Range Extender for electrified Vehicles: Combined Expertise

BRP-Powertrain and powertrain systems developer AVL collaborate on a research project to develop a range extender package for electrified vehicles.

Drivetrain electrification is growing in importance in the automotive industry, but the limitations of battery technology have created "range anxiety.” The two Austrian-based companies believe this can be eliminated with a range extender system powered by an internal combustion engine.

BRP supports the project in terms of providing components, manufacturing and assembling of prototypes, with the target of a potential serial production and is supposed to offer this Range Extender Package to automotive OEMs as a Tier 1 supplier.

In this model, the range extender is a "reserve fuel can" for the electric propulsion system – the internal combustion engine powers a generator to charge the vehicle battery.

The system is self-contained and mounted in the rear of the car under the trunk, leaving valuable under-floor space for the battery pack. It can even be offered as an option on electric vehicles.

Not only does this system alleviate range anxiety, it balances load power requirements with emissions targets, enables a small, lightweight, affordable battery for daily use (80-90% of range requirements) and has advantages in cold and traffic jam conditions.

According to studies by AVL, there are three factors that are critical to the success of such an approach: low cost, the smallest possible package and weight and low noise, vibration and harshness (NVH). BRP combines the expertise in advanced development, design, industrial engineering, tool-making, prototyping and production – all at one site.

Through this collaboration, the organizations found a BRP single-cylinder Rotax engine used in its Can-Am all-terrain vehicles (ATV) ideal, with some key modifications for NVH. As this engine is already industrialized, costs for tooling, manufacturing and components are minimized.

Initial market demand is seen between 20,000 and 50,000 units – relatively low volume for automotive suppliers, but a perfect fit of the production capability for the premium powersports manufacturer BRP.

The modified Rotax 1000 engine’s single-cylinder configuration especially facilitates a very flat system design, important to integrating it underneath
the trunk. It is mounted at 70° angle facing the right side of the vehicle and for easy serviceability the oil filter is accessible from the bottom.

The cylinder unit drives the crankshaft, which has been adapted for the range extender application for NVH reasons. The crankshaft is supported via three plain bearings and is connected to a gear stage driving the balancer shaft and the generator. The generator is mounted in a separate housing
which is assembled to the engine‘s crankcase to allow preassembling and testing of the generator at the supplier‘s facilities prior to system completion.

For achieving the desired specific-power targets a 4-valve cylinder head concept is required. Its single overhead camshaft with a roller supported
rocker arm not only ensures a small package size, but also has the advantages of low valve train friction and a cost-effective design.

NVH Innovations - Consumers appreciate the quiet and smoothness of vehicles propelled via electricity. They would not accept the significant vibration or noise from an internal combustion unit, so this is a critical area of focus.

With a single-cylinder engine such as the Rotax 1000, the main challenges are the long firing intervals, the balancing system design and the
comparable low inertia of crank train and generator rotor. BRP and AVL focused on the balancing system and engine rolling compensation.

Balance System - A system with two balance shafts has demonstrated much lower engine mount vibrations compared to a single balancer shaft variant.

The system for the Rotax 1000 is designed in a way that 25% of the oscillating forces are balanced by each balance shaft and 50% by the crankshaft. For optimum results, all three shafts must be positioned in a single plane. This optimum balancing principle was investigated in a multi-body simulation with AVL’s sophisticated EXCITE software code and it was found that the optimized two-shaft system reduced engine-mount forces by more than 50%, especially in the Y-direction.

Engine Rolling Compensation - The differential torque by the single-cylinder engine’s 720° crankshaft-angle firing interval vs. the uniform torque of the generator requires compensation measures.

The inertia forces by the difference of the engine’s torque peaks and of the uniform negative torque of the generator causes an external rolling force of the range extender system. This creates vibration that is transferred via the engine mounts into the vehicle structure. An effective compensation approach is using an inertia weight rotating against the drive unit‘s sense of rotation. With this system, the teams proposed a design where the generator rotor is driven by a coupling gear stage – essentially minimizing the vibration by rotating the generator counter to the crank.

Simulations showed a reduction of torque-bracket mount forces by more than 50%. Increasing the cranktrain‘s inertia will further reduce the
excitation forces.