Define "hold".

Two-phase mixtures of liquid and gas run the entire spectrum from 0% to 100% liquid. The distinction between droplets contained in a gaseous flow stream and bubbles of air contained in a liquid flow stream is not clearly defined. Carburetors frequently contain emulsion tubes, in which both air and fuel are metered in through jets and then the emulsion as a whole is metered through another orifice.

I don't remember with any specificity the specific flow path(s) of air and fuel required by your design.. but engines in RC aircraft have been atomizing fuel in quantities small enough for engines under 1cc since the 50s, using very, very simple metering. If I were in your shoes I'd be looking at their solutions.

IIRC RC engines are using a blended fuel containing nitromethane and alcohol, which is insensitive to air/fuel ratio because it carries a lot of its own oxygen. Simple fuel metering is possible when the air/fuel ratio doesn't matter much (and you are not concerned with emission regulations, or gas mileage or thermal efficiency).

I have a 125cc 4 stroke motorcycle (12 hp) which has EFI, but I can't think of a smaller engine that uses fuel injection. Lawnmowers, chainsaws, generators and the like are all carbureted.

There are sub-cubic-inch RC engines that run on diesel.

I don't think they run on pure kerosene diesel. They are compression ignition. Here's one recipe

Kerosene - 42.5%
Ether - 30%
Castor Oil - 25%
Octyl Nitrate - 2.5%

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

So those are running on starting fluid (ether). Compression ignition and glow plug ignition should be insensitive to air/fuel ratio and can also get by with simple fuel metering. For that matter, if memory serves, the original poster is also proposing a compression ignition concept. And the "diesel" RC engines use premixed air and fuel ...

The application is Homogeneous Charge Compression Ignition (HCCI) using diesel: Air and fuel are premixed then compressed to ignition. To work effectively, the mix must be very well blended and droplet size is key.

Both Ecotrons and PCRC make small engine fuel injection systems. The mix quality from these devices is unknown, and I doubt either of them is suitable for my needs (diesel, very homogeneous mix for HCCI, etc.), but I've contacted both with my requirements and await their reply. These injectors would only be viable for plenum style injection which is less than ideal (unless they can make a well mixed very rich mixture allowing direct injection of a very small amount).

I reached out to my engine consultant yesterday, and he informed me sapphire micro-orifices are available off-the-shelf, so I'm going to go back and take a look at the classic unit injector using these devices. The idea of feeding a single tiny orifice still worries me on a number of fronts, however (clogging, tolerances, etc.).

I recall that Paasche made a 'turbo' air brush where a needle, driven by an air turbine, is dipped into the paint and then pushed into the airstream where it is blown off. This allows extremely fine control under conditions where a venturi might be insufficiently precise. Perhaps a similar system with a tiny rod with an open eye was used to capture a known amount of fuel; since the operation is discontinuous the eye would be much larger than a continuous flow orifice would allow.

Thinking more - inkjet printing tech is also used for very fine dispersion of controlled amounts of fluid so that might also be a possibility. A very large filter area could be upstream of the nozzles in the low-pressure feed section.

Quote (OP)

My prototype engine is 49.5cc but, since it has six cylinders and each completes four full cycles per revolution, each cylinder volume is only 1.7cc.

Have I been figuring it wrong all these years?

Bill
--------------------
"Why not the best?"
Jimmy Carter

Quote (Waross)

Have I been figuring it wrong all these years?

I'm not sure. My fundamental cam driven architecture hasn't changed. What have you been figuring?

Since 49.5/6 <> 1.7, or any integer multiple of 1.7, there comes the question as to the exact relation among those three numbers.

3DDave,

The physical cylinder capacity is 1.7cc but there are 6 cylinders and each completes 4 full cyles per revolution. Thus, it moves 40.8 cc of air every revolution. Engines don’t run on air volume, however, but mass, and I operate with a small amount of boost. The total air mass moving through the engine is equivalent to a 49.5cc two-stroke engine at sea level (boost goes up with altitude). I go though all the fine detail only because “50cc” is a regulatory number in many states. My engine is very different, so I settled on air mass at sea level when specifying the prototype hoping that’s how regulators would see it.

