## A/f ratio for max temperature and pressure

## A/f ratio for max temperature and pressure

(OP)

Hello, I was working with one ICE simulation program, and noticed, that in diesel engine, where a/f is 1.75, max combustion pressure is 128.81bar, and max combustion temperature is 2118.1K, and in same engine, where a/f ratio is 1.5, max combustion pressure is 115.71bar, an max ombustion temperature is 1982.2K, if a/f ratio is leaner than 1.75, pressure and temperature also drops. I always thougt, that in diesel, if a/f is richer, pressure and temperature should be higher. So where is point in diesel engine, where combustion pressure and temperature is highest? And why pressure and temperature rops, when going to richer side?

## RE: A/f ratio for max temperature and pressure

## RE: A/f ratio for max temperature and pressure

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## RE: A/f ratio for max temperature and pressure

## RE: A/f ratio for max temperature and pressure

1.5=21.675 a/f ratio

## RE: A/f ratio for max temperature and pressure

## RE: A/f ratio for max temperature and pressure

## RE: A/f ratio for max temperature and pressure

A good clue would be an increase in cylinder out gas temperature, indicate that combustion phasing is being delayed, which correlates with lower peak pressures and temperatures in the cylinder.

In case you're not following me, with late heat release phasing, even though the area under the heat release curve may be the same or higher, when it occurs later in the cycle, the effective expansion ratio is less, so the bulk gas is at a higher temperature when it exits the cylinder as exhaust.

Incidentally, I believe it is the normal, if not universal convention, to define equivalence ratio as phi, i.e. the actual fuel/air ratio divided by the stoichiometric fuel/air ratio. The inverse is known as the excess air ratio, or lambda, i.e. the actual air/fuel ratio divided by the stoichiometric air/fuel ratio.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

## RE: A/f ratio for max temperature and pressure

Hemi. Your suggestions are all examples of varying other operating parameters. In a real diesel engine, if more fuel is added at the same engine speed, the injection timing, ignition delay, injection rate etc are all the same. The only change is injection duration and cut off point. Peak temperature will invariably be higher.

je suis charlie

## RE: A/f ratio for max temperature and pressure

Steve

## RE: A/f ratio for max temperature and pressure

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

## RE: A/f ratio for max temperature and pressure

Steve

## RE: A/f ratio for max temperature and pressure

with 1.5 a/f ratio

----------------- PARAMETERS OF EFFICIENCY AND POWER ----------------

4600.0 - RPM - Engine Speed, rev/min

48.386 - P_eng - Piston Engine Power, kW

6.6275 - BMEP - Brake Mean Effective Pressure, bar

100.45 - Torque - Brake Torque, N m

0.02104 - m_f - Mass of Fuel Supplied per cycle, g

0.23999 - SFC - Specific Fuel Consumption, kg/kWh

0.35296 - Eta_f - Efficiency of piston engine

8.3702 - IMEP - Indicated Mean Effective Pressure, bar

0.44577 - Eta_i - Indicated Efficiency

1.0898 - FMEP - Friction Mean Effective Pressure, bar

0.79180 - Eta_m - Mechanical Efficiency of Piston Engine

---------------------------- COMBUSTION -----------------------------

1.5000 - A/F_eq - Air Fuel Equivalence Ratio in the Cylinder

0.66667 - F/A_eq - Fuel Air Equivalence Ratio in the Cylinder

115.74 - p_max - Maximum Cylinder Pressure, bar

1982.1 - T_max - Maximum Cylinder Temperature, K

3.0000 - CA_p.max - Angle of Max. Cylinder Pressure, deg. A.TDC

14.000 - CA_t.max - Angle of Max. Cylinder Temperature, deg. A.TDC

5.3187 - dp/dTheta- Max. Rate of Pressure Rise, bar/deg.

Injection: Custom Fuel Injection System

617.30 - p_inj.max- Max. Injection Pres. (before nozzles), bar

12.363 - d_32 - Sauter Mean Diameter of Drops, microns

20.000 - Theta_i - Injection / Ignition Timing, deg. B.TDC

29.958 - Phi_inj - Duration of Injection, deg.

4.7253 - Phi_id - Ignition Delay Period, deg.

