Seems like a good research project. Have you any parameters? For example, in emergency vehicles, they are kept idling because that way they cannot fail to start, which could happen if they were shut off. They can also provide heating and cooling to the occupants. But without knowing why one is concerned it's tough to make a suggestion.

As a practical matter, with fuel injection, the amount of fuel to recharge the battery after starting is really small compared to the overall use in a motor vehicle, which I presume is also a parameter - the actual use for the motor.

Years ago I looked into this question. I was amazed to discover that my car (at the time) required a significant fraction of the fuel flow just idling, as compared to driving gently on the highway. For example, if a car is 8L/100km on the highway at 100 kmh, that's 8L per hour. For my example (at the time), the idling fuel consumption was as much as several L per hour.

So I wouldn't leave my car running to "warm up" (except in the most extreme exceptions). But I'm not going to manually turn it off at red lights. I would consider switching off in traffic jams.

Diesels are generally much better, requiring very little flow to idle.

Many modern cars have Start/Stop systems, because they can save fuel. They're annoying, but can save fuel and reduced CO2 emissions.

Thanks to both of you for the answer.

@3DDave
I know that many vehicles have to idle. I am thinking to the sleep trucks, that idle for 6 hours per day because people sleep overnight. But, my question is vehicle independent. In a website, I have found that if the stop is longer than 10seconds is more convenient to turn the engine off. However, there is no reference, so I do not have any clue to assess the data reliability.

Plus you have wear on the starter switch, the starter relay, and the starter motor to account for; possibly made up for by using far more expensive components if frequent starting is desired.

Your question is very vehicle dependent as there's no universal law that governs the tradeoffs.

I guess you are in the USA?

I am in Europe, currently, and a lot of cars have auto stop start of engines at standstill. So if the battery is charged, and you stop the car at a junction, then the engine stops, and starts again when the clutch is depressed.

I suppose this is because the emissions limits mist are in grams per kilometer, on a given duty cycle, and stopping the engine gives an emissions benefit.

"...stopping the engine gives an emissions benefit."

CO2 perhaps.

But a hot catalytic converter reduces traditional air pollutants to extremely low levels.

As opposed to letting it cool off and then re-starting while cold. Which can amount to a lot of the air pollution.

My brother has a newer ford with this feature but the human factor . he has the option of disabling that function so he does as his words it is so annoying and in 40 degree day the air conditioning isnt happy for him .

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare
www.sixstroke.com

I've got a new Escape that has Autostop. It also tells me it uses 1.4 litres per hour when idling. So in hot weather I can save 3 cents for every minute the engine is off (woohoo) and the a/c is not running. Except of course when the engine starts again the a/c has to come in with a mighty roar to get the temperature back down. So I'd imagine the fuel savings are rather less than 3c/minute in reality. I think you can guess where I'm going with this.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

I cant say I've had issues with a/c in any of the various vehicles I've had with start-stop, usually the vehicle restarts before the air blows warm regardless of driver inputs.

As mentioned, starting an engine requires relatively little low-cost electricity when compared to burning fuel at idle. It also puts out fewer emissions and isnt as hard on the engine. Win, win, win.

As is well known - SI engines with load control by throttle plate suffer badly from maximum pumping losses at idle - they use a surprising amount of fuel at idle.

I have seen testing of an engine with continuously variable valve opening duration with the idling controlled mostly by LIVC and the fuel saving at idle was a bit startling - like about 40%.

There is (or was) a video of it being demonstrated on the internet somewhere.

CWB1,
here in australia temperature of 40+ and tarmac at 70+ you dont want the Ac to cut out for 3 minutes at the stop lights

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare
www.sixstroke.com

Very interesting video about this topic https://youtu.be/dFImHhNwbJo

Link the ozy view ?

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare
www.sixstroke.com

Whether start/stop driving is efficient will depend on the driving conditions. Originally start/stop systems were designed to prevent pollution when waiting for a traffic light in buildup areas. That it also can reduce fuel consumption and CO2 emissions now is a added bonus.

If the car is equipped with a start/stop system then most likely the starter motor is designed in such a way that it can live with the far more frequent starts. A car without a start/stop system may well experience premature starter motor failure, because the starter motor will be operated far more then the designer anticipated.

