Two stroke diesel, turbo compound engines

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Viv
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Two stroke diesel, turbo compound engines

Post by Viv » Wed Dec 19, 2007 7:06 pm

Two stroke diesel, turbo compound engines.

Why post about two stroke diesel engines in the turbine forum? It’s a good question and the answer is historical in nature, between the development periods of the traditional piston aero engines and pure gas turbines there was a period when the turbo compound engine was under development.

A good example of this type of engine configuration would be the Napier Nomad, this was a 12 cylinder engine, a compressor section from a Napier Naiad gas turbine was slung underneath, here is a Wiki article.

http://en.wikipedia.org/wiki/Napier_Nomad

http://en.wikipedia.org/wiki/Image:Napier_Nomad.jpg

Nice detailed article with cut away drawing

http://users.bigpond.net.au/Shackleton/nomad.html

Petrol engines were also successfully adapted to turbo compound use such as the Wright R-3350; with exhaust gas energy recovery this was a significant engine with low SFC and reliable power output.

http://en.wikipedia.org/wiki/Wright_R-3350

http://en.wikipedia.org/wiki/Image:Wrig ... gine_2.jpg

http://en.wikipedia.org/wiki/Image:Wrig ... gine_1.jpg

See also the Allison 1710

http://www.pilotfriend.com/aero_engines ... _v1710.htm

Returning to the question of why diesels in the turbine section of the forum and hopefully after your background reading following the above links you will probably realise that if one follows a development path adding more and more blower capacity to the inlet of a two stroke diesel (or petrol) engine and energy extracting power turbines to the exhaust gas stream you will at some point end up with a situation were the piston engine is just a gas producer for the power turbines and compressor sections! Basically you have built a gas turbine engine with a complex piston operated combustor!

This is effectively what happened during the development of the original turbo compound engines, why make a horribly complex turbo compound engine when you could develop and build a much simpler full gas turbine design? Remember the Nomads 24 cylinder engine is effectively just a gas producer replacing the combustor cans of the Rolls Royce Nene that donated its compressors and turbine sections to the project.

All of this amazing work and thought came from the simple proposition that there is a lot of residual energy contained in the exhaust of a piston engine, attempts to harness that situation naturally lead to the Cyclone and Nomad engines, as we will see below the 2 stroke diesel has another interesting characteristic of its exhaust gases, a surplus of air! The 2-stroke diesel has a scavenging air blower as part of its cycle of operation so the exhaust is oxygen rich.

Today the turbo compound is not completely dead as the markets and needs of the 21st centaury have changed, flight in the 20 century involved hands on pilots; today UAV and remote pilots have radically changed the needs and requirements in the market place.

This opens up a possible new page in turbo compound engines, here is an interesting development, it may look a bit counter intuitive to begin with but read the article for the design goals before passing judgement, originally for heavy haulage applications but equally applicable to flight with its high power output for dash speed and lower power and economy for loitering.

http://www.cre8tivenergy.com/turbocompound.htm

Notice the auxiliary combustor in the exhaust outlet from the V6 piston engine.

http://www.cre8tivenergy.com/graphics/g ... ematic.jpg

Now if you have got this far you are probably thinking “yes but” “its all a bit esoteric really and not so practical” well maybe, maybe not, some modern research and figures will help,

http://stinet.dtic.mil/oai/oai?verb=get ... =ADA180007

Here we can see the US gov plonked down some ready cash to see just how well the numbers did stack up and this is a proper organisation doing the work not some vapour ware outfit, a search on Garrett turbine engine company gives some interesting results on its own.

Another modern two stroke diesel engine is under development by Delta Hawk engines, this remarkable design takes full advantage of modern techniques to do away with the need for separate cylinder heads and gaskets by casting the engine block complete with its cylinder bores and heads in one single peace, the reduction in weight and points of failure are obvious.

As a two stroke engine it requires scavenging air to be supplied by a blower and is also turbo charged to improve high altitude performance and overall power output, the SFC for this engine as presented is very attractive in its intended market place.

The scope for compounding this engine with out too much trouble can be seen.

http://www.deltahawkengines.com/drawin00a.shtml

My goal was to give the turbine guys cage a rattle and generate a bit of interest in an alternative gas producer than the more common propane or kerosene combustor normally used.

After all the Turbines they are working with to build their jet engines mostly started life bolted to piston engine exhausts so its kind of appropriate when you think of it that way.

Merry Christmas

Viv
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Post by racketmotorman » Thu Dec 20, 2007 12:43 am

Hi Viv

And a Merry Christmas to you :-))

LOL..."Rattle our cage" .......you're a bad man .

Cylinders and head in one casting ...............they were doing that back in the early 1900's for motorcycles , I've got a ~1912 JAP 550cc V twin engine with that configuration .

