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.

 

[gruntguru]

Rod. I understand your explanation but dispute your statement "P.dV doesn't fully describe the mechanism. Must use F.dX of each piston to fully describe it".

P.dV = F.dX

This relationship applies for each piston individually and for both pistons combined.

je suis charlie

 

[RodRico]

Grunt,

Yes, P.dV = F.dX both in theory and in my Excel workbook calculations. P.dV accurately predicts work and performance, but it is agnostic as to the direction of rotation. Each piston generates rotation force in a different direction than the other in my engine, and I need to know what those forces are so I know the engine won't counter-rotate. It eludes me how I would determine that examining P.dV (which represents the bulk condition of the combustion chamber of which each piston is only a part) rather than the P.dX for each piston alone ? Assuming it can be done, why would it be superior given we agree they are the same ?

You have repeated several times that I don't need the two pistons and that a single piston developing the same PV diagram would achieve the same result. I still disagree. Combustion occurs *during* compression in my engine... that's how I eliminate the vulnerability to factors affecting ignition delay that make it so hard for a "normal" HCCI engine to ensure combustion occurs shortly after TDC. If combustion were to occur before TDC in a single-piston engine, it would drive the engine backwards. In my engine, it occurs before TDC of one piston (which would be intolerable if it were the only piston) but not the other, and they are arranged such that the F.dX of the proper piston exceeds that of the other. Granted it's undesirable to continue compression during combustion, but this only occurs during start and warm-up. Once the engine is running and stable, I start shifting the timing of the combustion point near TDC of the ignition piston to minimize compression during combustion.

Rod

 

[gruntguru]

There is only one TDC. In multi-piston engines like yours, "TDC" is the point at which minimum cylinder volume occurs. If combustion occurs prior to that point, the pressure increase is tending to drive the engine backwards (Work, P.dV, Sum of F.dX, Sum of T.dTheta etc are all negative) - regardless of how complex you design the pistons and actuating mechanisms.

je suis charlie

 

[BigClive]

I don't know it anybody else has all ready said it - but I don't know how wise it is to try and develop a new engine architecture (if that is the word) and an experimental engine process (the HCCI) at the same time. Maybe develop each separately?

 

[RodRico]

gruntguru,

Top-Dead-Center describes a position. If you want to redefine it as Min-Cylinder-Volume, you can, but then you're left having to come up with terms for positions. No matter, it's semantics. We're just going to have to agree to disagree on the "you could do this with one piston" assertion. It's OK, it happens. I'll come back and let you know one way or the other if and when I get thru prototype.

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

BigClive,

You're right. I plan incremental hardware builds and will likely get the HCCI working first using stationary cylinders and rotating cams.

Rod

 

[SwinnyGG]

Top-Dead-Center describes a position. If you want to redefine it as Min-Cylinder-Volume, you can, but then you're left having to come up with terms for positions. No matter, it's semantics. We're just going to have to agree to disagree on the "you could do this with one piston" assertion. It's OK, it happens. I'll come back and let you know one way or the other if and when I get thru prototype.

You're correct in that TDC is a defined position of components- but Guru is correct in that this defined position of components is take as the point where cylinder volume is minimized. You are not the first person to design an engine with more than one cylinder acting on a single combustion volume, and those other people use this known, standard definition of TDC as well.

This is an industry convention that, if you want people that design and invest in engines to take you seriously, you should be using as well. You're already doing work to make your documentation align with known conventions, the terminology should align as well. It makes your ideas easier to understand and, more importantly to you, easier to sell.

 

[RodRico]

JgKRI,

I can't find that definition anywhere. Every one I see is a positional description. Nonetheless, I'll work on a description of my engine that doesn't violate the common use of TDC.

Thanks for the tip.

Rod

 

[LionelHutz]

Who here didn't understand the description of combustion occurring when one piston was past it's TDC point and the other piston was before it's TDC point?

 

[BrianPetersen]

I figured that out with the simplified diagram, and established (in my mind) that the adjustable phase-shift between the two pistons is essentially a variable compression ratio mechanism (despite the original poster's protestations), and I understand the point of having a variable compression ratio mechanism if you are attempting to use HCCI since it is one of the few means available to be able to (poorly) control the moment of ignition.

I just don't think the reality is going to work out the way it is imagined. I still consider this to be a SolidWorks exercise backed up by unrealistic theoretical calculations.

 

[berkshire]

I remember as a teenager using " Diesel" model aircraft motors ,and having to screw down on the compression screw to get the thing to start , then as the motor warmed up, backing the screw off, since adjusting the compression ratio was the only means of adjusting the timing and adjusting for best rpms, does this engine have that sort of adjustment?
B.E.

You are judged not by what you know, but by what you can do.

 

[BrianPetersen]

Evidently, adjusting the phase angle between the inner and outer cams, and therefore the pistons, is intended to have that effect.

I forgot about the model airplane engines - the "glow plug" engines. Those are a simple form of HCCI engine with (manually) adjustable compression ratio in order to coarsely tune the moment of ignition. (I refuse to call it "fine tune")

 

[RodRico]

Brian,

You haven't seen my calculations. They are indeed theoretical... as are about 90% of the hundreds of published papers I have read regarding HCCI. I will obviously have a more compelling story once hardware demonstations are completed. In the meantime, I'll take your meaning to be simply, "I haven't seen enough evidence to convince me it will work as claimed."

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

Berkshire,

One of the earliest HCCI engines was, in fact, a diesel model engine that used a premixed charge. Read the "History" section at

https://en.m.wikipedia.org/wiki/Homogeneous_charge_compression_ignition

.

