Applications of flywheels (AQA A-Level Physics): Revision Notes
Applications of flywheels
Flywheel design principles
To achieve optimal performance, a flywheel should have the majority of its mass positioned as far as possible from its axis of rotation. This maximizes its rotational inertia, making it more effective at storing rotational energy.
In practical applications, a complete hoop shape is not feasible due to attachment difficulties. Therefore, flywheels are typically designed as a rim connected by spokes to a central axis. The spokes are arranged symmetrically to ensure stability during rotation. While most of the mass should ideally be concentrated at the rim, structural integrity and safety requirements must also be considered in the design.
The design of a flywheel involves a trade-off between maximizing rotational inertia (by placing mass far from the axis) and maintaining structural integrity. The spoke design provides the necessary support while keeping most mass concentrated at the rim where it is most effective.
Main uses of flywheels
In industrial and transport applications, flywheels serve three primary functions:
- Smoothing torque and speed variations in vehicles
- Recovering and re-using kinetic energy that would otherwise be wasted during braking
- Acting as an essential component in machinery for certain production processes
Smoothing torque and speed in vehicles
Understanding engine torque fluctuations
In vehicle engines, power generation is not continuous. Power is only produced during the power stroke or combustion phase of the engine cycle. This intermittent power production causes the engine to generate torque that fluctuates significantly over time.
Torque is the rotational force that causes the wheels to turn, propelling the vehicle forward. When torque is uneven, several problems arise:
Problems caused by uneven torque:
- The vehicle experiences jerky motion
- Unwanted vibrations occur
- Passenger comfort is reduced
- Energy is wasted
How flywheels smooth fluctuations
When a flywheel is added to the engine system, it speeds up and slows down gradually due to its inertia. This property allows the flywheel to absorb sharp increases in torque and release energy during decreases, effectively flattening or smoothing the fluctuations.
During one complete engine cycle, torque varies erratically with pronounced peaks. The flywheel smooths these changes, creating a more consistent torque output that results in more comfortable and efficient vehicle operation.
Think of the flywheel as a mechanical energy buffer. During the power stroke when torque is high, the flywheel absorbs excess energy by speeding up. During other phases when torque drops, it releases this stored energy by slowing down slightly, maintaining a more constant output to the wheels.
Multi-cylinder engines
To increase total power output and further control torque fluctuations, modern engines typically use four or more combustion cylinders. These cylinders are staggered in their operation, meaning they fire at different times. All cylinders are connected collectively to a large central flywheel.
This configuration provides several advantages:
- The flywheel effectively smooths or averages out the torques produced by the separate cylinders
- More cylinders result in smoother overall engine operation
- The combined effect produces much more consistent power delivery
However, increasing the number of cylinders has trade-offs:
- Higher manufacturing costs
- Greater weight
- Increased mechanical complexity
Kinetic energy recovery system (KERS)
The concept of regenerative braking
Regenerative braking refers to methods used to collect and store energy from a vehicle's braking motion for later re-use, rather than allowing the kinetic energy to be wasted by conversion to heat through conventional braking.
A kinetic energy recovery system (KERS) is any system designed to capture and store this braking energy for subsequent use. This technology improves overall vehicle efficiency by recycling energy that would otherwise be lost.
Hybrid car systems
Hybrid cars combine conventional combustion engines with an electric motor. During braking, generators are spun by the vehicle's motion. These generators produce electricity, which is stored in batteries. When additional power is needed, these batteries power an electric motor (the generator working in reverse) to supplement the torque from the conventional engine.
However, hybrid systems have a notable drawback: the batteries have considerable mass, which means the overall fuel efficiency improvement is not as substantial as might be expected.
Flybrid design
A recent development in KERS technology is the flybrid design. The term "flybrid" combines "flywheel" and "hybrid" to describe this innovative system.
Operation during braking: When the vehicle brakes, some of the kinetic energy is used to spin a flywheel. This energy transfer can be achieved through two methods:
- A generator/motor method, similar to the hybrid system
- Direct mechanical linkage via gears connecting the rear driveshaft to the flywheel
Operation during acceleration: When the vehicle accelerates, the process reverses. The rotational energy stored in the flywheel is transferred back to the driveshaft as needed. This transfer uses either the direct mechanical method or electricity generation, which then powers motors attached to the car's wheels. The flow of power to and from the flywheel is managed by computerized control systems.
Advantages of flybrid systems:
Compared to battery-based hybrid systems, flybrid designs are significantly lighter because the flywheel assembly has much less mass than the battery packs used in conventional hybrids. Additionally, the flywheel can rotate at very high speeds (up to 60,000 rpm) in a vacuum to minimize energy losses from air resistance.
Future developments may involve combinations of both batteries and flywheels, particularly as battery costs continue to decrease.
Production applications
Sheet metal processing
Many industrial manufacturing applications, such as forming and piercing sheet metal, require continuous and non-fluctuating action to maintain precision and consistency.
Historical context:
Traditionally, leather belts driven by electric motors were used to drive presses. Flywheels were essential in these systems because:
- Leather belts could stretch or warp under different atmospheric conditions
- Belts could become inflexible or slip momentarily
- The flywheel helped compensate for these irregularities
Modern applications: Although leather belts were later replaced by motors directly attached to the presses, motors can still exhibit irregularities in their operation. The optimal solution is to have motors spin large flywheels, which reduce problems in the production process caused by fluctuations. This ensures that punch presses and similar equipment operate with the precision and consistency required for quality manufacturing.
Conveyor belt systems in mining
When extracting ores from mines, long conveyor belts are used to transfer material to other processing sections. Some of these conveyor systems can be extremely long, extending up to one kilometre and lifting ore through vertical heights of several hundred metres. These belts are made from one continuous piece of material.
Typical conveyor belt components:
- Two main pulleys: the head pulley (at the top) and the tail pulley (at the bottom)
- A motorized drive system
- A take-up pulley that maintains correct belt tension
The problem without a flywheel:
If a power fluctuation, failure, or emergency stop occurs, the belt's inertia attempts to keep it moving. However, different sections of the belt experience different tension levels, causing each section to react differently. This creates several problems:
- Some parts stop quickly while others cannot
- Stress waves are generated that travel along the belt
- The belt develops regions of sag and over-stretch
- Large-scale damage can occur to the belt structure
The solution: Adding a flywheel to the conveyor system solves this problem. The flywheel's rotational inertia helps maintain more consistent motion throughout the system, preventing the formation of damaging stress waves and protecting the belt from sudden changes in tension.
Key Points to Remember:
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Flywheel design: Maximum mass should be positioned far from the rotation axis for optimal performance; practical designs use a rim connected by spokes to the central axis.
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Three main applications: Flywheels smooth torque variations in vehicles, recover and re-use kinetic energy through KERS, and ensure consistent operation in production machinery.
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Smoothing function: Flywheels use their inertia to absorb and release energy, evening out fluctuations in engine torque and creating smoother operation, particularly in multi-cylinder engines.
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KERS technology: Regenerative braking captures energy during braking; flybrid systems offer advantages over battery-based hybrids due to lower mass while storing rotational energy for later use.
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Industrial reliability: In production processes, flywheels prevent problems caused by power fluctuations, protecting equipment like conveyor belts from stress waves and maintaining precision in manufacturing operations.