Does it take more gas to start engine , than to keep running.

July 10, 2005 7:22 PM

With the price of a gallon of gas now approaching/exceeding $ 2.50 , is the urban legend that it's cheaper to keep it running and that the car uses more gas in start-up than in running a few minutes at the drive through bank teller, railroad crossing, etc. , still accurate ( was it ever? ) I realize there are other mileage factors in the equation , but at some price point the theory must fail.
posted by Agamenticus to travel & transportation (18 answers total)

The Car Talk guys say it's wasting gas to leave the motor running for even a 30-second stop, though it's harder on your car to keep starting it.
posted by Airhen at 7:41 PM on July 10, 2005

Maybe a side question that applies: Do engines still get clogged up by idling too much, therefore, these standing/running times reduce efficiency?
posted by Goofyy at 8:12 PM on July 10, 2005

I think engines get clogged up from repeated, very short (5-10 minute) trips that don't allow oil to circulate through and lubricate the system. In turn you get engine wear and such that requires expensive repair work down the road.
posted by Rothko at 8:25 PM on July 10, 2005

Rothko, car people I know turn the engine on and wait a whole minute before they start driving for this very reason.
posted by banished at 8:54 PM on July 10, 2005

I don't understand how the price of gasoline could have any effect on the basic calculation. How much gas it takes to start your car and how much gas it takes to leave it running doesn't change with the price of gas, so the ratio would always be the same regardless.

It's only when you start factoring other things like engine wear and the cost of repairs, which aren't really considered by the original argument, that cost comes into play at all.
posted by jacquilynne at 9:24 PM on July 10, 2005

I think this theory started back in the days of carburated vehicles which used to require a healthy squirt of gas to get started.

Now, with fuel injection I doubt there is any savings at all.

The new fuel efficient hybrids shut off at stoplights if that is any indication.

Personally, the excessive engine wear and loss of air conditioning / heat are a deal breaker for me.
posted by vaportrail at 9:33 PM on July 10, 2005

Short version: It's ridiculous to think that idling for longer than 30 seconds is better than shutting off the engine and restarting. From the Washington State Department of Ecology Idling Reduction Fact Sheet:

- Drivers who shut off their engines, rather than idling for 30 seconds, benefit from both fuel savings and improved air quality. (average of recommended times from the U.S. EPA, Natural Resources Canada and Programs Europe)
- Frequent restarting has little impact on engine components like battery and starter motor. Wear caused by restarting is estimated to add $10 per year to the cost of driving, money likely recovered several times over in fuel savings. (Natural Resources Canada)
- Excessive idling can be hard on your engine because it isn’t working at peak operating temperature. Fuel doesn’t undergo complete combustion, leaving spark plugs dirty and contaminating engine oil. (Oregon’s Clean Air Action Day fact sheet)
- Idling isn’t an effective way to warm up your vehicle in cold weather. Modern engines need no more than 30 seconds of idling on winter days before starting to drive.

Read the whole thing. The link came from the local weekly here in Raleigh/Durham, where a columnist (who's also a pal) happened to discuss this very issue (a bit long-windedly) this week.
posted by mediareport at 10:09 PM on July 10, 2005

I don't understand how the price of gasoline could have any effect on the basic calculation. How much gas it takes to start your car and how much gas it takes to leave it running doesn't change with the price of gas, so the ratio would always be the same regardless.

It doesn't. But the price of gas does affect how much you can afford to waste and therefore your interest in not wasting it.
posted by Airhen at 10:52 PM on July 10, 2005

Idling isn’t an effective way to warm up your vehicle in cold weather. Modern engines need no more than 30 seconds of idling on winter days before starting to drive.

Whoever wrote that doesn't know what temperatures below 0 are (32 for those of you that measure temperature like the queen did :-P ). Of course, about 5-10 minutes of driving will heat up the engine enough to get heat from the heaters, but 30 seconds of idling will get you nowhere (about 20 mins will do it for full on heat). Not that I'm advocating it or anything (I'm not, I use an electric heater myself for those days) but don't take that 30 second advice!
posted by shepd at 11:04 PM on July 10, 2005

I don't understand how the price of gasoline could have any effect on the basic calculation. How much gas it takes to start your car and how much gas it takes to leave it running doesn't change with the price of gas, so the ratio would always be the same regardless.

It depends on if you are talking about the cost of fuel, or the cost of engine repair. I was a little confused at first, as well though.
posted by delmoi at 11:29 PM on July 10, 2005

I think this really depends on the car and it's software.

When a car starts up (and it's cold), the engine management software operates in whats called open-loop. In this mode, the software uses pre-programmed values to determine how much fuel is needed, how much spark advance to use, what the idle speed is, and more. These values are designed to insure that your car starts reliably, minimizes damage to the engine and allows it to be drivable immediately. They are not chosen to be fuel efficient.

