; Nama
Documents
Resources
Learning Center
Upload
Plans & pricing Sign in
Sign Out
Your Federal Quarterly Tax Payments are due April 15th Get Help Now >>

Nama

VIEWS: 18 PAGES: 3

  • pg 1
									Nama : Yohana Barbara

NIM      ;20030130107

                                      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,     One potential design: Each flywheel rotor is approximately two feet
                                                long and runs inside a casing which forms part of the 'spine' of the
the surge power for acceleration comes          car. The rotors are geared together to contra-rotate at identical
mainly from batteries. Imagine a car            speeds, to cancel out external gyroscopic moments. Each rotor is
                                                connected to a Continuously Varible Transmission (CVT), in turn
approaching a red traffic light. The driver     connected to a conventional differential and driveshafts. The
touches the brake pedal gently, and the car     transverse front engine runs on E85 or gasoline and drives through its
                                                own conventional transmission. The picture shows an AWD layout;
eases to a stop. In a conventional vehicle, all the FWD equivalent of a Prius would have the rear differential and
                                                CVT deleted.
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.

								
To top
;