Diesel vs miller cycle

[TFF]

Water/ meth on takeoff is common in turbines. If you want it in cruise, you can’t carry enough. One gallon for takeoff would be doable. More than 30 seconds and you didn’t put a big enough engine on your plane. If you depend on E85 there are lots of places that don’t have it. None around here. Closest pump with it I know of is 600 miles away.

 

[trimtab]

View attachment 118088
Nope, the Atkinson one is first to win. But it lacks power density. But - being 20-30% more efficient - all that it takes is to add the turbo and convert it to miller one.

Very true...the Otto cycle is merely more efficient than the diesel...

 

[stanislavz]

Very true...the Otto cycle is merely more efficient than the diesel...

At max power yes, but in car application - no, we are using no more than 10-20% of Otto cycle power, which is in low efficiency range. One of hypermilling strategy is to acclerated at wot, then coast, then again wot etc... Which brings superb economy. Just look weird.

But in our, aircraft use, we need 50-75% of take-off power for cruise.

 

[lelievre12]

The elephant in the room with fanciful dreams of high efficiency high compression SI engines is that they cannot run on low octane fuels without fuel preignition. In reality all SI engine compression ratio's are limited by the detonation margins of the fuel in question.

And before folks weigh in talking about how great their 10.5:1 engines are I expect none of these higher compression NA engines have been tested to AC 33.47-1 - Detonation Testing in Reciprocating Aircraft Engines (faa.gov).

These tests require that the engine be operated to within 10F of its maximum CHT and +20F hot day conditions. To pass the tests engines must then be set to operate at least 12% richer than the detectible detonation mixture at full power at max temps. As far as I know, no certified engines have passed this test on 100LL fuel at >9:1 CR, let alone E85. Dreaming of 14:1 CR and higher SI engines in planes is just that. Dreaming.

The AC 33.47-1 tests are performed at sea level however if the engine is turbo or supercharged then engines must also be tested at all operating alts. This results in turbo engines having even lower CR ratios than naturally inducted engines and so automatically their efficiency is even worse.

By way of comparison, Diesel engines are unaffected by these detonation limitations and can be tested to optimum stoichiometry without the need for the 12% fuel margin. They are also unaffected by turbocharging and can maintain high compression ratio's with subsequent high efficiency whether turbocharged or not. Their mission fuel consumption is therefore much lower as they can maintain full or partial power at Lambdas of 1 or less. For the CD155 Technify this equals around 0.36 lb/hp/hr. By comparison TCM's own SID-97-3E shows that the Continental TSIO550G has a BSFC at 0.65 lb/hp/hr at rated power or 82% higher than the Diesel. The TSIO550 does improve when leaned for cruise however the overall mission consumption is still hurt by the rated power fuel requirements which include the mandated detonation margins.

Expecting that partial power automotive peak stochiometry data can translate to real world aviation ignores the issues that come with continuous high power operation and the mandated detonation margins required therein. If experimental folks don't test to AC 33.47-1 in order to get better BSFC then they must accept the risks of running their engines that way. It follows that statements like "Otto/Miller/Atkinson is more efficient than Diesel" ignore the realities of AC 33.47-1.

 

[stanislavz]

As far as I know, no certified engines have passed this test at >9:1 CR. Dreaming of 14:1 CR and higher SI engines in planes is just that. Dreaming.

Nope. Atkinson have that same cr of 9:1 for for compression and 13:1 for expansion. And vulumes are different to..

 

[dwalker]

The elephant in the room with fanciful dreams of high efficiency high compression SI engines is that they cannot run on low octane fuels without fuel preignition. In reality all SI engine compression ratio's are limited by the detonation margins of the fuel in question.

And before folks weigh in talking about how great their 10.5:1 engines are I expect none of these higher compression NA engines have been tested to AC 33.47-1 - Detonation Testing in Reciprocating Aircraft Engines (faa.gov).

These tests require that the engine be operated to within 10F of its maximum CHT and +20F hot day conditions. To pass the tests engines must then be set to operate at least 12% richer than the detectible detonation mixture at full power at max temps. As far as I know, no certified engines have passed this test on 100LL fuel at >9:1 CR, let alone E85. Dreaming of 14:1 CR and higher SI engines in planes is just that. Dreaming.

The AC 33.47-1 tests are performed at sea level however if the engine is turbo or supercharged then engines must also be tested at all operating alts. This results in turbo engines having even lower CR ratios than naturally inducted engines and so automatically their efficiency is even worse.

