Carlo Caligiuri

I was wondering if there was a way of converting the example model to a GDI engine?
Is it a matter of defining different heat release laws?

Yes; from a 1D model point of view, I agree that basic difference between Diesel and petrol engines is in-cylinder heat release law.

If it is too complicated, is there a way of altering the combustion pattern of the diesel engine
so it is similar to that of a petrol one?

Diesel combustion process is usually modeled by using up to four Wiebe functions (more info at: http://openwam.webs.upv.es/manual/index.php?title=Four_Wiebes_simulation ). However it seems that for typical petrol engines, one Wiebe function is enough. So if I had to convert the example model I would:
– Go to the “Define heat release laws… ” option (when you right-click over the engine yellow box)
– Remove all the laws but one
– Select just “Diffusion” just as shown in the attached file
– Give values for engine speed, air and fuel mass flow (just for the model to run; they won’t be used since no interpolation procedure is calculated since you only have 1 law)
– Make Beta=1, since you only have one law
– Give reasonable values for Wiebe’s coefficients: m, C, combustion duration and combustion start. This can be a good paper to review:
F. Bonatesta, B. Waters, P.J. Shayler, Burn angles and form factors for Wiebe function fits to mass fraction burned curves of a spark ignition engine with variable valve timing, International Journal of Engine Research 11 (2) (2010) pp.177-186

Hope it helps.

Héctor

Last question one may thought before running a simulation would be: “How many cycles should I let the simulation run?”. Well, the simple answer is: “It depends”. In fact, it depends on a lot of things: engine type (turbocharged engine usuially need more time for getting a steady solution (convergency)), engine geometry (the ratio between engine displacement vs. intake and exhaust manifold sizes), flow initial conditions (if they are close to the final solution, the number of cycles needed for a converged solution is lower).
Here you’ll find some hints on how to choose the number of cycles a simulation should run in order to get a good solution (I’m assuming it is an engine steady running condition what you want to run, not an engine transient behavior):

• First time I run a simulation I use 100 cycles. OK, the simulation will take a lot of time but it is only one simulation.[/*:1vssc5ki]
• I always check the averaged air mass flow in the first intake duct and in the last exhaust duct (the comparison is simple: intake-mass + fuel = exhaust-mass). These values can be seen in the MED.DAT file but you have to include them before running the simulation. If the previous equation does not fit, do not blame the code, it means that intake or exhaust manifold flow conditions are not steady yet.[/*:1vssc5ki]
• Another way to see if the solution has converged is to check if averaged pressure values in different engine locations have achieved a steady evolution.[/*:1vssc5ki]
• If the engine is turbocharged, it is good to check and compare the average turbine and compressor work (in the MED file; again be sure to include them before running the simulation). Both works should be very similar (in pure theory they should be equal). If turbine work is greater/lower than compressor work you should detect that the turbo speed is increasing/decreasing. So turbo speed is also a good variable to check if a simulation has converged.[/*:1vssc5ki]
• Finally, you have to decide in which cycle the previous comments are well satisfied and choose it as a good value for the number of cycles. For example, I’ve attatched a plot containing intake and exhaust mass flows and it is observed that 20 cycles is enough for arriving to converge.[/*:1vssc5ki]
• Remember that a number of cycles which is appropriate for a given engine running condition may not be good for another one (mainly when changing engine speed or turbocharger operation).[/*:1vssc5ki]

aala, take a look at this videotutorial. I hope it helps:

[swf:3tqy05gr]http://personales.upv.es/hcliment/openwam/running_simulation.flv[/swf:3tqy05gr]

Cuando se cambia el sentido del flujo en un tubo desde WAMer no se intercambian diámetros inicial y final en la pestaña tramos de los datos del tubo. Si es complicado de implementar (quizá en un tubo con un solo tramo es fácil pero si tiene varios tramos se complicará la cosa) al menos debería de salir un mensaje al usuario indicándole que debería modificar él mismo estos valores.