Detroit Diesel 2 stroke- 92 series vs 71 series

[Dreadnought]

Some years ago I was involved in research of 2 stroke diesels and a lot of my work involved digging into the stuff that Detroit did. I also talked to a lot of former engineers involved in that era. I dug up the culmination of my work and summarised it into palatable chunks for those who are interested in this forum.

Along with technical conversations- the source of my work predominantly comes from the following SAE papers:

740037-8v92
881584
770255
870402
831202

It seems that the 92 series is an evolution of the 71 but the 'phase in' of the 8v92 was slow and not absolute.

The 8v92 uses wet liner configuration above the intake port compared to the 8v71. I don't usually like wet liners for boosted application and I have hear the earlier green 92 engines suffered from electrolytic corrosion problems- but that's probably also connected to inproper use or/changing of coolant etc.
Elimination of upper water jacket coring simplified the casting
While the water jacket cores surrounding lower cylinder bores are somewhat smaller in the 92 series



The Cylinder bore centres kept the same on the 71 series vs the 92, as were head bolt locations but fasteners increased from 5/8-11 to 11/16-11 for increased clamp load.

Struts that carry head bolt loading directly to main bearing bulk heads have proven 'entirely adequate (their words- sounds more like how Bentley or Rolls Royce would describe it)' at very high engine loading a comparable structure used for 92 series
Early prototype 92 series using the old style 5/8-11 bolts exhibited cap to block fretting and ultimately web cracking at the bolt holes. This is why the 92 series utilizes 1.12 inch wider caps, 11/16-11 bolts and stamped cap to pan stabilized plates.

I know from experience that clamp loads for fasteners are derived considering the peak cylinder pressure loading of the engine. That's what makes me slightly nervous about the 8v71 Turbo series- however a coach/RV application is probably not under very much load/duty cycle compared to HD trucks.





The 92 series has provision for an air to water heat exchanger-directly below the roots blower where the investment in tooling would have been too much for this on an 8v71 Turbo before- cooler effectiveness increased and quoted in the order of 70%.

92 series head design changes:
Valve head diameter of 1.38 inches. Cored water passage between traverse exhaust valves increased but stiffness also increased. Overall much less restriction over the exhaust posts AND intake port windows- this means less supercharger/blower gearing is required thus lowering parasitics


Crosshead vs Trunk pistons:

The 92 series utlises the later Crosshead piston design unlike my earlier trunk piston design of my 1964 71 series- this has proven much more robust- and we tested the two types of design in the labs back to back when I worked in San diego.



Better lubrication:

The V92 con rods incorporate a y drilled oil passage to supply piston cooling oil (to eliminate bearing shell steel fatigure failures which occurred) . Rods are good up to 4000 rpm testing- although this was under no gas load (even though the exh valves probably aren't!)


Above picture shows oil flow to piston 92 series vs 71



The upper main shell beariings on the 92 series are designed for better oil supply vs the earlier variants also.

There are other myriad improvements I haven't covered.

If I were to make more improvements with the benefit of hindsight and experience of modern engines- I would focus on combustion improvements- the BSFC or efficiency numbers are pitiful- and when I see numbers quoted of needing 32:1 air fuel ratio - I can see why, with the opposed piston 2 stroke I was involved in - we could get that number right down to 18:1. This would allow the supercharger blower ratio to be lowered even more thus lowering the parasitics further. I would also increase the injection pressure- both the 71 and 92 series only has a pressure of about 200 bar. The benefits of higher pressure aren't only emissions but also significantly on efficiency.

Then I would focus on the rootes type blower itself- the older style would get efficiency numbers of around 50-55%, where as the newer TVS style blowers attain 60-70% using bradable coatings to tighter tolerences and using a four lobe rotor design with 160 degree twist. I have experience of these design when we upgraded from the M112 super chargers on the old Jaguar XJRs to the later XF cars.

[daddysgirl]

This is too cool Dread...thanks!
I'm certainly going to leave the technical details to those of you more technical than I, but I am curious about something.

As most of you know, I have 8V71N. When dad (who was responsible for a large fleet of heavy equipment, buses, police cars etc.) ordered my current engine/transmission in 2000 he could have ordered anything, literally. He was not restricted by funding.
My specific engine is "customer specified", and I am learning what I can about those specifics. This type of information is very helpful.

