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Can someone give me a formula to calculate Oil and Fuel pressure of a car engine? Consider a small passenger car. I have information like Total distance travelled, speed, rpm, MAF, MAP, Intake Air Temp, fuel economy etc.

PS: I googled and googled, but ALAS!

  • Could you provide more information. Why do you need to know what the fuel pressure is? – vini_i Dec 17 '15 at 22:26
  • @soumyasen upvote some of these answers, they are very good. Participation in the site is predicated upon a system of reward, upvoting, awarding answers, things like that. With that, welcome to the site! – DucatiKiller Dec 18 '15 at 22:30
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Based on your history of other questions, I'm going to hazard a guess that you're trying to build an app functionality that can report a sudden drop in oil or fuel pressure.

I have seen fuel rail pressure reported in a freeze frame through OBD-II for one vehicle, a 2009 Mercedes GLK280, but not on other vehicles. I do not know what PID would provide this information though.

Oil pressure is less promising for reasons that @sweber has mentioned; it is not something monitored by the ECU beyond a binary threshold.


Footnote

I have to question why you would need an app that monitors oil pressure. It is a rare, rare thing for it to be needed.

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It's not possible to calculate them - I even have no idea how you could think so, or what you want to achieve.

The amount of fuel (gasoline) injected into the motor has to be controlled carefully to achieve an optimal combustion. So there are sensors to measure several parameters, which allow to determine how much fuel has to be injected. RPM, MAP/MAF, temperature are among those. So, you could also calculate the current fuel consumption from those values.

The oil pressure does not need to be controlled, so it is usually not measured. It has to be above a certain threshold to guarantee a sufficient lubrication, so most cars just have a switch sensor with that threshold.
Either pressure is OK, or it is NOT OK.
There's also no other way to calculate oil pressure. Oil pressure depends on the pump, resistance of all the oil pipes as well as on the viscosity of the oil - which depends on type, temperature, age and e.g. contamination by fuel. The only correct way to go is to measure the pressure itself. But as said, most cars don't do it.

For the fuel pressure, it's similar. There are different types of fuel supply with different pressure requirements: Carburetor, direct an manifold fuel injection. However, the exact pressure needed depends on the state of the art of the injector system. So, each system uses a different pressure. For some systems, the actual pressure is important, so they may be able to measure it, but most probably they won't broadcast this via OBDII. Also, there is no way to calculate fuel pressure - from what parameters should it depend?
So again: If the car doesn't tell you the value, you don't get it.

  • deatschwerks.com/resources/fuel-calculators/… how do these type of websites manage to calculate fuel pressure – Soumya Sen Dec 17 '15 at 10:13
  • @SoumyaSen They are NOT calculating fuel pressure. They have you measure fuel pressure and injector flow rate. Then they estimate what would happen to the flow rate when the pressure would change. – vini_i Dec 17 '15 at 10:44
  • @vini_i - Absolutely right. That page only helps you to figure out the differences when you increase the fuel pressure. At a given pressure a fuel injector will flow x amount. Output is determined by the pressure and the pulse width (how long an injector stays open). When you increase the fuel pressure, the output goes up accordingly as long as the pulse width stays the same. It's the reason why you can put Ford injectors in a Chevrolet and get better than rated output because Chevy runs their fuel systems at 43psi, while Ford runs theirs much lower. – Pᴀᴜʟsᴛᴇʀ2 Dec 17 '15 at 14:38
  • @sweber - Your last sentence is the most important one here: If a sensor doesn't tell you, you aren't going to get the answer ... so true in these two cases. – Pᴀᴜʟsᴛᴇʀ2 Dec 17 '15 at 14:40
  • @vini_i - 'At a given pressure a fuel injector will flow x amount. ' So, can I know this pressure, given the 'x' amount of flow – Soumya Sen Dec 17 '15 at 17:22
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You don't seem to be telling us why you want to know the car fuel pressure so it's hard to give you good advice but it also seems you have some misgivings about how a car actually works.

When a car starts after sitting overnight it is running in open loop mode because the oxygen sensor needs to be hot to give a good reading. The oxygen sensor is what closes the loop in the control system because it gives direct feedback on whether the correct amount of fuel has been added.

In open loop the car uses a multi dimensional look up table. The look up table accounts for RPM, intake air temperature, intake air flow, engine coolant temperature, load, barometric pressure, just to name a few. This look up table then gives the car the necessary amount of fuel to add to each cylinder. This look up table is painstakingly generated by the manufacture in a special engine dynamometer. This dynamometer can literally control every aspect of engine operation; load, coolant temperature, engine temperature, fuel pressure, etc... The dynamometer can also test the exhaust stream for the standard 5 gasses related to emissions. With this dynamometer engineers meticulously analyze every point in the look up table compensating it for everything they can think of, like the range of allowable fuel pressure. The values in the look up table are generally mid to worst case scenarios. For example if the fuel pressure is at the low end of the allowable scale.

When the oxygen sensor gets hot and starts giving a reading, that reading is used to verify that the correct amount of fuel is added and compensate for when it's not. This is reflected in the fuel trim numbers. When the sensor sees a rich mixture when compared to the open loop look up table it will take fuel away resulting in negative fuel trim.

Unless the car already has a fuel pressure sensor, the fuel pressure is compensated directly into the look up table and does not require calculation.

Theoretically I repeat Theoretically by comparing the delivered amount of fuel in closed loop to the open loop look up table you can infer how far away from the optimal fuel pressure you are. This depends on too many variables to truly be accurate. You need to know what that pressure that the engineers compensated the table for. The engine has to be in perfect running order. The injectors can't be even 1% plugged or your calculation would be off.

Also notice how i didn't mention oil pressure in there at all or whatsoever.

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