Flywheel Hybrids

The engine in a conventional car or truck is a clever compromise. On the one hand, it has to provide sufficient power for several seconds of strong acceleration up to freeway speeds. On the other hand, when the vehicle is cruising somewhere around 60 or 70 mph, it needs to convert gasoline into forward motion as economically as it can. The size needed for strong acceleration becomes a handicap, because a smaller engine is more efficient and perfectly suited for cruising.

The essential idea behind today’s hybrids is to have two power units rather than one, each optimized to do one of the tasks—acceleration from low speeds or running efficiently at high speeds—much more effectively than a conventional, compromised, engine. Toyota, Honda, and Ford believed that customers would pay a premium for two power units—one gas and one electric—to enjoy lower fuel consumption, both in the city and on the freeway. And sales figures, for the most part, have proven them right.

Energy Supply versus Surge Power

Hybrid engineers talk about Energy Supply Units (ESUs) and Surge Power Units (SPUs). ESUs can be gasoline engines, diesel engines, biofuel engines, fuel cell systems, gas turbines, or even plug-in batteries. Fierce arguments rage over the most appropriate choice for a particular application. In today’s production hybrids, the surge power for acceleration comes mainly from batteries. Imagine a car approaching a red traffic light. The driver touches the brake pedal gently, and the car eases to a stop. In a conventional vehicle, all its kinetic energy, i.e. the energy that is a function of its road speed and its mass, is thrown away, as heat from the brakes. This contrasts with a hybrid, in which the SPU collects as much of the vehicle’s kinetic energy as it can, causing the vehicle to slow down as it does so, with the disk brakes held in reserve for an emergency stop. The SPU then stores the energy, until the vehicle moves off again, when the ‘free’ energy from the SPU is used in preference to fuel-expensive ‘new’ energy from the engine.

The Problem with Electric Battery Storage

This saving of kinetic energy as electric/chemical energy can radically reduce fuel consumption even if the engine remains the same size. However, the battery-based solution seems to ignore the basic physics of the application. The key task of the SPU is to capture as much of the vehicle’s kinetic energy as practicable, and return it as kinetic energy a short time later. It is a fundamental of physics, reflected in the Second Law of Thermodynamics, that transforming energy from one form to another inevitably introduces significant losses. This explains why the efficiency of a battery-based hybrid drive system is so low. When a battery is involved, there are four energy-sapping transformations in each regenerative braking cycle:

  • Kinetic energy is transformed into electrical energy in a motor/generator
  • Then the electrical energy is transformed into chemical energy as the battery charges up
  • Later the battery discharges, transforming chemical into electrical energy
  • Finally, the electrical energy passes into the motor/generator acting as a motor and is transformed once more into kinetic energy

The four energy transformations undermine the overall level of efficiency. For example, if the motor/generator operates at 80% efficiency under peak load, in and out, and the battery charges and discharges at 75% efficiency at high power, the overall efficiency over a full regenerative cycle is only 36%, almost the same as the figure Toyota quotes for the Prius II.

Flywheels as a Solution

The ideal solution is to avoid all four of the energy-sapping transformations from one form of energy to another. This can only be achieved by keeping the vehicle’s energy in the same form as when the vehicle starts braking, and the form it must inevitably be in when the vehicle is back up to speed. In other words, less conversion equals less energy lost.

This requires the use of high-speed flywheels, popular in space and in uninterruptible power supplies for computer systems, etc., but novel in cars. High-speed flywheel energy storage is essentially a substitute for a battery system, in which the inputs and outputs are required to be electrical currents. For the space and computer applications, using high-speed motor/generators to add and remove energy from the flywheels makes sense. The use of flywheel technology is well known.

