Internal combustion engine backpressure

  • #1
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Hello everyone
I got diesel engine, and there is a backpressure applied in the exhaust. I know that when the exhaust pressure reaches approximately 6 atm. the engine stalls. The question is, how I'm able to calculate this precisely, especially when the backpressure didn't reach it's maximum value, for example 2 atm.
I've been told that it has to deal with mean effective pressure, but I couldn't find the right formula...
Thanks in advance
Oleg
.
 

Answers and Replies

  • #2
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Thanks for the post! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?
 
  • #3
SteamKing
Staff Emeritus
Science Advisor
Homework Helper
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Hello everyone
I got diesel engine, and there is a backpressure applied in the exhaust. I know that when the exhaust pressure reaches approximately 6 atm. the engine stalls. The question is, how I'm able to calculate this precisely, especially when the backpressure didn't reach it's maximum value, for example 2 atm.
I've been told that it has to deal with mean effective pressure, but I couldn't find the right formula...
Thanks in advance
Oleg
.
The formula for calculating the mean effective pressure of diesel and gas engines can be found here:

https://en.wikipedia.org/wiki/Mean_effective_pressure
 
  • #4
JBA
Science Advisor
Gold Member
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While BMEP is an element of the issue, the effect of backpressure on the engine's performance goes far beyond that.

Even for a single piston diesel engine, which is the simplest example calculating the effect of the backpressure on the BMEP of a cylinder with the ratio of combustion air vs exhaust gas at each backpressure value is going to be very difficult and will vary depending upon throttle position and possibly engine speed as well.

For a multi-cylinder engines it gets even more difficult because you have to include the pumping losses on the filling and exhausting of all other cylinders during the compressing and firing stroke of one individual cylinder; and this effect will be different for engines with different numbers of cylinders.
 
  • #5
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Thanks all for taking part in this disscution. The answer has been found. Here what I've found:
Cycle with backpressure may look like this :
Indicator-Diagram-for-a-Four-Stroke-Cycle-Petrol-Engines.png

There are two loops in there, the top one is the indicated work that we get from the cycle (W_ind) and the bottom one is called pumping work (W_pump), its actually energy that we lose. By dividing the work by displacement volume we will get mean effective pressures: bmep - brake mean effective pressure ; pmep - pumping mean effective pressure.
the pmep may be approximated as : (p_exhaust - p_suction)*V_displacment
Since backpressure reduces the total work, the new work will this:
W(kW)=V [m^3 / sec] * (bmep - pmep) [ Pa]
 

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