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Old 09-24-2013, 06:30 AM   #1
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Default P1 Drive Train / Motor Tech

Hi all,

As most of you know I asked a bunch of questions to McLaren higher-ups and techs because I felt there was confusion about the drive train tech used in the P1.

To start this thread, I think it is best for me to get a few basic physics explanations out there so words or things I use in the actual post will not be misunderstood or maybe understood better than before by some. Very low level... hope thats ok, not saying you guys are all physic-idiots, just better to establish a level playing field

Watt is a "unit of power". Short form: W
kW = kilo Watts, ie 1 kW = 1000 Watts
One Watt is the rate at which work is done when an OBJECT is kept at the velocity of 1 meter per second against the opposing force of 1 Newton.
Maybe a bit better example (which removes weird physics things like Newton that maybe not everyone understands): A person weighing 100kg, climbing a 3 meter high ladder in 5 seconds time does 600 Watts of work. {(kg * m^2) / s^3 - if anyone cares...}
1 HP is the old "unit of power" which was originally defined as the amount of work one horse could do pulling the wheel in a mill.
1 HP = 0.746 kW or 1 kW = 1.34 HP.

HP in a motor is calculated from its Torque and RPM. HP = Torque * RPM / 5252. So if an engine delivers 200 Nm of Torque and 2000 RPM, it delivers 76 HP at that RPM.

1 Watt is also 1 VA. Volt Ampere when looking at electrical units. So an electric motor converts VA into power and thus torque at a certain RPM of that motor. It can also be used as a generator by applying power to the shaft from outside which generates VA or Watt.

A BATTERY stores power. The power stored is measured in Wh. Meaning how many Watts it can produce over one hour period. So if you want to know how long it can deliver a certain amount of Watts, you divide the Wh by the Watts asked for. A battery has a certain max power output based on the Volts and Ampere it can deliver.

An electric motor converting electric power into kinetic power will always have a loss of conversion. So even if a battery can deliver 100kW and the motor can take it without issues (structural/heat issues in the end only) the power on the shaft will not be 100kW but a little less than that. How much depends on how well the circuits can convert the energy. How much heat is produced in the wiring and so on...

In my post below, note that when I talk about the ENGINE I will be talking about the PETROL ENGINE. When I talk about a MOTOR I will be talking about the ELECTRIC MOTOR.


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Last edited by ftyl; 09-24-2013 at 07:03 AM.
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Old 09-24-2013, 07:03 AM   #2
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As we all know, the motor (electric motor) in the P1 delivers around 176HP and 130Nm. Its 176HP converts to 132kW (131.2 converted exactly, but it produces a bit more than 176HP; the 132kW is the more accurate number).

The engine (petrol engine) delivers 727HP at 7500 rpm, and its top torque is 720Nm at 4000 rpm.

Now to the important stuff...

The electric energy needed for the motor to deliver 132kW is roughly 140kW.
The BATTERY stores 3.4kWh of energy.

Therefor, pretending you would find a long enough stretch the electric motor could be powered at full 132kW (needing 140kW electric) for 87 seconds. (3.4kWh / 140kW = 0.024h = 87s)

Within those 87s clearly the P1 can reach vMax, so I asked what will happen at that point with the drive train. The P1 does not need its full engine and motor torque to sustain vMax once reached, it needs to to get there faster. The ECU will therefor reduce torque of the motor to 0. It needs to increase the torque of the engine to compensate a little, which it can. vMax is maintained!

The moment the driver lifts its foot off the throttle, the ECU will IMMEDIATELY start charging the battery.

To recharge the battery, the P1 uses the motor as a generator, by applying negative torque to it through its set of 3 clutches between the motor, engine and gearbox. That means it turns the motor backwards generating energy. As a generator the engine, depending on the RPM applied, can regenerate as little as a couple kW all the way up to roughly 120kW. Which would mean, at max kW it would take 3.4kWh / 120kW = 102s to fully recharge the battery.

The biggest questions came from the comments regarding use and regen of the battery around the Ring, and the comment in their press kit stating that it does NOT use regenerative braking.

So to settle this once and for all, I have confirmation from McLaren on the following (btw, this is all exactly as I explained it before):

1) The battery supplies full power throughout the whole lap of the Ring! It never runs out of juice!
2) During one lap the P1 is able to put almost as much charge back into the battery as they take out of it.

It regenerates in _two_ ways:
1) The ECU applies negative torque to the motor, unless the battery is full, when you are off throttle. Some people call this "regenerative braking" but it is NOT! Because it does not depend on BRAKING. It simply depends on NO THROTTLE. To the driver, this simply feels like more engine braking than just the petrol engine would bring. The power taken depends on the motor speed, the gear and the power train mode the P1 is in. I did not ask for details but I guess the negative torque applied changes depending on power train mode, thus together with speed and gear it generates more or less power.
2) If you drive steady speed, the ECU will also, to regen, turn off the motor, then increase the engine torque and subtract that torque via negative torque applied to the motor, which will in turn generate energy stored in the battery. While driving the car you will notice this since the motor will rev higher without you asking for more power or the car speeding up.

The P1 DOES NOT however use REGENERATIVE BRAKING. What this means is, the negative torque applied to the motor does NOT CHANGE when you brake in order to keep brake-feel 100% based on mechanical braking instead of a mix of the two! They DID consider increasing negative torque when braking, but were concerned with brake feel so stepped away from the idea.

For an idea of how much energy the battery can truly regen via kinetic energy recuperation, in the vMax example above, when you lift off your foot from the throttle, the motor will generate about 100kW. It will not generate more in order not to do too much engine braking. It could do more and it will in certain circumstances, but the ECU would decide on roughly 100kW in that circumstance.


