constant mass flow rate supercharger performance for fsae

Constant Mass Flow Rate Supercharger Performance for FormulaSAE Vehicle

Second Lieutenant Adam Sean Leer1

University of New South Wales at the Australian Defence Force Academy

For the successful performance of a supercharger on a Formula SAE vehicle thereis a need to fully understand the performance characteristics of the superchargerunder the desired operating conditions. These include knowing the mass flow rate,volume flow rates, power required to drive the supercharger, adiabatic efficiency,actual efficiency, pressure ratios across the supercharger and intermediate temper-atures and pressures. Many papers have been written on and about superchargerscovering several areas such as their development, simulation, inlet and outlet de-signs, reduction of noise and vibration and comparisons between superchargers andturbochargers and the performance characteristics of the engines they are fitted tobut no paper has been written on the performance of a supercharger which mustcomply to Formula SAE rules. This initial thesis report looks at constant mass flowrate supercharger performance for formula SAE vehicles and the initial steps takentowards the final thesis paper. The project will be managed by project system engi-neering methodologies which will be used to meet the set deliverables. This reportwill cover previous work in this field and an analysis of the system to be designedto measure the performance of an M24 Eaton supercharger.

I Introduction

A Background

Roots type blowers have many applications in both commercial industry and automotive applications. Inindustries these pumps are used in applications where constant flow rates are required at varying dischargerates, such applications are cement plants and aquaculture aeration [1]. The supercharger that will be usedfor this thesis project is an M24 Eaton twin rotor roots type supercharger which has been specifically designedfor automotive use. The roots blower is a positive displacement pump which has two identical rotors whichare cycloid or involute in shape [2]. The pump works by pulling air through a pair of rotors which trapsthe air between the rotor and the supercharger housing Figure 1. As the rotor tips pass the inlet port theair in the inlet system is trapped in a constant volume system until the rotor passes the outlet port werethe trapped air is compressed in the outlet port of the supercharger [3]. This increases the amount of powergenerated per cycle in an internal combustion engine as compared to a normally aspirated engine, this isbecause by increasing the pressure and density of the air charge more fuel can be added to the combustionchamber and therefore more power is produced per cycle.[4].

B Aim

The aim of this thesis has been derived from the client brief which can be seen in Annex A. The projectis to design and build a system to measure the performance of a M24 supercharger which has been recom-mended by Eaton for use on formula SAE vehicles. The supercharger must be fitted with an inlet manifoldand have a restriction of 20mm in diameter as required by Formula SAE (FSAE) rules and the performancetested with a simulated engine on the outlet side of the supercharger with a range of volumetric efficiencies.Testing the performance of the supercharger will incorporate finding the mass flow rate, volume flow rates,power required to drive the supercharger, adiabatic efficiency, actual efficiency, pressure ratios across thesupercharger and intermediate temperatures and pressures. Tests will be conducted in accordance with amodified version of the SAE supercharger testing standard J1723 [5].

1School of Aerospace, Civil and Mechanical Engineering, (BE) Mechanical, Thesis, Part A, ZACM 4050

1

Initial Thesis Report 2009,ACME, UNSW@ADFA

Inlet port

Outlet port

Constantvolume

Compre

Ai

SuperchargerHousing Rotor

essed

ir

Figure 1: Supercharger gas flow

C Scope

The scope of this thesis is to design and build a test bench that will house the supercharger for testing. Thissystem will incorporate a motor to provide rotational power to the supercharger, a means of measuring thepower and a system of thermocouples and pressure gauges to record inlet and outlet manifold conditions.These measurements must be recorded while maintaining sonic flow through the 20 mm restrictor and asimulation of engine conditions on the outlet side of the supercharger with a range of volumetric efficiencies.The supercharger rpm must be able to be varied by mechanical or electrical means, this is required toachieve sonic flow through the 20mm inlet. It is also relevant to ensure that the system design be robustand versatile as to accommodate any further experiments that may be required such as fitting an aftercoolerto further test supercharger performance. Inlet and outlet manifolds need to be designed for the M24Eaton supercharger and must comply with the FSAE standards. The manifolds need to be designed sothat measuring instruments such as pressure gauges and thermocouples can be fitted without restrictingthe air flow and complies with the SAE supercharger testing standard J1723. The outlet manifold must bedesigned to simulate the valve system of an internal combustion engine and be designed in such a way thatfurther experiments must also be carried out such as fitting an aftercooler between the supercharger andvalve system.

D Deliverables

1. Design documents for the test bench system.2. Test bench3. Inlet and outlet manifolds4. Test results5. Processed supercharger characteristic diagrams.

II Previous work in this field

A Supercharger Performance

There are a great number of papers that deal directly with superchargers, these papers detail analyticalmodels, comparisons between supercharging and turbocharging, supercharger noise and vibration and su-percharger simulations. These papers do not cover the performance of a supercharger with a restricted inletand an engine with a range of volumetric efficiencies on the outlet side of the supercharger. These papersdo how ever do provide a good reference source to better understand the more complex workings of thesupercharger and applications to vehicles.

SAE have a standard for supercharging testing, the supercharger testing standard [5] which details themethod to which superchargers are to be tested. The scope of this standard is to test supercharges that donot have fuel added to the inlet air before supercharging. The standard only applies for bench testing and

2


specifies:

1. A standard basis for supercharger efficiency rating2. Reference inlet air supply test conditions3. A method for correcting observed efficiency to reference conditions4. A method for presenting these results in an accurate and usable way5. A method to compare superchargers without the effects of engine dynamics and intercooling.

The supercharger that is used for this thesis project is an M24 Eaton supercharger, it has a displacement of.393 L/rev and a maximum rpm of 18000; this gives a maximum flow rate of 7074 L/min. The superchargerstandard states that the inlet manifold conditions for the induction pipe must have the least amount ofrestriction as possible. The standard recommends a diameter for testing is to be 76.2 mm OD and shall be6 diameters in length [5]. This is significantly different to FSAE requirements which give specifications foran inlet which has a restriction of only 20mm. The outlet of the supercharger is also quiet different, thestandard requires a valve to be fitted to control the pressure ratio and when the valve is fully open thatminimal resistance is required. The requirements from the client brief specify simulation of an engine witha range of volumetric efficiency.

Sorenson in his paper ’Simulation of a Positive Displacement Supercharger’ [3] does not consider anyrestriction to the inlet of the supercharger or in his calculation, though a simulation of power required to runthe supercharger with an engine with a 75 percent volumetric efficiency running at twice engine rpm is calcu-lated [3]. Singer in his paper [6] has done a comparison of superchargers Vs turbochargers and bench testedsix supercharges and recorded their power requirements [3] in this case the supercharger inlets were main-tained at 100kPa by a variable speed blower and a throttle on the discharge side to create the required boost.

Eaton’s paper on the development of the Eaton supercharger [7] does not consider the restriction of theinlet or the volumetric efficacy of the motor on the outlet port. The paper is concerned with the inherentnoise levels of the supercharger due to uneven air flow and explores mathematical models of inlet and outletports followed by testing the most promising models. This paper does give a better understanding of theroots type blower and aspects that directly affect performance of the supercharger.

Other papers written on superchargers are mainly concerned with rotor profile to maximise performanceas well as inlet and outlet port design. These papers dealt with these issues to increase the efficiency of thesupercharger but to also reduce supercharger vibration and noise to an acceptable limit [7, 2].

A comparison between supercharges and turbochargers has been well documented on SAE engines. Attard,Watsom, Konidaris, Mohammad in there paper compare the performance and limitations of a downsizedformula SAE engine in normally aspirated, supercharged and turbocharged modes [8]. Similar papers Uthoffand Yakimow [8] compare supercharging and turbocharging but all look at the performance of the enginesthey fitted to and not the actual performance of the supercharger.

B Manifolds

The supercharger requires inlet and outlet manifolds to be designed for the Eaton M24 supercharger.The manifold systems for FSAE engines is well documented and it is not the intension of this thesis to dodetailed manifold research, however research into manifold systems is needed to ensure test data is accurate.The inlet manifold is to have a 20mm restriction as per FSAE rules and is to also have a section of the inletmanifold design specifically for test equipment, these being pressure gauges and thermocouples and needto be fitted as per the design requirements. For this research several papers were recommended by FSAEADFA members who have done or who are doing manifold research. These papers have been read and willbe used extensively for manifold design and analysis [9, 10, 11, 12]

III What has been done

To manage this project system engineering methodologies will be used to assist in the progression and thatthe desired outcomes for this project are met to the stakeholder’s requirements. As stated in the aim ofthis thesis it is required to design and build a test bench to measure the performance of an Eaton M24supercharger which can also be utilised in further experiments. The supercharger test bench system is

3


broken up into two phases, the acquisition phase and the utilization phase which will be further brokendown into activities Figure 2. The acquisition phase will be separated into four activities, conceptual design,preliminary design, detailed design and development and construction and the utilization phase into oneactivity the operational use and system support.

ConceptualDesign

PreliminaryDesign

Detailed Designand

Development

Operational Useand System

Support

Constructionand/or

Production

ACQUISITIONPHASE

UTILIZATIONPHASE

NEED

DISPOSDevelopment SupportProduction

D SAL

Figure 2: System Life Cycle

A Conceptual Design

1 Top down approach

Most engineering designs are based on bottom up designs where well known subsystems are assembledinto the system [13]. For this project a summarised top down approach will be used, this starts by lookingat the client brief and defining needs, goals, objectives and deliverables. After these points have been agreedto the system will be addressed as a whole which will include the initial allocation of requirements as well assubsequent analysis of the system and its interfaces [13]. Once the system levels are understood the systemwill be further broken down into subsystems until the whole system is understood. A system to test thesupercharger performance was conceived and can be seen in Figure 3.

Torque meterMotor Main shaft and Bearingpillow blocks

Supercharger

Drive pulley

Bench top with superchargeroutlet protruding

Test Bench Chassis75x50x3 RHS

Motor Support200x100x5 RHS

Torque Transducer tomain shaft CoupleMotor to Torque Transducer

Shear pin couple

Figure 3: Test bench concept

2 Need

To measure the performance of an M24 Eaton supercharger which is fitted with an inlet manifold whichcompiles to FSAE rules and an outlet manifold to simulate a range of engine volumetric efficiency.

3 Goals

Design the system to accurately take measurements from the inlet and outlet manifolds of the superchargerand to measure the actual performance of the supercharger which comply with a modified SAE J1723supercharging testing standard.

4 Objectives

The aim of this thesis project is to design and build a system to measure the performance of a M24 Eatonsupercharger which has a restricted inlet of 20mm in diameter as required by FSAE rules and an enginewith a range of volumetric efficiency on the outlet side of the supercharger. Testing the performance ofthe supercharger will incorporate finding the mass flow rate, volume flow rates, power required to drive thesupercharger, adiabatic efficiency, actual efficiency, pressure ratios across the supercharger and intermediatetemperatures and pressures. Test conducted in accordance with the modified Supercharger testing standardJ1723.

4


5 Deliverables

1. Design documents for the test bench system.2. Test bench3. Inlet and outlet manifolds4. Test results5. Processed supercharger characteristic diagrams.

6 System Requirements

The system must be accurate in order to get precise results from the testing and must be able to interfacewith computer software for analysis such as MATLAB or SIMULINK. For the system specifications anddesign a modified version of the J1723 supercharger testing standard will be used to correctly set up thesystem which ensures quality control and reduces the risk of incorrect analysis.

B System specifications

1 Torque meter

The torque meter must be fitted as close as possible to the supercharger input shaft as possible. Anylosses between the torque meter and the supercharger input shaft must be calculated and subtracted outbut cannot exceed 5 % of the total torque required to turn the supercharger. All losses must be quantifiedand noted in the test data [5].

2 Instrumentation Accuracy

The following minimum instrumentation accuracy is required [5].a. Torque ± 0.5 of the measured valueb. Speed ± 0.2 of the measured valuec. Temperature ± 1 ◦Cd. Air supply, inlet and exhaust pressures ± 0.5 kPae. Air flow ± 1 of the measured value

3 Inlet Pressure Measurements

Inlet pressure measurements shall be the average of the two absolute pressure measurements at 1 and 2diameters from the entrance transition of the supercharger. Pressure transducers must be inserted perpen-dicular to the walls of the tube and at 90◦ and 270◦ relative to the inlet thermocouples [5].

4 Outlet Pressure Measurements

Outlet pressure measurement shall be the average of two absolute pressure measurements at 1 and 2diameters from the exit transition of the supercharger. Pressure transducers must be inserted perpendicularto the walls of the tube and at 45◦ from the thermocouples and at least 90◦ from each other [5].

5 Inlet temperature Measurements

Two inlet temperature measurements shall be made between 2 and 3 diameters from the entrance tran-sition of the supercharger. The thermocouples must be inserted perpendicular to the walls of the tube and180◦ relative to each other. They must also be inserted at approximately 1/3 the diameter of the tube [5].

6 Outlet temperature Measurements

Outlet temperature measurement shall be made between 2 and 3 diameters after the exit transition ofthe supercharger; at 0◦, 90◦, 180◦, and 270◦. The thermocouples must be inserted perpendicular to the pipewalls with the thermocouples at 0◦ and 180◦ inserted 1/3 the diameter of the tube, the thermocouples at90◦ inserted 1/2 the diameter of the tube, and the thermocouple at 270◦ inserted 1/4 the diameter of thetube. The average of these temperatures shall represent the outlet temperature of the supercharger. No

5


cooling devices or fans may be used on the exhaust system to reduce the outlet temperature prior to theoutlet temperature measurement [5].

7 Manifold design

The inlet and outlet manifolds need to be designed for the M24 Eaton supercharger. The inlet manifoldmust comply with the FSAE standards and needs to be designed so that measuring instruments such aspressure gauges and air temperature gauges can be fitted without restricting the air flow. The exhaustmanifold must be designed in such a way that further experiments must also be carried out. The outletmanifold must be designed so a simulated valve system can be fitted to the outlet. Both manifolds are alsorequired comply to with the modified SAE supercharger testing standard.

C Preliminary Design

The supercharger test bench is broken up into system requirements subsystems. The first task of thepreliminary design is to calculate the estimated power required to power the supercharger to achieve sonicair flow through a 20 mm inlet restriction with the engine volumetric efficiencies of .6 to .9 and an enginerpm range of 6000 to 12000 rpm. From these estimated power calculations the worst case scenario is used asthe minimum power requirement with a safety factor of 10%. After this was done the test bench has beenbroken down into seven sub-systems as shown in figure 4. The subsystems were all researched and developwith many of the sub-systems having multiple designs and concepts to minimise risk and to increase thelikely hood of success of the project. A bibliography of annotated papers read for these designs and conceptscan be seen in Annex B Research and Development.

Supercharger TestBench

Test BenchChassisMotor

SuperchargerPosition And

Mounting

TorqueMeasurement

Size AndMaterial To BeA/C 3 Phase Torque

Top OfSupercharger

Protruding

Estimatepower

required

Main ShaftDesign

Material AndStress

Safety SystemTo Protect

SuperchargerAnd TorqueTransducer

Shear PinDesign And

DesignCouples

TorqueTransducer to

SupportBearings

Material To BeUsed11kw Transducer

Design AndBuild Mounts

For PillowBlocks

MaximumTorque, Rpm

Range

ProtrudingFrom Bench

Top

TorqueTransducer

Interface WithSimulink

Interface A/CMotor

Controller

75x50x3AS1163Chassis200x100

Motor Support

Design AndMaterial To Be

Used

StressAnalysis

Design AndTest

Motor PowerCut Off Switch

Material AndStress

Analysis

Transducer toMain Shaft

TorqueTransducer toMotor Shaft

Figure 4: Test Bench Subsystem Diagram

1 Motor

After consulting with several companies a motor was sourced from G and R National Electric MotorsPty. Ltd. in Pakenham Victoria. The motor purchased was a Teco 11KW A/C 3 phase electric motor 415v2950rpm. This motor was adequate in size to power the supercharger and was purchased at a good price.

2 Test Bench

The test bench chassis is made of 75 × 50 × 3 RHS and has a ply wood bench top. The AC motor, torquetransducer and main shaft assembly are mounted on two 200 × 100 × 5 RHS sections.

6


3 Torque Transducer

Torque measurement is to be achieved by a Vibrac optical torque transducer series 2 TQ5120. The unitspecifications can be found in Annex E.

4 Couples

The couples are designed to transmit drive from the motor to the torque transducer and the torquetransducer to the main shaft with the couples being made from 709-M steel. The couple between the motorand the torque transducer has been designed with a shear pin to reduce the risk of the torque transducerand supercharger being damaged if the system has a mechanical failure. The design for the shear pin couplewill not be finalised until results from shear pin testing are complete.

5 Safety systems

A risk assessment has been carried out on the supercharger test bench following the risk managementprocedure, the risk assessment can be seen in Annex D. From this assessment and consultation with MrSimon Parcell the test bench is to be located in the engine dyno bay area when testing is being carried outas the dyno bay is already enclosed. A heavy safety mesh is also to be placed around the test bench tocontain debris from mechanical failure. The motor and AC controller will have an emergency cut off switchfitted into the system, this is a manual means to cut power to the test bench in case of emergency. To protectthe supercharger and torque transducer from damage in case of a mechanical failure a shear pin system hasbeen designed into the motor to torque transducer couple.

6 Support Bearings

The support bearings for the main shaft were bought off the shelf to suit a 40mm shaft.

7 Supercharger Position and Mounting

From the risk assessment on the test bench it was decided that only the top of the supercharger neededto protrude from the bench top. This gives two advantages, one that the bench top is left uncluttered andthat the supercharger is contained under the bench were it could be surrounded by safety mesh.

8 Pulley Design

The main pulley which will be the driving pulley for the supercharger is required to be precisely machined.The standard for this type of pulley has been purchased so that these strict tolerances can be followed. Adesign of the pulley has been completed but has not yet been finalised.

9 Main Shaft

The main shaft is made from 709-M steel, a preliminary design of the shaft has been done but no stressanalysis has been performed.

D Detailed design and development

All of the systems are to be designed in catia, this gives an excellent visual representation of the testbench and also allows for individual components to be stress tested.

1 Motor

The AC motor and the two 200 × 100 × 5 RHS sections were measured and then modelled in catia. The200 × 100 × 5 RHS (motor support) was then designed to hold the motor, this is done by four holes whichprecisely match those that are on the motor, the reverse side of the RHS also had holes designed into itbut at a 3 times the diameter of the motor mount holes, this is allow for a socket to access the retainer nutthrough the RHS. The motor will be retained by four M12 Unbrako bolts fitted with washers and lock nuts.The motor is to be wired into the Zener VSC2000 high performance motor speed controller and have an

7


emergency stop bottom in the circuit. The main power supply used for the motor will come from the busbar in the vibration laboratory in building 17.

2 Test Bench

The test bench chassis is made of 75 × 50 × 3 RHS. The frame will support the motor support RHS, housethe supercharger and accessories and have a ply timber bench top. To move the bench lifting points will befitted and ample ground clearance given to allow pallet jacks to move the bench around. The bench heightis 740mm, this height is based on a good working height for a sitting position based on a 50th percentilemale [14].

3 Torque Transducer

The torque transducer will be mounted directly to the AC motor. The transducer will be held in placeby the couples and a transducer retainer bracket. The bracket has been designed but has not been finalised.The retainer bracket will be mounted onto a block which will be machined for the correct height at the timeof fitting to get the precise height. The block will be mounted directly to a 75 × 50 × 3 RHS cross sectionwhich will be welded directly to the motor support.

4 Couples

The couples have been designed and a stress analysis has been completed in catia. The motor to torquetransducer couple has been designed with a shear pin to reduce the risk of the torque transducer andsupercharger being damaged by a system failure, the designs have not been finalised as the shear testingrig experiments have not been completed. The couple’s minimum key length was calculated using a designprocedure for parallel keys [15].

5 Safety system

To decrease the risk of damage to the torque transducer and supercharger if the system fails a shear pinwas designed for the system. The shear pin is located in the motor to torque transducer couple. The pin is a1/48 taper pin made from 2011-T3 aluminium, the pin is designed to fail when a torque range is reached, inthis case 35 Nm, this figure is just below the maximum torque of the torque transducer. As the calculationsfor shear pins are not reliable an estimation of the average diameter was calculated and a test rig designed.Once the rig has been manufactured experiments will be carried out with the assistance of Mr Pat Nolan inthe Materials Testing lab.

6 Support Bearings

The support bearings for the main shaft were bought off the shelf to suit a 40mm shaft, this was thecheapest option.

7 Supercharger Position and Mounting

From the risk assessment report on the test bench it was decided that only the top of the superchargerneeded to protrude from the bench top. This is to keep all of the rotating parts underneath the benchwhich will be caged in heavy wire mesh. The mesh cage is yet to be designed. Preliminary designs for thesupercharger mounts have been completed but not finalised.

8 Pulley Design

The main pulley will have a diameter of approximately 300mm. This will be made from aluminium plateand will be machined to the SAE J1459 V-ribbed belts and pulleys standard. The pulley has been designedin accordance to the standard but the design has not been completed.

8


9 Main Shaft

The main shaft has had a preliminary design done but further work is required before this design can befinalised.

IV Future Work

With the conceptual design and preliminary designs completed work will continue on the detailed designand development of the project. This will include the main shaft, main pulley, supercharger mounts andthe drive belt pulley tensioner which still requires detailed design. The inlet and outlet manifolds have beenresearched but the manifolds still require detailed design and analysis performed on then before they can bemade, the milestone for which is on the 19 May 2009. The AC motor and the Zener motor speed controllerhave been tested and a job has been submitted for the technical support group to have this system wiredcorrectly with the safety switch. The 2nd of May milestone will see the commencement of the test benchchassis construction with the motor support and motor being fitted to the chassis. At the beginning of themid semester break the engineering system will be closely examined, this will include the altering of theGantt-Milestone chart as required to correct for design delays and misadventure, this will also include adetailed look at the test bench subsystems to ensure all of the system needs have been addressed to achievethe project aims.

V Summary

The first stage of the thesis project has been developed through system engineering to fully understand whatis required from this project and the client brief was used to define needs, goals, objectives and deliverablesfor this project. A literature review was then done on supercharger performance and many papers werefound on and about superchargers covering several areas but no paper had been written on the performanceof a supercharger which must comply with Formula SAE rules. After the review was complete a top downapproach was used to manage the project and to ensure that the stakeholder’s requirements are met. Thisincluded a system life cycle which has been used to manage the project. The life cycle is broken up into twophases, the acquisition phase and the utilization phase which is further broken down into activities. Thismethod has been used for the conceptual design, preliminary design, detailed design and development andconstruction and will be used for the utilization phase of the project to ensure the stakeholder’s requirementsare met.

References

[1] Everest Transmission Technical Team. Understanding twin lobe blowers roots blowers. 2005.

[2] Mimmi g. and Pennacchi .P. Analytical model of a particular type of positive displacement blower. Proceedingsof the Institution of Mechanical Engineers, 213 Part C, 1999.

[3] Sorenson S.C. Simulation of a positive displacement supercharger. SAE Technical Paper Series, 1984.

[4] Revised by a staff of specialist. Marks Standard Handbook for Mechanical Engineers. McGraw Hill, eleventhedition, 2007.

[5] The Engineering Society For Advancing Mobility Land Sea Air and Space. Supercharger Testing Standard. SAE,2005-08.

[6] Singer D.A. Comparison of a supercharger vs. a turbocharger in a small displacement gasoline engine application.SAE Technical Paper Series, 1985.

[7] Uthoff L.H. and Yakimow J.W. A development of the eaton supercharger. SAE Technical Paper Series, 1987.

9


[8] Attard W., Watsom H.C., Konidaris S., and Mohammad A.K. Comparing the performance and limitations ofa downsized sae engine in normally aspirated, supercharged and turbocharged modes. SAE Technical PaperSeries, 2006.

[9] Jawad B.A., Hoste J.P, and Johnson B.E. Intake system design for a formula sae application. SAE TechnicalPaper Series, 2001.

[10] Su H, Kempf J, Montgomery B, and Grimes R. Cae virtual test of air intake manifolds using coupled vibrationand pressure pulsation loads. SAE Technical Paper Series, 2005.

[11] Jawad B.A, DeGain M.D, and Young jr A.P. Design of restricted induction sytyem for a high speed four cylinderengine. SAE Technical Paper Series, 2006.

[12] Cauchi J and Farrugia M. Engine simulation of a restricted fsae engine, focusing on restrictor modelling. SAETechnical Paper Series, 2006.

[13] Faulconbridge R., Ryan, and Michael J. Engineering A System: Managing Complex Technical Projects. ArgosPress, 2005.

[14] Sutton Tools Pty. Ltd. Engineers Black Book. Pat Rapp Enterprises, second edition, 2004.

[15] Robert L. Mott. Machine Elements in Mechanical Design. Prentice Hall, fourth edition, 2004.

10


constant mass flow rate supercharger performance for fsae

Documents