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What do we need to do to make the car as fast as possible?

Greenpower schools education event, Castle Bromwich

1st May 2013

Dan Lloyd & Robin Boyd

Introduction

- Race Strategy is made up of our Powertrain and Electrical teams
- Our aim is to make the car go as fast and as far as possible during a race
- What can affect the car’s performance during a race?
- Batteries
- Gearing
- Aerodynamics
- Driver
- Other cars
- Pit stop?
- How do we know what needs improving?
- The experimental method

Data… Lots of data

- Before a Race:
- Test the batteries
- Look at data from other races
- Use simulation to help select gear ratio

- During Practice:
- Check whether gear ratio is correct
- Make sure everything on the car works properly
- During the Race
- Monitor car data
- Decide when to pit

- Simulation & Strategy

- Analysis

- Practice & Race

- Afterwards
- Use data to improve simulation, design or processes (such as pit stops)

Today’s Activities

Simulation & Strategy:

Driven vehicle simulation – Vicente

Analysis:

Motor dyno and losses – Dan

Understanding Batteries – Robin

- Simulation& Strategy

- Analysis

- Practice & Race

Race Strategy – Simulation & Strategy

Working out how fast and far we can go before we get there

Simulation & Strategy

- Target of the Race:
- Do as many laps as possible in 90 minutes…
- … using only 2 sets of batteries.

Go faster!!

Go slower!!

Simulation & Strategy

Gear Ratio

Simulation & Strategy

Gear Ratio

Simulation & Strategy

Gear Ratio

Simulation & Strategy

Gear Ratio

Simulation & Strategy

↑↑ Gear Ratio

-- Speed

++ Force

↓↓ Gear Ratio

++ Speed

-- Force

Which Gear Ratio?

?

- Every car is different:
- Different aerodynamics
- Different wheels
- Different losses

Test and simulation will help you decide

Simulation & Strategy

Motor calculations

Battery model

Vehicle losses

Track Data

Aerodynamic equations

Race Strategy – Motor and Losses

A quick guide on how to prevent your motor from melting

Motor Fields and Forces

- How does the Greenpower motor work?

Example of a 2 pole motor

Example of a (broken) Fracmo motor

Motor Fields and Forces

- There are 2 important fields at work:
- Electric field from windings
- Magnetic field from the magnets
- A current flowing through a wire in a magnetic field creates a force which makes the shaft spin.

BUT moving a charged wire through a magnetic field induces a voltage that opposes the supply.

These reach an equilibrium and a maximum speed is reached

Motor Power and Efficiency

- How do we calculate how much power the motor uses?

Power = Current x Voltage

(Watts) (Amps) (Volts)

For example:

For a 24V motor running at 20A the power used is 480W

This is higher than the motors rated power!

We can do the same to measure power out from the generator and calculate the efficiency.

Now it’s your turn!

Where’d the power go?

So the power in was more than the power out…

The energy must have gone somewhere

- You can’t create or destroy energy, only change it from one form to another.

Motor Losses

- Noise
- Heat
- From friction in bearings
- Electrical resistance in windings

- What about other losses on the car?
- Friction in the chain
- Friction in axle bearings
- Aero resistance
- Tyre friction (rolling resistance)
- Voltage drop across wiring

Motor Losses

All these losses mean that the motor draws more current to run at the same speed…

And more current means more heat!

How do we reduce current?

- Reduce losses and external loads
- Change gear ratio

Learning our lessons

- What do we do with this information?
- Try to minimise vehicle losses
- Use better bearings
- Keep chain clean and oiled
- Improve aerodynamics
- Make sure tyres are inflated properly

- Keep motor cool
- Don’t work the motor too hard
- Feedback results into the simulation

Q&A

Any Questions?

Squeezing the most from your cells

Source of Energy

- 32Ah - Amp Hours (what does this mean?)
- Can produce 32 Amps for 1 hour, at 12 Volts
- 384 Watts, for one hour (P = I x V)
- 192 Watts, for 2 hours…

How do they work?

- Chemical Reaction.

http://members.kos.net/sdgagnon/te33.jpg

http://hilaroad.com/camp/projects/lemon/1lemon_meter.jpg

How to get more?

- Change things about the battery
- Nature of the reactants
- Physical state
- Concentration
- Temperature
- Catalysts
- Pressure
- (source: Wikipedia)

How to get more?

- Temperature
- BUT
- Limited to 25 degrees by regulations…

How to get more?

- So we did some tests.
- Remember… 384 Wh
- Actual number measured? ~300Wh

Where has the power gone?

- Peukert'slaw
- The faster you discharge a Lead Acid battery,
- the less usable capacity you get

(http://en.wikipedia.org/wiki/Peukert's_law)

Can they Recover?

- Removing the load can cause the battery voltage to recover
- This doesn’t give you more energy, but it does change how the battery provides that energy
- Doesn’t last long…

What do we do with this?

- Combine the information with:
- Motor Info (Dyno)
- Track Info (Simulation)
- Car Info (Aero, drag)
- Race info (Competitors)
- And then use this to tune our choices and decisions

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