Fuel Economy: Where the Energy Goes
Only about 14%–26% of the energy from the fuel you put in your tank gets used to move your car down the road, depending on the drive cycle. The rest of the energy is lost to engine and driveline inefficiencies or used to power accessories. Therefore, the potential to improve fuel efficiency with advanced technologies is enormous.
City Driving
Energy requirements in this diagram are estimated for stop-and-go city driving using the EPA FTP-75 Test procedure.
Engine Losses
In gasoline-powered vehicles, most of the fuel's energy is lost in the engine, primarily as heat. Smaller amounts of energy are lost through engine friction, pumping air into and out of the engine, and combustion inefficiency.
Advanced technologies such as variable valve timing and lift (VVT&L), turbocharging, direct fuel injection, and cylinder deactivation can be used to reduce these losses.
Diesel engines have inherently lower losses and are generally one-third more efficient than their gasoline counterparts. Recent advances in diesel technologies and fuels are making diesels more attractive.
Drivetrain Losses
Energy is lost in the transmission and other parts of the driveline. Technologies such as automated manual transmissions (AMTs), double-clutch, lock-up transmissions and continuously variable transmissions (CVTs) can reduce these losses.
Parasitic Losses
Power steering, the water pump, and other accessories use energy generated by the engine. Fuel economy improvements up to 1% may be achievable with more efficient alternator systems and power steering pumps.
Power to Wheels
Braking Losses
Any time you use your brakes in a conventional vehicle, energy initially used to overcome inertia and propel the vehicle is lost as heat through friction at the brakes.
Less energy is used to move a lighter vehicle. So less energy is wasted from braking a lighter vehicle. Weight can be reduced by using lightweight materials and lighter-weight technologies.
Hybrids, plug-in hybrids, and electric vehicles use regenerative braking to recover some braking energy that would otherwise be lost.
Wind Resistance (Aerodynamic Drag)
A vehicle expends energy to move air out of the way as it goes down the road—less energy at lower speeds and more as speed increases.
This resistance is directly related to the vehicle's shape and frontal area. Smoother vehicle shapes have already reduced drag significantly, but further reductions of 20%–30% are possible.
Rolling Resistance
Rolling resistance is a resistive force caused by the deformation of a tire as it rolls on a flat surface.
New tire designs and materials can reduce rolling resistance. For cars, a 5%–7% reduction in rolling resistance increases fuel efficiency by 1%, but these improvements must be balanced against traction, durability, and noise.
Idle Losses
A vehicle spends significant time idling in city driving (stop and go traffic), using energy to run the engine and power the water pump, power steering, and other accessories.
Integrated starter/generator (ISG) systems, like those used in hybrids, eliminate idling by turning the engine off when the vehicle comes to a stop and restarting it when the accelerator is pressed.
Highway Driving
Energy requirements in this diagram are estimated for the EPA Highway Fuel Economy Test procedure (highway driving with an average speed of about 48 mph and no intermediate stops).
Engine Losses
In gasoline-powered vehicles, most of the fuel's energy is lost in the engine, primarily as heat. Smaller amounts of energy are lost through engine friction, pumping air into and out of the engine, and combustion inefficiency.
Advanced technologies such as variable valve timing and lift (VVT&L), turbocharging, direct fuel injection, and cylinder deactivation can be used to reduce these losses.
Diesel engines have inherently lower losses and are generally one-third more efficient than their gasoline counterparts. Recent advances in diesel technologies and fuels are making diesels more attractive.
Drivetrain Losses
Energy is lost in the transmission and other parts of the driveline. Technologies such as automated manual transmissions (AMTs), double-clutch, lock-up transmissions and continuously variable transmissions (CVTs) can reduce these losses.
Parasitic Losses
Power steering, the water pump, and other accessories use energy generated by the engine. Fuel economy improvements up to 1% may be achievable with more efficient alternator systems and power steering pumps.
Power to Wheels
Braking Losses
Any time you use your brakes in a conventional vehicle, energy initially used to overcome inertia and propel the vehicle is lost as heat through friction at the brakes.
Less energy is used to move a lighter vehicle. So less energy is wasted from braking a lighter vehicle. Weight can be reduced by using lightweight materials and lighter-weight technologies.
