Next we are going to talk about energy, why is energy important? Well does the vehicle have enough energy to let the drive take place? It is like fuel, energy is like fuel, does it have enough fuel? Does it store enough fuel? It will give you the range that to which you can travel and that becomes important role and we will talk about energy required. Another thing that we will come across throughout the course okay with all this known torque, power, speed, force and energy you have to now design motor, controller and batteries.
At what voltage should we design all this? You will see that depending on the voltages your currents will become very different, if you have high voltage your current will go down, to give us some power after all battery will give you some power, so if you suppose want a power P, if you want at a voltage V, then the current is that power divided by voltage, if I want a lower voltage my current will go, if I want higher voltage, my current will go smaller. Normally I prefer lower current. Why? Because current I have carry in conductors and when I carry current in the conductor there is a heat dissipation, there is I square R loss, now one would say well I square R loss should be small, depends on what the current is, we will see the currents in some of these vehicles can become 100 ampere, 200 ampere, 300 ampere I square can become very large, I therefore do not like this 200, 300 ampere.
If I use higher voltage my current maybe more like 30 ampere, 50 amperes, 70 ampere, 80 ampere, much less much easier manageable, ofcourse high voltage has another thing, lower voltage is safer, high voltage is less safe, it will require a complete isolation of all the electrical components with the rest of the vehicles, finally we will have to see where should we pay the cost of isolation and reduce the currents and where I do not want to get into this isolation, let me use low voltage let current go up a little bit. So, this voltage will be become very important.
A related thing will be, see if I know my peak power requirement, what is the current that I will draw? What is the average current that I will draw? What is the peak power current that I will draw? Currents are plays important role not just in terms of heat loss and conductors, but a battery also is normally designed to give you only so much current, if you try to draw higher currents from the battery, the life of the battery gets impacted we will study that in detail later on in a battery chapter.
So, this is another thing that we have to start discussing and then get into details later on. And the fourth thing that we will discuss start discussing, what are the losses in each sub-system? So, far we assumed everything is perfect, not so, motor never runs at a 100 percent efficiency, you given a certain amount of power, certain amount of energy, certain amount of power, part of that power is going to get lost into heat.
It has a double problem, problem number 1; some power and therefore some energy is wasted, so I will require more energy to drives the vehicle. Number 2; whatever losses are there heat is generated, that heat is going to heat up the battery, heat up the motor, heat up the controller and heat is not good for either the battery or for the motor or for the controller. You can allow a certain amount of heat after that you will have to do cooling, you have limit the heat, so we have to worry about the losses one is the losses due to I square R, but I square R is not the only losses, in motor there are other losses and heat is generated. So, wherever heat is generated we have worry, motors consists of a lot of coils, so there is going to be a lot of losses, it is not just in simple conductors, motor basically is a lossy, there are other
losses like iron losses, you will see in motors, so it will worried about this, magnetic loss, iron loss are magnetic loss. So, we will start looking at it, as we go on we will look at more and more, all these four things, in next section I am going to start with energy that it travels requires to travels.
Some assignment problems for a 2-wheeler, 3-wheeler and e-rickshaw, we have shown how to compute traction power, torque at different velocity, given a certain wheel radius, what would be the power and torque required? We had basically ask you to learn to compute get some feel of numbers.
Assuming the sedan is stuck on a climb at 12 degrees do not need this stuck, it has to start and during a start it will require a torque plus I will have to give a minimum acceleration for it to get moving, I have taken them a very small acceleration 0.5 meter per second square. So, now what do I have to do? For that vehicle at 0 speed have to worry about the torque then I have to worry about the acceleration and torque due to acceleration plus there is a drag force, I have to worry about the drag acceleration, ofcourse with the very 0 low velocity so it will be nearly 0, there will be some rolling resistance I have to worry about the force due to that and therefore torque due to do that.
