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Old 21st January 2012, 23:34   #46
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Default Re: How long a break at signal deserves engine to be switched off

Depends how deep your pockets are

Getting back to the topic - I remember reading somewhere that every 5 mins of idling is equivalent to about 1 km of running a vehicle and hence it makes sense to shut down if >1 min.

I switch off the engine even if it is 30 sec to go if there is a huge traffic pile up ahead of me. Usually it takes another 10-15 sec for me to move and hence it is still OK. But I have felt it is not that easy with a diesel car and usually I don't take a chance when I drive my friend's diesel. But then, it probably is so because I am not used to it.
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Old 22nd January 2012, 00:09   #47
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Default Re: How long a break at signal deserves engine to be switched off

I normally have a 60 second time limit. Over that.. I turn off the engine. Below that, keep it idling...
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Old 22nd January 2012, 00:35   #48
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Default Re: How long a break at signal deserves engine to be switched off

It is approximately 3 minutes of idling = 1 KM of travel. According to the FORD Driving Classes For Life.

Why are we asking this question?
1. Fuel Efficiency
2. Life / state of engine

1. We are wasting fuel if the engine is on and the car is stationary.
Hence from the first point, shut of the engine, if the wait is more than 30 seconds.

As has been shared by "null" above, it can take another 10 - 15 seconds for your turn to move once the signal has turned to green.

If the signal does not have a countdown timer, look to see if you are at the very begining of the signal or pretty close to it. This most times will tell you that you will have at least 30 - 60 seconds of wait. In which event switch off.

I have noticed that when i do not shut the engine off at the traffic lights at Bangalore, i have got 14 - 14.5 KM/L efficiency.
When i shut the engine off at the traffic lights, i have consistently been able to get atleast 17 KM / L.

A huge improvement in the FE. Either ways i do shut the engine off.

2. If we are looking at it from the life / state of the engine,
There are two ways to look at it, i am sure the gurus can add on to each in detail

a.This cannot be bad for modern cars, that is, the start stop function of the engine, think, the more expensive hybrid cars have auto start stop function which automatically would do the same.

b. I have read car manuals that state, if you drive in stop and go traffic or less than ~ 10 - 15 KM s, this is considered as stressful for the engine. I assume they also mean this because the engine might not have reached the operating temperature at all OR conversely it causes the engine to heat very rapidly.
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Old 22nd January 2012, 00:46   #49
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Default Re: How long a break at signal deserves engine to be switched off

Anything more than a minute and I switch off the engine completely ( Even the music system goes off )
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Old 22nd January 2012, 01:51   #50
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Default Re: How long a break at signal deserves engine to be switched off

I never shut off the engine, IMO its not worth the trouble lol and in chennai AC is also essential.
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Old 22nd January 2012, 02:13   #51
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Default Re: How long a break at signal deserves engine to be switched off

Quote:
Originally Posted by suhaas307 View Post
Also remember that when you start a car, it consumes more fuel than usual.
Only true in case of cold starts, which are not so common when you're stuck in traffic at a signal waiting for your turn. Different engine/coolant temperatures require different amounts of cranking fuel. The hotter the coolant, the lesser the amount of cranking fuel compensation.

Unless the signal's going green in 10-15 seconds and you're first in line, there's no reason to not shut off your petrol engine.
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Old 22nd January 2012, 05:39   #52
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Default Re: How long a break at signal deserves engine to be switched off

How does the whole equation changes if the car is a Diesel one. Unfortunately both my cars are petrol ones and i usually shut the engine off if i see timing anywhere above 30s.

If i have to keep it on for various reasons then i put the car in neutral/handbrake and then wait for the signal to turn green again. I dont think one should stop in a gear and keep the clutch pressed while waiting (When engine is running).

If its day time and hot i usually switch the AC on much before the signal so as when i am at the signal the car is cold already hence no problem for passengers if the AC is switched off for ~a minute.
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Old 22nd January 2012, 07:25   #53
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Default Re: How long a break at signal deserves engine to be switched off

Just to add to how we can judge when there is no indicator at a traffic signal, since i drive along the same route for the daily commute, we can anticipate "our turn" based on which side is passing. Ofcourse if we are way too behind, its a little difficult
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Old 22nd January 2012, 08:15   #54
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Default Re: How long a break at signal deserves engine to be switched off

I'm sorry to say that I'm one of the guys that never switch off, unless it really is a long wait (cars are stuck). In fact I usually idle with a/c when my wife does a quick run to get something from a shop (quick = 2 min to 15 and even 30 min sometimes!). Its usually neutral with handbrake on if the wait is more than 5/10 sec.

