HELP! Engine won't start

Mildly_Miffed

https://forums.moneysavingexpert.com/discussion/comment/82173606#Comment_82173606

If the gap's that small, then it was too tight anyway.

But it starts the engine when you lift off the brake, not press the throttle. And, in a well-maintained car, it'll start just about instantly.

finbaar

https://forums.moneysavingexpert.com/discussion/comment/82173727#Comment_82173727

People sitting with the clutch in and foot brake on in a manual, or foot brake on in an automatic when queuing really really boils my urine.

Veloflyer

https://forums.moneysavingexpert.com/discussion/comment/82173999#Comment_82173999

You don't need numbers to understand basic laws of physics. You can believe others if you like, I believe in first principles. Constant stop/start involves generating energy via the engine to provide electrical energy to the starter. It does not come for free from the magic energy tree. I'd argue that the fuel used on idle - which is next to nothing as the engine is already rotating, is far less than the equivalent fuel required to provide any constant stop/start demand for a stationery engine with a large reciprocating mass.

MattMattMattUK

https://forums.moneysavingexpert.com/discussion/comment/82174318#Comment_82174318

You could argue that, but you would be wrong.

QrizB

Constant stop/start involves generating energy via the engine to provide electrical energy to the starter. It does not come for free from the magic energy tree.

Starting an already-hot car engine might draw 200 amps at 12 volts for 2 seconds. That's 4800 joules (4.8kJ). If we assume that conversion of chemical energy into electrical energy is only 10% efficient, we'll need to burn 48kJ of fuel to replace the 4.8kJ used to start the car.

(10% efficient is similar to a cheap 2-stroke portable generator, where you might get 1kWh of electricity out for each litre of fuel you put in. Better designed systems are 2-3x more efficient than that.)

the fuel used on idle - which is next to nothing as the engine is already rotating

An idling engine burns around two litres of fuel an hour. A litre of fuel is around 36MJ (10kWh) so two litres an hour is 72MJ/hr, 1.2MJ/min, 20kJ/sec.

So two-and-a-half seconds of idling will use 50kJ of fuel, which is already more fuel than is required to recharge the battery following a start.

I'd argue

You're arguing against the report on the previous page from the SAE, the world's largest organisation for automotive engineers, who have undertaken both theoretical and practical studies with stop-start-equipped vehicles and who have concluded that stop-start improves fuel economy by a non-trivial amount in driving conditions where stopping and starting are frequent.

If you want to read the full report and not just the abstract, the US Government have published it here:

https://www.osti.gov/servlets/purl/2205440

chrisw

https://forums.moneysavingexpert.com/discussion/comment/82174282#Comment_82174282

Many/most autos and EVs automatically hold the car on the foot brake whilst stationary.

Nobbie1967

https://forums.moneysavingexpert.com/discussion/comment/82174177#Comment_82174177

The trouble is you select a gap that is safe and then the delay makes it unsafe.

MattMattMattUK

https://forums.moneysavingexpert.com/discussion/comment/82175697#Comment_82175697

The delay is not what makes it unsafe, the driver not understanding the performance of their vehicle is the issue, no difference to when drivers underestimate their breaking distance, overestimating their acceleration, their cornering grip etc.

The issue is not the start/stop system, the issue is the driver, as it is with 96-98% of all vehicle incidents.

forgotmyname

Taking my daughters car for an MOT the stop/start kicked in, on her journeys it's 99% of the time too short
for it to activate. Mine did coincide with pulling out onto a busy road when the stop start decided that was the
right amount of time to switch off. Not hearing the engine running was easy enough to spot so I waited for
another gap.

Roll on a couple of years and my car comes with stop/start but my issue is that it will turn off after covering
less than a mile when the engine/oil is still fairly cold. Add to the fact the alternator thinks that charging the
battery when it's above 12.2 volts is a waste of fuel and switches the alternator off.

I did my research before buying and it was a common comment from owners that getting more than a year
from the battery was going to be rare. It's almost a service item.

But it's the fact it shuts down long before the oil reaches it's correct operating temperature and it needs to
build pressure at each restart with no mechanism to retain oil pressure when stopped.

facade

Add to the fact the alternator thinks that charging the battery when it's above 12.2 volts is a waste of fuel and switches the alternator off.

I dislike smart alternator management (I'd prefer constant voltage like worked perfectly for a hundred years or so…). It means that I have to charge the battery on my low mileage car every month or so.

You'd think that every time the throttle is closed when the car is moving that the alternator would run full chat and get "free" energy into the battery, but AFAIK they only run full chat during warm up, when no-one cares about emissions, and do tickle a bit more when the ECU senses a significant electrical load.

I also fail to understand why you have to "code" a new battery, (which is just reset the BMS to a battery time of zero) or it will overcharge the new battery and kill it even quicker than it's complete failure to maintain an adequate charge level does already…….