So, I have to combine all this torque and say what is the starting torque required, before I start this new chapter, there were two questions that were asked to me and let me try to answer, the first question was asked is that, when a vehicle is starting is mu going to change? Is there a something called static friction versus dynamitic make friction? mu is related to function. Now, there are two ways of dealing with it, you can either have a different value of mu, mu due to during movement and mu due to starting, that is one way of dealing with it. Very often that is not what is done, what is in the vehicle computation what you assume, mu is same, but you require certain extract acceleration, minimal acceleration will require, so compute the torque required due to acceleration plus torque required due to the drag mu and that is the dynamic that is during the static. So, in fact in this problem that I talked about in here, I have not done it, I have done starting, but I have to say there is a slope, very large slope 12 degrees, but I also said include a sudden acceleration, it is at 0 speed but include acceleration.
So, take the acceleration of 0.5 meter per second square you can convert that into kilometer, whatever any other units. Now compute the torque due to this plus compute the torque due to slope that is basically a starting torque which could have other way also sort of say mu have changed, but we in electric vehicle or even for IC engine vehicle normally mu is assumed to be constant.
Ofcourse there is a small function of velocity, at high velocity it matters a lot, starting, starting torque is the same as moving torque plus acceleration required, without acceleration fine you cannot move, when you are 0 speed you have to get to some kilometre per hour that is a starting acceleration, starting acceleration is another term used for the change in value of mu. So, that is a question number one, which I think this problem when you do you will get an idea, we will give you more assignment problem, note down on something like this. There was a second question that was asked and the second question was, a (())(10:42)
Student: (())(10:42)
Professor: Suppose you have a long slope, in many hills the slope is constant and in continue for 2 kilo meters, is the energy requirement is the power requirement all that torque requirement, what will happen? We will actually deal with that to some extent in this chapter of concept of drive cycle, but let me point out, it is related to motor design and battery design, you normally talk about in a motor the power that motor has certain power, but it also has something called peak power.
A motor maybe have 5 kilowatt, but its peak power maybe 8 kilowatt, the same motor it can drive at 8 kilowatt, it can drive at 5 kilowatt so what is the motor kilowatt? What is a peak implies? Actually as far as the mechanical part of the motor is concerned or even the electronic part of the motor is concerned it is the same, a 5 kilowatt motor is 8 kilowatt motor is actually design for torque, it is a heat dissipation for at 5 kilowatt the heat dissipation is small, or whatever is the heat dissipation it is taken care of by the cooling system. 8 kilowatt this peak dissipation is going to be much higher, that at much higher heat dissipation, what do you do? If it is 15-20 second that extra heat dissipation, motor temperature will go up and 15-20 seconds will pass and now it will cool down so it will be alright. If on the other hand you require a constant 8 kilowatt then you require a very different kind of heat dissipation system, which will take out this P the losses the heat dissipation at 8 kilowatt.
So, I will say a peak power is related to that if it is 15-20 second I can handle it, the motor is actually designed for the nominal power or rated power you can say, because heat dissipation is designed for rated power. So, that is what and we will this look at this later on the heat dissipation.
To some extent same thing about the torque peak torque and rated torque, rated torque is you can keep on running with that torque and heat dissipation will be something that you will have to keep on removing such that the temperature does not go up, if you go to pick torque you will have extra heat generated which is alight for a very short period of time but it is not alright if you will apply it constantly.
Now, look at what does it mean in terms of driving of a vehicle. If I am trying to overtake somebody I need 15-20 second extra power and extra torque, because I will have moving behind I am behind I will actually move like this, this vehicle and then move up 15, 20, 25 second that is a time peak power and peak torque help, nominally I am not driving above the rated power, so that is fine.
What about in slope? And particularly the long slope question that was asked, in a long slope I am actually drive that at that slope for 5 minutes maybe even longer, ofcourse I can reduce my speed and all those kind of things for power, but for my torque speed does not matter. So, that cannot be done using peak torque or peak power, it has to be done at rated torque and rated power.
