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2 stroke verses 4 stroke?

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2 stroke verses 4 stroke?
Posted by Ulrich on Wednesday, January 23, 2008 6:14 PM
What advantages does one type have over the other? Cost fuel consumption etc.
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Posted by BigJim on Wednesday, January 23, 2008 7:52 PM
A two stroke EMD loads up today, a four stroke GE loads up sometime next week!
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Posted by Alan Robinson on Wednesday, January 23, 2008 10:31 PM

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.

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Posted by Ulrich on Wednesday, January 23, 2008 10:37 PM

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?

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Posted by erikem on Wednesday, January 23, 2008 11:01 PM
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.
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Posted by silicon212 on Wednesday, January 23, 2008 11:20 PM
 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.

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Posted by Ulrich on Thursday, January 24, 2008 8:52 AM
Thank you for the great responses...that's alot more detail than I expected and I appreciate it.
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Posted by Anonymous on Friday, January 25, 2008 8:45 PM
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.
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Posted by silicon212 on Friday, January 25, 2008 8:56 PM
Correct - two stroke Detroits were based upon EMD designs.
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Posted by CSSHEGEWISCH on Saturday, January 26, 2008 6:49 AM
All of Alco's various engines were four-stroke, FM's OP engine was also two-stroke.
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Posted by locoi1sa on Saturday, January 26, 2008 8:36 AM

   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
 

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Posted by gregrudd on Tuesday, April 01, 2008 5:55 PM

 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
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Posted by BigJim on Tuesday, April 01, 2008 9:21 PM

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!

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Posted by JonathanS on Wednesday, April 02, 2008 8:03 AM
 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. 

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Posted by BigJim on Wednesday, April 02, 2008 9:42 AM

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.

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Posted by JonathanS on Monday, April 07, 2008 12:35 PM
 BigJim wrote:

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.

Scavenge is a part of a two stroke cycle that does not have an equivalent in a 4 stroke cycle.  In a 4 stroke engine the piston comes up to its TDC position pushing out almost all of the exhaust gasses.  On a two stroke engine first you have a short power stroke, then the exhaust valve opens and releases the pressurized combustion gasses.  Then the piston uncovers the inlet ports and the blower or turbo must blow out the remainder of the exhaust gasses in the short time from when the inlet port is uncovered until the exhaust valve closes.

That brings another small advantage that the 4 stroke engine has.  It compresses over the entire stroke of the piston while a 2 stroke engine can only compress from the time it covers the inlet ports to TDC.  So for a given piston and crank, a 4 stroke engine has a higher compression than does the 2 stroke. The difference must be made up in a longer total stroke or by decreasing the size of the combustion chamber.  Both solutions introduce design problems.

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Posted by BigJim on Monday, April 07, 2008 4:47 PM
Scavenge is a part of a two stroke cycle that does not have an equivalent in a 4 stroke cycle. 

Well it certainly does on the exhaust side of things. Anyway, you haven't explained why the exhaust gasses from the GE's would cause problems and EMD's didn't.
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Posted by edbenton on Monday, April 07, 2008 7:34 PM
Because even at low engine speeds the EMD engine has a Roots style blower or the Turbo is Direct GEAR driven on it to provide the pressure needed to scavange the crankcase of the combustion gasses.  A 4 Stroke Engine at low speed would not need since the engine has the design advantage of having a dedicated stroke for both eshaust and then one for intake.
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Posted by JonathanS on Tuesday, April 08, 2008 7:21 AM

 BigJim wrote:
Scavenge is a part of a two stroke cycle that does not have an equivalent in a 4 stroke cycle. 

Well it certainly does on the exhaust side of things. Anyway, you haven't explained why the exhaust gasses from the GE's would cause problems and EMD's didn't.

Scavenge is the part of the 2 stroke cycle that happens when the exhaust valve is open and the inlet ports are uncovered so air is blowing into, through, and out of the cylinder.  There is not a time in a 4 stroke cycle that both the exhaust and inlet valves are open at the same time.  So there is no equivalent of scavenge in a 4 stroke engine.

