Plot inside a "for" loop

Hello. I am trying to write a program that generates the pressure-volume diagram for a vehicle engine. I came up with this idea:

nn=length(0:10:720);
P=zeros(nn,1);
V=zeros(nn,1);
for alfa=0:10:720
    cont=alfa/10+1;
    cos_alfa=cosd(alfa);
    x(cont)=76.8*((1-cos_alfa)/2+0.28/4-(0.28/4)*cos_alfa^2);
    if alfa<=180
        P(cont)=0.9;
    elseif alfa>180 & alfa<=360
        P(cont)=320.612/((x(cont)+8.93023)^1.32);
    elseif alfa>360 & alfa<=540
        P(cont)=1119.9/((x(cont)+8.93023)^1.24);
    elseif alfa>540
        P(cont)=1.1;
    end
    V(cont)= 0.0448882+x(cont)*pi*(80^2)/4*(10^(-6));
end
fprintf('\n For alfa %g [degrees], cos_alfa is %g, x is %g, P is %g [bar], V is %g [l] \n',alfa,cos_alfa,x,P,V);
figure(1)
plot(V,P,'-r'),title('Diagram P-V'),xlabel('Volume [l]'),ylabel('Pressure [bar]'),grid;
fprintf('\n')

But something in not quite right. The values that are generated by the program are incorrectly calculated. I cannot figure it out. Please help me. I also made an excel table and these are the correct values:

#   α  Cos(α)           x          P[bar]  V[l]
#   0  1          0          0,9  0,044888245
#   10  0,984807753  0,74548852  0,9  0,048635479
#   20  0,939692621  2,944675899  0,9  0,0596898
#   30  0,866025404  6,488624495  0,9  0,077503629
#   40  0,766044443  11,20512708  0,9  0,101211357
#   50  0,64278761  16,87172209  0,9  0,12969477
#   60  0,5          23,232          0,9  0,161665014
#   70  0,342020143  30,01355396  0,9  0,195752822
#   80  0,173648178  36,94580374  0,9  0,23059811
#   90  6,12574E-17  43,776          0,9  0,264930421
#   100  -0,173648178  50,28198379  0,9  0,297633062
#   110  -0,342020143  56,28070097  0,9  0,327785904
#   120  -0,5          61,632          0,9  0,354684466
#   130  -0,64278761  66,23781051  0,9  0,377835795
#   140  -0,766044443  70,03734031  0,9  0,396934315
#   150  -0,866025404  72,99937551  0,9  0,411823128
#   160  -0,939692621  75,11306918  0,9  0,422447711
#   170  -0,984807753  76,37872395  0,9  0,428809586
#   180  -1          76,8          0,9  0,43092715
#   190  -0,984807753  76,37872395  0,905871259  0,428809586
#   200  -0,939692621  75,11306918  0,923921965  0,422447711
#   210  -0,866025404  72,99937551  0,955514834  0,411823128
#   220  -0,766044443  70,03734031  1,003106372  0,396934315
#   230  -0,64278761  66,23781051  1,070571086  0,377835795
#   240  -0,5          61,632          1,163761373  0,354684466
#   250  -0,342020143  56,28070097  1,291446563  0,327785904
#   260  -0,173648178  50,28198379  1,466886302  0,297633062
#   270  -1,83772E-16  43,776  1,710494123  0,264930421
#   280  0,173648178  36,94580374  2,054404795  0,23059811
#   290  0,342020143  30,01355396  2,550357131  0,195752822
#   300  0,5  23,232  3,283087245  0,161665014
#   310  0,64278761  16,87172209  4,391348961  0,12969477
#   320  0,766044443  11,20512708  6,091903227  0,101211357
#   330  0,866025404  6,488624495  8,664639763  0,077503629
#   340  0,939692621  2,944675899  12,23115402  0,0596898
#   350  0,984807753  0,74548852  16,02792799  0,048635479
#   360  1  0  17,81724439  0,044888245
#   361  0,999847695  0,007485963  74,07242254  0,044925873
#   362  0,999390827  0,029940075  73,84231701  0,04503874
#   364  0,99756405  0,119699901  72,93523792  0,045489922
#   365  0,996194698  0,186960353  72,2686271  0,04582801
#   366  0,994521895  0,269098467  71,46935741  0,046240881
#   368  0,990268069  0,477834722  69,50837079  0,047290104
#   370  0,984807753  0,74548852  67,13210745  0,048635479
#   380  0,939692621  2,944675899  52,07581649  0,0596898
#   390  0,866025404  6,488624495  37,66976556  0,077503629
#   400  0,766044443  11,20512708  27,05627337  0,101211357
#   410  0,64278761  16,87172209  19,8942875  0,12969477
#   420  0,5  23,232  15,13799475  0,161665014
#   430  0,342020143  30,01355396  11,94082367  0,195752822
#   440  0,173648178  36,94580374  9,745657003  0,23059811
#   450  3,06287E-16  43,776  8,204814666  0,264930421
#   460  -0,173648178  50,28198379  7,102113193  0,297633062
#   470  -0,342020143  56,28070097  6,301157072  0,327785904
#   480  -0,5  61,632  5,714101556  0,354684466
#   490  -0,64278761  66,23781051  5,283192082  0,377835795
#   500  -0,766044443  70,03734031  4,969825313  0,396934315
#   510  -0,866025404  72,99937551  4,748002461  0,411823128
#   520  -0,939692621  75,11306918  4,600380691  0,422447711
#   530  -0,984807753  76,37872395  4,515899646  0,428809586
#   540  -1  76,8  4,488399041  0,43092715
#   550  -0,984807753  76,37872395  1,1  0,428809586
#   560  -0,939692621  75,11306918  1,1  0,422447711
#   570  -0,866025404  72,99937551  1,1  0,411823128
#   580  -0,766044443  70,03734031  1,1  0,396934315
#   590  -0,64278761  66,23781051  1,1  0,377835795
#   600  -0,5  61,632  1,1  0,354684466
#   610  -0,342020143  56,28070097  1,1  0,327785904
#   620  -0,173648178  50,28198379  1,1  0,297633062
#   630  -4,28802E-16  43,776  1,1  0,264930421
#   640  0,173648178  36,94580374  1,1  0,23059811
#   650  0,342020143  30,01355396  1,1  0,195752822
#   660  0,5  23,232  1,1  0,161665014
#   670  0,64278761  16,87172209  1,1  0,12969477
#   680  0,766044443  11,20512708  1,1  0,101211357
#   690  0,866025404  6,488624495  1,1  0,077503629
#   700  0,939692621  2,944675899  1,1  0,0596898
#   710  0,984807753  0,74548852  1,1  0,048635479
#   720  1          0          1,1  0,044888245

