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CI Core Engine Air Mass Flow and Torque Production

A compression-ignition (CI) engine produces mechanical power by injecting fuel into the combustion chamber near the end of the compression stroke. Since the combustion chamber pressure and temperature exceeds the fuel ignition limit, spontaneous ignition occurs after injection. Heat released during combustion increases the cylinder pressure. During the power stroke, the engine converts the pressure to mechanical torque.

Torque production relates to injected fuel mass, fuel injection timing, fuel pressure, and air system states. CI engines operate at lean air-fuel ratio (AFR) conditions, so the AFR is greater than the stoichiometric AFR. CI engines use exhaust gas recirculation (EGR). The exhaust gases recirculate back to the intake manifold, reducing engine-out nitric oxide and nitrogen dioxide (NOx) emissions.

Air Mass Flow

To calculate the air mass flow, the compression-ignition (CI) engine uses the CI Engine Speed-Density Air Mass Flow Model. The speed-density model uses the speed-density equation to calculate the engine air mass flow, relating the engine intake port mass flow to the intake manifold pressure, intake manifold temperature, and engine speed.


To calculate the engine torque, you can configure the block to use either of these torque models.

Brake Torque ModelDescription
CI Engine Torque Structure Model

The CI core engine torque structure model determines the engine torque by reducing the maximum engine torque potential as these engine conditions vary from nominal:

  • Start of injection (SOI) timing

  • Exhaust back-pressure

  • Burned fuel mass

  • Intake manifold gas pressure, temperature, and oxygen percentage

  • Fuel rail pressure

To account for the effect of post-inject fuel on torque, the model uses a calibrated torque offset table.

CI Engine Simple Torque Model

For the simple engine torque calculation, the CI engine uses a torque lookup table map that is a function of engine speed and injected fuel mass.

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