I've heard there's some effect of long periods of idling on the turbo. Would be grateful if someone can explain.

Last edited by dingolphie : 22nd January 2012 at 08:16.
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Old 22nd January 2012, 12:23   #55
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Default Re: How long a break at signal deserves engine to be switched off

You also need to consider the status of your battery. Starting poses a load on the battery. During the night, the lights consume a lot of battery power. Unless the battery is less than a year old, I would hesitate to switch the engine off at night. Don't want to struggle to start the car.
Even in day time, with an old battery, I would not switch off the car.

If you want to stop the engine at signals, especially at night, pl. make sure your battery is in excellent condition (as good as new).
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Old 22nd January 2012, 12:58   #56
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Default Re: How long a break at signal deserves engine to be switched off

If your car cannot charge the battery enough to restart an engine already at running temperature, you have bigger problems than having to worry about saving a few drops of fuel. Or you seem to have misconceptions about batteries and how they work.
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Old 22nd January 2012, 13:11   #57
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Default Re: How long a break at signal deserves engine to be switched off

What to do when traffic inches forward in a red light? One have to keep running the engine in such conditions.
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Old 22nd January 2012, 13:40   #58
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Default Re: How long a break at signal deserves engine to be switched off

30 seconds or above and a/c is not required; that's when I turn off the engine.

Quote:
Originally Posted by PM - B View Post
As has been shared by "null" above, it can take another 10 - 15 seconds for your turn to move once the signal has turned to green.
But do note that that 10-15 seconds delay is often because a LOT of people start their engines only when the car in front of them starts moving.

It is a good idea to crank the engine the moment the signal turns green; if everyone started saving those 10 seconds worth of fuel until it was time for them to move, the jams at intersections would clear much slower.

Quote:
I have noticed that when i do not shut the engine off at the traffic lights at Bangalore, i have got 14 - 14.5 KM/L efficiency.
When i shut the engine off at the traffic lights, i have consistently been able to get atleast 17 KM / L.
A difference of almost 2.5 kmpl?! I'm not really convinced by that logic. Agreed, the habit of not wasting fuel at intersections will result in better FE but not THAT much!
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Old 22nd January 2012, 13:54   #59
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Default Re: How long a break at signal deserves engine to be switched off

Looking up articles on fuel saving tips, I have seen that the recommended time period beyond which you should not idle the engine ranges from 30 seconds to 2 minutes.

This should depend on the type of vehicle. I have worked out a rough calculation to estimate this time period. I am not an automobile engineer. Please correct me if I have missed something here.

The fuel consumed by an engine should be a function of three factors:
1. Quantity of fuel injected per engine cycle
2. RPM
3. Time for which the engine runs

The product of these three factors will give us the total fuel consumption.

If we have the data, we can calculate fuel consumed during starting. Then we can calculate the time period of idling which consumes the same amount of fuel. Idling beyond this time period would be wasteful.

In the absence of data, I will make some assumptions here:
1. Fuel injected per cycle during start up is 2 times that of idling
2. RPM during start up is 3 times that of idling
3. Time taken to start is 2 seconds

These are my guesses. I think assumptions 2 and 3 would not be far from the mark. Assumption 1 is a wild guess.

Multiplying the first two factors, fuel consumed per second during starting would be 2*3=6 times that during idling. If starting takes 2 seconds, it will consume fuel equivalent to 6*2=12 seconds of idling. So we should not idle for more than 12 seconds.

I would appreciate some inputs to refine the above calculation.

My practice is to switch off the engine if I expect a halt of more than 20-30 seconds.
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Old 22nd January 2012, 18:26   #60
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Default re: Switching off the engine at traffic lights

@libranof1987

Amazing isnt it, so hard to believe, but for my driving pattern, this is the difference i see. 2.5 KMPL. If i do not switch of the engine at signals longer than 30 - 40 seconds.

I drive mainly in Bangalore city from 8.30 AM to 9.30 PM. You may or may not have observed that Bangalore city roads have the highest # of signals per KM of road.

So it will depend on your driving patterns and city, but make no mistake, you will see a substantial difference, which in my case is 2.5KMPL.

@highway_star has given an approach to the logic also.

On the other hand, waiting 10 - 15 seconds is an indicator and am ready to move the movement the ride in front of me rolls.
So no worries on holding up traffic.
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