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AskPablo: American Idle

| Monday April 2nd, 2007 | 8 Comments

tailpipe.jpg Is it better to let your car warm up for a bit or tear off down the road right after ignition? Having lived in Maine I know that automatic car starters are a hot commodity when it is -30F outside. It is understandable that people don’t want to get in their car if their hands are going to freeze to the steering wheel but some people take it a bit too far. Some have been know to use the running car to get rid of the ice on the windshield. This is what the ice scraper was invented for, suck it up! Here in sunny California people may run their car for other reasons; to get the AC going or because they think it is better for the car (or the environment).

To help me answer this week’s question I turned to my good friend Greg Rock, founder of the Green Car Company (www.greencarco.com) of Seattle, Washington. The Green Car Company is a green vehicle dealership specializing in biodiesel, electric, and hybrid vehicles and providing maintenance and conversion services. Greg and I studied engineering together at Cal Poly.
According to Greg “Traditionally warming up your can (sic) was an essential part of clean combustion. A cold engine would be running with the choke wide open and if throttle was applied it would be running rich creating a higher rate of air pollutants. While the choke is turned on you also burn fuel at a higher rate.” In the late 1980’s internal combustion engines with fuel injection replaced the need for a choke valve but they still burn “rich” for a short while after starting.
Greg goes on to say “today cars do not take as long to warm up and I think they are better at getting a complete combustion sooner. Still though, a cold car will burn more fuel I would guess (10-15%) for the first 1-2 miles” and “the catalytic converter will not warm up until 5 miles into your trip so many of the regulated emissions are not being reduced at all.”
This brings up an interesting matter. The air quality laws in the US, and particularly in CA, limit the emissions of local air-quality damaging gases. However, the catalytic converters that are needed to comply actually create stronger greenhouse gases. According to Wikipedia (here) “A three-way catalyst reduces emissions of CO (carbon monoxide), HC (hydrocarbons), and NOx (nitrogen oxides) simultaneously” but a catalytic converter can form “H2S (hydrogen sulfide) and NH3 (ammonia)”. Catalytic converters also account for 50% of total nitrous oxide (N2O, dinitrogen oxide, ‘laughing gas’) emissions to atmosphere. N2O is about 300x as potent a greenhouse gas as CO2 and accounts for around 7% of the overall greenhouse effect despite its small concentration in the atmosphere (Source: Wikipedia).
So, from a fossil fuel consumption standpoint it is not better to let your vehicle warm up. “Putting that fuel to use as soon as the car is running would be best,” says Greg. But from a local air pollution standpoint you could argue for letting the car warm up for longer, definitely until the choke turns off (or for fuel injection vehicles; until it starts burning cleaner), but there may even be an argument for letting it idle for even 5-10 minutes until the catalytic converter warms up. Greg’s final verdict is: “I would let your car warm up for 30 seconds to a minute before driving to get that choke turned off (or the modern car’s equivalent) and also to let all the engine oil circulate before applying heavy loads to it.”
In the coming weeks we will be examining other vehicle related shenanigans, so please send in your questions!
Pablo Päster
Sustainability Engineer


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  • Tyrell Sykes

    Hmm…. so if I have to go in for a smog test, i should drive around for about 10 miles before I stop in eh?

  • http://www.ecotality.com Janis Mara

    Thanks very much for the answer to this question, which I have often pondered. The 30-second-to-a-minute rule – does it vary according to the temperature or how long your car has been parked? For example, I’m assuming if you only left it parked about an hour after it had been driven quite some time, no warmup is needed, correct? Would you need to warm up longer than the prescribed maximum one minute on a cold day?
    Janis Mara

  • http://www.AskPablo.org Pablo

    If the car is warm then the catalytic converter will begin working much quicker. The engine will still burn a bit more rich upon startup though. The engine RPMs will also be a bit higher momentarily as the battery recharges to make up for the energy lost to the starter, this will use more fuel. Generally this isn’t very long though, so it makes sense to turn your engine off at a railroad crossing or a long red light than to keep it idling.

  • Pete

    Hmm – I always wondered when it made sense to turn off the engine. I remember my dad told me that the car uses more gas to start up than it does to idle, so if you’re waiting for someone in a store, leave the car running. But then I heard that gas/electric hybrids shut the motor off at stop lights, and I wondered if engines had gotten more efficient at starting up. I live in NYC, and tho’ I usually don’t use my car, when I do I’m stopped at lights for minutes at a time. Maybe I should be turning off the motor?
    I’ll try it out over the course of a tank of gas and see how the mileage works out. Yes, I check the mileage on every tank of gas, ’cause that’s how my dad raised me.

  • http://www.ecotality.com Janis Mara

    What! I’m really surprised to learn this. I had some vague notion that it was not a good idea to turn your car off whilst waiting at a red light or similar situation. Like, if you suddenly see some drunken idiot speeding right at you, it gives you even less time to react. But I guess I was wrong on that, eh?
    Janis Mara

  • Joseph Stetter

    If you are going to start your car and run it … you better make sure you have lots of fresh air circulation … otherwise you can build up CO and it can create an overexposure to this toxic gas .. goto http://www.transducertech.com and see how to test for CO with a small Pocket CO instrument

  • Rolly

    I have a 1999 venture van w/ 132k miles.
    service engine soon pop-up in the dash board
    high rpm or idle is not on normal run.
    when i step on the gas half way it will not
    continue to verb up.I added some additive in
    the gas tank.Same thing happen.
    Please help me any suggestion will appreciated.

  • simon

    Sorry but this is complete nonsense.
    The catalytic convertor will not warm up from idling at all.
    It works ok a few hunderd degrees celsius,
    it will not reach that when idling… definately not.
    the automatic choke does not go of when idling, it stays on, so drive when you have oil prssure which only takes like 10 seconds, no need to wait for a minute…that’s insane, the motor does not heat up one bit from idling one minute at all…
    Sorry but i know about cars, i work on them every day