Image: A race car from the 1930s. Credit: Bundesarchiv, Bild 146-1989-015-36A / CC-BY-SA 3.0. Mando Maniac
Breaking speed records was an almost daily occurence throughout the 20th century. Cars, ships, planes and trains became faster and faster, year after year. Because the power needed to push an object through air increases with the cube of velocity, this race to ever higher speeds raises energy consumption exponentially.
Engineers treat velocity as a non-variable, while in fact it is the most powerful factor to save a really huge amount of energy - with just one stroke, at minimal cost, and without the need for new technology. Lower speeds combined with more energy efficient engines, better aerodynamics and lighter materials could make fuel savings even larger.
The fastest car in the world reaches 10 times the speed of a normal vehicle cruising the highway, but it consumes 550 times more fuel.
Air resistance (drag) increases with the square of speed, and therefore the power needed to push an object through air increases with the cube of the velocity (see the formula here). If a car cruising on the highway at 80 km/h requires 30 kilowatts to overcome air drag, that same car will require 240 kilowatts at a speed of 160 km/h. Thus, a vehicle needs 8 times the engine power to reach twice the speed. In principle, this means that fuel consumption will increase fourfold (not eightfold, because the faster vehicle exerts the power only over half the time).
Over a distance of 1,000 kilometres, the slow car would consume 375 kilowatt-hours (12.5 hours multiplied by 30 kilowatts) and the fast car would consume 1,500 kilowatt-hours (6.25 hours multiplied by 240 kilowatts).
Speed is the key
However, this extra fuel consumption can be diminished or even negated by, most importantly, more fuel efficient engines, lighter vehicles, materials and better aerodynamics. Even though today’s cars are faster than those from decades ago, they consume a similar amount of fuel. This is the reason why almost everybody is talking about energy efficiency and aerodynamics, and not about speed.
But if you lower the speed, fuel consumption is decreased by the full 75 percent. More efficient technology can not change that – unless in a positive way. If you combine a lower speed with more fuel efficient engines and better aerodynamics, fuel savings can become much larger than 75 percent.
Drag can be partly offset by better aerodynamics: a boxy car like the Volvo 740 has a drag area (drag coefficient multiplied by frontal area) that is almost twice that of the most aerodynamic standard car, the Honda Insight. The Volvo needs almost two times the engine power of the Honda when driven at 120 km/h.
A boxy car vehicle at 60 km/h will consume much less fuel than the most aerodynamic vehicle driving at 120 km/h
Yet a Volvo 740 driving at 60 km/h will face less than half the drag and will need 4.6 times less energy power than a Honda Insight driving at 120 km/h. When compared to velocity, the potential of aerodynamics is limited.
Moreover, very good aerodynamics is incompatible with high speeds. Formula 1 racing cars have the worst drag coefficients of all vehicles on wheels, because of their large spoilers and very wide tyres. At higher speeds, it becomes important to minimize lift at the expense of better aerodynamics so that the car is not catapulted into the air.
Low speed trains
The blindness for the importance of speed leads to doubtful conclusions, like the environmentally friendly label of high speed trains. The French TGV that set the most recent speed record at 575 km/h for wheeled trains in 2007 has an engine output of 19,600 kilowatts. A contemporary “slow” train like the Siemens ES64 with a top speed of 240 km/h has a maximum power output of 6,400 kilowatts.
Travelling 1,000 kilometres, the “slow” train will consume 26,240 kilowatt-hours (over 4.1 hours) while the fast train will consume 33.320 kilowatt-hours (over 1.7 hours). A real slow train (like this one from 1956 with a top speed of 120 km/h) would consume only 20,000 kilowatt-hours over the same trajectory (and would do this in 8.3 hours, comparable to the travel time of a car).
Technology can limit the growth of energy consumption, but if we want to lower energy consumption, we have no other choice but to adapt speed
The French high speed train is definitely more energy efficient than the Siemens locomotive, and that one is definitely more energy efficient than the 1956 train, because in both cases power consumption did not increase exponentially with speed. But that does not take away the fact that the faster trains consume more energy than the slower trains. If, on the other hand, we would equip the 1956 train with the energy-efficient technology of today’s high speed train, it would consume much less energy than it did 50 years ago.
Time is money
High speed trains are labelled environmentally friendly because they are not compared to other trains but to planes (A Boeing 747 would consume around 65,000 kWh over the same distance, over approximately 1 hour).
In a way this makes sense, because if a passenger prefers the fast train over the plane, he or she will consume less energy for a similar trip. They might not make that choice when the train would be much slower than the plane. On the other hand, if passengers that normally would take a slow train now prefer a fast train, high speed trains do raise energy consumption. The problem is that people see a shorter travel time as an advantage, while it has no ecological value whatsoever.
Travelling from A to B would require twice as much time. But global > world oil consumption would be halved.
You could as well argue that airplanes are green because they consume less fuel than rockets. This sounds ridiculous now, but if rocket planes take off, their inventors will no doubt claim that their toys are environmentally friendly because they go faster than airplanes but consume less than rockets. Technology alone can limit the growth of energy consumption, but if we want to lower energy consumption, we have no other choice but to adapt speed.
Fixation on technology
A decrease of 75 percent in fuel consumption is not peanuts. More than 60 percent of world oil production is used for transportation, which means that total oil production would be almost halved (-45%). In combination with more efficient engines, better aerodynamics and lighter materials a 75 percent reduction of oil production is not unrealistic.
Yet, when the International Energy Agency argues that the average car sold in 2030 would need to consume 60 percent less fuel than the average car sold in 2005, it claims: “With current technologies, only plug-in hybrids are capable of this”. This statement is wrong. We could lower the fuel consumption of cars (and other vehicles) by at least 75 percent, we could do it today, and we can do it with present technology.