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Efficiency and performance with Energy recovery

The hybrid drive is not only electric:  Volvo uses in fact a flywheel that spins at high speed and recovers the vehicle’s kinetic energy under braking to return it when you need it

Nicodemo Angì

The word KERS is not unknown to anyone interested in engines, since it has risen to the headlines thanks to Formula 1: the Kinetic Energy Recovery System was in fact introduced a few years ago in Formula One.

The purpose of the KERS system is the same as that of an electric hybrid systems: storing energy during the deceleration and braking, to return it when needed with the aim of increasing performance and / or to optimize fuel consumption.

The reduction in fuel consumption is achieved through the "help" that the system of storage / restitution of energy supplies at the right time, ie in the stages where the efficiency of the engine is low: partial loads, accelerations, stop-and -go and the like.

In fact, to tell the truth, internal combustion engines are not highly efficient, and quite variable depending on the conditions in which they work: at low revs, with the throttle just open, major losses in the flow of intake air is observed and efficiency is also low at maximum rpm.

To increase the average efficiency  various "adjustments" have been introduced, such as forced induction – which allows to reduces the size maintaining the same performance - and the dynamic adjustment of the valve timing. The Fiat Multiair engine, regulates not only the timing but also the lift of the intake valves, which allows it to work without a butterfly valve.

A great contribution can be then given by the support of another unit with power output characteristics complementary to those of internal combustion engines and, especially, with reversibility.

This latter characteristic is rather appreciated, given that it is completely lacking in combustion engines: even forcing the rotation with an external force, they are not in fact able to "gather" heat, exhaust gas and mechanical work to obtain again  petrol or diesel.

Reversible,  able to generate and store energy when rotated passively, is what electrical power units and - precisely - the flywheels are.
Every internal combustion engine has a flywheel that stores kinetic energy during active phases of the cycle to return it when the engine (intake and compression phases) absorbs energy instead of generating it: in this sense we can say that the flywheel is an essential part of an engine, controlling (and, indeed, permitting) idling and working at low revs.



Racing in its DNA

The KERS that Volvo is testing is based on a flywheel but it is a distant relative of those that are currently used in engines: it is much lighter, more compact and, above all, capable of rotating at extremely high speeds.

The car used in the tests, an S60 T5, is equipped with a KERS in the rear axle, equipped with conventional differential gear and axle shafts connected to the heart of the system: a steel and carbon fibre flywheel  capable of rotating up to 60,000 rev / min!

The reason for this phenomenal rotation is obvious: since the formula that gives the stored energy from a rotating flywheel (E = ½ Iω2) shows the moment of inertia I and the square of the angular velocity ω, by increasing the latter the stored energy grows exponentially.

The Volvo KERS flywheel - which weighs about 6 kg, has a diameter of 20 cm and rotates under vacuum to reduce friction - has a steel hub and the outer part of carbon fibre, one of the few materials capable of resisting to the tremendous centrifugal force generated by such an extreme rotation.

Research for the next-generation Volvo road KERS began four years ago in partnership with Flybrid Systems, a highly specialized British company based in Silverstone (one of the legendary places of motor sports) part of the Torotrak group.

Flybrid has also developed systems for competitions: the unit developed for the 24 Hours of Le Mans reaches a 100 kW output while weighing less than 40 kg, while the F1 unit, which performs less due to the regulations that limit the power to 60 kW, weighs only 25 kg and has a volume content of 13 liters. The Volvo KERS can deliver up to 80 hp for short periods, and if used in order to optimize efficiency, can allow fuel saving up to 25%, compared to a engine with similar performance, by storing in the form of rotational kinetic energy  the mechanical energy created during braking and deceleration (otherwise dissipated as heat) and to release it when needed.



Team work

Wanting to use this surplus of energy to increase the performance the manufacturer states that the acceleration from 0 to 100 km / h is down to 5.5 seconds; the system delivers its best performance in city traffic and during dynamic driving situations characterized by frequent slowdowns and accelerations. 

The connection between the differential and the flywheel is made through a number of gears, a multi-disc clutch (that delivers enough torque to allow a standing start using only the energy of the flywheel) and a CVT - Continuously Variable Transmission - with variable speed drives.

The presence of these components is necessary because we need to adjust a few thousands (or hundreds) rev / min of the wheels to the 60,000 rpm of the flywheel: gears that multiply the speed aren’t enough, a clutch and a variable speed drive transmission, CVT in fact, are needed.
A few figures will clarify the concept: a tire compatible with the S60 - the 215/50 on a 17” rim - has a rolling circumference of 1.97 meters, which is equivalent to 846 rev / minute at a 100 km / h. It is therefore clear that "loading" the flywheel at that speed must be done very gradually.
The entire unit weighs about 60 kg, much less than a battery-electric motor system of comparable performance, and has a 90% greater overall efficiency.

Other benefits are an expected driving distance of 250,000 Km ( Flybrid data) without appreciable decline in performance, a volume comparable to an electric motor of a conventional hybrid and fully recyclable: the  more "sophisticated" material used in the system is in fact the carbon fiber.
Derek Crabb, Vice President of Powertrain Engineering ( Volvo Car Group ), talking about the Volvo KERS explained that "the energy stored in the flywheel is sufficient to move the car for short periods and this has a big effect on consumption. Our simulations indicate that it will be possible to turn off the engine for about half the driving time in agreement with NEDC approval. "

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