How big of a capacitor array would be needed to start a typical 6 cylinder automobile engine? (Assume only the starter motor is powered by the capacitors and the ignition and controls are powered by a battery as usual.) The idea is a simple hybrid that stops the engine when the car is stopped and restart it when it begins to move again, without having to use a large battery like current hybrids do.
1 Farad = 1 amp for one second. Figure on 3 seconds (minimum) for a reliable start and probably something like 30 amps for the starter motor (depending on engine size, compression, etc). Add in the losses for the ESR of the capacitor. Probably something like 100 Farads or so. Unless you need to try again, in which case you'll need to recharge the capacitors first. And don't forget that capacitors do self-discharge, so you'll have to factor that in.
Electrostatic field is a lousy way to store energy compared to chemical conversion methods, i.e., batteries. No doubt super capacitors will continue to advance, but IMO there's a limit to how much surface area you can pack in a given volume and support a given voltage. I think 100F is far too conservative for a practical application- think about what happens as it discharges.
There are certainly new and better ways to do things but a good read on energy fundamentals at Don Lancaster's site is highly recommended to anybody contemplating designing such things. He also has some interesting things to say about hydrogen and other alternates-
http://www.tinaja.com/
There are certainly new and better ways to do things but a good read on energy fundamentals at Don Lancaster's site is highly recommended to anybody contemplating designing such things. He also has some interesting things to say about hydrogen and other alternates-
http://www.tinaja.com/
The starter will not be a regular one. A special one will have to be used in order to withstand being used dozens of times every day, so you can make it just about any voltage you like (within reason).
My friends old Honda Insight only had a small (30AH?) battery for the engine..about the size of a small lawn/garden battery. I'd assume any cap bank able to supply that kind of juice would be larger than the battery.
I remember reading a while ago about truckers who would put some 'ultracaps' in parallel with the starter batteries to help kick the starter motor over. http://www.maxwell.com/ultracapacitors/
I remember reading a while ago about truckers who would put some 'ultracaps' in parallel with the starter batteries to help kick the starter motor over. http://www.maxwell.com/ultracapacitors/
star882 said:
Have you ever opened a NiCd D cell... I think it's the same Ah/weight as an AA. I'll have to look for the NiMH D cells.
I once measured the peak starting current on a normal compression and ambient temp V8 and it was interestingly an Amp for every Cubic inch. Not sure if there is any correlation but it was a good mnemonic for the time.
There are double layer super capacitors available, and have values of around 3000 Farads at 2 volts. So if you wired 6 in series for 12 volt operation, that would be equivalent to 500 farads. Another bank in parallel and you would have 1000 Farads
Interesting, but useful!
Actually I expect 6/8 cyl engines to be easier on the starter than 4cyls - simply put, there's much less torque ripple because of the decreased firing angle. Worst of the lot are 4cyl diesels... lore has it they can demand about 400A peak due to wide firing-stroke spacing and high compression (and are very common on this side of the pond). Plus the inital glowplug/preheat demand from cold .. another 60-80A!
Anyway, having recently left the parking lights on my car on all day, to the point where it couldn't *quite* turn over at all, simply adding the fully charged battery pack from a cheap 12v NiCd cordless drill in parallel was enough to effect an easy fix! Since the drill only cost #6 I don't care if it was a once-only trick - it's a 4litre V8 with a geared starter. The pack now appears OK, and contains 10 'C' size cells.
So - the long way round - I'd agree that small, quickly-charged secondary cell packs might be worth investigating rather than supercaps. Industrial 'D' cells can easily sustain repeated 30A discharges, infact the Honda Insight main battery pack is based on them...
Check out some of the Sub-C cells used for RC cars and planes. 30A is a piece of cake. You can even get matched/tested cells,with a detailed spec-sheet for a bit more $.
Some of the motors they use can draw some serious current,I've cooked a few speed controls like they were nothing when using 'modified' class motors.
Very true, but a lot of those are now LiPo whereas dumb old NiCd is very forgiving of charging regime, standing around unused etc...
The "BatCap 300" from the link I posted earlier uses spiral cell battery technology. It's essentially a battery (not a capacitor) designed for huge peak current delivery.
There was another company called Boulder Tech that used to make automotive jump start packs out of the same battery technology, but they went bankrupt some time ago.
Those "super caps" mentioned earlier have relatively high internal resistance making them unsuitable for high current discharge.
There was another company called Boulder Tech that used to make automotive jump start packs out of the same battery technology, but they went bankrupt some time ago.
Those "super caps" mentioned earlier have relatively high internal resistance making them unsuitable for high current discharge.
Based on quick calculations, 200A*12v*1s=2400J
That amount of energy is attainable with just 6 4700uF, 450v capacitors charged to 430v and discharged to 100v. And they'll very likely outlast the car since capacitors do not degrade much when used within specifications.
However, traditional starters are very inefficient (high resistive losses), so the actual energy needed is lower. High voltages reduce resistive losses. Add some more tricks like unload solenoids to deactivate some of the cylinders and electric oil pumps to prime the bearings and the energy needed is even lower.
Where capacitors really excel is regenerative braking. The problem is the sheer size of the capacitor array needed. A 3000lb car going at 45MPH would need almost 3F of 450v capacitors to store all of the energy from regenerative braking. (As a joke, one of my friends would say that's a "Peggy-sized array"!) So the capacitive engine starter is already practical, but not the capacitive regenerative brakes.
That amount of energy is attainable with just 6 4700uF, 450v capacitors charged to 430v and discharged to 100v. And they'll very likely outlast the car since capacitors do not degrade much when used within specifications.
However, traditional starters are very inefficient (high resistive losses), so the actual energy needed is lower. High voltages reduce resistive losses. Add some more tricks like unload solenoids to deactivate some of the cylinders and electric oil pumps to prime the bearings and the energy needed is even lower.
Where capacitors really excel is regenerative braking. The problem is the sheer size of the capacitor array needed. A 3000lb car going at 45MPH would need almost 3F of 450v capacitors to store all of the energy from regenerative braking. (As a joke, one of my friends would say that's a "Peggy-sized array"!) So the capacitive engine starter is already practical, but not the capacitive regenerative brakes.
Can't remember who but there was a small German manufacturer of boutique sports cars who uses a supercap in parallel with an ordinary car battery exactly because the engine can be started more reliably compared to the sole use of an old-fashioned and crappy car battery (especially when the car hasn't been used for longer or during winter).
Regards
Charlkes
P.S. And yes, there are boosters made by an American company that are used when you have a flat car battery - and these are based solely on super caps.
Regards
Charlkes
P.S. And yes, there are boosters made by an American company that are used when you have a flat car battery - and these are based solely on super caps.
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