Engine downsizing and downspeeding are two strategies that can be applied to gasoline and diesel engines to improve vehicle fuel economy. Forced induction is a necessary aspect of these strategies in order to maintain vehicle performance for customer acceptance. The primary methods of forced induction are turbocharging and supercharging, and the choice between these methods depends on the requirements for packaging, torque rise rate, fuel economy, and cost...Turbocharging tends to have an advantage in engine fuel consumption, whereas supercharging tends to have an advantage in torque rise rate, especially at lower engine speeds.

It is possible to leverage the greater low-speed torque rise rate afforded by the supercharger to enable a higher degree of downspeeding while maintaining vehicle performance metrics by optimizing the transmission shift schedule.

—Ostrowski et al.

In the study described in the paper—which was a continuation of previous work on pursuing fuel economy improvements via downspeeding—the team used a turbocharged 2.0L inline four-cylinder engine with a development-level Tier 2 Bin 5 calibration. The engine had a dual loop EGR system to provide high levels of EGR and correspondingly low levels of engine-out NO_x.

The team first tested the base turbocharged engine. The researchers then removed the variable speed turbocharger and replaced it with an Eaton R570 supercharger with TVS Supercharger technology. The supercharger was sized and geared to match the full load torque curve of the baseline engine, and was belt-driven by the engine at a speed ratio of 4.3:1.

A bypass valve controlled boost pressure, and the throttle was moved upstream of the supercharger to minimize the supercharger power consumption during throttled operation.

They adjusted the engine calibration to provide similar air-fuel ratio and EGR targets to the baseline configuration; this resulted in a less than 1% difference in engine-out NO_x. BSFC (brake specific fuel consumption) gradually increased with supercharging, up to a maximum of 12% at high load. Torque rise was faster with the supercharged configuration, especially at low engine speed; supercharging delivered reductions of up to 77% in time-to-torque.

Once the fuel consumption and torque response of both turbocharged and supercharged versions were measured, the team began working with transmission gear ratios and shift schedule optimization. They performed fuel economy and 0-60 mpg acceleration simulations for three shift schedule optimization strategies for 81 transmission gear ratio configurations of the supercharged configuration. The also created revised shift schedules for the baseline gear ratios for the turbo for comparison.

The downspeeding analysis showed that fuel economy could be improved with supercharging while retaining the baseline vehicle first gear grade-ability, top gear passing capability, and 0-60 mph acceleration time while adhering to industry-standard constant vehicle speed progression gear ratios. The hardware configuration paired with a balanced shift schedule optimization strategy increased the engine load sufficiently to more than overcome the increased specific fuel consumption inherent with the supercharged configuration. As a result, the downsped supercharged powertrain configuration was able to achieve better fuel economy than both the stock and downsped turbocharged powertrain configurations for this vehicle application.

—Ostrowski et al.

Resources

• Gregory Ostrowski Gary D. Neely, Christopher J. Chadwell, Darius Mehta and Philip Wetzel (2012) Downspeeding and Supercharging a Diesel Passenger Car for Increased Fuel Economy (SAE 2012-01-0704)