Is using auto start-stop bad?

ballyblack

I'm sure it will save money if your stuck in heavy traffic not moving

apart from that scenario the saving will be small

facade

It is a "gimmick" in the sense that the real world savings will probably never pay for the cost of the system, but it does reduce noise and pollution when cars are standing at traffic lights or in traffic jams.

It does save a teeny weenie bit of fuel, as if you have the engine off for 2 minutes whilst you sit through a traffic light cycle it saves more fuel than the small amount extra you use replacing the energy used restarting.

I read once in the comics that BMW were fitting a system that basically ran the battery down when moving, by disabling the alternator, and only charging the battery if it fell below say 50%.
They then used the alternator as a regenerative brake, by switching it on when the fuel was cut off = coasting downhill, or when the brakes were applied. This represents an increase in efficiency, as some of the kinetic energy that would have turned to heat in the brakes becomes electrical energy in the battery.

Don't know if it got to production or not though..

WellKnownSid

Paradigm wrote: »

Remember when they taught physics at school... you don't get anything for nothing!
Energy is energy & it comes from somewhere...does stop/start save fuel & emissions or is it a gimmick?

No, it doesn't because whilst the engine isn't running you're not burning fossil fuels.

Electrical energy is being used to run ancillaries with the engine stopped, you will need to replace this, but this is energy that you would be generating anyway with the engine otherwise doing no useful work.

When the engine restarts you will need additional energy. This will have been largely stored in the capacitor with virtually no drain from the battery. That capacitor was charged during the previous run / shutdown period.

You may need a little fuel during the startup period.

Total of startup electrical energy and fuel << energy saved by not running engine at idle and doing no useful work.

Appreciate that the NEDC is flawed, but engine stop/start can mean a 5g/km CO2 saving - which may equate to something similar if you're doing a lot of town driving,

If you never stop - the maths gets more complicated...

DUTR

Paradigm wrote: »

Remember when they taught physics at school... you don't get anything for nothing!
Energy is energy & it comes from somewhere...does stop/start save fuel & emissions or is it a gimmick?

There can be a saving but it is small

WellKnownSid

I think it's more accurate to say that it has a significant benefit in test cycles.

Thing is, as people are more aware this week, the test cycles used don't match real life.

But if that means you save £Xx / year by being in a lower CO2 band - plus you spend a lot of your time in traffic - then you will realise £££ in your pockets. Regardless of what is happening to the environment.

jellie

WellKnownSid wrote: »

Presumably indicators faced the same backlash. "I for one won't be using these new fangled self-illuminating trafficators. I'll just stick my hand out of the window like I've always done!"

Given the number of drivers who don't use their indicators these days, I guess there's still a lot who are wary of them. Although as they don't stick their hands out of the window either, there must be some other reason why they don't feel it necessary to let other drivers know their plans.

Alter_ego

peter_the_piper wrote: »

Isn't the battery always being recharged from the alternator anyway so its not really extra load.

No, there is a regulator to control it so that it supplies what is needed, and prevents overcharging.

Gloomendoom

WellKnownSid wrote: »

No, it doesn't because whilst the engine isn't running you're not burning fossil fuels.

Electrical energy is being used to run ancillaries with the engine stopped, you will need to replace this, but this is energy that you would be generating anyway with the engine otherwise doing no useful work.

You aren't generating any electricity anyway if there is no electrical load, and if there is a load you are using extra petrol/diesel to generate the electricity... even if the car isn't moving.

You don't get something for nothing.

WellKnownSid

Gloomendoom wrote: »

You aren't generating any electricity anyway if there is no electrical load, and if there is a load you are using extra petrol/diesel to generate the electricity... even if the car isn't moving.

You don't get something for nothing.

But that's my point you're not, when the car isn't moving you're NOT using extra fuel because the engine's already been turned off. It's only when you start moving.

Think of it another way. If it takes you 60 minutes travel up a motorway - and for 15 minutes of that the engine was not running - which was what happened to me on Friday night - you've avoided idling your engine for 15 minutes. Now, the electrical drain during that time was, what, 20 amps - so 5Ah lost from the battery. There might have been ten restarts during that time - if you ignore the capacitor and assume 5 seconds total at 900A that's 1.25Ah.
Your charging system is only 80% efficient so you need to put back 6.25/0.8 = 7.8Ah call it 8Ah.
It's a 12V system - so 96Wh. As a side check we want to put that ALL back in the last six minutes of the journey when I pull off at the junction. That's a load of 96/0.1 = 960 watts. 80A - about right for a big alternator.

My car uses 0.2 gallons / hour idling - so I've saved 0.05 gallons = 0.22 litres of fuel.

Energy density of diesel is 10kWh/litre - my 96Wh would take 0.0096 litres at 100% thermal efficiency or 0.024 litres of fuel at 40% thermal efficiency.

So I saved 0.196 litres or 21p of fuel on Friday.

DUTR

WellKnownSid wrote: »

But that's my point you're not, when the car isn't moving you're NOT using extra fuel because the engine's already been turned off. It's only when you start moving.

Think of it another way. If it takes you 60 minutes travel up a motorway - and for 15 minutes of that the engine was not running - which was what happened to me on Friday night - you've avoided idling your engine for 15 minutes. Now, the electrical drain during that time was, what, 20 amps - so 5Ah lost from the battery. There might have been ten restarts during that time - if you ignore the capacitor and assume 5 seconds total at 900A that's 1.25Ah.
Your charging system is only 80% efficient so you need to put back 6.25/0.8 = 7.8Ah call it 8Ah.
It's a 12V system - so 96Wh. As a side check we want to put that ALL back in the last six minutes of the journey when I pull off at the junction. That's a load of 96/0.1 = 960 watts. 80A - about right for a big alternator.

My car uses 0.2 gallons / hour idling - so I've saved 0.05 gallons = 0.22 litres of fuel.

Energy density of diesel is 10kWh/litre - my 96Wh would take 0.0096 litres at 100% thermal efficiency or 0.024 litres of fuel at 40% thermal efficiency.

So I saved 0.196 litres or 21p of fuel on Friday.

And 5 days a week is approaching £50 a year