Longitudinal
Control of Commercial Heavy Vehicles Equipped with Variable Compression
Brakes
Lasse Moklegaard, Maria Druzhinina, Anna G. Stefanopoulou
Although, service brakes can theoretically provide a retarding power ten times higher than the accelerating power of the vehicle, they cannot be used continuously because of the generated heat and associated wear of the friction contacts. The presence of delays associated with the pneumatic or the hydraulic actuation subsystem impose additional constraints on the longitudinal control of CHVs. Faced with these difficulties, fleet and engine manufacturers are introducing additional retarding mechanisms that can provide consistent magnitude and unlimited duration of braking force.Good and consistent braking for CHVs can be achieved by using an engine compression brake in addition to the conventional friction brakes. The compression brake is a retarding mechanism that enhances braking capability by altering the conventional gas exchange process in the cylinders of the engine and effectively converting the turbocharged diesel engine that powers the CHV, into a compressor that absorbs kinetic energy from the crankshaft. During compression braking, the engine dissipates the vehicle kinetic energy through the work done by the pistons to compress the air during the compression stroke. The compressed air is consequently released into the exhaust manifold through a secondary opening of the exhaust valve at the end of the compression stroke.
Compression braking increases the overall decelerating capability of the vehicle and, therefore, permits higher operational speeds. As a matter of fact, this retarder can potentially be used as a sole decelerating actuator during low deceleration requests. During high deceleration requests it needs to be coordinated with the friction brakes or gear selection to provide sufficient braking power. As a result, the application and intensity of the friction brakes can be reduced and the problems associated with wear and overheating of friction brake actuator can be mitigated. The compression brake isthetype of retarding mechanism that we focus on in our work.
Our previous work within MOU 372 contributed to modeling of CHVs. In particular, we developed a detailed crank angle based simulation model of a diesel engine equipped with a continuously variable compression brake. The variability of the compression braking torque was achieved through controlling a secondary opening of the exhaust valve of the vehicle's turbocharged diesel
engine using a variable valve timing actuator. By employing signal processing to the output of our simulator, we developed a linear reduced order model that is suitable for control design and analysis. That work bridges the gap between the detailed crankangle-based model developed in the engine design community, and the low order representation of engine torque response used in the vehicle dynamics community.
In this report we present our new results on employing classical and modern control techniques to develop longitudinal speed control algorithms that coordinate the variable compression brake with conventional service brakes and gear selection. Since the compression brake can be used continuously without danger of damage and overheating, it is, thus, a natural actuator to be used for speed control.
Specifically, we integrate the compression brake actuator with the service brakes and design a PI controller that emulates the driver's actions on long grades. The controller uses the engine speed measurement to activate the service brakes only when retarding power of the compression brake is insufficient. We also employ robust linear control technique using the concept of structured singular values to design a controller that is robust to parameter variations and model uncertainty in the CHV. Finally, we compare the performance of this robust controller with the performance of an adaptive control scheme that we have derived within MOU 393. The performance of all the controllers is demonstrated through extensive simulations on the 24th order nonlinear vehicle model developed in our previous work within MOU 372.