So, you have to look at the motors rated power and rated torque and you can continuously climb the slope at that rated torque and rated power. But if you are using the peak torque and peak power to drive to go up then it can be only for a short period of time, what about energy required which will be dealing in this chapter. Energy required will go up if it is a short period of time, it will go up, if it is a stays for a long period of time energy requirement will go up considerably because energy is integration of power over time.
So, sure in a long slope energy requirement will go much higher, but remember that we also talked about a concept of regeneration, so if you are climbing for long you require certain amount of energy, after that you are going to climb down to the same extent and if we had a regeneration efficiency, my energy requirement will not make any difference.
But to the extent that I do not have 100 percent regeneration efficiency I have to pay the penalty of extra energy, if I am only recovering let us say 30 percent of the energy during going down, so 70 percent of that has to be spent. All that issue of how much energy will be spent is exactly this concept of a drive cycle. And what is a drive cycle?
The question that we are asking in this how much energy will vehicle take per kilometre? How much energy will the vehicle take during certain drive? Per kilometre energy per kilometre is very important, it is like your fuel efficiency kitna deti hai, amount of petrol consumed per kilometre or amount of kilometre for one litre of petrol, the kitna deti hai it is a same thing, energy efficiency is defined in terms of watt hour per kilometre that depends on the vehicle design.
But it does not depend only on the vehicle design, it also depends on how is the vehicle travels, when you do the measurement, what is the speed at which it travels, remember that when it travels at higher speed you require larger power which means large energy is consumed, so you cannot talk about energy, efficiency or watt hour per kilo meter at all speeds it will be different. If vehicle accelerates, it consumes tremendous amount of power.
So, it will depend on how much you accelerate, how much time you are just idling, not moving at all, you are stopped on red light, how well it be decelerates, when it decelerate it gives you back some power, energy, probably fraction of energy. So, at what speed you travel? For how long? Then what is the acceleration, for how long? From what velocity to what how much do you decelerate, for how long? How much are you idling? All these things become important component in the amount of energy consumed.
Therefore, how do you not talk about watt hour per kilometre? You can talk about watt hour per kilometre by defining what is called a drive cycle, this by the way is done in a petrol engine is
also going to be done for electrical engineering. It is a standard a drive cycle a standard drive cycle, a drive cycle says a definition of how the vehicle is driven try to standardize the driving pattern. Vehicles are tested as per standard drive cycle, the standard drive cycle will tell you how long did you travel, at what speed you travelled for how long, at how much did you accelerate, how much did you decelerate, did you wait idol, it will define this and for a class of vehicle it will standardize it and that is called a standard drive cycle.
A standard drive cycle for a 2-wheeler, standard drive cycle for a 3-wheeler, a standard drive cycle for a 4-wheeler. What is the purpose of this? Well, a drive cycle will help you compare if you have made a vehicle, you have made a vehicle similar 2-wheelers and the I can compare what is the energy efficiency of yours vis-a-vis so I will take a example.
A sedan, which is driven at a constant speed of 50 kilometre per hour and let us assume this is the vehicle, this is the vehicle, the vehicle sedan it is in page number slide number 29 here not 28 because I probably added a slide here, area is 2.5 square meter, drag is 0.35, weight is 1200 kg and suppose it is driven at a constant speed of 50 kilo meter per hour for 5 minutes compute the distance travelled and energy used.
So, if I compute the distance travelled and energy used which I will do out here, the drag is what? 150 Newton meter that is what comes at 50 kilo meter per hour, 50 kilo meter per hour drag comes to 50, 150 Newton's, rolling resistance is higher 190 Newton's at 50 kilo meter. And if I am driving at a constant speed and let us assume no slope only the drag and the rolling resistance has to be taken into account.
