# Regen braking for Formula Electric race car



## Vee25 (Mar 17, 2012)

Hello

I'm a mechanical engineering student working on an electric vehicle design project in a group. Since September, the 4 of us (mech eng) + 3 electrical engineering students have been working at getting the bulk of the vehicle design finished, so that our school can build a Formula Electric race car and compete in the upcoming FSAE Electric competition. One of the events is an endurance race, and our energy efficiency plays a large factor in scoring.

On the project, I took on the feasibility study/design of the regenerative braking system. I know that racing doesn't provide the best conditions for regen braking, but:
1) We need to get as much energy efficiency as we can
2) Implementing regen would not cost us much more (other than time and brain power over a control/balancing strategy and algorithm)
3) Reducing our battery's energy requirement will reduce vehicle weight, leading to additional energy savings, and so on.
4) We've heard that some formula electric student teams in Europe smooth out their driving a bit, to get more regen 

My team members have been on the FSAE race team of our school for a while. We build an IC formula style race car every year and compete against other engineering student teams in Michigan. Luckily, we have race data from the IC car that I could analyse, getting an estimate of regen energy that we can expect to get. (We are designing the race EV to be very similar to the IC car). Using rear brake pressures, I calculated the rear braking energy and power of an endurance race. Since we're spec'ing a battery for 6.5kWh, we expect the max. charge power to be around 6.5kW. So when I added up the rear braking energy generated at 6.5kW and under, I got 0.1kW. This may not seem like much, but it's 9% of the total braking energy of the race, and would save us the energy for approx. 90% of one lap. I haven't accounted for losses as the braking energy is converted and stored, that's just a bit outside the scope of what I have time for right now.

One of the electrical students on my team said that if we can capture over 5% of the total braking energy, then regen is worth it. I'd like to get some additional opinions on this. I know adding ultracaps could help us store more regen, but I'm not sure if the added weight and cost is worth it. If there's anyone out there working on a student team for the Formula Electric competition, could you just give me a hint as to whether regen is worth it? I've done all this analysis on how much regen we can hope to get, but in the end, I don't have enough experience to tell if the expected regen amount is worth the trouble (and danger?) of programming a control algorithm for regen. 

I have a few concepts for the mechanical braking system, but my most promising one is using a relief valve and proportioning valve for regen/hydraulic transition.

This is my first time posting in the forums. I hope that I've given you enough information to show that I'm trying my best to learn this stuff. I'm not just here to get someone to tell me how to do everything! ;-)


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## WarpedOne (Jun 26, 2009)

I have a feeling you are thinking way to much about regen.

You are building electric race car. If you are using DC drive then forget about regen.
If on the other hand you are going AC route, regen capability will already be there. 
You only have to decide which pedal will activate it and how powerful it will be.

Also forget about ultracaps. They are only a distraction and bring no real value to the table. Regen power is not limited by battery charge current but vehicle traction and stability.

As of regen efficiency -.before any thorough calculations are done you can operate with 50% i.e. regen could in priniple capture up to about half of kinetic energy.


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## Duncan (Dec 8, 2008)

Hi Vee_

Since we're spec'ing a battery for 6.5kWh, we expect the max. charge power to be around 6.5kW. So when I added up the rear braking energy generated at 6.5kW and under, I got 0.1kW.

_You guys are engineering students - you NEED to get your units right_,
_Kw, KWh, 

Remember KWh is a "bastard" unit - I know we all use it but the correct unit is Joules

Are you using a 6.5Kw motor? - sounds a bit puny


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## Vee25 (Mar 17, 2012)

Duncan said:


> Hi Vee_
> 
> Since we're spec'ing a battery for 6.5kWh, we expect the max. charge power to be around 6.5kW. So when I added up the rear braking energy generated at 6.5kW and under, I got 0.1kW.
> 
> ...


Yes I realize that it's 0.1kWh energy...you'll have to forgive me on that because I wrote this at 3am last night after working on the project report for the whole day. (I've been writing it correctly in my report.)

We're using two 30kW motors I believe. We spec'd 6.5kWh for the battery because from our analysis, that's what we need to complete the endurance race. There are endurance race results from the European teams last year, and they used about 2.6 - 5.9kWh, so 6.5kWh sounds reasonable to us.