The turbo compound aero engines were an interesting execise , higher compression and expansion ratios have thermodynamic advantages especially the very efficient second stage of compression within the cylinders.

Superseded by lightweight , efficient, high compression multi staged axial compressors in flight engines .

Theres been a number of strange devices made over the years , all adds
to the "colour' of our hobby :-))

Cheers
John

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ah! you saw the hook

Post by Viv » Thu Dec 20, 2007 1:33 am

Hi John

Ah you saw the hook being dangled so enticingly;-)

I had forgotten about the old Jap motors, funny but lots of people still very interested in them for the Morgan three wheelers, when I was a kid it was all Jap 500-cc singles for grass track bikes, clever stuff stuff though doing all the machining in a single casting like that, the guys at Delta are doing water cooled so I think we can give them a few points for casting the water ways in to the jackets:-)

Yes axial turbines for flight but for the average garage hound that may be to high up the difficulty ladder, Johansen's little bike springs to mind though:-)

Self starting too!

Its just trying to get the best out of the cross over region between piston engine and turbine thats difficult!

Viv
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Post by racketmotorman » Thu Dec 20, 2007 3:12 am

Hi Viv

There was also a turbine that "free pistons" for the "compressor" , and another that had a large radial aircraft engine driving the compressor rather than a turbine .

Some of the truck manufacturers are now installing a freepower turbine downstream of their turbos (turbo compounding) , to suck the last bit of energy from the exhaust gases . The Volvo FH12 500 engine comes to mind as one such engine , I think Cat are also doing it .

Theres no end to human imagination :-))

Cheers
John

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a place between pistons and combustors

Post by Viv » Thu Dec 20, 2007 3:29 pm

Yes your right John

One of my goals was to remind our selves that there is a wealth of invention still to be found between the pure piston engine and the pure gas turbine, too free piston gas producers we can add pulse jets and all manor of arrangements.

I think the Volvo engine is very notable as a trend in raising the BMEP of diesels and in fact this whole thread is about that, the end of the curve as you said first is the pure high compression axial turbine but the beginning is a small diesel engine such as one of the new Gemini three cylinder two strokes aimed at small aircraft.

The Gemini is a three cylinder six piston design and really dates back to the 1940s for its heritage, I remember Bedford truck engines in the UK had a four cylinder unit using one crankshaft rather than the Gemini's two

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Gemini 2 stroker links

Post by Viv » Thu Dec 20, 2007 6:41 pm

Here are some links for the gemini engine mentioned in the last post

http://ppdgemini.com/_PDF/Gemini100_Spec_Sheet.pdf

http://www.airventure.org/2007/4wed25/diesel_power.html

I will see if I have time to find a Bedford T3 motor link

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Post by Bruno Ogorelec » Sun Dec 23, 2007 6:03 pm

I wonder if anyone will remember the old Junkers aircraft 2-stroke diesel as the gas generator for a compound engine. To me, it looks like a perfect candidate. The reason Junkers went for such a counter-intuitive layout -- pumping efficiency -- must surely be multiplied in the high-pressure environment of a turbo-compound. And, given that you have to have complex gearing between the turbo section and the piston section anyway, the relatively complex Junkers gearing between the two crankshafts is no loger an issue.
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take your Jumo raise you a Roots-lister

Post by Viv » Sun Dec 23, 2007 10:39 pm

Hi Bruno

Yes the Jumo is one of my favorites too and Napier I think also built the Jumo under license, it would have made a good compounding gas producer I think as well.

The other engines I was thinking of were the Roots opposed piston singe crankshaft units fitted to early Commer and Bedford trucks, I remember driving these when I was a kid working on farms in the summer.

http://www.oldengine.org/members/diesel ... al/TS3.htm

Another interesting arrangement

http://www.oldengine.org/members/diesel ... oxford.htm

http://en.wikipedia.org/wiki/Opposed_piston_engine

Viv
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Napier Deltic

Post by Viv » Sun Dec 23, 2007 11:14 pm

I know you like the deltic Bruno so heres one to look at.

http://en.wikipedia.org/wiki/Napier_Deltic

Now look at this baby

http://www.dair.co.uk/

Now thats 100 bhp from only 2 cylinders but in a Deltic configuration for three cylinders we only have to add one extra crankshaft and one cylinder for a nice increase in power, the cranks would be shorter too.

But its that big hole in the middle of the Deltic engines that makes me think its a perfect place for a turbine to live;-) it would be all warm and cosy surrounded by the cylinders and the exhausts can feed direct to it.