[Cristian]

Please help! I have no idea what I did wrong!

[Ahmed A. Selman]

As a start, replace (&) in

elseif alfa>180 & alfa<=360

and similar lines, with (&&) as

      elseif ( (alfa>180) && (alfa<=360) )

[Cristian]

Ok, thanks, but it doesn't change anything, any other ideas please? All suggestions are welcome.

nn=length(0:10:720);
P=zeros(nn,1);
V=zeros(nn,1);
for alfa=0:10:720
    cont=alfa/10+1;
    cos_alfa=cosd(alfa);
    x(cont)=76.8*((1-cos_alfa)/2+0.28/4-(0.28/4)*cos_alfa^2);
    if (alfa<=180)
        P(cont)=0.9;
    elseif ((alfa>180) && (alfa<=360))
        P(cont)=320.612/((x(cont)+8.93023)^1.32);
    elseif ((alfa>360) && (alfa<=540))
        P(cont)=1119.9/((x(cont)+8.93023)^1.24);
    elseif (alfa>540)
        P(cont)=1.1;
    end
    V(cont)= 0.0448882+x(cont)*pi*(80^2)/4*(10^(-6));
end
fprintf('\n For alfa %g [degrees], cos_alfa is %g, x is %g, P is %g [bar], V is %g [l] \n',alfa,cos_alfa,x,P,V);
figure(1)
plot(V,P,'-r'),title('Diagram P-V'),xlabel('Volume [l]'),ylabel('Pressure [bar]'),grid;
fprintf('\n')

[the cyclist]

This is a lot of code and output to look at. Can you be much more specific about what is wrong? Can you post the smallest possible example that illustrates the problem?

