Green Engine Technology - Alternative Fuel
 |
GM's EV1, the first mass produced electric passenger car
in the
world. It uses lead-acid batteries. |
Background
You might not know,
electic
cars appeared in the 19th Century just like motor cars, but they failed
to become popular due to many technical and practical reasons. For
example,
the battery was very heavy, stored little energy and took too much time
to recharge. As a result, electric cars recieved far less development
than
motor cars.
In the late 80s,
California
legislated a Zero Emission Regulation which requires large car makers
to
sell a certain percentage of ZEV (Zero Emission Vehicle), probably 10%
as I remember, before the year 2000 or they will be banned from the
state.
This regulation, although later postponed due to the inmature
technology
developed, pushed many car makers to accelerate their development of
electric
cars.
Battery
There are currently 3
kinds
of battery being used. Lead-acid is the most conventional one. Its main
advantages are cheap and highly recyclable, but poor in energy
efficiency
(i.e., generates less power per kg of weight) and takes a full night to
recharge. GM EV1 electric car is installed with 500 kg of such
batteries
!!
Another battery is
Nickel-Metal
Hydride ( Ni-MH ), currently being used by Honda EV Plus and many
others.
It is one time more efficient than Lead-acid, that means it can double
the range of the car or reduce the battery weight by half while
maintaining
mileage. Besides, it takes shorter time to recharge, and last longer.
Any
disadvantage? Yes, high price.
The latest newcomer
is Lithium-ion
battery, which was developed by Sony and has been installed in Nissan
Altra
EV. It is even more efficient than Ni-MH, even more durable and even
quicker
to recharge. Disadvantage is still high price and difficult to be
recycled.
|
Battery
Type
|
Energy
Density
(Wh
/ kg)
|
Specific
Power
(W
/ kg)
|
Recharge
time
(hours)
|
Life
(no.
of charge)
|
Energy
efficiency
|
|
Lead-acid
|
30
|
130
|
8
|
400
|
65%
|
|
Ni-MH
|
80
|
250
|
< 6
|
600
|
90%
|
|
Lithium-ion
|
100
|
300
|
< 3
|
1200
|
?
|
Motor
Most EVs use
traditional
D.C. brush motors. Two motors, one drive the right front wheels and
another
drive the left one, provides the power for the whole car. D.C. motors
are
cheap, but cannot provide sufficient power that a really fast EV
needed.
Therefore GM EV1 adopted a complicated 3-phase A.C. motor, which is
supplied
by an invertor which transforms D.C. supply to A.C. Since the motor is
induction motor, it has no friction that a d.c. brush generates,
therefore
it could be a lot more powerful.
All EVs do not need a
transmission.
The flat torque characteristic of electric motor eliminate the need for
gearing. Reverse gear is also saved because it can be simulated by
reversing
the polarity of the motor input.
Another special
feature of
EV appears during braking. Physical principles tell us that while
rotating
a motor by external force, the motor will become a generator. EVs make
good use of this principle to recharge its batteries during braking.
Electric cars, are they really green ?
In the foreseeable
future,
electric cars will still be inferior to conventional motor cars, no
matter
in performance, in price, in running cost (battery replacement is very
expensive) and in convinience (charging takes time). If Governments
insist
to promote electric cars, they must answer the following questions:
- Is there any
feasible and cost-effective
plan to recycle millions tons of old batteries ?
- At the same time
when EV is
developing, how much improvement in emission conventional motor cars
can
make?
- What about other
alternative
fuel technology, especially are Fuel Cell or Hybrid cars ?
Hybrid Cars - Toyota Prius
as an
example
Toyota
Prius is the first hybrid-powered production car in the world. It is
claimed
to cut pollutants by 90%, fuel comsumption by 50% and CO2
emission by 50%.
Prius' hybrid system
consists
of a 1.5-litre lean burn engine and an A.C. induction electric motor.
They
are so compact that they are mated in-line, mounted transversely in the
front and drive the front wheels like ordinary FWD cars. They can power
the car individually as well as simultaneously. The transition is
smoothly
implemented by means of planetary gears located between them.
