Experiences with start/stop systems

[BPHannes]

Hey everyone!

I'm doing a university project on low-cost technologies to improve fuel efficiency and we are focusing on micro hybrid powertrains (start/stop automatic with regenerative braking).

I've seen that there's a lot of experimental activity in the forum and some of you are even trying to build such systems on their own or are improving existing ones.

I was wondering if you have experienced any limitations of micro hybrid systems regarding their fields of application. Are they appropriate to achieve noticeable fuel savings?

//Hannes

[jmcc]

I'm doing a commercial project on the same. Start stop is a good fuel saver, especially on an auto. We are applying other technologies too - google the Technology Strategy Board for info on various UK government supported research projects. Hopefully they should have project info.

[pete c]

Welcome Hannes

I think mild hybrid systems offer the most bang for the buck. Kind of surprised we don't see much more of them now.

I think the way to go is to go with a higher voltage alternator which would also serve as a starter/engine assist. And don't get carried away with battery capacity. I think a Nimh pack of the same weight (or less) as the lead acid battery would work fine. Not sure if Li batteries offer enough of an advantage to offset their price.

[BPHannes]

@jmcc

Thanks for the tip, are you talking about the Clean and Cool Mission 2012? May I ask what other technologies you are applying?

@pete

We've done user research to figure out why these systems aren't as widely spread as they could be and the result so far is that

1) Fuel savings are not high enough to make a difference for the average driver

This of course depends heavily on traffic situation and driving style. Many new car models come with micro hybrid functions as a standard (especially in Europe; unlike in the US full hybrids are less common and micro/mild hybrids more), so the buyers can't really chose whether they want to have it or not. Those who can choose often go without it; those who can't often decide to deactivate it (either because they believe that car parts get damaged or because they are annoyed by the constant start and stop).

2) The functionality is limited by the battery

In many cases, the capacity is not high enough to ensure a proper functioning of the start/stop mechanism. E.g. when air condition, radio, etc. are switched on, the engine has to restart to maintain the charging level. Also, many drivers reported that the function stops working completely when the battery reaches an age of 2-3 years. Besides that, there are also issues with the ibs (intelligent battery sensor), which provides faulty capacity levels and hinders the start/stop function.

3) There are concerns about the maturity of the technology

Mostly these concerns are about the reliability of the system. In low outside temperatures (below 3° C/37° F) and in warm temperatures where the AC is on, the start/stop automatic doesn’t work. There are a number of factors that influence the functioning and drivers are not informed about them, which is why some of them deactivate the system permanently. Other problems regard the speed of restarting the engine, which is perceived as too slow by some.

4) The usability is limited

In certain traffic situations there is no added benefit. This is particularly true in traffic jams where periods of standstill are short and the constant stop and start becomes annoying. Some manufactures counteract these issues by requiring the car to exceed a certain speed before the mechanism is reactivated. Also, drivers in rural areas don’t experience stop-and-go traffic and can therefore not realize fuel savings.

5) Drivers are concerned about faster deterioration of car parts

Many drivers don’t trust micro/mild hybrid systems because they expect a faster deterioration of parts like alternator, starter motor and engine due to the considerably higher frequency of starts. Whether this is justified is difficult to say. We have heard different opinions on that; e.g. increased wear doesn’t seem to be a problem for the Toyota Prius, however some manufacturers stated that conclusions can only be made after the car has been used for a longer period.

All these concerns are of a technical nature and in order to achieve a higher technology diffusion of micro/mild hybrids, they have to be tackled. There are several approaches to do that (e.g. upgrading the technology to allow short-distance electric drive, making it possible to recharge the battery externally, pre-heating the engine, etc.), however the additional cost of such approaches must be weighed against the benefits.

What do you think about these issues? Can you think of ways to counteract them? There are countless other technologies that aim at the same goal of reducing fuel consumption and emissions. The big question is which one is most promising and how different technologies can be combined to achieve a significant improvement.

[jmcc]

We're working with a downsized engine, start-stop, advanced thermal control inc two separate cooling circuits, low friction lubricants, thermal energy storage and revised air system. In addition we have advanced aftertreatment so we can go on an emissions level. We are then recalibrating the engine maps to take advantage of the above systems, which have been selected because they work together.

[euromodder]

Originally Posted by BPHannes

We've done user research to figure out why these systems aren't as widely spread as they could be and the result so far is that

I think your analysis is pretty much complete.

A main factor in why it's introduced is the time spent at idle during NEDC.
These test cycles are hardly realistic at best, and when fuel is cut off during the idle periods, the test only becomes less realistic.

1) Fuel savings are not high enough to make a difference for the average driver

It's most effective in situations that typically increase fuel consumption, i.e. repeated stops and accelerations.
So it'd be hard to see the relatively small positive effect of stopping the engine, as it gets drowned in the increased consumption due to these situations.

With many diesels using around 0.5L/h at idle, shutting down for a minute only saves 0.0083 liters.
In order to save 1 liter, you'd need to be stopped for a full 2 hours.
On a 1000km tank, in a car averaging 5L/100km without start/stop, that'd mean an improvement of just 0.1 L/100km, to 4.9L/100km - and that's well within the variation noise from tank-to-tank.

2) The functionality is limited by the battery

Hence a second battery is sometimes used to provide power for the accessories - adding more weight and complexity.

4) The usability is limited

Because it shuts down the engine only when the car is stopped.

If it also shut down during periods when the engine is not required - i.e. gliding or coasting periods - its usability would improve dramatically for people who coast.
But in today's power-hungry cars, that'd require even more extra battery capacity during extended glides.

But coasting isn't on the NEDC.
It'll make one fail the driving tests to get a licence, and it's regarded as weird by most motorists - myself included until not too long ago.

What people fail to realise is that coasting is how an electric engine operates.
It shuts off when no power is demanded.

Failure to coast, and falling into the regeneration trap, is what prevents many people from getting the full range out of electric cars.
They'll drive them like they drive their ICE cars.

5) Drivers are concerned about faster deterioration of car parts

The starters are improved to make them handle the many more starts, but their durability will only be truly proven in real life.

e.g. increased wear doesn’t seem to be a problem for the Toyota Prius

Because it uses the very powerful electric engine and massive battery to start the ICE.

All these concerns are of a technical nature

Solving the technical side of it, will likely be the easiest part.
I see user acceptance - or lack thereof - as the main issue.

Most folks I know with a car that has a stop/start function, have shut it off.