I need your vote for a new Rotary HCCI engine ! 4

[RodRico]

Fellow engineers,

This post promotes my design, but it's also informative for those curious about new developments in engine design, so I hope it's OK. Please accept my apologies if not.

I have submitted my patent pending design for a "Hybrid Miller Cycle Rotary HCCI Engine for RQ-7 Class Drones" in the "Create the Future" contest. You may find it by googling the engine name above or by visiting

https://contest.techbriefs.com/2017/entries/aerospace-and-defense/8090.

Preliminary analysis indicates power density (3 HP per pound) and efficiency (45% with 0.300 BSFC) comparable to a turbofan when operated at full equivalency. When operated in Low Temperature Combustion (LTC) mode, the engine still produces nearly 1 HP per pound but creates very few emissions. Because of its small 10" diameter and 6.5" thickness, multiple engines can be arranged in a clover-leaf pattern around a common shaft to yield 380 HP in a 24" by 6.5" volume. Another set of engines can be arranged behind the first to yield 760 HP in 24" by 16" volume. Note it's not mechanically efficient to add a third engine set due to limitations in my design.

I would greatly appreciate your support of my contest entry. Viewing my entry helps, but voting for it (which requires simple e-mail verification) helps even more. As it stands, I'm only one vote ahead of a "free energy" device ! That's just wrong ! Please circulate the link as widely as possible and encourage all your engineering friends and colleagues to help me win this contest! If I win the contest 100% of the money will go to funding 3D modeling of CFD/Combustion/Heat Loss by a consultant.

Thank you very much for your time and any support you can offer. If you have questions or comments, please post them here or on the contest site and I will answer them to the best of my abilities. I view criticism as being more valuable than praise when it comes to design, so don't hesitate to challenge my design (but please keep it respectful per normal engineering tradition).

Respectfully,

Rod Newstrom

P.S. Some may wonder why I targeted my contest entry at military drones. I would have preferred to emphasize the efficiency and low emissions qualities of my engine when operating in Low Temperature Combustion (LTC) mode. I targeted the military application instead because the administration wants to zero funding at the DOE/ARPA-E (who would normally fund such advances) while simultaneously increasing military budgets. I mention the RQ-7 drone, an unarmed surveillance drone, specifically because my engine fits in the volume and weight envelope of its current engine (the AR-741 Wankel), and the Army issued a Request For Information in 2016 for a replacement engine.

 

[LionelHutz]

Unlike the silly argument over the use of TDC, you used the term compression ratio wrong. Compression ratio IS exactly as described by jgKRI.

The point I WAS making is that compression ratio is a mechanical characteristic, dictated by geometry. Theoretical compression ratio by piston bore/stroke and combustion chamber size; actual or dynamic compression ratio by piston bore/stroke, combustion chamber size, and timing of valve events.

What you wrote here

A modern spark ignition engine could delay spark causing combustion to occur later in the expansion stroke. The net result would be a reduction in compression ratio.

is completely wrong. Spark timing does not change the compression ratio. You probably could have said that retarding the spark timing has the same effect as reducing the compression ratio. You certainly can't say that retarding the timing reduces the compression ratio.

And your engine is variable compression ratio.

 

[RodRico]

Brian,

The paper I was referring to is titled "A reduced chemical kinetic model for HCCI combustion of primary reference fuels in a rapid compression machine" and it's available at

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.464.1483&rep=rep1&type=pdf

. Figures 8 through 13 figure prominently in my thinking. Note in reviewing this paper that my intake is at about 1.0 MPa and 770K at the start of the ignition process. By the end of the process, motoring pressure and temperature are around 1.4 MPa and 830K respectively. With this input state, predicted ignition delay is under 1 degree at 7800 RPM. At these given octane, intake pressure, and intake temperature, the chemical reaction rates are extremely fast and don't vary much over load. Altitude certainly affects the input state, but I regulate the supercharger piston output pressure/temperature via a computer controlled wastegate. Cylinder temperature should come up quickly and fluctuate pretty slowly due to the thermal mass of the rotor. All of this will be the focus of the next phase of work. My consultant will be building a multi-physics model (CFD, heat transfer, chemical kinetics, etc.), and engineering will construct controlled experiments to ensure everything tracks expectation (within reason). We'll iterate this process until we're happy then go into prototype to start peeling the next layer of the onion.

