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# A/f ratio for max temperature and pressure2

## 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

(OP)
1.5 and 1.75 is equivalence a/f ratio in cylinder

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

(OP)
For diesel 14.45:1

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

(OP)
1.75=25.2875:1 a/f ratio
1.5=21.675 a/f ratio

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

If the simulation program has a sophisticated combustion model, or if it uses the assumptions of a sophisticated combustion model, it may be taking into account the imperfect and time-based air/fuel mixing that is characteristic of diesel combustion. The overall combustion chamber may be lean, but local spots near the injector sprays WILL be rich. Black smoke (evidence of imperfect mixing and combustion) starts showing up well lean of stoichiometric.

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

(OP)
but why temperature and pressure in cylinder drops wen going to richer side?

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

Well, the fuel evaporates, which absorbs heat and thereby reduces temperature and pressure. Depending on the injection & combustion models, there could also be a comcomitant increase in ignition delay, that could have a further downward effect on peak pressure and peak temperature.
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

There is something wrong there. As you say the temperature should increase if more fuel is added - provided all other conditions are the same. One of the other operating parameters must also be different.

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

If you are running an ICE simulation, with a prescribed burn curve, you will be exploring hypothetical areas where real engines cannot realistically operate. Combustion will happen at whatever PHI value you give it. The burned mixture will an equilibrium mixture of all the ingredients and will contain ever larger amounts of H2 and CO as you make it richer and richer. The effective heating value will become less than the fuel's lower heating value and there will be a point where adding more fuel results in less overall heat release through combustion. Your temperatureas and pressures will drop. This is in addition to the evaporative cooling effects of the additional fuel.

Steve

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

#### Quote (gruntguru)

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.
That's actually what I was thinking. I was trying to give the simulation the benefit of the doubt. We don't really know yet from the OP what is being held constant and what is being allowed to change; and whether or not, as Steve points out, the simulation model is operating in a validated area.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

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

Actually you could could probably run a diesel engine right up to hydraulic lock if you had a suitably capable FIE system (unlikely). You might need a motoring dyno to keep it running at the top end. A plot of BMEP vs PHI for PHI between 0 and (say) 3 would be an interesting project. The neighbours would start to complain though

Steve

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

(OP)
gruntguru was right, there was something wrong, when i tried same thing today, i got normal results:
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

(OP)
How about petrol engine? When cylinder pressure and temperature should be highest? I think, that i'm doing some mistakes, because when using that ICE simulation program, i get max pressures and temperatures in SI engine when rpms are lowest, if everything else hasn't changed

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

Sounds reasonable - burn is completed closer to TDC (minimum volume) at low rpm. Max temp AFR should be stoichiometry.

je suis charlie

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

(OP)
But if i advancing ignition timing at higher rpm, to get max pressure at same (let's say 10deg atdc) like in lower rpms, i get highest pressures at low rpms anyway, it's ok? Shouldn't be vice versa?

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

2
You are running a cycle simulation. This gives you access to all the state variables you need to answer your own questions. What is trapped at IVC? How do heat release, pressure and temperature evolve with crank angle through the closed part of the cycle? How do these profiles change with speed? You are comparing single values when you have access to the whole story.

Steve

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

(OP)
Yes, i know, but how should be in ordinary SI engine? When pressure should be highest? When torque is highest? Or when?
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

Everything affects everything else ... and you haven't told us everything. Cam timing, for one thing. It's ALL in your model data. Look at it!

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

If you want to know why a specific metric (e.g. peak cylinder temperature) changes, you need to peel back the layers until you can see when the change starts to happen. Solve your problem by identifying and proving cause and effect, not by guesswork based on observed correlation between your chosen and other metrics.

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

(OP)

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

Cutoff ratio matters in thermal efficiency of a CI engine. How is that changing in your tests?

"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

(OP)
Panther140 - that program don't calculate cut-off ratio, so i don't know. But now i'm confused - cut-off ratio is ratio of cylinder volume when injection starts and when injection ends, or it is volume of cumbustion chamber and cylinder volume when injection ends?

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

(OP)
"doesn't calculate" - sorry for poor English :)

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

Cylinder volume when injection ends divided by volume of combustion chamber at TDC

je suis charlie

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