LHM (hydraulic fluid)
| Industry | Automotive |
|---|---|
| Founded | June 5, 1919 in France |
| Founder | André Citroën |
Key people | Paul Magès (inventor of hydropneumatic suspension) |
| Products | Automobiles, hydropneumatic suspension systems |
| Parent | PSA Group (historically), now part of Stellantis |
| Website | www |
Hydropneumatic suspension is a vehicle suspension technology created by Paul Magès and manufactured by Citroën, implemented in Citroën automobiles and licensed to other brands. This system merges hydraulic and pneumatic methods, utilizing gas to absorb shocks and fluid to transmit forces, resulting in a finely tuned, responsive suspension that offers exceptional ride quality. It commonly features both automatic leveling and driver-controlled ride height adjustments, improving ground clearance for challenging terrain.
The concept draws inspiration from aircraft pneumatic landing gear partly filled with oil for lubrication, with origins tracing back to a 1933 patent. Today, similar principles are applied extensively, including in aircraft oleo struts and gas-charged automotive shock absorbers.
Description
Developed by Paul Magès and produced by Citroën, hydropneumatic suspension was originally termed Suspension oléopneumatique, emphasizing its combination of oil and air components.
Beyond Citroën’s use, this technology was licensed to various manufacturers such as Rolls-Royce—notably for the Silver Shadow model—BMW (5-Series e34 Touring), Maserati (Quattroporte II), and Peugeot. It also appeared in Berliet commercial vehicles and Mercedes-Benz models under the brand Active Body Control. The 1991 Toyota Soarer UZZ32 "Limited" featured a computer-regulated hydraulic system known as Toyota Active Control Suspension with integrated four-wheel steering. Hydropneumatic suspension is also prevalent in modern military applications, including armored vehicles like tanks.
Effects
Designed to provide a highly responsive, adaptable suspension capable of managing substantial loads, the system ensures superior ride quality across diverse driving conditions. It generally incorporates both automatic leveling and driver-selectable ride height settings, allowing for improved clearance on uneven surfaces. Hydropneumatic suspension delivers notable benefits compared to traditional steel springs and is well-regarded within the automotive sector. The system operates by having gas absorb shock forces while hydraulic fluid transmits them, combining two key technologies:
- Hydraulics facilitate torque multiplication without relying on mechanical gears or linkages.
- Pneumatics utilize the compressibility of gas to mitigate impact forces.
Although often associated primarily with enhanced comfort by consumers, this technology also significantly improves vehicle handling and control capabilities, overcoming limitations inherent to steel springs. Despite these advantages, widespread adoption was limited due to patent constraints and perceived system complexity. As a result, manufacturers like Mercedes-Benz, British Leyland (with systems such as Hydrolastic and Hydragas), and Lincoln developed simpler compressed air suspension alternatives.
The implementation by Citroën required specialized maintenance tools and expertise, setting their vehicles apart from conventional designs. This system demonstrated particular effectiveness on France’s rough post-World War II roadways, exemplified by the smooth and stable ride of the Citroën ID/DS series and later models.
While hydropneumatic suspension naturally exhibits limited roll stiffness, innovations like steel anti-roll bars, adjustable suspension firmness (Hydractive), and active body roll management (Citroën Activa) have greatly enhanced its dynamic performance over time.
Fundamental Mechanical Configuration
The mechanism employs an engine-driven pump, powered either by a belt or camshaft, to pressurize a specialized hydraulic fluid. This pressurized fluid activates the brake system, suspension, and power steering. Additionally, it can control components such as the clutch, steering-responsive headlights, and power windows.
Nitrogen serves as the trapped gas within the system due to its excellent corrosion resistance. The nitrogen-filled spring reservoir is manipulated through incompressible hydraulic fluid contained in a suspension cylinder, allowing ride height adjustments by varying the fluid volume. A rubber diaphragm separates the nitrogen gas from the hydraulic oil inside the suspension sphere.