However, in ground vehicles it makes more sense to use mechanical, geared systems, which are much more efficient. For example, a typical conventional manual transmission is at least 97% efficient over most of its power and speed range. Of course, a mechanical solution to gearing a flywheel operating between, say, ten and twenty thousand rpm geared to road wheels operating at up to 2,000 rpm is much more complex, requiring a totally smooth continuously variable ratio transmission capable of ‘dictating’ whether the vehicle is accelerating or braking. Among other differences, the bearings must be optimized to deal with road shocks, rather than designed to minimize frictional losses, the priority for static or space borne battery substitutes. While the principles of using high-speed flywheels are similar in most applications, there are several other critical differences between battery substitutes and vehicle-mounted SPUs.

In general, a mechanically driven flywheel system has losses due to bearing friction, windage, etc, which will make it less efficient than a battery-based system in storing energy for more than an hour or so. However, over the much shorter periods required in cut-and-thrust traffic, a mechanically driven flywheel proves much more effective. Consequently, the ideal combination in a plug-in hybrid is a flywheel as the SPU, plus a battery optimized to store the plug-in electricity as efficiently as possible. The flywheel SPU then completely protects the battery from the shock loads of acceleration and braking, ensuring maximum battery life, and allowing optimally efficient discharge.

Almost every vehicle with a manual transmission is already fitted with a flywheel to smooth the flow of power from the engine and to provide a small store of energy to help prevent stalling on launch. Millions of toy cars are made each year which use a small flywheel geared up to spin fast enough to provide spectacular scale performance, to the delight of millions of small children, and quite a few adults too.

Engineers are now taking the geared high-speed flywheel concept and applying it to full-sized cars, trucks and buses. The prize is an SPU efficiency of at least 60%, with the possibility of 80% or more with further development. The result is a further dramatic improvement in fuel economy, at lower cost, without sacrificing acceleration.

Chris Ellis is Chief Engineer of the PowerBeam Company.


  • krishnadas

    According to me, it is duty of all responsible automobile design engineers to concentrate on following urgent fields of attention rather than on fancy vehicles and SUVs:

    1. The hybrid technology should be combined with even public transport buses which require perpetual braking and spend long time in accelerating after every stopping. Using a powerful engine reduces the mileage drastically.

    2. A four wheeler having ~ 8HP 4S petrol engine,flywheel+battery powered Hybrid, mostly composite structural construction, cruising at Max 70 kmph, with full computerization for power transfers and to carry small family in city is my dream.

    I am an aerospace Engr, middle aged and from a 3rd world country. So I think miserly about spending on fossil fuel. I am sorry if my thinking irritates new generation youngsters.

  • Gardner

    This doesn’t offend me in the slightest. I live in America and people such as myself were born and raised in an environment that encourages over-consumption and wasteage of our natural resources, while poisoning the environment. It’s pretty sad.
    Current vehicle hybridization tech is in its infancy, composed of only 2 technologies: ICE and EV with regenerative breaking.
    In my view, a truly efficient hybrid vehicle would add Flywheel energy storage, solar, hydraulic assist, as well as thermal recuperation. Biodiesel technology should be further developed, which would completely eliminate the need for the use of oil-based technology.
    It’ll take the coming together of multiple disciplines to produce a truly efficient and capable hybrid vehicle. It’s my belief this will probably take another 20yrs, depending on the level of commitment and investment in R&D. Of course, with the acceleration of global warming and disasters such as the one by BP in the Gulf, maybe that will help accelerate this technology. One can only hope.

  • ac delco flywheel

    You made some good points there. I did a search on the topic and found most people will agree with your blog. This idea will not store energy for a long time. My guess is about an hour at most. It will not help with a cold start on a Monday morning but by God, in a road car it will save a lot of gas pulling away from traffic lights.

  • Raisenbread
  • Gregg Weber

    Seeing the garbage truck stop and start reminded me of flywheel busses years ago. So I looked on the Internet and found this.
    I suspect that garbage pick up would be the most useful and efficient first use of this method. The biggest problem is scale. The mass of a full garbage truck vs. a SUV or other passenger car. If this can be solved by larger and/or many flywheels I don’t know. But apparently it was done on a bus many years ago.