We know we need to use a DC-DC converter, but as mech students, we're confused about how to use it, so we're "Recommending" in our report that our electrical student team needs to help with that next year. I'd appreciate any advice you have to give on that though. Thanks!


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## Duncan (Dec 8, 2008)

Ok so you have two 30Kw motors

The maximum driving power is 60Kw - the maximum re-gen power is also 60Kw

Just as you can pull power for a few seconds you can also dump that into the batteries

You will also need friction brakes - calculate how much power is being turned into heat on a max speed braking maneuver

Simulate a lap - remember in a track car you are always either accelerating or braking

Example
500Kg car - accelerating to 30m/sec (~100KPH) = 225Kjoules = 62.5 Whrs
You would do this ~ 5 times/lap for 10 laps = 3.125Kwh

- your 6.5Kwh sounds reasonable

So there would be 3Kwh available to be "harvested"

Simulate the lap - think of speeds - braking - acceleration - remember that you will "scrub off" a lot of speed in the corners


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## mizlplix (May 1, 2011)

Speaking as a lifelong racecar mechanic:

I was involved in a electric open wheel series here in the S/W USA, back in the early 1990's. We did considerable testing at P.I.R. (Phoenix). 

To get the fastest lap times on any racecar, you need to make each tire perform 25% of the total work. To that end, basic car design, chassis set up and driving style are most important. 

Cornering is quite tricky because deceleration, weight transfer and (again) driving style all affect car handling and exit speeds. 

Our 25% rule is easily upset by unintended events. (ones not predicted or compensated for by the driver). 

Drivers trained and experienced with conventional ICE powered racecars have a difficult time with "throttle off" regen braking. The effect is that the racecar goes instantly loose (oversteer).

Normal braking is a balanced affair among all four wheels and thus predictable.

Regen braking just affects the rear wheels upsetting the 25% balance rule.

Driver reaction was so ingrained that retraining them to get "used" to throttle-off regen was a dead end. We changed the mode to first stage hydraulic brake regen. (which was better, but under panic braking, still upset the car causing a loss of control.

Finally, we set up a paddle lever under the right side of the steering wheel to control our regen. This saw some success, as the drivers adapted/retrained to this smoothly. 

Unfortunately the series died an early death. Not enough cars to complete a whole venue was the culprit.

Regards, Miz

P.S>: The car should roll through the corners free. Corners are divided into three distinct sections: Entry-Transition-Exit. You are decelerating continuously down to the point of rotation where the car pivots and corner exit begins (the actual beginning of the straightaway) where you begin picking up the throttle. Any "scrubbing" is a bad thing...and a mistake. (maybe "lose speed" was meant, not actually scrub)


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## major (Apr 4, 2008)

mizlplix said:


> Speaking as a lifelong racecar mechanic:
> 
> I was involved in a electric open wheel series here in the S/W USA, back in the early 1990's. We did considerable testing at P.I.R. (Phoenix).
> 
> ...


Good points Miz.

Our experience was similar. Throttle off regen sucks for the race situation. We never used it and saw several who did try it end up on the hook coming back to the pits. 

I don't know about the paddle. We used the brake pedal, where one expects to find the brake control. The first portion of pedal travel gave proportional regen. Then further depression mixed in the hydraulic friction brakes which were heavily biased to the fronts. Inverter drive was torque control so the mix of electric and friction brake torque was seamless. It was completely intuitive for the driver. The harder the brake pedal was depressed, the more braking torque applied.

There was obviously a different mix of wheel (or axle) torques compared to the ICE racecars. But at least for our driver it was never a problem. It was much, much preferred over driving the electric without regen. Even top notch race brakes without power assist on a 3000 lb racecar suck.

I like your 25% rule. But you never do actually achieve that, do you  On acceleration, you power just the rear. And on braking and cornering, the dynamics always put different loads at each corner. 

Set-up was never my job on the crew but I did see what those guys struggled with. 

Regards,

major


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## Ziggythewiz (May 16, 2010)

Vee25 said:


> We know we need to use a DC-DC converter, but as mech students, we're confused about how to use it, so we're "Recommending" in our report that our electrical student team needs to help with that next year. I'd appreciate any advice you have to give on that though. Thanks!


What's confusing about a DC-DC? It takes your pack voltage and converts it to AUX voltage to run your contactor, headlights, wipers, radio, AC, possibly brake pump and cooling fans. How many of those do you have in your race car?