Rod

Your engine has a displacement of 10.2cc (1.7 x 6). "Gearing" between the cylinder and the crankshaft should not alter the displacement.

The correct definition of displacement is "how much volume is displaced by all cylinders during the time while one cylinder performs one complete thermodynamic cycle".

Some of you may be familiar with the incorrect displacement normally stated for Wankel engines due to confusion over their unusual "cylinder" to crankshaft motion ratio. eg a 13B rotary (commonly rated as 1.3 litres) is actually a 3.9 litre engine. (No wonder they make so much power)

je suis charlie

Quote (gruntguru)

...Wankel engines....confusion....actually a 3.9 litre engine. (No wonder they make so much power...

...and get such poor fuel economy). [For a "1.3L" engine.]

gruntguru,

Ignoring for the moment that my air pump piston produces some boost which increases the effective intake displacement of the 1.70cc cylinders to 2.06cc...

You are correct that my "four cycles per revolution" is essentially a reduction gear; my piston speeds and port timing support the view it's like an engine producing 1/4 the torque running at 4 times the RPM driving a 4:1 reduction gear. Taking this view, however, would allow me to increase the engine size by a factor of four while still calling it a 49.5cc engine for legal purposes. As it stands, I predict my engine would provide 11% more horsepower and torque than the Honda GXH50 while reducing BSFC by 43% and physical volume (~weight) by 77%. I'm pretty happy with that performance and feel regulators would accept my 50cc size without a fight. If I increase the size by a factor of four, it would provide 329% more power and torque while reducing BSFC by 40% with only a 13% increase in physical volume. That makes my performance appear in-credible and would likely result in a legal fight with regulators.

Rod

P.S. At the risk of leading the thread even further off on a tangent, "A Review of Heavy-Fueled Rotary Engine Combustion Technologies" by the Army Research Lab provides a nice analysis of Wankel deficiencies leading to poor fuel economy/emissions, and Chapter 6.4 of "The Wankel RC Engine" provides a comprehensive analysis of Wankel displacement. My engine design project actually started as an effort to address the issues identified in the Army report that cause the Wankel's poor economy and emissions. I (obviously) ended up abandoning the Wankel altogether.

Quote (RodRico)

Ignoring for the moment that my air pump piston produces some boost which increases the effective intake displacement of the 1.70cc cylinders to 2.06cc...

Not trying to be insulting here but... if a 2.0 liter I4 has a turbocharger that provides 14.7 psi of boost, we don't call it a 4.0 liter engine.

If the swept volume of an engine is exactly 2,000.00 cc, and that same engine has a peak volumetric efficiency of 95%, we don't call it a 1900cc engine.

Displacement is swept volume, period. There's no ambiguity.

Quote (jgKRI)

If a 2.0 liter I4 has a turbocharger that provides 14.7 psi of boost, we don't call it a 4.0 liter engine.

Of course we don't, but we're not regulators. If the law (or a Formula 1 rule) said a car engine could not exceed 2 liters, I doubt regulators would allow such a simple end-around as use of boost (especially if, as in my engine, the boost came from another piston attached to the crank such that one could rightfully view the intake volume of that piston to be part of the intake volume calculation).

Maybe I'm wrong; perhaps the regulatory authority would be fine with a 50cc engine with boost that happened to produce 4x the power of the typical two-stroke in existence when the laws limiting moped displacement and establishing engine classes for emissions requirements were written. I just don't want to fight regulators so I've used a pessimistic definition that produces power and torque on par with what they're used to and am featuring only reduced size, weight, fuel consumption, and emissions... a story that should play well to the CARB and EPA in regards small engines used for mopeds, powered bikes, and small lawn tools.

I'd appreciate it if we could drop the displacement discussion and return to how I might effectively atomize a very small amount of fuel.

Heat will atomize fuel to an individual molecular size.

