Idle or shut down?
 

I'm sure this is a well hashed topic, but I've often wondered what the fuel saving "cut-off" is for shutting down vs idling in a diesel. Surely 30 secs of idle uses less fuel than shutting down and restarting? What's the upper limit though? 1min, 2, 3? I'm sure it's temperature dependent as well, a warmed up engine probably uses less fuel to restart than a cold one. How much "extra" diesel is used during the starting process if any? I realize there is probably no end all be all answer considering the multitude of diesel engines and affecting variables; I'm just wondering in an honest attempt to save fuel. Facts, testimonies,...opinions???

What Vehicle is this question about?

I can only speak for My own Year and Model.
During Cold Weather there is no thermostatically controled Fuel Enrichment/Cold Start feature.

Glow Plugs which can be cycled for about 30 seconds of time if you ignore the Glow Plug Light before the Relay Timer turns it off; and you can repeat the Glow Plulg Cycles if you want to before starting dependant on how Good Your Battery is.
Using the Glow Plugs uses no Fuel and shortens how many times the Engine is going to Crank before it starts. Shorter Cranking time means less Fuel Used.

Using a Block Heater also costs no Diesel Fuel and aids in quick starting.

After that the Cranking time is realted to what is left in the Battery after using the Glow Plugs (during cranking the Glow Plugs are also on), the Condition of the Starter Motor, what type of Oil is in the Crankcase, how well tuned the Engine is (good Injectors, Fuel Injection Pump Timing, Valve Timing and in My case Valves adjusted), outside temp and Engine Compression.

If all of the above stuff is fixed and in good condition your wasted Fuel during cranking should be minimal.

I am not understanding other parts of the question concerning the Fuel used idling verses starting.

I can only make a guess based on what I remember from Calibrating Fuel Injection Pumps. The old Chevy Diesels used about 3.5-4 cubic centimeters of Fuel per 1000 rotations of the Fuel Injection Pump on the Test Stand.
So on the Engine that would be 3.5-4 cubic centimeters per 2000 rotations of the Engine Crankshaft.

Diesel. Leave it idling

Gas. Shut it down

Diesel911 so fuel use is linearly proportional to engine rotation? Is that what your saying? One rotation equals x amount of fuel use no matter the RPM's warm engine or cold, cranking or idling? That makes sense I guess, I always thought in order to start an engine you needed like a burst of extra fuel while cranking. So if this is true it would always be better to shut down and restart strictly from a fuel saving stand point?
What about a turbo charger? Do I even want to go down that road? lol
Sorry if I'm a diesel ignoramus, my knowledge is limited to what I've learned in the 9 month ownership of my car, 87' 300D.
Thanks

My question is whether the fuel savings add up to the new starter you're going to need so much sooner from all the extra use.

On Gasoline Car Engines of the Engine gets cold enough when you start it up it is in the cold start mode and you get Fuel Enrichment (or the Air volume is cut is reduced) until the Coolant Heats up till X degrees or the Computer times out the Cold Start mode. So that would use up more Fuel.

How fast the Engine gets cold enough to do that depends on the Outside Temp and Wind Chill.

I am going to modify what I said a little. The Governor in the Fuel Injection Pump at idle does control the Fuel to some extent once the Engine is started.
Once started and let us say you had 30wt Oil in cold Weather the Governor will give a bit of extra Fuel to try to compensate for that because the Governor is to try to keep the idle speed at a specific speed.

My own though on saving Fuel is that if I left My Engine running thinking that I was only going to do something for 30 seconds something is going to happen once in a while and it is likely that the Engine is going to end up idling minutes instead of 30 seconds. And, of course that would screw up the Fuel economy.

On the other hand frequent shut off and start ups wears the Starter out faster.

I agree. Can you imagine UPS Vehicles at every one of their stops having to make over 100-200 start ups a Day on their Vehicles. Every Month or so they would be changing the Starter.

They it has been said that Engines wear more during starting.

UPS trucks do shut down at delivery stops. I've never seen one that didn't. I think the latest trucks are hybrids.

Big rig drivers don't shut down because those big engines are hard to start from cold. NYC now limits idling to three minutes, you can actually get a ticket if you go longer.

Starting is damaging from a cold start, because the lubricants have leaked down and viscosity is high. If the engine is hot, not so bad. Hybrids turn on and off all the time.

The only time I let the engine idle is if I have to stop immediately after getting off a fast road. Letting the turbo temperature drop a bit prevents oil from boiling out of the bearings when it's stopped. a minute of cool down is a good idea.