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Fuel Efficiency and Losses in Car Designs

By: Thomas Yoon

Our previous article talks about careful planning in order to save unnecessary trips, adopting carpooling to share out the costs, avoiding traffic jams at peak hours to avoid unproductive burning of fuel, avoiding carrying unnecessary loads and other tips that will help in some way or another to save on fuel consumption.

I did touch a bit on more efficient smaller cars that are lighter and do not have to carry excess and unproductive weights in their frames, chassis and body. However, very often, these smaller and lighter cars are not as comfortable to ride compared to the heavier ones.

So, in this article, I won't compare lighter cars with heavier cars. I will explore with you the kinds of efficiency losses that are quite typical even within the same class of cars.

If you look through the sales brochures of new cars, very often you can find figures that indicate the fuel efficiency of the car. With that information, you can compare and find out the most fuel efficient vehicle that will meet your needs. Even within a specific size class, there is a tremendous range of MPG (miles per gallon) performance that you can find.

Just for example, for the same model year compact cars, we can find fuel efficiency ranges from 21 to 48 MPG. If you choose the 48 MPG car, you could be saving hundreds of dollars in fuel costs each year. I think that should be an important point to note when buying a new car.

Having decided on the model that fulfils your needs, you might be interested to know where the energy goes.

Idling losses could account for as much as 17.2% of all the losses. In urban driving, this much of energy is lost to idling at stop lights or in the traffic. There are technologies available such as integrated starter / generator (ISG) systems that help to reduce this type of losses. It does this by automatically turning off the engine when a vehicle comes to a stop and restarting it instantly when the accelerator is pressed.

Another very significant loss is through the engine itself. The internal combustion engine of gasoline-powered vehicles is very inefficient. Over 62.4% of the fuel's energy is lost through the engine combustion process of converting the fuel's chemical energy to mechanical energy. Energy is lost to engine friction, pumping of air into and out of the engine, and removing the wasted heat.

Advanced engine technologies have been developed to address these losses. Some of them are: variable valve timing and lift, turbocharging, direct fuel injection, and cylinder deactivation.

Diesel engines are about 30~35% more efficient than gasoline engines. New advances in diesel technologies and fuels are making these vehicles more attractive.

Accessories like air-conditioning, power steering, windshield wipers, and others use the energy generated from the engine. These uses up to 2.2% of the energy. Efforts in developing more efficient alternator systems and power steering pumps can improve the fuel economy by up to 1%.

Driveline losses can account for up to 5.6% of the total. The energy is lost through the transmission and other parts of the driveline. To reduce these, technologies such as automatic manual transmission (AMT) and continuously variable transmission (CVT) has been developed.

Next comes the shape of the vehicle. Aerodynamic drag accounts for 2.6% of the energy losses. As the speed of a vehicle increases, the drag caused by expending energy to push air out of the way increases. By designing the shape of a vehicle for smoother air flow, significant drag reduction can be achieved.

Rolling resistance is another loss encountered in a vehicle. It accounts for 4.2% of the losses. This rolling resistance is a measure of the force necessary to move the tire forward. To counter this, tires technologies like thread and shoulder designs, and the use of improved materials on the tire belt and traction surfaces are being developed.

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

Related to the driver's behavior is braking loss. This can account for as much as 5.8%. Each time a vehicle moves forward, the vehicle's drivetrain must provide enough energy to overcome the vehicle's inertia. This inertia is directly related to the weight of the vehicle. So for lighter vehicles, less energy is expended to overcome the inertia of the vehicle compared to a heavier vehicle. The less a driver brakes, the less energy is expended to move the vehicle again.

As you can see, there are many factors that can affect the fuel efficiency even of similar vehicles. Factors like the vehicle condition, tire pressures and design, driver's habit, planning trips, reducing excess loads, avoiding drag, idling at peak traffic and many others can affect the efficiency.

With a better understanding of the measures you can control, you should be able to achieve the best optimum efficiency in your vehicle and save money in the process.

Until next time...

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