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Discussion Starter · #1 · (Edited)
I've been compiling a research paper for the past year or so on different engine architecture efficiencies.

I understand there are politics at all levels in corporates and management decisions don't often relate to logic...When all is said & done the 2-stroke stacks up better in every way than 4-strokes...some would say the the natural process of evolution...thats is...the the most suited to the task will ultimately prove the strongest and therefore dominate...that doesn't often prove the case in business, where personalities are making emotive decisions on their prejudices and miss-information...Soooo, what I'm at a loss on, is if the likes of the Detroit type 71/92's rate higher fuel efficiencies than the best 4-strokes...are mechanically simpler and easier to manufacture...have more efficient thermal cycles with less heat rejection to the engine with lower Co emissions...a combustion cycle each revolution and for the same power ratings, lower BMEP's and associated lower Nox emissions...longer mechanical service life through less stressed components...Why are more of them around in all applications.

Oil carry over with cylinder ported scavenging had been an issue with the concept which has been dealt to loooong ago...look at the Fair Banks Morse 38D8 effectively unchanged through its 75 year life with inlet AND exhaust cylinder ports AND a frugal 0.08% oil consumption, far lower than most of its competitors.

Detroit Diesel seem to be the last manufacturer persisting with the principle, but even they have ventured into 4-stroke designs, It would seem to satisfy market interests in that direction...how is it 4-stroke engines retain their market place...is there something I'm missing in the real world?

Found a good site listing some history,
http://www.dieselduck.ca/historical/01 diesel engine/detroit diesel/index.html#.UUfwma5WySp
 

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...is there something I'm missing in the real world?
Yes sir, quite a bit.
..are mechanically simpler and easier to manufacture...
I don't know how. The 53, 71, and 92 series all have fuel passages machined into the cylinder heads. The heads are way more complex than a Cummins of the same era. Cracked 53 series heads (71 and 92's to a lesser extent) kept the scrape iron dealers in business. The blocks are relatively complex too. It's true, all 3 series have no intake valves but it has to have a supercharger (no small expense) plus it still has exhaust valves and therefore camshafts. In fact the V8's have 2 camshafts. The camshafts also have a 3rd lobe which fire the injectors. As someone posted the other day, setting the rack, buffer screw, governor, injectors (fuel system tuneup) on the 2 cycles is an art all in itself. Unlike a comparable Cummins, there's very few people nowdays that even know how to do it. All through the years I've worked with very few diesel mechanics, including myself, who didn't feel a Cummins of the same era was WAY easier to work on from fuel systems to complete overhauls.
As far as performance, the non-turboed 71 and 53 series couldn't help but lose power at higher elevations and the 92's weren't a lot better. I drove a 8V71 powered lo-bed semi, and logging truck and in later years a 92 powered semi cattle truck. I also drove a Cummins powered logging truck, lo-bed semi, and later transfer truck and tlr dumps (one was a Cat) and comparing power plants I much prefer the Cummins when it comes to power, noise, vibration and overall performance compared to the 2 cycles.
You hear crap all over the internet from the wannabees about the "oil leaking" Detroits. The Detroits have the same type valve cover, pan, front cover gskts, front and rear seals as any other engine and are not prone to leakage more than any other engine. What does drip on the older engines is the air box drains which they are designed to do, no big deal. Last, the old Cummins engines were very easy to "hop up" for more power, whereas the Detroits are not. I'm not brand bashing, just trying to give an honest realistic opinion.
On Edit: All through the years I never did hear that the Detroit 2 cycles EVER got better fuel mileage from owner/operators or written information. I never was around the 4 cycle 6 cyl inline series 60 Detroits, but a few guys I knew that bought them told me they loved them.
 

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Discussion Starter · #3 ·
Cheers, LMJD;
Thanks for the heads up...its what I'm looking for...unfortunately a LOT of the info I've been wading through has been academic focused, and you'd probably know where thats taking me...another delema I've been encountering following development trails has been the political decision making through many of the companies heading/leading developments (Rolls Royce seems particularly bade in that sense).

