The fuel consumption of motorcycles to a large extent correlates with their peak power output, although such power levels are only required for short periods during high-acceleration manoeuvres. Also, customers demand high torque reserves! However, in real-world driving, the measured load collective is concentrated in the low to medium power range, where aerodynamic drag and rolling resistance are low. This low load profile conflicts with the efficiency characteristics of naturally aspirated engines, whose optimal efficiency is generally located at three-quarter to full engine load.
Improvements in the efficiency map of internal combustion engines can be achieved by various principles. However, important requirements for motorcycle powertrains must be considered. These include low system mass, high low-end torque, and fast transient response characteristics. Furthermore, a compact overall vehicle design must be maintained while ensuring sufficient range without refuelling. Given the relatively low production numbers compared to automotive products, low development and manufacturing costs are essential.
Following a comprehensive evaluation of potential concepts, engine downsizing combined with mechanical supercharging was identified as a promising approach to reduce real-world fuel consumption. Based on the derived requirements, the target is to provide high torque (approximately 120 Nm) using a typical mid-size engine (approximately 800 cc), while benefiting from reduced friction and gas exchange losses associated with the relatively small displacement. A scroll-type supercharger was selected as the boosting device, as it can be decoupled during idle and part-load operation due to its low polar moment of inertia. In addition, its acoustically favourable behaviour eliminates the need for noise-insulating measures, which is advantageous with respect to packaging. The use of methanol as a fuel enables the exploitation of synergistic effects between mechanical supercharging and alcohol fuels, primarily attributed to methanol’s high latent heat of vaporisation and high-octane number. Moreover, methanol appears to be a suitable future energy carrier, as CO₂-neutral production is feasible with relatively simple facilities and reasonable energy efficiency. Beyond efficiency improvements, this approach could further reduce the ecological footprint of motorcycles.
The here presented powertrain is based on an existing production vehicle and was retrofitted with a scroll-type supercharger as part of a feasibility study. The powertrain was investigated both by means of one-dimensional engine process simulation and through experimental measurements on an engine test bench. The results demonstrate compliance with the highly dynamic requirements, achieving load build-up within only a few engine cycles. Key findings from the experimental investigations particularly relate to the gas-dynamic coupling of the two positive displacement machines - the scroll-type supercharger and the piston engine - which leads to pressure pulsations within the volumetrically small intake manifold. These effects necessitated measures such as synchronous operation via a toothed belt and gearbox. Pulsations were analysed with respect to the angular offset between the supercharger and the irregular gas exchange frequency from the crossplane crankshaft. The CAD design was successfully realised in the demonstrator vehicle.