I drive a Lexus hybrid that very frequently changes between engine and motor drive. The problem mentioned with AC in warm climates is overcome because the AC compressor is electrically driven, also the powersteering is electric. With a non-hybrid car in warm climates the temporarily stop of the AC may be a disadvantage - depending on the length of the stop.

At the moment some manufacturers experience problems with excessive camshaft wear on start/stop vehicles that are frequently operated in cold weather. A thinner oil might help prevent that to a certain extend. Both the carindustry and oil formulators are investigating what can and needs to be done about it. That might well result in future engine oil specifications that include a specific test to check what happens when operating the start/stop system under sustained driving in low temperatures in such a way that the oil will not reach the foreseen operating temperature. The problem with driving under those conditions is that the thermostatically controlled cooling system may reach the operating temperature quite fast but that the oil may need more then 20 minutes to reach a decent operating temperature - without the driver noticing because very few vehicles are equipped with a oil temperature gauge.

@Deividas,

That is what I was looking for! Data reported on a scientific paper. Thanks

UPS, one of the biggest fleets of delivery vehicles in the US has started shutting off all their diesel vehicles every time they stop to make a delivery. I'm sure they studied the economics down to last detail. Gasoline engines obviously take much less time to make shutting down cost effective.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

malbeare, I would have to ask around to confirm but believe there is commonly an algorithm to control restart based on cabin temperature for vehicles that don't have an electric a/c compressor.

thanks CWB1.
I think it would be good to combat the wear factor in engines with stop start that there should be a small electric oil pump to keep the oil flow whilst the engine is stopped . and also electric water pump for a similar purpose together with electric fan . maybe an accumulator for the oil that slowly keeps small oil flow for 3 minutes

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare
www.sixstroke.com

I'm cynical about the stop/start on my car. Not convinced that the money saved in fuel not wasted is worth more than the cost of replacing the bits that will fail prematurely as a result of the additional cycling. Or the anxiety it gives me when I'm surrounded by other cars at a junction and my engine stops. I've had starter motors fail on me in the past, but I've usually been somewhere safe, like at home or at my destination.

Steve

I agree entirely.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

Reportedly, some Stop-Start vehicles have Integrated Starter Generators (ISG) which would seem to address many of these concerns.

Furthermore, when combined with electric hybrid drive, then of course the ICE Stop-Start concept becomes nearly transparent.

I mentioned that the result could be seen in higher initial component cost. Looking at ISGs I believe that I am correct.

https://www.youtube.com/watch?v=YnLcP2fpEQ4

Some vehicles like the RAM 1500 with 5.7l Hemi use a 48 volt mild hybrid system for the start-stop. So a conventional 12 V starter engages the flywheel for a cold start but after that the belt driven motor/generator re-starts the engine and adds 130 ft-lbs of torque to the bottom of the powerband. It does not increase peak torque because it's no longer active when the engine is turning at higher speeds. It handles charging both the 12 V battery and the 48 V lithium battery during decel. So you are definitely paying $1450 more for the system but it's completely optional.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

malbeare,
How would oil flow while the engine is not running reduce wear? If this pump primed the oil lines immediately before starting, that might have some effect, by getting the oil circulating that little bit quicker. A dry start really isn't a thing though. There's always some amount of oil film on parts and it should be adequate to protect until the flow gets to where it needs to be.

Andrew H.
www.mototribology.com

Prelube pumps do exist for engines in harsh environmental conditions and those picking up load quickly upon starting but the automotive cycle is fairly gentle and conditions rarely extreme in most markets.

Quote (3DDave)

...higher initial component cost. Looking at ISGs...

Governments regulate tailpipe CO2 emissions, but they don't regulate embodied resources (such as "$1450" worth of optional ISG to save a wee bit of energy, with potentially dubious Eco-ROI).

ISG's and their mild hybrid role do a lot more and save a lot more fuel than the mere "stop start" function.

je suis charlie

Indeed, adding an ISG and a mild hybrid system to a gasoline-engined vehicle can tangibly improve the economy (albeit with high initial cost). But so can switching to a simple diesel engine.

