AskPablo: American Idle

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 ( 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