[Ahmed A. Selman]

So interesting.

I've ran the code you posted, and got some nice figure. Then I've copied your data into a text file, imported it to Excel, re-imported it to Matlab, and draw P vs. V, and got the very same, very nice, figure. SO where is the problem.. :)

If you think there is some problem with the code, then re-check your math, and your excel data as well.. please.

Answer 1

[bym]

Please see comments in code. I checked a couple of values and they agree with your excel table, but it is up to you to verify that it is indeed providing the correct calculations

clc;clear,close all

nn=length(0:10:720);
P=zeros(nn,1);
V=zeros(nn,1);
for alfa=0:10:720
    cont=alfa/10+1;
    cos_alfa=cosd(alfa);
    x(cont)=76.8*((1-cos_alfa)/2+0.28/4-(0.28/4)*cos_alfa^2);
    if (alfa<=180)
        P(cont)=0.9;
    elseif (alfa<=360)   % you do not need the && statement, if it greater than 180 don't test for it
        P(cont)=320.612/((x(cont)+8.93023)^1.32);
    elseif (alfa<=540)
        P(cont)=1119.9/((x(cont)+8.93023)^1.24);
    else                       % last statement is an 'else' since other options are exhausted
        P(cont)=1.1;
    end
    V(cont)= 0.0448882+x(cont)*pi*(80^2)/4*(10^(-6));
end
%fprintf('\n For alfa %g [degrees], cos_alfa is %g, x is %g, P is %g [bar], V is %g [l] \n',alfa,cos_alfa,x,P,V);
figure(1)
plot(V,P,'-r'),title('Diagram P-V'),xlabel('Volume [l]'),ylabel('Pressure [bar]'),grid;
fprintf('\n')
fprintf('x\t\t\tP[bar]\t\t\tV[l]\n')         %table output might be clearer
fprintf('%-6.4f\t\t\t%-6.4f\t\t\t%-6.4f\n',[x' P V]')

Answer 2

[Sysenso Systems]

You program generates P and V values for the alfa=0:10:720. That has an increment of 10. But the Excel table has the expected values for alfa=[0:10:360 361 362 364 365 366 368 370:10:720].

Here are the two solutions: 1.If you compare the generated data with the Excel file by ignoring the rows for alfa = 361 362 364 365 366 368, then everything works fine!

2. Modify the code to work for alfa = 361 362 364 365 366 368. Here is the modified code.

 nn=length([0:10:360 361 362 364 365 366 368 370:10:720]);
 P=zeros(nn,1);
 V=zeros(nn,1);
 cont = 1; 
for alfa=[0:10:360 361 362 364 365 366 368 370:10:720]
    cos_alfa=cosd(alfa);
    x(cont)=76.8*((1-cos_alfa)/2+0.28/4-(0.28/4)*cos_alfa^2);
    if alfa<=180
        P(cont)=0.9;
    elseif alfa>180 & alfa<=360
        P(cont)=320.612/((x(cont)+8.93023)^1.32);
    elseif alfa>360 & alfa<=540
        P(cont)=1119.9/((x(cont)+8.93023)^1.24);
    elseif alfa>540
        P(cont)=1.1;
    end
    V(cont)= 0.0448882+x(cont)*pi*(80^2)/4*(10^(-6));
    cont = cont +1;
end
fprintf('\n For alfa %g [degrees], cos_alfa is %g, x is %g, P is %g [bar], V is %g [l] \n',alfa,cos_alfa,x,P,V);
figure(1)
plot(V,P,'-r'),title('Diagram P-V'),xlabel('Volume [l]'),ylabel('Pressure [bar]'),grid;
fprintf('\n')

By the way, there is no Plot inside a "for" loop in the code!!!

[INDRANIL RANJIT SALVI]

Please give me some pointers on how to start plotting cylinder pressure vs swept volume graph for which we have the cylinder pressure and the total swept volume.How to incorporate engine load settings