AVL Concept Turbohybrid Mild Hybrid System Approaches Full Hybrid Fuel Savings at Lower Cost; Improved Driveability
|Comparison of Fuel Consumption, Fun to Drive and System Cost of the different powertrain concepts. Click to enlarge.|
AVL has developed and patented a mild hybrid system and engine operating strategy that combines downsizing with a turbocharged gasoline direction injection engine; downspeeding (the application of a longer gear ratio); and a relatively low power electric machine to deliver a reduction in fuel consumption approaching that of a full hybrid but at lower cost, while improving the driveability of the vehicle.
In testing on a BMW demo 3-Series vehicle, a 1.6-liter AVL Turbohybrid prototype delivered a 24% reduction in fuel consumption on the NEDC compared to the 2.0L naturally aspirated baseline; a 1.6L full hybrid (power-split) system delivered a 36% compared to the baseline. However, the system cost of the Turbohybrid was 150% that of the baseline, while the system cost of the full hybrid was more than 300% that of the baseline. In addition, the Turbohybrid delivered a “fun to drive” rating of 7.81—higher (better) than both the conventional vehicle and the full hybrid.
|Stage 1 of the Turbohybrid powertrain concept. The clutch module contains a second clutch, currently not activated, which can disconnect the combustion engine from the drive train to enable electric-only driving in a future phase. Click to enlarge.|
Compared to a naturally aspirated 2.0-liter direct injection engine equipped with a stop-start system, the AVL Turbohybrid delivered a 10% improvement in fuel consumption; the power-split hybrid delivered a 26% improvement. Cost of the Turbohybrid system was less than 150% of the baseline NA GDI system, while the cost of the power-split system compared to the GDI system was still more than 300%.
The Turbohybrid system. Downsizing a gasoline engine (e.g., with the use of turbocharged GDI) and downspeeding can result in a significant increase in fuel efficiency, but with a negative impact on traction force reserve and driveability. AVL reasoned that merging turbocharged GDI with a mild hybrid function can result in excellent driveability even with the combination of strong downsizing and downspeeding.
By the combination of downsizing and downspeeding the balance is shifted even more towards maximum fuel efficiency, however, also towards an unacceptable driveability. Merging turbocharged GDI with a hybrid, excellent driveability can be obtained even in combination of extreme long gear ratio and strong downsizing. Furthermore the hybrid provides the additional advantages start-stop and recuperation. Nevertheless, the most essential fuel economy aspect of the Turbohybrid concept is the possibility of a high level of downsizing and downspeeding.—Fischer et al. (2008)
|The demo Turbohybrid powertrain. Click to enlarge.|
To maintain the battery charge required to supplement the low-speed torque with the e-drive, AVL developed a special strategy to guarantee a consistently high level of charge by using the overboost capability of the turbo-charged combustion engine.
AVL says that its smart recharging concept guarantees the filling of the “torque gap” at low speeds under all circumstances of acceleration even with a small energy storage system
|Full load performance of the Turbohybrid. The overboost capability of the 1.6L engine (green hatched area) is applied to charging the energy storage system rather than in directly delivering torque. Click to enlarge.|
In cooperation with Bosch, LUK and BMW, AVL applied the Turbohybrid powertrain in the BMW 3-series demo vehicle. The initial powertrain consisted of the 1.6-liter turbocharged GDI engine; a crankshaft hybrid module with two clutches (only one of which was activated in this phase of the project) and a 20 kW motor electronically limited to 15 kW; a “diesel type” 6-speed manual transmission with a long axle ratio; a prototype hybrid controller; and a high voltage double layer capacitor module (334 kJ / 93 Wh), which can be easily replaced by a lithium-ion battery as alternative.
AVL is continuing to benchmark the initial Turbohybrid system. In addition, it plans to evaluate electric driving after changing the electrical storage module to a Li-ion battery pack. Use of a dual-clutch transmission, which offers further fuel efficiency, is also under consideration.
Robert Fischer, Richard Schneider, Peter Ebner (2008) The Turbohybrid – The Realization First Results (20th International AVL Conference)