0.05101 - x_e.id - Fuel Mass Fraction Evaporated during Ignit. Delay

With 1.75 a/f ratio

----------------- PARAMETERS OF EFFICIENCY AND POWER ----------------

4600.0 - RPM - Engine Speed, rev/min

41.286 - P_eng - Piston Engine Power, kW

5.6551 - BMEP - Brake Mean Effective Pressure, bar

85.714 - Torque - Brake Torque, N m

0.01801 - m_f - Mass of Fuel Supplied per cycle, g

0.24080 - SFC - Specific Fuel Consumption, kg/kWh

0.35177 - Eta_f - Efficiency of piston engine

7.3812 - IMEP - Indicated Mean Effective Pressure, bar

0.45914 - Eta_i - Indicated Efficiency

1.0765 - FMEP - Friction Mean Effective Pressure, bar

0.76615 - Eta_m - Mechanical Efficiency of Piston Engine

121.60 - Phi_z - Combustion duration, deg.

3.1677 - Rs_tdc - Swirl Ratio in the Combustion Chamber at TDC

1.5000 - Rs_ivc - Swirl Ratio in the Cylinder at IVC

35.108 - W_swirl - Max. Air Swirl Velocity, m/s at cylinder R= 24

---------------------------- COMBUSTION -----------------------------

1.7500 - A/F_eq - Air Fuel Equivalence Ratio in the Cylinder

0.57143 - F/A_eq - Fuel Air Equivalence Ratio in the Cylinder

113.97 - p_max - Maximum Cylinder Pressure, bar

1957.5 - T_max - Maximum Cylinder Temperature, K

4.0000 - CA_p.max - Angle of Max. Cylinder Pressure, deg. A.TDC

11.000 - CA_t.max - Angle of Max. Cylinder Temperature, deg. A.TDC

5.1814 - dp/dTheta- Max. Rate of Pressure Rise, bar/deg.

Injection: Custom Fuel Injection System

519.59 - p_inj.max- Max. Injection Pres. (before nozzles), bar

13.074 - d_32 - Sauter Mean Diameter of Drops, microns

20.000 - Theta_i - Injection / Ignition Timing, deg. B.TDC

28.010 - Phi_inj - Duration of Injection, deg.

4.7428 - Phi_id - Ignition Delay Period, deg.

0.05650 - x_e.id - Fuel Mass Fraction Evaporated during Ignit. Delay

92.400 - Phi_z - Combustion duration, deg.

3.1536 - Rs_tdc - Swirl Ratio in the Combustion Chamber at TDC

1.5000 - Rs_ivc - Swirl Ratio in the Cylinder at IVC

34.952 - W_swirl - Max. Air Swirl Velocity, m/s at cylinder R= 24

## RE: A/f ratio for max temperature and pressure

## RE: A/f ratio for max temperature and pressure

je suis charlie

## RE: A/f ratio for max temperature and pressure

## RE: A/f ratio for max temperature and pressure

Steve

## RE: A/f ratio for max temperature and pressure

P.S. I'm not an engineer, i'm just a student, and english is not my native language, so sometimes i got some troubles when i wanna find out some "deeper" info about engines. In our country there is no forum like this, so i decide to register there. And Thank You all for answers and patient :)

## RE: A/f ratio for max temperature and pressure

The mass trapped at intake valve closure will be very different if intake valve closure is at 0 degrees ABDC, or at 70 degrees ABDC, for example.

The former, will trap maximum intake charge at negligible (cranking/idle) RPM and only get worse, and that's why real engines aren't built that way. The latter will trap barely enough to keep going at cranking/idle but will probably work really well as the car is going through the timing lights at the end of the drag strip. You haven't told us which case you have. But of course, the latter will only work right if the port and valve configurations and sizes are optimized, and you haven't told us that, either. Over-aggressive cam timing combined with terrible ports will result in an engine that neither idles nor makes power.

## RE: A/f ratio for max temperature and pressure

I suspect that your trapped conditions are different. You may have more or less mass in the cylinder at IVC than you expect. If this is the case, you can forget about what happens next (the combustion) and try to understand why the trapped mass is different (what Brian describes above). If not, is your equivalence ratio what you think it is? Peak temperature and pressure are way down the line from these two important things.

As before, use your tool. That's why you have it.

Steve

## RE: A/f ratio for max temperature and pressure

## RE: A/f ratio for max temperature and pressure

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

## RE: A/f ratio for max temperature and pressure

## RE: A/f ratio for max temperature and pressure

## RE: A/f ratio for max temperature and pressure

je suis charlie