Super-efficient engines ditch the spark
NEW YORK (CNNMoney.com) -- Car companies are working to perfect a new
generation of gasoline engines that could get up to 20% better fuel
efficiency. The system, which burns gasoline without spark plugs, relies
in massive amounts of speed and power - the computer kind - to work.
It's called Homogenous Charge Compression Ignition, or HCCI. General
Motors (GM, Fortune 500) has been allowing journalists to drive a test
vehicle with this technology. It clearly isn't quite ready for the mass
market, but GM insists that it's finally in the home stretch after years
"You're basically getting the best of gas and diesel all together," said
Kelly Mundt, director of gasoline system marketing for auto parts
supplier Bosch, which is working on some components needed for this new
That means more fuel efficiency, like a diesel, but with cleaner
exhaust. But it's not easy to get a gasoline engine to behave this way.
This technique for burning gasoline requires loads of on-board computing
power and the ability to control to the tiniest degree everything that
goes on inside the engine in real-time. The ideas behind HCCI have been
around for decades, but the parts have only come together in the last
In normal gasoline engines, the injector squirts fuel into the cylinder
as the piston rises, and at just the right moment, a spark ignites the fuel.
Diesel engines compress air inside a cylinder until it's hot enough to
ignite fuel. At just the right time, a fuel injector squirts in a spray
of diesel fuel. It burns immediately in the hot air, pushing the piston
In an HCCI engine, the injector squirts fuel while the piston is on the
way up, like a regular gasoline engine. But the fuel doesn't need a
spark. It combusts from pressure and heat alone, like a diesel engine.
Now here's the tricky part: getting the fuel to burn at just the right
time without a spark or fuel injection to trigger it.
To set the stage, the engine's exhaust valve closes early, trapping some
hot exhaust gas in the cylinder. A series of small jets of fuel squirt
into the cylinder, raising the temperature and pressure to a critical
point. When the time is right, the fuel ignites. The combustion doesn't
start at one point in the cylinder and move outward, like it does in
regular gasoline or diesel engines. It's a key reason HCCI is so efficient.
All this is tricky enough in a stationary engine humming along at a
constant rate inside. But keeping it all going under the constantly
changing demands of real driving is especially hard.
HCCI engines still have spark plugs to use in situations where HCCI just
isn't very efficient. At high speeds, HCCI doesn't work well, so when a
car is accelerating hard to merge onto a highway, the sparks plugs take
Where HCCI fits
During a recent test drive in a specially prepared Saturn Aura sedan
with an HCCI engine, it was clear that the technology wasn't quite ready
for the consumer market.
First of all, it's noisy. For those used to a quiet gasoline engine or
an equally noiseless modern diesel engine, the popping and cracking
noise from the Aura's HCCI engine would be simply unacceptable.
That noise doesn't mean the system isn't working well, GM engineers
insist. It's just an unpleasant byproduct that will diminish with
further tweaking and more sound insulation, said GM engineer Paul Najt.
More work is also needed on "transitions." During normal driving, the
engine has to switch over to regular spark-ignition then back to HCCI.
In the test car, the change was noticeable.
For it to become economical, engineers still need to push HCCI's
performance range farther while making engines behave and sound more
like regular gasoline engines. For now, HCCI won't work at highway speeds.
But it could make a lot of sense in a gas-electric hybrid car. With the
help of an electric motor, an HCCI engine could spend more time in its
optimal operating range, said GM engineer Vijay Ramappan.
"It's not something you'll see next year," said Hakan Yilmaz, manager of
advanced systems engineering for Bosch.
With gas prices topping $4 a gallon, though, the incentive to bring it
all together has never been greater.