As a range extender generator is provided to supplement the EV capability on a series hybrid, it is not a system in constant operation, indeed on many short or low speed journeys it will not operate. Therefore the attributes and requirements of such an engine are slightly different to a conventional engine.
The range extender engine and generator system needs to be as lightweight and compact as possible (fewer cylinders) with a fast engine warm up (low thermal mass and close coupled catalyst), low back pressure exhaust and a simple mounting system. The ideal operating strategy should be for maximum efficiency.
One of the major expectations for users of all vehicles is consistency in its operation and behaviour. This means not only range, performance and speed but also the noise performance. This is a key issue for users of hybrid vehicles where the expectation of these vehicles is a capability of very low interior noise levels.
The intermittent and unpredictable operational noise of a small highly loaded engine is a major concern for customers and the requirement for an acceptable cost effective solution to this problem is a serious issue for manufacturers.
We are aware of several manufacturers who run the range extender in different modes and efficiencies at different vehicle speeds to maintain an acceptable level of refinement.
The conventional NVH solutions for powertrain noise would all tend to be prohibitive in terms of cost, weight, package or efficiency.
Active noise control offers an effective solution as shown in the following example of the Lotus Evora 414E series hybrid concept car.
The Lotus designed range extender range extender as fitted in the Lotus Evora 414E series hybrid is a 1.2 litre, 35 kW, three cylinder aluminium monoblock engine, integrating cylinder block, head and exhaust manifold with a close coupled catalyst. The range extender generator and inverter is an axial flux synchronous motor-generator with the engine flywheel integrated into generator rotor which is directly mounted to the crankshaft.
It is primarily designed and operated for efficiency, not refinement. Above 12 kW output the engine runs at wide open throttle and the output power is regulated by changing engine speed (3,500 rpm max speed).
With regard to the NVH performance of the engine it is not surprising that a small lightweight 3 cylinder engine operating at full load has less than ideal NVH characteristics.
The interior noise is dominated by high noise levels at engine firing, noise acceptability is further reduced due to the fact that the engine operation is intermittent and is linked to battery state of charge and system load demand rather than vehicle speed and driver demand. The large NVH difference between when the engine is and is not running serves to highlight this issue.
This makes the 414E an ideal platform to demonstrate the benefits and performance of the HALOsonic active noise technology, specifically for the application of active engine order cancellation (EOC) within the cabin.
The graph shows a linear interior noise spectrum up to 200 Hz for the engine operating at 3,000 rpm (50 Hz) generating around 30 kW electrical power.
The black line shows the interior noise with EOC switched off and the black line with EOC
Initial system tuning was for the main dominant orders 1.5E (75 Hz) and 3.0E (150 Hz) which are at levels of more than 20 dB above the background noise. The test result shows that these dominant orders have been completely cancelled.
The residual noise peaks at 2.5E (125 Hz) and 3.5E (175 Hz) could also be reduced if required as the system has the capability to cancel up to 4 separate engine harmonics.
The fact that the engine is designed to run at moderate speeds and has only 3 cylinders produces a characteristic dominated by low frequency noise which is an ideal application for active noise control which is most effective in the range up to 250 Hz.
Following on from this initial result the development plan is to maximise the overall cancellation at all relevant engine orders and then to optimise the performance for the transient engine switch on and off events.
The 414E will also be equipped with interior sound synthesis to provide an enhanced driving experience with full gear change simulation giving the capability for improved driving control and energy efficiency though simulated engine braking and performance boosting. It is a fundamental requirement of this technology to ensure that the customer only hears one engine sound that is linked to his driving inputs and behaviour and is not confused by the residual drone of the range extender when it is operating.
In conjunction with Harman International production ready solutions have been developed for this technology so it is ready and available for integration into vehicle programmes.
Writer: Colin Peachey⎢Group Chief Engineer (EEI)