New 2-stroke Diesel engine, cool link to Engineering TV

[bryn]

do not confuse a 2 stroke gas engines problems with a 2 stroke diesel. they are very different engines.

[rmay635703]

Originally Posted by bryn

do not confuse a 2 stroke gas engines problems with a 2 stroke diesel. they are very different engines.

Not sure who this is directed toward but I know most here already know that.

Historically they were always considered more efficient than the equivalent 4 stroke diesel (generally higher compression, lower weight, more power). In other threads I have stated I wished that 2 stroke gassers would be redesigned in a similar way as the 2 stroke diesels.

Now they are considered dirtier (2 stroke diesels), current detroit 2 stroke models do not meet the restrictive US Tier II req. (nor do many diesels) Many modifications were needed to get most motors in spec especially on 4 stroke HD motors which I deal with everyday. In fact we only have one motor that meets spec for our large equip from Cummins, CAT doesn't seem interested, Detroit seems lost on the subject.

When I state the water wash method I am talking going nearly 0 emission save CO2 and water vapor. Ultimately we need to get to there. Or better yet 0 emissions period. (electric/solar/wind/human )

Also note that the water wash really only works on diesel emissions, minimal effect on most gas emissions.

[UK Mark]

Big Dave has hinted that this design of engine is old and is based on the 1940s Junkers Jumo aircraft engine. In fact, the opposed piston diesel engine is much older than that. The Jumo is probably the most famous opposed piston diesel worldwide, but there are plenty more about, and some predate the Jumo by a good few years.

One was built by Junkers as a stationary plant, light marine and automotive engine and mirrors the design of the OPOC very closely, except that it has two cylinders inline instead of two opposed pistons, but both are operated from the same crank with the upper piston on a crosshead (its not a skotch yoke as described above, the skotch yoke is a crankshaft & rod arrangement designed to keep the con ron in line with the cylinder at all times). Another well known engine that uses the same single crank and crosshead arrangement is the Doxford ship engine. This is a very large engine designed to propell large ships and was first built in the 1930's. Again, its a vertical engine (about 3 storeys tall!) but differs from the Doxford and OPOC in having crossheads half way along the con rods so they are effectively in two parts (very common for large ship engines). Info on the Doxford can be found here.

Another engine that had only one crank was the Rootes/Commer TS3. This engine was based on an earlier Sulzer design and had the crankshaft located centrally below the cylinders with large rockers linking the pistons to the crankshaft via two con rods each. A very strange looking unit, but actually very effective and used extensively in Commer trucks during the late 1940's until the early 1970's. It was a pretty powerful unit for its time. From just 3.3 litres (203ci) it developed 120bhp at 2400rpm, which for a truck engine was pretty impressive. The nearest equivalent 4 stroke truck engines of that power were closer to 8 litres (450ci). More info here.

The Junkers Jumo had, as was stated, two crankshafts, one for each piston, geared together to keep them in sync and deliver the power from both to the aircrafts propellor. It is not the only opposed piston engine with multiple crankshafts. Probably the most exceptional opposed piston diesel is the Napier Deltic, which is an extremely compact, extremely powerful engine designed for marine, and later, rail locomotive use. It consists of three 'banks' of opposed pistons arranged to form a triangle. Think of it a bit like forming a V engine with opposed pistons, the putting another bank across the top. Each corner of the triangle had a crankshaft for the pistons in the two banks coming together at that point. In its standard 18 cylinder form (each bank having 6 cylinders) it was an 88.9 litre engine (5425ci) and delivered approximately 4000 horespower in its most powerful version. However, it was not as big or heavy as a conventional 4000 horsepower engine. In fact, it was less than half the size and weight. You can get an idea what this engine was about here.

It is also interesting to note that in the UK an opposed piston diesel has already been used by the British army for a tank. The engine was built by Leyland (the truck maker) and featured 6 cylinders and looked like a horizontal version of the Junkers engine or one bank of a Deltic.

So, as you can see the new OPOC engine isn't really that new either. Also, many of the technical issues mentioned in the video were overcome around 50 years ago. As for the engine being perfectly balanced, frankly its not. He has forgotten that the very long rods for the outer pistons change and very short rods for the inner pistons mean they don't move in perfect unison. The outer piston will have relatively even movement, whilst the inner piston will tend to move very rapidly around TDC but very slowly at BDC, meaning the pistons won't balance each other perfectly. Also, to achieve the port timing he demonstrated, the inner and outer pistons are not working on exactly opposed crankpins. The crankpin for the inner pistons will be advanced compared with the pins for the outer pistons, which again affects balance. The result is that, whilst the engine has reasonably good balance, it is far from the perfectly balanced machine claimed, and thus will suffer from the same vibration problems experienced with more conventional engine designs.

In terms of emmissions, most 2 stroke diesels failed to meet the new regulations because they were relatively old designs running traditional mechanical fuel injection and had received minimimal development work. As a result, combustion was generally poorer than the 4 stroke that had been gradually developed over the years whilst the 2 stroke pretty much stood still. If the same development is applied to a modern 2 stroke diesel and is coupled with modern electronically controlled common rail or unit injector technologies, there is no reason why a 2 stroke could not run as clean, if not cleaner than a 4 stroke diesel. More importantly, if done right the 2 stroke has the potential to be more fuel efficient than a 4 stroke of the same output.

