2-cycle vs. 4-cycle diesels

[Murphy Siding]

A lot of what I read about first and second generation diesels talks about 2 & 4 cycle engines. What are the advantages of one over the other? Thanks

[edbenton]

2-cycle will load up faster since there is a power stroke everytime the piston comes up in the cylinder. 4-strokes offer better fuel usage but load slower. A 2-stroke requires positve pressure to remove all the exhaust in it since there is no dedicated exhaust stroke. It all depends on what you are looking for in the way of usage.

[CSSHEGEWISCH]

EMD (except for the 265H) and FM used 2-cycle diesels, the other builders all used 4-cycle diesels.

[Murphy Siding]

QUOTE: Originally posted by edbenton

2-cycle will load up faster since there is a power stroke everytime the piston comes up in the cylinder. 4-strokes offer better fuel usage but load slower. A 2-stroke requires positve pressure to remove all the exhaust in it since there is no dedicated exhaust stroke. It all depends on what you are looking for in the way of usage.

Don't the railroads use both in (mostly) the same type applications?

[edbenton]

They used to anymore the primary switch engine is a rebuilt geep. GE's are not used to switch normally. The older switch engines were made by all manufacters but now a GP series is the primary engine used. Now for road engines anything and everything is used I have seen GP-9's mued with Dash-9's now that was a consist to see. Three Dash-9's with a 1300 series geep the geep was not isolated since it was at night and it was giving quite the light show.

[adrianspeeder]

Two stroke gas or diesel's are tougher to meet emissions standards.

Adrianspeeder

[Paul Milenkovic]

Don't know why a 4-cycle would be slower to load. Locomotive Diesels work in the 500-1000 RPM range -- for a 12 cylinder 4-cycle, you get 6 power strokes every revolution or about 3000-6000 per minute or one every 20-10 ms. You may notice hesitations in the 20 ms range tramping on the accelerator of a car and expecting it to leap from a stop, but in a locomotive? The slow-loading GE must be an attribute of their control system.

On the other hand, I watched an Amtrak consist of P42, 4 Horizon cars, and an F40P cab car, and that thing pulled out of the station smartly at a rate comparable to a Silverliner MU car.

The 2-cycle of course gets a cylinder power stroke for every revolution as opposed to half as often for 4-cycle, and you would think that a 2-cycle would give twice the power for the same displacement. The hangup on the 2-cycle is scavenging -- they have to cram the scavenging time into the compression stroke rather than having a complete stroke to do it, and properly tuned and valved 4-cycle engines are making inroads into traditional 2-cycle territory -- lawn mowers, outboards, the EMD 4-cycle H engine. I suppose people are hanging on to 2-cycle for very slow-speed big-cylindered marine Diesels and I also don't know of any 4-cycle chain saws.

[wabash1]

just remember the old buses the old grey hound buses were all 2 cycle detroit engines

[Tulyar15]

In Britain before privitization, just about all the diesel engines used were 4-cycle (with the notable exception of the Deltics, also the short lived Metro Vick Co-Bo's - the Thomas the Tank enginer character "BoCo" is based on one of these).

Now the all conquering EMD class 66's with their 2 cycle engines are everywhere but I'm pretty sure the equally ubiquitous Cummins engine that's in just about every DMU is 4 cycle.

I'm glad we dont have Greyhound buses here - some of our 4 cycle ones are bad enough!

[daveklepper]

Other advantages of two cycle: less weight and bulk for given horsepower

less noise for given horsepower --except that with some models the turbocharger is the high noise item anyway and when I say less noise, that is before intake and exhaut silencers are applied, sometimes incorporated into air filters on the intake side.

Lower vibration levels for a given horsepower since power is more continuous

As already numerated, 4 cyle naturally gives better economy and lower emmissions

[Anonymous]

QUOTE: Originally posted by daveklepper

Other advantages of two cycle: less weight and bulk for given horsepower

That is one of the fairy tales EMD touted. It simply is not true. On any engine all you need to do to increase horspower is raise the RPM and/or cram more air into the cylinders. The Westinghouse/Beardmore diesel engines that predated the 567 by well over a decade weighed just over half of what a 567 does per horsepower. In fact Beardmore diesels were so light per horsepower that they were used in Blimps and Airships.