From an engineering view point the 2-strokes should have an advantage both in performance (Lbs/hp & hp/cu") and efficiency...I have some research material that backs it up...but really trying to add reasoning to it all...being wary of academic type info...they don't typically follow common sense thinking.

Definitely on a hp per cui basis and hp/g-hp-hr fuel efficiency all things being equal (tuning etc) the 2-stroke adds up to be ahead and in durability...but at the end of the day build quality is a big factor as with everthing...and thats a point very little material factors in or accounts for.

When you look into the monumental problems turbo-jet engines had in the early days (before either Wwar)...and even later years up to the 80's its a wonder the concept got off the ground (so to speak).

Problems encountered have been quite notable prejudices...such as exist with the Wankel engine...anyone and everyone I've had dialog with still believe the rotor seals are the problem...not at all. The principle design is seriously flawed in combustion chamber surface area (vs squish/quench area and the profile at TDC) and will never be able to meet ever restricting emissions.

Similar prejudices appear on the 2-stroke Si arguement...efficiencies in direct injection...ha...the problem still exists of lubricating oil carry over...with crankcase scavenging...being total loss oil systems...its all got to go out the exhaust port eventually...they need some form of condensers and/or particulate filters... Browsing the forums etc...bugger all commentators have considered the point of low ash oils and that 100:1 ratio the manufacturers have been targeting for ages...1% seems to be a magic number for some reason but it needs to get a whole lot lower than that...then they blow up!

I find it bloody funny...with all the emissions std's and engine oil consumption...there's NOTHING...that I can find where the rule makers ever consider...what happens to the oil when people change it...a LOT I've spoke to that change their own...chuck it in the wheely bin...consider the oil leaching from new seal on the roads...all those tires building up...about 5ltrs or oil per tire :-?

Anyway...got a bit off the track there!!!

Nothing quite like experience, and thats getting rather hard to find these days...bit like common sense!
 

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Your post is interesting, lots to think about. As far as a cubic inch versus efficiency discussion, the 2 cycle 8V92 is 736 cubic inches and it's close competitor and comparable engine at the time was the Cummins 400 which is the same size as the decades older versions, 855 cubic inches. But there again, I think most guys would agree, the Cummins (4 cycle inline 6)wins out in overall performance. Same with the 6 cyl Cats, but I don't know their cubic inches.
...what happens to the oil when people change it...a LOT I've spoke to that change their own...chuck it in the wheely bin..
The last shop I worked in was huge and we filled a big tank with used oil frequently. Years ago I talked to the tanker driver when he was sucking out the oil and he thought it eventually was used to power big ocean ships many of which I guess are turbine powered. Personally, my local landfill takes my used oil (it goes in a recycle tank) plus Walmart and a parts house or two do also.
 

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First of all, I will state that all I know about diesel 2-strokes is what I have read on this forum. I raced mx from my youth until I was 30, which was the beginning of the modern 4-stroke mx machines. From my mx experience, I know that 2-strokes rev a lot higher, produce right at twice the power, but do not have the reliability of the 4-stroke. When 2-strokes were the machines everyone used, they constantly HAD to be rebuilt to avoid catastrophic failures while the 4-strokes of the time could literally last years before piston/ring replacement/service.

My question: Is the same true for the diesel 2-stroke vs. 4-stroke engines (generally speaking)? I am not talking about the emission issues, just the performance and maintenance issues. Thanks for your replies!
 

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My question: Is the same true for the diesel 2-stroke vs. 4-stroke engines (generally speaking)?
Based only on my experience back in the stone age when I had a Yamaha 250 dirt bike, no, IMO 2 stroke bikes and 2 stroke diesels don't have much in common other than they are both two strokes. The diesels have exhaust valves, but DO have intake ports in their cylinder liners (sleeves):

Air is forced into the ports by a low pressure blower (supercharger) rather than crankase pressure (through a reed valve?) in a bike engine. Engine for engine, the Detroits ran at the same max RPM as a Cummins, 2100 or 2250 RPM. I think it's safe to say the Detroits don't have more power, if even as much, as a Cummins. Personally, I can think of many specific cases where they both equal each other in reliability. They're both tough "industrial" engines designed to be under constant load.
 

