Are adiabatic/ceramic engine concepts being researched nowadays?

[DaveShoe]

I understand that ceramic "adiabatic" engines were popular research and development topics in the 1980s. The 1979 oil crisis brought the ideas into vogue, and major engine manufacturers around the world were constructing and testing conceptual ceramic engines that required no cooling system. I have read some of the old sae.org technical papers on these experiments, and it seems the downfall of the ceramic engine is ceramic fatigue. At least that is my impression.

I am interested in learning more on the latest in published adiabatic engine technologies. I am having fun developing a concept, posted at

http://www.insulatedpulseengine.com,

and am looking to learn more and move beyond this level of understanding. My present plan is to purchase more recent technical papers from sae.org, but would be interested in finding a forum that discusses the topic. I just realized this eng-tips site is a great resource, and am now checking it out.

Thanks.

 

[DaveShoe]

Hi CCycle,

That makes sense to me, and is something I've pondered when thinking thick vs. thin ceramic.

The issue is energy absorption of the cylinder walls, as they reradiate on cooling, and the higher the temperature change, the higher the reradiation.

I surmise that iron probably reradiates about the same energy as an insulating ceramic for the first instants after the temperature delta. This is sensible, though I've hoped the gradient math might work out in favor of an unusual efficiency somewhere along the way.

Damned molecules. They all seem to behave with similar thermal character when looked at individually.

I'll keep looking into this, with a new scepticism. It sounds like the IPE would work well at less than 10RPM and with pistons larger than 500mm diameter. But so would a non-IPE. Might make prototyping a little bit tougher, but prototyping (for me) is done only on paper, so cost is no object.

Thanks for the fresh insight. The staged gradient factor is a new concept for me, and will certainly help me figure out the limits of this engine a bunch sooner. I know the IPE works, but just don't know the efficiency. I am now seeing it might be a surprise to learn how inefficient it is. This is a fun perspective to recognize.

Thanks,
Shoe.

 

[BrianPetersen]

A few posts back you criticized my hypothetical but relatively conventional 1.0 litre engine as "noisy" and "hot" compared to the engine you are proposing. Perhaps, but it can also be described as "compact" and "lightweight", and with half the cylinders (and each cylinder being smaller) it has less friction and much, much less surface area. The BSFC map of such an engine, if it were done per usual principles, will have a best efficiency point at probably three-quarters of rated torque, and half of rated speed but will be relatively insensitive to the RPM. You compare this to your own engine, which you describe as quiet and cool. BE CAREFUL - Be objective and don't jump to conclusions about unproven concepts. It's fine to use intuition as a design tool, but it's equally important to not be blinded by it.

By the way, in MY world, 60 horsepower from a 1.0 litre engine is extremely under-tuned. In my world, a 1.0 litre displacement engine makes 160-ish horsepower, and that's with OEM-level reliability, excellent driveability, and emissions-legal operation (typical 1000cc sports motorcycle engine). Admittedly, this type of tuning does not give the best BSFC numbers ...

Back to topic, if you are using 20:1 effective compression ratio, the end of compression pressure will be around 66 times the end of intake pressure. It's plausible that the peak pressure could reach 200 bar, depending on the increase in temperature from combustion. To expand 200 bar down to 1 bar adiabatically requires a 44:1 mechanical expansion ratio (assuming ratio of specific heats is 1.4; if you use 1.3 it changes things a bit). If you work backwards from reaching 1 bar at the end of the power stroke, you will find that you should be using a little less than half the intake stroke - not a quarter of it. Obviously this is back-of-notepad calculations here but I hope you get the idea.

BUT, we are still talking idealism here. In reality, pressure drop through the intake port will mean the pressure in the cylinder is less than expected (translation, valve has to stay open longer than the ideal world predicts to let the same amount of air in). The other thing is that there will inherently be back pressure from pushing the exhaust out the port and down the exhaust pipe. This is not necessarily "friction" loss - in fact the pressure will most likely be dominated by pressure wave effects. And, due to inherent mechanical friction, it's not worth utilizing the last little bit of expansion (the extra friction and losses from having to make the engine bigger to utilize it outweigh the possible gain). Translation, the target pressure at end of power stroke should probably be more like 1.2 - 1.5 bar (guesstimate) not 1 bar. Working backwards, that means you can let more air into the engine on the intake stroke (in the interest of downsizing the engine and reducing friction and heat-transfer area).

 

[SomptingGuy]

Shoe,

Do some web searching for "isoengine". This project pushes the boundaries somewhat and exsts in real hardware.

- Steve

 

[DaveShoe]

Thank you.