NYT: Rubbing Out Friction in the Push for Mileage

Messages
616
Location
Texas
October 14, 2011 Rubbing Out Friction in the Push for Mileage By PAUL STENQUIST Detroit WITH more than three decades of intensive engineering efforts devoted to the cause, automakers have depleted the store of cheap and easy solutions to improving fuel economy. Yet with federal mileage standards rising to 50 miles per gallon and above in coming years, the pressure to do better is only increasing. That is why reducing friction inside engines is becoming crucial. “We have to hit every system on the engine that slides or rotates,” said Mike Anderson, General Motors’ global chief engineer and program manager for 4-cylinder engines. “We have to go after all of these interfaces.” Friction reduction is proving to be an effective pursuit in the struggle to improve engine efficiency, complementing innovations like direct fuel injection, variable valve timing, turbocharging and cylinder-shutdown systems. In recent years, all have contributed to the development of engines that produce more power from less fuel. 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. 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 E.P.A. highway rating to 28 m.p.g. from 25 for the previous version. Among the improvements were measures that cut engine friction by 4 percent, good for 0.15 m.p.g. — a small but worthwhile gain, said Paul DeHart, senior powertrain engineer for the Odyssey. Of all the power-robbing parts in an engine, Mr. DeHart said, the piston assembly was the source of the most significant losses. As the piston moves in the cylinder bore, the piston rings press against the cylinder wall, providing a seal for combustion gases and keeping lubricating oil from being burned. All the while, the piston’s sides, or skirts, slide against the cylinder wall. 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 — 4 to 6 one-thousandths of an inch — 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. For example, Honda has developed a technique it calls plateau honing for the cylinder walls of its engines. Rather than a standard single-step machining process with a honing tool — an abrasive that smoothes the cylinder to the required finish — plateau honing 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, said in a telephone interview that his group was developing a technology that used a laser to burn a honing pattern into cylinders. But the best cylinder-finishing practices are wasted if the bores are subsequently pulled out of round when bolts are tightened during engine assembly or by the expansion of the metal at running temperatures. Powertrain engineers emphasize the importance of designing an engine block that will not exhibit cylinder distortion when in use. Mr. Anderson, the G.M. engineer, said that when designing the engine block for the company’s new 2.5-liter 4-cylinder, engineers did extensive computer modeling of the block’s structure. The modeling included simulations of the conditions that could cause cylinder distortion. “Because thermal and assembly loads handshake with one another, we do coupled analysis,” he said, referring to the practice of doing the two simulations concurrently. “It requires some of the world’s most powerful computers.” With cylinder bores held to tight tolerances and engineered to remain as round as possible in operation, the tension exerted by the piston rings can be reduced, lowering the drag as the piston travels through its stroke. The rings can also be made narrower, reducing their contact area. Further efforts to cut friction losses include shrinking the size of the piston skirts and applying friction-reducing coatings. Pistons in the 1.8-liter engine of the 2012 Honda Civic carry a molybdenum treatment applied in a polka-dot pattern. Such slippery coatings are not just for gas-sippers; they have been used in powerful engines like the 7-liter V-8 of the Chevrolet Corvette Z06. Automakers have also shifted the positioning of the engine’s crankshaft in respect to the cylinder bores to gain an advantage. For a new 1-liter 3-cylinder engine, Ford engineers specified a crankshaft position that was offset, rather than centered, under the cylinders. Ford’s technical leader for gas engine systems, Stephen Russ, said this reduced the force of piston skirts against the cylinder wall, letting the piston slide with less resistance. Oil splashing against the engine’s rotating parts — even in the form of an oil mist — can also create drag that cuts efficiency, so engineers work to minimize lubrication. The flow of oil draining back to the sump from the top end of the engine is carefully routed to keep it away from the spinning crankshaft. Because thick oil requires more power to move, automakers are specifying lighter viscosities. But other steps to keep oil flowing freely are being taken as well. Mr. Anderson noted that G.M. had designed a computer-controlled two-stage cooling-system thermostat for its new 2.5-liter engine. By letting the engine run hotter when conditions are not extreme, the oil remains thinner. Friction savings have been realized on the engine’s top end as well. Many automakers have abandoned once-common bucket-type valve lifters, an arrangement where the lifter is pressed directly by the cam lobe rather than a rocker arm. The sliding motion of the cam across the lifter is a source of considerable friction; today, roller-tipped cam followers are becoming the norm, as they generate less friction. Automakers that have retained the bucket-type valve lifter fight friction with hard and slippery coatings. Nissan, for instance, uses a coating called Diamond-Like Carbon on piston rings, piston pins and cam followers. The hard film binds well with engine oil and, according to Nissan, reduces overall engine friction by 25 percent. 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 m.p.g. world. How successful has that effort been? It’s difficult to quantify the gains achieved by each technology because the components and systems interrelate. For example, more uniform cylinder bores enable the use of low-tension piston rings and tighter tolerances. Reduced clearance allows the use of lighter-grade oil. Putting a percentage number on any single change is difficult. However, General Motors has quantified the overall friction reduction for three of its 4-cylinder engines. The 2007 2.4-liter 4-cylinder generated 46 percent less friction in low-speed driving than G.M.’s 2-liter 4-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 4-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.
 
Messages
14,505
Location
Top of Virginia
Neat. In a few Toyota inline engines, they use an offset crankshaft. The crankshaft is offset a bit to one side, toward the downstroke side of the piston, so on the powerstroke, there's less sliding pressure on the cylinder wall from the piston. Others may be doing this as well. I, too, would like to hear more about GM's new 2.5L engine. 2.5L seems to be the "new" 4-cylinder displacement. Toyota and Ford/Mazda have 'em. Nissan has one. Now GM will have one as well. I guess "2.4" is out the door.
 