Here is what I don't get, and I am being literal, no embellishing I have also driven a few much newer buses with 4 strokes:
For all the various discussion of HP and Torque (the latter in my mind, for some reason, as being more important on the bus), I accelerate quickly when necessary; climb mountains; pass the trucks, some cars and on the flats, I have been well over 85MPH several times. I have never come close to overheating. I have never seen any smoke whatsoever. When not torn apart, she's NOT light weight.
I do need to lube up the throttle, as it's heavy enough to kill my leg on long trips with mountains.

But how is this possible? Other than the injectors and the governor, where would the most efficient modifications be made so that an 8V71N performs like it's not an 8V71N?

[Dreadnought]

This is too cool Dread...thanks!
I'm certainly going to leave the technical details to those of you more technical than I, but I am curious about something.

As most of you know, I have 8V71N. When dad (who was responsible for a large fleet of heavy equipment, buses, police cars etc.) ordered my current engine/transmission in 2000 he could have ordered anything, literally. He was not restricted by funding.
My specific engine is "customer specified", and I am learning what I can about those specifics. This type of information is very helpful.

Here is what I don't get, and I am being literal, no embellishing I have also driven a few much newer buses with 4 strokes:
For all the various discussion of HP and Torque (the latter in my mind, for some reason, as being more important on the bus), I accelerate quickly when necessary; climb mountains; pass the trucks, some cars and on the flats, I have been well over 85MPH several times. I have never come close to overheating. I have never seen any smoke whatsoever. When not torn apart, she's NOT light weight.
I do need to lube up the throttle, as it's heavy enough to kill my leg on long trips with mountains.

But how is this possible? Other than the injectors and the governor, where would the most efficient modifications be made so that an 8V71N performs like it's not an 8V71N?

Andrea, to get more power/torque out of a diesel you need air and fuel. The air in the case of our Naturally aspirated engines come from the blower or supercharger and the fuel from upgraded injectors.

On engines that have good efficienct clean combustion- they need less air (oxygen) and are able to burn cleanly.

Above- when I said: "the BSFC or efficiency numbers are pitiful- and when I see numbers quoted of needing 32:1 air fuel ratio - I can see why, with the opposed piston 2 stroke I was involved in - we could get that number right down to 18:1"- I was touching on this and comparing it to my experience- according to the papers- the 8v92 needs a lot of air (hence 32:1 Air fuel ratio) to burn cleanly- this means that its combustion isn't great. When combustion isn't great, the engine will black smoke quicker than 'brand b' engine with better combustion when you inject the same amount of fuel.

In your case- the injectors are probably pretty potent. It would be interesting to see what they are. Mine are C60s, I wonder if yours are 75s? But it still doesn't smoke. The thing they do on the 71s is advance the timing to mitigate potential to smoke.

The only way to supply more air is to change the blower speed but that's very hard on a gear driven type of equipment like ours and its a precarious balancing act- if one does this , they also increase parasitic losses which is bad for efficiency.

[lostagain]

Big injectors like 80 or 90, timed so they don't smoke, and a cooling system in good order, probably explains your circumstance.

JC

[luvrbus]

The technology DD used on the 92 with the liners having 2 inches of water at top carried over to the DD 4 stroke engines cooling the upper cylinder to produce more HP.
The 92 series are not really a wet liner if you compare it to other wet liner engines like Cummins or Cat the 92 just has a 2 inch space that contacts the coolant.
The 8v92 will always be one of my favorite engines second to only the CAT 3406.FWIW I have in my shop one of the double O's 8v92TA engine that was produced between the old green engine and the Silver 92 so far I found very little difference in it and a Silver 8v92,I do like the single wide oil control ring they used instead of the 2 thin ones used today       

[daddysgirl]

Thanks for confirming what I was thinking...one aspect of it at least. Air and fuel I understand. I know my injectors are larger, I have the invoice for them somewhere. I can't remember what size...I'll dig it up.

Your responses do lead me to what might be the most stupid question ever asked. If so, my apologies...it just popped into my head. I know nothing about 8V92s.

You know how there is so much discussion about the 8V92 and overheating in coach applications?
Is it possible that the increased air demands and the configuration could be partially responsible for that problem? If so, could the intake point of the cycle be modified in some creative (exterior) way to introduce cooler air?

[Dreadnought]

Thanks for confirming what I was thinking...one aspect of it at least. Air and fuel I understand. I know my injectors are larger, I have the invoice for them somewhere. I can't remember what size...I'll dig it up.

Your responses do lead me to what might be the most stupid question ever asked. If so, my apologies...it just popped into my head. I know nothing about 8V92s.