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Old 09-24-2013, 07:31 AM   #3
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Thanks very much Fabian.... Great explanation..clears up a number of points that were not well understood.

903 for the whole lap when needed. Amazing.

Clearly the acid test is going to be who does the fastest consecutive second lap....

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Old 09-24-2013, 07:36 AM   #4
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Originally Posted by Mikeyb View Post
Thanks very much Fabian.... Great explanation..clears up a number of points that were not well understood.

903 for the whole lap when needed. Amazing.
More importantly, also for the 2nd, 3rd, 4th, 5th, 6th lap...

I wonder however how fast the petrol engine runs out of fuel..

But clearly faster than the battery runs out of power with that little loss per lap...


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Old 09-24-2013, 07:58 AM   #5
 
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wonder how many laps you actually could potentially do with the full 916ps available?
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Old 09-24-2013, 08:03 AM   #6
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It's interesting equally that the recharge time from flat they day is 10 mins after e mode one assumes that this is assuming significantly less torque delivered in town driving,..,

Anyway a true hybrid. Not another electrical boost system..

I wonder how much quicker it will be down dottinger hohe at vmax

It's amazing when you think it doesn't need full power for vmax chat but seeing as we know this is load limited with an electronic top speed not intuitively surprising.. Shows you how pointless top speed figures really are from a useable basis..
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Old 09-24-2013, 08:04 AM   #7
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wonder how many laps you actually could potentially do with the full 916ps available?
I would guess 4 or 5

Before the petrol ran out!
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Old 09-24-2013, 08:31 AM   #8
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What shocks me is that I actually understand what you have written Fabian,thanks for the concise explanation in laymans terms.This clears things up and needed an engineers brain to put it across,rather than some of the guff that has been banded around,thanks again !!


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Old 09-24-2013, 08:32 AM   #9
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I would guess 4 or 5

Before the petrol ran out!
Probably similar to the 12c don't you think ?


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Old 09-24-2013, 08:36 AM   #10
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Yes although dont have a clue how much the torque overrun will consume
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Old 09-24-2013, 08:47 AM   #11
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Thanks mate, very intriguing and fascinating information delivered in a concise and direct-to-read manner. Good job
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Old 09-24-2013, 09:48 AM   #12
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Interesting.. however... The 918 was clearly shown to have run out of electric power per lower top speed down dottinger hohe compared to some earlier straights. And the 918 has considerably higher battery capacity despite a greater electric power output and I think the maths ends up to being full battery power for 115s or something like that (I'm a bit lazy to find the exact figures sorry...) which is longer than the 87 seconds you have for the P1. Given the 918 has the capability to charge the battery through braking as well, it would seem strange the P1 will be able to maintain it's electric capability throughout the lap compared to the 918?

Unless the torque differential between the turbo v8 and the NA engine of the 918 is such that it can charge the battery that much more off throttle.....or the 918 is able to use far more of the full electric power available being awd etc so depletes it's batteries to a far greater extent and is not able to recharge it.
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Old 09-24-2013, 12:55 PM   #13
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Quote:
Originally Posted by Mikeyb View Post
It's interesting equally that the recharge time from flat they day is 10 mins after e mode one assumes that this is assuming significantly less torque delivered in town driving,..,

Anyway a true hybrid. Not another electrical boost system..

I wonder how much quicker it will be down dottinger hohe at vmax

It's amazing when you think it doesn't need full power for vmax chat but seeing as we know this is load limited with an electronic top speed not intuitively surprising.. Shows you how pointless top speed figures really are from a useable basis..
If you think about it, 10min is a lot to recharge actually... At 100kW the negative torque needed is not a lot, it takes only 102 seconds to recharge. Meaning for 10 minutes, all it needs is about 21kW. 21kW is only 28HP. So all it needs to take is 28HP from the engine to recharge the battery in 10 minutes... Thats not really a problem to do. Especially in the city.


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Old 09-24-2013, 01:00 PM   #14
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Interesting.. however... The 918 was clearly shown to have run out of electric power per lower top speed down dottinger hohe compared to some earlier straights. And the 918 has considerably higher battery capacity despite a greater electric power output and I think the maths ends up to being full battery power for 115s or something like that (I'm a bit lazy to find the exact figures sorry...) which is longer than the 87 seconds you have for the P1. Given the 918 has the capability to charge the battery through braking as well, it would seem strange the P1 will be able to maintain it's electric capability throughout the lap compared to the 918?

Unless the torque differential between the turbo v8 and the NA engine of the 918 is such that it can charge the battery that much more off throttle.....or the 918 is able to use far more of the full electric power available being awd etc so depletes it's batteries to a far greater extent and is not able to recharge it.
The extra negative torque added through increased negative torque while braking is very little to keep your brake feel! I think the biggest problem is that it needs much more HP from the battery than the P1. And on top of that, above 260kph or something like that the front motor is DISCONNECTED. Its RPM would be too high, so also as a generator it can not be used in high speeds. So although in the most important speeds, ie the lower end below 200kph you need both motors, only one is effective at regenerating at all speeds. This loses regen power from engine/motor braking and the regen brakes does not recoup that.

So the 918's battery is bigger, but is used more and regens less than the P1. P1's regen is much closer to its usage which helps it not to lose too much power during any normal circuit's lap.


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Old 09-24-2013, 01:37 PM   #15
 
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Thank you very much Ftyl for that knowledge. Can't wait to see what this equals to in lap times around the ring.
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