Hydropneumatic suspension in a truck
Installation of a hydropneumatic suspension for a Scania truck
Hydropneumatisk hjulupphängning i lastbil
Installation av hydropneumatisk hjulupphängning till en Scania lastbil
Rasmus Karlqvist
Fakulteten för hälsa, natur- och teknikvetenskap Högskoleingenjörsprogrammet i maskinteknik Examensarbete 22.5 hp Handledare: Anders Wickberg Examinator: Anders Biel Datum: 2020-05-18 Version: 3.0 Foreword
This thesis would not be possible without the help of several people. First and foremost I would like to direct a sincere thank you to my mentors at Scania CV AB Olle Alsing and Johan Parsons for the time and engagement they have directed towards helping me out with this project. I would also like to dedicate a thank you to the all the colleagues at RTC and RTCB for a very pleasant experience and warm welcoming to the group it’s been a blast to be around you throughout this time with the activities, fika and lunches. Last but not least I would like to thank my mentor from Karlstad Universitet Anders Wickberg for guiding me through this project.
Abstract
Investigation and testing of hydropneumatic suspension systems has previously been done at Scania between the year 1992 and 2000. Interest has aroused at Scania CV AB to further test a hydropneumatic suspension. The reason being the new ventures of decarbonised, clean, electrified, automatized and digitalised vehicles. If electrified trucks are to be adopted in the market as an alternative to trucks with combustion engines, solutions for this type of vehicle’s capacity need to be presented. The vehicle’s weight needs to be reduced; the effectiveness of the components needs to be increased and alternatives to increase battery storage needs to arise if it’s going match the traveling distance of a combustion engine.
The mission of the project is to present an installation solution of a hydropneumatic suspension that retains the performance of the current air suspension. The presented material will contain CAD-models of all the brackets that will be designed to fit the suspension, as well as the placement in the vehicle assembly for said brackets.
The results show that as for the front suspension the best solution is a placement of the hydraulic cylinders in front of the vehicles front axle. Furthermore the rear suspension is best suited for a placement of the hydraulic cylinders behind the vehicles rear axle. However it was concluded that the rear suspension will not be able to retain the current stroke of the vehicle without sacrificing its ground clearance. Parts of the suspension could however be terminated when the air suspension system was replaced by the hydropneumatic system namely: The front suspension anti-roll bar, shock absorbers, air springs and their coexisting brackets.
Sammanfattning
Undersökning och testning av hydropneumatiska upphängningssystem har tidigare gjorts i Scania mellan 1992 och 2000. Intresset har väckts hos Scania CV AB för att ytterligare testa en hydropneumatiska hjulupphängningen. Anledningen är de nya satsningarna på avkolade, rena, elektrifierade, automatiserade och digitaliserade fordon. Om elektrifierade lastbilar ska anta marknaden som ett alternativ till lastbilar med förbränningsmotorer, måste lösningar för denna typ av fordon presenteras. Fordonets vikt måste minskas; komponenternas effektivitet måste ökas och alternativ för att öka batterilagringsutrymme måste uppstå om det skall kunna matcha förbränningsmotorernas räckvidd.
Målet med projektet är att presentera en installationslösning av den hydropneumatiska hjulupphängningen som erhåller den aktuella luftfjädringens prestanda. Det presenterade materialet kommer att innehålla placeringarna av hydraulcylindrarna till det hydropneumatiska systemet i fordonets sammanställning samt CAD-modeller av samtliga fästen som kommer att utformas för att passa till hjulupphängningen.
Resultaten visar att den bästa lösningen för främre hjulupphängningen är en placering av de hydraulcylindrarna framför fordonets framaxel. Den bakre hjulupphängningen är däremot bäst lämpad för placering av de hydraulcylindrar bakom fordonets bakaxel. Det drogs dock slutsatsen att den bakre hjulupphängningen inte kommer att kunna bibehålla fordonets nuvarande slaglängd utan att ge avkall på markfrigången. Delar av hjulupphängningen kunde emellertid avlägsnas när luftfjädringssystemet ersattes av det hydropneumatiska systemet nämligen: Den främre fjädringens krängningshämmare, stötdämpare, luftfjädrar och deras samexisterande infästningar.
Nomenclature
4x2 truck - An axle configuration of two axles were one is the drive axle.
6x2 truck - An axle configuration of three axles were one is the drive axle.
FMEA - Failure modes and effects analysis.
WBS - Work breakdown structure.
Accumulator - A pressure storage reservoir, accumulators enables the coping of extreme demands using less power and smooths out pulsations in a hydraulic system.
ECU - A box of electronics that provides rules and logic to a specific area of function.
Load handling – The action of controlling the vehicle’s height through the suspension system.
Roll – The load transfer of a vehicle towards the outside of a turn.
Lateral force – The force that acts in the direction parallel to ground and perpendicular to the direction of gravitational pull of the earth and the length of the a body.