[Big Dave]

I'm not from Missouri, but when you claim a two-stroke engine can be cleaner and more efficient than a four-stroker, you are going to have to show me. Detroit Diesel tried for years. EMD tried for years. No joy.

If you say a two-stroker makes more power per unit weight and is more reliable I would tend to agree. If you told me they are more fuel flexible, I'd tend to agree.

I alluded to the Junkers Jumo because its power-to-weight ratio was very high for a diesel and power-to-weight ratio is important for vehicle engines. I knpow opposed piston two-strokers were used in marine andstationary applications. Fairbanks-Morse diesels built in the 1930s are still in continuos-duty service. If I'm not mistaken, the MAN engines in the Graf Spee were opposed-piston nine cylinder two stroke diesel engines.

I am still completely mystified as to how you would package the engine shown in the original video in a motor vehicle.

[UK Mark]

Dave, sorry I've not replied sooner, and I think my rather sweeping statement does deserve some explanation.

The first thing is to understand there are many different factors that affect how efficient a diesel engine is and therefore how clean it will run. The biggest single factor however is how well the fuel and air mix when injected. Poor mixing results in poor combustion and that means a dirty exhaust and frequently poor fuel consumption. The real problem is how to get the fuel and air mixed. Back in the early days of diesels this was done in many different ways. Alongside the direct injection system which dominates today where the combustion chamber is formed as a bowl or toroid in the piston crown, there were numerous other direct and indirect injection systems ranging from the swirl chamber (the longest lasting indirect system found on most smaller engines until relatively recently), pre-combustion chamber, cylinder head mounted air cells (such as the Lanova system used on some Fairbanks Morse engines), piston mounted air cells (extensively used by Cummins) and a host of others. These all aimed to achieve one result. To intimately and evenly mix the injected fuel with the air in the cylinder in a few thousandths of a second. The indirect injection systems were extremely successful at this, and as a rule of thumb in the 1930s, 40s & 50s the cleanest and most powerful engines used some form of indirect injection system. More recently (from the 1940s onwards), the direct injection system was progressively developed to become almost as clean as the indirect systems, but it still has not quite reached the same levels. So, if the indirect is so much cleaner and has better combustion, why has it been replaced by the direct injection system? In a nutshell, indirect injection systems tend to be less efficient as their combustion chambers tend to have a very large surface area and they rely in forcing air through small passages to create high degrees of turbulence, which means they lose a lot of heat from compression and combustion to the cooling system. As a result they can be difficult to start and have higher fuel consumption. So, direct injection has taken over. Today, the best direct injection engines have highly developed combustion chambers that create a lot of turbulence towards the end of the compression stroke along with fuel injectors operating at very high pressures ensuring good atomisation and penetration (i.e. they can fire smaller droplets right across the combustion chamber whereas older low pressure systems had to fire larger droplets to ensure they would get across). The result is that many modern direct injection engines run cleanly and efficiently.

How does this relate to 2 stroke engines? First, it has to be understood that the GM 2 stroke was designed in the 1930s and continued with relatively few changes until it ceased production some 50 years plus later when they found it impossible to make it meet the latest emmissions standards. GM are not the only ones to have done this. Gardner in the UK built probably the best 4 stroke diesel truck & bus engines in the UK based on a 1930s design. They were so well regarded that truck buyers would pay quite a lot extra to have a Gardner engine in their trucks or busses in place of cheaper alternatives from Cummins, Perkins or the truck maker themselves. However, in the early 90's Euro emmissions regulations on diesels started to tighten up, and Gardner found they could not get their engine through the new tests and would need to start again from scratch. But, instead of designing a new engine, they decided it was better to simply stop making new engines and concentrate on supporting all the existing engines instead. Bearing in mind both of these engines were very old designs towards the end of their lives it is little wonder that they could not be brought into line with the new regulations. It is also worth bearing in mind this happened before diesel engines gained the benefit of electronically controlled unit injectors and common rail injection allowing more precise and more flexible fuel metering and timing.

Now for a little guesswork. I suspect that one of the problems with making the Detroit run clean is that it smokes under load at low revs. This would be due to poor scavenging and air charging at low speeds meaning that with the original mechanical injection system too much fuel was being injected for the air in the cylinder. As the revs rose, the scavenging and air charging would improve and the engine would get cleaner until it reached the point where it was going too fast for the air to get through the cylinder and properly scavenge it. If this is the case, a lot could be cured by using an electronically controlled injection system that could better match the fuel quantity to the oxygen in the cylinder. In addition to that, with a uniflow engine like the Detroit, modern variable valve timing technology (a very simple one that merely advances and retards the cam timing based on speed is all that would be required) would be able to match the exhaust valve operation to the inlet ports to give optimum scavenging and cylinder charging at all speeds. The final result would be an engine that would be capable of running a lot cleaner than a traditional 2 stroke. I believe that combining modern combustion chamber designs (based on those from 4 stroke engines), modern electronically controlled high pressure injection and variable valve timing would result in an engine that could be made to run as cleanly as a 4 stroke. If the blower is made a little bigger so that it delivers too much air and some of that is allowed to pass right through the cylinder and into the exhaust that excess air could be used to clean things up further by providing additional oxygen to the catalysts to complete combustion. The overall result would then be a 2 stroke engine that, in theory at least, could run cleaner than a 4 stroke. Of course, much of this is theory and there's a large chunk of guesswork in there, but I believe the prinicples are sound enough and cannot see why a clean 2 stroke could not be built.