Isn't it the goal, what is the minimum amount of fuel that can be mixed with air to sustain combustion.
For your small engine you have to scale everything down to that size. Injectors ? You will have some small holes in them. The RC stuff is all a simple carburetor, is that going to work on your engine? Even the small diesels or compression ignition RC engines are carburetor, they don't have a fuel pump.

enginesrus,

My original statement was meant to ask how much evaporated fuel a given volume of air can carry. My intent is to create as rich a mixture as possible then inject it into the cylinder along intake air such that the final mixture is 0.4 x stochiometric. As others have implied in their answers, a fuel spray isn't evaporated, however. I think I'm just looking for the maximum density fuel spray possible.

A carburetor won't work for me. In the current design, the air path is shared by intake and scavenge. With an injector, I can time fuel delivery so none of it makes it out the exhaust ports before they close. I can't imagine how to do that using a carburetor.

The amount of fuel that needs to be injected is so small, it's hard to imagine a classic injector made to tolerances that would be effective. That's why I'm looking to create a dense mix so that the injected volume is larger. I can't make the injected too large, however, or the injector becomes too large.

Rod

Then you need to combine heat for evaporation and something similar to the water fogging machines. Oh and don't forget to add me to the patent for my idea's.

Quote (RodRico)

The amount of fuel that needs to be injected is so small, it's hard to imagine a classic injector made to tolerances that would be effective. That's why I'm looking to create a dense mix so that the injected volume is larger. I can't make the injected too large, however, or the injector becomes too large.

You keep mentioning that intake and scavenge air share an intake path- is there a diagram in one of your other posts that explains what you're saying? I'm not familiar enough with your design to visualize this.

In any event.. I think with the fuel quantities you're talking about here, it's going to be very very difficult to come up with a cycle-metered solution (like a scaled-down traditional fuel injector). As you already know.

I mentioned the RC stuff because that's at least at the same scale; if I were you, I'd be looking at how a mass-flow metered system could be made to work, at least as a starting point to get the thing producing power on a stand.

Developing an entirely new method of fuel injection on top of all the stuff you're already doing would be quite an additional goal... but based on your past posts that probably doesn't scare you much.

jgKRI,

Here's a functional diagram of one cylinder set (of six) in the engine. A pair of opposed pistons share a common cylinder with the upper piston gating the intake ports and the lower piston gating the exhaust ports (uniflow scavenging). A third piston in another cylinder of substantially larger bore pumps air into the intake port. The pump piston's timing is arranged so it is at max volume at the start of the scavenge period. After a bit of time (determined by port flow calculations), the exhaust port closes signaling the end of the scavenge period. The pump piston continues its stroke to complete the intake period, reaching its min volume just as the intake port closes.

From the above, it's clear that an injection system is required to ensure no fuel is passed out the exhaust port. Use of a carburetor would require that scavenge and intake be segregated, and that can't be done without introducing a number of other challenges.

The most complex possible path I'm pursuing is a scaled down version of a classic fuel injector of the unit type; one cam surface driving the piston generating pressure while another controls the pressure bypass valve. This approach requires very fine tolerances and will likely be vulnerable to clogging due to the very small orifice required. In spite of the challenge, I'm currently running CFD on a version of this nozzle scaled down to a .0023" orifice assuming the Swiss Jewel Co. can make the precision parts affordably. Note this injector will have to use pressure multiplying pistons in order to operate with a piston stroke large enough to be feasible within tolerances.

The rich mixture approach (in which a very rich mixture from a common manifold is injected into the cylinder where it mixes with the larger volume of air to yield the final mixture) should be much simpler to construct, but I'm finding it harder to quantify the design criteria.

Rod

P.S. All of this scares me. I just don't let my fear dictate my actions when working with something that's possible but outside the bounds of my current knowledge or skills. I simply learn what's needed and press on.

So your plan is a direct injection scheme using this newly designed injector??

Ultimately I'm not an expert in fuel injector design so I don't really have much to add on that front.

I guess the approach I was envisioning was what I would do in your shoes- which is prioritizing getting the thing running and producing power, as opposed to getting every detail correct on the first try.

That is, of course, just a difference in approach.

jgKRI,

I appreciate and share your sentiment about getting things working with minimum fuss.

My grand plan is to get the engine working with performance sufficient to attract outside investment. This requires the prototype be close to the desired end-state in terms of architecture (functional block diagram).