Longitudinal force – The force that acts in the direction of the length of a body.
Vertical force – The force that acts in the opposite direction of the gravitational pull.
Level sensor – A component in a vehicle which determines angle of change relative to the point of calibration.
CAD - computer aided design.
Hz - Hertz. kN - kilo newton. Table of contents
1 Introduction ...... 8 1.1 Background ...... 8 1.2 Problem ...... 9 1.3 Project object ...... 9 1.4 Project mission ...... 9 1.5 Project delimitations ...... 9 2 Theory ...... 10 2.1 Hydropneumatic suspension ...... 10 2.2 Air suspension ...... 13 2.3 Current layout ...... 17 2.4 Comparison ...... 23 3 Method ...... 24 3.1 Planning ...... 24 3.2 Concept & Design ...... 26 4 Results ...... 30 4.1 Replacement and removal of the front suspension ...... 30 4.2 Concept generation of the front suspension ...... 31 4.3 Concept evaluation of the front suspension ...... 40 4.4 Final concept and further development of the front suspension ...... 42 4.5 Replacement and removal of the rear suspension ...... 45 4.6 Concept generation of the rear suspension ...... 45 4.7 Concept evaluation of the rear suspension ...... 47 4.8 Final concept and further development of the rear suspension ...... 48 5 Discussion ...... 51 6 Conclusion ...... 52 7 Future work ...... 53 References ...... 54
Appendixes Appendix A: WBS Appendix B: Gantt-Schedule Appendix C: Equilibrium calculation Appendix D: FMEA
1 Introduction
1.1 Background
Investigation and testing of hydropneumatic suspension systems has previously been done at Scania between the year 1992 and 2000. Conclusions have been drawn that the air suspension is superior according to driver’s subjective experience and the project has since then been resigned. Interest has aroused at Scania CV AB to further test the hydropneumatic suspension. The reason being the new ventures of decarbonised, clean, electrified, automatized and digitalised vehicles.
If electrified trucks are to be adopted in the market as an alternative to trucks with combustion engines, solutions for these vehicles’ capacity need to be presented. The vehicles weight needs to be reduced, the effectiveness of the components needs to be increased and alternatives to increase battery storage needs to arise if it’s going match the traveling distance of a combustion engine.
According to Wöhrmann et al. (2013) Citroën was the first company to introduce hydropneumatic suspension in the 1950’s. The suspension was used on passenger cars and was available in the market a while before air suspension was introduced. Now the hydropneumatic suspension is only manufactured by Citroën in larger quantities with respect to passenger cars. The interest of the suspension type diminished when many deficiencies were noticed. Leaks in the system caused parked cars to sink to the ground, which forced the driver of the vehicle to restore the pressure level before departure. It was also known that the hydropneumatic system requires installation of more components than the conventional compression spring system, namely oil pump, tank and accumulator.
Wöhrmann et al. (2013) goes on to explain that there has recently been an increased interest in hydropneumatic suspension in commercial off-road heavyweight vehicles and that the aforementioned problems with leaks in the system now are negligible. Compared to the conventional compression spring system and air suspension, hydropneumatic suspension offers high energy density at low surface area, therefore providing the possibility of placement near the wheel centre, which is normally a very tight space. Placement near the wheel centre is advantageous as it can eliminate torque that otherwise presents because of the lever effect of the distance between the wheel centre and the spring.
8 1.2 Problem
Today's trucks use compressed air to carry loads in the suspension. Such a solution is space consuming and has relatively high energy consumption. For future chassis, a more efficient solution in terms of space, energy consumption and weight are required.
1.3 Project goal
Present concept solutions for the installation of hydropneumatic suspension system that fits into an existing test truck with minimal alteration of surrounding parts. Where in future work Scania can evaluate this type of suspension solution, meet market demand and expand the company's growth opportunities. The presented material will contain the position in the truck assembly for the hydraulic cylinders as well as CAD-models of all the brackets that will be designed to fit the suspension.
1.4 Project delimitations
The project focuses on concept generation for the positions of the hydraulic cylinders as well as design of brackets for the hydraulic cylinders installation possibilities for a truck with air suspension as existing solution. The project will not contain:
Testing of the hydropneumatic suspension and the installation of it. Optimization of the design for production. Market research. Mechanical drawing of the solution. Installation of remaining articles in the hydropneumatic system. (see section 2.1)
9 2 Theory
2.1 Hydropneumatic suspension systems
The hydropneumatic suspension system that is supplied contains a hydraulic cylinder, accumulator, damping valve block, level sensor, ECU, Oil tank and an Oil pump. Every axle in the truck will require two hydraulic cylinder, two accumulators and two level sensors. The components around the axle share a single damping valve block. The whole system is then controlled by an ECU (electronic control unit), which provides logic and functions to the system. Since the truck for testing is a 4x2 truck the total of the components used will be the following:
4x hydraulic cylinder 4x accumulator 4x level sensor 2x damping valve block 1x ECU. Oil tank Oil pump
2.1.1 Function
Each side of an axle is mounted to a hydraulic cylinder which suspends the axels of the vehicle. The vehicle can adjust the ground clearance by increasing or decreasing the volume of oil in the hydraulic cylinder chambers. In a hydropneumatic suspension the springing medium is the gas, which in most cases as well as this system is nitrogen. The gas is stored in a chamber in an accumulator, as the hydraulic cylinder is compressed by irregularities in the road the gas in the accumulator also compresses from the oil that mediates the motion. The increased pressure from the compressed gas naturally causes a reactive force which springs the vehicle. The return motion of the accumulator causes the oil to pass through the damping valve block which acts as a conventional shock absorber by restricting the flow of the oil which in turn suppresses the hydraulic cylinders motion. (Supplier internal material 2020).