-------------------------------------

jgKRI,

Sorry for misunderstanding your comment. My engine is, in fact, "variable compression" during the very short transition period between cold start and run state. Making a variable compression engine with opposed-pistons and a third supercharger piston is not complex (or at least not much more complex than building the same engine with fixed compression). There are two ways to accomplish this: Manipulate the supercharger piston's wastegate to change intake charge pressure/temperature, or change the phase of the two cams. The engine already needs a wastegate for other reasons, so manipulating that wastegate is cheaper and is the preferred approach. Nonetheless, let's look at how one can make a comparatively simple and robust variable compression engine using opposed pistons alone without a supercharger piston...

Forget my cams for a moment and just imagine the two opposed-pistons are on separate crankshafts. If the two cranks are in phase, maximum compression will occur when both are at maximum stroke. If they are 180 degrees out of phase, the compression ratio will be zero as the pistons will just move a fixed volume up and down in the cylinder. The phases between 0 and 180 degrees yield different compression ratios. The mechanism to control the phase of one crank relative to the other is where the mechanical challenges come in. My engine, however, doesn't use crankshafts, it uses cam shafts, and their relative phase can be adjusted using a cam phase adjuster identical to those used in several modern engines (i.e. Ecotec). If I have trouble with attaining the desired result via manipulation of the wastegate, I will use the cam phasor approach.

My engine has more compression than needed to initiate autoignition even under lean cold start conditions. The first few combustion cycles are inefficient because compression continues after combustion. I eliminate this inefficiency once running by dumping some supercharger pressure or by shifting the relative phase of the two cams. Once established, this setting should change very slowly (certainly not cycle to cycle) per my response to Brian above. Thus, once running in steady state, compression ratio remains stable.

--------------------------------

BigClive,

See my discussion above for two comparatively simple ways to effect variable compression ratio in an opposed-piston engine. One requires presence of a supercharger, the other does not.

The reasons diesels produce more torque and better efficiency are related, as you know... High compression ratio yields efficiency and torque but the long stroke of the engine (and, to a lessor degree, the time to complete fuel injection) yields detrimental impacts on RPM and power. I address these issues in two ways. First, the radial arrangement of my cylinders naturally facilitates a supercharger piston bore that is much larger than the bore of the other two pistons which form the combustion chamber, and this facilitates very high compression ratio with little stroke. Second, HCCI ignition delay and heat release rate are extremely fast when the intake pressure and temperature are as high as they are in my engine (see the paper I linked to in my response to Brian above). By my analysis at 7800 RPM, combustion completes within a degree of crossing the autoignition temperature. This rapid combustion also improves efficiency as it is about as close to constant volume as possible using petroleum fuels. HCCI also, of course, improves efficiency simply due to its lack of a flame front. Watch the video at

https://www.youtube.com/watch?v=oNCBbcINHuk

to get an idea of how much faster HCCI is than spark combustion with a flame front (imagine how much faster than a diesel fuel spray it would be). The rapid heat release does, of course, come with a price. It yields extremely high combustion pressures. I had to downsize the pistons and use steel to get stresses in bounds with margin. The downsized pistons cost me a bit of efficiency, but it appears the other gains outweigh the cost of the small cylinders with high surface area to charge ratio.

Rod

 

[SwinnyGG]

No harm done on the misunderstanding- your interpretation of my comment wasn't what I intended to convey, but it was a reasonable interpretation of the actual phrasing I used. My comment was a little bit vague in terms of what I actually meant, which is why I felt I should explain.

