Fresh Air Engine

[judifer]

Hi Endless Sphere,
I would like to present here my design for a compressed air engine that incorporates regenerative braking. I designed this engine almost 20 years ago. I would eventually like to build a proof of concept prototype. The design is open source, and is explained in the attached pdf. There are many aspects of this project that the Endless Sphere community can help me with. Just to start with, 1/ Complete the 3D cad model and select engineering materials and components, 2/ Model the thermodynamic process of the engine, 3/ Build and program a microprocessor to control the engine. To preempt some, the energy density of compressed air is about 30kwh/ton or 30wh/kg, and I am not bothered that your Tesla battery does a bit better.

[SlowCo]

Welcome but why would you want to invest time and money in such a design and build? There are loads of compressed air engines to be found on the internet.

[spinningmagnets]

I wish you luck with this. A compressed air engine has much in common with steam engines. The more cylinders you have, the smoother an engine will run, however, extra cylinders add complexity and cost.

Although steam engines found their profitable applications in many fields, I found their applications as factory engines to be interesting. If you needed an engine of "X" amount of power, the most cost-effective type was typically a very large single-cylinder engine, and to smooth-out the power pulses, it had a very large and heavy flywheel.

In applications where size was a constraint, you could make the piston "double-acting", so it was pushed one way, and then pushed back the other way. In one rotation of the shaft, you obtained two power-pulses with one cylinder. Most steam locomotives used two double-acting cylinders, so one shaft rotation provided four evenly-spaced power pulses. The Stanley Steamer automobile also used a compact set of two double-acting cylinders, directly connected to the rear axle.

If you plan to use the engine to spin a generator, higher RPM's are helpful for generating higher voltage which has a beneficial effect on the amount of watts generated. For higher RPM's you might try a wider bore with a shorter stroke. The design shown is appropriate for this, but I would suggest prototyping a 3-cylinder radial first, as a faster and cheaper way to iron-out the development.

However, if the job you wish to accomplish can be done at lower RPM's, then a long stroke will get the most work out the available compressed air. If this would be useful, then a good engine layout might be an "opposed piston" using two crankshafts at each end of one cylinder. The two pistons start at the center of the cylinder, and are pushed outwards towards their respective cranks.

Regardless of the engine design, it can help efficiency to draw air from the storage reservoir and pass it through a heating accumulator first. As an example, if your engine is run off of 100-PSI air, the large storage tank might contain a mass of compressed air at 3,000-PSI. A small mass of heated air can provide that same 100-PSI compared to a larger mass of cold air at 100-PSI.

[spinningmagnets]

A few more thoughts. Air-driven cars that are small and light have been made. There are communities for retirees where vehicles similar to a golf cart can be used to travel around the compound to the various facilities, restaurants, and stores. Since they are not driven on city streets, they do not require the normal licensing.

There were several benefits noted. Although they did not run at high speed, they had fairly short range, however....

They could be refilled as quickly as a gasoline car.

There was no fire hazard, compared to lithium batteries.

Batteries might last ten years, but...they are expensive whenever they eventually need to be replaced. The air-car was expected to last many decades.

Although air compressors are typically expected to run on electricity, some of the compressed air can be harvested directly on-site with a wind-driven compressor.

[Chalo]

Compressed air propulsion is inherently a boondoggle because of the huge inefficiencies introduced by adiabatic heating and cooling. You lose something like half of your input energy before the motor even gets to start wasting it directly.

Don't get me wrong-- it's a cool hobby project. Making exciting new stuff is almost always worth it for the maker. It's just not something that has an economically viable application that I can see. Maybe for spark-free industrial environments?

[JackFlorey]

Compressed air propulsion is inherently a boondoggle because of the huge inefficiencies introduced by adiabatic heating and cooling.

On the plus side, in hot climates you get air conditioning for free.

[judifer]

Spinningmagnets, short range golf cart type vehicles are a good use for this engine. Ideally the car will have at least two compressed air storage tanks, one tank at near operating pressure (say 150 psi) and another tank at greatly higher pressure. During use air will be released from the high pressure tank to an expansion tank, this expanded air provides cabin cooling, and the heat from the atmosphere absorbed in this process provides some recovery of the energy lost in the compression phase. In a stationary compressor it would be criminal to not harness the heat generated during the compression phase for hot water heating or some other use.

