OK, I've always been told that it takes more gas to restart the engine than it takes to let it idle if you're just waiting for 'a few minutes' but no one has ever been able to tell me what that amount of time is.

So, question is: Say wife has to run into the gas station, just to grab a coffee. If you expect her to take 2 minutes, would you use less gas idling, or turning off and restarting?

Anyone know the an approximate 'break even time' between idling and shutdown/restart?

Just something goofy I've wondered about <img src="/forums/images/graemlins/confused.gif" alt="" />

In most municipalities that I know of, leaving the vehicle idling is illegal. If ticketed, that translates to a lot more gas wasted. <img src="/forums/images/graemlins/pfft.gif" alt="" />

EDIT: Also, if someone runs off with your vehicle, that translates to a whole lot more gas wasted. <img src="/forums/images/graemlins/butwiggle.gif" alt="" />

I read somewhere that if you have to idle for more that 10 seconds you may as well turn it off. Not very convenient in traffic I would imagine <img src="/forums/images/graemlins/pfft.gif" alt="" />

If I have to get out of the car or wait by the side of the road etc I turn it off <img src="/forums/images/graemlins/kewl.gif" alt="" />

i heard that every time you start your car/truck it uses as much gas as idling for 1 minute.


but bad idea to leave your car running when going in a store, cars get stolen like that all the time.

That sounds about right, especially with the precision control offered by todays fuel injection systems...

If restarting a hot engine took a lot of gas (why would it?), then the Prius Hybrid would be getting terrible mileage because it constantly stops and restarts the engine, based to power demand requirements..

True. I don't think it takes any more gas to restart.

The only time it would take more gas is on a cold start, on an efi truck. The ECU reads coolant temp as cold, and sends out full fuel to all the injectors plus your cold start injector, and leaves it that way until the cold start timer runs out (not sure how long that is, 15 seconds maybe?)

True.. You really see the effects of a cold start in a Prius (I own one).. It not only gives you instantaneous and average consumption rates, but also mpg averages at 5 minute intervals (the bar graphs).. The mpg on the first 5 minutes of a cold start is always low.. burning energy to get everything up to operating temperature, including lubrication oils.. The second 5 minute interval is higher but still low compared to the overall average. By the third 5 minute interval, it is up to normal mpg.

Research by Oak Ridge National Laboratories analyzed and made public many findings about the Prius design.. One of them is that when the lubricating oil in the HSD drive (the planetary gear set and the two oil mist spray cooled motor/generators) is cold, the parasitic loss from the cold lubricating oil is significant, quite high.. As the oil comes up to temperature, the losses start dropping dramatically..

Same thing happens in conventional vehicles.. it just that no one has published a lot of about it.. I sure mpg is the reason, later model vehicles have been redesigned to operate with low viscosity lubricating oils.. and good reason for running synthetic oil..

The pic below while not of a cold start, shows the mpg readout... on Nevada US50 btw.. <img src="/forums/images/graemlins/kewl.gif" alt="" /> <img src="/forums/images/graemlins/kewl.gif" alt="" />

OK so it just takes a matter of seconds worth of idle to equal a warm start, anytime the wife runs in the store and I'm still sitting in the car (I also never leave car running unattended) then I might as well just turn it off <img src="/forums/images/graemlins/kewl.gif" alt="" />

Very cool place. I spent a semester there in college working in a lab. Also went over to UT Knoxville and saw Payton Manning play when he was just a wee college QB back in '94.

Ahhhh, college days, what fun.

I'll stop now so I don't hijack my own thread <img src="/forums/images/graemlins/zombie.gif" alt="" />

The one thing not brought up is the wear and tear on the starter, starter relays and ignition switch. Constantly stopping and starting the starter wears out the contacts and the motor on something that is only designed to operate so many cycles.

If you have an automatic and the starter just up and quits working, you're screwed. So, whatever ounces of fuel that might have been saved is probably not worth the inconvienence.

Stopping at the gas station and turning off the car once a week is probably not going to make a difference, but I've seen people turn off their cars at stop lights, through drivethroughs every day, etc.

Also, the starter uses up a lot of amps. It would be interesting to see how much fuel has to be used to recharge the battery back to normal levels after using the starter even once. I'm sure it's minimal, but if you're constantly starting, after 1 or 2 minute stops, then it probably adds up.

The prius uses the electric motor to start the engine it's much heavier duty, not to mention, much more powerful than the typical starter motor and design to turn on the motor many thousands of times with minimal wear and tear on the system.

Another thing to consider is that even though the engine is warm and the computer generally retains memory, I wonder if most cars still have operate on a closed loop for a few seconds or minutes after each start just to review all the sensors, before it settles down to the most effecient mode. In other words, are you burning more gas as the engine decides what the parameters should be by restarting it, then if you left it running for 1-2 minutes with the engine running at peak effeciency.

I'm sure the Prius has that problem solved, as it's just designed to pick up right where it left off and the sensors probably don't turn off when the motor does.

It's an interesting discussion. I would imagine the hybrids are able to save a significant amount of fuel by virtue of the fact that they do turn off the engine at every stop.