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For people who drive, gas money is undoubtedly one of the most important expenses of daily driving. The fluctuation of oil prices always involves the nerves of old drivers. Therefore, "how to drive and save fuel" is always a topic that drivers talk about when communicating.
Written by | Li Shiyuan
Review | Wang Yu
There are many factors that affect the fuel consumption of driving. For example, reducing unnecessary loads on the car, avoiding aggressive driving behavior with frequent acceleration and deceleration, and turning off the engine while waiting for a long time can save fuel consumption.
Compared to these trivial factors, it seems simpler to discuss "how fast to drive is the most fuel-efficient", especially if driving at a uniform speed. So is there a so-called "optimum speed" or the most economical speed that minimizes fuel consumption?
There is a simple answer to this question: it is best not to be fast or slow, too fast or too slow will increase fuel consumption, and the relationship between fuel consumption and speed is roughly U-shaped. However, specific to how much speed to drive is the most fuel-efficient, different cars will have certain differences.
It is best not to be fast or slow
For the most common fuel vehicles, the chemical energy released by the oil burning inside the engine provides the full source of power. After passing through the layered drivetrain, the energy is finally transferred to the wheels and converted into kinetic energy for the vehicle to travel.
Image source: Hyundai Motor's official website
During the driving process, the efficiency loss in the vehicle transmission system, the rolling friction between the wheels and the ground, and the air resistance (wind resistance) when the vehicle is driving will continue to consume energy, which is why the car still consumes fuel even at a uniform speed. If the complex model is simplified, the road load of the car (that is, the resistance to which the car is subjected) can be expressed as a quadratic function in the case of uniform speed v and no gradient:
F corresponds to the resistance of the car, but also the traction required for uniform driving. Among them, the coefficients A, B, and C are all characteristic attributes of the vehicle, and there are differences between different cars. A corresponds to the rolling friction of the wheel, the braking system and the friction of the wheel bearing, B corresponds to the rolling friction of the wheel and various pump devices, and C corresponds to the wind resistance that we are most familiar with.
Obviously, due to the existence of wind resistance, there is a quadratic function relationship between road load and vehicle speed, and when the speed is very fast, the driving resistance will increase sharply. Therefore, high-speed driving itself is very fuel-intensive. Studies have conducted experiments on 15 different cars and found that the fuel economy of the vehicle (the distance that can be driven per unit volume of fuel) at 120 km / h is reduced by 15% to 44% compared to the speed of 90 km / h; in other words, driving the same road, 120 km / h costs 18% to 79% more fuel than 90 km / h. The U.S. Department of Energy's recommendations also point out that high-speed driving can reduce fuel economy by 15 to 30 percent.
Conversely, why is it more expensive to drive too slowly? This is related to the efficiency of energy transfer in mechanical systems. In fact, only a small fraction of the chemical energy stored in the fuel can ultimately be actually transferred to the wheels to maintain the kinetic energy of the car. Studies have found that at low speeds, the efficiency of the engine, transmission and clutch is significantly lower than that of medium and high speeds, because the gear (that is, gear ratio) affects the efficiency of the mechanical system.
Among them, the engine is the component with the greatest loss of efficiency, with an efficiency of only about 10% at low speed, and only about 30% at high speed. The efficiency of the transmission and clutch in the transmission system is more affected by the gear, especially the efficiency of the gearbox shows obvious step-by-step changes with the shift, in some models, the transmission efficiency in the low gear is less than 50%, and the efficiency in the high gear is close to 100%.
The efficiency (%) of a Ford Focus mechanical system changes with vehicle speed (km/h). (Image source: https://dspace.mit.edu/handle/1721.1/58392)
Therefore, at low speeds, although the car is subjected to less external resistance, the reduction in internal efficiency not only offsets this difference, but also leads to a surge in fuel consumption. The speed of the car increases, the resistance increases slightly, the efficiency is significantly improved, and the overall fuel consumption will be reduced, at which time the car is in the economic speed range. Once the speed is too high, the wind resistance will increase sharply, and the efficiency will reach the bottleneck or even decrease slightly, so the fuel consumption will also increase.
How fast is the most fuel efficient?
As mentioned earlier, the most fuel-efficient speeds of different models are not the same. A 1988 study tested 15 different models and found that their optimum speeds ranged from 28 to 78 km/h, with the lowest speed being the Toyota Corolla in 1982 and the highest Being the Chevrolet Citation in 1982. In general, there are 10 models with a maximum speed of between 40 and 70 km/h.
More recent studies have yielded similar results. In 2008, a study by Tsinghua University tested 10 different cars and found that the most suitable speed was between 50 and 70 km/h. These cars cover some of the more common models, including the Volkswagen Santana, Jetta, Passat, etc., and the results are more informative for ordinary people. For example, the Volkswagen Jetta produced in 2001 has a maximum speed of 65 km/h.
However, in actual driving, it is almost impossible to always control the speed of the car to a specific value. Fortunately, when the speed is floated up and down within a certain range relative to the optimal speed, the loss of fuel economy is not very large. The 1988 study found that, on average, if the floating amplitude was within 8 km/h, the loss of fuel economy did not exceed 5%, and if the float was within 19 km/h, the loss of fuel economy did not exceed 15%.
The 100 km fuel consumption (liters) of the Honda Accord (square), Honda Civic (triangle), Ford Explorer (diamond), and Ford Focus (round) changes with speed (km/h). (Image source: https://dspace.mit.edu/handle/1721.1/58392)
A 2010 study simulating the Honda Civic, Ford Focus, Honda Accord and Ford Explorer found that fuel consumption per 100 km could be controlled to be less than 5% higher than the optimal speed for a certain speed range – the narrowest range was 63-70 km/h for the Ford Focus, and the widest range for the Honda Accord, at 48-63 km/h.
