To boost fuel efficiency, car engines are benefiting from innovations such as direct fuel injection, turbocharging and cylinder-shutdown systems. But to squeeze every mpg they can get, automakers are also working to reduce friction inside the engines.
"We have to hit every system on the engine that slides or rotates," says Mike Anderson, General Motors' global chief engineer and program manager for four-cylinder engines. "We have to go after all of these interfaces."
Internal-combustion engines are by nature high-friction devices. A multitude of mechanical parts turn or rub in metal-to-metal contact, separated only by a thin oil film. Friction nibbles away at engine power, producing heat that is lost to the atmosphere.
- Other efforts to cut friction losses:
- Designing engine blocks that will not exhibit cylinder distortion when in use.
- Shrinking the size of the piston skirts and applying friction-reducing coatings.
- Shifting the positioning of the engine's crankshaft in respect to the cylinder bores.
- Working to minimize lubrication, as oil splashing against the engine's rotating parts can create drag.
- Specifying lighter viscosities of oil, as thick oil requires more power to move.
Limiting those losses is a goal of every automaker, with huge efforts directed toward tiny returns. For instance, the 3.5-liter V-6 engine of the 2011 Honda Odyssey benefits from a suite of advances that improves the minivan's EPA highway rating to 28 mpg from 25 for the previous version. Among the improvements were measures that cut engine friction by 4 percent, good for 0.15 mpg — a small but worthwhile gain, says Paul DeHart, senior powertrain engineer for the Odyssey.
Of all the power-robbing parts in an engine, DeHart says, the piston assembly was the source of the most significant losses. Considerable progress is being made in reducing the friction losses resulting from piston movement. Mercedes-Benz, for example, has developed a slippery cylinder-coating technology it calls Nanoslide, first used in 2006 on the 6.3-liter AMG V-8.
In the Nanoslide coating process, the cylinder walls are sprayed with an ultrathin layer of a molten iron-carbon alloy. A special finishing process puts a smooth surface on this extremely hard coating, at the same time opening tiny pockets in the metal that retain oil for lubrication. In the diesel V-6 engine of the ML350 BlueTEC model, Mercedes says, Nanoslide has reduced fuel consumption by 3 percent.
Other automakers have taken different approaches to producing cylinder surfaces that are as smooth and wear-resistant as possible.
- Automakers are making other efforts to cut fuel usage in small ways that add up:
- Replacing spare tires with lighter tire-inflator kits
- Making cars more aerodynamic
- Employing start-stop technology
For example, Honda has developed a technique it calls plateau honing for the cylinder walls of its engines, which uses two stages of grinding to produce a surface that is ultrasmooth yet leaves a pattern of very fine grooves to hold oil.
Even that may soon be outdated. The director of advanced powertrains at Chrysler, Chris Cowland, says his group is developing a technology that uses a laser to burn a honing pattern into cylinders.
Other techniques, including smaller crankshaft bearings and straighter camshaft chain drives, are being applied in the effort to keep gasoline engines alive in a 50-mpg world.
How successful has that effort been? It's difficult to quantify the gains achieved by each technology because the components and systems interrelate.
However, General Motors has quantified the overall friction reduction for three of its four-cylinder engines. The 2007 2.4-liter four-cylinder generated 46 percent less friction in low-speed driving than GM's 2-liter four-cylinder of the early 1980s, despite having more valves, a more complex camshaft drive and a pair of counterbalancer shafts. Friction reduction alone resulted in about 7 percent better fuel economy over that 24-year period.
In the new 2.5-liter four-cylinder that will power the 2013 Chevrolet Malibu, friction has been cut another 16 percent, resulting in a 2 percent engine efficiency gain compared with the 2007 2.4-liter.
Each engine developed more power per liter than its predecessor, doing more with less.