Control strategy hierarchy
The control strategy is arranged in a layered architecture. At the highest level, there are two alternatives:
1. Throttle position controls engine torque while the transmission controls engine speed
2. Throttle position controls engine speed while the transmission controls engine load
Either strategy can be successfully applied but option 2 has advantages where shuttling is required and the transmission is working close to ‘geared neutral’. Option 1 has advantages if the engine is of a type which is slow to respond to speed change requests.
All other strategies and control decisions follow from this first decision and seek to operate the engine along the line of best fuel economy on its load-speed map. Low level strategy details vary with different applications and can be very sophisticated; for example, during engine overrun the system may select a high overdrive ratio for refinement and economy to suit a cruise condition or a low ratio to maximize engine braking if a braking event is detected.