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Discussion Starter · #7 · (Edited)
01f25073
My question: Is the same true for the diesel 2-stroke vs. 4-stroke engines (generally speaking)? I am not talking about the emission issues, just the performance and maintenance issues. Thanks for your replies!
I think i've got this "quote thing worked out :-?
Where do we start...what LMJD says is probably conservative...the only similarity is they fire on every stroke...and that in its self delivers the power.

On the dirt bike front...2-strokes will always deliver more torque and hp all things being equal...HOW CAN I SAY THIS...ever been interested in motorcycle trials...2-strokes ruled, and even Honda couldn't develop a 4-stroke to compete in a sport where torque is everything...rpm is only constrained by piston speed...except... in 2-strokes the piston has to interact with holes in the cylinders, called ports. This limits cylinder ported 2-strokes from having the ultra short strokes that the likes of F1 and Indy engines run, because of piston rock in the cylinder bore.

A problem for ALL 2-strokes is managing piston and ring temperature. Next to combustion detonation, this is the biggest issue causing seizing. Most of a pistons heat is transfered through the piston ring to the cylinder...Overheating rings typically on the exhaust port side burn off lubricating oil and begin to scuff...and pistons overheat, swell and themselves scuff and jam in the bores. That rattly "ring" 2-stroke bike & chainsaws make as they get older is piston slap...eventually piston slap causes the piston to lip out on the port and bang, you're off :-! I've seen it happen at idle!

We ALL know combustion efficiency is directly influenced by combustion chamber design, higher compression ratio (up to a fuels auto ignition) and intake charge swirl.

2-stroke crankcase scavenged engines of the type found in chainsaws, outboards and motorcycles, are more "efficient" in combustion than 4-strokes but less "efficient" in scavenging...HOW IS THIS!!...without playing with words... it depends on your interpretation of "efficient". In this case they burn the fuel/mix more effectively (quicker and more of the mix) than 4-strokes (Fact!) due to very compact and ideal shaped combustion chambers, no valves in the way dictating chamber design...squish band lands...or plug placement...also...2-strokes are more efficient at light load part throttle, as long as they're not 4-stroking (which indicates a richer than ideal mix for that engine state).

Fact! 2-stroke engines run at their designed C/R ALL THE TIME
Consider! What is the compression ratio of your car...when your trundling along the road...at 1/4 throttle. Think about it! if the enf=gines designed C/R is for arguements sake 10:1 uncorrected, and at 1/4 throttle the engines breathing in 1/4 its volume...that would be about 2.5:1...doesn't make for efficient combustion! and your car spends most of its life at this engine state.

Because a 2-stroke "can't" evacuate the cylinders volume of gas...any gas...it always traps a cylinders full volume...even at idle...a 4-stroke only traps what it sucks in because its expelled the cylinders corrected trapped volume. At idle some exhaust gas is sucked back into the cylinder before the exhaust valve closes, because there is a high vacuum on the piston side of the throttle...so the idle speed charge is affected by residual hot inert gases...

If efficient scavenging means, all the exhaust is expelled for a clean unpolluted! fresh intake charge...then 4-strokes are more efficient, ahh, that is...only at full throttle...because at part throttle, this condition is never met.

2-strokes are actually more efficient in scavenging...and a result of this is they can be tuned much leaner than 4-strokes. The principle of HCCI Homogenous Charge Compression Ignition is a factor of a 2-strokes lean burn capabilities...because with the fuel mix passing through the crank case, it absorbs heat and reaches a finer point of evaporation and higher volatility which enables better burn and leaner mix as a result. In the days of aircooled 2-stroke bikes (DT250) and you've just thrashed your bike down the beach...screeched to a stop and turned your key off...it would some times continue to run, and for some time...unless you clicked into gear and dropped the clutch...sometimes it would even run backwards...that made experts ask what was happening...HCCI...because the engine (piston & head) had reached a critical temperature and spontaneous combustion was happening...not necessarily detonation, though that could have been happening too.