Or how about a Jumo 223 with four banks? I found a nice picture for you that shows the general layout and how compact this configuration is, originally for 2500 hp output the next in the series did not get off the drawing board at 4500 hp

Viv
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Last edited by Viv on Mon Dec 24, 2007 4:05 am, edited 1 time in total.
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some interesting engines

Post by GRIM » Mon Dec 24, 2007 1:46 am

Hi all first post here at gas turbines , and just some interesting old engines , I remenber the old commer trucks, glorious sound, and worked on a couple of ships that had sulzer main engines , awesome things, the turbo alone is about the size of a small car

heres a cute vid , sorry no turbines :oops:
http://www.youtube.com/watch?v=pizDIl0qluM

peace?

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Post by Ash Powers » Mon Dec 24, 2007 9:19 am

What I dont quite understand in those designs is how the combustion chamber linked to the exhaust stream of the piston engine is capable of supporting combustion at all. In all "boosted" piston engines, you must run a relatively rich mixture to keep combustion temps down low enough to prevent overheating the components. Because of this, there is virtually no latent oxygen left in the exhaust stream. I suppose the block diagrams shown are missing some elements, or they are just plain incorrect. Perhaps what they excluded was a feed tube from the compressor into this combustor to supply atmospheric air into it to support combustion - but it is hard to imagine they would have "forgot" to put that in the drawing, LOL.. What's up?

Thanks for the entertaining post. :)

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Post by Bruno Ogorelec » Mon Dec 24, 2007 10:03 am

Ha-ha-ha... Viv, you always knew which buttons to press with me...

The deltic animation is totally hypnotic.

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Post by Bruno Ogorelec » Mon Dec 24, 2007 10:06 am

Sorry, an erronneous posting deleted.

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Crankless Deltic

Post by Viv » Mon Dec 24, 2007 2:38 pm

Bruno Ogorelec wrote:Ha-ha-ha... Viv, you always knew which buttons to press with me...

The deltic animation is totally hypnotic.
Hi Bruno

Its my bit of Christmas fun to give people some thing to worry over the holiday season;-)

Imagine the Deltic with out the crankshafts on the corners and the turbo shafting in the middle, rockers would give the same linear motion to the pistons as cranks;-)

Enjoy!

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Blowers, turbines and scavenging

Post by Viv » Mon Dec 24, 2007 3:26 pm

Ash Powers wrote:What I don't quite understand in those designs is how the combustion chamber linked to the exhaust stream of the piston engine is capable of supporting combustion at all. In all "boosted" piston engines, you must run a relatively rich mixture to keep combustion temps down low enough to prevent overheating the components. Because of this, there is virtually no latent oxygen left in the exhaust stream. I suppose the block diagrams shown are missing some elements, or they are just plain incorrect. Perhaps what they excluded was a feed tube from the compressor into this combustor to supply atmospheric air into it to support combustion - but it is hard to imagine they would have "forgot" to put that in the drawing, LOL.. What's up?

Thanks for the entertaining post. :)
Hi Ash

Thanks the whole post is a bit of intellectual fun for Christmas but it also follows a more serious train of thought for propulsion;-)

Now to answer your question, its all to do with one aspect of 2 stroke diesels that may not be too familiar, I do understand your point about fuel, temperature and mixture control but thats more to do with petrol engines than diesels I think.

Let me introduce you to "BMEP" or break mean efficiency pressure, this is the holy grail figure for diesels and I will search out a separate article on it for you as it really gives you the meat and vegetables for this meal, in tractor pulling contests this is the real reason why they can take a 175 horse power John Deer tractor and add multistage LP, HP, blowers with water injection inter-stage-cooling to boost the same engine to 1500 horse power!

On 2 stroke diesels they to have scavenging air supplied by an engine driven blower, for a part of the cycle, both the inlet and the exhaust ports (there are no valves:) are deliberately kept open and the blower (Roots type crankshaft driven supercharger) blows about twice the cylinder volume of clean air through the ports to remove all traces of exhaust gas from the cylinder.

This gives a significant power improvement to the 2 stroke diesel cycle as it removes all traces of exhaust gasses from the next combustion stroke, and it also means the exhaust is oxygen rich at all engine load conditions, when you add to this a turbocharger (power turbine driven compressor) in the exhaust stream to recover some of the pressure energy you end up over boosting the engine and start to get even more scavenge air flowing through to the exhaust in the blow down part of the cycle.

The exhaust gasses passed in to an auxiliary combustor are better than you would expect as they are preheated to near combustion temperatures and they contain some unreacted combustion products from the diesel engine (soot unburned fuel etc) all it needs is a bit more fuel to burn with the excess oxygen and the exhaust stream can be burnt and expanded through the turbine section for power recovery and air compressor drive.

Hope that helps, I tried to make it clear for people unfamiliar with this subject that superchargers and turbochargers are different beasts, I will have a look round and see if I can find some nice articles to give some more detail, if you notice I used the old terminology for superchargers by calling them blowers, when you read about oil engines (diesels) this is what they will call them or scavengers.

Viv
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