Investigation into the trade-off between the part-load fuel efficiency and the transient response for a highly boosted downsized gasoline engine with a supercharger driven through a continuously variable transmission

Abstract

Downsizing is an established trend in the development of passenger car engines. However, the benefits of an improved fuel economy are often obtained at the expense of the engine's dynamic response (owing to increasing demands on the boosting system) and, consequently, the vehicle driveability. The use of a continuously variable transmission in the supercharger driveline offers increased control flexibility over the air path, which could allow more suitable calibrations to be developed. This paper gives details of a co-simulation-based investigation into the trade-off between the steady-state part-load fuel efficiency and the resulting tip-in transient response for a highly boosted downsized gasoline engine. The engine was a 2.0 l in-line four-cylinder unit, designed to replace a 5.0 l, naturally aspirated V8, equipped with a positivedisplacement supercharger in a sequential series arrangement with a fixed-geometry turbocharger with an external wastegate. The supercharger can be de-clutched and bypassed, and therefore three separate supercharger engagement regimes were investigated for part-load operation, defined as follows: with the supercharger disengaged and bypassed; with the supercharger engaged with a fixed drive ratio; with the supercharger engaged using a variable ratio (i.e. through a continuously variable transmission). For each of these supercharger engagement regimes, design-of-experiments and optimisation techniques were used to find the best settings for the key engine control parameters such as the intake and exhaust valve timings and the exhaust gas recirculation rate. Using these calibrations as a starting point, the transient performance was then assessed in fixed-speed tip-in simulations. The trade-off situation was found to be highly complex; identifying the best overall balance of the steady-state efficiency and the dynamic performance requires a subjective assessment. However, the continuously variable transmission does provide the best potential for dynamic response combined with a satisfactory fuel economy. It is suggested that the most suitable solution would be to have multiple userselectable calibrations, such as the 'economy' and 'sport' modes used on many modern vehicles.

Observations on and potential trends for mechanically supercharging a downsized passenger car engine:a review

“…For example, in the simulation study by Rose et al, 80 the clutch acceleration torque can be seen in Figure 13. The engine torque dip phenomenon can also be seen in Figure 14, in which it can be observed that the economy mode (labelled SC disengaged) has a significantly lower engine torque during the first part of a transient event in comparison with the sport mode (labelled SC engaged).…”

“…Although some passive bypass valves can function similarly to active bypass valves by tuning the action of 80 For reference, the BMEP target, 90% of the BMEP step demand (i.e. full load) and the equivalent BMEP for the baseline experimental results are also shown.…”

“…where T is the torque, I is the inertia and ω · is the angular acceleration, or generates a torque dip (due to the referred inertia of the supercharger rotors) during a transient event affecting the driveability. For example, in the simulation study by Rose et al, 80 the clutch acceleration torque can be seen in Figure 13. The engine torque dip phenomenon can also be seen in Figure 14, in which it can be observed that the economy mode (labelled SC disengaged ) has a significantly lower engine torque during the first part of a transient event in comparison with the sport mode (labelled SC engaged ).…”

“…To the present authors’ knowledge, there does not seem to be any literature covering this topic. However, from some simulations and test projects conducted at the University of Bath, 10,38,46,49,65,79,80 it is still possible to summarise some key challenges for developing a viable control strategy for the supercharger system (it should be noted that these follow only some suggestions arising from the engine simulations and test results from the University of Bath and that these should not be interpreted to represent or be calibrated for any specific application).…”

“…Supercharger input torques for tip-in simulations in the supercharger-engaged regime, in the supercharger-disengaged regime and for a CVT-driven supercharger regime. 80 …”