Why hybrids?

Thursday, September 27, 2012

Tags :  alternative-energy

For a number of years, there have been hybrid roadcars available. Beloved of celebrities wanting to cultivate a tree-hugging image, they can't be said however to have captured the public's imagination. Technology pundits had predicted a long transition from a production passenger car market dominated by internal combustion engines to one dominated by electric vehicles, via increasingly 'hybridised' vehicles. In fact, without hybrids ever having become very popular, the number of fully electric cars coming to market these days might make hybrid vehicles seem like a flash in a niche pan.

The fact is that cars with better optimised engines have tended to outperform hybrid roadcars in terms of fuel consumption. The basic principle of a 'normal' hybrid system, such as that in a Honda or Toyota hybrid, is that the kinetic energy which would normally be converted to heat during braking, is stored and then re-used at an appropriate time. This is called regenerative braking, and is a laudable aim with real environmental benefits. Efficiency is increased, and less fuel should be used.

However, a driver who is genuinely concerned for the environment and seeks to maximise fuel economy will already do relatively little braking and will be very gentle on the throttle. It is quite possible then that the driver who is most likely to buy an efficient hybrid car is also the driver who is least likely to make the best use of such a system. The irony is that hybrid systems on roadcars would give the greatest improvement on those that are driven aggressively.

There are some vehicles that would benefit enormously from a hybrid system - city buses, garbage trucks, taxi cabs, some trains and racing cars. All of these have a very 'start-stop' drive cycle characterised by regular heavy braking events where a large amount of energy is converted, followed soon after by a requirement for strong acceleration. If we measure the 'success' of a hybrid system by the improvement in fuel economy rather than by the absolute fuel economy achieved, then these drive cycles are surely the natural targets of regenerative braking hybrid systems.

There is a type of hybrid system that predates the 'eco-hybrids' and one that is probably more relevant to most drivers, including those whose drive cycle is quasi-steady-state, and that is the motorway driver or long-haul trucker. Turbo-compounding is a system where excess energy is extracted from an exhaust gas turbine and transferred back to the driveline, increasing torque.

Formula One will embrace this concept in 2014, when the new engine rules take effect. The Formula One rules are published in draft form on the FIA website, and define the energy recovery system (ERS) as "a system that is designed to recover energy from the car, store that energy and make it available to propel the car", and as part of this it specifies that a "Heat Motor Generator Unit is the electrical machine linked to the exhaust turbine of a pressure charging system as part of the ERS".

The Formula One system will be able to store recovered energy rather than having to deploy or waste any energy available to it. The ERS will combine recovery of heat from the exhaust flow and from braking. Note that the heat recovery is linked to the use of a turbocharger. Unlike regenerative braking hybrids, this is not a piece of equipment that can be adapted to an existing, normally aspirated engine for an instant performance increase.

For once, motorsport will be a worthwhile development arena for relevant technologies for passenger and haulage vehicles. The FIA ought to be warmly applauded for bringing in such a brave set of rules. Unfortunately, at this early stage, involvement appears to be limited to wealthy automotive manufacturers. The costs of developing a new energy recovery system alongside a new Formula One engine to compete against the likes of Renault, Mercedes and Ferrari are immense.

Written by Wayne Ward

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