The Science Behind CO2 Cars

CO2 cars are small, rocket-like model vehicles propelled by the rapid release of pressurized carbon dioxide from a cartridge; they move forward by Newton’s third law, and their performance is governed by gas dynamics, aerodynamics, rolling resistance, and mass. In school competitions and lab settings, these cars use a CO2 canister as a compact energy source to create thrust, converting the gas’s stored pressure into motion over short straight tracks—typically around 20 meters—in roughly a second.

What Exactly Is a CO2 Car?

CO2 cars are lightweight models, often made from balsa or similar materials, that carry a small CO2 cartridge at the rear. When launched, a pin pierces the cartridge seal, releasing high-pressure CO2 through a nozzle. The gas rushes backward; the car rockets forward. The setup is purpose-built to study core principles of physics—forces, energy conversion, fluid dynamics, and design optimization—in a controlled and dramatic way.

How the Propulsion Works

Compressed Gas and Thrust

A CO2 cartridge contains liquid CO2 in equilibrium with gas at high pressure (roughly 50–60 bar at room temperature). Piercing the seal allows the gas to expand and accelerate through a small orifice at the back of the car. The high-speed jet generates thrust: the momentum of gas expelled backward pushes the car forward. In many launches, gas flow reaches “choked” conditions at the nozzle, meaning it exits at about the local speed of sound for CO2, and the thrust remains relatively high for a brief burst—typically a few tenths of a second—until the pressure drops.

Energy and Thermodynamics

The cartridge stores energy as pressure. When released, part of that energy becomes kinetic energy of both the gas and the car. Because expansion is fast and close to adiabatic, the gas cools sharply (Joule–Thomson effect), often frosting the nozzle. Some CO2 may flash from liquid to gas during discharge, absorbing heat and reducing pressure more quickly. The useful “impulse” (thrust integrated over time) is usually on the order of a newton-second for classroom cartridges, enough to accelerate a lightweight car to impressive speeds in a few meters.

Forces and Motion on the Track

Once the car launches, its motion reflects the balance of forward thrust against opposing forces: aerodynamic drag and rolling resistance. The car accelerates rapidly early on, then continues to gain speed as thrust declines, with drag rising roughly with the square of speed. On a 20-meter course, peak speeds commonly exceed 20 m/s (about 45 mph), and recorded run times are often near or just above one second in well-optimized builds.

Aerodynamics

Drag grows quickly with speed, so minimizing the drag area (CdA) matters. Body shaping, smooth surface finishes, and careful attention to the frontal area all contribute. At the typical small sizes and speeds involved, the Reynolds number usually sits in the 10^5 range, where flow separation and surface texture meaningfully affect drag. Even wheel shape, wheel gaps, and the interface around the nozzle can add or subtract measurable milliseconds.

Rolling Resistance and Alignment

Wheel and axle friction, bearing quality, wheel concentricity, and alignment determine how much energy is lost to contact surfaces. Slight misalignment can cause scrubbing against a guide line or rails, wasting energy. Larger-diameter, lightweight wheels with low-friction bearings and precise axle holes reduce rolling losses, while consistent weight distribution helps stability during the thrust phase.

Design Levers You Can Control

Successful CO2 cars depend on a handful of tunable design choices that trade weight, stability, and aerodynamic efficiency against each other. The items below outline the major levers designers adjust and why they matter.

• Body mass: Lighter cars accelerate faster for a given thrust, but too little mass can compromise stiffness and straight-line stability.
• Shape and surface finish: Streamlining reduces drag; smooth surfaces decrease skin friction and delay flow separation.
• Nozzle alignment and rear geometry: Clean, centered flow reduces yaw-inducing forces and avoids wasted thrust.
• Wheel design: Trueness, diameter, tread width, and material affect rolling resistance and aerodynamic drag.
• Axle bearings and lubrication: Low-friction interfaces cut energy loss and improve consistency across runs.
• Center of mass and track: Balanced weight distribution reduces fishtailing during the thrust burst.
• Manufacturing precision: Symmetry, straight bores, and tight tolerances prevent alignment drift and rubbing.
• Cartridge seating: Secure, repeatable mounting keeps the thrust line consistent from run to run.

Together, these choices determine whether the car turns the brief gas burst into clean, straight acceleration—or loses precious time to drag, friction, and instability. The best designs find a balance that preserves stability without adding unnecessary mass or surface area.

Testing, Data, and Optimization

Teams typically validate designs with repeatable timing systems, photogates, or onboard sensors. Even simple A/B tests—swapping wheels, changing axle treatments, or rebalancing the chassis—can show measurable differences over 20 meters. While professional CFD isn’t required, smoke visualization, tuft testing with fans, or smartphone-based accelerometer data can reveal alignment issues and airflow separation zones that deserve attention.

Common Build Pitfalls

Many performance setbacks come from a few recurring mistakes. The following list highlights frequent issues and why they matter.

