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
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.
EMD (except for the 265H) and FM used 2-cycle diesels, the other builders all used 4-cycle diesels.
Don’t the railroads use both in (mostly) the same type applications?
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.
Two stroke gas or diesel’s are tougher to meet emissions standards.
Adrianspeeder
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.
just remember the old buses the old grey hound buses were all 2 cycle detroit engines
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!
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
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.
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.
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?
“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.
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).
Sarah, You rock !!!
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.
"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.
As I read Sarah’s post, I was thinking the same thing![:)]
I haven’t seen anybody mention how much faster an EMD power assembly can be changed compared to a GE.