Last week in
Alternative Cars Part 5 - Steam
we looked at the possibility of cars powered by steam. This week, we look at
hybrid cars.
Unlike most of the other types of cars covered in
this series, hybrid cars – those that mix battery electric power with
conventional internal combustion engines – already exist. The Toyota Prius is
the best known hybrid, but nearly every manufacturer has hybrids in development.
So are hybrids the way of the future?
Hybrid cars attract widely – and wildly –
differing claims as to their merit. Rather like the rotary engine story of the
1970s, some see them as a panacea to the ills affecting conventional engines
while others see them as quite pointless. Again rather like the rotary, the
on-going success of hybrids will depend on how manufacturers choose to the use
of the technology, and whether or not alternative automotive technologies
overcome the advantages that hybrids currently have.
Function
A hybrid car uses a driveline that consists of a
largely conventional internal combustion engine, one or two electric
motor/generators, and a battery pack. In the more sophisticated hybrids, the
electric motor/generator and the internal combustion engine (ICE) are linked by
a special transmission that allows the wheels to be driven by electric power
alone, ICE alone or a stepless combination of the two power sources.
The battery pack of current production hybrids
consists of a large number of nickel metal hydride cells in series, providing a
high voltage but relatively small total capacity. Electronically commutated (ie
brushless) electric motor designs are used, with the high power switching
circuitry sometimes cooled by a fluid system separate to the ICE’s cooling
system.
Electronic control determines the mix of electric
and ICE power used in any given driving situation. Control strategies take into
account aspects such as driver throttle input, high voltage battery level, road
speed and emissions requirements. Nearly all hybrids drive as two pedal
automatic transmission cars.
The high voltage battery is charged by
regenerative braking (where braking energy that would otherwise be wasted in the
brakes is returned to the battery pack as charge current) and if this is
insufficient, by direct charging via the motor/generator being powered by the
ICE. No mainstream commercially sold hybrid allows external mains-powered
charging. (People often wonder why this is so – the answer is easy: the battery
capacity in current hybrids is too small to make this a worthwhile
strategy.)
When stationary and/or slowing for a stop, hybrids
turn off the ICE. Automatic re-starting occurs through the use of one of the
motor/generators acting as a very high power starting motor. To run the
air-conditioning compressor, power steering and brakes when the ICE is not
operating, electric assistance is used for each of these systems.
So-called mild hybrids employ smaller battery and
electric motor/generator capacities. In these cars, the electric motor cannot
power the car; it can act only as an ICE assist. Mild hybrids still use
regenerative braking and turn the ICE off when the car is stationary. Converting
an existing car into a mild hybrid is a far easier and cheaper design exercise
than building a full hybrid from scratch.
Advantages
In city traffic, hybrids achieve excellent fuel
economy and very low emissions outputs. This is especially the case in
stop-start traffic, where for much of the time the hybrid will use electric
power alone. In fact, no other production cars can match hybrids for economy and
emissions in slow city conditions. With the ICE off, the hybrid is also smooth
and quiet.
For their total driveline power output, hybrids
accelerate quickly from a standstill. This is largely because of the
characteristic strong low-speed torque curve of the electric motor.
The magnitude of hybrid advantage depends very
much on the manufacturer’s approach to the use of the technology. For example,
fitting a mild hybrid driveline to a large SUV gives advantages in fuel economy
over conventionally-powered SUVs but in absolute terms, the mild hybrid SUV
still achieves poor fuel economy and emissions.
Disadvantages
When used for open road, relatively high speed
driving, most hybrids return fuel economy and emissions that are only a little
better than the very best of conventional cars. However, note that this is
depends on the design of the hybrid system and car – the 1999 hybrid Honda
Insight has world’s best open-road fuel economy.
The high voltage battery pack has a life that is
likely to be substantially shorter than the rest of the car. The pack, that has
a high replacement cost, is not currently being factored by manufacturers into
the consumer’s lifecycle cost of the car.
Solution
Hybrid technology is currently undergoing a major
change in direction. The first modern hybrids (‘modern’, because hybrid
petrol/electric cars existed in the 1920s!) were the Toyota NHW10 Prius and the
Honda Insight. Both cars were purpose-designed hybrids that were also very
efficient cars in their own right – even if they’d been fitted with conventional
drivelines. The design of the NHW20 Prius (the current, very successful model)
built upon the philosophy of the original Prius.
However, manufacturers - including pioneers Toyota
and Honda - are now frequently fitting much more powerful hybrid drivelines to
otherwise conventional cars. These cars achieve performance similar, or in some
cases even better, than ICE-powered equivalent cars but have relatively poor
absolute fuel economy and emissions figures.
The advantages of hybrid drivelines remain,
irrespective of the size and/or power of the car. However, it is confusing to
the public when a hybrid car has fuel economy that is simply nothing special. To
convince the public that a hybrid car is worthy of purchase not because of clear
fuel economy and emissions benefits, but because of its
performance/economy/weight compromises, is to try to establish a whole new
paradigm in the minds of buyers.
Honda has not found great success with its Civic
and Accord hybrids (and its Insight was too expensive), and Toyota’s success
with its high-powered Lexus hybrid models has so far been limited. We will be
surprised if the hybrid SUVs being manufactured by US car makers prove to be a
major success.
It is rumoured that Toyota will capitalise on the
success of their Prius brand-name by releasing three different Prius models
aimed at different markets, including performance, load carrying and pure
economy/emissions cars. Unless each car has exceptional fuel economy, this
strategy risks diluting the Prius brand – a brand that is currently the envy of
the world’s carmakers.
Manufacturers need to focus again on environmental
and consumer cost efficiencies in their hybrid models – aerodynamics, rolling
resistance, ICE design, battery technology and battery lifecycle costs.
The Prius, a hugely brave and innovative car, has
created a new word in the automotive lexicon. But what the world is now waiting
for is the next step – 10 years after the release of the first Prius, a new car
that uses the best of existing and new technologies to achieve a fuel economy,
emissions and total lifecycle consumer cost that is superior to anything on
offer. Such a car would use an aluminium body, have a drag co-efficient close to
0.25, have good ride and handling, 5-star crash testing safety, be under 1200kg,
would carry four adults comfortably and achieve city and country economy of
around 4 litres/100km. In addition, the battery pack would have a nominated,
nominal replacement cost for the life of the car.
Conclusion
When they were released, auto makers outside of
Japan dismissed the Prius and the Insight as irrelevancies. Now hybrids are
appearing thick and fast. However, there still remain important steps to be
taken if hybrids are to be a long-term success.
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