The fuel tank is
unusually
small, since Prius drinks 50% less fuel than conventional cars.
Electric
motor is supplied by the 40 pieces of Ni-MH (Nickel-Metal Hydride)
batteries
located above the rear axle. The batteries weigh a lot less than pure
electric
cars because of the help from petrol engine. Moreover, they will be
automatically
recharged by the engine when electricity level is low.
Operation
STARTING : Powered by
electric
motor only. Prius does not employ the engine during starting, because
starting
is a heavy load action which greatly increases the emission pollutant.
Moreover, this arrangement also benefits cold start emission,
because
the electric pre-heated catalyst has sufficient time to heat up before
the engine intervene.
ACCELERATION : When
the car
gets up to speed, the engine joins and provides power together with the
electric motor. The engine provides the neccessary power that pure
electric
motor cannot provide. On the other hand, electric motor help easing the
load taken by the engine, so emission level remains low.
STEADY SPEED : Still
engine
+ electric motor. However, under light load, the engine will be
switched
off.
BRAKING / DOWNHILL :
This
is the most important advantage of hybrid car. Conventional car will
eliminate
the kinetic energy by braking, that means transferring to heat loss.
Prius
will make good use of the kinetic energy to recharge its batteries
through
electric motor (now act as generator), and by the way generate braking
force. This double the mileage.
Honda Insight
The
world’s second hybrid power production car is Honda Insight. Starting
from
December 1999, this 2-seater "Sports car" will be sold in United States
for just $20,000.
The exterior shape
reminds
me the late CRX, however, although being also a 2-seater the Insight is
not sporty at all. It can’t match GM EV1 electric car’s 0-60 mph time
of
7.9 sec, let alone the late little Honda rocket. The marketing people
of
US Honda didn’t supply any data about performance or power, but from
Honda’s
Japanese headquarters I know this car weighs about 800 kg, with a
maximum
78 horsepower generated from the hybrid power unit, thus I estimated 10
sec for 0-60 and a top speed of 105 mph without deliberately limited.
The
IMA hybrid power unit consists of a 1.0-litre 3-cylinder engine and a
DC
brushless motor. The former, just like the ULEV Accord engine,
incorporated
4-valve per cylinder and a version of VTEC designed to give lower
emission
rather than higher power. The VTEC gives the 2 intake valves different
timing and lift, thus create swirl to the intake air flow hence a
perfect
pulverization of fuel.
The DC brushless
motor is
so compact that it actually acts as the flywheel of the engine. The
thickness
of the disc-shaped motor is just 60 mm, so combining the 3-cylinder
engine
they take no more space than a 4-cylinder. However, the electric motor
alone is not powerful enough to pull the whole car (unlike Toyota’s
Prius)
even though Insight is already a very light car. Here Honda sacrifice
the
name of ZEV (Zero Emission Vehicle) and pursuit a lighter motor and
smaller
battery pack. In other words, the engine always takes the
responsibility
of pulling the car while the electric motor just adds more punch or
recharge
the battery during braking.
|
Power and torque curves of the IMA unit.
Black curves : petrol engine only.
Green curves : petrol engine + electric motor.
|
Weight saving mostly
come
from the smaller Ni-MH battery (just 20 kg, versus GM EV1’s 500 kg !),
the impractical 2-seat layout and small dimensions and the use of light
weight material. When you know the Insight has an aluminium space frame
chassis (comprising of extrusion, stamped and die-cast parts),
aluminium
body panels, plastic front and rear fenders and even magnesium oil pan,
you may doubt that the car will be profitable as claimed earlier.
If it can’t earn
money, if
it can’t be a real ZEV as Prius, can’t match Prius’s accommodation and
practicality, at least it should out-run other cars. Nevertheless,
Honda
claims a fuel consumption of 70 mpg, which seems not specially
impressive
today. A Volkswagen Lupo TDi achieves 94 mpg even though it is made in
conventional way.
A
fuel
cell is an electrochemical device that produces electricity silently,
without
combustion. Hydrogen fuel, which can be obtained from natural gas or
methanol,
and oxygen from the air are electrochemically combined in the fuel cell
to produce electricity. Heat and pure water vapor are the only
by-products.
