The powertrain describes the mechanism that generates power and propels a vehicle. Over the years, standard linear powertrains – engine, gearbox, driveshafts to wheels – have been improved to make cars more reliable, more fuel efficient, less polluting, and so on. But fine tuning one particular system can only go so far, before each adjustment puts something else out of kilter.
The real step change happens when somebody decides to work up a completely fresh system. Thanks to modern technology, the scene has been set. Enter somebody like Hugo Spowers. Hugo is founder and Company Architect at Riversimple. He is an Oxford University trained engineer who previously worked in the world of racing cars. We could also mention the bungee jumping or the piano on skis, but the focus of this blog post is the Rasa powertrain. Inspired by the Hypercar work of Amory Lovins at the Rocky Mountain Institute, the Rasa powertrain has been designed to provide everything you need and nothing you don’t in terms of power – resulting in a car that can cruise along very efficiently on a tiny fuel requirement, but can also pack a big punch when asked to accelerate or climb a hill.
Meet the Capacitors
At the heart of this car, is the hydrogen fuel cell. We describe how a fuel cell generates electrical energy from hydrogen here. But you can take the fuel cell to a whole new level with a bank of super-capacitors.
A capacitor is basically something that can store electrical energy. Capacitors also used to be known as condensers, used in a traditional ignition distributor. They consist of conductive plates separated by a non-conductive layer called the di-electric. When an electric current enters the capacitor, the di-electric stops the flow and a static charge builds up and is stored in an electric field between the plates. Batteries, in contrast, store electrical energy in a chemical form. This means that capacitors have some advantages over batteries: they can be charged and discharged very quickly and many times without wearing out.
But ordinary capacitors don’t hold very much charge; super-capacitors are an evolutionary leap. The prototype super-capacitors in the Rasa have a coating of charcoal and lithium ion to give them much greater capacity to hold energy – the supercap bank has a capacitance many times the capacitance of two concentric spheres the size of the planet.
Everything you need and nothing that you don’t
The unintended consequence of conventional powertrain architecture is that the engine is sized for peak acceleration, which is available all the time but is only needed for less than 10% of the time – so the car has to carry a heavy engine, and gearbox, that is 80% redundant for 90% of the time. In contrast, the Rasa powertrain will be able to recover energy that is traditionally wasted during braking, whilst allowing recaptured energy to flow precisely where it is needed, when it is needed, and for as long as is needed – but no more. There is no redundant capability, which otherwise translates into extra cost, weight and fuel consumption.
This is pretty revolutionary, and is managed by purpose-designed Riversimple software – that’s the integrated brain of the car – ensuring that:
- A small, light fuel cell maintains cruise
- When braking, the motors in each wheel hub recover the kinetic energy and feed it into the bank of super-capacitors
- This energy stored in the super-capacitors provides most of the power (approx. 80%) required for acceleration and hill climbing
- The consequence is that cruise and acceleration are ‘de-coupled’ – no longer does better acceleration unavoidably increase the top speed
As a result, the Rasa is incredibly nippy with great acceleration (and that means fun to drive), whilst the size of fuel cell and fuel tank can be kept small and light. We call it a Network Electric system.
The powertrain needs further rigorous testing and adjusting – continuing with the Beta trials – but ultimately this, coupled with the overall lightness of the design, is where groundbreaking efficiencies are being made. We anticipate that the commercial Rasa will be more than 3 times as efficient as other fuel cell cars currently on the market.