Prototype Fuel Injection - How Much Fuel Can Air Hold? 4

[RodRico]

Guys,

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. There's only 2.76E-6 kg of air mass in the cylinder and the stoichiometric ratio for diesel is 14.7, so there's only 1.88E-7 kg or 0.0002 cc of fuel required. There's no feasible way to atomize that little fuel... the nozzle orifice and tolerances required by a classic fuel injector design are simply unrealistic.

The obvious solutions are to either build a much larger engine or use a shared intake manifold with a single injector or carburetor. Neither approach is attractive... the larger engine would be costly to build and too big to allow shipment of an engine and test bench to third party evaluators, and the shared intake manifold would require I segregate intake and scavenge air paths which significantly increases complexity.

My current thought is to use a shared manifold to route premixed fuel/air to a simple injector. This is essentially a way to segregate intake and scavenge without adding a lot of complexity. To be feasible, the fuel/air mix must be *very* rich to allow use of a small injector similar to what I had envisioned. I did some research and found the easiest way to make a dense mist with small droplets is using a piezoelectric atomizer, preferably one using a piezo mesh disk as these produce very small droplets. This $20 mesh disk produces 8um droplets at 480cc/hr versus my whole-engine peak requirement of 812 cc/hr, so two such devices should do the job. The output of these devices can be electronically controlled (droplet size is determined by the driving signal's frequency and the density by it's amplitude), so there's no need to control the injector at the cylinder; it can just be a simple piston with fixed stroke that draws in the charge then shoots it at high velocity into the cylinder to further aid mixing. Here's a picture showing how dense the spray from these type devices is...

My question is this: Does anyone have any idea how much fuel a given volume of air can hold?

Thanks for your help!

Rod

 

[mighoser]

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!

 

[Screwman1]

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.

 

[CWB1]

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.

 

[RodRico]

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

 

[djhurayt]

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

 

[gruntguru]

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

je suis charlie

 

[RodRico]

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