If there were a way to use a carburetor that didn't differ significantly from the desired end-state, I would gladly go that route. It does me no good, however, to prototype something that isn't representative of the end-state. Since segregation of intake and scavenge air flows adds a lot of complexity, it's not representative, so I'm not comfortable taking that path.

I believe I'm stuck with some form of injection whether it be a classic fuel injector or the alternate approach which I'll call a charge injector. If the volume of air included in the charge of the second approach is similar to the full intake volume, then the charge injector is really just a segregated intake path, and it would be too large. If, however, I could make the injected charge very rich, the volume of the injected charge would allow use of a small injector piston.

I'm only running the CFD on the classic injector with the sapphire 0.0023" orifice because my engine design consultant suggested I do so before rejecting the option. He feels, with the extreme precision assemblies made by Swiss Jewel Co, the remaining assemblies can be built with attainable tolerances. His approach employs pressure multiplying pistons so that the stroke of the injector and pressure relieve valves can be large.

Rod

I guess I still don't understand where this:

Quote (RodRico)

Since segregation of intake and scavenge air flows adds a lot of complexity

Comes from. Intake air is scavenge air and scavenge air is intake air, unless you think that injecting fuel anywhere other than direct injection into the combustion volume somehow means you have to evacuate the cylinder volume twice.

Is your plan to spray fuel directly into the combustion volume, after the exhaust valve is closed, using this jewel orifice injector?

If the pump piston's intake is drawn from a manifold with a carburetor, then fuel will be passed out the exhaust during scavenge. Preventing this would require two pump pistons (one for scavenge and another for intake), and that would add a lot of complexity. Even then, fuel would wet the walls of the passage from the pump piston to the intake port on the opposed pistons resulting in fuel in the scavenge air, so the engine would need separate channels for scavenge and intake. This approach is infeasible given the fact that scavenge and intake have different timing and there's limited real estate for separate pistons and channels to the intake ports.

I do plan to inject directly into the cylinder. This gives the most room for mixing to occur without wetting the walls and ensures there's no fuel on the intake passage walls that would subsequently be passed out the exhaust during the subsequent scavenge cycle. I'm even considering injecting just below the intake ports so injection can occur after the intake port closes. This would have the benefit of recovering the work from pressurizing the fuel (and wouldn't subject the injector tip to harsh exhaust gasses as the intake port doesn't open until the exhaust port is open long enough for pressure to settle near ambient).

Agree entirely with jgKRI. There are a multitude of hurdles to overcome with your design. Put a carburettor on it and get it running. Who cares if the prototype scavenges a bit of fuel out the exhaust? You will be able to recognise and start addressing the REAL issues with your engine. Fuel metering can wait.

je suis charlie

Less cylinders will make your injectors easier to source.
Take a look at the Detroit 51 series. You don't need two pistons to port both intake and exhaust.
These engines were noted for good fuel efficiency and light weight.
One rotary blower has a lot of advantages over six piston pumps.
1.8 L with 50s era mechanical injectors.
Run the numbers on a similar design scaled down to 49 cc.
Link

Bill
--------------------
"Why not the best?"
Jimmy Carter

Bill,

Yes, larger injectors would be easier. I believe I've said as much.

The Detroit diesel isn't even close to what I'm shooting for in terms of performance.

Yes, one rotary blower is simpler. It is not, however, as efficient as pushing air only when it is needed as I do. I've run thermodynamic models on both. Properly timed pistons win hands down.

I'm sure if you fully understood what I'm doing you wouldn't be recommending I look to 1951 for the answers. There is nothing predating the turn of the century that even comes close to what I'm trying to achieve. Granted, I may (or even be likely to) fail, but I'll not be wasting my time building what's already been proven not to meet today's needs.

Rod

Here's a look at the nozzle for a micro fuel injector using Solidworks flow simulation. The colors are mass fraction of fuel; red is 100% fuel while dark blue is 100% air. For this simulation, I used ethanol rather than diesel because the Solidworks liquid library already had a full definition of ethanol characteristics over temperature and its density is close to diesel (at some point, I'll need to build a library entry for diesel).