All you need to do is find the peak power used in your AUX system and size the DC-DC accordingly.

You might also consider not using one, as it will add a bit of extra weight and inefficiency.


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## Arlo (Dec 27, 2009)

vee25 do you have any video or pics or a build thread?


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## ishiwgao (May 5, 2011)

Vee25 said:


> We know we need to use a DC-DC converter, but as mech students, we're confused about how to use it, so we're "Recommending" in our report that our electrical student team needs to help with that next year. I'd appreciate any advice you have to give on that though. Thanks!





Ziggythewiz said:


> What's confusing about a DC-DC? It takes your pack voltage and converts it to AUX voltage to run your contactor, headlights, wipers, radio, AC, possibly brake pump and cooling fans. How many of those do you have in your race car?


I agree with Ziggy. What problems are u facing with the DC-DC? Wiring it and using it couldn't be any simpler than 4 wires, 2 for pack voltage 2 for aux voltage?


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## Vee25 (Mar 17, 2012)

I guess we just haven't had the time to sit down and learn about DC-DC converters - how they work, how to use them, how to install them, etc. We've been focusing on more of the mechanical aspects of the project, such as chassis and drivetrain. Since we have an "electrical side" to the team, we've been relying on our electrical engineering friends to take care of most of the electrical systems. It's been a real challenge working on this multi-disciplinary project because some of our mechanical designs depend on the designs for the electrical system. 

On the mechanical side, we started working on the battery mount and case that protects the driver from the battery accelerating forward. But to do that, we needed to know the battery size and weight first, so we started analysis on traction energy and power required during the endurance race to spec the battery. We have a peak power requirement of 30kW and an energy requirement of 6.5kWh. Trying to spec the battery, we realized that too many cells (too much volume & weight) would be needed to satisfy these requirements. Then our electrical team told us we need a dc-dc converter so that we can use a lower amount of battery cells and still meet our requirements. This is when we started to look into what to do with the dc-dc converter, but with our last term of undergrad wrapping up, working on the FSAE IC vehicle, and our final report due, we just haven't been able to put in the time to figure out the dc-dc converter.


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## Vee25 (Mar 17, 2012)

No we don't have videos or pictures at this point. We're still in the design phase. Our goal is to design a formula style race car to enter a competition like this one: http://www.formulastudent.de/fse/ This might give you a better idea of what we're designing.


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## Vee25 (Mar 17, 2012)

Duncan said:


> Ok so you have two 30Kw motors
> 
> The maximum driving power is 60Kw - the maximum re-gen power is also 60Kw
> 
> ...


Yes, we've used endurance race data from the IC car to simulate traction and braking energy. At first I was going to just estimate a percentage of the total braking energy to be recoverable - to account for only being able to recover braking energy from the rear (driven axle), but then I realized that I also had rear brake pressure data, so I calculated braking energy off of that instead. (using total piston area, etc.) Since I was able to determine the rear brake energy directly, I think my results for rear braking powers seen during the race are also more reliable that just estimating a percentage off of the total brake energy.

Yes, we plan on using friction brakes in a hybrid brake system with regen.

When you say that we could dump 60kW energy into the battery for a few seconds, do you have a reference that can verify that? It'd be great if we could do that, but in my discussions with the electrical engineering students, they have said repeatedly that we'd only be able to charge at 6.5kW max. From what I've read about batteries, aren't they limited by a max. charge rate of 1C or 2C? And multiplying the max charge rate through with the voltage and current that can be run through the battery will give us the max. charge power. I thought that the max. charge power is limited by the max. charge rate, no matter what?


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## Vee25 (Mar 17, 2012)

mizlplix said:


> Speaking as a lifelong racecar mechanic:
> 
> Drivers trained and experienced with conventional ICE powered racecars have a difficult time with "throttle off" regen braking. The effect is that the racecar goes instantly loose (oversteer).
> 
> ...



One of our design decisions is whether to allow regen when the throttle is off, or to get the motor controller to replicate coasting by decelerating at a constant rate. This way, we might avoid the over-steering problem when throttle is off? The driver would avoid coasting of course.

Is there potential for better vehicle performance if regen is allowed when throttle is off, provided that the driver learns how to use the different braking system?


