Where do the MPGs go? in town/highway consumption breakdown

[pfunk678]

Thyme,

I was being careful to avoid speaking in absolutes. My point being about "reducing" idling and braking. I personally follow the traffic laws, but it is very achievable to brake less and do more coasting as well as adjust my speed (up or down) to make more green lights. These are things that certainly can't be controlled all of the time, but there are many opportunities to be taken.

[some_other_dave]

Originally Posted by Thymeclock

Just out of curiosity, how many times in the course of a day do you shut off and restart your car at traffic lights?

The typical break-even point for where more fuel is consumed idling than re-starting is on the order of 10 seconds. More for some cars, less for many.

In my CRX, I shut the car off probably a dozen times on the way to work, and a dozen more on the way home. Probably much more than that, as I frequently used pulse-and-glide, with engine-off coasting on the glide. I put about 40 thousand miles on the car doing that.

Wayne Gerdes over on cleanmpg.com has a couple of hundred thousand miles worth of re-starting constantly.

Wayne and I (not sure about PaleMelanesian) have a good bit of time spent in heavy traffic coasting with the engine off to stops. So far, nobody has tried to shoot either of us. Nor have we left a string of wrecks in our wake.

Some braking is unavoidable, especially in emergency situations. Most braking that most drivers do is avoidable, as long as you keep planning ahead. Gas-to-the-light-then-brake is immensely wasteful. Even gas-to-maintain-speed-to-the-light-then-brake is somewhat wasteful.

Most idling is avoidable, unless there are mechanical considerations that prevent you from turning the car off. (E.g., many automatic transmissions can be damaged by a lot of moving with the engine off; some cars dump a whole lot of fuel after a re-start; some cars have starters that are "iffy"; some cars have batteries that are too weak or small to keep their lights on at night for any real amount of time with the charging system off...)

Some of the choices are ones that you will not make. That does not mean they aren't feasible choices for other people.

-soD

[jamesqf]

Err... Could some of you folks explain exactly what this "idling" thing is? Those of us with well-designed cars, like the Insight, just don't know. Then maybe you could explain how to hand-crank a Model T :-)

[euromodder]

4-way stops are not universal.

While the braking issue will not apply there, all others do just as they apply elsewhere.

Idling at a light won't have that much effect on a tank as it has in the NEDC (where stop/start systems account for some 8% fuel savings).
The acceleration after the light will be far worse than some idling at the lights.

[Thymeclock]

Just to illustrate how many traffic lights and stop signs are in my vicinity, here are some examples. To go from my home to my workshop is one mile. The most advantageous route (with the least possible number of traffic control devices) comprises 8 stop signs and 2 traffic lights. The alternative, which is about one and a half miles has 10 traffic lights and 2 stop signs. The speed limit is 30 MPH for any possible route. That is just the short but essential trip that I must drive once or more, daily. If I need to drive to service a customer, the number of lights and signs encountered increases enormously.

Out of curiosity I made a tabulation of all the lights & stop signs I encountered today in a typical 12 mile round trip in my local area. The total was 62 traffic lights and 10 stop signs. Typically a good portion of the lights (often a majority) are not well timed and will require stopping. The big problem is that it is often impossible to predict how long you will be waiting at a red light. At the few lights that are predictable (and known to be long) I do shut the engine off. But there is no way I'm going to do that for every red light I come to. It would be inconsiderate to keep cars behind me waiting while I'm restarting my engine. I don't like comatose drivers who hold up traffic by not moving after the light turns green, so I don't do it to others.

We now have photo enforced red light cameras here and I found that the best way of avoiding a ticket is to speed up as needed to be sure to get through that intersection while the light is still green. The length of the yellow light is not absolute; it can vary and the enforcement powers are enjoying the uncertainty it generates and the cash cow revenue it brings the county.

I do engine-on coasting (in drive) always, and neutral coasting whenever possible, meaning approaching stop signs, as it's an automatic. But the opportunities are relatively few. I refuse to do engine-off coasting as there is so much traffic here that to do it would be an increased risk at the least, or even a safety hazard.

[Cd]

Thank you for posting this very useful information !

[Fat Charlie]

Originally Posted by jamesqf

Err... Could some of you folks explain exactly what this "idling" thing is? Those of us with well-designed cars, like the Insight, just don't know. Then maybe you could explain how to hand-crank a Model T :-)

It's something that people with manuals can allow the car to do if they feel like wasting gas.