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

 

[BrianPetersen]

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.

 

[SwinnyGG]

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.

 

[BrianPetersen]

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.

 

[SwinnyGG]

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

 

[GregLocock]

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

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

 

[BrianPetersen]

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 ...

 

[RodRico]

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.).

 

[3DDave]

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.

 

[waross]

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

 

[RodRico]

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?

 

[3DDave]

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.

 

[RodRico]

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

 

[gruntguru]

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

 

[VEBill]

...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.]

 

[RodRico]

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.

 

[SwinnyGG]

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.

 

[RodRico]

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.

 

[Compositepro]

Heat will atomize fuel to an individual molecular size.

 

[enginesrus]

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.