Technologue: Driving the ultimate bastard engine

Our beloved gasoline engine is taking heavy flak. Plug-ins and hydrogen fuel cells are gunning for it, pricey new clean diesels are making inroads, and congress is threatening to repeat failed history by cranking up CAFE again. The pressure's so great that mad scientists around the globe are at work on unnatural cross-breading of engines to combine the brawn and efficiency of Rudolph Diesel's brainchild with the cheap, high-revving power of Nicholas Otto's spark-ignition engine. The idea of auto-igniting gasoline without a spark-as in a diesel-has been kicked around in lab engines for decades, but never proved sustainable in the cold, cruel world. Recent combustion-control advancements have made driveable prototypes viable and inspired a new marketing moniker: DiesOtto. At least that's what Mercedes-Benz is calling it. GM still uses the cumbersome handle "homogeneous-charge compression ignition" (HCCI).

At start-up and during acceleration, it's a typical modern spark-ignited engine. At cruising speeds, the exhaust valves close early and the intake valves open late trapping lots of exhaust in the cylinder. The plugs shutoff and the direct injectors squirt one dose of gasoline (or an ethanol mixture) into the trapped exhaust, then another into the intake charge. The whole process is timed so that the cylinder pressure and temperature rise to the point of auto-igniting a lean mixture (two and a half or three times too much air) just as the piston hits the top of its travel. The burn happens spontaneously throughout the cylinder, with no spark-ignited flame front. This all-over burn is 97-98 percent complete and cool-about 900-1000 degrees below the threshold for NOX formation, so no pricey lean-NOX catalyst is required and less waste heat is generated.

It's tricky maintaining such combustion as weather conditions, fuel quality, and a zillion other variables change. GM does it by using a piezo-electric sensor that tracks the pressure rise in each cylinder in every cycle. (Similar sensors will first appear on GM's 2009 Euro-market 2.9-liter V-6 diesel.) By comparing these pressure traces with ideal lab values, the computer twiddles the electric phasing of the intake and exhaust cams within a 100-degree range, swaps between two valve-lift profiles on each cam, and times the central fuel injection-all on a per-cycle basis to achieve that ideal burn.

With about 60 percent of the cylinder filled with exhaust, there's a fairly low limit to the amount of torque that can be produced during HCCI combustion, and in GM's experimental 2.2-liter naturally aspirated Ecotec engines running 12.2:1 compression, 55 mph is about as fast as the car can cruise in HCCI mode. Intake boost or electric hybridization can broaden HCCI's operating envelope, and Mercedes's DiesOtto concept employs both (eroding its cost advantage over diesel).

HCCI-mode fuel savings come primarily from running lean, cool, and unthrottled, and amount to a 15-percent improvement in EPA-combined economy in a Saturn Aura (diesel's advantage is typically 20 percent or a little more). Cost savings relative to diesel include the lack of a turbo or exotic exhaust catalyst, cheaper 2000psi (versus diesel's 20,000 psi) fuel injection, and lighter-duty hardware.

I drove the HCCI Aura and noted the unmistakable ringing sound of a diesel engine when operating in compression-ignition mode. Some transitions between modes prompted head-bobbing torque-interruptions, others were unobtrusive. Driving with an egg on the pedal, I sustained HCCI operation for 70 percent of my three-mile loop. Overall driveability seemed close to production feasibility, but it's still an R&D project, and Powertrain honcho Tom Stephens pegs the likely production date at around 2015. That's plenty of time to smooth it out and dream up a better name-maybe RudNich? Okay, maybe not.