This engine is designed to operate without changing the compression ratio. It does have the ability to shift the relative phase of the two pistons to adjust timing, but this does not neccesarily change compression ratio; I'm really just shifting the whole process such that combustion occurs when both pistons are further along in their trajectory... the expansion piston is further into its expansion stroke and the compression piston closer to its full stroke. This costs a small bit of expansion but reduces negative work by the compression piston and allows the engine to accomodate varying fuel characteristics, engine temperatures, operating load, etc. just as spark advance is used in conventional gas engines.

Rod

 

[gruntguru]

shift the relative phase of the two pistons to adjust timing, but this does not necessarily change compression ratio
It does actually.

je suis charlie

 

[btrueblood]

shift the relative phase of the two pistons to adjust timing, but this does not necessarily change compression ratio
It does actually.

Depends on what the valve timing does.

 

[RodRico]

guru,

We're debating terms again.

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. Modern spark engines don't generally do this, however. Instead, they adaptively move spark timing to try and hold it at the optimum position at the start of the expansion stroke. The fact that they *could* delay the spark enough to yield reduction in compression ratio doesn't make them variable compression engines.

Like a spark engine, my engine *can* vary it's compression ratio, but in actual operation, it has one and only one compression ratio, that required to cause auto-ignition. Any more and it's wasted energy, any less and it doesn't work. At start-up, it's uncertain what compression ratio is necessary, so the engine is designed to ensure the proper value falls somewhere in the range of the compression piston stroke. The margin used to encompass uncertainty inevitably leads to some "spare" compression, meaning the compression piston continues compressing after combustion, resulting in lost efficiency. As soon as the engine fires, it delays the start of the compression piston stroke relative to the receding expansion piston to shed the "spare" compression. The compression ratio at which combustion occurs is constant within a narrow window regardless of when it occurs. If you want to focus on one limited case, start-up, you could say the engine is variable compression. It is, however, operating at a fixed compression ratio with very little variation over time the overwhelming majority of the time.

Note, by the way, that the optimization of operating point after start up can also be effected using the waste-gate on the output of each supercharger piston. This is, in fact, the cheapest and simplest way to accomplish the desired state. Though its latency is greater than that of the shaft phase adjuster, the rate of variation while running is low, so I'm pretty confident this method will be determined sufficient during testing. Many engines employ waste-gates, as you know, and they effectively change compression in aircraft engines in order to compensate for reduced air density at altitude. We don't call *those* engines variable compression either.

Rod

 

[BrianPetersen]

I suspect you will find that the "one and only one compression ratio, that required to cause auto-ignition" will be different depending on altitude, intake air temperature, lambda, residual exhaust percentage, temperature of surfaces in contact with the charge, and fuel properties. Lambda, residual exhaust percentage, and temperature of surfaces in contact with the charge are dependent on how much load is on the engine. The temperature of surfaces in contact with the charge is additionally dependent on HOW LONG a given load is on the engine.

 

[SwinnyGG]

'Compression ratio' has nothing to do with when combustion does or doesn't occur.

No one that designs engines uses the term 'compression ratio' to describe the combined net effect of mechanical dimensions, valve event timing, and ignition timing in the way that you're using it.

You're not having a 'debate' about the meaning of compression ratio- you're using the term incorrectly.

 

[RodRico]

Brian,

I'll post a link to a paper on HCCI response to the conditions you mention tomorrow (gotta dig the link back up). As I recall, it shows HCCI at high intake pressure to be less sensitive to many of the conditions you mention than one might expect. I'll elaborate when I post the link.

-----------

JgKRI,

You apparently aren't familiar with HCCI. "Homogeneous Charge" (HC) means fuel and air are premixed at intake (like a gasoline spark engine). "Compression Ignition" (CI) means the mixed charge is ignited by compression. Compression has *everything* to do with when combustion occurs in an HCCI engine.

We weren't debating the meaning of compression ratio. We were debating whether my manipulation of piston stroke phase is rightly called a "variable compression ratio" process.

I never said valves had anything to do with compression ratio in my engine (though they cetainly can in some engines... consider the modern direct injection engines with variable valve timing that hold the intake valve open in the early portion of what would normally be the compression stroke to reduce intake volume and compression while increasing expansion ratio). Note, by the way, that my engine is a uniflow scavenged opposed-piston two-stroke (ala Junkers) augmented with rotary side ports. It uses ports, not poppet valves, and it has no means to vary port timing or duration.

Rod

 

[SwinnyGG]

I am intimately familiar with HCCI. I've bit my tongue a lot in this thread, mostly because Brian has frequently said very nearly exactly what I would've said had he not posted first.

The point I was making is that your interpretation of what the term 'compression ratio' means is incorrect.

I said this:

[quote='Compression ratio' has nothing to do with when combustion does or doesn't occur.][/quote]

Which you apparently interpreted as 'combustion timing of compression ignition engines is not affected by compression ratio'.

That statement would be incorrect, but that isn't the point I was making

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.

Changing ignition timing does not change compression ratio, period, regardless if the engine is an HCCI design or not. If you think it does you're simply not talking about compression ratio.

The only spark ignition engines that can change compression ratio are those with complicated mechanical arrangements that change some or all parts of the geometric factors listed above.

If you're changing BDC or TDC volume, you're changing compression ratio, unless you're doing it in a way maintains the relationship between them.

It's a little pedantic to go back and forth about terms in such detail- but if you want people to understand your concept, and you want to appear to know what you're talking about, you can't redefine technical terms on the fly.

 

[BigClive]

"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.
A glow engine with adjustable CR would be quite possible - but I have never heard of it being done.