So can someone explain to me how these stop start engines actually save fuel?
As far as I understand it everytime you start the engine you need to inject fuel into the cylinders, and light it therefore constantly restarting the engine uses a lot of fuel as each time.
How do these new engines get around this?
They get round it by your assumption being wrong. Cold engines use more fuel to start, not warm engines.
To keep an engine running also involves injecting fuel into the cylinders and lighting it - but more often. I think you are assuming the old addage that you have to inject a load of extra fuel to get a car started (like the old days of pushing the throttle when starting) which is not true with modern engines and anyways the balance of stopping and restarting is less than the leaving running in most cases
So slight thread hijack, is my manual stop / start that I do at lights I know the phasing of where I will be waiting more than a minute worth the effort?
richmtb - Member
So slight thread hijack, is my manual stop / start that I do at lights I know the phasing of where I will be waiting more than a minute worth the effort?
probably worth the effort, although the engines that are designed to do this will probably have more powerful batteries to cope with repetitive starting. I think they also won't stop/start until the engine is warm & will also cancel stop/start if the battery voltage gets a bit low.
I tend to do this if I get to a level crossing and know it's going to be down for a while.
Yes, there is a big warning in the manual saying don't replace the battery with a normal battery must be a special one with lots of juice.
The manual advises you to knock it out of gear (hence allowing the engine to switch off) if you suspect you will be stationary for more than 5 seconds.
Starts in 0.3 seconds from dipping the clutch. It's also quite clever (have it on two Fords) in that if you knock it out of gear in a queue on a hill say, engine goes off. Take handbrake off and let the car roll and once speed gets above 5mph(ish) the engine starts (so you have power steering or brakes). Similarly if you brake it starts. It also doesn't turn off if you have climate control on and cabin temp is not at the desired temp. Also doesn't do it until the engine is warm.
Yup. S/S engine dont stop until they are warmed up. Doing it manually is fine as stumpy said if your battery is crisp so engine fires up immediately, ideally the engine is warmed up and you let the ECU start the engine - i.e. dont push the accelerator
I think they also won't stop/start until the engine is warm & will also cancel stop/start if the battery voltage gets a bit low
Correct - we have a pool car which does this. Leaving the hospital, there are a couple of stop start situations (traffic lights, T-jubction) and the engine never stops at these. It seems to take 3-4 miles to get uip to temperature then starts to stop.
Indeed, both warm starting pulse widths and after-start enrichments are practically idle and zero respectively, so start-stop only really affects oiling issues and starter longevity. Since *75% of engine wear occurs during a start, start-stop systems worry me a bit but I'm sure they've been designed around rather than just an afterthought!
*Taken from an oil manufacturer, so possibly questionable but probably based on sense.
Coffeeking - your quote from the oil manufacturers has been taken out of context "Since *75% of engine wear occurs during a start". High levels of engine wear do occur during the initial engine starting as it warms up. Once it has reached its operating temperature turning it on and off will have no additional affect on engine wear.
Out of interest do these ss engines do it when you're parallel parking or 3 point turning? That would just be stupid. I know a woman who would need an uprated starter as well as battery if that was true.
Coffeeking - your quote from the oil manufacturers has been taken out of context "Since *75% of engine wear occurs during a start". High levels of engine wear do occur during the initial engine starting as it warms up. Once it has reached its operating temperature turning it on and off will have no additional affect on engine wear.
Not sure that's strictly true. The wear in discussion would be bearing wear (caused by oil starvation after draining) and wear on cams as well as piston/bore wear. Of course reducing the bearing clearances on all these parts with heat does help prevent some of the impact damage, it doesn't stop the oil starvation perspective. I know it's not all drained in the space of a minute at a set of lights, but to take it to an extreme, if I may, many older really worn engines will knock like mad from the mains and big ends on *hot* startup and silence themselves pretty instantly when oil starts flowing a few second later. I have one in the drive that does it. This knocking is metal on metal impacts with limited oil bearing layer to help protect them. While I'm sure it's not 75%, I'm sure it's significantly higher than 0.
Out of interest do these ss engines do it when you're parallel parking or 3 point turning? That would just be stupid. I know a woman who would need an uprated starter as well as battery if that was true.
No - they only do it when you take it out of gear and release the clutch*, so if that's something you do as part of a parallel park / 3 point turn they will, but I doubt you do!
*and engine is warm, climate control isn't working too hard etc.
I've just had a new car with S. S. , my first and it's quite unnerving at first and yes like the OP you just can't help thinking for normal light stops (<30 seconds), it must be using more fuel. I guess that this has all be calculated though.
One thing did strike me the other day was I came tamping down a slip road off the M5 and at the bottom, with 2 cars in front the car immediately stopped. Having always driven turbo diesels I understood you had to be careful to let the engine idle so the turbo spooled down to prevent damage by oil starvation to the turbo. Do you think they oil pump continues in stop start mode?
I used to have cars that stopped at traffic lights when I was a student and paid £150 for them. Now they break down on purpose. Weird.