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 Hassaneen August 15, 2000 11:22

compression stroke in piston engine

Dear all, The pressure values I'm getting from STAR-CD at the top dead center in the compression stroke of piston engine is almost one half of the values calculated from the isentropic compression process, does any body know why. The temperature values seem to be O.K. Thanks

 John C. Chien August 15, 2000 16:20

Re: compression stroke in piston engine

(1). Is this true only at the TDC? (2). Or at any instant, from the begining of the compression stroke? (3).Compute your isentropic pressure from the begining of the compression to the TDC position. And check against the pressure computed from the code at different position of the piston. (4). Make sure that both use the same initial conditions, same dimensions, same units, etc. (and also same definitions of the pressure). (5). It is likely that, the initial conditions are not the same, the real dimensions are not the same, or the definition of the pressure are not the same. And hope you are using the same units. (6). If everything's checked out all right, then find out whether there is a tutorial sample which you can get the right pressure at the TDC position. good luck.

 Hassaneen August 16, 2000 04:07

Re: compression stroke in piston engine

The units are fine. The pressure values are half the calculated values all the way from the BDC to TDC. The only thing is the initial condition: I'm putting the initial pressure = 0 Pa. besause the reference pressure is 1e+5 Pa, is that true or not. It seems to be true because the density calculated by STAR-CD at BDC is correct for AIR with 0 initial pressure and 293 K for Temperature. Ahmed

 J. Y. Luo August 16, 2000 05:30

Re: compression stroke in piston engine

I don't know how you get the pressure from the calculated results. The pressure data in the post file is relative to the reference pressure. Try to get absolute one by using PROSTAR command getc, p, abso.

By the way, STAR-CD has been extensively tested (and validated) in the real engine applications for many years - I think the pressure is the one global variable which can be simulated relatively accurately.

 Hassaneen August 16, 2000 05:47

Re: compression stroke in piston engine

I'm not saying that something wrong with STAR-CD. One of the settings is wrong so that I'm just trying to consult the others who meight face the same problem (don't take it personally to STAR-CD). The calculation is based on the very simple isentropic process.

 John C. Chien August 16, 2000 09:05

Re: compression stroke in piston engine

(1). For beginner to use several black boxes blindly can easily hit a sink hole, even if it is at the ground level. (2). So, it is likely that "this pressure is not that pressure". Remember the famous NASA's Martian Probe incident? "this unit is not that unit" This is a common problem.

 Hassaneen August 16, 2000 09:13

Re: compression stroke in piston engine

It is not your exact problem (the tutorial) but if you understand how they create the events file, you meight create your own !!!. Did I get your point???

 Hassaneen August 16, 2000 09:21

Re: compression stroke in piston engine

Sorry,the previous message is not for you John. Your message is: do you realy understand thermodynamics?? this is my final word.

 John C. Chien August 16, 2000 09:28

Re: compression stroke in piston engine

(1). Many users have hit this "pressure "sink hole frequently, not just the code you are using, but many codes based on the similar approach. (2). If the user is familiar with the method, inside and out, then these method-related pressures are not a big issue. (3). In other words, the code interface is not suitable for the beginners who are not familiar with the methods used.

 John C. Chien August 16, 2000 09:37

Re: compression stroke in piston engine

(1). I am just trying to give you a real example. (2). That is why to drive a car ,one needs a driver's license. (both the written test and the road test are required).

 Hassaneen August 16, 2000 09:48

Re: compression stroke in piston engine

Good lecture

 Hassaneen August 17, 2000 04:47

Re: compression stroke in piston engine

The problem was the cells number, it was very small. With increasing it to almost 10000 cells the calculation of the pressure is now reasonable.

 b.saravanan December 29, 2000 13:30

Re: compression stroke in piston engine

sorry friend ,

i am not aware to this side. so i can't help you.

but i am very interest to read this.

thankyou.

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