[Anonymous]

Unlike most 2 cycle gasoline engines, 2 cycle turbo-supercharged diesels have valves, camshafts, etc. so any potential weight savings isn't as great. Another big difference is they scavenge with fresh air, so emissions isn't as much of a problem. The fresh air charge is blown in through ports at the bottom of the cylinder(not the crankcase) and scavenged through exhaust valves at the top, 4 on most EMDs. More power strokes mean more heat, so that must be dealt with accordingly.

An interesting 2 stroke gasoline engine that meets the new emissions requirements is the Ficht design outboard motor that Outboard Marine went broke trying to get right. The company was bought out by Bombardier and they seem to have ironed out most of the problems. It uses a computer controlled high pressure direct injection system to inject the fuel just prior to ignition.

[Murphy Siding]

Does a 4-cycle *wear* faster, based on having to do twice the motion, to get the same results? Or. am I not perceiving that correctly?

[daveklepper]

"Simply not true?" I would say true in general. That is not to say there may be some 4-cycle engines that weigh less and take less cubage than some 2-cycle, but in general a 2-cycle will be smaller and weigh less than an equivalent 4-cycle.

We already know that marine engines can be less rugged than equal horsepower railroad engines because they are subject to a kinder operating environment in terms of fewer changes in output over a given time, a more stable platform support, etc.

[CSXrules4eva]

Oh man 2 stroke vs. 4 stroke Diesels this is going to be a long one.

A 2 Stroke Diesel basiclly preforms the same acts as a 4 stroke diesel would in one revolution of the crakshaft. However there are some differneces. The two stroke diesel preforms the intake and exhaust function during part of the compression and power strokes, hensse the 2 stroke. On 2 stroke diesels you also have to use a blower which forces air into the combustion chamber. This is commonly called scavenging. This is were the downsides of 2 stroke diesels comes into play. Since the 2 stroke has to complete the Intake, Compression, Power, and Exhaust strokes in half the time of that of a 4 stroke diesel it "rushes" the combustion chamber scavanging process which, in turn doesn't allow the 2 stroke engine to effeciantly produce "all" the power it could give. Here are some other diavantages of 2 stroke diesels: they produce too much pollution, poor power output at low speeds, require more service, must have an oil mix with the fuel, lastly they just aren't as effecient as a 4 stroke diesel.

Ok...now
The 4 stroke diesel engine has the Intake, Compression, Power, and Exhaust stroke. (They are in order by the way) In this case the crankshaft must rotate 2 times to complete one cycle of the engine. So, it takes 4 strokes to equal 1 cycle, which in turn produces engine power. This process is very fast, it is also more efficent than the 2 stroke diesel. The 4 stroke diesel doesn't have to be serviced as often as a 2 stroke. Another advantage of the 4 stroke diesel is that it doesn't emitt as much NOx, SOx, PM, or HCy as 2 stroke diesels would due to the nature of the combustion. In todays world a locomotive manufactur wouldn't be able to use or build a 2 stroke diesel that would meet EPA requirements.
MAKE ANY SENCE???? (Don't worry it took me a while).

[chad thomas]

Sarah, You rock !!!

[espeefoamer]

Is a two stroke diesel inherintly more reliable than a four stroke?The two stroke EMDs were more reliable than other builders four stroke engines.

[silicon212]

"Here are some other diavantages of 2 stroke diesels: they produce too much pollution, poor power output at low speeds, require more service, must have an oil mix with the fuel, lastly they just aren't as effecient as a 4 stroke diesel. "

You're right about the efficiency - but let me add a couple of things here.

They use slightly more fuel due to the fact there's twice as many power strokes per RPM as with a 4-stroke.

To meet Tier 2 Emission certification, GE had to design and build a new 4-stroke engine (the HDL in use in the GEVO). EMD is still using its 710 prime mover, which is a 2-stroke of the same design as the 70+ year old 567 engine. This engine meets Tier 2 in the SD70ACe.

