Top 4 Important Reasons Why Idling is a Big Mistake

VW Beetle

There's a common misconception that you should idle your car first to warm up the engine. This belief can be traced back in the 80s, when idling was needed then--back when carburetors had to be warmed up first to make the engines' combustion process truly efficient.

But what carburetors lacked then, we already have now. Modern cars today have electronic fuel injection systems with sensors that allow cylinders the proper "air-fuel mixture" for effective combustion.

What's more, most cars today have high compression ratios that, when combined with better technology, tougher starter engines, alternators, and batteries, allow for excellent fuel control and better combustion.

So this makes idling unnecessary.

Still, we understand if you're not convinced. Idling, which occurs when a stationary car has its engine running, has been a common practice since way back then. Waiting for your kids at school, waiting in line during a drive-thru, or sleeping in your car while waiting for your next class are examples of idling. And yes, many are still guilty of them.

And to prove to you that idling is a waste of time, we're giving you the top four reasons why this common practice is unnecessary in this time and age:

1. Idling wastes fuel

It has always been one of the most common beliefs that starting your car's engine consumes more fuel compared to idling--even for a short period of time.

But idling for more than 10 seconds actually uses more fuel than just restarting the engine, and was later proven in a study made by Argonne Laboratory, a division of the U.S. Department of Energy. Their results are seen here: Taken from the report: Which is Greener: Idle, or Stop and Restart by Argonne National Laboratory (U.S. Department of Energy)

What's more, a study made by the Environmental Defense Fund in 2009 showed that an average car burns around 0.11 gallons per hour of fuel while idling, which is a conservative figure in our opinion. Others, like the Natural Resources Canada and AAA New York reported around .48 gallons (or 1.8 liters) to one gallon per hour*.

Large trucks use more, around 0.37 gallons per hour. Still actual numbers vary, with some falling at one to 1.65 gallons of diesel fuel for every hour of idling. And this goes up if you turn the air-conditioner on.

Argonne Laboratory and the U.S. Department of Energy also made a study on idling time with respect to fuel consumption. They used the engines of Honda Civic, Ford Fusion, and Chevrolet Malibu for the experiment, and their results are seen here: Taken from the report: To Idle or Not to Idle (That is the Question) by Argonne National Laboratory (U.S. Department of Energy)

Their findings show that fuel consumption increased in direct proportion to idling time  for all the cars used in the study. Fuel consumption also increased as engine size and accessory loads were added in the variables as well. You can see it on the table below: Taken from: Idling Reduction Savings Calculator by Argonne National Laboratory and Clean Cities (U.S. Department of Energy)

2. Idling also creates emissions (at lower levels), with Carbon Dioxide (CO2) taking the biggest portion of the mix

They also did a study on fuel use when idle and its emissions using a Ford Fusion sedan. With an EPA fuel-efficiency of 10 km/L city/ 14 km/L highway (11 km/L combined), they isolated four significant emissions (compounds) in their experiment:

  • Total hydrocarbons (THC)
  • Nitrogen Oxides (NOx)
  • Carbon Monoxide (CO)
  • Carbon Dioxide (CO2)

Their results revealed that idling your vehicle has the lowest emissions of the three. The second lowest was doing stop-start cycles (or start cycles), while the biggest emission comes from starting a cold engine. Taken from the report: Which is Greener: Idle, or Stop and Restart by Argonne          National Laboratory (U.S. Department of Energy)

Moreover, Carbon Dioxide (CO2), which is a key component in global warming, has the most number of emissions (0.59 g) for  every .279 cc of fuel used up by an idle vehicle. Taken from the report: Which is Greener: Idle, or Stop and Restart by Argonne National Laboratory (U.S. Department of Energy)

And while idling takes up the least emissions compared to stop-starts and cold engine starts, the fact that it still emits harmful substances and volatile compounds can still affect people's health in subtle ways.

Health complications related to idle car emissions

According to the Environmental Defense Fund, these gaseous emissions can cause childhood respiratory diseases like asthma and stunted lung development.

Cancer risks also go up, with a "40 percent increase in lung cancer risk for the group with the highest average traffic-related exposure to NO2 (nitrogen dioxide, a prevalent vehicle pollutant).

