The math seems so straightforward: as engines get smaller and fuel economy improves, the cost of driving will fall, right?
Not necessarily. In a growing number of new vehicles, the methods used to squeeze equivalent horsepower from engines with smaller -- or fewer -- cylinders have automakers pointing customers toward the pump that dispenses premium-grade gasoline. That's a setback for drivers who hoped their new car's better mileage rating would mean savings with each fill-up.
It's also a reminder that the label of premium, meant to signal the gasoline's octane rating -- its ability to forestall the destructive effects of erratic combustion -- also applies to the price. According to the Energy Department, premium averages 30 cents a gallon more than regular-grade gas, or $4.50 for each 15-gallon fill-up.
It's an extra expense that Micah Highland, a 35-year-old photographer in La Grange, Ill., knows well. Last year, he bought a 2008 Acura MDX, a sport utility roomy enough for his wife, daughters and Rosco, their Great Dane.
Mr. Highland was aware of the MDX's fuel requirements before he made the purchase. Still, when the dealer handed him the keys and reminded him to fill up with premium gas, he thought to himself, "Oh, man. Are you serious?"
Mr. Highland, whose previous S.U.V. also required premium, said he considered vehicles that did not require the more costly fuel, but after seeing the MDX, "fuel wasn't a factor." Filling the tank costs about $80; today's MDX still requires premium, according to Edmunds.com, a Web site that offers car-buying guidance.
In the last 10 years, the number of models that require a fill-up with premium unleaded gasoline has actually fallen slightly, says Edmunds.com. Ten years ago, 21 percent of all models required premium, which in the United States typically carries an octane rating of 91 or higher; today, the number is 17 percent.
But the number of vehicles for which automakers recommend premium -- guidance typically found in the owner's manual or on the fuel-filler door -- is growing. Premium was recommended for only 2.5 percent of all models in 2003, says Edmunds.com; for 2013 models, it's 12 percent.
"What we are seeing now is a higher penetration of smaller engines that are developing more power and torque," Bill Visnic, a senior editor at Edmunds.com, said. "As you get to the higher-output engines, manufacturers are probably going to at least recommend premium fuel, if not require it."
Filling the tank with regular-grade gas when premium fuel is recommended has drawbacks. Horsepower, torque and fuel economy may suffer, though modern engine-management electronics prevent internal damage. Sensors in current models detect knock, or detonation in the cylinders, that was heard in older cars as "pinging," a distinctive rattle that let drivers know when the gas didn't meet the engine's needs.
The premium label does not always carry over to vehicles whose diet calls for the more expensive gas. While there's little surprise in a BMW or Mercedes having a preference for high-octane blends, 2013 models whose makers recommend premium include economy-focused vehicles like the Fiat 500, Mini Cooper and Nissan Juke, as well as some versions of the Dodge Dart, Subaru Impreza and Volkswagen Beetle -- even the Chevrolet Volt.
The higher pump price of premium can have a wet-blanket effect on sales of vehicles in the lower price classes. Mazda learned this lesson with its CX-7 compact crossover, introduced in 2007. Originally equipped with a 2.3-liter turbocharged engine that required premium fuel, sales lagged despite the boom in crossovers.
"What we found was that there was either a rejection of the car when people were shopping, or it proved to be a customer satisfaction problem if they discovered that the running costs were higher," said Ruben Archilla, a manager for advanced engineering at Mazda North America.
The automaker modified the engine to make premium gas recommended rather than required, which sacrificed power and fuel economy, but it was too late. By the time Mazda rolled out a 2.5-liter nonturbo engine that ran on regular gasoline, consumers were already shopping elsewhere.
The CX-7 was discontinued last year to make way for the more fuel-efficient 2013 CX-5, which takes regular fuel and wrings performance out of its SkyActiv engine using a high 13:1 compression ratio.
"The whole premium-fuel question is not a problem if you are BMW, but it's a big issue if you are trying to sell cars that are in the compact car range," he said.
Not wanting to risk the loss of customers to other makes, engine designers at high-volume brands are stepping up efforts to squeeze more power from downsized gasoline engines that will run on regular grade.
"There has always been a desire by the chief engineers to keep the consumer fuel cost as low as possible and to design the vehicles around regular unleaded," Bill Studzinski, a powertrain fuels specialist at General Motors, said.
That has become increasingly challenging, because as engine sizes are shrinking, cylinder pressures are rising, either as a result of using higher compression ratios -- squeezing the air and fuel tighter during the combustion process -- or through the addition of turbochargers or superchargers. Each method can have the effect of increasing the likelihood of engine knock.
At the same time, consumers want small engines to perform like big ones.
"As we continue to reduce the displacement of the engine, the customers' expectations of the performance of the vehicle aren't changing," Doug Skorupski, technical strategy manager at Volkswagen of America, said. "So you have to run higher and higher boost pressures from your turbochargers, and as you do that, higher octane plays into it."
Engineers have several tools to prevent engine knock, but there's no cure-all. Knock sensors, which can signal the engine-control computer to delay the ignition spark when detonation is detected, inevitably cut back performance. Owners of cars that carry a premium-fuel recommendation can safely operate their vehicles with regular unleaded fuel; however, the engine will lose roughly five horsepower and one to two m.p.g., Mr. Skorupski said.
"You are reducing your fuel economy and your performance, but there is no detrimental damage to the engine," he said.
Another increasingly common technology, direct fuel injection, sprays gasoline into the cylinder at high pressure; the cooling effect of the atomized gasoline helps to keep knock in check. In effect, direct injection tricks the engine into thinking it is running a higher-octane gas -- up to four to six octane numbers, Rick Davis, a combustion specialist at G.M., said.
Putting regular-grade fuel in a car for which premium is required, rather than just recommended, is far riskier, experts say. "Under extreme driving conditions, the engines aren't necessarily protected from degradation or damage if somebody used regular fuel," Mr. Davis said.
Fluctuations in the price gap between regular and premium gasoline will continue to confound consumers.
Late last year and through the first quarter of this year, the average price difference between premium and regular unleaded gasoline in the United States rose to more than 30 cents per gallon, according to the Energy Information Administration. In some locales, the gap is closer to 40 cents.
Fuel experts attribute this growing spread partly to the increased use of ethanol, which acts as an octane booster, while others say that ethanol blending has actually kept premium prices lower than they otherwise might be. Ethanol, which has an octane rating of 115, is blended at a 10 percent concentration into nearly all of the gasoline available in the United States.
"On a percentage basis, the price spread between premium-grade and regular-grade gasoline has actually remained relatively stable," said a May 6 report from the Energy Information Administration.
Sensitivity to the extra cost of premium gas is less of a concern, predictably, for drivers of luxury-class vehicles. Patrick DeLisle, a 36-year-old business analyst in Indianapolis, says he spends an extra $3 each time he fills his 2009 BMW X3.
"It's a cup of coffee," Mr. DeLisle reasons. "If I drove more, then I would just not drink coffee outside my house. You save $3 a day just brewing it yourself."autonews
This article originally appeared in The New York Times.