Messages
11,196
Location
NY Capital District
Originally Posted By: Hokiefyd
Neat. In a few Toyota inline engines, they use an offset crankshaft. The crankshaft is offset a bit to one side, toward the downstroke side of the piston, so on the powerstroke, there's less sliding pressure on the cylinder wall from the piston. Others may be doing this as well. I, too, would like to hear more about GM's new 2.5L engine. 2.5L seems to be the "new" 4-cylinder displacement. Toyota and Ford/Mazda have 'em. Nissan has one. Now GM will have one as well. I guess "2.4" is out the door.
Get on the floor, walk the dinosaur?
 
Messages
8,576
Location
Ohio
It is interesting but some of it is also kind of silly at the same time. They'll spend all this money (or I should say you as the car buyer and maintainer will) to increase gas milage sometimes .1 mpg. Meanwhile, engine size and power is being increased eg 2.4 to 2.5L. On a societal fleet scale it looks good, but for the individul's bottomline it's not a good deal. To get to 50 mpg average it is going to be necessary to go to smaller cars with smaller turbo 4 or 3 cylinders.
 
Originally Posted By: hattaresguy
They need to concentrate on drive trains more. Transmissions suck power, and AWD is a big powersoak.
They cannot do that because it would mean going back to basics, i.e light weight, simple drive trains and small displacement engines. Public does not want cars like that, they want all the toys and options. Those suburban warriors truly need their all wheel drive minivans and CUVs to haul around their statistical 1.5 kids LOL
 
Messages
1,746
Location
Rochester, NY
Originally Posted By: KrisZ
They cannot do that because it would mean going back to basics, i.e light weight, simple drive trains and small displacement engines. Public does not want cars like that, they want all the toys and options. Those suburban warriors truly need their all wheel drive minivans and CUVs to haul around their statistical 1.5 kids LOL
Automakers are certainly going toward lighter weight drivetrains and lower-displacement engines. Ford Explorer is now FWD by default. The F-150 has two V6 engines available, and the 3.5 Ecoboost is very popular. Chevrolet's compact entry has moved gone from having a 2.2L/2.4L engine to a 1.8L/1.4L turbo engine. The new Sonata and Malibu only offer 4-cylinder engines. I'm sure there are other examples too.
 
Messages
1,746
Location
Rochester, NY
Originally Posted By: hattaresguy
They need to concentrate on drive trains more. Transmissions suck power, and AWD is a big powersoak.
They are definitely looking at drivetrains. 6-speed autos are common now, and 8-speeds are coming in. Ford, VAG, and probably others have automatic transmissions that use clutches instead of torque converters. You're right about AWD, but except for Subaru, AWD is usually optional, so people are free to make the tradeoff for themselves. I'm sure there are some gains to be had with AWD systems though.
 
Messages
19,686
Location
Sunny Florida
Mercedes has been using a computer controlled clutch plate stack instead of a torque converter for years in their AMG models. Combined with a high efficiency design like that newer ZF 8 speed you get great gains over any regular slushbox. Ford and others are quite late to this party. AWD is a power sucker, but the safety benefits outweigh the small efficiency loss, IMO.
 
Messages
14,505
Location
Top of Virginia
Today's AWD systems don't carry near the weight penalty that conventional part-time 4x4 systems carry, and are generally very efficient at what they do. And for most uses and purposes for which they're used, they're nearly as effective. For instance, our 2008 Honda CR-V is rated 20/26 city/highway, and ours is the "AWD" version, or what Honda calls Real-Time 4WD. The FWD 2008 CR-V, with exactly the same engine, same tires, same ride height, etc, is rated 20/27 city/highway. So you can see the fuel economy penalty for the AWD system is statistically very small (even with the margin of vehicle-to-vehicle variability), and the benefits gained are many...depending of course on where you live and how you intend to use the vehicle.
 

Astro14

$100 Site Donor
Staff member
Messages
13,378
Location
Virginia Beach
Still - there are a lot of folks buying more car (bigger, heavier or AWD) than they can really use. AWD is just the most obvious/egregious example...I see folks all the time who have 4WDs that have never, and I mean never, been off road...so...why have it? That 4WD Explorer isn't any better in the rain...and it's not really any better in the snow (people here crash because they can't stop, not because they can't go) but it's heavier and sucks gas as a result... Sigh...
 
Messages
8,576
Location
Ohio
True they are looking at improving engine and transmission efficiency but still power keeps going up and vehicle weight and to some degree size is still increasing. Vehicle weight is a biggie, they are being made heavier and stiffer for a quiet and rattle free ride, better handling and for crash safety. From a gas milage stand point vehicle weight is counterproductive and expensive to get increased gas milage although I can't say it is a bad trade off. A car's chassis integrity is almost everything and I do like a solid, quiet ride that holds up over the life of the car. Increased safety is a plus too. Those cheap 80's gas milage kings often would get pretty rattly after a few miles.
 
Messages
10,908
Location
Jupiter, Florida
Interesting information. Of course, engineers have had a clear understanding of internal frictional losses for a very long time. Piston ring swept area has always been a major contributor to friction. The Lycoming aircraft engines designed in the 40's have large cylinders, direct drive and low RPM in an attempt at reducing internal friction. The result is an engine with far lower fuel burn than most modern engines today! (per HP produced). Same goes for cooling system losses. Air cooled engines run hotter and can, at times transfer less heat to the cooling system, resulting in more efficient use of the combustion energy.
 
Top