Back to technical discussion...

How exactly are you calculating compression ratio?

and this facilitates very high compression ratio with little stroke

This phrase is.. confusing.

I also noticed when I looked at your submission the first time, that your stated compression ratio is very high and the stated expansion ratio is lower than I would have expected given the quoted compression ratio.

So.. How exactly did you calculate this theoretical 70:1 ratio?

And is that compression ratio the value you are using to calculate efficiencies? Lumping the various stages of your combustion process together seems to me to be a little dubious.

I understand your combustion phasing concept; what I don't understand at this point is how the combustion volume and 'supercharger' piston interact.

If the 'supercharger' piston is never exposed to combustion pressure, than its expansion does not contribute to positive work- in fact it represents pumping loss and thus negative work- in which case it does not contribute to dynamic compression.

 

[RodRico]

LionelHutz,

Imagine you're doing thermodynamic analysis of a classic four stroke engine. What compression ratio do you use in working through to efficiency ? Do you use that establihed by the compression stroke or that at time of combustion ? It matters.

I apologize if I have deviated from normal use of some terms. That being said, are you trolling on terminology or truly trying to understand how my engine works? If it's the former, I will stop responding to you. If it's the latter, please read everyting and tell me what you need clarified.

Rod

 

[djhurayt]

Regardless of terminology used, how is the voting going RR?

 

[BrianPetersen]

You have to use terminology that is consistent with what is used in industry and in combustion-engine research if you want to be taken seriously.

Obviously, in a real internal combustion engine, the "instantaneous", or "effective", compression at the moment that a given fuel particle burns is what effects the thermodynamic analysis and, at least in theory, the total is the integration of that over the entire fuel mass. But this isn't what is normally considered to be the "compression ratio". The nominal compression ratio is the ratio of maximum chamber volume to minimum chamber volume - period. (No consideration of moment of ignition or speed of combustion.) The effective, or dynamic, compression ratio is the ratio of trapped chamber volume (at the moment that it becomes trapped - whether by valve closure in a poppet-valve engine or port closure in a piston-ported engine) to minimum chamber volume - period. Again there is no consideration of the moment of ignition or speed of combustion. Forced induction is not taken into account.

I think we could use a better diagram of how the gas-flow works relative to the motions of the various pistons, ports, and valves. Focus on one cylinder's piston motions.

 

[btrueblood]

Rod,

"Figures 8 through 13 figure prominently in my thinking. Note in reviewing this paper that my intake is at about 1.0 MPa and 770K at the start of the ignition process. By the end of the process, motoring pressure and temperature are around 1.4 MPa and 830K respectively. With this input state, predicted ignition delay is under 1 degree at 7800 RPM. "

The paper gives curves for initial pressures up to 0.1 MPa, so you are extrapolating the ignition delay curves from those shown (typ. 1 to 10 ms) to your expected pressure. By my math, (7800 rev/min)*(360 degrees/rev)/(60 sec/min) = 46,800 degrees/sec, or the reciprocal about 2x10^-5 seconds per degree. I'm not sure I buy a 5 orders of magnitude reduction in ignition delay for a one order of magnitude pressure increase? Can you explain your reasoning here?

 

[RodRico]

btrueblood,

I extrapolated the curves of Figure 8 for PRF63, the midpoint betwwen PRF75 and PRF50. Note PRF50 looks to average around 1 ms, so I think your orders of magnitude is too large. Anyway, I contacted the author to see if he had data further down and he replied he did not. I asked if he felt extrapolation would be valid and he replied yes, for first order estimates. I have hired a consutant experienced in all aspects of combustion (worked on Skyactive team at Mazda) to check the fuel characteristics in his chemical model and review my thermodynamics work. He should be done soon.

