This is a 2 part question based on the average car:
1. How much gas is consumed when starting up the engine?
2. How much gas is consumed when left at idle spead for half an hour?
-
Too little to be able to measure, for all practical purposes. And, after all, if you don’t start your engine, you can’t drive anywhere.
-
This depends, to a very great extent, on the engine. Is it a 1964 Chrysler 426 c.i. hemi V-8 with twin four-barrel carbs? Is it a brand-new Honda Fit? No matter which it is, idling any engine for more than 30 seconds or so at start-up is a waste of gas, and the amount of wasted gas is directly proportional to the displacement of the engine and the condition of the engine.
If you are in a traffic jam, turn off the engine. If you are stopped at a RR crossing, turn off the engine. Don’t use drive-up windows at banks and fast food joints. In other words, use common sense and don’t idle the engine when it is not necessary for the engine to be running.
amen to the drive-up windows
I recently pulled into a bank lot in a small town (I usually walk to the office in the city) and was amused to see 10 cars lined up at the drive-in window, all idling. I parked and went inside, 3 tellers, no wait.
It takes very little extra fuel to restart a hot engine. I have heard that the break even point is only 10 seconds or so. The engine’s EFI does go into an open loop mode for a couple of seconds until it can register readings from the O2 sensors. It’s nothing like starting an ice cold engine and waiting for it to warm up.
For the 1.5 liter four cylinder engine used in the Toyota Yaris, people who own scan guages report .17 gallons per hour idling in drive. Diesels reportedly have a much lower idle fuel consumption because there is no closed throttle to suck air past. Not having that manifold vacuum to fight on the intake stroke means the engine doesn’t have to burn so much fuel to keep running.