For how long should I idle at a stop?

Question

As I approach a stretch of road that has been single-laned due to roadworks, the sign-flipper spins the sign to STOP. Having been through these roadworks before, I know I'll probably be waiting at least 3 minutes, probably more.

If I leave it running, then fuel consumption and engine wear will continue at a constant rate. If I turn off, then there will be no fuel consumption or engine wear while the engine is off, but there may be greater fuel consumption and wear during restart.

Should I leave the engine running, or turn it off? What is the time threshold whereby it is better to leave the engine idling, vs switching off?

I'm not asking for any specific model, but in general for any reasonably modern (say post 2000) fuel-injected internal combustion-engined car.

Answer 1

fuel injected vehicles don't require a crazy amount of fuel to get started, in terms of fuel economy alone anything over 30 seconds should be safe (although we are talking about levels of fuel only eco-modders care about). If you are concerned about wear, there's no good way to tell which is better. If you change oil regularly I wouldn't worry about the wear aspect as long as this isn't multiple times in a driving day.

Answer 2

wear is increased at engine start

When you have no oil pressure and you start the car, that is when the most wear occurs on your bearings.

Crank bearings in most cars are not roller bearings anymore. They are called plain bearings and rely on hydro-dynamic lubrication.

Here is answer related to crank bearings with illustrations so you can understand how these bearing function and how they can suffer from increased wear at engine start.

• What are crankshaft bearings and how are they different from camshaft end bearings?

These fluid bearings are also called 'fluid dynamic bearings' and 'hydrostatic bearings'

They rely upon a thin layer of oil between the crankshaft journal and the hydrostatic bearing. The crankshaft rides on the oil much as a pane of glass would sit on a layer of water if you poured it on your countertop and sat the glass down on it. Breaking this fluid barrier wears the bearing surface down. Once the bearing is worn down too much the integrity of the fluid barrier can get broken due to too much gap between the journal and the bearing. When that occurs then engine will begin to eat itself up. A condition called rod knock will begin to occur.

This answer describes 'rod knock' very well.

• What is a rod knock?

Although saving fuel is important to our wallets as well as the environment you want to be careful with stopping and starting your engine all the time. Certainly technology has become better and vehicles are less prone to this issue than any other time but there is a balance to doing this too frequently.

I don't think your describing a constant start/stop of the engine like a delivery truck but this is something to keep in mind. I typically leave my engine idling in situations like that.