Road vehicles — Liquefied natural gas (LNG) fuel systems — Part 1: Safety requirements
1Key Takeaways
This standard establishes safety requirements for liquefied natural gas (LNG) fuel systems in road vehicles. It outlines specifications for the design, installation, operation, and maintenance of these systems to ensure compliance with relevant safety regulations. The document covers essential aspects such as material …
2Expert Interpretation
ISO 19723-1:2025 provides comprehensive safety requirements for liquefied natural gas (LNG) fuel systems for road vehicles, covering key technical specifications such as system design, refueling, leak control, and installation protection. This article provides an in-depth analysis of the main updates in the second edition of this standard, including requirements for electronic control units and minimum refueling port clearance specifications, offering professional implementation guidance for LNG vehicle manufacturers and conversion companies.
ISO 19723-1:2025 Standard Framework and Technology Evolution Analysis
The second edition of ISO 19723-1, released by the International Organization for Standardization in December 2025, marks a new stage in the development of safety specifications for liquefied natural gas (LNG) vehicle fuel systems. This standard replaces the first edition from 2018 and incorporates the 2021 revisions, reflecting the latest achievements in LNG vehicle technology development and safety practices over the past seven years. The core objective of the standard is to establish a unified safety benchmark for LNG on-board fuel systems, ensuring that fuel systems operating at ultra-low temperatures of -162°C meet the safety requirements for various types of road vehicles.
Scope of Application and System Boundary Definition
The scope of this standard is clearly defined as all types of motor vehicles as defined in ISO 3833, including original equipment manufacturers (OEMs) and modified vehicles, applicable to single-fuel, dual-fuel, or multi-fuel application scenarios.
The standard specifically emphasizes system boundaries—covering only LNG system components and their connections, including tanks, valves, fuel lines, etc., up to the vaporizer. The downstream portion of the vaporizer is considered the compressed natural gas (CNG) component, regulated by the ISO 15501 series of standards. This clear boundary avoids standard overlap and provides manufacturers with a clear compliance path.
Key Terminology Definitions and Safety Concepts
Chapter 3 of the standard clearly defines 23 key terms, among which several core concepts are crucial for understanding the standard requirements:
| Terms | Definition Highlights | Safety Significance |
|---|---|---|
| Liquefied Natural Gas (LNG) | A cryogenic liquid with a temperature dropping to approximately -162°C at atmospheric pressure | Ultra-low temperature characteristics determine material selection and insulation requirements |
| Operating Pressure | The maximum pressure that the component design can withstand, serving as the benchmark for strength calculations | Ensuring the structural integrity of the system under extreme operating conditions |
| Electronic Control Unit (ECU) | Control engine LNG demand and shut off automatic valves under specific conditions | New requirements reflecting the trend of electronic safety control |
| Airtight enclosure | A device to guide gas leaks to the outside of the vehicle, with a minimum opening of 450mm² | Prevent the accumulation of combustible gases in confined spaces |
System Design Requirements and Component Specifications
4.1.1 General Design Principles
Standard requirements: LNG on-board fuel system components must comply with the ISO 12614 series and ISO 12617 standards. All components should be designed based on the manufacturer's declared operating pressure, a requirement that is more stringent than simply considering the operating pressure. The system design must meet seven basic conditions, including environmental tolerance, installation location safety, grounding continuity, etc. Of particular note is the standard's requirement that all connections be placed in easily accessible locations, reflecting a safety philosophy of preventative maintenance.
