Requested boost is not delivered torque
Bosch's diesel boost control compares a desired pressure with the pressure the sensor reports. The desired value is a function of engine speed, injected fuel quantity, coolant temperature, fuel temperature and the surrounding air pressure. If they differ, the ECU drives either a wastegate transducer or the vanes of a variable-turbine-geometry turbo. The point of that control, in the source, is the full-load torque curve and the part-load exhaust and filling cycle. Bosch, boost-pressure control.
Torque still has to wait for the air. In the torque-controlled description the management can only produce the external torque together with the correct injection quantity, the correct injection point, and the intake conditions, of which boost pressure and EGR rate are the examples given. Bosch, torque and the air path. A boost target that steps up with the pedal does not move the compressor instantly. Until the measured air arrives, a diesel smoke limit that follows air will hold fuel, and the car feels soft or hollow. If fuel is not held, the same delay is smoke and heat. That split is diesel smoke limits.
Overshoot, undershoot and a target the hardware cannot hold
Three different traces get called lag.
- The target rises and the pressure follows late. The request left the pedal map. The air system has not caught up. On a diesel, read air mass beside pressure, because the smoke limit uses air. On a petrol engine, read load or cylinder charge beside manifold pressure.
- The pressure overshoots, then the controller or a protection pulls it back. The ECU asked, the turbo delivered more than the target, and something reduced it. ME7 documents that reduction as a difference between requested and actual pressure, with separate proportional, integral and derivative action, an integral limit, and over- and under-pressure checks that can cut load. ME7 torque, load and boost.
- The target itself never rises. A torque limit, a temperature limit, a compressor-speed or pressure-ratio limit, or a regeneration mode kept the request down. Raising the wastegate or vane map will not change a target that was never issued. See which limit wins.
ME7 also documents a learned integral term. Two identical requests can produce different actuator duty because the adaptation moved. Temperature and altitude change the available command as well. The hardware page for compressor and turbine types is forced induction. Charge cooling changes the density those pressures represent: charge cooling.
Petrol torque can move before the turbo does
Simos examples separate a fast path, ignition and fuel, from a slow path, air charge, throttle, turbo actuator and cams. A short torque change can be spark even while boost is still climbing or still falling. MG1 and ME7 use the same split in different words: ignition is quick, the turbo is not. A hesitation that is gone before boost has moved is not a vane or wastegate problem. See Simos 18 and Simos 19 and ME7 ignition and knock.
What to log through the delay
Log pedal, requested torque or load, the limited request, requested boost, measured boost, air mass or cylinder charge, actuator duty or vane position, and injected quantity or lambda. Start the log before the pedal moves. A single peak-hold boost number cannot show whether the target, the hardware or the fuel limit was late. Repeat hot and in the gear the complaint uses, because quantity, temperature and ambient pressure are all in the diesel desired-boost calculation.
Sources
- Bosch Electronic Diesel Control (EDC), 2001, independent PDF mirror. Desired boost, wastegate and variable-geometry control, and boost as a condition of delivered torque. Printed pages 73–77. Accessed 2026-11-11.
- ME7 torque, load and boost and Simos 18 and 19. Petrol request, limit and fast/slow response examples.
- Forced induction. Compressor and turbine hardware, separate from the control loop.