[dhanushs]

Originally Posted by noopster

Is it a function of the engine technology and environmental conditions or a constant amount?

Yes, technology plays a major role in fuel consumed while starting. As I mentioned above, carburetor engines consume more fuel than efi ones on starting. And mind you, on cold start it gets worse.

Also, environmental conditions too play a major role. For example, cold start will consume more fuel than a warmed up engine.

Again, as I mentioned earlier, newer IC engines start very frugal. If you look at hybrids, which use electric motor during idle/low load conditions and fire up the IC engine when pedal to metal, you might get the point.

[dot]

Originally Posted by ilangop

Yes Dot, more fuel is consumed in cold start. But in the drive cycles you have compared above, there chance that you drive the car in 5th gear is more in the 8 km stretch. So this could also be a factor in the fuel economy increase.

Agree, you have an important point. However even in the one kilometer stretch, I could drive the car in 5th gear for sometime as traffic conditions of the commute is pretty good.

One of the perks of staying in Pune!

[honeybee]

Since most modern petrol cars are MPFI I would like to make an assumption here:

Excluding cold starts where the engine is cold (not the ambient temperature), the fuel consumed would be more than when the engine is already warm/in its normal operating temperature.

When the engine is already warm/in its normal operating temperature, the fuel consumed at start up would be the same as when idling (since the engine would more or less be at the same RPM level).

Now, cold starts are out of the equation because most of the times we switch off the engine in traffic jams or at traffic lights when the engine has already reached its operating temperature.

Assuming the engine requires cranking for about three seconds, it's probably equal to three seconds of idling.

Starter motor is out of the equation because I have yet to see any hard proof that frequent start/stop affects the life of the starter motor.

The only component that now remains unknown is the battery. A good battery can easily provide enough juice to crank the engine repeatedly for about three to four times (my own experience when stuck with different engine starting problems). However the important point here is how much time does it actually take for the battery to replenish this power once the engine starts and the alternator kicks in. Is it five second? Is it ten seconds? This is the minimum amount of time the engine should be kept running once it is started.

Assuming the cranking requires three seconds, I can switch off the engine for any interval of more than three seconds to save some fuel. Assuming it takes my battery ten seconds to recover the juice it spent on cranking, I must keep the engine running for ten seconds once I start it.

All we now need to figure out is the time taken for the battery to recover its juices.

[dhanushs]

Originally Posted by honeybee

When the engine is already warm/in its normal operating temperature, the fuel consumed at start up would be the same as when idling (since the engine would more or less be at the same RPM level).

Umm.. I have no concrete info, but still I think the fuel required for starting is definitely more than idling.

Originally Posted by honeybee

All we now need to figure out is the time taken for the battery to recover its juices.

AFAIK, a normal 1.5 sedan might take about 200-250 amps for starting. Replenishing the battery also depends on the electrical load requirement of the entire system. So, it depends actually. ie, on a rainy night, if you have your wipers, AC, Headlights, foglamps, and ICE on, then whist idling, the battery might lose power and not actually charge.

[honeybee]

Originally Posted by dhanushs

Umm.. I have no concrete info, but still I think the fuel required for starting is definitely more than idling.

AFAIK, a normal 1.5 sedan might take about 200-250 amps for starting. Replenishing the battery also depends on the electrical load requirement of the entire system. So, it depends actually. ie, on a rainy night, if you have your wipers, AC, Headlights, foglamps, and ICE on, then whist idling, the battery might lose power and not actually charge.

Unless the engine runs at a higher RPM I doubt if it consumes more fuel.

About the battery, on a rainy night I doubt if people are going to switch off the engine (as they can't open the windows, and have to keep the AC running) so we can classify that under exclusions. Similarly if you are using the AC, it's highly unlikely you will switch the engine off and on within a span of a few seconds and suffer the discomfort of the heat. So we are talking of a condition where there's no load on the battery once the engine has started.

[goingout]

I usually turn off my car if the wait is going to be for more than 60 seconds. In Delhi almost all traffic lights have timers so it is quite convenient.

Apart from saving some fuel in the long run we should also not forget the environmental help we will do to our planet.
Imagine if every one starts turning off their vehicles at traffic lights then how much fuel will be saved overall and it will also contribute to making our air cleaner.

Thankfully all new age cars are coming with start/stop function. Maybe in 3-4 year's time most cars will have this function. Although its a bit too late I think it is still an essential function and I hope we will soon get these features in our desi cars.