You know how there is so much discussion about the 8V92 and overheating in coach applications?
Is it possible that the increased air demands and the configuration could be partially responsible for that problem? If so, could the intake point of the cycle be modified in some creative (exterior) way to introduce cooler air?

Intake or ambient air could be cooled. That's what a charge air cooler would or could do. The problem is - that it needs to be tied up to some sort of heat exchanger like a radiator. And....if you cool the intake charge- it WILL have some impact on combustion temps and therefore coolant temps but it wont be as good a method to reduce temps as cooling the coolant further. Modern engines cool intake charge for other reasons- to cool the EGR (for emissions reasons) so this is how I know- if you go too cool here you can end up with internal engine condensation and then corrosion- you typically want to target about 55 deg C.

Other ways to tackle it- would be:

-Advancing injection timing- if you do this- the exhaust temps get cooler but the piston temps go up- the problem with doing this is that your peak cylinder pressure goes up and you could go above what the engine components were designed for. So this rules that out
-You could BLOW more air through- using the blower- i.e changing the blower ratio. However in the real world- how do you do this? Will we get a gear set made up and somehow matched? In addition- doing this would make the engine specific fuel efficiency (BSFC go down the toilet due to increased parasitic loss.
-You could mess around with the port timing- but this would involve getting liners with new port windows custom made- whos going to do that?


In the above pic- the green curve is the intake opening area and the red is exhaust. The plot on the right shows a conventional diesel engine like a Cummins ISB and the plot on the left shows a 2 stroke prototype engine. You'll notice the big where the red and green curves overlap is where both intake and exhausting are simultaneously open. A 2 stroke characteristic is to have that simulataneous opening period very large- its called the scavenging period. If this is increased- it can help cool the engine but is very wasteful and can shoot fresh intake charge through the engine and again waste energy/efficiency. So its very complicated.


What I would do is simply try to find ways of increasing the cooling ability of the vehicle/cooling system. I have measured heat rejection data for a series 60 and various 4 stroke Cummins engines. I have friends who have sized up radiators and cores so I could probably come up with a rule of thumb as to what radiator core would dissipate what. The problem I have is- how do I find out how much a 550 bhp 8v92TA produces in terms of heat. I will continue to do some digging and extrapolating. Something tells me it wont be the same rule of thumb or ascertained by simply scaling 4 stroke data I already have. I will continue digging. Once I get that I can find a way to size up radiators and sort it. That's what us engineers do.

[RJ]

Dread -

I went with Scott Robertson on Thursday to help him pick up an MC-6 with the original 12V71 engine in it.

Unlike the 5s, 7s, 8s, 9s, etc., the 6 uses a HUGE, hydraulically-driven, radiator on the LH side, like GM, Prevost & Eagle and now current MCIs.

Only reason I mention this is that the 12V71 hp & torque figures aren't that different from an 8V92TA, so you might be able to come up with some rough heat load numbers that could help with your extrapolations.

You might have to look outside the normal transportation numbers for the 12V, possibly marine or industrial applications, to find what you're looking for.

FWIW & HTH. . .



PS:  I drove the 6 for about a mile - that 12V has got TONS of torque!  I remember reading somewhere that with 60 injectors, it produces 1350 ftlbs of torque at 1200 rpm!  Insane (for it's day, of course!)  Fun, none the less!   

[Utahclaimjumper]

Andrea,, probably the reason you perform so well is the location your in.. the 8V71 does very well near sea level but runs out of breath quickly above 5 or 6 thousand feet.>>>Dan

[Iceni John]

Very interesting, thank you for those insights.

I often wonder how much improvement a 6V92 would derive from an air-air CAC instead of that air-water aftercooler that probably heats up the coolant more than it helps the engine's intake air temperatures.   I think a lot of our perennial cooling problems are directly related to that aftercooler.

During your research did you have any chance to compare the venerable Commer TS2 with Detroit's 2-strokes?   Is the TS2's opposed-piston design advantageous to a non-OP 2-stroke?   (How does the Deltic also compare?)   TS2s used to get quite a lot of power from a relatively small displacement, but I suspect they weren't torquey.

You mention the difficulty of producing liners with different port windows.   Didn't Don Fairchild do this with his CCTS-modified 2-stroke Detroits that could achieve later emissions standards?

John

[Dreadnought]

Dread -

I went with Scott Robertson on Thursday to help him pick up an MC-6 with the original 12V71 engine in it.