We are running out of Crude oil...HCCI is a serious solution.

2-strokes of old could run for ever...they could compete performance wise with 4-strokes, with small ports, though they were typically made as budget machines due to their few parts (machining) and simple design...modern high performance 2-strokes have very large ports and typically have port bridges just a millimeter or two for pistons and rings to slide over...that is a problem for ring and cylinder life. Motorcycle trials bike engines seem to have quite long lives...I don't know the overhaul life cycle of modern MX 4-strokes. I do know they can't pump out the power of the 2-strokes though.

A Question to ponder! ever wondered why...when you're riding down the street and then button off...the engine goes ping......ping.....ping and does the same at idle. Play with the secondary jet mixture needle on a chainsaw and you can make it idle smooooth as...and firing on every revolution, unfortunately you can't get it to pick up rpm...think about it!

Thats not a problem of the engine or its design, more of the carburetor...

Boaties have been hailing the praise of direct injection being the halyluya of the 2-stroke outboard...sure they are hp/hp lighter and more fuel efficient...BUT... the problem of oil carry over with the total loss lubrication still remains a difficult emission to resolve...uniflow 2-strokes don't have this problem with the same lubrication systems as 4-stroke engines.

Uni-flow 2-stroke diesels are a way different beast and their principle functions have very different roles, their design criteria and operation are very different. Factor in the different design criteria and issues, like for like, of cylinder porting (Bristol Hercules & Centaurus sleeve valve 4-stroke radial Aero engines) vs poppet valves...look how long it took to develop reliable poppet valves...The problem Bristol encountered which halted development, was the turbojet, even though radial engines persisted unexpectedly in manufacture for another 30-40 years.

My Question; Why haven't the Detroit Diesels been refined...common rail injection, single overhead cam & rockers, integrated dry liners and modern porting concepts to the same level as modern 4-strokes...has it been corporate politics...definitely the Commer TS3 & TS4 opposed piston 2-stroke run head on into politics when Rootes got bought out in 1966/67 by Chrysler, they dumped the TS3 to prevent it competing with the new (yet to be realised, disastrous) 170hp Cummins Vale engine they'd just finished spending a huge sum developing ready for production! and was installed in Leylands new cab over 10 tonner.

A KEY difference between Spark ignition engines and diesel...is diesels don't, and can't run a throttle, because they have to run their designed compression ratio to promote compression ignition, so they're running full volumetric efficiency all the time and purging as near, all exhaust gases.

At low power settings (throttle) combustion is running in a very lean state (excess oxygen) however pumping losses can be high relative to throttled Spark ignition (Si) engines. A consequence of this is diesels need to have high charge swirl designed in, to promote effective fuel air mixing, at the moment of combustion, no pre-mixing as with your bike. Cylinder ported 2-stroke diesels are much more effective and efficient than poppet valved designs in the this regard.

Excess swirl transfers heat into surrounding cylinder walls, pistons and heads (blast effect or chill factor in reverse). A reason diesel pistons are built so heavy/solid yet still need oil cooling and or fire plate piston crowns and fire rings either in the piston or around the top of the cylinder.

But I've run out of time and gotta go for now
 

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As a truck driver fot 40 years and a school bus driver for 15 I have a lot of experiance with Detroit 2 strokes VS Cummins, Cats and Mack 4 strokes.
There are only 2 advantages I have found with the Detroit?GMC engines. (1) Lighter engine weight= more payload. Make good horsepower in narrow rpm bands.
Disadvantages (1) despite your ascertations, I have never seen a Detroit that got close to the 4 strokes for fuel mileage. (2) Torque and pulling hills- I told on boss who had a fleet of Detroit v6 two strokes that I needed a roll of duct tape the next time he sent me through PA. He wanted to know why and I said, I want to cover the headlights, that truck gets scared when it sees hills ahead and slows up before I get to them.
(3) Engine durability- Not hauling heavy loads, I have seen much longer life out af Cats, Macks and Cummins in that order.
 
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