Development Timeline
Citroën first introduced this technology in 1954 on the rear suspension of the Citroën Traction Avant. Its initial four-wheel application debuted on the innovative Citroën DS model in 1955. The suspension design drew inspiration from pneumatic aircraft landing gear systems that used oil for lubrication and gas retention, a concept patented by Citroën in 1933. Later breakthroughs included the 1960 patent for a "Double stage oleo-pneumatic shock absorber."
Significant historical points include:
- During World War II, Paul Magès, a Citroën employee without formal engineering credentials, devised an oil-and-air suspension system combining comfort with vehicle stability and automatic leveling.
- 1954: Implementation of rear suspension utilizing LHS hydraulic fluid on the Traction Avant 15H.
- 1955: The Citroën DS integrated suspension, power steering, brakes, and gearbox/clutch systems all powered by high-pressure hydraulics supplied via a belt-driven seven-piston pump.
- 1960: The United States granted a patent for a double-stage oleo-pneumatic shock absorber influencing aircraft oleo struts and gas-charged shock absorbers.
- 1965: Rolls-Royce acquired licensing rights to Citroën’s technology for use in Silver Shadow suspensions.
- 1967: Introduction of LHM mineral hydraulic fluid with non-hygroscopic properties.
- 1969: Production of the Citroën M35 coupé featuring a Wankel engine and hydropneumatic suspension by Heuliez.
- 1970: Adoption of hydropneumatic suspension in the compact Citroën GS; launch of Citroën SM with DIRAVI variable-speed power steering and hydraulically controlled directional headlights.
- 1972: BMW offered optional hydropneumatic rear suspension on its E12 5-series; this system remained in use through various BMW models into the early 1990s before transitioning to air suspension.
- 1974: Height-adjustable suspensions were prohibited by the US National Highway Traffic Safety Administration (NHTSA), though this ban was lifted in 1981.
- 1974: The Citroën CX combined hydro-pneumatic self-leveling suspension with DIRAVI power steering and flexible subframe mounts to improve ride comfort.
- 1974: Maserati Quattroporte II incorporated hydropneumatic suspension based on an extended Citroën SM chassis following Citroën’s acquisition of Maserati.
- Mid-1970s to 1980s: Mercedes-Benz integrated hydropneumatic suspensions with adjustable height and self-leveling features alongside coil springs in select S-Class and E-Class vehicles.
- Late 1980s to early 1990s: BMW’s E30 Touring and E32/E34 series employed hydropneumatic rear suspensions sharing hydraulic circuits with power steering pumps.
- 1989: Citroën launched Hydractive Suspension on the XM model, featuring electronic control governed by acceleration sensors.
- Early 1990s: Peugeot introduced rear hydropneumatic suspension on the 405 Mi16x4; JCB Fastrac tractors adopted similar systems for rear suspension; Toyota Soarer UZZ32 utilized hydraulically controlled struts with multiple sensors for enhanced handling dynamics.
- Mid-1990s: The Citroën Xantia featured Hydractive technology combined with active anti-roll bars to virtually eliminate body roll, achieving exceptional lateral acceleration during moose tests.
- Late 1990s to early 2000s: Mercedes-Benz debuted Active Body Control—an electronically managed hydropneumatic system—on CL-Class and S-Class models; Citroën introduced Hydractive 3 on the C5 with integrated pump/sphere units and electric height sensors; the C6 model featured AMVAR (Hydractive 3+).
- From mid-2000s onward: Hydractive systems became optional on Citroën C5 II; JCB Fastrac expanded hydropneumatic suspension to both front and rear axles on newer models.
- 2019: Mercedes-Benz unveiled eActive Body Control for GLE-Class SUVs, eliminating mechanical anti-roll bars and enhancing dynamic performance.
- 2023: BYD Auto released advanced active hydropneumatic suspension technology on its Yangwang U8 SUV and U9 sports car, offering unique functions such as three-wheel drive stability and level parking jumps.