Unlike the 5s, 7s, 8s, 9s, etc., the 6 uses a HUGE, hydraulically-driven, radiator on the LH side, like GM, Prevost & Eagle and now current MCIs.

Only reason I mention this is that the 12V71 hp & torque figures aren't that different from an 8V92TA, so you might be able to come up with some rough heat load numbers that could help with your extrapolations.

You might have to look outside the normal transportation numbers for the 12V, possibly marine or industrial applications, to find what you're looking for.

FWIW & HTH. . .



PS:  I drove the 6 for about a mile - that 12V has got TONS of torque!  I remember reading somewhere that with 60 injectors, it produces 1350 ftlbs of torque at 1200 rpm!  Insane (for it's day, of course!)  Fun, none the less!   

That's insane and amazing! I love the idea of the 12 cylinder however I must keep sane and I know the Turbo has benefits at high altitude!

Does the MC6 with 12v71 have just ONE radiator on the Left hand side or in this radiator in addition to the two that are there on the MC5s?

I just talked to my friend whos still at the Opposed piston 2 stroke company and we tried to figure out,qualitively if not quantitively, if the Detroit 2 stroke would be worse in terms of heat rejection than a series 60 4 stroke, or a Cummins 4 stroke, then where it stood relative to an opposed piston 2 stroke etc.

We decided that the fact that it has 4 exhaust valves and a coolant jacket around that made it worse than a 2 stroke opposed piston (this rejects less heat to coolant and more to the exhaust gases themselves) and likely worse than a the 4 strokes.

[Dreadnought]

Very interesting, thank you for those insights.

I often wonder how much improvement a 6V92 would derive from an air-air CAC instead of that air-water aftercooler that probably heats up the coolant more than it helps the engine's intake air temperatures.   I think a lot of our perennial cooling problems are directly related to that aftercooler.

During your research did you have any chance to compare the venerable Commer TS2 with Detroit's 2-strokes?   Is the TS2's opposed-piston design advantageous to a non-OP 2-stroke?   (How does the Deltic also compare?)   TS2s used to get quite a lot of power from a relatively small displacement, but I suspect they weren't torquey.

You mention the difficulty of producing liners with different port windows.   Didn't Don Fairchild do this with his CCTS-modified 2-stroke Detroits that could achieve later emissions standards?

John

I mentioned the difficulty of producing a liner- yes, as I'm a single independent guy- trying to save my money! At Achates with opposed piston engines- we made them all the time, but I'd build a one off. Believe me, I'm not going into some sort of business trying to pedal this. I don't see a viable market.

We bought a Tillings Steven engine from New Zealand (the commers seem to have a following down there) but on the first run one of the huge rocker arms broke and we never got any useful data out of it. :-(

What I have found with opposed piston engines is the following:

The- the OP layout allows for a long stroke which has advantages in terms of scavenging and thermal efficiency but this is pretty much lost due to the friction from the extra piston- also figure that the piston skirt grows ALOT with increased stroke to maintain the port timing.

We found that less heat was rejected to the coolant and more went to the exhaust system. As I extrapolated or hypothesised above- I reckon our Detroit diesel style would possibly reject more heat to coolant- as it has a water jacket around 4 exhaust valves...

[luvrbus]

Very interesting, thank you for those insights.



You mention the difficulty of producing liners with different port windows.   Didn't Don Fairchild do this with his CCTS-modified 2-stroke Detroits that could achieve later emissions standards?

John

Yes Don did do that,you can buy different port size liners aftermarket or from Detroit the   
95 and 105 are the 2 most popular now

[TomC]

Starting with an 8V-71N with C65 injectors, that gives me 300hp and 800lb/ft torque. By increasing to 7G75 injectors (same 18.7 two piece pistons), custom air to air intercooler, by pass blower, but same injection timing, power was increased to 375hp and 1125lb/ft torque. While fuel mileage remains about the same (5-6), my performance has improved and smoking in high altitudes has been reduced to almost zero.
I had a OO-92 (stood for Owner Operator) 8V-92TA in my first truck in 1980. While it was a great performer and loved the engine sound, maintenance on it was intense and fuel mileage was about half of what trucks get today. The only 2 stroke Diesel I know that are extremely efficient are the giant low speed engines used in container ships. They run at 82rpm and have a 39" bore by a 135" stroke (yes thats over 11ft!). Those engines are the most efficient engines ever made. Good Luck, TomC

[luvrbus]

 Tom Detroit did the smart thing letting you truckers find the trouble spots for the 8v92 they only built 11,000 of the double O engines