The alleged experts claim that the engine Nicolaus Otto invented in 1876 is running out of gas. They’re wrong. The beauty of Otto’s internal-combustion gasoline engine was its unmatched developmental potential. Superior means of pumping the working fluids, lighting the fires, and twisting the crank keep arriving to sustain the Otto engine’s vitality. The latest breakthrough holding fuel cells at bay is cold combustion.
Japanese researchers investigating ways to clean up two-stroke engines stumbled across this alternative form of combustion in 1979. Since then, practically every major manufacturer who’s picked up the cause has added a pet name. Honda called it Activated Radical Combustion. Mercedes-Benz’s moniker is DiesOtto. Volkswagen calls it Gasoline Compression Ignition. Most engineers favor the clunky Homogeneous Charge Compression Ignition descriptor. The nickname we prefer, cold combustion, was coined by professor Dennis Assanis, head of the University of Michigan’s Automotive Research Center.
Nuances of diesel and gasoline engines peacefully coexist in cold combustion. Like a diesel, no spark is needed to trigger ignition. Like a gas engine, the fuel is ordinary, regular-grade gasoline, although bio-fuels and hydrogen are other possibilities. The key benefit with cold combustion is higher efficiency: fifteen percent better mileage than the best gasoline engines without the more expensive fuel or the more complicated fuel injection and emissions controls necessary with diesels.
The innovation that helped Otto’s engine surpass every existing form of internal combustion was adding a compression stroke. Likewise, cold combustion depends on that phase of the four-stroke process when the piston rises in the cylinder to ready a carefully prepared fuel and air mixture for spontaneous ignition.
Ironically, heat is cold combustion’s crucial ingredient. Closing the exhaust valve early during the exhaust stroke traps a portion of the previous combustion cycle’s burned gases in the cylinder. This residual exhaust heats the fresh charge of fuel and air. When just the right pressure and temperature conditions are achieved at the end of the compression stroke, the mixture self-ignites. The entire charge lights off in an instant – diesel-like – without the usual flame front sweeping through a gasoline engine’s combustion chamber.
Efficiency gains are due to three factors. Both burn time and peak temperature are reduced, so the heat lost to the cooling system is lower. The mix of fuel and air is leaner (less fuel, more air) than what’s necessary with spark ignition. And holding the throttle wide open during the intake stroke greatly diminishes pumping losses. A major side benefit is that peak combustion temperatures are too low for NOx formation.
Cold combustion requires no drastically different hardware. The pistons, blocks, heads, cranks, and catalytic converters already in production work just fine. Engines do require direct fuel injection and a fast-responding valvetrain with variable timing. A cylinder-pressure sensor is also needed to monitor what happens during one cycle so that fuel delivery and valve operation can be adjusted for the next. The speed and authority of the engine control computer must be increased as well.
The chief hindrance keeping cold combustion in the lab is that it doesn’t work in all driving regimes. The conditions necessary for automatic ignition cannot be achieved immediately after a cold start or when the accelerator is floored to extract the engine’s full power. When this technology arrives in five to eight years, it’s most likely to be in a dual-mode application with cold combustion for cruising and hot combustion used the rest of the time. Another possibility is a hybrid configuration with the engine running only with cold combustion and electric propulsion picking up the slack.