Operation
At the core of the system lie the spheres—typically five or six in number, with one dedicated to each wheel, an essential main accumulator, and occasionally a separate brake accumulator. Advanced Hydractive or Activa variants may incorporate as many as ten spheres. Each sphere consists of a hollow metal shell housing a flexible Desmopan rubber diaphragm that partitions the interior; the upper section contains high-pressure nitrogen gas (up to 75 bar), while the lower part interfaces with the hydraulic fluid circuit. A high-pressure pump driven by the engine pressurizes mineral hydraulic fluid (LHM) between 150 and 180 bars, first supplying the front brakes through a safety valve, then depending on the vehicle’s configuration, powering steering, clutch actuation, or gear selection.
Hydraulic pressure forces fluid into suspension cylinders and spheres, compressing the nitrogen gas to provide damping via a two-way leaf valve that regulates fluid movement. This straightforward yet highly effective damper mechanism also facilitates ride height adjustment through height corrector valves connected to anti-roll bars. When additional load is applied, these valves introduce more fluid into the suspension cylinders; when load decreases, surplus fluid returns to the reservoir. Rear braking is supplied from the rear suspension circuit, with braking force adapting proportionally to load due to corresponding pressure changes.
Hydraulic Fluid Types
From 1954 until 1967, Citroën employed a red synthetic hydraulic fluid known as LHS (Liquide Hydraulique Synthétique). However, its vented reservoir design allowed moisture and dust ingress, leading to corrosion and operational problems.
To resolve these issues, Citroën introduced LHM (Liquide Hydraulique Minéral), a green mineral oil-based fluid akin to automatic transmission fluids. Being hydrophobic, LHM prevents moisture-induced sponginess in brakes and incorporates corrosion inhibitors. Its effectiveness prompted adoption by other manufacturers such as Rolls-Royce, Peugeot, Mercedes-Benz, Jaguar, Audi, and BMW who transitioned to mineral oil-based hydraulic systems.
To reduce dust contamination risks, filters were installed on reservoirs; routine fluid replacement and filter upkeep are essential for maintaining system durability. It is crucial to use the appropriate fluid type—mixing LHS and LHM systems necessitates comprehensive flushing with specialized cleaning agents prior to refilling.
Newer Citroën models equipped with Hydractive 3 suspension utilize orange LDS hydraulic fluid, which offers extended service intervals and complies with DIN 51524-3 HVLP standards.
Production
High-pressure components are fabricated from narrow steel tubing linked to valve control units via Lockheed-style pipe unions sealed with Desmopan—a polyurethane thermoplastic compatible with LHM fluid. Dynamic elements such as suspension struts and steering rams employ contact seals between cylinder and piston surfaces to ensure pressure containment. Plastic and rubber parts include return tubes from valves like brake control and height corrector valves; these also capture any fluid seepage around suspension push-rods. Critical components including height correctors, brake master valve spools, steering valve spools, and hydraulic pump pistons are engineered with extremely tight tolerances (1–3 micrometres), minimizing leakage. Metal and alloy parts generally remain reliable even after extensive mileage; however, elastomer components exposed to air tend to harden and develop leaks over time—common failure points within the system.
Spheres gradually lose pressure due to nitrogen diffusion through their membranes but can be economically recharged. The Hydractive 3 system introduced spheres featuring nylon membranes that significantly reduce gas loss; these are distinguishable by their grey coloration. Traditional green- and grey-hued spheres typically endure for 60,000 to 100,000 km. Original spheres included threaded plugs for recharging purposes; newer “saucer” style spheres lack these plugs but can be retrofitted for gas replenishment. Membranes have an indefinite lifespan unless subjected to low-pressure operation causing rupture. Recharging every three years is recommended to sustain optimal performance. A ruptured membrane results in suspension failure on that wheel but does not affect overall ride height. Without proper spring action beyond tire compliance, striking potholes with a deflated sphere can damage suspension components or wheels. Failure of the main accumulator sphere leaves only the high-pressure pump supplying braking pressure to front wheels; certain older power steering models incorporated separate accumulators for front brakes.
Earlier LHS and LHS2 (red) systems utilized elastomers incompatible with green LHM fluids. Vehicles equipped with Hydractive suspensions employ orange LDS fluid which is also incompatible with other types of hydraulic fluids.
