The all-new 2012 Subaru Impreza made headlines for a number of reasons - one of which was its expected highway fuel economy rating of 36 miles per gallon with the Lineartronic CVT (Continuously Variable Transmission). But fuel economy is not only a function of engine and transmission; here's a look at Subaru technologies that contribute to improve fuel economy.
Overview
Whether or not a vehicle achieves good fuel economy is determined by its design, engineering, and manufacturing. It has to do with how vehicle components are made, what they're made of, and how systems function.
While every aspect of a vehicle contributes in some way to its level of fuel economy, we'll focus this discussion on how Subaru has improved fuel economy by addressing these areas:
- Vehicle aerodynamics - how a vehicle moves through the air
- Reduced weight - a lighter vehicle tends to achieve better fuel economy
- Body
- Drivetrain components and systems - Component and system design - engineering for light weight, smooth operation, and aerodynamic efficiency
- Efficient functionality - operating with low losses of energy to friction
- Engine
- Transmission
- Drivetrain
| The Cycles of Technology |
| Automotive manufacturers introduce technologies as their products cycle through their launches. A new engine design will be introduced in one model, lightweight suspension components in the next model (which also integrates the new engine design), aerodynamic technology in the next model (which also integrates the new engine and suspension design), and so on. |
| The engine technology in the new 2012 Impreza is an example. All Subaru models have not been converted to the new design, and it was not introduced first in the Impreza. Rather, it was first used in the 2011 Forester. |
| The same is true of the CVT in the 2012 Impreza. That transmission technology was introduced in the 2011 Outback and Legacy. |
Aerodynamic Shapes
How a vehicle moves through the air helps determine the amount of fuel that must be used by the engine to overcome drag - friction caused by the air. A measure used by manufacturers that indicates the aerodynamic efficiency of a vehicle's body is called "coefficient of drag," or Cd. Multiplying that figure by the frontal area of the vehicle gives a numeric indicator of its aerodynamic drag.
A vehicle's overall shape, the finish of its surfaces, and how air flows through cooling and ventilating systems all affect its Cd, which can be measured in wind-tunnel tests. The lower a vehicle's Cd, the better it moves through the air, and the better its fuel economy.
The accompanying chart indicates the coefficient of drag for 2012 Subaru models.
| Coefficient of Drag (Cd) | |
| 2012 Subaru Models | |
| Cd measures aerodynamic efficeiency - the lower the number, the better | |
| Impreza | 0.31-0.33 |
| Legacy | 0.32 |
| WRX/WRX STI | 0.34-0.35 |
| Outback | 0.37 |
| Forester | 0.375-0.382 |
| Tribeca | 0.38 |
Perhaps you've never considered the aerodynamic design of a Subaru. Aerodynamics are part of every model, and the emphasis that Subaru puts into the aerodynamics of its products has been reflecting its engineering prowess for years. As an example, in the mid-1980s, Subaru introduced its XT Coupe, which had the lowest Cd of any mass-produced car at that time - 0.29. (See article on the 1986 XT Coupe in Summer 2011 Drive.)
The Cd measurements of 2012 Subaru models are not much more than the XT Coupe's 0.29. All are within 0.10 Cd of it.
Along with their smooth exterior shapes, all 2012 Subaru models have aerodynamic underbody panels. These smooth panels cover the bottom of the engine and some driveline and suspension components to help prevent them from disrupting airflow under the vehicle. Front wheel liners and wheel opening aero flaps also aid airflow efficiency.
On the 2011-2012 Legacy and Outback as well as the 2012 Impreza, the angles of the windshield have been lowered compared to previous generations. The lower angles promote airflow over the vehicles. The front bumpers on the 2012 Impreza also have an optimized aero shape.
Lightweight
Besides shape, weight is one of the prime obstacles to good fuel economy. It takes more power - and, hence, more fuel - to start up a heavy vehicle than a light one and to keep it at speed. Perhaps you've noticed the difference in mileage between driving an empty car and one loaded with passengers and cargo. Even the difference of 100 pounds can affect fuel economy.
As a result, manufacturers focus on weight reduction with each succeeding model, despite a corresponding increase in vehicle content (i.e., added and evolved safety systems, audio systems, comfort and convenience equipment).
In order to maintain low curb weights and reduce them wherever possible, Subaru uses technologies in its vehicle bodies and mechanical features that allow lower weight with increased strength. Greater proportions of high tensile strength steel are employed in all Subaru bodies, and strong, lightweight metals are used in components throughout the model lineup.
In addition, lightweight unit-body structures are the foundation for Impreza, Legacy, Outback, and Forester models. These same models have window glass of reduced thickness.
Measures taken under the hood reduce weight, too. All Subaru engines have aluminum-alloy blocks and cylinder heads and all Subaru transmissions have aluminum-alloy cases. Not only does this construction reduce overall weight, it helps to improve the front/rear balance of the vehicle for improved handling. The engines' pistons have a lightweight design, and the intake manifolds are made of resin, which also contributes to weight reduction.
The 2012 Impreza pushes the envelope farther with optimized lightweight aluminum-alloy wheels and aluminum-alloy rear brake calipers.
Design and Operation
Notice that we've only lightly touched upon engines and transmissions. We'll have more to say about them in the Winter 2012 issue, in which we'll present additional ways in which Subaru vehicles are built to achieve good fuel economy. In Part Two, we'll look at vehicle design and how components and systems are engineered to function efficiently.