Steve

I'm curious is there data on the longevity of start stop engines? Both CI and SI. How much start enrichment is applied?
Cat converter heat loss and HC emissions? I've heard of cases of cars starting when unoccupied, though that could be more of a remote start system fault.

In an engine stop-start situation, there are no more cold starts than without it ... stop-start only operates with a warm engine. So, there is no enrichment on start-up. Generally there is a mathematical temperature model of some sort in the ECU, so the catalyst works immediately (the internal parts of catalysts stay hot for quite a while when there is no flow through them). Indeed, there can be issues with people forgetting to turn the car off, thus leading to surprise engine restarts, because it was never actually switched off. Keyless pushbutton starting takes the blame for that one.

SomptingGuy - It seems like "simple diesel engine" is an oxymoron these days with cats, particulate filters and DEF. Diesel seems to be on the decline and possibly on the way out for light duty. A number of inner city areas are banning them completely.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

That was my first though as well, dgallup.

Bill
--------------------
"Why not the best?"
Jimmy Carter

I agree dgallup. Who'd buy a new small diesel car these days? My last one (2004) was only Euro 3 and it had expensive bits on it that broke. Interestingly (and back to the initial topic), it was on its third starter motor when I got rid of it.

Steve

I'm going to say the mere fact that so many automakers incorporate start-stop tells me there's some small gain to it. That being said, they don't really have to add anything to the engine to make it work, so it wouldn't take much of a gain to motivate them to incorporate it.

I wouldn't count diesel out just yet. It's always held low market share in the US, but it still represents a significant share of sales in Europe. It's declining in popularity there, but industry analysts expect it to plateau soon. A key factor in this is that much of US oil is light crude preferred by gasoline refiners while diesel as opposed to the heavy crude preferred by diesel refiners. This is why diesel costs so much more than regular gasoline these days in the US... why go diesel when it's improved efficiency is offset by increased fuel cost at the pump?

Interesting that diesel sales in Spain are still above 50%. Any idea why?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

"How much start enrichment is applied?" - on new engines there is almost none, even on a cold start. The manufacturers have the EFI systems very well refined.

In many jurisdictions farmers and home furnaces access a substance that is remarkably diesel like but on which no or little excise is payable.

It may be that in less policed jurisdictions that diesels have an economic benefit to the owner not available to their gasoline burning competitors.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

I have read that idling is detrimental for the engine life. Why?

Running an engine is detrimental to the engine's life with regard to time.

Idling an engine is detrimental to an engine's life with regards to miles because you are producing wear without any added miles.

There are more intricacies to each point, but I'd venture a guess that's the talking points what you read were referring to.

Andrew H.
www.mototribology.com

Many states here in the US have taxed heating oil to be nearly the equivalent of onroad diesel in cost. Many hobby farmers in my family's locale ran "offroad" fuel in their personal vehicles for years, in recent years most have given up as maintaining tanks and pumps is more of a pain than they see beneficial.

In this part of the world, the premium cost of a diesel equipped vehicle is next to impossible for a typical commuter to recoup via fuel savings. The only way to actually save money would be by finding a source of the fuel without taxes as Greg alluded to, but that is not easy to accomplish for the typical commuter.

At idle, oil pressure and flow are at a minimum, valve spring pressure "is what it is" (the springs don't magically apply less force just because the engine is only turning slowly) so the hydrodynamic conditions at sliding-type valve lifters/tappets/buckets are at their worst. If the idling is following a cold start, idling doesn't generate as much heat so the engine takes much longer to warm up. If the engine in question is a modern emission-controlled diesel engine, it will have trouble keeping catalysts up to temperature, so it will probably be attempting to do screwy things with the fuel injection to try to raise exhaust temperature.