By way of comparison, Diesel engines are unaffected by these detonation limitations and can be tested to optimum stoichiometry without the need for the 12% fuel margin. They are also unaffected by turbocharging and can maintain high compression ratio's with subsequent high efficiency whether turbocharged or not. Their mission fuel consumption is therefore much lower as they can maintain full or partial power at Lambdas of 1 or less. For the CD155 Technify this equals around 0.36 lb/hp/hr. By comparison TCM's own SID-97-3E shows that the Continental TSIO550G has a BSFC at 0.65 lb/hp/hr at rated power or 82% higher than the Diesel. The TSIO550 does improve when leaned for cruise however the overall mission consumption is still hurt by the rated power fuel requirements which include the mandated detonation margins.

Expecting that partial power automotive peak stochiometry data can translate to real world aviation ignores the issues that come with continuous high power operation and the mandated detonation margins required therein. If experimental folks don't test to AC 33.47-1 in order to get better BSFC then they must accept the risks of running their engines that way. It follows that statements like "Otto/Miller/Atkinson is more efficient than Diesel" ignore the realities of AC 33.47-1.

I am assuming you are speaking of AIR COOLED engines?

 

[tallank]

Thermal efficiency is just one problem to solve. Raising the compression ratio to 30 or more, insulating the cylinder and piston from heat loss, etc., make it possible. At the expense of weight, life, and especially NOx.

A 70% thermally efficient ICE is largely an academic exercise, unfortunately.

For a given compression ratio, the Otto wins in thermal efficiency.

What does NOx have to do with anything?

 

[rv6ejguy]

Dave Anders ran a 12.5 to 1 IO-360 in his RV-4. Worked great, especially up high and never blew up. Set some CAFE records with it. Proper ignition timing is key to avoiding detonation WOT near SL and still getting good efficiency up high where MAP is low so you don't use fixed timing mags here. Modern, programmable EI is the way to go with high CRs.

Unturbocharged CI engines are complete dogs, completely unsuitable as aero engines with very poor specific output and low power to weight ratios.

Single stage turbocharged CI engines are effectively limited to below 20,000 feet due to the high PRs required to make decent power. They also won't relight up there and need to carry at least 40 inches in the descent up high to avoid flame outs.

Cruise BSFC figures for the IO-550 running LOP are within a whisker of the aero diesels. Cruise is where aero engines spend the majority of their lives.

If we compare modern liquid cooled designs like the Adept V6, which I did in a YT video, mission fuel burn was less than existing CI aero engines though due to lower JET A costs vs. avgas costs, the CI operating costs were still lower than the SI engine.

With the advent of unleaded avgas coming likely next year from GAMI, we'll be able to effectively use modern closed loop feedback control as cars have been doing for decades. This will offer further improved mission fuel burns for SI aero engines.

 

[reo12]

20 or so years ago I had the experience of driving a Ford econoline running on 50% ethanol and 50% water - using catalytic igniters. Same guy who has developed the ultra-efficient diesel.

An M.E. prof I knew once tried an experiment of injecting water into a normally aspirated diesel (actually a John Deere farm tractor). It ran better and better as they added more and more water. In the end, he described it as pretty much a garden hose into the air inlet and it STILL kept running....better and better.

Anecdotal, but true. We still have a LOT to learn about what does and can go on when we burn hydrocarbons.

Water injection was done by technicians at Garrett testing turbocharging 2-stroke Detroit Diesels and adding water injection. They were attempting to attain turbine engine power outputs. If I recall correctly it was for a business jet application. There was so much water going into the engine that the oil was an emulsion of water and oil in a short time. Diesels really don't care how much water is added - short of hydro locking and air starvation as the water being incompressible - increases compression and some turns to steam for added expansion. The turbocharger on the other hand can not withstand free water in the exhaust stream due to blade and scroll erosion.

 

[PMD]

Very true...the Otto cycle is merely more efficient than the diesel...

2 cycle cathedral diesels run at 52-54% all day long. No production SI engine can do that. One of the reasons we WILL see 2 cycle CI engines come back into use (since enough has been learned about oil control to make this possible).

The question about NOx was brought up. The answer is that we have to thing much longer term in having emissions profiles that will allow us to continue to fly in the air we breathe. Also, since any scaling of technology involves the automotive world, NOx is indeed VERY important. That is the beauty of introducing water into the combustion process: drop in peak temperatures allows the O2 available to be used to make more water and CO2 instead of NOx. The reduced heat is converted to cylinder pressure by change of state from liquid water to steam. Double benefit.

Regarding engine efficiencies in general: What Ross has pointed out is that you need full engine management of mixture, fuel distribution and timing to make ANY engine run to modern standards, and the strangle hold for certified legacy engines is the cost of certifying that technology within the existing regulatory framework. While you CAN do it with air cooled engines, the use of mass production water cooled SI engine cores uses hardware that is already designed to do this and just needs software written to optimize their use in an aviation environment.

