Start-Stop and Turbocharged Engines: What They Do for MPG

Stop-start systems switch the engine off when a vehicle is stopped and restart it when you are ready to move; a turbocharger forces extra air into the engine so a smaller engine can deliver the power of a larger one. Both are efficiency technologies that show up on many EPA-rated vehicles. Neither changes how the label is calculated, but both can affect the number it shows.

Key takeaways

• Stop-start saves fuel by avoiding idling. fueleconomy.gov lists a potential efficiency improvement of 2%, and the benefit is concentrated where a vehicle would otherwise idle.
• Turbocharging with engine downsizing has a potential efficiency improvement of up to 8%, according to fueleconomy.gov.
• Turbocharged engines are one of the cases where higher octane may be required; check the manual.
• A technology’s benefit depends on how you drive. Look at the vehicle’s own rating rather than the feature list.

How stop-start works

fueleconomy.gov explains that stop/start technology conserves energy by shutting off the gasoline engine when the vehicle is at rest, such as at a traffic light, and automatically restarting it when the driver pushes the gas pedal to go forward. An electric starter-generator and a battery handle the restart. fueleconomy.gov adds that stop/start hybrids are not true hybrids, since electricity from the battery is not used to propel the vehicle; those are different from the full hybrids in hybrid vs plug-in hybrid vs EV, which can move the car with the motor.

The benefit scales with how often you would otherwise idle. fueleconomy.gov says idling can use a quarter to a half gallon of fuel per hour depending on engine size and air conditioning use. A system that cuts that waste at lights and in traffic helps most in the city and least on an open highway. fueleconomy.gov lists a potential efficiency improvement of 2% for stop-start systems, against 3% to 6% for mild hybrids and 27% to 35% for full hybrids.

How turbocharging and downsizing work

A turbocharger is a compressor that forces additional air into the engine’s cylinders, allowing more fuel to be injected as well, which creates more power. That lets an automaker use a smaller engine, which is the “downsizing.” fueleconomy.gov’s example: replacing an 8-cylinder engine with a turbocharged 6-cylinder, or a 6-cylinder with a turbocharged 4-cylinder, can save fuel and still provide the extra power when needed. It rates the potential efficiency improvement at up to 8%.

Other engine technologies fueleconomy.gov lists, with their potential improvements, include cylinder deactivation (up to 5%), valve timing and lift technologies (3% to 4%) and gasoline direct injection (1%). All percentages on that page are cited to a 2015 National Academy of Sciences report, with most values rounded down conservatively. They are potentials, not guarantees for any vehicle.

Does it show up on the label

The EPA tests the finished vehicle, with its actual engine and controls, on the same test cycles as any other; see how the EPA tests fuel economy. If the technology saves fuel in those cycles, the rating reflects that. If it helps most in conditions the cycles only partly capture, such as long idling at traffic lights, the on-road difference can be larger or smaller than the label implies.

For turbocharged engines, the EPA’s 2017 update to its calculations is relevant: it used data from model years 2011 to 2016 so the method better reflects the fleet, which includes technologies such as hybrids and turbocharged engines. The details are in why model year ratings change.

Octane and turbochargers

fueleconomy.gov says higher octane is required for engines that use a higher compression ratio, supercharging or turbocharging. Many turbocharged engines specify premium fuel, and others do not. The difference between required and recommended is covered in regular vs premium gas. Check the owner’s manual, since the fuel cost estimate changes with the grade.

Practical points

• Driving style still dominates. fueleconomy.gov says aggressive driving can lower gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic, a larger swing than the technology gains above. See how to lower your fuel costs.
• Lab and road can differ. Loads, speed and weather change what any engine uses; see real-world MPG vs the EPA label.

Where to go from here

Compare models using EPA ratings in the car database, and look at the vehicle’s full rating rather than one feature.