Auto math question

For some reason this problem sticks in my head from time to time and there's a key piece of info I can't find that would help me solve it:

How much energy does it take to crank and engine to the point where it can start?

Basically, I'm obsessed with knowing how long you would have to run your engine after starting your car (assuming a typical 65amp alternator) to charge your battery enough so you could just keep starting it and shutting it off in perpetuity (assuming no battery degradation). 

No, I'm not high.

I can say one thing- the cranking time of a modern engine is about .5 sec @ 70F.  

One important piece of data needed for this calculation is how hot is the motor?  Cranking speeds and the amount of power to spin the engine changes a HUGE amount between hot and cold.  And it's worse the colder it gets.

But I'll have to look at some data for stop-start- which doesn't use much energy...  For sure, less energy than the same amount of neutral idle. 

Is there a way to datalog the amperage draw of a starter?

If the starter draws say 195 amps (probably way high) for 0.5 sec, then a 65 amp alternator would replenish that in 1.5 seconds.

I'm sure there are some inefficiencies to deal with and you get into an issue of how fast will the battery accept that charge and how fast will the output of the alternator respond to the current request.

Also, an alt. can't actually charge at 65 amps, can it?  I think a motor could draw 65 amps from it, but you need to charge a 12v battery with 14v.

edit:  65 amps @ 2v would be the charge rate (0.13kw) and 195 @ 12v  would be the discharge rate (2.3kw)

It depends on the starter.  SBC starter power consumption ranges from 1.1kw to 1.8kw (2.5hp)  1.8kw = 150amps but then there's in rush current is likely 2 to 3 times that number and how long do you crank it?

Yeah, but if you are in motion you may not even need electrons?  Diesel engines fire on compression once warm enough and you can push start a manual transmission car without a battery.  So if the OP is talking about improving running efficiency while in motion as an extreme case of displacement on demand this is a somewhat different case.

Oh, crap...I just referred to JG as "the OP"...  Ooops.

ProDarwin said:

Is there a way to datalog the amperage draw of a starter?

If the starter draws say 195 amps (probably way high) for 0.5 sec, then a 65 amp alternator would replenish that in 1.5 seconds.

 

I'm sure there are some inefficiencies to deal with and you get into an issue of how fast will the battery accept that charge and how fast will the output of the alternator respond to the current request.

 

Also, an alt. can't actually charge at 65 amps, can it?  I think a motor could draw 65 amps from it, but you need to charge a 12v battery with 14v.

 

edit:  65 amps @ 2v would be the charge rate (0.13kw) and 195 @ 12v  would be the discharge rate (2.3kw)

I will tomorrow- given what it is, I'm sure it's data I can share- it will be with a '19 2.0l....  Just as a reference point.

If you measure voltage and current during cranking you generally see the battery voltage dropping to 11-9.5 volts depending on battery health and other factors. Current varies dramatically based mostly on the size of the engine (or the huge jump in compression ratio when you get to diesels) but it's usually 150-400 amps for gas engines.  With those two numbers you can ballpark the power requirement. 

So the cold start was about 1.5kJ

The first hot start was about .9 kJ, after that, each re-start was about .8kJ.

Roughly.  Hard to get it super close, as there's power to run the rest of the car (which I tried to take out).  

At the very nominal charge rate, it's recharged in 10-15 seconds.  And when you consider that you can do some extra smart charging and you decelerate, you can make much of that up pretty easy.

ProDarwin said:

Is there a way to datalog the amperage draw of a starter?

If the starter draws say 195 amps (probably way high) for 0.5 sec, then a 65 amp alternator would replenish that in 1.5 seconds.

As alfadriver points out, it is very dependent on a lot of factors.  195a might be a small four cylinder at operating temp.  A cold older V8 with a direct drive starter might be 400+ amps.

And yes, it is simple to datalog with an amp clamp and practically any digital 'scope.  Same for the charging system side of things. 

Also, an alt. can't actually charge at 65 amps, can it?  I think a motor could draw 65 amps from it, but you need to charge a 12v battery with 14v.

Heck, you can use the amp clamp on the battery cable and forget the math, and just datalog amps out when cranking, and amps in after it starts.

Also, an alt. can't actually charge at 65 amps, can it?  I think a motor could draw 65 amps from it, but you need to charge a 12v battery with 14v.

 

edit:  65 amps @ 2v would be the charge rate (0.13kw) and 195 @ 12v  would be the discharge rate (2.3kw)

It totally can. You have to remember that even though the alternator is only making a small voltage over the battery, the resistance of the large wires in between (and the battery itself) is very low.  I've seen as much as 180 amps using an inductive clamp on alternator charge wires.  

