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Originally Posted by stovie
Ok using the bmep method I only need 68psi to equal the same torque my engine has now.
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That is mean (average) pressure. You might bleed your air into the cylinder to maintain the pressure constant as the piston descends and that would be (ideally at least) a valid condition, as distinct from a simplification.
You might consider though, that a constant pressure expansion extracts less of the energy from the gas than does adding all the gas you are going to expand (with the same internal energy and at a higher pressure) at TDC.
I would encourage you to have a play with some Pressure:Volume (PV) plots to see the effects of different ways of doing things. You don't need calculus, just basic high school math, to determine the areas if you plot on graph paper and count the squares between the boundary lines. The units matter less for the insight than the do the relative magnitudes. Maybe buy or borrow an introductory text on thermodynamics. It's all been done before, in various forms.
There is still the fundamental problem of there not being much energy contained in the stored air. The standard heating value of just one kg (~1.3 liters) of gasoline is 43MJ. That compares with the 420kJ available (assuming that is somewhat close) in the 100 liters of air at 450psi.
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I was also thinking of using liquid nitrogen under pressure as the compressed air part. From what I've read liquid nitrogen will expand from one cubic mm to one cubic meter, there's suppose to be 1000 cubic mm to a liter of liquid nitrogen so I believe that means it has 27,000cf and I found one liter of it for $2 the only other problems I can think of at that point is the pressure, the temperature it will be at and the cost of tanks that can withstand that pressure!any more ideas???
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CO2 might be easier, with lower storage pressure and the ability to be stored at ambient temperatures. Maybe safer and cheaper also. It's quite common to use CO2 to power rams in Robot Wars so maybe you'll find some info. around them.
CO2 has virtually identical thermodynamic properties (at least at the temperatures and pressures likely with this) to N2O, so readily available N2O system parts might be used with CO2.
With a phase change working fluid, like N2 or CO2, there has to be heat available from somewhere to allow the phase change. There are charts around on the web for both materials that will give you the pressures, temperatures and energy required for the phase change across a particular temperature and pressure change. To start:
Psychrometrics - Wikipedia, the free encyclopedia
Temperature
Enthalpy
With what you are looking to do, it might be (theoretically) possible to use the heat in the coolant from the cylinders that are operating as an ICE. You'll need some idea of the power you want to generate (= the mass flow of N2 or CO2 you want to change phase) to assess that.
Multiply the specific heat of vaporization by the mass of N2 (or CO2) you can store and that's you maximum available stored energy (to be extracted from the environment). without even calculating it, I expect it's still much less than the energy in gasoline.
Given an ICE cylinder can be modelled approximately as adiabatic (no heat transfer), I think you may have difficulties in getting sufficient heat to transfer fast enough into your working fluid to generate a useful amount of power.
That's why combustion engines are
internal combustion engines. It's not possible to transfer heat fast enough into and out of the working gas to produce useful power so we use combustion to heat the gas, then mass transfer, exchanging the hot working gas for cold, then start again.
I suspect there may also be issues with trying to operate part of the engine very cold and part of it hot (differences in thermal expansion/contraction).
Using the compressed air for supercharging a downsized engine looks better
.