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I guess spark plug life will be reduce not engine life.

In addition to spark plugs, pistons, rods and cams also are affected due to high initial force affecting them. Hence engine life does get affected due to frequent stop/start cycles.

Much more than pistons, rods and cams, it is the bearings that suffer most.
The only time metal to metal contact occurs in bearings is during stop/start, ie during engine run-up from stop and during engine run-down to stop.
It takes a few seconds for the hydrodynamic oil film to form. Once this film forms there is no metal to metal contact in bearings, and hence no wear.

This may be true when the engine is started from cold. If it is already warm (example in traffic light)I think thin layer would be present to avoid such damages.
Yes it is not advisible to speed up as soon as you start from cold as to give time for the oil to circulate

I would think that actually MPFI engines would waste a lot less fuel at idle than carburettors. So 2 min to 7 sec seems sort of unreal to me. Only way seems to be to try it out and see over 1 cycle of the top-top fill of fuel.

Also it is not idling that is the problem. We waste more fuel to get the car into momentum again.

Aka_iitd, it is not sufficient to have a thin layer of oil in bearings. What is needed is the hydrodynamic wedge, which is a load-bearing wedge of oil. It takes a few seconds for oil pressure to build up, and then another few for the hydrodynamic film to form.
Till the process is complete, some wear does take place in bearings, due to metallic contact.
Theoretically, if an engine were to be kept running perpetually, the bearings would never wear out!
Mind you, this applies only to bearings that are rotating and not the ones which merely oscillate, like rocker arm bearings. Oscillating bearings can never form hydrodynamic films and work on a far less efficient lubrication called boundary lubrication.

It would be very difficult to obtain figures for HOW MUCH difference is made to the life span of an engine by frequent stop-starts. Theoretically the wear & tear is much higher. :)

Bearings are not exactly made up of titanium or diamond. So don't worry if minute theorotical wear and tear happens due to stop/start cycles. Replacement of these parts won't really drain ur pocket as well. And you owe that much to keep the environment clean. So please shut off your engine wherever possible.

So in this context, could you throw some light on the matter of catalytic converters. I am told these require the car to be at a steady-state temperature to do their job.

As it is, in our stop/start traffic our engines are often unable to reach steady state temperatures. If we switch off at each signal, will not the emissions far exceed what the system is designed for, since the catcon cannot function to optimum ability?
This presents a dilemma from the environment point of view.

The problem is there are so many hypothesis and theories around. Whether all these practically work out is still dazy and not all have been concretely nad convincingly proved. Guess sometimes its better to keep things simple.

yes, we do waste energy to roll on from a standstill. but that is not in our control. what *is* in our control is the ignition switch. it would be naive to assume a stop/start cycle did not affect anything, but the point I think we should be thinking about is that is the additional wear and tear worth the saved fuel? and the reduced pollution?
if everybody shut off their engines whenever they could, it would equate to a lot less pollution, not to mention, additional FE.
the government, instead of educating people sits silent about this. the least they can do is to install properly functioning timers on all signals instead of stuff that looked like the Predator's self destruct timer.

and by the way, there still has been no definitive answer to my original question. how much idling time equates to a stop/start - fuel wise?

Engine wear at start up is right... but on what basis do you say there is metal to metal contact on turning it off?? Never heard of this....

Wish it were that simple. Replacing these parts means having clumsy mechanic hands inside your engine. Something I would definitely do everthing to avoid!! Lol!

I for one dont switch off my car or bike engine unless the stop is for more than 2 mins approx.

Your concerns about the well being of your car are absolutely justified.
As said earlier wear and tear could be equated by reduced fuel consumtion. And there shouldn't be any doubt about reduced pollution.
Sad thing is if you have made up your mind on shutting ur engine only for more than 2 mins stops,then I think probability of encountering such signals would be dismally low. And 95% of time you would be wasting oil.
Just look at the bigger picture. If each one of us could join and contribute, it could really make a difference.

"No fuel' as in not even a drop of fuel ? I don't think so. Maybe much lesser amount, but definitely there has to be some fuel injected to keep the engine cycle running.

The kill-engine-if-you-stop-for-2min-or-more guideline was many years ago for carb engines. Rule of thumb for today's engines is if the stop is for 15sec or more, it makes sense to kill engine. And while I don't want to deny any of the effects mentioned by various tbhp-ians on pistons, con-rods, Catcons etc etc, let us also be aware that cars are built to last today and can take this and much more without breaking into pieces.

Yes. Not only is the environment helped via less emissions, we save money on fuel, country saves in foreign exchange and it also reduces health hazards. If you bike even occasionally, you can understand the last benefit. While in a traffic jam with many cars/bikes/buses running, you might have experienced a kind of dull headache/nausea, which is due to CO. If we kill the engines at signals that have timers, this can help a lot.

the catcons go out with an FFE installation anyway. so there's one less thing to consider.

@SB- I wouldnt mind if some of the bikers around here get a little dull headache and nausea.

Switching off the engine for stops of 15secs is kind of overkill anyway.