Canada's Ballard
Power System
Inc. is currently the leader in Fuel Cell technology. Not only produced
many Fuel Cell buses servicing in Europe, it also won Mercedes-Benz and
Ford's contract to develop fuel cell power system for their passenger
cars.
Mercedes invested $450M in Ballard and plan to produce 100,000 fuel
cell
cars in 2005. Ford also invested $420 and hopes to sell its fuel cell
cars
starting from 2004. It seems that Fuel Cell technology will be no
longer
a research topic like other alternative fuel technology.
Proton
Exchange Membrane (PEM) Fuel Cell
PEM fuel cell
consists of
a polymer membrane layer and 2 flow field plates on either side of the
layer. The membrane layer is coated by electrodes on both side. When
methanol
flows through the fuel field plate, hydrogen in which will be
catalytically
dissociates into proton (positive hydrogen ion), the free electron will
be absorbed in the + electrode as useable current (which provides the
power
the electric motor need). The proton migrates though the membrane layer
to the other side and react with the oxygen in air flow, the result is
pure water and heat.
The water vapor is
normally
in 85 degrees Celius. It is emitted through "exhaust pipe" without
causing
any pollution and green house effect. The sub-product, heat, can be
water-cooled
easily.
A single fuel cell
generates
little power, so many fuel cells must be stacked together to provide
the
amount of electrical power required.
Mercedes NECAR 4
 The
NECAR 4 is the fourth experimental fuel cell car Mercedes-Benz created.
Unlike all its predecessors, it is virtually production ready and has
practicality
like any conventional cars because it is based on the A-class. Only the
high cost of the fuel cell power unit - some £12,000 versus
piston
engine’s £1,900 - prevent it from going into production.
It is nonsense to
say NECAR
4 performs as good as ordinary cars, although it is already the most
advanced
fuel cell car ever appeared. The 75 hp fuel cell stack plus all the
accessories
like electric motor and high-pressure fuel tank put some 410 kg over
the
slowest petrol A class, yet the A140 output 7 more horsepower than the
NECAR 4. Although the electric motor has constant torque at any rpm, it
still fails to compensate the weight penalty - considering it weighs as
heavy as a well-specified E320. Top speed barely reaches 90 mph, or 15
mph lower than A140.
More questions
about the
fuel supply should be raised. Fuel cells can drink hydrogen as well as
methanol (see its theory in the above). The former is the more
favourable
as it generates only pure water during the reaction, hence no air
pollution
at all to our cities. However, the highly explosive liquid hydrogen
should
be stored in a strong, high-pressure tank cooling at minus 230ºC,
thus arouse concerns about safety. In particular, collision from behind
will hit right on the fuel tank.
Methanol doesn’t
make the
car a ZEV, but it is cheaper to produce (from waste) and safe to store.
It generates CO2 30% less
than
petrol cars and without all other unwanted pollutants. Fuel consumption
is 77 mpg, versus Hydrogen’s 88 mpg. This isn’t too remarkable compare
with Volkswagen’s 94 mpg Lupo TDi. The company is planning to offer
both
versions.
 The
A-class is a perfect platform for the fuel cell. Thanks to its sandwich
structure, the large and heavy fuel cells can be installed under the
floor,
not only engage no cabin space but also help improving A-class’
roll-resistance.
The electric motor and fuel tank are positioned near front and rear
axle
respectively. However, to make mass production really possible in 2004,
the development team is working closely with Canada’s Ballard Power
System
Inc., the supplier of those fuel cells, to take another 160 kg out of
the
car while increase power to 94 hp. They should also bring down the
production
cost and reaction time dramatically (now 2 min to start the car from
cold).
All these mean
the fuel cell
cars have so many inherent limitation and are unlikely to rival
conventional
cars in the foreseeable future, let alone sports cars. However, the
continue
tightening of emission regulations all over the world guarantees the
future
of these cars in a long term basis.
|
Copyright©
1998-2000 by Mark Wan
AutoZine
Technical School
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