CNG gas temperature right after it exits the DI nozzle

[Azmio]

Hi everyone,

I need some information on the items below: -
1) Compressed CNG from a high pressure tank at 200 bar, 25 degree C being injected into an engine cylinder through a DI injector. No regulator to bring down the feed pressure. What will be the temperature of the methane gas right after it gets out of the DI nozzle
2) What will be the gas pressure during this time?

The complication that I have is that the gas expands a lot as it goes out from the high pressure line to the cylinder. However, there is also some heat transfer from the tip of the nozzle which can go up as high as 250 degree C. Will the methane gas drops below 0 degree C?

 

[Zapster]

Depending on the design of the injector it might be a constant entropy process. In contrast if the injector design drops the pressure with significant friction, it would be a constant enthalpy process. It really depends on the design of the injector. The process will be bracketed somewhere between the two extremes. Go to

http://webbook.nist.gov/chemistry/fluid/and

look up the properties of methane for your conditions. Note that methane will be close enough. Most commercial natural gas is between 90 to 98 percent methane.

For example throttling valves are typically considered to have a constant enthalpy pressure drop across them. An example of this is found in a car’s air conditioning system and is called an expansion valve.

An example of a constant entropy designed valve would be a properly engineered pressure safety valve. These valves are designed such that they approach an isentropic process.

With the above information, you should be able to answer your own question. I do not have detailed knowledge of a DI injector; however, I would guess that it might approach a constant entropy design. If you carefully look at the injector’s design, it should have a diverging nozzle to expand the gas if creates an isentropic drop.

 

[Bribyk]

Finding the gas temp after expanding is easy (as per Zapster above) but the heat transfer from the tip will make it difficult to find your initial temperature before expansion. Because this is DI, I assume your injection duration will be short (a low percentage of 720 deg crank angle) so the gas in the tip will have significant time to absorb heat. You'll also have to consider the volume of gas in the tip vs the amount injected and the surface area available to transfer heat. Running at or near stoichiometric AFR, your fuel mass is only around 5-6% of the charge so the gas temperature will have a minimal effect on combustion chamber temps.

 

[Azmio]

Zapster,

I have the property table of Methane but I couldnt figure out on how to calculate the temperature change as it exits the nozzle.

 

[Bribyk]

You need a table that shows temperature (T), pressure (P) and enthalpy or entropy. Find your enthalpy/entropy at State 1 using a known temp, T1, and pressure, P1. Then, you have to know either temperature or pressure at State 2. Find the same enthalpy/entropy value at T2 or P2 to find the other value. You will find the temperature of the gas drops about 1 degC (or 2 degF) for every bar of pressure drop (~15 psi). Example:

P1 = 30 psig, T1 = -40 degC, then h1 = 12.285.
if P2 = 0 psig and isenthalpic process (h1 = h2), then T2 = -41.5 degC.

 

[amorrison]

A Pressure-Enthalpy graph and data for methane is available in
ASHRAE Handbook, Fundamentals Volume - available in any engineering library.

ASHRAE = American Society of Heating, Refrigerating and Air-Conditioning Engineers

 

[Zapster]

You can generate a graph or tabulated data from the link I provided if you drop the “and” off the end of the link.