The nozzle geometry is scaled from that presented in the paper "Enhancement of Atomization of High-Viscous Liquid Jet by Pressure Atomized Nozzle" using sapphire orifices such as those from the Swiss Jewel Co.. The spray pattern developed using Solidworks looks quite similar to that shown in the paper, but is also scaled. It's tilted a bit due to the proximity to the wall formed by the piston face and appears to break up to zero fuel fraction in only 3.3 mm. The pressure is only about 100 psi to deliver 5.3E-5 kg/s fuel and, at this rate, the entire charge raises to stoichiometric in only 3.7 ms.

The micro injector will obviously need a good fuel filter. I think it would simply be a screen (to catch large particles) followed by a comparatively large disk of sapphire with many holes of the same diameter as the orifice (0.06mm). I'll be calling the folks as Swiss Jewel to find out what they would charge to make these assemblies in small quantities. If the price isn't too crazy, I'll then proceed to design the full injector around the nozzle.

Rod,

I don't know all the details about your injector geometry but just looking at the orifice diameter you are trying to achieve (0.06mm) and the fact you might want arrays of these holes it seems you are starting to get down into MEMS territory. MEMS foundries can work with sapphire wafers and can etch very fine features so you might want to start researching MEMS and the deep reactive ion etch (DRIE) process that can create higher aspect ratio holes that I think you might need. Be warned though, this isn't a cheap process, especially with sapphire substrates.

hendersdc,

The nozzle as simulated is comprised of three disks. The lower disk with the cone is off the shelf at https://www.swissjewel.com/product/aperture-wafers... . The middle disk creates the auxillary chamber, and the last disk has two holes. The middle and last disks are similar to those shown at https://www.swissjewel.com/products/orifice-jewels.... For all I know, they make these using a MEMS process and if that's the case, I would hope they could run a few customs on part of a shared wafer. In any case, if they want too much for the three disks, I'll explore using one of the many micro-machining shops. The one at http://www.owensind.com/CNCServices/WireEDM offers suitable preceision using wire EDM, for example. There's really no need for sapphire in the prototype and, given the low pressure required, perhaps not even in a production product.

Rod

I stand corrected.
Thank you for your courteous reply.
Others may not have been as patient nor as polite.
Good luck with your design.

Bill
--------------------
"Why not the best?"
Jimmy Carter

Bill,

Thanks for your best wishes! If you have any spare prayers, wood to knock on, etc. I'd appreciate their use as well; I'm going to need as much luck as anyone has a right to expect if this is to work before I die.

Rod

P.S. I'm actually pretty proud of all the work I put into those air pump pistons. The run on the same cam as the expansion piston of the *next* cylinder in the radial set. This required I coordinate the timing of the expansion stroke on one piston with the scavenge and intake period of the next cylinder. Given the many constraints already imposed on the cams, adding the timing requirement really complicated things. I ultimately wrote some VBA in Excel to iterate the cam design until all constraints were met. In the end, the scavenge and intake charge aren't really presurized much (intake is only at 1.45 bar regardless of altitude), so the air pump isn't creating pressure so much as simply moving air when it's needed. All this means there isn't much work going into moving the scavenge and intake charge. The air pump piston, cylinder, and rings are all Vespel with no lubrication required, by the way.

The better printer inkjet nozzles disperse 10 micron droplets so do your calculation on that number. I am hoping to put together a list of inline four cylinder diesels that are relatively easy to turbo. In that process, I hope to learn what the typical CFM to HP ratio is, what the optimal boost pressure is likely to be, what the practical rpm limit of a diesel engine is and what strategies to use in raising that number.

gmaslin,

Thanks for the pointer regarding droplet size.

The information you seek on diesel turbos is too general IMHO. The "optimal boost pressure" would vary depending on your objective; you might boost only enough to ensure 100% volumetric efficiency or you might boost to the limit of the structural integrity. The typical "CFM to HP ratio" is pretty straightforward once you know the boost pressure and charge temperature after the intercooler; assuming a constant air/fuel ratio (which demands the injectors be cable of the higher flow required with boost applied), it's just a matter of calculating the mass of the air in the cylinder with the boost in place versus that present when no boost is applied. The practical RPM limit is definded by component strength versus reciprocating forces and fuel injector performance. You may be able to find information on fuel injector speed/performance, but it's going to be very hard to determine the structural limit of a given engine without knowledge of the detailed design and materials employed. Modern engines are the product of thousands of hours of engineering, and messing with the design without all the required information and tools is fraught with risk.

