2 stroke verses 4 stroke?

|
Want to post a reply to this topic?
Login or register for an acount to join our online community today!

2 stroke verses 4 stroke?

  • What advantages does one type have over the other? Cost fuel consumption etc.
    Replies to this thread are ordered from "oldest to newest".   To reverse this order, click here.
    To learn about more about sorting options, visit our FAQ page.
  • A two stroke EMD loads up today, a four stroke GE loads up sometime next week!
    .
  • One big difference is that a two stroke design has a power stroke in each cylinder for each crankshaft revolution. A four stroke design has a power stroke in each cylinder only every other revolution. Thus you can get twice the power from each cylinder for any engine displacement and for any given RPM setting. This is one reason why EMD's seemingly slow revolutions at maximum power (less than 1000 RPM) is really not so slow. All other things being equal, a two stroke powerplant might be preferable for several reasons. Higher power at lower revolutions means lower friction losses. The engine itself can often be made lighter for the same power output. Diesels have to be built comparatively robustly anyway because of the high compression ratio and the rapid rise in pressure at ignition that weight can be an important concern in engine design.

    One big disadvantage for a two stroke might be that in the past, two stroke designs didn't scavenge combustion products as well as four stroke designs did. However, modern engine designs incorporating advanced fluid flow concepts allows two stroke designs to match four stroke types in this department.

    Alan Robinson Asheville, North Carolina
  • Thank you for the answers...and what would the advantages of a 4 stroke be?

     Would some of the environmental concerns about two stroke outboards and lawnmower engines also apply to locomotive engines?

  • A 4 stroke engine tends to be a bit more efficient than a two stroke, partly because the engine can use all of the ignition/expansion/power stroke, where the 2 stroke will have the intake ports uncovered before the piston reaches bottom dead center. 4 stroke engines are easier to turbocharge and efficiency gains with turbocharging may be a bit more than with a 2 stroke.
  •  Ulrich wrote:

    Thank you for the answers...and what would the advantages of a 4 stroke be?

     Would some of the environmental concerns about two stroke outboards and lawnmower engines also apply to locomotive engines?

     erikem posted the advantages of a 4-stroke over a two stroke, but there's also a turbo lag situation that doesn't occur with the two stroke engines by design - that's why GE locomotives take so long to load.  If you could make a GE load as fast as an EMD, it will belch thick, black smoke due to turbo lag (where the injectors increase fuel delivery before the air can keep up). 

    The environmental concerns of two stroke gasoline engines do not apply to two stroke diesel engines.  On a gasoline two stroke engine, cylinder scavenging is accomplished by pressurizing the crankcase with the downstroke of the piston, forcing an air/fuel mixture (drawn into the crankcase by the vacuum of the piston on its upstroke) through ports in the bottom of the cylinder.  Due to the role of the crankcase in this design, the engine cannot be oiled conventionally and oil must be added to the fuel. 

    During the scavenging phase, this air/fuel mixture pushes the spent exhaust through an opposing set of ports in the cylinder sidewall.  Some of this unspent air/fuel mix will scavenge out of the exhaust port along with the spent gases.  Also, the oil gets burned in the cylinder during normal operation and both of these factors contribute to the increased hydrocarbon content of the exhaust on a two stroke gasoline engine.

    --

    On a two stroke diesel engine, the fuel is direct injected into the cylinder, so during scavenging only air will mix with the exhaust - no fuel available at this point in the cycle.  Also, since the crankcase on a two stroke diesel does not play a role in engine scavenging, the engine can be lubricated in a conventional, pressure type delivery system (with oil in the crankcase as well as in a separate tank).

    Since the crankcase does not play a role in scavenging, a two stroke diesel must be fed air in a slight pressure.  This means employing either a Roots type blower (the 6-71 Roots blower so lovingly embraced in automotive drag racing owes its existence to a two stroke diesel), or a Roots blower with a turbocharger (as in some Detroit Diesel designs) or a hybrid turbocharger driven by the engine through an overrun clutch (as used on turbo EMD engines).  Without this positive displacement of air, the engine cannot scavenge and will not run.

    Typically, a two stroke diesel cylinder employs intake ports on the low end of the cylinder (above BDC), usually surrounding the entire cylinder.  Exhaust is removed through camshaft-operated poppet valves in the cylinder head (like a 4-stroke engine).

    The two stroke engines also, for a given power rating, tend to have less total displacement and lower maximum RPM than the equivalent 4-stroke engine - i.e. the EMD 16-710G3 (two stroke) runs at 900 RPM (Notch 8) vs. a GE GEVO (4-stroke) which runs @ 1050 in Notch 8.