So, the total force that I have is 340 Newton’s, power consumed is known, therefore 340 Newton's multiplied by the velocity, velocity is 50 kilo meter per hour I have to convert it into meters per second by dividing by 3.6 and actually I am consuming 4.72 kilowatt to overcome this 4.72 kilowatt, I am consuming throughout for 5 minutes I am consuming 4.72 kilowatt, what is the energy that I am consuming?4.72 kilowatt for 300 second, this was the velocity 300 second, but I want to 300 second divided by sorry 5 minutes 500 minutes is 300 second, but actually why do I divide by 3600? Converting
into hour, because I want to actually right down not in terms of watt second but watt hour and if I
do this calculation I get 393 watt hour. So, every I am consuming 393 watt hour, if I continue drive this from 1 hour, it will be 393 watts, watt hour, if I continue to, well for 5 minutes I am consuming 393 watt hour, if I continue to drive for 60 minutes I will consumed 12 times this, what is the distance travelled? Well, 50 kilometre divided by 3.6 for meter per second into 300 seconds that gives me so many meters or 4.16 kilo meters.
So, what is the energy used per kilo meter? So, I consuming 393 watt hour, (())(24:29) 4.6 kilo meter, so per kilo meter and consuming 94 watt hour per kilo meter. So, if I am taking a ideal vehicle and just overcome drag and rolling resistance at 50 kilo meter per hour, I will consume 94 watt hour per kilo meter.
Now, is this the energy efficiency of the vehicle? No, this is assuming I am traveling constant speed at 50 kilo meter per hour, but what if I accelerate or decelerate? What if my velocity goes to 30 kilo meter and then 70 kilo meter? I will consume different watt hour per kilo meter. So, this is a watt per kilo meter for that drive, for a standard drive it will be something else and therefore I have to define standard drive.
I am giving you a assignment problem, where I change the drive, I have taken the same sedan, this time first I am accelerating from 0 to 50 kilo meter per hour in 20 second, now when I am accelerating at 0 to 50 kilo meter per hour in 20 second, I can actually compute the power requirement due to acceleration and that is coming close to 6000 watts for 20 seconds. Then it travels for 50 kilo meter per hour for 5 minutes, then it decelerates 20 second and as I compute the energy required assuming R equal to 1, R equal to 1; 100 percent regeneration, what will be the energy required? Can someone tell me?
Student: (())(26:32)
Professor: It is exactly the same 393 watt hour, why? Whatever energy I spent during climbing up during accelerating I am recovering that during deceleration, because R is 1, but in reality R is not 1, let us take R equal to 0.3, now I have to actually compute the energy required during reacceleration.
I have to add the acceleration force plus the drag force plus the rolling resistance force, compute the power required for all these three, I have to add the power, that is a power that I will be requiring for accelerating, figure out how much time will I require to get to 20 second (interval) in 20 seconds I am going to get it.
And I compute the energy and then I also compute the kilometre travel, well the previous kilometre travel what I had was assumed earlier the 4.16 kilometre was in the when I was traveling at 50 kilometre, but during acceleration also, I will be traveling some distance. So, I compute that and then divide it I will get watt hour per kilo meter.
I take a second problem again assignment problem this time I am traveling starting from 0 to 25 kilometre for 15 seconds, travel at 25 kilometre or 2 minutes and then I speed from 25 kilometre
per hour to 50 kilo meter per hour in another 15 seconds travel for 4 minutes at 50 kilometre per hour and then decelerate to 0 kilometre per hour in 20 seconds, compute the energy required. So, method is same, compute also the distance travelled, compute what is watt hour per kilometre, these problems are somewhat not very difficult problem that will get you used to the concept of a drive cycle, what is pointed out is very little correct, we have assumed what is assumed that only the acceleration force is reversed the drag and rolling resistance is not reversed.
So, ideally only for the acceleration force or the gradient force you should apply R, but you know acceleration force and gradient force are so much more than the rolling resistance and drag that all that is taken into account in R itself the regeneration efficiency and you do not separate them you just compute that and just resumed the regeneration efficiency if it was only for acceleration deceleration may have been 0.35 due to combine it is only 0.3. So, you just assume that and in reality in computation R is assumed for all; all the energy we are assuming that during climbing up and climbing down net energy consumed is 0. So, not exactly correct with R equal to 1, but if you want you can do this detailed calculation, but that gets into a little bit of trouble we just assume that is same.
