Down-Speeding and the Powertrain

down-speeding and the powertrain

Engine development has been evolving in recent years and Tech Know looks at the effect of down-speeding and the powertrain, and how it significantly alters the engine torque curve. The rate at which a low emission engine reaches full-rated torque is much quicker, which places a much greater load on the drivetrain system. 

The torque carried through the drivetrain system reaches and, in some cases, exceeds the structural integrity of the drivetrain system. As a result, drivetrain components are being subjected to an over-stress condition that is manifesting into sudden component fracture. These fractures occur more often during vehicle start up and acceleration, positioning manoeuvres and docking events at a terminal or during winter time ice and snow conditions. 

Historically, the method used to calculate steady-state drivetrain torque was satisfactory for sizing drivetrain components. Drivetrain data collected from trucks equipped with modern engines having tall rear axle ratios and direct drive transmissions shows 20 per cent greater torque than traditional steady-state calculation methods.

A dynamic simulation model was developed to better understand the transient inertial effects of the drivetrain system. The simulation model is a tool used to predict the large differences between calculated steady-state torque and vehicle dynamic torque events.

Several truck tests were conducted. The purpose of the tests was to measure the amount of torque produced during various truck manoeuvring events. Test results show there are events that are not uncommon that expose the drivetrain to torque values that exceeds the rated torque limits of the components. These events can over=stress and fracture the drivetrain components. 

Torque and system response data measured during truck tests correlated closely with values predicted using the dynamic simulation model. The close correlation between test data and predicted data validates the usefulness of the simulation model. The model can now be used to more accurately predict the behaviour of a truck system for future down-speeding applications.

Into the Future

The trend to down-speed engines will continue as a way to improve fuel efficiency and lower emissions. The new engines will be combined with rear axles having ratios lower than today and direct drive transmissions. 

The amount of torque transmitted through these new drivetrain combinations is much greater than former combinations. As a result, the drivetrain components will be more frequently exposed to over-stress conditions that can fracture a drivetrain component.

The dynamic behaviour of a vehicle is dependent on the characteristics of the drivetrain components. The characteristics include mass/inertia of the system, system stiffness and system dampening. 

Transient torque behaviour varies by vehicle configuration. Changes to any drivetrain component or subsystem can significantly impact the transient behaviour of the drivetrain system.

down-speeding and the powertrain

Dynamic simulation modelling and vehicle tests confirm that the transient behaviour of the drivetrain system can produce peak torque values far in excess of calculated steady-state torque. Peak transient torque becomes the origin of drivetrain component fracture.

Vehicle tests confirm the need for controls that effectively manage the powertrain behaviour to limit drivetrain peak transient torque. Successful vehicle test events produced results that turned the problem on by fracturing drivetrain components. Reproducing the same vehicle test events using revised engine controls to mitigate peak transient torque turned the problem off by preventing drivetrain component fractures.

Truck drivers reported no perceived impact on start-up or drivability with engines having controls activated that effectively managed the peak transient torque.

Truck OEMs, engine manufacturers and drivetrain component manufacturers need to continually work together to develop and implement control strategies that satisfactorily protect the drivetrain components. 

A control strategy is more effective than simply up-sizing fractured drivetrain components. Up-sizing, though it will solve the problem with that particular component, will only transfer the problem to the next weakest component in the drivetrain system. The outcome is larger, heavier and more costly components to overcome the unintended effect of down-speeding for fuel efficiency gains and greenhouse gas emissions standards. 

For more information go to Meritor. 

down-speeding and the powertrain