Efficiency of internal combustion engine

November 23, 2021

A Toyota Prius hybrid car

Only about 15 percent of the energy from the fuel you put in your tank gets used to move your car down the road or run useful accessories, such as air conditioning. The rest of the energy is lost to engine and driveline inefficiencies and idling. Therefore, the potential to improve fuel efficiency with advanced technologies is enormous.

Engine Losses - 62.4 percent

In gasoline-powered vehicles, over 62 percent of the fuel's energy is lost in the internal combustion engine (ICE). ICE engines are very inefficient at converting the fuel's chemical energy to mechanical energy, losing energy to engine friction, pumping air into and out of the engine, and wasted heat.

Advanced engine technologies such as variable valve timing and lift, turbocharging, direct fuel injection, and cylinder deactivation can be used to reduce these losses.

In addition, diesels are about 30-35 percent more efficient than gasoline engines, and new advances in diesel technologies and fuels are making these vehicles more attractive.

Idling Losses - 17.2 percent

In urban driving, significant energy is lost to idling at stop lights or in traffic. Technologies such as integrated starter/generator systems help reduce these losses by automatically turning the engine off when the vehicle comes to a stop and restarting it instantaneously when the accelerator is pressed.

Accessories - 2.2 percent

Air conditioning, power steering, windshield wipers, and other accessories use energy generated from the engine. Fuel economy improvements of up to 1 percent may be achievable with more efficient alternator systems and power steering pumps.

Driveline Losses - 5.6 percent

Energy is lost in the transmission and other parts of the driveline. Technologies, such as automated manual transmission and continuously variable transmission, are being developed to reduce these losses.

Aerodynamic Drag - 2.6 percent

A vehicle must expend energy to move air out of the way as it goes down the road—less energy at lower speeds and progressively more as speed increases. Drag is directly related to the vehicle's shape. Smoother vehicle shapes have already reduced drag significantly, but further reductions of 20-30 percent are possible.

Rolling Resistance - 4.2 percent

Rolling resistance is a measure of the force necessary to move the tire forward and is directly proportional to the weight of the load supported by the tire. A variety of new technologies can be used to reduce rolling resistance, including improved tire tread and shoulder designs and materials used in the tire belt and traction surfaces.

For passenger cars, a 5-7 percent reduction in rolling resistance increases fuel efficiency by 1 percent. However, these improvements must be balanced against traction, durabillity, and noise.

Overcoming Inertia; Braking Losses - 5.8 percent

To move forward, a vehicle's drivetrain must provide enough energy to overcome the vehicle's inertia, which is directly related to its weight. The less a vehicle weighs, the less energy it takes to move it. Weight can be reduced by using lightweight materials and lighter-weight technologies (e.g., automated manual transmissions weigh less than conventional automatics).

In addition, any time you use your brakes, energy initially used to overcome inertia is lost.

Source: www.consumerenergycenter.org
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