Look at the car wiring diagram. Some show the starter as simply a HP load.  For a Volvo 1800 it's about 1 HP, which is about 746 watts. A little over 62 amps at 12V. So in theory the 35A alternator typically fitted to an 1800 would recharge the battery in about 2x as long as it took the starter to get the engine running. Actual numbers and times would be a bit different since automotive systems tend to operate at a bit more than 12V.

JG Pasterjak said:

Basically, I'm obsessed with knowing how long you would have to run your engine after starting your car (assuming a typical 65amp alternator) to charge your battery enough so you could just keep starting it and shutting it off in perpetuity (assuming no battery degradation). 

No, I'm not high.

Are you trying to see what the stop/start cycle of cars that turn themselves off instead of idling at rest is doing to things?

Alfadriver has given the most specific answer so far but the big-picture answer is that the OEMs have done the math and that's why stop-start exists even on vehicles that don't primarily sell on their mpg. 

Another thing that hasn't been talked about is that if you have enough monitoring and control (i.e. a motor/generator instead of a regular starter & alternator) you can stop the engine in a specific place where it is easier to restart than if it had stopped in some other position. When you get full control of the valvetrain you can also make the cylinders have ~0 compression (and 0 fuel, 0 ignition which are all energy coming from somewhere) until the engine is spinning fast enough to plausibly start. Once you're committed to motor/generator it makes sense to step up system voltage at which point it becomes feasible to also run your ac compressor electrically so you can have full AC performance for a limited time while the engine is stopped. Most of this existed on a Prius 20 years ago. The valvetrain thing is still not commercially viable in mass production AFAIK.

Back when i was driving my Honda Insight I always wondered how nasty it would feel to 'bump start' the engine by simply rolling forward in first gear with the electric motor (there are mods to take control of the electric portion via a joystick etc) but i never did get around to trying it. Would probably have felt pretty nasty without being able to 'freewheel' the cylinders at first. But then again with precise enough motor control you can also apply opposite-phase torque waves to counteract the 'vibration' you feel from a low cylinder count engine operating at very low rpms. That level of precision is what's already on the market in stuff like Mazda's HCCI.

I am rabbit hole, insert brain.

In reply to Vigo :

The benefits of stop-start is pretty easy to show- I've even done some unintentional testing showing the benefit- it was a lot more than I expected.  And given the cost to do it, I'm actually surprised that it hasn't come earlier.  

The next thing that I think will become more universal are the starter/gen sets.  They are really getting out there- but the cost and benefit of a 12V system is tough to justify over a 42v system.   So at some point, 42v battery packs to help move the car will start moving into cars.    There is some pretty easy low hanging fruit of the basic stage of hybrids that should be pretty universal in a few more years.

I'll still lean toward Toyota for the company who seems to have the best direction.  Direction that delivers both for the cycles we test and the driving you do.

The next thing that I think will become more universal are the starter/gen sets.  They are really getting out there- but the cost and benefit of a 12V system is tough to justify over a 42v system.   So at some point, 42v battery packs to help move the car will start moving into cars.    There is some pretty easy low hanging fruit of the basic stage of hybrids that should be pretty universal in a few more years.

One interesting thing is that after all the years since the brief flurry of GM cars dried up and were forgotten, 'mild hybrid' has so little positive name recognition that RAM doesn't advertise their 48 volt E Torque system as a hybrid at all even though that's exactly what it is.  

alfadriver said:

In reply to Vigo :

The benefits of stop-start is pretty easy to show- I've even done some unintentional testing showing the benefit- it was a lot more than I expected.  And given the cost to do it, I'm actually surprised that it hasn't come earlier.  

I thought part of it was that in order to implement it initially, the OEM had to submit to two EPA tests... one with the start/stop system on, and one with it off... and the cost of that was a deterrent to bringing those systems over (for example MINI had an ECU that did it in Europe, but disabled it in the US)

sleepyhead the buffalo said:

alfadriver said:

In reply to Vigo :

The benefits of stop-start is pretty easy to show- I've even done some unintentional testing showing the benefit- it was a lot more than I expected.  And given the cost to do it, I'm actually surprised that it hasn't come earlier.  

I thought part of it was that in order to implement it initially, the OEM had to submit to two EPA tests... one with the start/stop system on, and one with it off... and the cost of that was a deterrent to bringing those systems over (for example MINI had an ECU that did it in Europe, but disabled it in the US)

It really has nothing to do with the testing- that's fast and easy to do, and it's quite cheap in the whole scheme of things.  It's more that BMW didn't calibrate the MINI system good enough to meet the US requirements.

Ford is constantly submitting multiple test configurations to demonstrate that a system working or not will not change the emissions- every single shift change logic gets tested.  As does anything where the OEM gives the consumer the choice of activating it or not- like stop start.