Report: Smaller-Displacement Engines May Not Deliver Expected Economy Gains
Testing Shows Significant Variance Between Rated and Real-World MPG
Downsized, and often turbocharged, engines are all the rage lately. Automakers have adopted them on a large scale in the quest to satisfy ever-higher government fuel-economy mandates, while also delivering the amount of power and performance that customers have become accustomed to. The official window-sticker ratings make it appear that this new technology has managed to successfully balance the competing attributes of performance and economy. However, a report released by U.K.-based Emissions Analytics shows significant real-world variances between rated and real-world economy in downsized engines.
The variance is most pronounced in smaller-displacement European-market cars, where sub-1L engines performed as much as 36 percent poorer than their official economy ratings. Relatively speaking, larger-displacement engines performed better, with 5.0L and larger engines only showing a -1 percent variance compared to official ratings in the U.K. test. The report says variances for U.S. market models were much smaller, with the EPA test being revised in 2008 to better reflect real-world use with higher test speeds and greater use of commonly used accessories and features such as air conditioning. Ironically, among U.S. models, 5.0L to 6.0L engines showed the highest variance at -3 percent.
The contributing factors identified in the report for the poorer-than-expected performance is more aggressive throttle application in smaller engines for acceleration, resulting in higher loads on the engine and higher RPMs, both of which affect fuel consumption. Turbocharged engines under load can consume significantly higher quantities of fuel than under light-load operation, when their smaller displacement gives them a hypothetical efficiency advantage.
Source: Emissions Analytics