Hybrids, plug-in hybrids, and electric vehicles use regenerative braking to recover some braking energy that would otherwise be lost.
Wind Resistance (Aerodynamic Drag)
A vehicle expends energy to move air out of the way as it goes down the road—less energy at lower speeds and more as speed increases.
This resistance is directly related to the vehicle's shape and frontal area. Smoother vehicle shapes have already reduced drag significantly, but further reductions of 20%–30% are possible.
Rolling Resistance
Rolling resistance is a resistive force caused by the deformation of a tire as it rolls on a flat surface.
New tire designs and materials can reduce rolling resistance. For cars, a 5%–7% reduction in rolling resistance increases fuel efficiency by 1%, but these improvements must be balanced against traction, durability, and noise.
Idle Losses
Highway driving includes little to no idling. The EPA highway driving cycle (HWFET) includes no idling.
Combined City/Highway Driving
Energy requirements in this diagram are estimated for 55% city and 45% highway driving. See the estimates for city and highway driving for more information.
Engine Losses
In gasoline-powered vehicles, most of the fuel's energy is lost in the engine, primarily as heat. Smaller amounts of energy are lost through engine friction, pumping air into and out of the engine, and combustion inefficiency.
Advanced technologies such as variable valve timing and lift (VVT&L), turbocharging, direct fuel injection, and cylinder deactivation can be used to reduce these losses.
Diesel engines have inherently lower losses and are generally one-third more efficient than their gasoline counterparts. Recent advances in diesel technologies and fuels are making diesels more attractive.
Drivetrain Losses
Energy is lost in the transmission and other parts of the driveline. Technologies such as automated manual transmissions (AMTs), double-clutch, lock-up transmissions and continuously variable transmissions (CVTs) can reduce these losses.
Parasitic Losses
Power steering, the water pump, and other accessories use energy generated by the engine. Fuel economy improvements up to 1% may be achievable with more efficient alternator systems and power steering pumps.
Power to Wheels
Braking Losses
Any time you use your brakes in a conventional vehicle, energy initially used to overcome inertia and propel the vehicle is lost as heat through friction at the brakes.
Less energy is used to move a lighter vehicle. So less energy is wasted from braking a lighter vehicle. Weight can be reduced by using lightweight materials and lighter-weight technologies.
Hybrids, plug-in hybrids, and electric vehicles use regenerative braking to recover some braking energy that would otherwise be lost.
Wind Resistance (Aerodynamic Drag)
A vehicle expends energy to move air out of the way as it goes down the road—less energy at lower speeds and more as speed increases.
This resistance is directly related to the vehicle's shape and frontal area. Smoother vehicle shapes have already reduced drag significantly, but further reductions of 20%–30% are possible.
Rolling Resistance
Rolling resistance is a resistive force caused by the deformation of a tire as it rolls on a flat surface.
New tire designs and materials can reduce rolling resistance. For cars, a 5%–7% reduction in rolling resistance increases fuel efficiency by 1%, but these improvements must be balanced against traction, durability, and noise.
Idle Losses
A vehicle spends significant time idling in city driving (stop and go traffic), using energy to run the engine and power the water pump, power steering, and other accessories. Highway driving, however, includes little to no idling.
Integrated starter/generator (ISG) systems, like those used in hybrids, eliminate idling by turning the engine off when the vehicle comes to a stop and restarting it when the accelerator is pressed.
Data Sources
Baglione, M., M. Duty and G. Pannone. 2007. Vehicle System Energy Analysis Methodology and Tool for Determining Vehicle Subsystem Energy Supply and Demand. SAE Technical Paper 2007-01-0398, 2007 SAE World Congress, Detroit, Michigan, April.
Bandivadekar, A., K. Bodek, L. Cheah, C. Evans, T. Groode, J. Heywood, E. Kasseris, M. Kromer and M. Weiss. 2008. On The Road in 2035: Reducing Transportation's Petroleum Consumption and GHG Emissions. MIT Laboratory for Energy and the Environment, Report No. LFEE 2008-05 RP, Cambridge, Massachusetts.
Baglione, M. 2007. Development of System Analysis Methodologies and Tools for Modeling and Optimizing Vehicle System Efficiency. Ph.D. Dissertation. University of Michigan.