# Computing Cylinder Pressure from Temperature Trace

• tangodirt
In summary, the conversation discusses the process of creating a basic engine model with limited combustion. The model includes isentropic compression and expansion, as well as calculating peak cylinder temperatures and exhaust gas temperature. However, when converting these temperatures to cylinder pressure, there seems to be a discrepancy, possibly due to the use of isentropic equations instead of polytropic ones. The speaker is looking for confirmation and suggestions for improvement.
tangodirt
I'm just trying to put together a very basic engine model with extremely limited combustion. Basically, I am modeling isentropic compression, combustion (using LHV to calculate the delta T), and isentropic expansion.

At the moment, I am calculating a peak cylinder temperature under motoring of ~400 C and a peak cylinder temperature under firing (5 deg BTDC) of ~2800 C. This puts my exhaust gas temperature around ~1200 C, which all seems to make sense. I am using a compression ratio of 8:1 and air standard conditions.

However, when I try to convert these numbers to a cylinder pressure, my ~2800 C point is exploding to ~3700 bar! Obviously, this is not correct. I am using an isentropic expansion equation (P2/P1 = (T2/T1)^(g/g-1)).

Any ideas? Can anyone double check my reasoning and let me know what you get?

Part of the discrepancy might be that in a real engine you don't have isentropic compression or expansion, where PV^k=constant, k being 1.40. Considering heat losses, it's polytropic expansion and compression, so PV^n=constant, where n is about 1.30.

## 1. How do you calculate cylinder pressure from a temperature trace?

The calculation for cylinder pressure involves using the ideal gas law, which states that pressure is equal to the product of the gas constant, temperature, and volume, divided by the number of moles of gas. In this case, the volume is the displacement of the cylinder, and the number of moles can be calculated from the mass of the fuel and air mixture.

## 2. What is the purpose of computing cylinder pressure from a temperature trace?

Computing cylinder pressure from a temperature trace is important for understanding the efficiency and performance of an internal combustion engine. It can also help identify any issues or potential problems with the engine's operation.

## 3. Can cylinder pressure be accurately calculated from a temperature trace?

While there may be some margin of error, cylinder pressure can be accurately calculated from a temperature trace using the ideal gas law and other factors such as fuel properties and engine specifications. However, it is important to note that cylinder pressure can also be affected by other variables, such as engine load and speed.

## 4. What types of data are needed to compute cylinder pressure from a temperature trace?

In addition to the temperature trace, other data that is needed includes the engine displacement, mass of the fuel and air mixture, and the gas constant for the specific fuel being used. Other variables such as engine load and speed may also be necessary for more accurate calculations.

## 5. Are there any limitations to computing cylinder pressure from a temperature trace?

One limitation is that the temperature trace may not accurately reflect the actual temperature within the cylinder due to factors such as heat loss. Additionally, the ideal gas law assumes an ideal gas, which may not always be the case in real-world conditions. Therefore, the calculated cylinder pressure may not be entirely accurate, but it can still provide valuable insights into the engine's performance.

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