Once the engine has warmed up, the engine moves to closed-loop operation. By now, the O2 sensor is working and the car adjusts the fuel/air ratio based on how much oxygen remains in the exhaust after combustion. The knock sensors also come into play, allowing the engine to advance the spark to the bleeding edge of knocking, meaning more power and efficiency.

If the car is already warm when it is started, it may move very quickly to closed-loop. However, some open-loop operation still happens, the length of which can vary.

Anyway, let's assume that all that closed/open loop stuff doesn't matter and do some back-of-the-envelope calculations:

Assume your car gets 25 miles to the gallon on the highway. Assuming the highway rate is 60 mph, this means you burn 2.4 gallons an hour, during which time you are probably using moderate-low throttle. Let's assume you car uses 25% of this rate at idle, 0.6 gallons an hour or 0.01 gallons a min. If gas costs $2.50 a gallon, thats 2.5 cents a min. If you spend 5 min idling every week day, 260 days a year, 1303 min a year, you spend $32.59 a year in gas idling.

Fixing your worn-out starter would cost $300 or more, so you would have to hope your starter would last 9 years before needing replacing, which is unlikely considering the extra 2394 starts it's going to have to do, assuming you start the car one extra time a day.

I'd just leave it running...
posted by darkness at 1:22 AM on July 11, 2005

I think engines get clogged up from repeated, very short (5-10 minute) trips that don't allow oil to circulate through and lubricate the system. In turn you get engine wear and such that requires expensive repair work down the road.

The problem with very short trips isn't with oil flow, it's with moisture in the exhaust system.

Getting the car completely warmed up, including the exhaust system, takes about 20 minutes (just like human cardio exercise!). This allows inevitable water in the exhaust to get completely cooked out. I don't recall how it gets there in the first place, although I can think of a few ways. Without the full heatup, the persistent water eventually rusts/corrodes out the system.

Note: I'm not saying anything about stopping and restarting the engine, sounds like the posts above have that covered.
posted by intermod at 4:40 AM on July 11, 2005

intermod and darkness,

I live about 7 miles from where I work. So I often drive about 10 minutes, sometimes 15. My dad was telling me that that short driving time was difficult on my car, for the reasons that intermod described. Should I start the car early one day a week to dry out the exhaust? Should I be thinking of changing the oil more frequently (right now I change it at about 3000 miles)? Side question - do either of you subscribe to the idea that you should change the oil every six months as the oil breaks down anyway, even if you haven't driven 3000 miles?

Car people on AskMeta! What don't we have?!
posted by Slothrop at 5:18 AM on July 11, 2005

intermod,

good point about the water in the exhaust system.

If you want to know where the water comes from, this is how my dad explained it to me:

We call fossil fuels hydrocarbons. Hydro=hydrogen. We call burning "oxidation" oxi=oxygen. Take to of H and add one of O and you get the drip out of the tailpipe. You're left with a bunch of C that either comes out as CO (deadly, if engine not burning well) or CO2.

That, and he once illustrated how a fluid clutch works with a glass of water, some grapes, and a stir stick.
posted by sol at 5:36 AM on July 11, 2005

sol,

There's quite a bit of C that comes out as C, as well. Lots of short trips and idling fouls your sparkplugs with carbon soot.
posted by electroboy at 6:53 AM on July 11, 2005

The lab right beside mine does vehicle emissions studies, so I asked their head chemist. Out webpage seems to be down right now, but if you go here in a few days and follow the vehicles link, you can find the test results for some of the vehicles they've done.

From what I understand, 30 seconds to 2 minutes is the cross-over point for fuel consumption. Older cars take more gas to start than newer ones, so turn off your late-model cars earlier.

The trade-off is for increased tailpipe emissions. After 30 s to a minute the catalytic converter cools off. On restart, the car produces higher levels of NOX and CO until the converter reheats.

To summarize: turning off saves fuel but creates more smog. Idling burns more fuel, but keeps the air cleaner.
posted by bonehead at 7:03 AM on July 11, 2005

Slothrop--

You should get your car up to temperature at least once weekly. Depending on where you live (temperatures), starting it early may not accomplish the task. You're best off taking the scenic route every now and then.

"Blowing out the carbon" (driving at high RPMs for extended periods) is not necessary, but a half-hour of spirited driving every now and then is a good thing.

Persistently running the car without getting it up to temperature will indeed increase oil contaminants (as well as cylinder contaminants, resulting in fouled plugs). Changing the oil every six months isn't a bad idea for an infrequently driven car. Learn to do it yourself if you don't know how and need to save money.
posted by Kwantsar at 8:16 AM on July 11, 2005

Thanks for the debate fodder. I especially liked the article link from Raleigh, NC from mediareport.
posted by Agamenticus at 8:08 PM on July 11, 2005