1. Over-sanding or thin walls: Weakens the body and leads to flex, wobble, or failure under thrust.
2. Rough finishes: Increase drag and promote early flow separation, costing top speed.
3. Misaligned axles: Cause scrub, heat, and unpredictable steering moments.
4. Poor nozzle seating: Off-center thrust induces yaw and track contact.
5. Unbalanced wheels: Introduce vibration and rolling losses at speed.
6. Loose tolerances: Variability between runs masks whether a change actually helped.

Addressing these pitfalls early usually yields larger gains than exotic body shapes—precision and consistency multiply the benefits of good aerodynamics and low mass.

Safety and Rules

CO2 cartridges are pressure vessels and must be handled with care. Use approved launchers, keep faces and hands clear of the nozzle path, wear eye protection during testing, and never heat, puncture, or modify cartridges outside of sanctioned equipment. Most competitions specify allowed materials, minimum dimensions, wheel constraints, and approved cartridges to keep runs fair and safe; always verify the current rule set before building.

Real-World Connections

CO2 cars are scaled-down demonstrations of rocketry and high-performance vehicle engineering. The gas jet and impulse mirror cold-gas thrusters used for spacecraft attitude control, while the car’s aerodynamic and rolling-resistance challenges echo those of land-speed record vehicles and efficient road cars. Although “CO2 car” refers to the propellant, the emissions involved in a single educational run are very small; many programs encourage reusing materials and recycling spent cartridges to reduce waste.

Summary

CO2 cars accelerate because compressed CO2 expands through a nozzle, generating thrust that propels a lightweight, low-drag chassis down a short track. Their performance hinges on the interplay among gas dynamics, aerodynamics, rolling resistance, and mass. Thoughtful design, precise construction, and methodical testing turn a brief burst of pressurized gas into a fast, straight, and safe run—making CO2 cars a vivid, hands-on way to learn core physics and engineering principles.

How does a CO2 car work?

A CO2 car works by using the principle of Newton’s Third Law of Motion to propel itself forward. A compressed CO2 cartridge is punctured, releasing carbon dioxide gas, which shoots out the back of the car. This action creates an equal and opposite reaction, pushing the car forward down the track. The gas escapes rearward, providing the “action,” and the car moves forward, fulfilling the “reaction”. 
Here’s a breakdown of how a CO2 car works:

1. CO2 Cartridge: A sealed cartridge contains high-pressure carbon dioxide gas. 
2. Puncturing Mechanism: At the start of the race, a device punctures a hole in the CO2 cartridge. 
3. Rapid Gas Release: The pressurized CO2 gas rapidly expands and escapes from the punctured cartridge. 
4. Rearward Thrust: The escaping gas creates a rearward force, pushing the gas in one direction. 
5. Forward Motion: Due to Newton’s Third Law, this action generates an equal and opposite reaction, propelling the car forward. 
6. Overcoming Inertia: The force of the escaping gas is strong enough to overcome the car’s inertia, causing it to accelerate down the track. 

The design of the car itself is crucial, focusing on factors like: 

• Lightweight Design: Keeping the car light reduces the effort needed to accelerate. 
• Aerodynamics: The car’s shape is designed to reduce air resistance (drag), allowing it to go faster. 
• Reduced Friction: Smoothly functioning axles and wheels minimize friction with the track and the car’s body. 

What makes a CO2 race car go faster?

Simply put, the less weight your dragster has, the faster it will go. This is the most important factor that will figure into your design. Keep it light! Thrust: The gas escaping from the CO2 cartridge in the car.

How fast do CO2 cars go?

CO2 dragsters can reach speeds from around 35 to 70 miles per hour (56 to 113 km/h), with some reaching up to 79 km/h (49 mph) or even faster depending on the specific competition. These lightweight, CO2-powered cars are propelled by a compressed CO2 cartridge down a flat, level track and can cross a 20-meter (65.5-foot) track in just over a second. 
Factors influencing speed

• Weight: Opens in new tabThe lighter the car, the faster it will go, making mass a crucial design factor, according to HowStuffWorks. 
• Aerodynamics: Opens in new tabThe cars’ design, particularly a rounded front and tapered rear, helps reduce wind resistance and improve speed. 
• Track Length: Opens in new tabThe cars are designed to operate on a standard 20-meter track, which helps dictate their typical speed range. 

Common speeds

• Typical Speeds: Many CO2 dragsters can reach speeds in the range of 35 to 40 mph. 
• Higher End: Some high-performing cars can achieve speeds of 60 to 70 mph or even more. 
• Record Speeds: During one competition, a dragster was clocked at a speed of 68 mph. 

What is the science behind CO2 dragsters?

Aerodynamics play a critical role in the performance of a CO2 dragster. The shape and design of the vehicle are engineered to reduce air resistance, directing airflow around the body to minimize drag.