Thread: FA20DIT Thread
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Old 04-24-2014, 03:48 PM   #78
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Diesel engines have a throttle now, basically for EGR/emissions control. However the load is still basically controlled by fuel mass injected (in addition to EGR rates).

When they talk about "dethrottling" on a gas engine, whether it's through valve events or through lean stratified combustion, it's really running a higher manifold absolute pressure. The goal is lower pumping mean effective pressure--you'll see it in a cylinder pressure trace. This is something that can't easily be done without careful coordination of an electronic throttle, spark control, and other actuators. You still need a throttle valve for failsafe operation and certain conditions where the engine won't tolerate dethrottled operation.

The software uses a gasflow model (Ideal gas law and/or saint venant flow equation, basically variation of navier-stokes) to control the AVCS/cam phasers and throttle valve for a target MAP. So if normally you'd run around 40 kPa MAP fully throttled, you can raise your MAP to anywhere between 40-90 kPa at a given speed and load point.

The simplest way to do that is with internal EGR from dialing in overlap by phasing the cams. You might get 60 or 70 MAP, maybe higher, with cam phasing alone. Your upward limit on MAP is about 90 kPa on a gas engine, because some vacuum is still required to drive evaporative purge, PCV, and other functionality. 90 kPa is typical on a lot of systems with continuously variable valve lift. However, in reality the dethrottling is limited by combustion stability. Combustion stability depends on a bunch of parameters like DI rail pressure, injection timing, spray pattern, intake port tumble characteristics, and piston bowl design.

Lean stratified combustion might run at lower MAP than 90kPa but the charge air is highly diluted like a diesel and thermal efficiency is higher.
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