I am trying to understand engine parameters on a typical OBD system. Let us take an example with 3.0 V6 engine with MAF. I can monitor bunch of parameters such as air intake in gm/sec, fuel consumption gal/hour, load in percent, rpm, throttle position sensor, fuel trims etc.
Which sensor monitors the engine load? As it turns out there is no single sensor. The load is "computed" by the ECM using other sensors such as rpm, TPS, MAF. What confuses me is that the ECM uses this computed load to determine the ignition timings and fuel injector pulse width.
For my engine, how would I compute fuel consumption? I understand that UltraGauge/ScanGauge type instrument use MAF and engine size to compute fuel consumption by assuming average 14.7 stoichometric ratio.
Engine air intake consumption is 3 liter every other revolution. So at 600 rpm idle, it is ingesting 300 * 3 liters / minute = 15 liter/sec. What is the weight of a liter of air? One cubic meter of air weighs about 1.2kg which means 1 liter of air weighs 1.2 gm (at room temp / sea level)
So my engine should be taking in (1.2 * 15) = 18 gms/seconds of air.
Unfortunately, the actual MAF reading is at 3.0 gm/sec. The shop manual says that the range is 2--6 gm/sec for the idle MAF. The 3x range also surprise me. It tells me that the ECM does not really care about the actual MAF value but only the relative value. I do not understand how that works.
I also understand that I am missing something important because my calculation is based upon *only* RPM and nothing else. We know for sure that fuel consumption shoots up very high if the engine is under load at the same rpm.
How do I account for that?
- Vikas
Which sensor monitors the engine load? As it turns out there is no single sensor. The load is "computed" by the ECM using other sensors such as rpm, TPS, MAF. What confuses me is that the ECM uses this computed load to determine the ignition timings and fuel injector pulse width.
For my engine, how would I compute fuel consumption? I understand that UltraGauge/ScanGauge type instrument use MAF and engine size to compute fuel consumption by assuming average 14.7 stoichometric ratio.
Engine air intake consumption is 3 liter every other revolution. So at 600 rpm idle, it is ingesting 300 * 3 liters / minute = 15 liter/sec. What is the weight of a liter of air? One cubic meter of air weighs about 1.2kg which means 1 liter of air weighs 1.2 gm (at room temp / sea level)
So my engine should be taking in (1.2 * 15) = 18 gms/seconds of air.
Unfortunately, the actual MAF reading is at 3.0 gm/sec. The shop manual says that the range is 2--6 gm/sec for the idle MAF. The 3x range also surprise me. It tells me that the ECM does not really care about the actual MAF value but only the relative value. I do not understand how that works.
I also understand that I am missing something important because my calculation is based upon *only* RPM and nothing else. We know for sure that fuel consumption shoots up very high if the engine is under load at the same rpm.
How do I account for that?
- Vikas