Hodgkin's Disease also increase by 51 percent for children whose mothers were exposed to NO2 during their pregnancy.

3. Idling your car during cold weather actually shortens your car's engine life

According to the Environmental Protection Agency and the Energy Department, cold weather makes your car at least 12 percent less efficient at burning fuel.

Stephen Ciatti, who is a former drag racer with a PhD in mechanical engineering, sat down with Business Insider and explained:

“Idling your car in the cold not only wastes fuel, but it’s also stripping oil from critical components that help your engine run, namely the cylinders and pistons."

Argonne Laboratory also has a study to back this up. Using a Jetta TDI and a Ford Focus in their experiment, they "cold soaked" the cars and recorded their emissions until they reached "operating temperature." They again repeated this procedure after they were "hot soaked." Taken from the report: To Idle or Not to Idle (That is the Question) by Argonne National Laboratory (U.S. Department of Energy) Taken from the report: To Idle or Not to Idle (That is the Question) by Argonne National Laboratory (U.S. Department of Energy)

Their studies revealed the following:

  • Warm-up times decrease and cool-down times increase as the temperature rises.
  • In temperate weather, the gasoline vehicle’s catalyst (converter) is ready to work immediately on hot start, but it takes about a minute at idle.
  • In general, the diesel vehicle’s catalyst warms up more slowly than the gasoline vehicle’s, around 2 minutes longer to warm up at 0°C;
  • The catalysts warm up much faster when the car is being driven than when it is idling. This means that driving rather than idling at start-up minimizes start-up emissions as well as fuel use and CO2 emissions. This is true for both hot and cold starts.

4. Modern Cars are built to handle stop-starts / start cycles compared to the old ones

Many ask if the frequent start cycles can damage the the starter battery, alternator, and even motor.

According to a U.S. Department of Energy report about the stop and restart effects of modern vehicles, other factors have more impact on their entire life span. For instance, studies show that accessory use actually shortens the life of a starter battery, and stop-start cycles have very small impact in its life.

Starter motors are more prone to failure in high temperatures, and increased start cycles have limited impact unless it's taken to extreme (more than 20 cycles per day).

Alternators are also not affected by increased start cycles, unless the battery is overly discharged in-between starts.

Also, there is chance that light- to medium-duty commercial fleet vehicles can lead to premature failure if you do more than 20 start cycles in a day.

Here is the approximate scale that the U.S. Department of Energy's laboratory created to show the effects on the starter system based on total engine start cycles per day: Taken from the report: Stop and Restart Effects on Modern Vehicle Starting System Components (Longevity and Economic Factors) by Argonne National Laboratory (U.S. Department of Energy)

Recommendations and Best Practices

1. Make sure to turn off your engine if you're going to wait more than ten seconds.

2. The best and FASTEST way to warm up your engine and the cabin interior is to drive it slowly for 30 seconds. Just remember not to rev up the engine.

3. Remember: Cars with modern fuel injection and engine control systems can handle restarts. To optimize your fuel savings, limit your engine start cycles to ten (or less) each day.

4. If you do not exceed seven engine start cycles per day, you can still save on fuel even for any shutdown that exceeds even a minute.

5. Try not to use your electronic accessories during shutdowns, especially those on longer periods.

6. Batteries aren't that affected when it comes to engine start cycles and the distance traveled between each cycle. However, it's recommended that you drive your car more than eight kilometers between engine start cycles to make sure that your battery is fully charged.

7. Turning off the engine is still the best practice to take whenever possible. In fact, for every 10 minutes your engine is not working, you are preventing one pound of Carbon Dioxide (CO2) from getting released into the atmosphere.


*Daily fuel use by idling vehicles was calculated based on the estimated idling emissions of CO2, and assuming that 8,482 grams of CO2 are released by burning one gallon of gasoline and that 10,272 grams of CO2 are released by burning one gallon of diesel fuel.

**Note that the shaded area under the blue line is the idling fuel rate and the red line before the engine was restarted (at 10.1 s) represented the quantity of fuel that the engine would have burned if it were idling instead of being off. Meanwhile, the shaded area between the lines after the engine is restarted represents the fuel excess on restart.

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