Rod

 

[RodRico]

djhurayt,

Not as well as I hoped. All of my contacts are engineers, and most said it looked interesting, but they couldn't vote for it until they did analysis. LOL! Typical engineers! The guys pitching free energy machines obviously don't suffer the same problem. No matter. Votes and views can get me a small prize, but the big one is determined by a panel of judges. Last year's winner had fewer votes than I currently have.

Rod

 

[Lou Scannon]

Glow-plug" model engines don't have adjustable CR - only model "diesel" engines do - the CR can be as 40:1. The model "diesels" are notably more fuel efficient and quieter than "glows" but usually less powerful.

That's consistent with the model (airplane) engine landscape when I was fooling around with them in the 70s. By then, the diesel variant was obsolescent, I guess. I never saw one first hand, but the technology was still well known at the time. The sealing mechanism for the variable CR screw boggles the mind.

From a thermodynamic perspective, there are at least two additional interesting volume ratios in addition to the mechanical compression ratio. One has been discussed already, the "dynamic compression ratio". The other is effective expansion ratio (EER). Engine layouts and combustion systems that can at least partially decouple these two ratios are interesting, because a high EER is always beneficial for efficiency, while keeping the dynamic compression ratio to a lower value is beneficial for reducing peak temperatures and hence NOx formation, in all engines; and thereby also adding knock margin, in spark ignited engines. The Miller cycle concept is probably the best known and simplest engine layout commercially applied today that achieves this goal. Lest anyone think I'm ignorant of the Atkinson cycle, as applied today it is simply a variation on the Miller theme, albeit that Atkinson predates Miller.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[BrianPetersen]

Atkinson/Miller cycle is relatively simple to emulate with a 4 stroke engine of conventional layout ... either close the intake valve really early (VW) or really late (Toyota) and make the mechanical compression ratio quite high. The effective compression ratio is lower (i.e. "normal") because part of the charge either never gets into the cylinder to begin with (early intake valve closure) or part of it gets pushed back out again (late intake valve closure).

It's not normally so easy if you use piston porting, but I wonder if this is what our OP (original poster) is attempting to achieve but has not been able to explain correctly. If the piston that is handling the exhaust ports is bigger in displacement (either via bore or stroke) than the one that is handling the intake ports, AND the piston that is handling the exhaust ports "leads" the timing of the piston that is handling the intake ports (i.e. exhaust opens first and closes first, intake opens second and closes second but by which time the "big" exhaust-side piston has come partway up) it will have the effect of a two-stroke Atkinson cycle.

I read the paper that was linked to which concerned HCCI combustion in a rapid-compression device. The long delay between initiation of slow combustion and the "explosion" (~ 10 ms in some cases) is something of an issue - in an engine spinning at just 3000 rpm, the expansion stroke is done in that time. The variable amount of that delay depending on fuel characteristics and intake air temperature is another headache to deal with. If that's the situation with HCCI combustion, I don't see much hope for it (and it may be why, since that paper was published in 2003 and it is now 2017, we have not seen HCCI in production yet). I realize that the delay can be shortened by overcompressing, but then if operating conditions (fuel, temperature, load) change so that the delay is less, combustion will happen before TDC and peak cylinder pressure will skyrocket. I don't see a hope for this without variable compression ratio of some sort ... Which the original poster's concept design is capable of doing, even though the original poster himself is denying its use

 

[BigClive]

Hemi - most of the sealing of these small model engines is by the excess oil from the fuel mixture after combustion. These engines are notable "droolers" of oil from the exhaust ports.

 

[RodRico]

Guys,

I found an old movie I made showing engine operation on my phone. It doesn't reflect the current configuration in a couple of ways (there are no cam valves for the intake and scavenge paths as each now has its own side port, and all of the numbers relate to a different instantiation of the engine), but it should help you visualize what's going on. I'll be updating the movie when I get back to the states to reflect the same configuration as shown in the contest entry.

Someone asked how I calculated compression and expansion ratios and suggested I use them to calculate performance.