 

[lelievre12]

Dave Anders ran a 12.5 to 1 IO-360 in his RV-4. Worked great, especially up high and never blew up. Set some CAFE records with it. Proper ignition timing is key to avoiding detonation WOT near SL and still getting good efficiency up high where MAP is low so you don't use fixed timing mags here. Modern, programmable EI is the way to go with high CRs.

AC 33.47-1 requires full power runs on a +20F day with CHT at Max allowable (490F). This is hard to do. With mapped electronic ignition 13.5:1 could work as long as CHT/OAT inputs were mapped into the advance curves so ignition could be heavily retarded in those conditions. However the problem arises that the rated power of the engine then drops as the engine 'protects' itself from detonation by retarding ignition. Its unlikely that a 13.5:1 engine could be both 200HP and comply with AC 33.47-1. It's one or the other. On a hot day your max gross takeoff tables would be worthless.

Unturbocharged CI engines are complete dogs, completely unsuitable as aero engines with very poor specific output and low power to weight ratios.

Yep, total dogs. Without a turbo CI has limited use due to very low specific power. However in some low power applications (Gazaile etc) they make sense as direct drive is possible without a PSRU and fuel consumption is insanely low (~1.5GPH)

Single stage turbocharged CI engines are effectively limited to below 20,000 feet due to the high PRs required to make decent power. They also won't relight up there and need to carry at least 40 inches in the descent up high to avoid flame outs.

Most power curves show SI will fly >26K without compound turbos however, as you say, most are only certified to <20K. I dont think the ceiling is because of difficult relight, but more because the engines make <50% at 26K which is low and not enough to maintain a climb margin in a heavy certified airframe. By way of comparison SI TSIO engines can maintain higher power % at those alts and therefore tend to be certified higher. So if you fly experimental you can fly SI at 26K but not certified.

Cruise BSFC figures for the IO-550 running LOP are within a whisker of the aero diesels. Cruise is where aero engines spend the majority of their lives.

Yes again. At around 0.38lb/hp/hr the naturally aspirated IO-550 is the current gold standard in cruise. Low alts only. The disappointment is when this same engine is turbo-normalized for higher alts (say the Vitatoe TN550 conversion) it needs to be run massively rich to avoid detonating at climb power. As one example, the Vitatoe IO-550 turbonormalized TN550 (Cessna T/P210N) is normally set at 37GPH at climb power (310HP) which yields 0.71lb/hp/hr. Abyssmal.

Herein lies the problem. You can't have your cruise cake and eat it too. As soon as you add higher CR to gain cruise efficiency, the engine becomes more prone to detonation and therefore needs to be retarded and run very rich at rated power. Certainly so when turbocharged. With CI you don't get this issue. You get other issues such as weight and higher boost requirements at alt. Whilst I think Technify deal with the Diesel weight issue very well (aluminium blocks etc), Austro took the 'other' route and retain cast iron blocks etc to make engines that are heavy but supposedly more reliable.

By way of comparison, I have not seen any Adept data (they dont publish?). However I expect liquid cooling allows detonation limits to be more manageable as 220F cylinder head temps are a lot easier to manage with AC 33.47-1 detonation testing than Lycosaur 490F. The lower CHT's should allow higher permissible CR and subsequent better rated power BSFC. However before we all jump for joy, it's worth remembering that the liquid cooled TSIOL550A only made 0.51lb/hp/hr at rated power and the later TSIOL550C was worse at 0.63/lb/hp/hr. So liquid cooling didn't help those engines at all when it came to rated power BSFC in turbocharged engines. Hence its worth remaining skeptical until we see some actual data.

 

[stanislavz]

Herein lies the problem. You can't have your cruise cake and eat it too. As soon as you add higher CR to gain cruise efficiency, the engine becomes more prone to detonation and therefore needs to be retarded and run very rich at rated power.

You can. Variable valve timing/ variable valve lift. New engines withe near 40% thermodynamic efficiency do it and in wide area of load. But - it is complicated and heavy.

 

[stanislavz]

But - it is complicated and heavy.

Rephrasing. All for SI.

1. High compression - + for cruise, - for climbing power.
2. Lower compression without turbo - - for cruise, + for climbing power.
3. Low compression with turbo - - for cruise, +++ for climbing power.
4. Atkinson - + for cruise, --- for climbing power.
5. Miller ?
6. Variable valve timing and lift + cruise + climbing - in complexity.

 

[dwalker]

Cylinder head technology has come a long way on the last 50 years.

 

[Martin W]

.

