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Four cycle vs Two cycle diesel engnes

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  • Member since
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Four cycle vs Two cycle diesel engnes
Posted by cessna 310 on Tuesday, February 16, 2016 9:58 AM

Would someone help me to understand the differences between these engines and the benefits? I thought EMD tried this technology before and did not have success.

Thanks, Cessna310

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Posted by Wizlish on Tuesday, February 16, 2016 5:34 PM

In a two-stroke engine, every 'down' stroke of the piston is a power stroke.  To make this work you need to arrange some method of blowing the burned gas out of the cylinder and replacing it with fresh air (in a diesel engine, the fuel is usually injected into the charge air at the end of the compression stroke and therefore you don't need a fuel-air mixture, just air.

The EMD engines (567, 545, 710) accomplish this with pressurized charge air introduced through a ring of ports in the cylinder wall low down.  These open "automatically" whenever the piston is low enough.  There are four big valves in the cylinder head that open to facilitate the sweeping out of the combustion gas quickly enough that 'sufficient' clean air is in the cylinder for the next fuel injection.

Note that this is not like the sort of reed-valve two-stroke gasoline engine in your lawnmower or chain saw, which needs lube oil mixed with the fuel.

In a four-stroke engine, the intake charge (air or mixture) is drawn in by moving the piston down, then compressed by moving it up.  Subsequent ignition and combustion drives the piston down, and it then rises ejecting the burned gas.  This is the cycle used in most modern automobiles.

Advantage of the two-stroke is that it produces somewhat more power than a four-stroke of comparable displacement operated at comparable speed, and that its construction can be somewhat lighter.  One advantage of the four-stroke is that it generally has better fuel efficiency; another is that it can run with lower emissions.  I believe you can boost/intercool a four-stroke design to higher pressures (and therefore burn more fuel effectively per stroke).

EMD's problems with four-stroke technology were not related directly to the type of engine; they were related to the crankcase construction selected (in large part a choice of the German designers of the 'inspiring' engine).  This was a cast piece, not fabricated like the EMD two-strokes, and featured some areas of relatively complex contour and thin walls.  Resonance from the relatively sharp 'knock' of polynucleate diesel ignition set up ultrasonic vibrations that reinforced at key locations, causing little bubbles to form and then collapse 'explosively' in a process known as cavitation.  The H engine (265) was not the only engine in that general time period to have issues like this; I believe some cavitation effect was a principal factor affecting the general non-success of the prime movers in the 6000 hp GEs (also German-originated, from Deutz).  I also believe some of the Ford Powerstrokes had this kind of issue, but I don't know which ones. 

It's interesting to comtemplate how the 'new' 1010 engine will do in this respect; I know Entropy has distinctive competence here and I suspect there are others with hard knowledge.  Peter Clark and some others have commented on the general 'worth' of the 265-H. including details in a few (now old) threads here. 

 

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Posted by M636C on Tuesday, February 16, 2016 7:35 PM

Wow, one post into a thread and I've been referred to already...

While the EMD 265 certainly had problems, its early demise was more due to the unexpectedly good performance of the two stroke 710 engine in Tier 2 and Tier 3 emissions testing and the desire of the railroads to stick with a proven product.

GE had less choice in that it appears the FDL could not realistically meet Tier 2 and Tier 3 emissions standards in railroad application, (although it could do so in a marine environment where there was much more cooling water available, and the FDM became the V228).

The GEVO was developed from the HDL and I recall references to a thicker and stiffer crankcase casting being one of the changes. The Tier 4 version of the GEVO is said to be 8000 pounds heavier which suggests even more rigidity combined with a stated slightly greater length. The HDL had a very distinctive "hollow" exhaust note compared to the FDL which went away when the AC6000CWs received  GEVO-16 engines.

EMD 265s have no such distinctive sound, for whatever reasons.

Cavitation is the result in a sudden pressure drop in a liquid which allows dissolved gases to form bubbles. These bubbles collapse as the pressure rises again and the collapse of a bubble against a metal surface results in a tiny high pressure water jet that erodes the metal surface.

The main area of interest in cavtation was ship propulsion and screw propellers which tended to erode away under high power since cavitation bubbles formed at the trailing edge where the high pressure behind the blade met the lower pressure in front of it.

The SS United States, arguably the fastest conventional ship in the world (although US Navy Aircraft Carriers wouldn't be far behind, if at all) had its four huge bronze propellers swapped each year during the winter low season to repair the serious erosion due to cavitation.

On my first trip to sea on the Australian Aircraft Carrier "Melbourne", I stood on the quarter deck looking at the helical cavitation trails clearly visible in the wake, partly since we left in the early evening. That ship had twin three bladed screws and about a fifth of the power of the "United States" and was at low power at the time described.

But to return to the subject at hand, both GE and EMD had problems with their new four stroke engines. EMD were able to continue using their existing engine while GE had no choice but to turn the HDL into the much more successful GEVO. There is no reason to think that the 1010 will be any less successful than the GEVO even though EMD didn't need to use it until Tier 4 standards arrived. Certainly Caterpillar have years of experience in four stroke engines, and have their own engine in that power category, the C280. That engine was tried without success in locomotives previously, but quite some time ago.

But if Caterpillar felt the 1010 had serious problems, they could have gone forward with the C280 instead. I'm sure there were some within Caterpillar that wanted to use the C280.

But the problems experienced by the 6000 HP engines were due to pressure to get the product on the market quickly as much as any differences between two stroke and four stroke engines, since both EMD and GE had problems at that time.

GE have built and sold a lot more GEVO engines, but the current engines are quite different from the first units that followed the HDL. Equally the 1010 is substantially different from the 265H. EMD haven't built as many 1010s, but they have had time to work on the design, and there are around 400 265Hs working in China (but rather more GEVOs there and elsewhere).

We can only wait and see.

Peter

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Posted by cessna 310 on Wednesday, February 17, 2016 11:16 AM

Thank you for the information.

Cessna 310

  • Member since
    March 2009
  • 62 posts
Posted by cessna 310 on Wednesday, February 17, 2016 11:18 AM

Just the info I was looking for. Great avatar!

Cessna 310

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