I don't use compression and expansion ratio in my performance calculations. I calculate in-cylinder port state (intake, exhaust) as well as side port state (pump out, scavenge and intake in, exhaust) along with volume, pressure, temperature, ratio of specific heats, and work in 0.5 degree steps. I then sum work and derive performance from there.

I only *report* compression and expansion ratio. I calculate compression ratio by dividing the max volume of the air pump by the volume at first burn and expansion ratio as the volume when the exhaust side port first opens divided by volume at first burn. The calculation of compression ratio is pretty course; it doesn't include incremental losses as the gas moves through all the ports from the air pump to the combustion chamber. The expansion ratio number should be solid as it involves no transfers.

Someone said my pump piston has a cost. Of course it does. I calculate a loss of 1.8 joules over that piston's cycle. Another 5.2 joules is lost between the compression and expansion piston, and 18.8 joules gained during combustion/expansion. The result is a net gain of 11.8 joules out from 25.1 joules of fuel energy in.

Rod

 

[BrianPetersen]

To me, that looks like a sealing nightmare, a gas-transfer nightmare, a heat-loss nightmare, and a friction-loss nightmare all rolled into one. Maybe I'm wrong.

 

[GregLocock]

Yah but it works in Solidworks animations, you old cynic you.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[RodRico]

Brian, It's not going to be easy, that's for sure !

Greg, Every design goes through the steps I'm taking. Nobody exits System Concept Review saying "were're done!" All they can say is "we haven't found anythng in concept development that indicates it won't work, so we're going to proceed into preliminary design."

 

[GregLocock]

The thing is anything that looks like a pump can be made into an engine. This not exactly earth shattering revelation means that you can start from almost any pumping concept and then try and make an engine of it. Thermodynamics give some pretty harsh limits on what you'll be able to achieve efficiency wise, and the other operating parameters are more a matter of detail than concept. Read up on this one, its been running for years

http://www.revetec.com/latestnews.htm

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[RodRico]

Greg,

I guess you missed the part where I described what thermodynamic and mechanical analysis has (and hasn't) been completed thus far. I have hired an expert in combustion, heat transfer, chemical kinetics, etc. to reveiw my analysis. If it passes muster, I'll proceed to full 3D multi-physics analysis. Only after that will I cut metal.

I'm fully aware that many have tried and failed. I examined the many engines that came before including Wankel, Revetec, Duke, OPOC, Scudiere, Nautilus, Achates, etc. I wouldn't have taken it this far if I didn't believe it offers something different and valuable. Mine is comparatively simple and offers efficiency and power density of a turbofan.

I have long experience in risk driven technology development. Some pretty radical concepts that few experts beleived were possible suceeded, and a number that seemed simple failed well into development due to unknown unknowns. My engine has passed my own thermodynamics and stress analysis. If it fails that of my consultant it stops. If it fails in 3D multi-physics, it stops. If it is too costly to manufacture it stops. If at any point it fails to offer performance vs cost value at least twice that of competing engines, it will stop.

I have no illusions that my engine is a slam dunk. Heck, even if it *does* work as well as hoped and proves to be manufacturable and affordable, it may be too late to suceed commercially in a world where many see only electric vehicles.

Rod

 

[CWB1]

I believe the point that many in this thread are attempting to make Rod is that you are going about this contrary to accepted engineering practice while lecturing others on basic concepts and verbiage which you are mistaken about. In engine development an initial combustion analysis via CFD is literally the first step, it drives hardware design, gives a rough estimate of possible performance, and also lends credibility to your design. Without this you are still in the realm of the many hobbyist dreamers that attempt to break into engine development every year, insisting on all manner of free energy and easy efficiency. I am sure you put a lot of effort into your paper study however without correlation to significant testing its highly unlikely to even get you in the ballpark due to efficiency losses and combustion trends beyond your current understanding and analysis capability. JMO as someone who works in this niche but you are still at step zero in a very long process.

 

[BigClive]

Roller followers might be better than your "plain" followers.