At the traffic lights, instinctively, many people do turn off their engines. Perhaps, to save some fuel. Fact is, with today’s engines rocking electronically controlled fuel injectors you actually do not save much. One thing is for sure, idling uses fuel. Your fuel economy is zero because the car is not moving. The less time spent idling or warming up the engine, the less fuel used, meaning more money in your pocket.
It is a general misconception that warming up a car by idling it is good for the engine. It turns out that idling is not an effective way of warming up fuel-injection engines and is actually bad for the engines and their components, including cylinders, spark plugs and exhaust systems. The science behind this is that idling does not keep the car at optimum temperature for combustion, so the gasoline is only partially consumed, leaving residue on the cylinder walls. This damages the parts and increases fuel consumption.
One argument is that stopping and restarting the engine creates wear and tear on the battery, ignition and engine. Indeed some extra stress is created though probably not as much as you would think, unless, of course, your battery is on the way out. Many car owners justify idling while using their sound systems or air conditioning, remarking that those pieces of equipment would drain the battery if the engine was not running. Indeed the AC shall only work if the engine is running.
However, the amount of power consumed by the radio is negligible compared with the capacity of the battery, and the alternator recharges the battery when the engine is running, making up for the loss of power unless of course your battery is old and really down or your alternator has a problem.
Still back to the issue of how good your battery is. To complicate things a little, physics with the law of conservation of energy dictates that the alternator shall still use engine power to recharge the battery with every start, but wouldn’t want to get into that.
Internal engine friction is a huge waste of fuel. Keeping parts lubricated adequately is one method of reducing this friction, but some engine parts depend on splash or spray to receive lubrication. When an engine is idling, these parts receive less lubrication because the speed of the crankshaft is not fast enough to splash ample oil onto the parts.
Idling can wear parts faster than when driving. The engine really only needs about 30 seconds of idling time to get oil moving to all parts. After that, the car is warmed up better by driving it. Avoid high revolutions per minute (RPM) or hard acceleration, but otherwise drive it normally.
By driving the car you are warming up the transmission, drive axle, tyres, steering, suspension and wheel bearings. None of these parts warm up just by idling the engine, so your car will actually last longer if all parts are warmed up at the same time. Idle time is great for a person to relax and perhaps read a good book. Idle time for an engine is not good. Save your money and your car by keeping idle time to a minimum.