Looks like I need to split my response across multiple entries as it errors;
The short answer to the original question is - yes. You do save fuel on switching the engine off.
A longer answer would be:
Turning an engine over, i.e. keeing it running, uses a certain amount of energy per unit of time. The amount of energy used is the same (or as shall see, very similar) whether the engine is turned over by its normal combustion power or by an electric motor (starter).
So from this notion it is reasonable to assume that switching the engine off is economical as long as it will remain off longer than it takes to start the engine again. The average modern car engine will need about 2 seconds on the starter motor to start. In theory this means that you need to leave the engine off for 2 seconds or more save fuel.
In reality we should of course allow for some losses in a: the starter motor b: the alternator. This may add up to 20% losses. That means if you need to leave the engine stopped for say 3 seconds or more (50% longer). For practiclity we can say 5 seconds(150% longer).
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