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 Post subject: Help with understanding differential coupled configuration
 Post Posted: Sun Jul 24, 2011 9:26 am 
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I read in many team reports that they are using differential coupled drivetrain. One of the posts in this clarified its meaning as fitting 2 sprockets on a chain drive differential. Unfortunately am still confused with the idea. How can one differential housing be turned by 2 different power sources? Is it that both the sources are running at the same speed? What happens when only the motor is running i.e won't the engine chain turn too?

Kindly clarify this configuration to me.
Thanks in advance. :)


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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Sun Jul 24, 2011 11:10 pm 
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Location: Texas A&M
Rakesh Murali wrote:
Is it that both the sources are running at the same speed?


Yes, both sprockets will be turning the same RPM. If your motor is direct drive with just a chain, it's speed will always be directly proportional to the car speed. If your motor is connected with a CVT or other transmission, its speed may very, especially if there is a free wheel in the system.

Rakesh Murali wrote:
What happens when only the motor is running i.e won't the engine chain turn too?


Yes, the engine chain will spin, but you can simply put the engine transmission into neutral to keep the piston from moving.

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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Wed Jul 27, 2011 2:25 am 
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Thanks for ur help mr.mojave. Kindly clarify me on the following too:

-->In the case of electric mode, what do you opine about usage of an overrun clutch on the engine chain?
-->Am I right in saying that the engine, motor (after reduction) and the wheels will be turning at the same RPM in linear motion?
-->What will be the total torque at the differential? I suppose it will be the sum of the torques from engine and motor(after multiplication). Am I right?


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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Wed Jul 27, 2011 8:06 am 
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Location: Texas A&M
Rakesh Murali wrote:
Thanks for ur help mr.mojave. Kindly clarify me on the following too:

-->In the case of electric mode, what do you opine about usage of an overrun clutch on the engine chain?
-->Am I right in saying that the engine, motor (after reduction) and the wheels will be turning at the same RPM in linear motion?
-->What will be the total torque at the differential? I suppose it will be the sum of the torques from engine and motor(after multiplication). Am I right?


I assume you mean a 1-way clutch, ie the engine can turn the differential, but the differential can't back-spin the engine. The main issue I see with this configuration is the lack of engine braking. Drivers might take some practice to get used to the car free wheeling when they lift off the throttle on corner entry. You can feel this in your street car by putting it in low gear in a parking lot, running the engine up to high RPM then letting off the gas pedal.

Yes, everything will be going the same speed.

Yes, you can sum all the torque.

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- 1st place FH '09: 981 pts


Last edited by Mojave on Wed Jul 27, 2011 11:50 am, edited 1 time in total.

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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Wed Jul 27, 2011 11:33 am 
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Thanks a lot mojave. By the way am working for the formula hybrid team of VIT University, India. We are newbies to the competition and hybrids. Your answers were quite reassuring and helped us well.


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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Wed Jul 27, 2011 11:49 am 
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Rakesh Murali wrote:
Thanks a lot mojave. By the way am working for the formula hybrid team of VIT University, India. We are newbies to the competition and hybrids. Your answers were quite reassuring and helped us well.


One tip I would give a new team is to start simple! You don't need a super-complex hybrid to do well. Get the car done before showing up in NH and get as much test time as possible. This is a race car competition, not a science-project competition.

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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Thu Jul 28, 2011 3:38 pm 
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Mojave wrote:
One tip I would give a new team is to start simple! You don't need a super-complex hybrid to do well. Get the car done before showing up in NH and get as much test time as possible. This is a race car competition, not a science-project competition.


I agree – that is really excellent advice... both for new teams and for veterans!


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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Tue Aug 02, 2011 11:15 am 
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Thanks for the advice. I thought this differential coupling is among the easier ways of making the car.
One more doubt:

when the engine alone is powering the wheels i.e. the motor is charging and the wheels are turning, how will the torque split?
will it be 50% to motor and 50% to differential housing or will it depend on the charging rate of the battery? If the 2nd case is true, then on what basis will the torque split?


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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Tue Aug 02, 2011 12:08 pm 
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Location: Texas A&M
Rakesh Murali wrote:
Thanks for the advice. I thought this differential coupling is among the easier ways of making the car.
One more doubt:

when the engine alone is powering the wheels i.e. the motor is charging and the wheels are turning, how will the torque split?
will it be 50% to motor and 50% to differential housing or will it depend on the charging rate of the battery? If the 2nd case is true, then on what basis will the torque split?


