In last week’s post we discussed the addition of the new User Engine Torque Limits function in the ECU and that Strat modes are selected. These “Strat” modes determine how the ECU limits the torque output. Screenshot 1 shows where these “Torque Limit Strategies” are configured. There are a total of five (5) types of Strat modes that can be configured. At the time of this post, the functions that can be configured and linked to these strategies are – Traction Control (Torque Mode), Launch Control (Torque Mode – Static), Launch Control (Torque Mode – Moving) and User Torque Limits. Using the same example of from our last post we will look at the strat mode used to achieve that example 600Nm.
Screenshot 1: Engine Torque Limit Strategy Setup - Torque Limit Stat 1 Setup
Screenshot 2 shows the Torque Limit Strat 1 Setup. This is the core setup dictating how the ECU will reduce torque from the engine. There are a total of 3 limit priorities that can be configured. The ECU will systematically go from Priority 1 to Priority 2 to Priority 3 in this order to achieve the Torque Target. As seen in Screenshot 1, there are a series of clamp tables for each torque limiting method that limit to what extent the ECU can use it before reverting to the next Priority in line. In this example only Torque Limit Priority 1 and 2 are used. The Priority 1 being “Throttle Area” and Priority 2 being “Ignition Retard. When a torque limit is required, the ECU will calculate where to reduce the throttle to achieve the torque target using the Throttle Mass Flow calculations. It will then monitor the “throttle Area Min Clamp Table” and any error in the airflow model that causes the Engine Torque to be reported above the target. If the Torque Target = Engine Torque before the min clamp has been reached, then only Torque Limit Priority 1 method is required. If the Engine Torque is still higher than the Torque Target, the ECU will revert to the Priority 2 method for torque reduction to make up the balance required. In this case Ignition Retard is the next in line. Assuming there is enough retard allowed by the Ignition Retard Max Clamp Table the engine torque will meet the target. If not, the ECU will then revert to the final Priority 3 method for reducing torque. In this example, Priority 3 is set to OFF so it is important the clamps are set such that Priority 1 and Priority 2 allow for enough torque reduction to reach the target. Not used in this example but highly effective at limiting torque is cutting. It must be noted that the “Ëngine Torque” channel is generated from the “Air Mass Final” in the ECU and therefore if there are errors between the Throttle Mass Flow calculated and the Air Mass Final then this will affect the torque reduction calculations. This is most important when a lot of the torque reduction is generated from reductions in throttle area. It is advised that Air Mass Final uses a strong blend of the Throttle Mass Flow Calculated when using throttle reductions as the primary source of torque reduction, to minimalize errors in the system.
The ECU can also leverage cutting by Fuel, Ignition, or a combination of both. Cutting is considered fast torque reduction method. This can very useful when looking to generate instantaneous torque reduction and usually for short periods if possible. We will go over such examples in future posts.
“Boost Target Override” is another feature option of the Strat Modes and allows the ECU to calculate a reduced boost target to be used during torque reduction. Explanation of this is not in the scope of this post, but it should be noted that it is very useful in reducing the need to heavily reduce torque due to a large margin of airflow generated torque to targeted torque. In other words, it is not practical to produce 1000Nm worth of airflow and request the ECU generate 50Nm torque output. It is also useful at reducing throttle closing levels when the torque limit is active, as the airflow is reduced from the driver demanded levels.
Screenshot 2: Setup – Torque Limit Strat 1 Setup