[neptronix]

Compressed air propulsion is inherently a boondoggle because of the huge inefficiencies introduced by adiabatic heating and cooling. You lose something like half of your input energy before the motor even gets to start wasting it directly.

It's unfortunately higher losses than that.. on both input and output..
The power density of this solution is so poor that it makes batteries look good..

You may as well use a tiny direct drive motor ( with as lowest drag/highest efficiency possible ) and a tiny lipo battery for an ultralight KERS style bike... ideally you only take a 10% loss on the way in and a 10% loss on the way out.. and have very good power density ( this can easily be done with 7-8lbs of battery/motor/etc )

I don't see the point of choosing this system over other alternatives.

[SlowCo]

Spinningmagnets, short range golf cart type vehicles are a good use for this engine. Ideally the car will have at least two compressed air storage tanks, one tank at near operating pressure (say 150 psi) and another tank at greatly higher pressure. During use air will be released from the high pressure tank to an expansion tank, this expanded air provides cabin cooling, and the heat from the atmosphere absorbed in this process provides some recovery of the energy lost in the compression phase. In a stationary compressor it would be criminal to not harness the heat generated during the compression phase for hot water heating or some other use.

But for what actual application/vehicle would you want to build an engine like this? I love radial piston engines but other than being wonderful pieces of mechanical engineering I don't see the use for one in current (pun intended...) times.

[spinningmagnets]

Don't let the obscure nature of compressed air engines make you reluctant to experiment. The Amish are famous for avoiding modern tech, however, that is not exactly correct. They are fiercely independent, and do not wish to become dependent on outside infrastructure, like electrical generation or gasoline/diesel being provided by a far-away company.

In fact, the two ways in which they actually embrace tech is solar-panels providing light after sunset, and using air-tools. They are savvy enough to understand that they can bring-in cash to their community by selling quilts and other crafted items, so they are complicit in "acting Amish" to maintain what tourists want to see. In fact, the LED lights they use are fake candles with a "flicker" feature. Solar panels charge batteries, and the batteries supply lights at night and in the winter.

They do not use wind-generators to charge a battery that powers an air-compressor. They use a wind-driven compressor to fill a large tank. The compressed air in that tank runs a variety of shop-tools.

The turbines that generate electricity use three large slender blades for technical reasons that provide the best efficiency when generating electricity. For pumping water up from an underground well, the "high solidity" turbines are lower-speed and higher torque. They are not the best for generating electricity (much of the wind goes around them instead of through them, ie "Betz Limit"). However, they are perfect for compressing air.

I used to drive an 18-wheel truck, and for a while I operated a pneumatic trailer that used an air-pump to discharge powdered goods out of the trailer, like cement powder, and coal ash. After a certain number of years, the trailers are no longer fit to pass inspection or be driven on public highways. At that point they can be purchased cheaply (at scrap metal prices) and used as an enormous compressed air-tank.

Large expired propane tanks also provide an affordable option for air storage.

[Hillhater]

This is obviously a “hobby” design exercise, with little to no chance of being commercially practical.
So good luck developing and refining your design.
Air drive systems are not only poor on energy efficiency, but also have the same fundamental drawback as electric drives....
...... the size, cost, and weight of the energy storage device.

[zzoing]

That's very good CAD work and good engineering. I'd suggest to channel your time and efforts into technologies which are fairly recent and currently changing and growing in user base, things that you spend a lot of time on and want a fix for, things that you find very inconvenient and you know others want a fix for. I.e. archaologists spend 1000s of hours digging and end up throwing 50,000 year old stratigraphic chemistry information of the dig, design a 3d printer based digger rig that has a spectrometer, and voxel maps archaeology sites to micron precision using the 50 most common elements and builds trillion voxel maps of archaological places. lol.

[judifer]

For further reading I have attached here a Lund University study of the Cargine hybrid compressed air regenerative braking system. The study is of good depth and quality in what it covers, it does not cover the thermodynamic processes completely, and the Frankenstein type lab setup is not pretty. One issue is to find (on ebay) valve actuators to operate at a decent speed, 1000 Revolutions per Minute is 16.66 rev/sec or 0.06 seconds per revolution. Ideally I want the valves to go from fully closed - to fully open - to fully closed in less than half a revolution, ie less than 0.03 seconds, or at 5000 rpm 0.006 sec. I have pulled out an Arduino Mega 2560 that I bought some time back and am looking for actuators for it . Not looking for high speed in the initial experimental setup.