  • Thank you for the great responses...that's alot more detail than I expected and I appreciate it.
  • GM & EMD + Detroit Diesel ( ie the 71 series) versus ALCO & GE, 1 chug per rotation of crank for 2 cycle,1 chug per 2 rev for 4 cycle, this sound even when they are running same range of rpm always made the Detroits really seem like they were moving fast. Its really obvious in a loco, at trackside a GE just chugs wide open, EMD scream. I think it was the 71 Detroits that were a mini copy of 567/645/710 EMDS. ALCO & FM were great chuggers.
  • Correct - two stroke Detroits were based upon EMD designs.
  • All of Alco's various engines were four-stroke, FM's OP engine was also two-stroke.
    Paul The commute to work may be part of the daily grind, but I get two train rides a day out of it.
  •    Great thread.

       One of the things not covered in the 2 stroke debate was the uncontrolable engine.(run away) I have seen many a Detroit 318 or 6-71 run on their own oil. Thats why they have the emergency shut down flap to shut off the air. Working on trucks and heavy machines for forty years I have only seen one 4 stroke run away. Just a couple years ago I had the wonderful opertunity to rebuild a 6V92T that had burned its oil till it siezed. Theres something about those slobbering two strokes. The oil outside of them keeps them from rusting but sure stains your skin.

       Also the 2 strokes need a larger cooling system. A lot of heat with every downstroke.

         Pete
     

     I pray every day I break even, Cause I can really use the money!

     I started with nothing and still have most of it left!

  •  BigJim wrote:
    A two stroke EMD loads up today, a four stroke GE loads up sometime next week!

     

    It seems stange but it is well known in Australia that the Goodwin/ALCO DL531 loads up a lot faster than what the equiv Clyde/EMD G8 does.  So it is just not prime mover charactaristics but excitor/ governor response.

    For those Americans who have not seen a baby alco 

     

    http://www.csse.monash.edu.au/~ajh/trains/nsw/48class/48112+4820-1.jpg

     

     

    Let me reiterate, what I was saying to you previously -Rex Mossop
  • One of the things not covered in the 2 stroke debate was the uncontrolable engine.(run away) I have seen many a Detroit 318 or 6-71 run on their own oil.

    I must say that I have never seen that happen with a diesel locomotive.

    It seems stange but it is well known in Australia that the Goodwin/ALCO DL531 loads up a lot faster than what the equiv Clyde/EMD G8 does.

    I'm not surprised. ALCO's here were fast loading. But, since they have been gone for years, I never gave them much thought for a modern day question.

    That said, it is my understanding that the F-M Trainmasters hold the loading up record, thus beating the trusty ALCO!

    .
  •  Alan Robinson wrote:

    One big difference is that a two stroke design has a power stroke in each cylinder for each crankshaft revolution. A four stroke design has a power stroke in each cylinder only every other revolution. Thus you can get twice the power from each cylinder for any engine displacement and for any given RPM setting. This is one reason why EMD's seemingly slow revolutions at maximum power (less than 1000 RPM) is really not so slow. All other things being equal, a two stroke powerplant might be preferable for several reasons. Higher power at lower revolutions means lower friction losses. The engine itself can often be made lighter for the same power output. Diesels have to be built comparatively robustly anyway because of the high compression ratio and the rapid rise in pressure at ignition that weight can be an important concern in engine design.

    One big disadvantage for a two stroke might be that in the past, two stroke designs didn't scavenge combustion products as well as four stroke designs did. However, modern engine designs incorporating advanced fluid flow concepts allows two stroke designs to match four stroke types in this department.

    You don't get anywhere near a full power stroke from a two stroke engine.  You only are able to get power from the combustion for a fraction of the stroke.  The exhaust valve MUST open before the piston uncovers the inlet ports.  You do not want the exhaust from this cylinder forcing its way into the inlet air of any of the other cylinders.  Once the inlet ports are uncovered the blower must clear the cylinder of the air from the combustion using the remainder of the "power stroke" and the beginning of the "compression stroke".  The compression does not begin until the inlet ports are covered again.  So a "2 stroke" engine only gets a partial power stroke and a partial compression stroke.  And it MUST have a blower of some type to provide air to the cylinders.  That blower requires power that the engine provides. 

    A four stroke engine has a full power stroke and a full compression stroke. 

  • This is one reason why EMD's seemingly slow revolutions at maximum power (less than 1000 RPM) is really not so slow.

    Alan,
    The GE engines don't turn any faster than that.

    One big disadvantage for a two stroke might be that in the past, two stroke designs didn't scavenge combustion products as well as four stroke designs did.

    Now by scavenge, do you mean exhaust? If so, EMD two strokers had it all over the GE four strokes. The exhaust gases from a GE would come out of the stack at such slow speeds that they would curl up right into the windows and make everyone sick when running long hood forward. It wasn't too bad if you were in the eighth notch going uphill at 12 - 15 mph, but any other time when the throttle wasn't worked so high, or notching up, or drifting, or in dynamic the fumes could be terrible. Even switching in yards could make you sick.

    I never had a problem with exhaust gasses from an EMD running long hood forward. The gasses seem to move fast enough to get up high enough that they weren't a problem.

    .