I think the split will greatly depend on the particular motor, batteries, motor controller, and motor controller programming. On the car I helped design and build (Texas A&M '09 car), our motor never charged the batteries, so we never messed with this scenario.

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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Tue Aug 02, 2011 12:25 pm 
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Rakesh Murali wrote:
will it depend on the charging rate of the battery? If the 2nd case is true, then on what basis will the torque split?


Most motor controllers are set up to control torque. So if you set up the motor controller to charge the battery, using torque supplied by the engine, you would program it to do so with a particular torque value, or send it a signal from a computer or from the driver to specify that torque value. If you set that at, for example, 20% of full engine torque, if you ran the engine at 40% of full torque, you'd get a 50:50 split. If you ran the engine at full torque, you'd get 80:20. Or if you ran it at 30%, you'd get 33:67.


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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Tue Aug 02, 2011 12:31 pm 
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Thanks a lot Mr.Charlie. The idea sounds brilliant. So is the torque setting for battery charging done electrically or mechanically? In either case, kindly brief me the method of tapping the toque as it seems wierd without a differential !


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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Tue Aug 02, 2011 12:41 pm 
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A good question is with a parallel hybrid design: why are you using the engine and spending fuel to charge the batteries?

Wouldn't it be more efficient to take that mechanical energy to drive the car directly, rather than convert it to electrical (into motor) then to chemical ( into batteries) then back to electrical (out of batteries) then back to mechanical (out of motor).

Nothing in the system, not even the motor controller, is 100% efficient, so a non-insignificant portion of energy that was already in mechanical form has been lost in the multiple conversions.

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- 1st place FH '09: 981 pts


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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Wed Aug 03, 2011 1:47 pm 
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Rakesh Murali wrote:
Thanks a lot Mr.Charlie. The idea sounds brilliant. So is the torque setting for battery charging done electrically or mechanically? In either case, kindly brief me the method of tapping the toque as it seems wierd without a differential !


It is done electrically. The theoretical explanation, for a permanent magnet DC motor, is that torque is proportional to current, and so what one needs is an electronic circuit that controls the current in the motor. If that electronic circuit successfully controls the current, and keeps it at a set value, that will keep the torque at a set value.

The practical explanation is that a motor controller with the capability like that will usually have a signal input that you used to tell it what torque you want, and then it does what it needs to do to get that torque. (For a PM dc motor, that would mean that it controls the current; for other types of motors what it does internally may be more complicated, but assuming you are buying a motor controller rather than building your own you don't need to worry about that.)


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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Wed Aug 03, 2011 1:59 pm 
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Mojave wrote:
A good question is with a parallel hybrid design: why are you using the engine and spending fuel to charge the batteries?

Wouldn't it be more efficient to take that mechanical energy to drive the car directly, rather than convert it to electrical (into motor) then to chemical ( into batteries) then back to electrical (out of batteries) then back to mechanical (out of motor).

Nothing in the system, not even the motor controller, is 100% efficient, so a non-insignificant portion of energy that was already in mechanical form has been lost in the multiple conversions.


In a general-purpose vehicle, made for driving on public roads, one answer to that starts with the fact that if you open the throttle pretty wide and send all the power to the wheels, you will rear-end the vehicle in front of you and/or exceed the speed limit. So your choices are to skim off some of that power and send it to the batteries, or to throttle back the engine. Either way, you lose efficiency. Figuring out how to adjust that extra degree of freedom to maximize overall efficiency is one of the most interesting and challenging aspects of hybrid vehicle design.

I'm not an expert race-car driver, but I know enough to know that one generally drives with the throttle wide-open much more of the time. So there may be fewer opportunities to do that kind of thing. But without looking at the efficiency curves of the various components, and the detailed operating cycle in a race, it's hard to say for sure.

The other reason that one might charge the batteries using some of the engine power would be if the batteries were getting low and it was important to have battery energy available to assist acceleration at a later point in the race, such as for acceleration out of the next turn, which might help improve lap time more than using the engine power directly to accelerate at the end of the straightaway, just before braking.


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 Post subject: Re: Help with understanding differential coupled configurati
 Post Posted: Mon Aug 08, 2011 5:35 am 
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Thanks a lot lot :D mr.charlie. Mr. Mojave, we are trying to make a series-parallel hybrid. Think that should clarify things for you. The motor is way more efficient than the engine and the torque output is very good. Ours is a powerful motor which will take away the battery charge at a much faster rate. The motor can be used later for increased acceleration. Just regenerative braking wont be enough for charging the battery. I hope Mr. Charlie's explanation along with this would have made things clearer.


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