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Horsepower from acceleration

  1. Nov 29, 2006 #1
    I'm trying to calculate a vehicles bhp based on acceleration.
    I'm using RPM time stamped in milliseconds.

    Tire Dia= 0.62738 meters
    R&P= 3.23
    3rd Gear= 1.66
    Mass= 1500 Kg

    Wheel speed= Engine RPM/(R&P*Gear)
    Velocity= WS*Tire Dia*0.0523598
    M*(Dia/2)*1000*((v2-v1)/(t2-t1))=Nm
    Nm*0.001341022/5.3618= Ft.lbs
    Ft.lbs*RPM/5252=hp

    Does this seem correct? Obviously this does not factor for aerodynamic losses.

    Time RPM
    426273 860
    426897 984
    427193 1066
    427487 1143
    427785 1245
    428080 1350
    428375 1463
    428669 1577
    428964 1705
    429258 1823
    429556 1951
    429850 2093
    430145 2238
    430439 2400
    430735 2541
    431030 2715
    431324 2852
    431617 3013
    431912 3156
    432207 3314
    432503 3471
    432798 3646
    433094 3827
    433388 3956
    433684 4154
    433977 4299
    434273 4458
    434569 4613
    434865 4789
    435160 4950
    435456 5085
    435751 5240
    436053 5382
    436348 5521
    436643 5657
    436938 5770
    437234 5919
    437529 6039
    437824 6176
    438120 6307
    438414 6403
    438709 6538
    439005 6616
    439300 6721
    439596 6799
    439891 6920
    440192 6981
    440780 7228

    Thanks,
    Joshua
     
  2. jcsd
  3. Nov 29, 2006 #2

    berkeman

    User Avatar

    Staff: Mentor

    The times are not in milliseconds, but other than that your approach looks correct. When you correct the units on the timestamps and graph the HP versus time, what does it look like? Are you making your own in-car dyno? Cool.
     
  4. Nov 29, 2006 #3
    Thanks for the reply. It's for a pda application that reads info from the OBD2 port. The time stamp should be ms... It's not relative to real time, just a timer. It does not reset when logging begins, perhaps that's the confusion?:confused:

    What's kind of odd is that one car is providing very believable numbers but 2 others do not. They read very low at all speeds so I don't think it's a wind resistance issue. I have real dyno numbers from all the vehicles for comparison.

    These are the specs on the 2nd car:

    Tire Dia= 0.60452 meters
    R&P= 1
    3rd Gear= 5.3865
    Mass= 1200 Kg


    Time RPM
    45620087 874
    45620704 969
    45620998 999
    45621293 1056
    45621585 1125
    45621879 1187
    45622169 1236
    45622462 1297
    45622755 1382
    45623048 1472
    45623341 1519
    45623632 1598
    45623925 1685
    45624219 1768
    45624511 1866
    45624804 1955
    45625099 2053
    45625395 2133
    45625687 2224
    45625982 2311
    45626275 2405
    45626569 2479
    45626862 2583
    45627156 2671
    45627449 2760
    45627744 2860
    45628049 2950
    45628343 3059
    45628636 3148
    45628930 3237
    45629223 3328
    45629517 3425
    45629811 3518
    45630104 3614
    45630397 3704
    45630692 3783
    45630986 3876
    45631281 3971
    45631575 4065
    45631870 4172
    45632164 4261
    45632458 4356
    45632753 4447
    45633049 4525
    45633344 4611
    45633639 4702
    45633933 4798
    45634228 4893
    45634522 4973
    45634817 5059
    45635112 5145
    45635406 5306
    45635702 5333
    45635994 5400
    45636288 5492
    45636583 5570
    45636878 5672
    45637172 5737
    45637466 5845
    45637760 5919
    45638053 5987
    45638349 6068
    45638643 6131
    45638938 6200
    45639231 6284
    45639525 6356
    45639819 6434
    45640114 6501
    45640407 6574
    45640702 6649
    45640997 6706
    45641292 6789
    45641588 6833
     

    Attached Files:

  5. Nov 30, 2006 #4

    berkeman

    User Avatar

    Staff: Mentor

    Yes, that makes more sense that it's just the delta that matters. I couldn't understand the absolute ms reference. So you are saying that in the following:

    that the delta-T for the 2nd and 3rd datapoints is 617ms and 294ms? That's pretty low-performance sampling, eh?

    As for the graphs, I'd isolate the torque and HP graphs for each situation, to help debug the problem. It's pretty easy to look at the torque and HP graphs together to see if they look reasonable or not.
     
  6. Nov 30, 2006 #5
    The sample rate is only about 3hz :bugeye: That's the best the OBD2 has to offer on ISO vehicles....
    The first sample is usually junk. I posted the raw values but for graphing that initial value is discarded.

    Delta ms.
    617
    294
    295
    292
    294
    290
    293
    293
    293
    293
    291
    293
    294
    292
    293
    295
    296
    292
    295
    293
    294
    293
    294
    293
    295
    305
    294
    293
    294
    293
    294
    294
    293
    293
    295
    294
    295
    294
    295
    294
    294
    295
    296
    295
    295
    294
    295
    294
    295
    295
    294
    296
    292
    294
    295
    295
    294
    294
    294
    293
    296
    294
    295
    293
    294
    294
    295
    293
    295
    295
    295
    296


    Those screen shots are each of one vehicle and compare the actual BHP to the acceleration based HP. As would be expected at ~60mph a significant divergence begins. The second chart never lines up though. I think the formulas are correct and either the gear ratio is off or data from the vehicle is being interpreted wrong.

    Best regards,
    Joshua
     
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