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Why the unburnt hydrocarbon emission is zero in Port fuel SI engine simulation using Ansys Forte ?

    • sridharsahoo366
      Subscriber

      Recently, I was simulating the tutorial file "SIEngine_PortInjected_AMG_Tutorial.ftsim". After the simulation completed I got some results. I have some basic questions that make me doubt the solver, maybe I am wrong.

      The questions are as follows:

      1. 100% combustion efficiency. ( On which basis combustion efficiency is calculated? ) I calculated fuel mass multiplied by LHV which is greater than the chemical heat release rate. How is that possible? This seems like an ideal case.
      2. UHC is almost zero. How is this possible?
      3. Fuel Mass per Engine Cycle is showing  2.07651E-002 [g], whereas in the excel sheet iC8H18 concentration is 2.18E-002 gram before combustion. Why there is a deviation?
      4. Fuel Lower Heating Value per g  44.00000 [kJ]. This LHV value remains the same if I change the combustion from gasoline to CNG also. I guess it may affect results also.


      Please answer these questions.

    • RK
      Ansys Employee
      Hello 1) 3) and 4): Believe it or not, it is not very straightforward to let the software detect how much fuel is trapped inside the cylinder in a fully automatic way, mainly because there are many different use cases in which fuel is introduced into the domain and simulation is set up. The fuel mass reported in the summary data is the hydrocarbon species that comes into the computational domain through open boundaries. It could be different from the actual amount of fuel trapped in the cylinder. The mass reported in the table is 2.07651E-2 g. While if we check the ic8h18 mass in speciesmass.csv after IVC but before combustion, the mass is 2.18E-2 g. This is exactly what you observed. Also, it is not straightforward to obtain the heating value for each fuel (hydrocarbon) species automatically unless the fuel is injected through spray. This is because the gas phase chemistry mechanism does not contain LHV inputs for each hydrocarbon species. This is why 44 kJ/g is used as approximation. The LHV of isooctane is 44.3 kJ/g, which is higher than the assumed 44 kJ/g. These two factors make the accumulated heat release rate higher than the heating value derived based on the table. In summary, the accumulated chem heat release rate is correct, but the fuel mass and LHV are approximated. We added several "disclaimers" in FORTE.log above the engine performance data table, but we are working on a different way for reporting these data such that it can be more flexible and can cover more use cases more accurately.
      2) The fuel/air mixing is simplified in this setup - fuel and air are perfected mixed when coming into the domain through the inlet. This is why the combustion is complete and leaves little unburned HC. If port fuel injection is modeled and if the engine geometry contains crevice volume and other fine features, such as spark plug recess etc., there will be unburned HC emissions.
    • sridharsahoo366
      Subscriber
      Thank you.
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