For all the energy required to propel a vehicle, not all of it makes it to the wheels. Some of it is lost to friction and heat. Vehicle inefficiency can be classified into two categories of losses: road-load and energy conversion. At Tesla, careful attention is given to both to achieve the maximum range. The Tesla Roadster leverages both an incredible electric powertrain and an engineer’s obsession with efficiency to be the most efficient production sports car on the market today.
All vehicle types, regardless of what powertrain they use, need to overcome road-load. It includes wind resistance, mechanical friction (bearings, hubs, driveshaft, etc.), and tire rolling resistance. Road-load affects all vehicles. As a car speeds up, wind resistance increases; there is more air for the car to “push” out of the way. Therefore, road-load is greater at higher speeds and is dependent on the aerodynamics of the vehicle. Tesla engineers are focused on decreasing aerodynamic losses while achieving beautiful styling. Road-load can be minimized by designing brakes, bearings and other rotating components with less friction. It is also important to use tires that have low rolling resistance and make the vehicle as light as possible. Energy saved by decreasing road-load can have a significantly positive impact on range. The Model S will be one of the most aerodynamic sedans ever built with all components tuned to minimize friction and achieve the highest possible range.
Energy Conversion Losses
Two theoretical cars with identical road-load could have very different overall efficiency based on how effectively they convert energy before it turns to the wheels. Electric vehicles have the lowest overall energy conversion losses.
Electric Vehicle Efficiency
In an electric vehicle, chemical energy is stored in a battery. Lithium-ion batteries are used in Tesla vehicles because of high energy density. Converting the chemical energy to free electrons (electrical energy) can be greater than 90% efficient – some energy is lost to heat in cells and other battery pack components such as current conductors and fuses. The remaining components of the Tesla powertrain – the drive inverter and motor – are also extremely efficient. Overall, drive efficiency of the Tesla Roadster is 88% - almost three times more efficient than an internal combustion powered vehicle.
Internal Combustion Vehicle Efficiency
Chemical energy is stored as gasoline in a conventional car. Combustion is used to convert the chemical energy into thermal energy. Pistons convert the thermal energy to the mechanical work that turns the wheels. The conversion process is, at best, 35% efficient. The majority of the energy stored in the gasoline is lost as heat.
Hybrids and Plug-In Hybrid Vehicle Efficiency
The energy conversion process in a hybrid is a combination of the process in an internal combustion vehicle and battery electric vehicle. Overall efficiency for a hybrid is a bit higher than a conventional vehicle because it can recapture some road load energy, but is still much lower than an electric vehicle. When Hybrids operate in all-electric mode – overall efficiency can be quite high. However, once the gasoline engine kicks in, the overall powertrain efficiency includes losses due to both the ICE conversion process and the battery. A world of 100% hybrid vehicles is still 100% dependent on oil.
Comparing an example of each technology reveals that Tesla electric technology uses energy most efficiently. As utility companies build more efficient power plants and bring more renewables online, the well-to-station efficiency will significantly increase. In turn, the overall vehicle efficiency increases to levels never to be recognized by internal combustion or hybrid technologies.