Gregory's ServerOne way this could have potential performance advantages (though probably not enough to justify it) is that by strategically positioning the solenoid coil in the cylinder (assuming you're using the piston head as the plunger) you could change the torque profile to get more torque when it's mechanically advantageous, not just top dead center.
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A solenoid coil positioned to yank on the piston at that prime spot after the compressed air has done the first bit could have an interesting bonus effect for the power output of the engine. Another way would be to use the solenoid to move the cylinder head down as the piston descends, basically keeping the good compression ratio until the crank is in an ideal orientation. That idea (sands solenoid) has been proposed for internal combustion engines. Oop, just realized a modified timing on the solenoids (overlap their activation on the top hlaf of the upstroke) would yield better upstroke performance. Why did my brain pick *this* to hyperfixate on today?  So I should probably explain the thinking behind this rather than just clown on myself. This design is a dual-expansion air engine, integrating in a stroke-offset shuttle piston that acts as an intermediary between the high and low pressure volumes. Every stroke is a power stroke, and the solenoids help extend that power stroke to include the half of the upstroke as well. The design uses the thermal inefficiency of solenoids as a method to counteract working gas cooling. Working in concert, the engine would theoretically enjoy better efficiency that either a purely pneumatic or solenoid engine would on its own. That being said, it is likely the engine could run (poorly) off of either power source in a pinch. The electromagnetic nature of the solenoids allows for in-operation tuning of timing and pull, allowing some fine control. In fact, when run in reverse and with different solenoid timings, the engine can work as a fairly high pressure ratio compressor. The adiabatic efficiency wouldn't be great, but the system could theoretically recharge an air reservoir. In terms of operation, high pressure air enters into the first expansion volume, and is heated by the solenoid waste heat as the volume is moved into an optimal crank position by the shuttle piston. Once there, the heated air is allowed to expand, driving the power piston down through bottom dead center. The intake valve switches at this point to direct the high pressure exhaust gasses up into an intermediate chamber, where they are trapped and reheated... ...by the waste heat from the shuttle return solenoid. At the beginning of the power piston downstroke, this reheated air flows into the low pressure expansion volume, helping to push the shuttle piston and power piston down past top dead center. Once the power piston passes bottom dead center, the exhaust valve opens and the expanded air is pushed out of the engine. Solenoids help pull the shuttle and power piston to their top positions to reset the cycle. How useful an engine like this is really depends on the availability of compressed air and electricity. It would likely be a lot quieter and cleaner than an internal combustion engine, but given how affordable and efficient electric motors are, you'd need a strong case not to just use one of those. There's no rare-earth minerals in this engine though, which is a possible selling point in extreme resource constraint scenarios. Ok, looks like I was a bit off on the compressor angle. It wouldn't take running it in reverse, it just requires offsetting the valves (in purple) by 180 in the cycle. If you have a mechanism that can do that, then the engine can switch into "regenerative" mode mid-operation. That's got a bit of potential for vehicles with regenerative braking. Just to elaborate a bit on that possible use case, let's imagine a relatively low speed vehicle that has a hybrid electro-pneumatic drive train. It's overall air storage capacity isn't huge, but it takes the acclerative power demand off of the electric system which is there to recharge the air tank and augment the engine through different power modes. Overall vehicle weight could be lower, and cost could probably be lower too, given less batteries. Because the electric system wouldn't be providing all the torque, you could get away with batteries which are energy dense but aren't as power dense. The vehicle would still be rechargeable like a purely electric system. Range wouldn't probably be fantastic, but could probably be adequate for certain kinds of applications where high torque, start and stop driving, and frequent opportunities to recharge could be possible. A bus or trolley comes to mind.  @skyfaller Feel free to use the concept! Compressed air storage is great if you can spare the space for the tanks and are okay with the quirks pneumatic systems have. |
Expanding gasses lose their "oomph" the more they expand, which isn't great in a piston engine, because that means that the crankshaft is in a terrible position (top dead center) for receiving that oomph when the cylinder is at it's most compressed. It's about 1/3 to 1/2 through its downstroke that you want that big push on the crank.
Think about it like pedaling a bike, you don't want to push on a pedal at the very top. You want it down a bit.