For hour first course will assume it is same, it is a combined and we will not separate out, otherwise things will get complicated. (())(30:35) just like that, is the road always same? Is the rolling resistance always same? It is not, it will vary, we do not take that into account, so we have somewhat simplified, in reality you will get slightly worse result and you will say regeneration efficiency is lower, that is fine, okay.
Let me come to the concept of standard drive-cycle. A standard drive cycle is standardized and standard up by some body not normally motor vehicles authority in a country and it is standardized for different vehicles, two wheelers, three wheelers, four wheelers, e-rickshaw, autos, so that vehicle by two manufacturers can be compared that is a purpose and also saying that while you are not unnecessarily wasting petrol because it is important because if you are wasting petrol, it is actually converted into more and more emissions. So, just like those emissions testing, etc. is done, the drive-cycle tests are always done. And of course, the slogan (())(1:15) has made this extremely important because I will buy a Maruti because it gives me higher mileage. So, each vehicle have its own drive-cycle. In fact different cities can have different drive-cycle. Why? Because depending on the condition, in the city, the drives standard drive is different. It tries to typically picture a standard drive. But very often, so there is a daily drive-cycle.
But very often in cities use the same kind of drive-cycle, in a country, countryside on a, if you are mostly driving countryside drive-cycle is different. Usually climbing the slope, or climbing down is not standardized as a drive-cycle.
But if you are actually driving a vehicle in a place like , which has huge slope going up and down most of the time, it does not make sense to have a daily drive-cycle which is on flat road, you have to define a drive-cycle for which we will have to take into account the slope up and slow down. So, it is up, one can define, in fact in the things that I am going to give you. While I will mostly talk about flat road, I am going to give you some examples of some assignments where I will say let us have a slope up and down what is the energy consumed per kilometer. Different countries have different drive-cycles or different continents for example, in Europe vehicles drive at 150 kilometer per hour. In India, they rare they do not drive more than 90 kilometer per hour. So, Europe will have a different drive-cycle, we will have a different prep cycle, US will have different, in fact within US also states have different drive-cycles, because they have different kilometer per hour limits. It will take into account also the average roads, what are your speed limits; they are not supposed to drive at higher speed than speed limit.
Normally drive-cycle is never defined for 100 kilometers, is defined for smaller distances, 2 kilometers, 2.5 kilometers small time and then you keep on repeating that cycle you take because you do not want to take measurements on one cycle because maybe some slight extra energy was used or less energy is used. So, you repeat that drive-cycle 10, 20 times and then take the measurement. So, measurement is taken over multiple cycles.
With this let me come to the 1st definition of a drive-cycle. This is called India Drive Cycle for 2-wheeler, it is called IDC. It is a drive-cycle defined for 2-wheeler and it is very commonly used. And remember that this drive-cycles have been defined for petrol engine for electric vehicle same thing will be used. But certain things you will see there is practically no reason. It assumes that after you start, your idling for 15 seconds. This idling, why are you defining idling? Idling means you are not, you are just waiting, zero speed. In electric vehicle, during zero speed we will consume zero energy. By the way auxiliary things like lights etc. are never used in standard drive-cycle measurements, that is extra. Electric vehicle at zero speed will consumes zero energy. In a petrol engine, engine is turned on, kept on and you are resuming, consuming certain amount of energy. So, this is a part of a drive-cycle idling. Of course, today, vehicles are designed more and more to consume less and less during idling, even petrol engine, they are designed to even turn off and then have a electronic turning on, those are things there.