General rules
Due to the different factors such as models, test methods and conditions in different studies, the results obtained in these studies are also relatively different, but in general, the most suitable speed is roughly within the range of 40 to 70 to 80 km / h, and the speed of driving on the main roads, outer ring roads, secondary roads and other roads in the city with smooth road conditions is similar.
In addition, the study also found some common laws related to the optimal speed. For example, the optimum speed of large cars seems to be higher than that of small cars, and the most suitable speeds for cars with 4-, 6- and 8-cylinder engines are on average 44 km/h, 59 km/h and 68 km/h, respectively; when going downhill, the optimum speed increases by about 5 to 10 km/h for every 1% increase in gradient. The 2010 study summarized the effects of wind resistance, work-to-weight ratio and gear levels on fuel consumption (see table below).
As the metric for the corresponding column increases, the metric for the corresponding row increases, indicating a decrease. (Source: https://dspace.mit.edu/handle/1721.1/58392)
You might feel that many of the previous results are sporadic data that don't seem to have universal significance or are easy to remember. It is indeed very difficult to build a purely theoretical model between fuel consumption and vehicle speed, but there are still studies that have made approximate estimates through a large number of tests. In a 2013 study published at Oak Ridge National Laboratory, researchers used 74 models that were tested at uniform speeds in the range of 50 to 80 mph (about 80 to 129 km/h) and built several models to fit the vehicle's fuel economy.
One of the simplest models uses only 2 variables: in addition to the speed, there is the vehicle's nominal MPG value, which is a fuel efficiency indicator evaluated according to the regulations of the U.S. Environmental Protection Agency (EPA), which is not equal to the actual fuel efficiency of the vehicle, but reflects the fuel efficiency of the vehicle to some extent. The formula for this model is also relatively simple:
The MPG to the left of the equal sign is the actual fuel efficiency in miles per gallon; the X on the right is the nominal MPG, and the v is the speed of the vehicle, which is in miles per hour. If you replace the variables with the commonly used metric units, the formula approximates:
Take the latest Toyota Corolla as an example, its nominal MPG under highway conditions is 40, and after substituting the formula, the variable is converted to be expressed in meter-based units
That is, within the range of speeds considered by the study, the higher the speed, the worse the fuel economy. Considering that the speed range of the study is beyond the usual optimal range we mentioned earlier, it is normal for this monotonous change to occur. It can be seen from the formula that for every 1 km increase in speed, the mileage that 1 liter of fuel can travel is reduced by more than 200 meters; compared with the fuel consumption of 100 km / h (3.88 liters), the fuel consumption of 100 km / h (4.65 liters) is 20% higher, and the fuel consumption of 120 km / h is 50% higher than that of 120 km / h (5.81 liters).
Image credit: Taras Makarenko/Pexels
Suppose you live in a big city, the daily commuting mileage is 50 kilometers, according to the legal working days of 250 days a year, the oil price of 9 yuan / liter, compared to the speed of 120 kilometers per hour, driving this Toyota Corolla at a speed of 80 kilometers per hour, you can save nearly 2200 yuan a year, enough to buy a mobile phone with good performance.
Time is money?
Perhaps some readers will feel that although it costs fuel to open fast, it can save their precious time, and this time can be used to earn money from part-time work, and maybe in the end, they can offset the excess oil money, or even make a profit. Rhett Allain, an associate professor at southeastern Louisiana University, also expected this. So he proposed that it is best to take into account the price of oil and the level of personal income, and it is the most economical to comprehensively calculate how fast the car is driven.
Assuming that the time saved on transportation can be fully converted into your own normal work income, the total cost of driving a commute can be expressed as:
In other words, the total cost per commuting distance is:
Where Δx is the distance traveled, G is the oil price, and R is the income per unit time. After deduction, it can be concluded that the greater the ratio of G and R, that is, the higher the oil price, the lower the income level, and the slower the most economical speed.
Professor Allain also provided simple Python codes that could easily calculate the most economical speed (accurate to 1 km/h). For example, if the oil price is 9 yuan / liter, if the hourly wage is 30 yuan, the most economical speed is 90 km / h; if the hourly wage is 60 yuan, the most economical speed will reach 107 km / h; if the hourly wage is 150 yuan, the most economical speed will reach 128 km / h.
Of course, it should be emphasized that such a model is oversimplified after all, and many of the assumptions in the text do not hold true in real life. Whatever the outcome, the most important thing is to obey traffic laws and never drive fast. If you really want to save fuel, you may want to change some of your driving habits, or, where feasible, switch to public transportation, bike or walking to commute, or live closer to where you work.
Or change to a well-paid job and stop thinking about it.
Reference Links:
https://www.wired.com/story/is-there-an-optimal-driving-speed-that-saves-gas-and-money/
https://www.fueleconomy.gov/feg/driveHabits.jsp
https://doi.org/10.4271/2013-01-1113
https://doi.org/10.1016/0191-2607(88)90036-2
https://doi.org/10.1016/j.trd.2008.09.002
https://dspace.mit.edu/handle/1721.1/58392
https://www.fueleconomy.gov/feg/Find.do?action=sbs&id=44074
This article is reprinted from the WeChat public account with permission: Global Science Author: Global Science
The reproduced content represents the views of the author only
It does not represent the position of the Institute of High Energy of the Chinese Academy of Sciences
Edit: Chu Mo
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