• Why don’t pint-size cars get better economy?

    Chevrolet Spark
    Date:24 March 2012 Tags:, , ,

    Logically, you’d think that the main reason to drive a car as small as Chevrolet’s 3,6-metre Spark would be: (a) that it’s cheap and (b) that it gets great fuel economy. You’d be right about the first part, but a little off the mark about the latter.

    The Spark’s official figures at highway speed are slightly lower than that of other larger cars in the line-up. Go up one size class to the Chevy Sonic/Aveo and you can get 5,9 litres/100 km on the highway. Another step up yields 5,8 litres/100 km in the Cruze Eco. This scenario is not unique to Chevy. The Fiat 500, for instance, gets 6,2 litres/100 km on the highway – again, higher than in significantly larger cars.

    Many aspects affect fuel economy. The biggies are engine size and efficiency, vehicle weight and aerodynamic drag. How large a role each plays depends on how you drive. On the national roads, aero drag is the dominant player, and it turns out that a vehicle’s length has a huge effect on drag. And counter-intuitively, short cars have higher drag than longer ones.

    “The Spark’s short length impacts the drag in two main ways,” explains GM’s small-car development manager, Dan Molnar. “First, the grille and windscreen are more upright to allow greater passenger space, and second, there’s simply not enough length to guide air around the car smoothly.”

    The drag occurs at both ends: the air hitting the front of the car, and the vacuum created as the turbulent airflow spills off the car’s rear. Longer cars may weigh more, but they can better approximate a teardrop shape, resulting in slightly less resistance as they travel, which helps highway economy. Engineers attempt to reduce drag on shorter cars with various body features, such as smaller wheel openings and sometimes a rear lip spoiler, but there’s only so much that can be done. The Spark’s drag coeffcient is a few percentage points higher than the Sonic/Aveo’s.

    So why drive small? Tiny cars weigh less, so in stop-go city driving they require a lot less fuel to get around. Where many 6 litres/100 km cars have much worse city consumption (Hyundai’s Accent gets 7,7), the puny brigade typically fares far better. Every car design is a compromise. The trick for those looking to save fuel is, first, to figure out where you do most of your driving, and then to choose the car that best matches your profile. – Larry Webster

     

    The Spark’s windscreen angle is about 33 degrees, closer to vertical than most larger cars. Although that design yields more interior space, it also increases aerodynamic drag and reduces highway fuel economy.

    Turbulence at the back of the car causes a low-pressure zone that pulls the car rearward. More turbulence means greater drag, which the engine has to work harder to overcome.

     



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