Secondly, while most two-stroke gasoline engines require you to add oil to the fuel, EMD's engine uses a dry-sump oil system (as found on aircraft and race cars), which operates independently of any fuel system. In a two-stroke gasoline engine, the air/fuel/oil mix is introduced into the crankcase and then blasted into the combustion chamber through crankcase pressure generated by the downstroke of the piston. EMD's design does not run the air through the crankcase, but through "airboxes" surrounding the cylinder banks (see the handhole covers mounted on them). This is why these engines must be force-fed air - either with a Roots-type blower or a turbocharger that's engine driven at low speeds where exhaust pressure is insufficient to drive the turbo.

Also, the two-stroke engine does not need maintenance any more than a 4-stroke does, and tends to be more reliable due to its simpler design. However, EMD's two stroke design isn't as elegantly simple as a gasoline engine due to the fact that it uses exhaust valves and overhead camshafts to run both the valvetrain and the injectors.

[Murphy Siding]

QUOTE: Originally posted by chad thomas

Sarah, You rock !!!

As I read Sarah's post, I was thinking the same thing![:)]

[BigJim]

I haven't seen anybody mention how much faster an EMD power assembly can be changed compared to a GE.

[M636C]

QUOTE: Originally posted by Paul Milenkovic

Don't know why a 4-cycle would be slower to load. Locomotive Diesels work in the 500-1000 RPM range -- for a 12 cylinder 4-cycle, you get 6 power strokes every revolution or about 3000-6000 per minute or one every 20-10 ms. You may notice hesitations in the 20 ms range tramping on the accelerator of a car and expecting it to leap from a stop, but in a locomotive? The slow-loading GE must be an attribute of their control system.

On the other hand, I watched an Amtrak consist of P42, 4 Horizon cars, and an F40P cab car, and that thing pulled out of the station smartly at a rate comparable to a Silverliner MU car.

The 2-cycle of course gets a cylinder power stroke for every revolution as opposed to half as often for 4-cycle, and you would think that a 2-cycle would give twice the power for the same displacement. The hangup on the 2-cycle is scavenging -- they have to cram the scavenging time into the compression stroke rather than having a complete stroke to do it, and properly tuned and valved 4-cycle engines are making inroads into traditional 2-cycle territory -- lawn mowers, outboards, the EMD 4-cycle H engine. I suppose people are hanging on to 2-cycle for very slow-speed big-cylindered marine Diesels and I also don't know of any 4-cycle chain saws.

It is indeed the GE control system and design features of the two designs of engine that affect the speed of "loading up", rather than inherent features of two and four stroke engines.

The GE control system is set up to slow "loading up" to prevent the production of smoke (and other unfavourable gases) due to turbocharger lag, the turbocharger not supplying sufficient air during the early stages of acceleration. On the other hand, the EMD engine has a turbocharger driven directly from the engine crankshaft until the exhaust gases develop enough power for it to run free. This feature allows earlier "loading up" of the engine.

While again there is no cycle related reason for a weight difference, the GE FDL with a cast crankcase is significantly heavier than the fabricated EMD 710G3 and this might influence judgements where a "2 cycle vs 4cycle" question actually relates to two specific engines. I don't have any figures for the GEVO engine.

M636C

[oltmannd]

One more difference. Because the 2 cycle engine gets twice as many "bangs" per crank rev, those "bangs" are about 1/2 as powerful. So, the whole engine design can be much less robust. A good example would be to compare the cross se.ction of an EMD con rod with that of a GE. EMD can get away with a fabricated engine frame vs GE's cast one, too.

[Murphy Siding]

QUOTE: Originally posted by oltmannd

One more difference. Because the 2 cycle engine gets twice as many "bangs" per crank rev, those "bangs" are about 1/2 as powerful. So, the whole engine design can be much less robust. A good example would be to compare the cross se.ction of an EMD con rod with that of a GE. EMD can get away with a fabricated engine frame vs GE's cast one, too.

Trust me when I say I'm not playing dumb about this- I am dumb about this. Wouldn't one downstroke, and one upstroke equal one revolution of the crank, regardless whether it's a 2-cycle or a 4-cycle?

[Paul Milenkovic]

Murphy Siding:

I believe the argument is that while each rev produces one downstroke, the 2-stroke is producing a bang with every downstroke while the 4-stroke is producing a bang with every other downstroke -- kind of like bang-bang-bang vs BANG-nothing-BANG-nothing.

Besides, doesn't a GE produce a kind of "chug-chug-chug" sound while an EMD produces more of a "brrrrrrr" sound when they start up?