List of Required and Optional Components
The standard clearly distinguishes between required and optional components, providing flexibility for system configuration:
| Component Type | Required Components | Optional Components | Key Requirements |
|---|---|---|---|
| Tank Related | LNG Tanks, Pressure Relief Valves | Pressure Control Regulators, Level Gauges | Compliant with ISO 12991, Securely Installed |
| Valve System | Overflow Limiting Devices, Manual Valves, Automatic Valves | Relief Pipe Lockout Device | Automatic valves must remain normally closed when not inactive |
| Control Monitoring | ECU, Fuel Indicator | Gas Temperature Sensor | ECU Delay Shutdown Not Exceeding 2 Seconds |
| Safety Devices | Category M Vehicles Require Natural Gas Detector or Gastly Sealed Enclosure | Gastly Sealed Enclosure | Preventing Gas Accumulation in Passenger Compartment |
New Requirements for Electronic Control Units (ECUs)
The second edition of the standard adds detailed requirements for ECUs, reflecting the trend of automotive electrification. The ECU must shut off the automatic valve within 2 seconds after the engine is turned off; this time requirement balances safety response and system stability. Temperature tolerance ranges are divided into two categories: -40℃ to 105℃ and -20℃ to 120℃, to meet the needs of different climate zones. Electromagnetic compatibility requirements reference the ISO 11451/11452 and ISO 7637 series standards to ensure reliable operation in complex electromagnetic environments. The standard specifically specifies limits for transient conducted interference: positive pulses for 12V systems should not exceed +75V, and negative pulses should not exceed -100V; the corresponding limits for 24V systems are +150V and -450V. These stringent requirements ensure the stability of the ECU during fluctuations in the vehicle's power system. Chapter 4.2 of the standard provides detailed requirements for the refueling system. The refueling port should be installed in an easily accessible location, preferably on the side of the vehicle, and installation in the engine compartment is explicitly prohibited. This provision is based on two safety considerations: firstly, to avoid the impact of high-temperature environments on low-temperature components, and secondly, to prevent interference with the battery or high-voltage ignition circuit.
Minimum Clearance and Ergonomics
The newly added minimum refueling port clearance requirement (referencing ISO 12617:2015 Figure 1) ensures a safe connection between the refueling nozzle and the refueling port. The standard also requires consideration of ergonomic factors in the connection process, reflecting an extension from purely technical safety to operational safety. The installation must be able to withstand the load generated by an accidental disconnection of the refueling hose to ensure that the airtightness is not affected.
Leakage Control and Ventilation Systems
Leakage Prevention and Monitoring
Chapter 4.3 of the standard emphasizes a systematic approach to leakage control. All pressurized gas systems must be designed to be leak-free under operating stress and must be leak-tested after assembly. For tanks installed in the passenger compartment or poorly ventilated spaces, valves and piping must be housed within an airtight enclosure to direct leaked gas outside the vehicle.
An important note: LNG-powered vehicles use odorless LNG, and leaks cannot be detected by odor.
Therefore, Category M vehicles must be equipped with a natural gas detector or an airtight enclosure, and Category N vehicles have the same requirement when the storage tank is located inside the cargo hold.Ventilation System Design Specifications
Chapter 4.7 details the design requirements for the ventilation system. The outlet of the main pressure relief valve (PRV) must be connected to the high-altitude venting system to prevent exhaust gases from entering the enclosed area. The inner diameter of the ventilation duct must not be smaller than the PRV vent outlet, and the minimum burst pressure must be at least 1.5 times the operating pressure of the LNG storage tank.
The newly added requirement for a vent pipe interlock device reflects consideration of the actual operating environment. If a flap-type interlock is used, the flap must be able to open greater than 45° and be fully closed when not in operation. The design must prevent freezing in the closed position, a requirement crucial for operations in cold regions.
Storage Tank Installation and Mechanical Protection
Installation Strength Requirements
Standard Chapter 4.4 specifies different acceleration tolerance requirements according to vehicle category, reflecting the risk classification concept:
| Vehicle Category | Acceleration in Travel Direction | Lateral Acceleration | Vertical Downward Acceleration | Application Scenarios |
|---|---|---|---|---|
| M1/N1 | 20g | 8g | 5g (Undercar Installation) | Passenger Cars/Light Trucks |
| M2/N2 | 10g | 5g | 5g (Underbody Mounting) | Medium-sized Bus/Truck |
| M3/N3 | 6.6g | 5g | 5g (Underbody Mounting) | Large Bus/Heavy-duty Truck |
These requirements are based on typical collision scenarios for different vehicle types to ensure the tank remains intact in an accident. The standard explicitly prohibits on-site welding on the tank to prevent damage to material properties.
