-- Don't knock it until you try it.

A very simplified way to look at General Motors Corp.'s HCCI engine is to think of it as the old engine knocking. Remember turning off your car and listening to the engine still running in that clunk, clunk kind of way?

Instead of using a sparkplug to ignite the fuel, the heat trapped in the cylinder and pressure created by the piston pushing up would ignite the remaining fuel. It was never a pleasant experience as it sputtered and coughed.

For decades, GM has been working on ways to capitalize on many of the inherent efficiencies from compression ignition. Now, it's realized many of those advantages in a 2.2-liter EcoTec four-cylinder HCCI engine.

GM estimates that the homogenous combustion compression ignition engine can create a 15 percent increase in fuel efficiencies. These improvements come from a variety of things such as direct injection, variable valve timing and other features that create a more efficient engine. The homogenous combustion also helps by using compression to ignite all of the fuel at once instead of a typical spark-ignited combustion, which burns the fuel from the starting point (the spark) out.

Not only is the HCCI engine more efficient, it stays cooler, which prohibits the formation of NOx and releases less CO2. According to the EPA, NOx can create ground-level ozone, which can cause serious respiratory problems and contribute to global warming, acid rain and other environmental concerns.

During my short test drive of a Saturn Aura with the 2.2-liter HCCI engine, it behaved much like a regular gasoline engine. The only time it tended to sound different was when the engine transitioned from HCCI to regular spark ignition, when the engine would rumble like a diesel.

Not to sound too Bill Nye, but, the reason for the sound was the engine adjusting back to the traditional spark ignition. The system tries to change from one combustion method to the other in a single cycle. If an engine is running at 2,000 rpm, a cycle takes about 1/16 th of a second. Sensors in each cylinder measure the pressure inside and adjust things like fuel mixture as quickly as possible.

While there was a noticeable sound difference, nothing else seemed impacted by the transition. The vehicle never lost torque and continued to power through, whether accelerating aggressively or driving a typical around-town pace.

In fact, the system was pleasantly boring. Acceleration was decent and the car drove like a typical 4-cylinder Saturn Aura.

While there's still much work to do on HCCI before it makes it to a production engine, there are some significant advantages that need consideration. The lack of NOx emissions is one, as well as the improvements to city driving. Additionally, GM has developed the HCCI system to run from idle to about 3,000 rpm. That gives the engine a top speed of 60 mph before it changes over to regular spark ignition, which doesn't have the same efficiencies and emits more pollution.

Some of the drawbacks for the HCCI engine include it will only work on lighter-load vehicles. While the engine may act like a diesel, it does not produce that low rpm torque diesels generate. For most drivers, I don't think this is a concern.

People use cars to move around, not to move big heavy stuff. And if someone needs the power, there are other vehicles available. While no one would say when an HCCI engine would arrive in a production vehicle, it's something that should arrive sooner rather than later.

Uwe Grebe, GM's executive director for GM powertrain advanced engineering, said it would happen in the coming decade, citing competitive constraints on more specific timing.

The cost to consumers should be about half of what it would cost to buy a diesel engine.

It's a cost well worth paying, and the sooner GM can get this system in cars, the better.