After many years of development and research, General Motors has brought a completely driveable and street-worthy HCCI (Homogeneous Charge Compression Ignition) test mule (working concept vehicle) from the proving grounds of Detroit to the streets of major metropolitan areas like Washington D.C. and greater New York City. I have had a strong interest in, and have been following this somewhat obscure and little understood technology on-and-off for several years. I'd follow it, and then news of its developments would dry up, then it would reappear and I'd follow it some more. Finally, GM took on HCCI development in a serious way, and when I got the chance to be one of the first automotive journalists to drive the Saturn Aura HCCI on the streets of New York, you can bet I made a beeline to the venue.
The HCCI Concept
In a brief nutshell, HCCI is an engine design that falls somewhere between a diesel and a spark ignition gasoline engine. Instead of a rich fuel mixture ignited by a spark plug in an engine's combustion chamber (like almost every gasoline car out there), HCCI uses a super lean (high air-to-fuel ratio) homogeneous gasoline or E85 mixture ignited by compression ignition (heat triggered much like a diesel, but without using diesel fuel). So what's the big deal? Why would GM consider it worthy of investing many millions of dollars of R&D money? Why not just stick with diesels? The answer, friends, is fuel efficiency (up to a 15 percent gain) AND clean emissions--two of the most difficult to achieve (simultaneously) parameters in all of engine design. For a thorough explanation of how HCCI works and its many benefits (as well as some drawbacks), see our detailed article "What is HCCI and how exactly does it work?"
Driving Impressions of the HCCI Saturn Aura
So our HCCI article explains how this idea works, and that's great, but on paper it's not much more than theoretical. It's when the rubber meets the road (literally) that ideas are borne out, and that's the purpose of this article. Take a journey with me as I climb behind the wheel of the Saturn HCCI Aura and push on the GO pedal.
- The Look
- Cold start—
- Merge into traffic—
- Cruise with traffic at moderate speed—
- Stomp on the gas—
On the exterior, aside from the splashy graphics (GM really does want the attention), the HCCI Saturn Aura looks every bit the part of the run-of-the-mill Aura sedan. On the inside, it was pretty much the same except for the engineers' laptop computer plugged into the engine's computer and the HCCI feedback display mounted on the dash. (Don't look for these options when the car hits production).
As with diesels, cold starts require a bit of special treatment for HCCI engines--it's a function of heat. When cold, compression ignition engines need an initial heat source. Diesels supply initial startup heat with glowplugs, whereas HCCIs use traditional spark plugs for cold fire. My test Saturn HCCI mule was reasonably warm from previous runs that morning, but it did still initially start-up in spark mode--and then stayed there for a minute or so during idle. After that, it automatically switched to HCCI mode (as evidenced by the operation mode display) . When that happened, I did notice a slight change in the engine's timbre, an ever so faint diesel-like clack just after switchover.
I was fortunate enough to ride with two of the main engineers dedicated to GM's development of the HCCI engine: Vijay Ramappan, Senior Project Engineer, and Dr. Matthias Alt, HCCI Program Manager. I peppered these guys with questions as I drove, while forcing myself to pay simultaneous attention to the HCCI system as I put it through its paces. By the time I pulled out into traffic, the engine was fully in HCCI mode and it accelerated quickly and smoothly into the fold with other vehicles without one bit of spark ignition assistance. I reminded myself that this was gasoline burning inside this engine, not diesel fuel--that ignition was coming from compression heat, not a spark plug. All of my mechanical instincts told me that this wasn't possible--to burn gasoline without a spark--that this wasn't supposed to be able to happen, at least not so smoothly and effortlessly.
As we cruised along, Vijay kept pointing at the operation display and his laptop while explaining to me the sundry HCCI control adjustments that were happening nearly instantaneously. Throughout the cruise, fuel delivery pulses were ever fluctuating (more fuel, less fuel), the variable valve lift dimensions continuously changed (a little more valve lift, a little less valve lift) and electromechanical cam phasers rotated back and forth among all manner of early-open, late-close and late open-early close modes to keep the engine's valves (and subsequent cylinder pressure) in perfect harmony with whatever load and speed requirements prevailed at the moment. These continuous micro adjustments really are the heart and soul of HCCI. Powering a highway-traveling vehicle with its myriad and ever changing load, speed, temperature and atmospheric condition parameters is perhaps the greatest challenge that can be presented to an engine. That probably goes double or triple for the HCCI process. So far, I was smiling away.
Well ya knew I had to do it! Knowing that compression ignition engines aren't the most responsive to droppin'-the-throttle-and-takin'-off, I decided to lay back a bit from the car ahead of me, and then at first chance, nail the gas to see if I could make this complicated system buckle under--or at least stumble. I must admit that I could not, and I was all at once disappointed--and quite impressed. Matthias, Vijay and the development team decided long ago that they'd engineer-in dual mode capability to this package so that it could do diesel-like efficiency and emissions, but still pound out spark ignition-like instant response. When I nailed the Saturn's gas pedal, it took but a few brief nano seconds for the onboard computer to detect a change in engine dynamics and elevated cylinder pressure readings and kick the 2.2-liter 4-banger into spark ignition mode ...
Continued on page 2: Read how the Saturn performed on the highway, during stop/idle, along with Scott's conclusions.