In the interest of making sure we all know they've got a mapped strategy for powertrain technology, GM is taking its message on the road. Recently, we got the chance to talk to a few of GM's experts on their "newest" fuel economy improving technology: Homogeneous Charge Compression Ignition (HCCI). Simply put, it uses diesel technology on a gas engine. From my point of view, anything that makes a gas engine run and sound like a diesel is probably a good thing.
Without getting into too much detail, gasoline engines, which typically run cooler as a result of a lower fuel-mixture ignition temperature use spark plugs to ignite the fuel/air mix that has been pumped, pulled, or pushed into the combustion chambers. In the past, because of the nature of the fuel mix and the relatively unsophisticated nature of engine management computers, as well as the tech behind the plugs themselves, the burn efficiency of most gas motors is not so good, to put it mildly. In fact, that's why diesel engines have been improving by leaps and bounds lately: They're using some of the most sophisticated techniques to get better fuel mixes and smarter injectors taking care of business.
Add to that the fact that diesel engines have always used compression ignition inside the cylinders to ignite the mixture, which is the reason they need much higher compression ratios (typically around 17:1, whereas gas engines are usually somewhere around 9:1). Those particular facts have a lot to do with why diesel engines are typically more massive and heavier than equivalent-size gas engines; they need stronger blocks, heads, and other parts to deal with the bigger forces.
From a power output standpoint, there is no competing with the efficiency of a compression combustion diesel motor, but from a cost view, gas engines with spark ignition are simpler and less complicated. However, that seems to be changing. The theory of HCCI has been around since the beginning of the engine, but it's never really been cost-effective because of the difficulty managing the precise pressures and temperatures in the combustion chamber (in real time), but with fuel prices up and diesel prices leading the parade, GM thought it might be a good time to revisit this technology and see if it might not make more sense now.
The principle is pretty easy: Allow the engine to use compression ignition to burn a more efficient air/fuel mix to get the same amount of power from less fuel, with cleaner emissions out the tailpipe. To meet current NOx and particulate standards, today's diesels have to use expensive, complicated, and heavy after-treatment technology. With HCCI technology on an equivalent gas engine, no emissions catalysts or filters are necessary as long as the gas is clean. In fact, from some of their preliminary testing, the HCCI engines, depending on how they set them up to operate, burn much cleaner. Previous iterations of the HCCI motors had them only working within a restricted rpm range, say between 1700 and 3200 rpm, which meant they only produced benefits in very specific situations. Recently, though, Matthius Alt, lead GM engineer for the current HCCI program, found a way to get HCCI to work off idle, all the way up to 55 mph, or around 3000 rpm. This allows the system to be even more efficient than previously thought. However, we should put this into perspective.
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