给汽车降温，开车窗还是开空调更省油？

夏天开车最难以忍受的就是高温。

好在汽车空调的存在大大提升了驾驶舒适度。不过有些人也因为担心油耗问题而选择开窗降温。在一定温度条件下，开窗和开空调，都可以达到降低车内温度的目的。

究竟哪一种方式更加省油？ 

Photo/pexels

There are two main factors to consider when approaching this question. The first deals with how the air compressor in your car works and how much extra fuel the engine has to use to keep it running. The second is what is known as air resistance or drag. Drag is the resistance that cars, and all moving objects, encounter when moving through the air at any speed. Most modern cars are designed to be relatively aerodynamic, which allows them to pass through the air with minimal resistance.

探讨这个问题时需要考虑两个主要因素。第一个与汽车中的空气压缩机的工作方式有关，以及引擎需要使用多少额外的燃料来保持其运行。第二个因素是所谓的空气阻力或阻力。阻力是指汽车和所有移动物体在任何速度下移动时所遇到的空气阻力。大多数现代汽车的设计都是相对符合空气动力学的，这使得它们能够以最小的阻力穿过空气。

However, when a vehicle has its windows down, air passes into the car where it was formerly allowed to flow over it, causing resistance that didn't exist when the windows were up. You can think of it a bit like a parachute. When a skydiver opens up the parachute, it cups the air and causes a massive amount of drag, enough to slow the speed of the skydiver and allow him or her to land safely on the ground. Unlike the parachute, you definitely don't want a lot of drag on your car because it makes your engine work harder to get your vehicle up to the same speed.

然而，当车辆将车窗摇下时，空气进入车内，而以前允许空气通过的地方现在产生了阻力。你可以想象成一把降落伞。当跳伞者展开降落伞时，会兜住空气并产生强大阻力，足以减缓跳伞者的速度，使他们安全降落。与降落伞不同的是，我们不希望车辆遇到太多阻力，因为在车速不变的情况下这会让引擎消耗更多燃料。

So, does drag really affect a car's fuel economy more than air conditioning? There are times when using the AC will save you gas.

那么，开窗的空气阻力会比开空调更耗油吗？什么时候开空调更省油？

Back in 2004, the Society of Automotive Engineers (SAE) conducted a study at a General Motors wind tunnel and on a desert track. In the wind tunnel, air was forced over the front of the car and also from an angle on the front of the car to simulate a cross wind. In the desert, temperatures and vehicle speed were factored into the study. Two vehicles were used in the test, one was a full-size SUV with an 8.1-liter V-8 engine and the other was a full-size sedan equipped with a 4.6-liter V-8 engine. Overall, both studies showed that driving with the windows down has a significant negative effect on the fuel efficiency — more than using the vehicle's air conditioner.

早在2004年，美国汽车工程师学会(SAE)在通用汽车公司的风洞和沙漠赛道上进行了一项研究。在风洞中，空气需要穿过汽车前部，从汽车前部的一个角度来模拟侧风。在沙漠中，测试研究了温度和车速的影响。测试使用了两辆车，一辆是8.1升V-8发动机的大型运动型多用途汽车（SUV），另一辆是配有4.6升V-8发动机的全尺寸轿车。总体而言，两项研究都表明，将车窗摇下来对燃油效率的负面影响要大于使用车辆的空调。

For the sedan, when the windows were down, the efficiency was reduced by 20 percent, while the SUV fuel efficiency was reduced just 8 percent [source: Hill]. These differences are an important factor in determining just how much the windows down option will affect the fuel efficiency of your vehicle. The study concluded that the more aerodynamic the vehicle, the more drag open windows will create.

当车窗摇下来时，轿车的燃油效率降低了20％，而SUV的燃油效率仅降低了8％。这些差异是决定车窗关闭选项对车辆燃油效率影响程度的重要因素。研究得出的结论是，车辆的空气动力学设计越强，打开车窗产生的阻力就越大。

However, with newer, more efficient vehicles the difference is likely to be negligible. 

然而，对于更新、更高效的车辆，这些差异可能可以忽略不计。

Since no tests have been conducted on an extensive array of vehicles, here are some :

由于没有对大量车辆进行测试，以下是一些概要指南：

When you're driving around town at relatively low speeds, you'll use less gas by switching the air conditioner off and rolling down the windows. It's more efficient to drive with the windows down at slow speeds as opposed to faster speeds because there's less aerodynamic drag when you're driving slower.

当市区以相对较低的速度行驶时，关闭空调，将车窗摇下来可以节省燃油。在低速行驶时，打开车窗比在高速行驶下燃油效率更高，因为在低速行驶时的空气阻力要小一些。

If you're searching for a good rule-of-thumb number for when it's best to open the windows and switch off the air conditioner, the cut-off should be around 40 mph (64 kph) for larger vehicles [source: Arthur]. With smaller vehicles, like the Corolla, you could use the AC and get up to 70 to 80 mph (assuming that's a speed limit where you are) without using extra gas.

如果你需要一个经验法则，来确定什么时候打开车窗并关闭空调最合适，那么大型车辆的速度上限应该在64公里/小时左右。对于卡罗拉等小轿车，低于113-129公里/小时(假设你所在的地方的限速为此范围)的速度开窗行驶不额外消耗燃料。

编辑：董静