Thanks for the replies!
PeterH:
Interesting project.
I'm trying to get my head around the 8-stroke cycle. Are you going to leave the cam alone and actually pull in a fresh charge at the start of the 'steam' power stroke? In this case you'd be throwing away the hot exhaust charge that contains most of the energy you're trying to recover, and the only thing powering your 'steam' phase would be the hot metal. In this case I'm sure that skipping half the power strokes and running water through the cylinder would cool the engine a lot, but I'm sceptical that you'll gain enough power during the steam phase to offset the friction and pumping losses. I suspect you'll end up with a very rough engine with about half the power and a dead loss cooling system.
But it's interesting enough as a concept that I'd like to see how it works in practice.
I suppose that to do this properly you'd need to replace the cam drive system with one that ran at 1/3 crankshaft speed instead of half crankshaft speed, and you'd need to change the cam phase/durations. Getting a custom cam grind isn't technically difficult but needs money and engineering skills. Changing the chainwheel/pulley sizes to change the cam speed is something that is easy in theory but may not be possible in practice, depending on the design of your cam drive. It seems to me that this is the only approach that is likely to work well, though.
How are you implementing the water injection? Are you injecting into the cylinder? What sort of injectors and water supply system are you planning?
What sort of duty cycle are you envisaging for the water injectors? The petrol injectors are probably batch fired, but your water injectors would need to be sequential and accurately phased, so they'll need to be sized to use a much shorter pulse. This may mean you can't simply drive them from the petrol injector signal. You could use the ignition LT pulse to give you a crude engine phase sensor but you still need some logic to decide which part of the engine cycle you're in and switch the fuel and water injection systems on and off.
Needing sequential injection also makes it harder to choose the ECU. There are several aftermarket systems that support sequential injection up to 6 cylinders, but it rules out the experimental systems such as Megasquirt/VSMS. It would be worth doing some research though to find what state MS is up to now - last time I was involved with this they were developing a SMD controller that was planned to support lots of drivers and probably do what you need here.
If you're running this setup in economy mode I guess you'll be around 3000 rpm which corresponds to 20 ms per crank revolution. If you wanted to complete the water injection within say a 45 degree period, that gives you 2.5 ms water injection duration. I wouldn't expect any difficulty achieving that with an Arduino. The pulse wouldn't need to be calibrated accurately as long as you have some scope to dial it up and down a bit to find the best compromise between water consumption and power. You'd also need outputs to disable the corresponding injector, and inputs for crank position (crank trigger wheel?), cam position (#1 spark LT?) and throttle (MAP?) so you could decide when to enable water injection and phase it correctly.
Yes, I'll be leaving the cam alone. The 6 stroke engines do have a regular exhaust stroke, so they lose much of the heat in the exhaust as well. Having the second intake/compression strokes will pull in fresh air and compress it, leaving it pretty warm. I know this isn't as ideal as a specially built engine, but I have a feeling it should work, at least a little bit. At highway speed, a dead miss on one cylinder isn't typically even noticeable on these engines, so hopefully having a little bit of a power stroke from the water will help. I have also considered the possibility of disabling the system on idle and acceleration, but I'll figure that out if I come to it.
Another thought I had was to convert it to electronically controlled hydraulic valves, Fiat is working on those for the purpose of varying the lift and duration of the valves. That would be expensive and complicated, but may be feasible in the near future.
I'm thinking about the same water injector pulse width as the fuel injectors. Most of the 6 cycles I read about used about the same quantity of fuel and water, but as the water injectors will probably flow more, the pulse could be shorter. I will probably use fuel injectors for a direct injection engine as they are designed for high temperature and pressure. They are becoming more and more common in newer production cars. I won't be doing this for a while (but I'll probably start on the code), I just bought this car to have something to drive while I build up the engine in my street/track car and it is a cheap one to experiment on once my new engine is built. I might need to find an additive for lubrication and corrosion protection, I was thinking maybe methanol and the lubricant they add in the engines that burn it. I would need something for freeze protection in colder months (Iowa, USA) anyway, and that should probably work. I would machine a port for the water injectors into the top of the head. I'm an automotive technician and have worked on and modified lots of engines, and I know I can do the mechanical and electrical parts, but I will likely need a great deal of help with the programming part. I will detail the entire process if I end up doing this. This engine is sequentially injected, so I won't have to do anything with the stock ECU. I also have HP Tuners, a PCM tuning system that lets me control almost everything about how the engine runs. I can even set it to not turn on the check engine light for the fuel injector circuit trouble codes that would probably set from opening the power supply to disable the injectors.
I don't think I should need a way to directly measure the crank position. The stock PCM does this already, and fires a coil at "x" degrees before TDC. At higher RPMs they have more advance because the fuel takes time to burn and build pressure, I think the water would take time to vaporize as well. I'm hoping for the water to be injected as soon as the spark occurs for that cylinder. For disabling the injectors, I'll be using a depletion mode MOSFET for each fuel injector. Currently the fuel injectors have one power wire from a relay with a splice under the upper intake manifold that has 6 wires coming out of it, so all I have to do is run individual power to each injector. For enable and disable RPM, I can just measure the time between coil firings. On this engine, 3 coils are used that fire on two companion cylinders at the same time. Companion cylinders reach TDC at the same time, one on exhuast and one on compression, so coil pulses on one coil per minute equals RPM. I don't think I should have to measure the throttle or MAP, because the time the fuel injectors are commanded on is already calculated by the PCM based on these inputs. I'm hoping to just multiply that by a number to get how long to fire the water injector (after measuring the flow rate), and it's possible it could even be the same time.
dc42:
You'll need some additional way of telling whether the spark is for a particular cylinder, perhaps a flip-flop readable by the Arduino that is set by a spark to a particular cylinder and cleared by a spark to the next cylinder.
I think I should be able to tell what cylinder the spark is for by looking at which fuel injector fired most recently.