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## Vee25 (Mar 17, 2012)

major said:


> Good points Miz.
> 
> I don't know about the paddle. We used the brake pedal, where one expects to find the brake control. The first portion of pedal travel gave proportional regen. Then further depression mixed in the hydraulic friction brakes which were heavily biased to the fronts. Inverter drive was torque control so the mix of electric and friction brake torque was seamless. It was completely intuitive for the driver. The harder the brake pedal was depressed, the more braking torque applied.


I think I'm looking to use a similar control strategy:
low vehicle velocity: pure mech
low deceleration: pure regen
med deceleration: mech + regen
high: pure mech

This would just be for the rear brakes, and the front brakes would be completely hydraulic all the time. I think that the mechanical system and regen control will have to be designed so that appropriate brake bias occurs. Do you have any advice on my proposed control strategy?


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## Duncan (Dec 8, 2008)

_but then I realized that I also had rear brake pressure data, so I calculated braking energy off of that instead. (using total piston area, etc.)_

I would be interested in that calculation - please show us what you have done

Charging your battery
Most batteries are charged at 1C - but its a symmetrical reaction if you can draw 10C then you can charge at 10C (just not for very long) 

Some cells can be charged/discharged at 20C - some only 3C (I assume you are not using the 3C ones for a track car)

You seem to be confused about the DC-DC - this is used to charge your 12v battery from your main pack - you may not even need one - just charge your auxiliary battery befor each run

What you do need is a motor controller - this is determined by the type of motor you are using
AC, Series DC, ....

Series DC motors are the most common in DIY EV's 
Their controllers act as Power in - power out devices - 150v x 20amps battery - 15v x 200amps motor

DC series motors can take massive overloads - for short time periods - so a motor rated at 10Kw (1 hour) - could produce 200Kw - for 20 seconds


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## major (Apr 4, 2008)

Vee25 said:


> I think I'm looking to use a similar control strategy:
> low vehicle velocity: pure mech
> low deceleration: pure regen
> med deceleration: mech + regen
> ...


Drop it and do as I said. Always use the regen first and get into the friction only if/when faster deceleration is needed. Use this regardless of vehicle speed.

We always included an adjustment pot on the dash for the driver to adjust the severity of regen and also to turn regen off in case a wet track or conditions warranted it. The driver also had the standard front/rear bias control valve for the hydraulic brakes.


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## Ziggythewiz (May 16, 2010)

Vee25 said:


> Trying to spec the battery, we realized that too many cells (too much volume & weight) would be needed to satisfy these requirements. Then our electrical team told us we need a dc-dc converter so that we can use a lower amount of battery cells and still meet our requirements.


Sounds like your electrical team needs to go back to electrical class. No converter will change the amount of power you need to meet your requirements. A converter only converts one voltage (and the accompanying AH capacity) into another voltage with a different capacity.

Your controller already acts as a converter, adding another is not going to help efficiency. There are plenty of battery options out there to get whatever reasonable configuration you need.


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## mizlplix (May 1, 2011)

In oval track racing, having the regen controlled by the brake pedal works well. Especially when there is a trim pot to set the percentage of regen over friction braking.

Oval racing is done in mostly good conditions, whereas road courses are ran in all weather.

It is incredibly difficult for a road course driver to "trail brake" with regen functioning off of the hydraulic pedal as this creates an artificially "loose" condition for the car. 

Trail braking is the act of carrying too much speed into the corner and using it to transfer extra weight to the front of the car to force greater tire compliance.
There can be as much as 80% of the car weight momentarily on the front suspension just prior to transition. With a fixed increase of rear brake over front brake, the rear tire contact adhesion would be over loaded and the result is as if the driver had dialed in too much rear brake, loss of car control.

Each corner is a unique situation and requires a unique set of operational parameters (car wise). That is one reason why we ended up with a paddle under the steering wheel. It provides the driver a means to dynamically adjust the car's handling, allowing them to match suspension dynamics to most all situations in real time.

Major is spot on when he sez to "Use regen first and most". Every time you use the pedal, you are wasting energy.

As an aside: Although we only had three different drivers, they all agreed, Heavy regen was good in early braking and tapered off until it needed to be cut just prior to lateral chassis weight transfer. While braking effort was somewhat different. It started out as a linear progression, holding on to the point just after car rotation at the apex and the throttle is picked up for corner exit.

Regards, Miz


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