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Posted by erikem on Tuesday, April 08, 2008 10:59 PM
 JonathanS wrote:

Scavenge is the part of the 2 stroke cycle that happens when the exhaust valve is open and the inlet ports are uncovered so air is blowing into, through, and out of the cylinder.  There is not a time in a 4 stroke cycle that both the exhaust and inlet valves are open at the same time.  So there is no equivalent of scavenge in a 4 stroke engine.

I beg to differ - most four cycle engines have a bit of valvle overlap where both the intake and exhaust valves are open at the same time - the exhaust valve would be in the process of closing while the intake valve would be opening. Having said that, you are correct in that there is no equivalent of scavenge in a 4 cycle engine. 

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Posted by BigJim on Wednesday, April 09, 2008 9:04 AM

Scavenge is a part of a two stroke cycle that does not have an equivalent in a 4 stroke cycle. 

Having said that, you are correct in that there is no equivalent of scavenge in a 4 cycle engine. 

It is obvious that youz and I are talking about completely different aspects of scavenging.

As I keep repeating about the exhaust side of things, or in other words, what happens from the exhaust valve out toward atmoshpere, or in other words again, the ability of one exhaust pulse to help suck another exhaust pulse and another and another out toward the atmosphere thereby helping the engine to breathe.

edbenton,
Thank you very much for jogging my brain and pointing out the obvious. Somewhere in the back of my mind I knew that blower was the key to the EMD success.

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Posted by edbenton on Thursday, April 10, 2008 2:13 PM
Valve Overlap has more to do with keeping the vlaves cool than scavenging the cylinder out.  Think about it the exhaust gasses are at 1200 degrees about Steel weakens at 1300 or so.  By using a bit of overlap you can keep the Valves coole enough to keep them from burning the heads off and leaking.  Now here is somthing interesting about a Detroit motor you put in a reverse flow oil pump and the sucker would run backwards and not even care EXCEPT FOR ONE THING it would be sucking the air from the Exhaust side and out the INTAKE.  Had it happen to me on a skidpad training run with the guy next to me driving I knew right away to shut it off he paniced and RAN.
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Posted by silicon212 on Friday, April 11, 2008 8:12 PM
Most exhaust valves for big diesel engines, two stroke or not, are made of inconel for that reason.
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Posted by erikem on Saturday, April 12, 2008 11:37 AM

 edbenton wrote:
Valve Overlap has more to do with keeping the vlaves cool than scavenging the cylinder out.

I'm pretty sure that valve overlap has more to do with inertia of the valves amd gas flow rather than valve cooling. For example, racing cams have a lot more overlap than cams intended for street driving. The overlap on a diesel engine is presumably lower than an equivalent gasolene engine due to the lesser clearance at TDC due to the higher compression ratio. Also bear in mind that when the piston is near TDC, it is barely moving and there isn't much impetus to either pushing exhaust gases out or drawing in intake air.

Something else to keep in mind is that on almost all 4 cycle engines (gas and diesel, is that the exhaust valves open before the piston reaches BDC of the power stroke and the intake valves close after the piston reaches BDC of the intake stroke. 

The high compression ratio of a diesel engine pretty much ensures that almost all of the exhaust gases in te cylinder will be pushed out during the exhaust stroke of a frou cycle engine. The pressure inside the cylinder at the bottom of the power stroke is typically much higher than ambient even without some form of supercharging - which is why turbochargers can improve the efficiency of a diesel engine (also why the Wright turbo compounds were more efficient than the non-compounded engines).

An interesting tidbit about the EMD engines - the exhaust valves close closer to TDC on the compression stroke than they open after TDC on the power stroke. This improves scavenging by allowing more time for the exhaust to leave the cylinder and also gives the engine a higher expansion ratio than compression ratio (which is what the Miller cycle does on the engine of the Toyota Prius).

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