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.