  • eddie chong

    Back in the mid 50′s as a school boy I read a popular magazine that featured an article that spoke up for “Flywheel Hybrids” especially for British buses. That was almost 60yrs ago when fuel cost was not much of a problem. However, in the post-war years people were conscious of where money was spent and saving it wherever it could be done is more important than cutting on quality of service or quantity of goods sold.
    As a Social Observer, I note that the human spirit is perpetually
    bogged down with things it needs to do on one hand and on the other hand, the companion spectre which holds back and sternly deters any action.
    Then in the 60′s, as an executive with a British oil marketing company, I discovered that the company was deeply into Solar Energy research! This contradiction is now cleared as I found out recently that this Petroleum company is now a world leader in Solar energy technology.
    Mankind can learn from this lesson. 60 years can make a new technology base of great significance. Nobody heeded the early warnings of Global Warming 60 year ago either.

  • KevinM

    Will it save as much gas in pulling away from those stoplights as it costs to add the extra weight to the car all the time?

    Also – what happens in an accident? How would a flywheel behave then?

  • soessex

    Battery type hybrids are poor at saving energy; and, advancements in battery technology are most likely terminally limited. Flywheel technology is here now and power-flow is shock free and as smooth as though from an electric motor; see below. Here-to-fore the problem with flywheels has been complex and inefficient power management to and from the flywheel. This patent completely solves this problem with maximum possible efficiency (85 -95%).

    PRESS RELEASE
    January 3, 2012
    Reference: US Patent 7,931,107 B2
    VEHICLE KINETIC ENERGY UTILIZATION TRANSMISSION SYSTEM. (KERS)

    This recent patent enables the reduction of fuel consumption in motor vehicles by the storage of kinetic energy for reuse. This technology incorporates an infinitely variable transmission (IVT) in the form of an eddy current induction device (called a Modulator) coupled to a gear system to conquer the torque flow management problem caused by infinitely varying bi-directional energy flow between a moving vehicle mass and an associated rotating flywheel mass created by the fact that the respective mass velocities move in an inverse acceleration relationship.

    To illustrate this phenomenon, observe that as kinetic energy passes from the moving vehicle to, and is captured by, the flywheel it is caused to accelerate, however the vehicle is consequently caused to slow; but to function efficiently, the flywheel requires an ever increasing input-speed factor from a source which is ever slowing. This always changing speed dichotomy can only be effectively managed by an infinitely variable transmission, and, other than that offered by the above patent, none have been successful for the subject purpose.

    The technology reflected in this patent involves very few parts, and is therefore economical to manufacture. It is in addition, long lived, requires little maintenance, and is very durable. Importantly, this system is suitable not only for passenger car use, but also for delivery vans, trucks, and buses.

    The conservation of kinetic energy through the use of battery energy-storage technology is exceedingly inefficient while such a mechanical approach is well known to be very high in efficiency. As may be realized, existing battery hybrid technology was developed because it was a way around this, now solved, torque-management problem. As these complicated and costly battery-related electric energy arrangements only avoid, and do not solve this problem, the penalty for this has been the great loss of efficiency as compared to a mechanical storage system such as that proposed by the subject patent.

    Thank you,
    South Essex Engineering

  • southgate

    Flywheel energy is a very good idea , smaller lighter cars and small generators that can be used at time of low battery if necessary, people have to get places and not all the world is connected with congested roadways and slow buses ,I was discussing this technology with an engineer 20 years ago. they thought it could not be done WRONG ,, It is the way to go for all sizes of vehicles , and keep the trucks lighter and the trailers smaller , and set speed limits that are reasonable according to the size of the vehicles 100 for anything 3000 kilos ans heavier and 110 for 2000 – 3000 kilos and 120 for anything lighter , this would do a lot to save fuel..
    Time traffic lights in cities to encourage a steady state flow of traffic
    I am a automotive tech who continually gets very good mileage in all my vehicles with special tuning ,m exhaust and aerodynamic improvements. My next project I would like to do is a vehicle with an ICE in the front ,an electric with flywheel in the back for all wheel drive and 2 modes of power.