine, Steelpillow mentioned.

Here is where my money would go. I would like to see someone contribute from the other side, so that we will have a donation in any case.

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[TomS]

I think this conversation has reached the point where this image is necessary (and Lord knows, I've been this guy myself once or twice. I suspect we all have):

 

[Nicknick]

its soley for a good purpose

 

[iverson]

I am by no means an expert. Just widely read. And, if I may paraphrase the august philosopher Monty Python, I am NOT here for an argument.

I think the argument that has been simmering here depends solely on the definition of "pumping device", which I was taking to mean an external supercharger or scavenge blower, not a crankcase or a tuned inlet or exhaust system. If one defines valve timing, split cylinders, exhaust manifolds, and/or funny pistons as "pumps", then there is no argument.

I think we do have to distinguish scavenging from compression, though. All these engines compress mixture using the pistons, sometimes in the cylinder above the piston, sometimes in the crankcase below the piston with a transfer port. Some engines supplement the cylinder/crankcase compression with a separate, supercharger blower. Some use a separate blower solely for scavenging. Most do not use an external blower at all.

The simplest engines use vertical-loop/cross-port scavenging. This works by means of a physical divider (the piston crown) and the inertia of gases flowing between the inlet/transfer and exhaust ports with valves in the head. They do not require external scavenge blowers, as near as I know.

Uniflow engines use a combination of piston controlled ports low in the cylinder and poppet or rotary valves in the cylinder head. Combustion gas pressure forces the exhaust out of the port in the head when the exhaust valve opens and fresh charge enters at the bottom of the cylinder when the descending piston clears the exhaust port. Opposed piston engines are Uniflow engines that use pistons to control both the exhaust and the inlet ports.

Split-single-cylinder designs (with two barrels and two pistons with a common combustion chamber) work like a hybrid of vertical loop scavenging and uniflow designs. The inlet port is in one cylinder and the exhaust in the other, to improve scavenging and maximize fuel efficiency. In the 1930s, DKW motorcycles used this system very successfully, in both supercharged and unsupercharged form. In the late 1930s, Arnold Zoller used the split-single design for a 12-cylinder,1500-cc, engine fitted with twin Zoller superchargers that allegedly produced 200 bhp.

In each case, the most sophisticated two-stroke engines use valve overlap and resonance effects in tuned exhaust pipes, in specially designed exhaust expansion chambers, and in the design of the inlet arrangments to improve scavenging and volumetric efficiency.

For convenient online explanations, please see

• https://en.wikipedia.org/wiki/Uniflow_scavenging
• https://en.wikipedia.org/wiki/Loop_scavenging,
• https://en.wikipedia.org/wiki/Split-single_engine
• https://en.wikipedia.org/wiki/Expansion_chamber

 

[Nicknick]

For new readers, this discussion is no longer understandable, since a lot of the original postings have been deleted or altered. Here ist the part which started this discussion:

"once had a little Cox BabeBee .049 two-stroke glow plug engine. It had a port on either side, and each acted as both intake and exhaust port. Gas exchange was accomplished via the dynamics of the cylinder gases as the piston rose and fell. The entire engine had just two moving parts, the piston and the crankshaft. My dad had an equally valveless two-stroke mower, a Hayter but I do not know who made the engine. Air was sucked in through the intake port via a carburettor and combustion gases expelled via the exhaust port through a silencer. Gas exchange was again induced by the cylinder gas dynamics, with the help of a slipper piston crown. No scavenging pump, no force of any description outside the cylinder was present on either engine. So please stop repeating unsupported claims that fly in the face of the evidence and either quote us supporting sources or leave off altogether. "