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Where the Energy Goes: Hybrids

About 25%–40% of the energy from the fuel you put in a hybrid is used to move it down the road, depending on the drive cycle. Hybrids are more efficient than comparable conventional vehicles, especially in stop-and-go driving, due to the use of regenerative braking, electric motor drive/assist, and start/stop technologies—see How Hybrids Work for details.

Still, much of the energy is lost to engine and driveline inefficiencies or used to power accessories.

Energy requirements in this diagram are estimated for stop-and-go city driving using the EPA FTP-75 Test procedure.

Like conventional gasoline-powered vehicles, most of the energy in the fuel that goes into a hybrid 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.

more...

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.

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.

Braking Losses

When you apply the brakes in a conventional vehicle, energy initially used to overcome inertia and propel the vehicle is lost as heat through friction at the brakes.

Hybrids use regenerative braking to recover some energy that would otherwise be lost in braking. This makes hybrids more efficient than comparable conventional vehicles, especially in stop-and-go traffic.

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.

more...

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.

more...

Hybrids reduce idling by turning the engine off when the vehicle comes to a stop and restarting it when the accelerator is pressed.

This makes hybrids more efficient than comparable conventional vehicles in city driving, which includes a significant amount of idling.

Hybrids use regenerative braking to recover energy typically wasted in braking. Since more braking takes place in stop-and-go traffic, hybrids are most efficient in city driving.

When you apply the brakes, the vehicle's inertia turns an electric motor-generator, producing electricity that is then stored in a battery. The electricity can later be used to power the electric motor, which supplies power to the wheels.

This also allows the vehicle to sometimes use the electric motor to power the vehicle at lower speeds, where a combustion engine is typically less efficient.

Energy Requirements for City (Stop and Go) Driving: Engine Losses (66%-72%), Parasitic Losses (5%-7%), Power to Wheels (25%-40%), Drivetrain Losses (3%-5%), Idle Losses (near 0). Energy Requirements for City (Stop-and-Go) Driving Engine Losses: 66%-72% Idle Losses: Near 0 Power to Wheels: 25%-40%, which is used to overcome wind resistance (6%-11%), overcome rolling resistance (6%-11%), and in braking (13%-20%). Parasitic Losses: 5%-7% Drivetrain Losses: 3%-5% Energy Recovered by Regenerative Braking: 8%-14%

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).

Like conventional gasoline-powered vehicles, most of the energy in the fuel that goes into a hybrid 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.

more...

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.

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.

Braking Losses

When you apply the brakes in a conventional vehicle, energy initially used to overcome inertia and propel the vehicle is lost as heat through friction at the brakes.

Hybrids use regenerative braking to recover some energy that would otherwise be lost in braking.

Since there is little braking in highway driving, regenerative braking offers little advantage over a conventional vehicle on the highway.

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.

more...

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.

more...

Highway driving includes little to no idling. The EPA highway driving cycle (HWFET) includes no idling.

Hybrids use regenerative braking to recover energy typically wasted in braking.

When you apply the brakes, the vehicle's inertia turns an electric motor-generator, producing electricity that is then stored in a battery. The electricity can later be used to power the electric motor, which supplies power to the wheels.

The electric motor can be used to power the vehicle at lower speeds, where a combustion engine is typically less efficient.

However, since there is little to no idling or low-speed driving, hybrids offer little advantage over conventional vehicles on the highway.

Energy Requirements for Highway Driving: Engine Losses (63%-66%), Parasitic Losses (2%-4%), Power to Wheels (29%-36%), Drivetrain Losses (3%-5%), Idle Losses (none). Highway driving does not include significant idling. Energy Requirements for Highway Driving Engine Losses: 63%-66% Idle Losses: None Power to Wheels: 29%-36%, which is used to overcome wind resistance (17%-23%), overcome rolling resistance (8%-11%), and in braking (3%-4%). Parasitic Losses: 2%-4% Drivetrain Losses: 3%-5% Energy Recovered from Regenerative Braking: 2%-4%

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.

Like conventional gasoline-powered vehicles, most of the energy in the fuel that goes into a hybrid 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.

more...

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.

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.

Braking Losses

When you apply the brakes in a conventional vehicle, energy initially used to overcome inertia and propel the vehicle is lost as heat through friction at the brakes.

Hybrids use regenerative braking to recover some energy that would otherwise be lost in braking.

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.

more...

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.

more...

Hybrids experience negligible energy losses from idling in combined city-highway driving.

In city driving, hybrids reduce idling by turning the engine off when the vehicle comes to a stop and restarting it when the accelerator is pressed.

In highway driving, there is little to no idling.

Hybrids use regenerative braking to recover energy typically wasted in braking.

When you apply the brakes, the vehicle's inertia turns an electric motor-generator, producing electricity that is then stored in a battery. The electricity can later be used to power the electric motor, which supplies power to the wheels.

The electric motor can be used to power the vehicle at lower speeds, where a combustion engine is typically less efficient.

Energy Requirements for Combined City/Highway Driving: Engine Losses (65%-69%), Parasitic Losses (4%-6%), Power to Wheels (27%-38%), Drivetrain Losses (3%-5%), Idle Losses (near 0). Energy Requirements for Combined City/Highway Driving Engine Losses: 65%-69% Idle Losses: Near 0 Power to Wheels: 27%-38%, which is used to overcome wind resistance (11%-16%), overcome rolling resistance (7%-11%), and in braking (9%-13%). Parasitic Losses: 4%-6% Drivetrain Losses: 3%-5% Energy Recovered from Regenerative Braking: 5%-9%

Data Sources

Energy requirement estimates are based on analysis of over 100 vehicles by Oak Ridge National Laboratory using EPA Test Car List Data Files.

Lohse-Busch, H., M. Duoba, E. Rask, K. Stutenberg, et al. 2013. Ambient Temperature (20°F, 72°F and 95°F) Impact on Fuel and Energy Consumption for Several Conventional Vehicles, Hybrid and Plug-In Hybrid Electric Vehicles and Battery Electric Vehicle. SAE Technical Paper 2013-01-1462. DOI:10.4271/2013-01-1462.