Combustion engineers mounting a last stand against electric propulsion harbor bizarre fantasies. The gasoline guys dream of the diesel's potent torque curves and exemplary thermal efficiency. Diesel devotees yearn for cheaper fuel and relief from expensive injection systems and complex emissions controls.
But what if both camps collaborated on one super engine combining the best of both technologies? You'd get what Hyundai and Delphi call Gasoline Direct-Injection Compression Ignition (GDCI): a gas engine needing no spark plugs.
Engineers have been studying this alternative for more than a decade. GM and Honda both demonstrated cars powered by homogeneous-charge compression-ignition engines running on gasoline. More recently, Hyundai and Delphi advanced the cause by switching to stratified charge (a rich mixture in part of the cylinder) in a 180-hp 1.8-liter four-cylinder using auto ignition from idle to a 4500-rpm redline. When the study moves out of the lab into two test cars later this year, it should be clear whether the combination of diesel efficiency and gasoline convenience is within reach.
Hyundai's experimental engine—equipped with direct injection, variable valve timing, a turbo, a supercharger, and exhaust-gas recirculation—looks fairly normal on the outside. What's weird are pistons with soup bowls cast into their crowns. With no spark plugs in the way, the injectors can squirt fuel into the exact center of each bowl. GDCI achieves auto ignition by heating intake air with carefully controlled amounts of exhaust gas followed by squeezing the dickens out of the mix with a 14.8:1 compression ratio. This high compression-ratio figure is a key enabler to the GDCI engine's operation. Its 14.8:1 ratio is closer to that of VW's 2.0-liter TDI diesel (16.5:1) than it is to Hyundai's production 1.8-liter gas engine (10.3:1).
Injecting a small dose of gas just before top dead center, and the main fuel squirt just after that point, yields cylinder pressures that rise far more gently than those found in any diesel. This improves efficiency, since combustion pressure is working against a descending piston. Lean fuel-air mixtures, minimal heat lost through the cylinder walls, no throttling, and the large expansion ratio (the flip side of compression ratio) deliver fuel efficiency comparable to a diesel, according to Hyundai's GDCI expert, the suitably named Nayan Engineer. (Mark Sellnau served as Delphi's engineering manager on this project.) Best results derive from minimal swirl in the piston bowl. Fuel-injection pressures are in the gasoline-engine range, or only a fifth of what's required in a diesel, yielding major savings in cost, lower parasitic losses, and quieter operation versus diesels. The supercharger delivers intake air at low speeds and loads when there's insufficient exhaust energy to spin the turbo.
Cleaning up what exits the combustion chambers poses no major hassles. The typical diesel bugaboos—particulates and high NOx emissions—are not an issue because of GDCI's low combustion temperatures. The lean mixtures reduce the standard three-way catalyst's effectiveness, a concern addressed by fitting a second oxidation catalyst to diminish carbon-monoxide and hydrocarbon emissions.
It all adds up to a 10-to-15-percent efficiency improvement without switching to a troublesome fuel. Strides like this will keep the internal-combustion engine eligible for stand-alone or hybrid-propulsion duties for decades to come.