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Bearleener · 11-25-2009, 05:42 PM

I've been having problems with my car's anti-theft system and wonder if it's got something to do with my frequent engine-off coasting (EOC) stops and starts (see separate thread). Maybe I should do engine-on coasting instead.

So I'm looking for excuses why it wouldn't be so bad to leave the engine on during coasting and at stoplights, if I could get myself to do it.

The fact is, you pay a little bit for the zero consumption during EOC the next time the engine is on:

During EOC the ignition and all electrical accessories are still on, say the ECU, fuel pump, etc. (200 W), headlights (200 W), blower fan on 1st stop (50 W), radio (50 W ?), i.e. a total of about 500 W. You're not using fuel during EOC, but the engine has to make up for the electrical power over the alternator the next time it's on.

Assuming a gasoline engine with an efficiency of 29%, running an alternator with an efficiency of 55%, charging a battery with an efficiency of 90%, the total power consumed by the engine to run all the electric stuff is:
500 W / 0.29 / 0.55 / 0.90 = 3.48 kW, wow, that's 4.7 hp!
(BTW, when the engine's on the battery efficiency factor wouldn't be there because power is coming directly from the alternator.)

So per hour you need 3.48 kWh of energy. Gasoline has an energy density of about 8.9 kWh/l, so that's 0.391 l/h (0.1 gph).
So if the engine needs about 1 l/h to idle (albeit without headlights, fan & radio), then a whopping 39% of it is for running electric stuff; without lights & accessories it's 16%. But that also means that having the engine off uses at least 84% less.

On my 24-km commute the engine is off for about 9 of the 30 minutes. I'm currently getting about 6 l/100km, or about 1.44 litres per commute. The difference between engine-on and engine-off coasting & idling is then
(9/60 h)*0.84 l/h = 0.126 l, which is 8.8 % more with the engine on. In my case that's 0.53 l/100 km or 3 mpg worse. (Disregarding the above electrical power effect it would have been 10.4 %.)

Moral of the story: even considering the electrical power effect, if you can, shutting off the engine is significantly better than letting it idle. But we already knew that.

What the heck, I'll just let my anti-theft system konk out periodically.

11-25-2009, 05:42 PM	#1 (permalink)
Bearleener Efficiency freak Join Date: Mar 2008 Location: Berlin, Germany Posts: 58 Frugolf - '96 VW Golf wagon 90 day: 40.36 mpg (US) Thanks: 0 Thanked 15 Times in 6 Posts	EOC <> zero fuel consumption I've been having problems with my car's anti-theft system and wonder if it's got something to do with my frequent engine-off coasting (EOC) stops and starts (see separate thread). Maybe I should do engine-on coasting instead. So I'm looking for excuses why it wouldn't be so bad to leave the engine on during coasting and at stoplights, if I could get myself to do it. The fact is, you pay a little bit for the zero consumption during EOC the next time the engine is on: During EOC the ignition and all electrical accessories are still on, say the ECU, fuel pump, etc. (200 W), headlights (200 W), blower fan on 1st stop (50 W), radio (50 W ?), i.e. a total of about 500 W. You're not using fuel during EOC, but the engine has to make up for the electrical power over the alternator the next time it's on. Assuming a gasoline engine with an efficiency of 29%, running an alternator with an efficiency of 55%, charging a battery with an efficiency of 90%, the total power consumed by the engine to run all the electric stuff is: 500 W / 0.29 / 0.55 / 0.90 = 3.48 kW, wow, that's 4.7 hp! (BTW, when the engine's on the battery efficiency factor wouldn't be there because power is coming directly from the alternator.) So per hour you need 3.48 kWh of energy. Gasoline has an energy density of about 8.9 kWh/l, so that's 0.391 l/h (0.1 gph). So if the engine needs about 1 l/h to idle (albeit without headlights, fan & radio), then a whopping 39% of it is for running electric stuff; without lights & accessories it's 16%. But that also means that having the engine off uses at least 84% less. On my 24-km commute the engine is off for about 9 of the 30 minutes. I'm currently getting about 6 l/100km, or about 1.44 litres per commute. The difference between engine-on and engine-off coasting & idling is then (9/60 h)0.84 l/h = 0.126 l, which is 8.8 % more* with the engine on. In my case that's 0.53 l/100 km or 3 mpg worse. (Disregarding the above electrical power effect it would have been 10.4 %.) Moral of the story: even considering the electrical power effect, if you can, shutting off the engine is significantly better than letting it idle. But we already knew that. What the heck, I'll just let my anti-theft system konk out periodically. __________________ You ever notice that birds pulse & glide, too?