Air leakage rate for a combination vehicle: what it is and why it matters

Introduction

Air leakage rate describes how quickly air moves through gaps and openings in an enclosure. In a combination vehicle—a common term for a tractor and trailer—sealed space quality matters for comfort, safety, and efficiency. Even with closed doors, imperfect seals around cabs and trailers let air pass at varying speeds as the vehicle moves.

What is the air leakage rate?

Defining leakage rate

The air leakage rate is the amount of air that leaks through gaps in the vehicle envelope for a given pressure difference. In practice, this describes how easily air can infiltrate or escape the cab, sleeper, or cargo area when the vehicle is pressurized or depressurized.

Units commonly used

Leakage rate is often expressed as volume per time (for example, cubic feet per minute, CFM, or liters per second) at a specified pressure difference. In automotive work, you may also see terms like infiltration or exfiltration rate used to describe how air moves through the vehicle shell.

Air leakage in a combination vehicle

Common leakage paths

Typical leakage points in a tractor-trailer setup include:

• Cab doors and sleeper doors and their seals
• Windshield and side-window seals
• Cab-to-trailer gap and weatherstripping
• Trailer doors and curtain-seal surfaces on curtain-side trailers
• Roof, vents, penetrations for HVAC ducts, and access hatches
• Perimeter joints around fuel- and cargo-area openings

Consequences for a tractor-trailer

Air leaks can increase interior drafts, raise HVAC energy use (heating or cooling), contribute to higher fuel consumption due to extra drag and pressurization work, and affect moisture and dust control inside the cargo area.

Why air leakage matters

Fuel efficiency

Unwanted air exchange creates a pressure differential that the vehicle’s HVAC and propulsion systems must overcome, reducing overall efficiency and increasing fuel use.

Cabin comfort and cargo protection

Leaks can cause drafts, temperature inconsistencies, and moisture or dust ingress, affecting driver comfort and, in some cases, cargo quality.

Noise and moisture

Air leaks can amplify wind noise and bring in moisture, which may impact comfort and long-term component wear.

How air leakage is measured

Stationary testing

technicians can seal the vehicle and apply a known pressure difference using a blower or pump, then measure the airflow required to maintain that pressure or observe the rate of pressure decay. This provides a leakage rate for the vehicle envelope.

Dynamic testing

Leakage can also be assessed under real driving conditions by monitoring interior pressure changes as the vehicle encounters different aerodynamic loads, speeds, and weather. This helps capture leaks that only occur during motion.

Identifying leaks

Common methods include visual inspection, smoke pencils or tracer gas to reveal draft paths, thermography or acoustic imaging to locate leaks, and targeted measurements at suspected joints and seals.

Ways to reduce air leakage

Inspect and replace seals

Regularly inspect weatherstripping around cab and sleeper doors, cargo doors, windows, and windshield. Replace worn gaskets and adjust door alignments as needed.

Optimize door and seal performance

Ensure doors latch securely and seals sit evenly without gaps. Consider upgrading to higher-quality or oversized seals where appropriate.

Seal around cab-to-trailer connections

Check and improve seals in the cab-to-trailer area and around any penetrations between the two units. In some cases, aftermarket gap reducers or aerodynamic seals can help reduce leakage paths.

Trailer door seals and curtain surfaces

Inspect curtain-side trailers and roll-up doors for worn or damaged seals. Replace to maintain a tight seal when doors are closed.

Aerodynamic and design considerations

Where feasible, employ aerodynamic gap reducers, proper under-cab seals, and other design features that minimize pressure differences across the vehicle envelope.

Routine maintenance and testing

Incorporate leakage checks into regular maintenance schedules and use fuel economy or interior comfort complaints as signals to investigate potential leaks.

Practical tips for fleet operators

• Build leakage checks into quarterly or semiannual maintenance routines.
• Train drivers to report drafts, temperature inconsistencies, or unusual cabin noise, which can indicate leaks.
• Track fuel economy and HVAC load over time; sudden drops can hint at worsening seals or new leaks.
• Prioritize seal replacement and door alignment as cost-effective upgrades with clear returns in comfort and efficiency.

Takeaways

• The air leakage rate measures how quickly air crosses gaps in a combination vehicle’s envelope and affects comfort, fuel use, and cargo protection.
• Common leakage paths include doors, windows, and cab-to-trailer joints; regular inspection and sealing can reduce leaks.
• Measuring leakage involves pressure-based tests and, when possible, dynamic assessments during operation.
• Proactive maintenance and sealing improvements offer a practical path to better efficiency and driver comfort for fleets.