Engine Off Coasting - When to do and when not to.
 

Hi all,

I have used Engine on coasting vs fuel cut off when I lift off the pedal. I have done some research on an empty road into what works and what doesn't when my engine is off and the car is moving. Basically it comes down to brakes, steering and lights.

Important note to anyone doing this - check your steering column lock is free when your engine is off with the key. I did this stationary first.

Brakes - assistance is gone and without it you could probably stop the car a few times, but you need to body-build (yourself) to make it work in a hurry. Moderating the effort is harder too - its like getting into an older car (I have an MGB w/o assistance so I'm OK with it).

Steering - PAS goes. But none-PAS in my car is 5-10x heavier than PAS. Any sharp adjustements needed, like avoidance and so on is not going to happen. The engine weighs a lot in this car (1.9 litre, turbo etc.).

Lights - mine go off with the ignition so at night (I'm as north as St Petersburg here) its not an option in the total dark.


As a supplementary question I would like to ask about turning your engine off at lights. I hit a lot of them that just cannot be "timed", so I've started to do this - seems to work well. Except Ford have launched the new Focus Econetic with Stop-Start (excellent as it gets into Band A in uk.gov taxation which was reserved for Hybrids - I feel happy about this as driving one annoys Gordon B who cannot collect tax on me ;-) but one of the changes they specified was an uprated starter.

So, question is - does my own stop-start regime risk breaking the starter ? I have had cars of >200K miles and despite a lot of them having Lucas (aka - Prince of Darkness) electrics I can count the number of starters I have replaced on the fingers of no fingers. Nope, never. I don't stop for anything more than 5 mins so the radio and sidelights will be OK - I think, and this is usually after a longer battery charging journey. And the battery is "man" enough to start my 1.9 Diesel from cold.

You do turn the key back to the "ON" position after it dies - right?

The beefed-up starter motors in cars with the start-stop option is so that starting the engine takes only an instant. Normally it can take 0.5-1 second, but you anticipate it. An automatic system has to ready in when the driver needs it - if there is a lag, then drivers won't want it.

One of the things I've been thinking about lately with the cold weather (last 3 weeks have been mostly between -5 and -20*C, the car sleeps outside) is whether EOC, or even coasting in idle, is a good idea with a cold engine. My reasoning is that when driving in gear the engine heats up faster, which is good. When coasting it produces less heat, in EOC none, lengthening the time it takes to finally reach operating temperature. I guess this doesn't make a difference in the summer, but now my temp never gets over 80*C with grille block.

About 90% of my starts are bump starts with the clutch, not using the starter motor. I've only replaced one starter motor in the past 30 years, so it's not something you should worry about, unless you have Lucas electrics.

There's no additional wear on an engine during EOCs. The engine essentially has to run the same 3 minutes or whatever time to reach normal operating temperature. I'm sure it's much easier on the engine to run 3 minutes at 20 mph than it is to run 3 minutes at 60 mph on a cold engine.

Could you possibly replace the A/C compressor with a large electric motor/starter?

/not intentional thread jack.

What are the advantages of bump starts over conventional ones ?

I've just replaced clutch in mine - not pretty, my CC still wimpers when I take it out...

Conventional starting wears your starter motor and key ignition, bumping wears your clutch, but less than normal daily driving, especially city driving. You can reduce ignition wear with a start button. I think the really difference is in electrical load: a starter motor takes so much current that everything else (for example lights) gets dimmed/turned off for a second. Bump starting does take away some momentum, but that can be minimalized with practice. The power drain from using the starter has to be made up for with the alternator and I'm willing to bet that would take more power from your forward movement than a bump would.

Also, in a multiple EOC-starter-accelerate-EOC-starter-accelerate-... sequence the alt wouldn't charge the battery enough for more than a few starts.

Arragonis, you need a kill switch so you don't have to turn the key. That will eliminate concerns about the steering column lock and headlights, and it allows your radio and fuel economy instrumentation to run during EOC.

On my Subaru, I'd usually get one assisted braking event before vacuum was depleted. So my braking method during EOC required a little bit of planning ahead, since you don't want to back off the pedal until you're done braking.

@MadisonMPG: That's what GM did with the Malibu hybrid, eh? Bosch makes belt-driven integrated starter/generators, and I think a 12V version exists, but I've never seen a DIY retrofit.

Arragonis,

We need to be sure you're properly doing EOC. Your ignition should have four positions: OFF, ACC, ON, and START. START is the same as ON, except it cranks the starter motor. When you're EOCing, you want to turn the engine OFF long enough to stop it from idling, then back to ON. The best way to do this is to let the engine drop to idle speed before momentarily switching the ignition to OFF, then back to ON. When you're in the ON position, you should retain your steering and lights.

You should retain two or three power-assisted brake usages during EOC before losing the power assist.

You will lose power steering during EOC, but with your tires inflated to maximum sidewall pressure, you should not notice much difference in steering effort during EOC at normal driving speeds. Our parents drove without power steering, and we can do it, too.

My cars keep the headlights on with the ignition switch in the ON position. Since you should be EOCing with the ignition in the ON position, your headlights should be on during EOC. My alternator keeps the battery charged during normal pulse and glide 1/3 on to 2/3 off cycles at night with the lights on. I monitor voltage to make sure the battery voltage does not drop below 11.6 V.

Hi,

Thanks for the replies. :thumbup:

SentraSE-R - My experiences match yours except for the steering - its much harder than before. A severe need to steer a lot (and who can predict it) would be tricky. I'm also wondering what happens to things like ESP with the engine off. I'm willing to try it but I just want to explore the potential consequences.

RobertSmalls - I'm reading and re-reading the kill-switch Diesel threads on and off. At the moment I'm 50/50 about keeping this car vs swapping it for a more economical one - i.e. < 1.4 litre Diesel. Or maybe a Yaris D4-D.

Piwoslaw - I think I will preserve my clutch - brakes are cheaper than clutches. As for starting my thought is that the Alt should level demand from the electrical system (inc battery) over a journey. I remember reading (about 20 years ago though) that each start would cost 7 miles in charging for the battery. The auto-stop/start part of my journey is usually after 20 or so miles of motorway.

I have my grill block built but not installed (we are not used to a week of snow here in the UK unlike you poles :thumbup:) so I have not ventured out much over the holiday. Once I do I may find similar issues to you.

Thanks all. :turtle: