New fuel system allows V6 engine to run leaner, improving fuel economy.

Every tiny bit counts as automakers attempt to squeeze the most fuel efficiency possible out of engines. To that end, Mercedes-Benz will launch a technology called stratified direct fuel injection this year, which should provide a small gain in economy.

Bart Herring, Mercedes-Benz USA general manager for product development, says that the stratified system gives engine designers much more accurate timing of the injection spray. Compared to standard direct injection, which generally sprays fuel during the engine’s intake stroke, the stratified system will allow the engine to inject fuel during the compression stroke – almost like a diesel engine.

“The injection is actually happening on the upstroke,” he says.

This increased fuel-injection precision means Mercedes can run much leaner fuel mixtures at low-load situations, Herring says, squeezing out a bit more fuel economy with, “basically no sacrifice in power.”

There’s one major downside to running these ultra-lean mixtures: Increased emissions of nitrogen oxides (NOx), a major pollutant. As a result, the Mercedes stratified injection engines will need special NOx traps – not unlike the particulate filters on a diesel car – to help reduce harmful emissions. Mercedes has already said it will start equipping some gasoline engines with particulate filters from 2017. Otherwise, the traditional direct injection and stratified injection systems “are very similar,” with changes mostly limited to unique injectors and new software.

The technology will launch on a V6 engine when Mercedes refreshes a vehicle with that engine before the end of the year, though Herring wouldn’t offer any more specifics about the application. Horsepower and torque ratings for the engine will be unchanged after the upgrade, but its fuel economy should rise by a small amount.

The big enabler for stratified direct injection is the more widespread of ultra-low sulfur gasoline in the U.S. Starting in 2017, all gasoline sold in the country must have a much lower sulfur content than before (from 30 parts per million to 10 parts per million), which Herring says is key to being able to use the more precise, later injection strategy. Until that switch happens, “the fuel’s the biggest problem in America.”

Gallery: Mercedes-Benz launching stratified direct injection engine