In places where it gets below freezing in the wintertime, weather reports always include both the air temperature and the wind chill—that is, how cold it feels with the wind blowing. Wind makes the temperature feel colder because it carries heat away from our bodies. In this Science Update, you'll hear how the wind affects car engines in a similar way.
Cranking a cold car. I'm Bob Hirshon and this is Science Update.
The start of winter makes this question from David Yount of Duluth, Minnesota, especially timely.
I'd like to know why it is that an automobile left outside in subzero temperatures has more difficulty starting than an automobile left in an unheated garage.
Well, David, we asked Naeim Henein, director of the Center for Automotive Research at Detroit's Wayne State University. He says if you drive your car around and then park it in the garage, it won't get as cold as if you kept it outside.
When you stop the engine, the engine is hot. It takes a long time for the internal components of the engine to cool down. It can take up to 8 or to 12 hours.
That's because there's no wind to carry the heat away. So even if it feels cold in your garage, the engine may still be warm.
Henein adds that regardless of whether your car is inside or out in the winter, it'll start more quickly if you use thinner, low-viscosity oil, and make sure your air and fuel filters are clean.
If you've got a science question, we'll get cranking. Just call us at 1-800-WHY-ISIT. If we use it on the show you'll get a free Science Update mug. For the American Association for the Advancement of Science, I'm Bob Hirshon.
Making Sense of the Research
To understand why cars don't start when it's cold out, you need to understand how cars start in the first place. Briefly put, it's a kind of chemical reaction called a combustion reaction. In other words, a fire (in this case, a very small and controlled one).
To put it very simply: When you turn a car's ignition key, it makes the engine's spark plug spark, which on a good day ignites the mixture of fuel and air—yes, air—inside the engine. The oxygen in the air is an essential part of this reaction. Without it, the combustion reaction can't take place, so the fuel won't ignite, and the car won't start. (It's the same reason you can put out a fire by smothering it—fires can't burn without air.)
In the winter, the air is there, but it's very cold. And since all chemical reactions slow down in cold temperatures, it's harder to set off that combustion reaction. What's more, the car's battery is also cold, which means its chemical reactions have slowed down, which means it won't be able to send power to crank the engine very efficiently either. As a result of all this, if the temperature drops below a certain critical point, the combustion reaction may not happen at all.
Now, if you drive your car around all day, and park it in a garage, this is less likely to happen. As Henein points out, that's because the engine itself takes a long time to cool off. And as long as the engine components are warm, the air surrounding them will be warm too.
But if you're parked outside, the rushing wind carries that warm air away from the engine. That makes the engine lose more heat, which in turn is carried away by the wind, and so on. The end result is a cooler engine, with cooler air inside it, and a rougher start.
The process by which objects transfer their heat to moving air is called convection cooling. Convection cooling also explains the "wind chill factor" that makes windy winter days feel colder than calm ones. On a windy day, the wind constantly carries heat away from your body; on a calm one, a thin layer of heated air surrounds you and helps insulate you from the elements.
Now try and answer these questions:
- What are the essential ingredients in the combustion reaction that starts a car?
- How does temperature affect the reaction?
- How does wind affect the temperature of the engine?
- If you left a car in an unheated garage for a month during a cold snap, would you expect it to start? Why or why not?
- The principle of convection also explains how a warm radiator heats up a cold room. Can you guess why? Be specific.