Legacy
The fundamental concepts behind hydropneumatic suspension have impacted various sectors, including aviation with oleo struts and automotive shock absorbers filled with gas—initially patented in the United States in 1934 by Cleveland Pneumatic Tool Co.—and are extensively utilized in contemporary military vehicles such as tanks.
Hydractive
Hydractive Suspension represents an automotive innovation introduced by Citroën in 1990, building upon the original 1954 hydropneumatic suspension by incorporating electronic sensors and driver-adjustable controls. First showcased in the 1988 Citroën Activa concept, this system enables drivers to toggle between a firm "sport" setting and a softer, more comfortable ride. The suspension continuously receives input from sensors tracking steering, braking, throttle, gearbox activity, and body movement, which are processed by an onboard computer to adjust suspension spheres for either enhanced handling or ride comfort. Although the Activa models exhibited a slight one-degree inward lean during cornering, Citroën acknowledged that ideally, no lean would occur.
A notable commercial benefit of Hydractive suspension is its ability to decrease fuel consumption and reduce tire wear by limiting tire scrub caused by negative camber during turns.
Hydractive 1 and Hydractive 2
Citroën implemented Hydractive and its successor Hydractive 2 on vehicles such as the XM, Xantia, and advanced Activa variants. Hydractive 1 featured two modes: Sport, which maintained a consistently firm suspension, and Auto, which automatically switched between soft and firm settings based on sensor data including accelerator use, brake pressure, steering angle, and vehicle body dynamics.
Hydractive 2 refined this approach by offering Sport and Normal modes; in this iteration, Sport no longer enforced constant firmness but lowered sensor activation thresholds to provide firm handling during cornering and acceleration without sacrificing ride comfort.
In cases of sensor failure, the system defaulted to a firm suspension setting for safety reasons. Starting with the Xantia in 1994 and the XM in 1995, an additional SC/MAC sphere served as a hydraulic pressure reservoir for rear brakes equipped with locks, preserving normal ride height for extended periods without engine operation—referred to as the 'anti-sink' sphere—enhancing vehicle dependability and customer experience.
Hydractive 3
The 2001 launch of the Citroën C5 featured an advanced Hydractive suspension system called Hydractive 3, which electronically maintained standard ride height even when the engine was off. This model also introduced orange synthetic LDS hydraulic fluid to replace the traditional green LHM mineral oil used in earlier hydropneumatic systems.
An upgraded version named Hydractive 3+ appeared on higher-end C5 models and became standard equipment on the Citroën C6 starting in 2005. This variant includes extra spheres activated via a Sport button to deliver a firmer driving experience.
Hydractive 3 offers two automatic height adjustment modes:
- Motorway mode: lowers vehicle clearance by 15 mm at speeds exceeding 110 km/h
- Rough road mode: raises vehicle clearance by 13 mm when traveling below 70 km/h
Its BHI (Built-in Height Integration) system dynamically adjusts ride height based on vehicle speed and front-to-rear height distribution.
Hydractive 3+ introduces a third mode allowing adjustable suspension firmness ('Comfort' or 'Dynamic') and incorporates additional sensor inputs for enhanced precision:
- Vehicle speed
- Front and rear ride heights
- Steering wheel rotation speed and angle
- Longitudinal and lateral acceleration forces
- Suspension movement speed
- Accelerator pedal position
Applications by model:
C5 I (2001–2004)
- Hydractive 3: EW7J4, DW10TD engines
- Hydractive 3+: EW10J4, EW10D, ES9J4S, DW12TED4 engines
C5 II (2004–2007)
- Hydractive 3: EW7J4, EW10A, DV6TED4, DW10BTED4 engines
- Hydractive 3+: ES9A, DW12TED4 engines (prior to RPO No 10645)
C6 (2005–2012)
- Hydractive 3+: Standard across all versions
C5 III X7 (2007–2017)
- Hydractive 3+: Availability depends on market region and trim level