Quote:
Originally Posted by JRMichler
The fundamental problem with any compressed air vehicle is the inefficiency. A one hp industrial air motor needs about 4 hp of air compressor to run it. That's because compressing air makes it hot, and expanding air in a motor makes it cold.
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That's part of it. The amount of energy it is possible to store is the other. My previous post assumed
very ideal "conditions" as a simplified feasibility calculation and even that shows that there's not a lot of energy to had from a reasonable tank volume.
Quote:
Originally Posted by RustyLugNut
Brake Mean Effective Pressure (BMEP). With some manipulation you can calculate an approximate power output for a constant pressure above your pistons.
Here is a good discussion that uses algebraic manipulation rather than the standard calculus based derivation.
Brake Mean Effective Pressure (BMEP): The Performance Yardstick |
The Wikipaedia page on Adiabatic Processes shows a worked example of a compression process that uses very similar conditions to what is being proposed; 362psi, 10:1 CR, 1 liter swept volume. Being adiabatic, it's reversible, so using 362 (~400psi, even 500psi) will only allow a similar power output to that required to compress the air in the 4-stroke cycle. That's much less than is released after combustion.
A compressed air car might use regeneration during braking, which will extend the range a bit. It helps a lot to use a small engine (at least on paper), whether it's air powered or supercharged using compressed air.
Exploring the supercharging idea further. If you converted half the cylinders into an air compressor (and maybe that should be every second cylinder, rather than the front or rear 3 - have to think about that), maybe only compressing at light engine load or on decel., you could then work the other 3 harder and reduce light load pumping losses. The compressed air is then available for supercharging when higher power is required.
The compressed air could be cooled to near ambient i.e. near perfect intercooling.