Opposed Piston Diesel Engine

Half-baked idea.

I've been looking at Opposed-piston_engines and I'm interested in them, particularly the M4+2. I really like this one and this too, oh and this one. This is my try at designing one.

A different approach: http://www.gizmag.com/duke-engines-axial/33631/ or this: https://www.youtube.com/watch?v=aZB0Go3Kj-k

Goals

The goal of the engine is to simplify design. With a focus on fewer parts and easier to fix. Because of CNC manufacturing, the solid parts can be complex and precise, so long as replacing them is simple.

Pistons

The pistons are opposed in a single chamber. To be lighter, the pistons should sort of cup shaped.

The pistons have 3 rings, the outer two consisting of permanent magnets with opposing poles. I'll probably make the core out of steel to help with the magnetic field. This design allow for acceleration powering the windings like this:

 ---NNSS
 --NNSS-
 -NNSS--
 NNSS---

I'd like to see if the pistons can be machined with enough precision to not need rings. A v2 improvement would be looking at ways to reduce friction with something like fluid bearings.

Cylinder

The outside of the cylinder will be steel pipe. I'll make an inner core of wound enable copper wire, with several stages (I'd guess 6). The inner layer will be smoothed with high temperature epoxy. I think the best approach is to make a wax cylinder the exact size of the piston, and then melt it out after the epoxy has set.

Valves and Air Flow

There are 4 air states; closed (CL) high pressure (HP), atmosphere out (AO) and atmosphere in (AI). High pressure goes to air tank, though I need to look at this because the exhaust may be too dirty to use directly. The two options are to either filter it or use it drive a secondary pump that won't be sensitive to the particulate. I'm leaning towards the filter though, because it should be easier and it would make this a bit cleaner to operate.

There are two valves, one inside (I) the pistons, with the two outer (O) sections connected together to a single valve. The outer valves are located just past the last winding, but a gap is left beyond that with will turn into a high pressure spring if the pistons over shoot.

Atmosphere In is separate for I and O. For I, fuel is fed at a constant rate (constant with result to engine timing, but adjustable overall).

Combustion Operation

Intake

I:AI/O:AO

The windings are used to drive the pistons apart, drawing and air fuel mixture from atmosphere in.

Compression

I:CL/O:HP

The windings plus the high pressure air is used to drive the pistons together. This initiates diesel combustion.

Combustion

I:CL/O:HP

The windings are used to gather generated energy. I need to consider how this is wired. By leaving O connected to HP, we recover the pressure loss from driving the pistons and provide an air brake to prevent the pistons from slamming into the ends of the cylinder.

Exhaust

I:AO/O:CL

The outside of the piston will be at high pressure, so the windings can be used to collect energy at this stage.

Cooling?

I:HP/O:AO

This may not be necessary, but a cooling stage may help efficiency. By using I:HP, we should get a lot of cooling from the pressure drop. Also, we can use this to get two more power strokes.

Compression Ratio

This design provides a lot of control over the pressure ratio. How far the pistons are separated during intake, how much energy is put into the driving coils.

Compressor

The engine can be used as an air compressor by using the windings and disabling the fuel injector.

Inner Compression

I:HP/O:AI

Outer Compression

I:AI/O:HP

Generator

This mode uses the compressed air to generate electricity

Stroke 1

I:HP/O:AO

Stroke 2

I:AO/O:HP

Electronic Control

I'd really like it if this could be controlled by an AT-Tiny-85, 6 IO pins. But I don't think that's possible because of the complexity of the winding control.

Mode Select

I'd use one IO pin, with the ADC to choose the mode. I need to map this out, but the idea is that it's hooked up the battery it's charging and a pressure sensor and it will feed back it's mode so that if the battery is low and the tank is full, it will run as a generator, if the tank is low and the battery is full, it will run as a compressor, if both are high, it will be off and if both are low it will run as an ICE.

Compression Ratio

Another ADC will be used to set the compression ratio. It looks like diesel engines typically have compression ratios in the range of 14:1 - 24:1, but I'd like to extend this range just to be safe (and I'd like it to work with non-conventional fuels), so probably anywhere from 10:1 - 30:1, maybe even a little higher.

I'd also like a way to set this to auto find the compression ratio. Which it should be able to do using the windings as sensor and inspecting the current draw.

Valve Control

Charlie-plex the valve solenoids

States:

1. I:AI/O:AO
2. I:AI/O:HP
3. I:AO/O:HP
4. I:AO/O:CL
5. I:HP/O:AI
6. I:HP/O:AO
7. I:CL/O:HP

I'll need to pin this out, but I'm sure I can do it with 4, I may be able to do it with 3.

Winding Control

This actually gets pretty complex. Each winding has 3 modes; driving, powering and sensing.

The power stroke is very brief so I need to get as much energy out of it as possible. I think each coil needs it's own bridge rectifier, with the DC side of all of them wired in series. Then I think I need something like a charge pump, but I'm not sure. Fortunately power mode is all or nothing, so I can just disconnect the windings from the control circuit when that happens.

The driving circuit needs to be adjustable, because different driving operations will require different amounts of power. I also need to deal with discharge and rapidly switching polarization. I'll probably have to look at resonant capacitors. Good excuse to look at Gauss guns.

Sensing will be used during compression to get the position of the pistons. I'll have to experiment with this because I think there will be a current induced from the neighboring coil, but I need to detect when the piston is entering the coil.

I think the best design to cope with this is to use a single PWM pin to set the power level and then assign each wire to a pin. I'm not sure how make a single pin work for sensing and driving.