Re the valve spring and valve lifter hydrodynamic lubrication situation ... one would hope that the engine manufacturer has sorted this out, but it doesn't always happen. A couple of vehicles ago, I had a 2006 VW diesel (last of the type before the emission controls went crazy), which was an engine type that had electronically-controlled but mechanically-operated individual unit injectors. 4 cylinder 8 valve, thus 2 valves in line with each other per cylinder. They needed an extra cam lobe to operate that unit injector, which forced the lobes for the intake and exhaust valves to be narrowed relative to historic VW 8-valve engines. VW's method of compensating for this was to specify the use of a very specific engine oil. They still had frequent issues with camshafts and lifters - and yet, I put 430,000 km on that car without having that trouble. Part of this is me living in a relatively cooler climate (which keeps oil temp down and viscosity up - better for hydrodynamic lubrication - the issue appears to be more prevalent in hot climates). Part of it is my usage pattern which frequently saw that engine spinning 2000 rpm in top gear on cruise control and hardly any city driving. And I used the right oil. Of course, if you ask VW, they never had any issues with worn-out camshafts and lifters, but I digress.

Diesel cost? Back in the old days in the US, diesel was always half the cost of gasoline, I remember as a kid diesel at .14 a gallon and gas was around .25 or so(that is cents). It has magically gone up in price, in some areas it can be a few cents or more less cost than gasoline. Depending on the state there are special petroleum taxes so price varies.

The sign outside my window prices diesel at 21.5% more than gasoline almost wiping out any efficiency benefit from the diesel cycle.

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" diesel at .14 a gallon and gas was around .25 or so(that is cents)"
Holy crap .25 cents per gallon!

je suis charlie

The difference in price between gasoline and diesel is very different in Europe. I have read that the US difference is largely due to the fact that we're awash in light crude (preferred for gasoline) and comparatively low on heavy crude (preferred for Diesel).

Per https://www.globalpetrolprices.com/...

Unofficial test on my 2018 Ford F150
V-6, turbo, 10 speed
Location Denver Colorado area
On May 1, I had about 5000 miles on the truck.
I kept the auto-stop turned on, drove in "eco" mode most of the time, with "sport" mode turned on less than 5 times for traffic light starts, then back to "eco" mode.
Total miles for the month of May approximately 1000 miles.
For the month of May 20.7 mpg.

I turned off auto-stop June 1. I will report my mpg after June 30.

Variations in driving pattern, traffic patterns etc will cause more variation in mpg than the auto stop function.

je suis charlie

You asked for a study, here it is: http://www.iwilltry.org/b/projects/how-many-second... . The conclusion is that fuel is saved for idle periods longer than 7 seconds. Since most traffic lights are far longer than that, it would yield an improvement over the long term. The low cost P0 hybrid configuration is sufficient to recapture the energy used in starting. While the focus here has been on efficiency impact, stopping rather than idling also results in reduced emissions (which is why idle emissions are tested in most emissions tests).

Video on the same subject is here: https://www.youtube.com/watch?v=dFImHhNwbJo

Video mentions the same "7 seconds" as mentioned just above.

Honda even does this with their scooters in Japan. The good old nps50 and perhaps some more

After about 1100 miles on my Ford F150 for the month of June and 1st week of July, I had 21.4 mpg. A slight increase from 20.7 mpg in May.
Same routes, no change to driving patterns, only difference is an increase in AC usage.

Have you quantified your measurement accuracy and repeatability?

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

LOL
"Have you quantified your measurement accuracy and repeatability?"
As I said in my earlier post - an unofficial test.
For my driving habits and routes, the auto-stop/start does not do anything for me.

What's your typical route? City, suburban, or rural?

I know in general driving, your mood affects mpg much more than silly things like drafting or turning the AC off. I imagine if you drove primarily in areas with stoplights auto-stop would be more beneficial.

Here's an obvious variable: your local jurisdiction's assumptions about optimum traffic signal light timing.

I've been to quite a few cities in the USA where a red light stays red for minutes on end. Monumentally boring.

At the other extreme, I was just visiting Toronto and I noticed that the traffic signal light cycles there tended to be fairly quick. The rental vehicle had Auto Stop-Start, and it wasn't even getting 7 seconds. Each red light was merely a short pause. Often not even enough time to check the navigation.

YMMV.

My drive is suburban, freeway and a little city.
My drive to work is a 4 miles one-way, one stop for a right hand turn and one stop to show my badge at the plant entrance. Most driving on the weekends, with every Wednesday evening about 20 miles one way mostly freeway.

That route almost entirely removes the use of auto-stop so I'm not surprised the mpg was the same.