The media invitation came at the last minute: the
next day I was invited to the Mount Cotton driver training complex near Brisbane
to view – and ride in – the GM Allison hybrid diesel electric bus. It was one of
the few media invites that lobs into my in-box that really got my attention,
especially when I did some cursory web research to find that more than 400 of
these buses are already roaming the streets of US cities.
So what makes these buses so interesting?
Well, firstly, the hybrid part of the bus is
almost a standalone unit. It comprises a very special transmission (which
contains two electric motor/generators), a battery pack, two AC/DC inverters and
the control electronics. And that’s all.
So?
Well, the transmission can be bolted to any
commercial diesel engine that runs an SAE J1939 communication protocol for its
diesel engine management system! The control electronics, inverters and battery
pack can be distributed almost as you please around the rest of the vehicle and
then, hey presto, you have a diesel electric hybrid.
That’s a radical approach in the time of the
Honda, Toyota, Ford and GM hybrids which are either very ‘soft’ (say running
just a small combined starter/alternator unit) or alternatively, use a dedicated
internal combustion engine that’s often pretty weird in one aspect or another
(eg an Atkinson cycle design). In fact, the left-hand drive demo bus was a
Canadian-built New Flyer equipped with a 209kW Cummins diesel - but the
manufacturers of the chassis, engine and body could all have been other
companies!
The other really tricky aspect of the hybrid
system is something that has major implications for upcoming hybrid cars from
GM, DaimlerChrysler and BMW. (All three manufacturers have signed
an agreement to share this technology. Well, you gotta do something – even get
into bed with erstwhile enemies – when you’re just on a decade behind Toyota in
your hybrid technology!) The transmission comprises two electric
motor/generators (or, more precisely, motor/alternators), no less than three
epicyclic geartrains, and two auto trans style clutches. So how do all these
combine?
The answer to that is incredibly complex (if you
really want to struggle through the detail, go to US patent 5931757, available
from the search page at http://www.uspto.gov/patft/index.html)
but in short, the system operates as what GM Allison call a “compound split
parallel drive unit”.
The key point is that at two different road speeds
(in the bus they were 40 and 100 km/h) the transmission acts as a direct
mechanical drive, giving highest efficiency. These speeds are called the
‘mechanical points’ of the drive system – it’s no coincidence that they match
the most common speeds of urban and country driven buses, respectively. Away
from those speeds, the electric motor/alternators are spinning, so there’s a
flow of electric power in or out of the transmission. For example, off the line
the so-called electric launch mode uses one alternator/motor as an alternator
and the other as a motor, giving a heap of electric torque when moving away from
a standstill. The change in mode occurs only once in the time the vehicle takes
to move from nought to about 100 km/h (probably to be a much higher speed in car
applications) with the mode change activated by the hydraulic clutch packs
releasing or stopping components of the planetary drive system.
So the trans works as an infinitely variable ratio
drive unit blending battery power, regenerative power (gained in braking) and
engine power. But depending on the software approach adopted, the system can
work in dozens of different modes, all predicated around the two ‘mechanical
points’ at which efficiency is highest. (Don’t forget that of all the juice you
put into a battery, you generally get only half of it back out again. So any
hybrid system that’s generating electricity for later retrieval is immediately
dropping a lot in potential efficiency. The trick is to be a mechanical trans
except when you need all that electric torque in drive or braking!)
The bus system uses two 75kW electric
motor/alternators, a roof-mounted nominally 600V NiMH battery pack manufactured
by Panasonic (who also make the Prius batteries), two 160kW AC/DC oil-cooled
inverters and two ECUs. The trans containing the three planetary geartrains and
the two electric motors is sized similarly to a conventional heavy duty
truck/bus trans and weighs 428kg.
Driving the Bus
About a dozen people arrived at the media day –
however, it appeared all but me were state government employees assessing the
bus from a perspective of government purchase. The bus was slowly driven around
the Mount Cotton ‘country road’ circuit, which has hills, straights and corners.
It’s nothing like a race track – much more like a real road. From the passenger
seat there wasn’t a helluva lot different to normal – it sounded and felt pretty
much like a smoothly driven bus. But then the opportunity came to drive the
vehicle – and that was absolutely startling.
Drive is selected by a pushbutton on a panel
(apparently it’s the same panel as used with other electronically-controlled
Allison transmissions); there is zero indication provided to the driver that
they are driving a hybrid. No high voltage battery level, no showing of regen
braking power flows or the amount of electric assist. (One of the two US
engineers suggested that in fact municipal authorities in the US actively sought
a near blank instrument panel for their drivers...) A traditional push-cock
deactivates the air brakes and then it’s as simple as putting your foot down on
the throttle.
Whooaaa!
You want torque, I hear? OK, well this thing has
TORQUE.
Absolutely effortless, smooth torque off the line.
The bus has a mass of about 18 tonnes but it doesn’t feel like it. Not when
moving away from a standstill, anyway. But if the wall of torque is impressive,
wait until you hear what the engineers have to say.
“We’ve got the power of the driveline turned right
down,” they say. “This one’s really in economy mode.”
So what’s it like in full power mode?
“Oh”, they smile, “then you can just smoke up the
wheels!”
But even more impressive is the braking. If you
haven’t experienced regen braking, you’re missing out. Especially when it’s
calibrated so incredibly well that it’s smooth, powerful and progressive. And
all activated just by a lift of the throttle. Yes, rather than do a Prius (where
the throttle-lift regen is so small it just feels like the decel of an ordinary
car), the Allison approach is to give you plenty of braking with a lifted foot.
How much then? Well, the engineers suggest 0.48g is available! When you consider
that a car at full emergency braking pulls about 1g of decel, you can see that
nearly half of that amount is a helluva lot.
But again the engineers just smile when I comment
on the power of the braking.
“Oh”, they casually say, “if we want to, we can
calibrate it so that the wheels will lock under regen!”
Waft off the line to 20 km/h or so than work the
diesel a bit harder to get to 50 or 60 km/h. Then just lift the throttle. The
decel is a giant hand wielded with a lightness and deftness of touch that is
amazing. Listen hard during decel and you can hear the two motor/alternators
altering speed up and down (one goes each way) as the control system juggles
regen rates. Then, when you’re nearly stopped, the regen switches off. (It’s a
legal requirement in some US jurisdictions that the vehicle moves forward if the
foot is not on the friction brake.)
The thought of this much torque and this much
regen deceleration in a car is awesome. But what if you don’t want to slow down
so fast? Easy – just don’t lift your foot so far.
Results
In service the hybrid buses are claimed to reduce
emissions of particulates, hydrocarbons and carbon monoxide by up to 90 per
cent, and of oxides of nitrogen by 50 per cent. Fuel consumption improvements of
20 – 40 per cent are being recorded, with the hybrid buses already having
travelled nearly 23 million kilometres. The hybrid parts of the driveline cost a
cool US$160,000, but in the context of a whole bus that’s probably not
excessive.
Conclusion
The Allison diesel electric hybrid is one of the
most impressive vehicles I have driven. The effortlessness of the initial
acceleration and the giant regen braking are both characteristics that could be
immediately effected in cars. The use of these two attributes in stop/start
urban driving (think not only buses but also garbage trucks and delivery
vehicles) make a lot of sense, while the two-mode transmission with its high
efficiency mechanical points has the potential to lift the hybrid bar a long way
in cars. The first cars with the technology of the system are scheduled for release in 2007...
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