Thermal Protection and Ignition Risk Control
Chapter 4.5 requires that components maintain a minimum distance of 100mm from the exhaust system; otherwise, a heat shield must be installed. Chapter 4.6 details measures to minimize the risk of gas ignition: In areas where gas leaks may occur, electrically operated valves and sensors must be intrinsically explosion-proof, wiring must be adequately protected, and connections must be secure. The protection level requirements for electrical connections are divided into two levels: IP40 in the luggage compartment and passenger cabin, and IP54 in other locations. These requirements reference ISO 20653 and IEC 60529 standards to ensure electrical safety under different environmental conditions. For vehicle manufacturers and conversion companies, the following aspects should be emphasized when implementing ISO 19723-1:2025: Early design intervention: Consider the installation location and space requirements of the LNG system during the vehicle design phase to avoid safety hazards caused by later modifications. **Component Selection Verification:** Ensure all components comply with the ISO 12614 series standards, especially the newly added requirements for ECUs and temperature sensors. **Test Verification Plan:** Establish a complete test plan according to ISO 19723-2, including tests for mechanical strength, leakage, and electrical safety. **Documentation and Marking:** Prepare user manuals that meet the requirements of Chapter 5 and mark the system according to the requirements of Chapter 6.
Technical Solutions Reference
Appendix A provides three feasible technical solutions for ventilation of valves, connections, and piping:
- Place the tank and its fittings within an airtight enclosure with permanent ventilation
- Seal the tank neck and its fittings with a specially designed durable cladding that provides airtightness to the compartment and permanent ventilation
- Install a self-exhausting valve to expel all possible leak sources through an internal channel and enclose the exhaust ports of piping, connections, and valves in an exhaust pipe that directs the gas to a safe location outside the vehicle
These solutions provide flexibility for different vehicle configurations, especially in space-constrained retrofit scenarios.
Industry Application Cases
In the development of an LNG model for a European heavy-duty truck manufacturer, key challenges in implementing this standard included: integration of the ECU with the existing vehicle network, material compatibility under cryogenic conditions, and misoperation prevention design of the refueling system.
By adopting a modular tank installation system, dual-redundant pressure relief valve configuration, and electrical connectors compliant with IP54 requirements, the manufacturer successfully passed EU whole-vehicle type approval, and the vehicles have safely operated for over 500,000 kilometers within an ambient temperature range of -30°C to +45°C. In the operation of an LNG bus fleet of a public transport company in an Asian city, the focus of implementing this standard was on leak monitoring and ventilation systems. By installing multi-point natural gas detectors in the engine compartment, optimizing the vent pipe route to avoid ice blockage, and conducting regular pressure tests, the fleet achieved a zero-major safety incident operating record, demonstrating the effectiveness and practicality of the standard requirements. These changes reflect three trends in the development of LNG vehicle technology: **Electronic Integration:** The refinement of ECU requirements marks the shift from mechanical to electronic control in LNG systems. **Operational Safety:** Ergonomic considerations and anti-misoperation designs reflect a user-centric safety philosophy. **Environmental Adaptability:** Anti-freezing requirements in cold climates expand the geographical applicability of LNG vehicles. Looking ahead, with the development of alternative fuels such as hydrogen, LNG standards may further harmonize with relevant standards. Simultaneously, the specific needs of autonomous vehicles may drive further evolution of LNG system safety requirements, including new requirements for remote monitoring and automated emergency response. ISO 19723-1:2025 provides a unified safety benchmark for the global LNG vehicle industry, and its implementation will promote technological innovation and market expansion. Manufacturers, operators, and regulatory agencies should deeply understand the standard requirements and develop implementation strategies tailored to specific application scenarios to jointly promote the continuous improvement of LNG vehicle safety levels.