How much fuel does it take to re-start a warm engine?

[MetroMPG]

The question of how long to leave an engine idling at a stop vs. restarting it - and which uses more fuel - came up in this thread:

Pulse and Glide questions..Totally new to this

But I figure it's worth its own thread.

IWillTry wanted to know the same answer, so he systematically measured fuel consumption @ idle, and compared it to the fuel consumed performing a warm restart.

His observations (based on a 1.0L gas engine with throttle-body injection):

If you begin to extract useful work from your engine within 1 second of starting it, then the additional fuel consumed by restarting your engine is equivalent to about 0.2 seconds of idling. So you don't even need to consider how long you might be stopping for. If you're interested in reducing fuel consumption, turn your engine off whenever you can, and don't turn it back on till just before you need it. If you're a typical city commuter this can increase your mileage by around 10%.

He makes the same caveats about starter wear.

See his full report on iwilltry.org.

[DifferentPointofView]

On Jeeps website (I can't remember if it was the official one or a forum) they said that if you know your gonna be stopped for over a minute, you should turn off the engine, because you'll be using more fuel than idling it. That was with typical Jeep engines, which are normally atleast 2.5 liters for the 4 bangers in the wranglers. but on average a 3.8L or bigger.

[roflwaffle]

There's way more starter wear from excessive cranking/undervolting than from normal starts. IIRC the number of starts most starters can do before replacement/rebuilding is in the hundreds of thousands.

[robchalmers]

from what i've got off my friends in the engine dept of certain auto makers it works out that depending engine age, wear, temoperature, proximity to service, cranking time it can soak up 12sec of fuel at idle in worse case on a 4 banger to restart providing it catches in the first few turns. most modern cars a i think safe margin is 4s@idle. so 4 sec after you've switched off at the lights, a smug grin can start spreading across your face! (as long as the lights are still red!!!! )

[orange4boy]

I don't have to worry about this because I'm not running an alt, but the AH used for starting should be accounted for because your alt has to replace that energy into the battery using fuel. Anyone have starter amps/warm engine info?

One of the benefits from running at 18V is the almost instant start when the engine is warm.

Added: I see this is taken into account in his wikki. I think the higher idle speed after restart is probably because of the load of the alternator recharging the battery.

[JacobAziza]

Originally Posted by orange4boy

One of the benefits from running at 18V is the almost instant start when the engine is warm.

How and why do you run 18v w/o an alternator?
I removed my alternator, but haven't found any drawbacks to running 12v instead of 14.

[Christ]

Originally Posted by JacobAziza

How and why do you run 18v w/o an alternator?
I removed my alternator, but haven't found any drawbacks to running 12v instead of 14.

I dunno about 18V, but you're probably not seeing a difference because you're not using even close to the load that a "normal" vehicle sees.

The ECU that you don't have, in some cases a TCU, BCM (Chryslers), high-tension ignition system, etc. all have to work on that measly 12VDC, so there is an advantage to overvolting to 14VDC, in that there is less voltage sag should everything decide to harmonize the battery draw at the same time.

If you've ever driven a gasser with a bad alternator, you know that it starts running like a wet dog right about the same time your headlights get dim. If you turn off the headlights, you get power back, once you turn them back on, you lose all power again. I drove my 1994 Hyundai Excel GL with no alternator for about 2 months back and forth to work... took 2 batteries each way.

[orange4boy]

I'm running 18 volts because it's the simplest way to keep my voltage up. I have a 6V and a 12V in series.

12V (11.5 under load) is too low to keep voltage to the coil and fuel pump high enough for my car. Keeping 18V seems to add a bit to my MPG. I have not done any good "scientific-ish" testing so take my numbers with this in mind.

See this thread:

Attention! Alternatorless modders. Get your voltage up

[Christ]

Originally Posted by orange4boy

I'm running 18 volts because it's the simplest way to keep my voltage up. I have a 6V and a 12V in series.

12V (11.5 under load) is too low to keep voltage to the coil and fuel pump high enough for my car. Keeping 18V seems to add a bit to my MPG. I have not done any good "scientific-ish" testing so take my numbers with this in mind.

See this thread:

Attention! Alternatorless modders. Get your voltage up

Do you have a stepper or something to make sure that you're not putting too much voltage to your ECU and related components?

[orange4boy]

Sorry about the hyjack...

I don't have any stepdowns. I will, at some point, tap just the coil, fuel pump and starter for 18V and tap the rest of the car at 12V. My only casualties have been my dome lights which I will swap for LEDs when I get a chance.

Cars are designed to able to take much higher voltage in case of alternator/regulator over voltage. Having said that, running at the higher voltage over time has overheated those dome lights. Do the tapped setup if you try this so you don't possibly let the smoke out of your ECU. Laziness can be expensive.

Idle less and save the world! (to keep on topic)