But anyway, we will not consider that we are going to talk mostly about electric vehicle. We will assume that (())(6:05) has 1st 18 seconds is 0 speed and let me go through this. So, if you see 16 seconds, 1st 16 seconds it is idling, then if you see you are accelerating from time 16 seconds, to 22 seconds, 6 seconds you are accelerating and your acceleration is 0.65 meter per second square, it should have been, I made a mistake here, meter square per second I have taken, so meters per second square, please correct this meters per second square, then you are actually after 22 seconds, I have to say after that you are actually decelerating. Well, here itself it is broken into two. It is assume 16 to 22 second it is at a certain speed 0.65, then your acceleration slightly decreases, does not show very well in the curve. Why does not show? Because it is not a very fine curve, but this is the important thing acceleration is 0.65, acceleration goes smaller. Then for the next 4 second, it is actually speed is going down then there is a steady speed for a short period of time, constant speed neither 0 acceleration for almost 2 seconds, then you are again accelerating this part, but you are accelerating at a certain speed, then you are accelerating faster or slower, from point 0.56 you go to 0.44.
Then, you are again decelerating for 3 second, then you are running at constant speed for 4 seconds, then you are decelerating, then you are decelerating for two seconds, then you are again running at constant speed, then you are again decelerating. So, this is how the whole thing is defined for 108 seconds and after that for 12 seconds again you are idling and up to 108 seconds and then you just keep repeating keep repeating.
So, total is defined for 108 seconds and then you actually have to drive it 6 times with the same pattern. Total test time is therefore 6 into 108, 648 second, total distance if you travel you just integrate this kilometer per hour, find out the distance traveled, will come out to be 3.9484 kilometer and maximum speed is 42 kilometer per hour. This is the drive-cycle, it is actually idling for what, 15 percent of time, 16 seconds, steady speed at for 12 percent, acceleration for 42 seconds, deceleration for 37 seconds, this is average speed is 21.93 kilometer per hour. This is a standard drive-cycle. Now, you drive the vehicle either electric vehicle or a petrol vehicle exactly as per this cycle 6 times and you compute. So, earlier actually there used to be a vehicle track in which you had to drive to do the measurement. Now, there are instruments where the vehicle is kind of made to drive.
There is a track on which it is made to drive, it is actually not moving, and there are instruments which will capture all the data. What are these instruments called? Dynamometer, so they are these dynamometer, vehicle dynamos. So, what do you do? What is the mechanism that you use to? Now, given this, I want to 1st compute, I know my forces, I know my power for every speed, for every acceleration, I know my drag, I know my rolling resistance, I know my acceleration force, I know my climbing force, in this case, of course there is no climbing, what do I do? Actually, this can be nicely computed on a spreadsheet. And I will, in fact give you a assignment problem to computed on a spreadsheet, you actually can take every second or every half a second, so 0 to 108 second, if you do it 108, 1 second each, so you take 108 intervals of delta T of 1 second, you calculate the average velocity during that.
Or it may be actually increasing, if you want to not take that into account, you take 0.5 seconds, number of points will go up number of lines in a number of rows in a excel sheet will go up, but typically 1 second with average velocity gives you very good result. So, find the average velocity. And the distance travel in that you can compute as velocity into delta T, velocity is given by the drive-cycle, average velocity for that 1 second, you can compute the delta T, you start writing down every second, what the velocity should be, take V final minus V initial divide by 2, that is average velocity. Acceleration is what? You have calculated the delta velocity with every second, it divided by delta T, it will give you acceleration meter per second square. Every second now, you compute the force, acceleration force is more mass into acceleration, mass of the vehicle is known acceleration.
So, you have all the 120 value of the acceleration every second, you are doing that, you now compute the rolling resistance, you know your mass, you know that g, you know the value of mu, find out the rolling resistance, you can assume mu to be constant. Also, compute the drag force, 0.5, Cd, rho, A is given, v is changing every second it is changing, average velocity, take average velocity and compute al of that. You go on, now since you have computed the acceleration force, rolling resistance, drag, this you compute what is called Total traction force. You also compute the traction torque, you know the force multiplied by field meter. So, every second what is the torque that are required. So, you have data for every second for the traction force, you have for traction torque.
You have the power consumed; it is a force multiplied by velocity. So, you already have got the total traction force, you take the average velocity multiply that you get every second, what is a power consumed, and if there is a deceleration and you want to take that into account, take the value of R, if R is negative, then you take R into F track