So I’m driving a modern (let’s say built within the last five years) conventional gasoline-powered car (not a hybrid of any sort). Let’s say that it’s a reasonably standard 2.0 or 2.4 liter four cylinder (as opposed to a high performance, high compression v-six in, say, an Audi or something). I’m driving on city streets and come to a stoplight. As soon as my car has stopped moving, I turn off the engine. When the light turns green, I turn the engine back on and proceed on my way.
Many years ago (probably I last heard it discussed during the gas crunch of the seventies), the conventional wisdom was that for typical duration traffic lights, this would actually use more gas than just leaving the car running. The supposed reason being that it took some substantial amount of gas to get an engine running.
So what is the situation today, with modern engines? Is the threshold at which turning the engine off and back on again uses less gas than leaving it running somewhere in the few seconds, or over a minute, or even longer?
Does it make a difference if the car has push-button start vs. conventional turn-the-key-to-start?
Based upon instrumented tests I’ve conducted with my own vehicle, you’ll save gas by turning off the engine at the stop light.
Factors that will rob you of some savings:
short lights
light is very close to where you started your trip (restarting hot motors uses less juice)
same goes for really cold days
My results aren’t published, the savings is (obviously) small and it’s hard to get NASA-quality data on surface streets, but I’ve never seen any results that trended in the opposite direction.
On edit:
This procedure may be illegal in your jurisdiction.
There are a growing number of cars that do this automatically, precisely for the fuel savings. AFAIK the engine is entirely standard, and there aren’t many significant modifications to anything else. You’ll probably see similar fuel savings if you do it manually. Wiki has more about start-stop systems. In short, these systems really just use a specially designed starter, and don’t have accessories that run off of a belt from the engine.
Here’s a Slate articlefrom 2008 that says the break-even time is as low as 10 seconds. They go on to say that the “engine starting uses lots of fuel” idea may have been true with a carbureter but is now untrue with fuel injection systems. Oh and that more frequent starting isn’t particularly bad for your car.
Things to consider is the wear on the ignition switch, starter motor, etc. Also consider that A/C’s don’t like the start-stop the compressor, and some electrical systems go off during starting.
Need to replace just one item more then a fuse that wore out due to this and there goes all your gas savings you will ever achieve doing this the rest of your driving life.
Not to mention getting rear ended while you dawdle getting launched. Failing to move out promptly from a green light when there is any other traffic is extremely selfish.
I once heard on a radio show (Quirks & Quarks, on CBC) that the fuel savings kick in when you turn off the engine for at least 17 seconds at a time (IIRC). This was for a small, 4-cylinder non-hybrid car. It wasn’t very specific.
Slightly off-topic, from the same source :
However, when restarting the engine, the vehicle will emit more pollutants for the first few seconds because the catalytic converter will have cooled off. Turning off for at least 1 minute at a time yields an overall decrease in both fuel consumption and emissions.
I can avoid this by only killing the engine at lights where I can anticipate green returning.
once the opposing light turns yellow, I know to take the engine back online.
Thanks for all the informative replies. Just to be clear, I am not planning on doing this, it’s just something I’ve been wondering about (I bought a new car recently, and I’ve been paying a lot of attention to the gas mileage). The place where I am inclined to do it is while waiting in the drive-thru of a fast food joint. Presumably no laws against that, little chance of the need for emergency reaction, and less likelihood of the person behind me getting annoyed or impatient if I take an extra few seconds to get moving.
Not necessarily. I used to have a high-performance sports car. It was vey responsive to even a slightly depressed gas pedal. Had I hit the gas as soon as the light turned green, I’d have run my car right up the tailpipe of the car ahead of me. I used to get honked at a lot by drivers behind me, but I knew what I was doing and ignored them.
Off topic but tangentially relevant. I think one of the reasons people like taxi drivers also turn off the engine is to help keep the automatic transmission cool.
I don’t know if this is still true, but when the car is idling, the torque converter is still spinning causing heat to build up. Plus since the car isn’t moving, there is less air to cool the hydraulic fluid radiator. If this isn’t true any more, I’d be interested in knowing how they get around these problems. Thanks.
And in other jurisdictions, it may be illegal not to do it. In the city I live in, a lot of red lights have signs instructing drivers to shut their engines off.
I have a car that does this automatically. It is a high efficiency diesel turbo thingy. It also has an energy recovery system and fancy bells and whistles electrics that run off the recovered kinetic energy where possible. When you come to a halt, put it in neutral and take your foot off the clutch the engines dies.
When you dip the clutch again to put it into 1st, the engine starts automatically (and does so very quickly so there is no delay in getting away)
It also has low rolling resistance tyres and other energy saving bits and bobs and I get an average of 60mpg from it. So that’s very welcome in these times of very high UK fuel costs. As I’ve mentioned in another thread it is by far the best car I’ve ever owned. It’s a Skoda Yeti Greenline if anyone is interested.
My car, the Honda CRZ, does the same things. The autostop feature used to annoy me, but now I really like it. It starts right up as soon as I depress the clutch, so there’s no delay off the line at a light.
I’m getting a little over 43 MPG, so no where near as good as your car, but much better than most cars in the US.
The stop-start systems I’m familiar with are only on manual transmission cars. At a stop light, you shift to neutral and let the clutch out, the car shuts off. When you press in the clutch again, the car starts and is ready to go by the time you shift into 1st. For these cars, there is almost no difference in delay, compared to a normal manual trans car.