The ultimate piston engine would have
--- a short intake stroke
--- a short compression stroke
--- and a very long power stroke

Such an engine was built around 1980 in Winnipeg Manitoba by engineer and university professor Hoken Kristiansen and was known as the K-Cycle engine or the Kristiansen-Cycle engine. I lived in Winnipeg at the time and followed it closely.

It produced the equivalent horsepower of a Chevy 350 V-8 .... but weighed only 125 pounds .... and mileage efficiency was improved over 30% which was a major breakthrough.

It had 6 pistons arranged in a circle similar to a revolving cylinder in a 6-shooter pistol ...... instead of a crankshaft the connecting rods rode on a roller-coaster shaped swash-plate which allowed for the various length of stroke.

The theory was solid and several prototypes were built including installing one in a Ford Mustang and driven to the Ford factory as a demonstrator. .... it was a low-rpm engine producing huge torque and burned the fuel so efficiently the exhaust pipe was only warm to the touch.

Couple of show-stoppers prevented further development .
--- Kristansen unexpectedly died of a sudden heart attack shortly afterward
--- But the main issue was the pistons and cylinder block all rotated around the output shaft ... that is a lot of moving mass ... which could be accommodated because it was a low-rpm design
--- However 2 remaining issues were not resolved .... the centrifugal forces caused the pistons to move outward and caused heavy wear on that side .... plus all the crankcase oil was flung to the outside of the crankcase instead of lubricating the connecting rod mechanisms etc.

 

[rv6ejguy]

Dr. Kristiansen was our neighbor and we became good friends with the whole family, staying in touch for years. He was a very smart and kind fellow. The engine design was brilliant. Basic problems would have a better chance of being solved today. It would be great to see that happen and for it to make it into production.

 

[stanislavz]

The ultimate piston engine would have
--- a short intake stroke
--- a short compression stroke
--- and a very long power stroke

And i think i have seen similar one made from ordinary 4 cylinder inline one.

 

[stanislavz]

Found it:

(PDF) New design of the fivestroke SI engine

PDF | The paper describes design of the five-stroke engine developed in the Chair of Combustion Engines at Cracow University of Technology. The idea of... | Find, read and cite all the research you need on ResearchGate

www.researchgate.net

 

[PMD]

The K-cycle engine was inspired but let's not forget that fixed block, rotating swashplate/wobble plate engines have a very long history and have been certified for aviation use (Dynacam). What Kristiansen was developing was really an extension of the original Atkinson Differential engine (1881) but arranged as a swash plate engine. Noteworthy is that it used opposed pistons (as have many very successful engines over the last 140 years) but differentiated displacement for a 4 stroke cycle. This IMHO was one thing Herrmann missed (in 1941) designing the Dynacam.

I have had some exposure to some ongoing R&D using swashplate barrel designs, and from what I can gather from the very real success of the Dynacam but much ongoing difficulty with other R&D projects it is getting the cam and rollers to work reliably and durably is where there is still more work to be done. Yes, my interest in such obscure things has a lot to do with being a ME student at UofM a very long time ago.

The "5 cycle" engines are really just a normal 4 cycle with an integral positive displacement power recovery stage - which limits aviation potential as it would be severely altitude challenged. Much simpler to just add a turbocharger and let it cover the need for more air with altitude.

Which brings us to why CI vs. SI. No aspirated charge means no such detonation limits that drive current fossil fuel/fossilized tech engines to burn stunning amounts of avagas to try to overcome fixed antique mag timing issues and lack of cooling air at altitude. Much simpler to just throw the mags in the dust bin and use compression to ignite. Again: one of the reasons that pretty much ALL newly certified piston engines for some time are diesels, not gassers.

 

[rv6ejguy]

Which brings us to why CI vs. SI. No aspirated charge means no such detonation limits that drive current fossil fuel/fossilized tech engines to burn stunning amounts of avagas to try to overcome fixed antique mag timing issues and lack of cooling air at altitude. Much simpler to just throw the mags in the dust bin and use compression to ignite. Again: one of the reasons that pretty much ALL newly certified piston engines for some time are diesels, not gassers.

Easily solved with electronic ignition/EFI and cruise BSFCs become as good as aero diesels on engines like the Adept line and within a few percent of CI engines even on Lyconentals fitted with this tech and slightly higher CR pistons running LOP. Witness the amazing performance Dave Anders RV-4 gets with a modified IO-360 using high CR, EFI, EI, tuned induction and exhaust systems. Estimated at .35-.36 LOP.

The Rotax 912iS and 915iS are SI engines where the EFI/ variable EI tech replaced carbs and fixed timing EI, resulting in a 15-20% reduction in fuel flow for the same TAS. These are the best selling aero engines in the world where CI engines have barely scratched the surface of market share in the past decade.