[tormadel]

QUOTE: Originally posted by CSXrules4eva

Oh man 2 stroke vs. 4 stroke Diesels this is going to be a long one.

A 2 Stroke Diesel basiclly preforms the same acts as a 4 stroke diesel would in one revolution of the crakshaft. However there are some differneces. The two stroke diesel preforms the intake and exhaust function during part of the compression and power strokes, hensse the 2 stroke. On 2 stroke diesels you also have to use a blower which forces air into the combustion chamber. This is commonly called scavenging. This is were the downsides of 2 stroke diesels comes into play. Since the 2 stroke has to complete the Intake, Compression, Power, and Exhaust strokes in half the time of that of a 4 stroke diesel it "rushes" the combustion chamber scavanging process which, in turn doesn't allow the 2 stroke engine to effeciantly produce "all" the power it could give. Here are some other diavantages of 2 stroke diesels: they produce too much pollution, poor power output at low speeds, require more service, must have an oil mix with the fuel, lastly they just aren't as effecient as a 4 stroke diesel.

Ok...now
The 4 stroke diesel engine has the Intake, Compression, Power, and Exhaust stroke. (They are in order by the way) In this case the crankshaft must rotate 2 times to complete one cycle of the engine. So, it takes 4 strokes to equal 1 cycle, which in turn produces engine power. This process is very fast, it is also more efficent than the 2 stroke diesel. The 4 stroke diesel doesn't have to be serviced as often as a 2 stroke. Another advantage of the 4 stroke diesel is that it doesn't emitt as much NOx, SOx, PM, or HCy as 2 stroke diesels would due to the nature of the combustion. In todays world a locomotive manufactur wouldn't be able to use or build a 2 stroke diesel that would meet EPA requirements.
MAKE ANY SENCE???? (Don't worry it took me a while).

I thought the SD70ace's were 2 strokers that meet EPA regs?

[silicon212]

Yes they are.

[daveklepper]

Again, because you get twice the number of power strokes per number of revolutions of the crank shaft, for a given RPM and cylinder size and all other things being equal, a four-stroke will have half the horsepower of the two-stroke. That is why GENERICKLY, a four-stoke is usually heavier and larger than a two stroke. Obviously if the four-stroke runs at twice the PM, has a far more efficient power train (electrical system) and supercharger, and just better design, and is designed for a less rugged environment, you can have a situation where the reverse is true.

I am sure EMD has or will have the technology to allow two-stroke engines to meet future emission requirements.

One possibility is the adoption of the old steam locomotive blow-off valve principle and have exhaust valves built into cylinder walls other than just the valves at the top of the cylinder, overcoming the piston-ring wear that would occur as the rings pass the seated valves throuigh advanced netallurgy and using the latest computer technology with reduncancy to assure accurate valve timing.

[Leon Silverman]

The engine structure of a 4-stroke has to be heavier than a 2-stroke engine because higher pressures are developed in the 4-stroke. The compensating factor is that the cooling requirements for a 4-stroker are far less than that of a 2-cycle engine producing the same horsepower. Being less fuel efficient translates into the requirement that the heating value of the excess fuel has to dissappated in the cooling system instead of appearing as useful work.

[Anonymous]

All else being equal, the 2 stroke diesel looses some power and boost because the intake port opening and closing is controlled by the piston and symetrical around bottom dead center i.e. the ports & exhaust valves open before the piston completes a full power stroke. When the ports and exhaust valves are both open, some of the boost pressure is being blown right through the cylinder in order to scavenge the cylinder. A 4 stroke can use the full power stroke and because it has a seperate exhaust stroke, all of the boost can be used to increase effective compression.

Because it has one power stroke per revolution instead of one every 2 revolutions, a comparable 2 stroke will have more power, but I don't think the theoretical difference is a factor of 2, that would depend of valve/port timing, compression ratio, boost pressure and turbo size. I suspect the GE's are running much higher compression and effective boost to make up the difference, thus the need for a beefier bottom end, broken con rods, etc.[:)]

Does anyone have actual specs for GE's and EMD's?

[daveklepper]

On a very old thread, several years back, our great mentor/teacher/referee, now in Iraq, put a full posting of all EMD engines up to that date. I think I managed to copy it to a floppy and mabe I can locate it.