Rod

Test Socket Polymers Sheet (MDS-100 or GC-100) can be machined to have 0.05mm holes.

• https://files.engineering.com/getfile.aspx?folder=6fcd974f-0f57-4aae-bc95-0

mighoser,

I’d be surprised if a polymer was strong enough for a traditional injector operating at thousands of psi. Besides the pressure, cavitation erosion is a big deal. Bird makes ruby jewels with 0.001” orifices, and they’re the basis of my current approach.

The polymers might be handy in other ways. Thanks for the info!

Rod

Those are some good points. Single Crystal Ceramics sound like a much better fit. If you have issues with your Vespel consider a PBI grade like CU-60. The surface finish tends to improve with wear which may have added benefits. Good luck!

There was an early comment about ink jet technology and I think that you should spend some time looking at that. They have very precise volumetric and directional control of extremely small volumes of material. The technology is well understood and I bet you could adopt it to your application without have to reinvent the wheel.

JMO but you're making the classic entrepreneur mistake chasing a specific fuel system - biting off way too much. If you look at most research engines you'll see a stripped long-block, the on-engine systems being limited to combustion only. Typically lube, charge, and ignition are fed to the engine via independent test benches off-engine so that they can be quickly and easily adjusted through a much greater range with a finer control than otherwise possible without swapping on-engine components. From what I understand of your system, an oversized carb or (I prefer an) injector block, a VFD driven blower, a bypass (to limit charge flow), and a hydrocarbon measurement setup (ie Horiba etc) should be all that's needed to validate the core combustion. If you're successful in that regard then you'll also be miles ahead with data to choose or develop a properly sized injector vs seriously questionable simulations.

CWB1,

I've worked R&D for 30 years, so I understand what you're saying in regards risk. On the other hand, if the prototype doesn't reflect the actual end-state design, then it's somewhat meaningless; many others have shown how well Homogeneous Charge Compression Ignition works in test rigs. I need to prove I can solve its peculiar problems, and failing to incorporate the design features that accomplish that goal doesn't mean much to me. That being said, I still need to mitigating the injector risk so...

I'm currently working an alternate design using separate pump pistons for intake and scavenge so that the intake pumps can be fed from a shared intake manifold. This should improve the probability of a homogeneous charge, save cost (one injector per engine vs one per cylinder set), and reduce risk by allowing use of the fuel system from a small diesel engine. The new prototype design will be sized for compatibility with the selected fuel system. While this would have resulted in a significantly larger engine than I planned, I found a way to rearrange things that results in tolerable size.

I'm also considering how I might make a version of the engine in which the cylinders are stationary as this would aid instrumentation. A stationary cylinder design will use rotating cams (similar to those of a normal engine) rather than spin the cylinder block within lower complexity stationary cams as planned. I'm not sure how well this is going to work out yet, but it's certainly worth pursuing as it will simplify testing. The cam timing will be the same, so I feel it would be representative, and I can always revert to the rotating cylinder design after the basics are working.

As you can tell, I'm not committed to anything yet. I found a challenge in the fuel system, spent some time pursuing options, am now convinced the "injector per cylinder set" is not a reasonable start position, and have moved on to looking at other approaches.

Thank you for your continued inputs!

Rod

Rod, have you looked at urea injectors used on diesel systems? They are generally pretty low flow.

CWB1 is 100% right. When developing an engine with a very unconventional combustion system, you build a prototype with one cylinder.

je suis charlie

djhurayt,

Thanks for the tip! I never considered urea injectors. I just tried to find one with specifications and failed, however. Even if I could find specifications, they'd be for urea rather than diesel fuel, so I'm not sure they'd be of much use in any case. If I can successfully mechanize my engine with two air pumps (one for intake and one for scavenge), I could just use a carburetor or small engine injector, and I think that's likely the best approach until such time as it's reasonable to tackle the custom micro-injector.

Rod