10
Figure 2.1.1. Schematic of the hydropneumatic suspension.
2.1.2 Secondary spring
Contrary to conventional hydropneumatic suspensions, theses accumulators are equipped with secondary springing. The secondary springing is derived from the extra smaller cylinder compartment on top of the accumulator. The compartment is filled with gas and is therefore pre-loaded. The preloaded accumulator should decrease the difference in stiffness of the system for when the vehicle is laden versus unladen. As the vehicle is unladen the secondary springing compartment is forcing the piston in the accumulator to maintian it’s position, putting pressure on the lower gas compartment making for a greater stiffness of the system in an unladen condition.
Figure 2.1.2. Secondary spring in accumulator.
11 2.1.3 Damping valve block
The black accumulator membrane above the damping valve block in figure 2.1.3 saves information of the load magnitude and varies the damping intensity depending on the static and dynamic pressure in the system. (Supplier internal material 2020).
Figure 2.1.3. Damping valve block.
2.1.4 Previous Testing
Results acquired from previous testing in 1992 of a hydropneumatic suspension at Scania yield that the test drivers experienced the truck to be too stiff while driving on “good” roads were high frequency vibrations occur. The drivers described it as they could feel the road structure up in to the cab.
During cornering the drivers explained that they experienced the truck to be swaying more comparing to the air suspended axle. Although while increasing the cornering intensity the problem seemed to fade.
Scania’s testing also noted the load handling to be significantly faster for the hydro pneumatic suspension. As well that the use of a hydropneumatic suspension on a 6x2 truck could eliminate the need of six air tanks in the truck. They also measured the natural frequency of the front and bogie axle to 1,2 respective 1,4 Hz versus 1,6 respective 1,8 Hz for the air suspension. (Hellström 1992).
12 2.2 Air suspension
An air suspension system often consists of
• air spring • compressed air tank • compressor • level valve or level sensor + ECU
There are many adaptations on air suspensions, where the two most common are of the type roll bellow or convoluted bellow.
The air spring is the component that links the truck’s axle and frame and springs the vertical forces that the truck is exposed to. The roll bellow type air spring displayed in figure 2.2.1 will be focused on in this section as it is the type Scania uses in their trucks. The roll bellow type consists of a rubber bellow which is clamped between a piston in the base and a mounting plate at the top. (Nygren 1991)
Figure 2.2.1. Reversible sleeve air spring.
2.2.1 Function
During operation, the bellows are filled with air that is compressed and expands as the vehicle travels over uneven roads. The compression is caused by the piston pushing up underneath the bellow. The increased pressure inside the bellow pushes the piston back. Thus, forcing the piston to return which causes the air spring to expand to its regular state. (Nygren 1991, p14)
13 2.2.2 Carrying capacity
The carrying capacity of an air spring is dependent on the effective area and the internal overpressure. (Nygren 1991). The carrying capacity F is defined by the formula:
F=p A (1)
Where:
• Po is the internal overpressure
• Ae is the effective area • F is the carrying capacity
(Nygren 1991)
2.2.3 Effective area
The effective area changes during the spring movement, since it is dependent on the dynamic load acting vertically on the top of the bellow and the overpressure from the gas within the bellow. The effective area is defined by the formula A = D (2) where Ae is the effective diameter. In order to measure the effective diameter, the suspension is subjected to a relatively low load. With the load applied the bellow rolls over the edges around the piston and form a fold similar to a semicircle. The effective diameter can be approximated during this process to the distance between the midpoints of the opposing folds. (Nygren 1991)
Figure 2.2.2. Effective diameter of a roll bellow.
14 2.2.4 Spring characteristics
Air springs differ from conventional leaf springs in terms of suspension characteristics. The conventional suspension characteristic has a stiffness which is linear to the deformation in the spring and is described by the relationship between the l oad F and the deformation of the spring s, 푘= (3). The characteristics of air suspension are more complex as the stiffness varies depending on the dynamic overpressure within the air bellow and is therefore progressive. The spring characteristic is described in theory by the formula: