ATE93929T1 - Verfahren und vorrichtung zum bremsen eines viertaktverbrennungsmotors. - Google Patents

Verfahren und vorrichtung zum bremsen eines viertaktverbrennungsmotors.

Info

Publication number
ATE93929T1
ATE93929T1 AT90903446T AT90903446T ATE93929T1 AT E93929 T1 ATE93929 T1 AT E93929T1 AT 90903446 T AT90903446 T AT 90903446T AT 90903446 T AT90903446 T AT 90903446T AT E93929 T1 ATE93929 T1 AT E93929T1
Authority
AT
Austria
Prior art keywords
stroke
exhaust system
pct
exhaust
combustion chamber
Prior art date
Application number
AT90903446T
Other languages
English (en)
Inventor
Ulrich Gobert
Ola Danielson
Original Assignee
Volvo Ab
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Family has litigation
First worldwide family litigation filed litigation Critical https://patents.darts-ip.com/?family=20375056&utm_source=google_patent&utm_medium=platform_link&utm_campaign=public_patent_search&patent=ATE93929(T1) "Global patent litigation dataset” by Darts-ip is licensed under a Creative Commons Attribution 4.0 International License.
Application filed by Volvo Ab filed Critical Volvo Ab
Application granted granted Critical
Publication of ATE93929T1 publication Critical patent/ATE93929T1/de

Links

Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02D—CONTROLLING COMBUSTION ENGINES
    • F02D13/00—Controlling the engine output power by varying inlet or exhaust valve operating characteristics, e.g. timing
    • F02D13/02—Controlling the engine output power by varying inlet or exhaust valve operating characteristics, e.g. timing during engine operation
    • F02D13/04—Controlling the engine output power by varying inlet or exhaust valve operating characteristics, e.g. timing during engine operation using engine as brake
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01L—CYCLICALLY OPERATING VALVES FOR MACHINES OR ENGINES
    • F01L13/00—Modifications of valve-gear to facilitate reversing, braking, starting, changing compression ratio, or other specific operations
    • F01L13/06—Modifications of valve-gear to facilitate reversing, braking, starting, changing compression ratio, or other specific operations for braking
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01L—CYCLICALLY OPERATING VALVES FOR MACHINES OR ENGINES
    • F01L1/00—Valve-gear or valve arrangements, e.g. lift-valve gear
    • F01L1/02—Valve drive
    • F01L1/04—Valve drive by means of cams, camshafts, cam discs, eccentrics or the like
    • F01L1/08—Shape of cams
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02B—INTERNAL-COMBUSTION PISTON ENGINES; COMBUSTION ENGINES IN GENERAL
    • F02B75/00—Other engines
    • F02B75/02—Engines characterised by their cycles, e.g. six-stroke
    • F02B2075/022—Engines characterised by their cycles, e.g. six-stroke having less than six strokes per cycle
    • F02B2075/027—Engines characterised by their cycles, e.g. six-stroke having less than six strokes per cycle four

Landscapes

  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • Output Control And Ontrol Of Special Type Engine (AREA)
  • Valve Device For Special Equipments (AREA)
  • Electric Propulsion And Braking For Vehicles (AREA)
  • Hybrid Electric Vehicles (AREA)
  • Regulating Braking Force (AREA)
  • Control Of Throttle Valves Provided In The Intake System Or In The Exhaust System (AREA)
AT90903446T 1989-02-15 1990-02-15 Verfahren und vorrichtung zum bremsen eines viertaktverbrennungsmotors. ATE93929T1 (de)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
SE8900517A SE466320B (sv) 1989-02-15 1989-02-15 Foerfarande och anordning foer motorbromsning med en fyrtakts foerbraenningsmotor
PCT/SE1990/000102 WO1990009514A1 (en) 1989-02-15 1990-02-15 A method and a device for engine braking a four stroke internal combustion engine

Publications (1)

Publication Number Publication Date
ATE93929T1 true ATE93929T1 (de) 1993-09-15

Family

ID=20375056

Family Applications (1)

Application Number Title Priority Date Filing Date
AT90903446T ATE93929T1 (de) 1989-02-15 1990-02-15 Verfahren und vorrichtung zum bremsen eines viertaktverbrennungsmotors.

Country Status (13)

Country Link
US (1) US5146890A (de)
EP (1) EP0458857B1 (de)
JP (1) JP2931090B2 (de)
KR (1) KR0158458B1 (de)
AT (1) ATE93929T1 (de)
AU (1) AU637352B2 (de)
BR (1) BR9007110A (de)
CA (1) CA2047219C (de)
DE (1) DE69003094T2 (de)
DK (1) DK0458857T3 (de)
ES (1) ES2044564T3 (de)
SE (1) SE466320B (de)
WO (1) WO1990009514A1 (de)

Families Citing this family (91)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
IT1255447B (it) * 1991-11-08 1995-10-31 Iveco Fiat Motore provvisto di un dispositivo di frenatura continua, particolarmente per un veicolo industriale.
SE470363B (sv) * 1992-06-17 1994-01-31 Volvo Ab Förfarande och anordning för motorbromsning med en flercylindrig förbränningsmotor
SE501193C2 (sv) * 1993-04-27 1994-12-05 Volvo Ab Avgasventilmekanism i en förbränningsmotor
US5406918A (en) * 1993-08-04 1995-04-18 Hino Jidosha Kogyo Kabushiki Kaisha Internal combustion engine
SE502614C2 (sv) * 1994-03-29 1995-11-20 Volvo Ab Anordning för reglering av motorbromseffekten hos en förbränningsmotor
US5540201A (en) 1994-07-29 1996-07-30 Caterpillar Inc. Engine compression braking apparatus and method
US5647318A (en) 1994-07-29 1997-07-15 Caterpillar Inc. Engine compression braking apparatus and method
US5526784A (en) 1994-08-04 1996-06-18 Caterpillar Inc. Simultaneous exhaust valve opening braking system
US5718199A (en) * 1994-10-07 1998-02-17 Diesel Engine Retarders, Inc. Electronic controls for compression release engine brakes
US5479890A (en) * 1994-10-07 1996-01-02 Diesel Engine Retarders, Inc. Compression release engine brakes with electronically controlled, multi-coil hydraulic valves
US5537975A (en) * 1994-10-07 1996-07-23 Diesel Engine Retarders, Inc. Electronically controlled compression release engine brakes
US5511460A (en) * 1995-01-25 1996-04-30 Diesel Engine Retarders, Inc. Stroke limiter for hydraulic actuator pistons in compression release engine brakes
US5619965A (en) 1995-03-24 1997-04-15 Diesel Engine Retarders, Inc. Camless engines with compression release braking
JP4129489B2 (ja) * 1995-08-08 2008-08-06 ジェイコブス ビークル システムズ、インコーポレイテッド カムと電子液圧エンジンバルブとの組み合わせ制御を有する内燃機関
US6125828A (en) * 1995-08-08 2000-10-03 Diesel Engine Retarders, Inc. Internal combustion engine with combined cam and electro-hydraulic engine valve control
US5829397A (en) 1995-08-08 1998-11-03 Diesel Engine Retarders, Inc. System and method for controlling the amount of lost motion between an engine valve and a valve actuation means
US5746175A (en) * 1995-08-08 1998-05-05 Diesel Engine Retarders, Inc. Four-cycle internal combustion engines with two-cycle compression release braking
US5537976A (en) * 1995-08-08 1996-07-23 Diesel Engine Retarders, Inc. Four-cycle internal combustion engines with two-cycle compression release braking
IT1280907B1 (it) * 1995-08-11 1998-02-11 Iveco Fiat Motore endotermico per un veicolo e relativo metodo di controllo
US5626116A (en) * 1995-11-28 1997-05-06 Cummins Engine Company, Inc. Dedicated rocker lever and cam assembly for a compression braking system
US5586531A (en) * 1995-11-28 1996-12-24 Cummins Engine Company, Inc. Engine retarder cycle
GB2341638B (en) * 1995-11-28 2000-06-07 Cummins Engine Co Inc An engine retarder cycle for operation of an engine brake
US5724939A (en) * 1996-09-05 1998-03-10 Caterpillar Inc. Exhaust pulse boosted engine compression braking method
DE19637999A1 (de) * 1996-09-18 1998-03-19 Daimler Benz Ag Verfahren zum Betreiben einer Motorbremse und Vorrichtung zur Durchführung des Verfahrens
SE507506C2 (sv) * 1996-10-03 1998-06-15 Volvo Lastvagnar Ab Turbocompoundmotor med kompressionsbroms
US5787858A (en) * 1996-10-07 1998-08-04 Meneely; Vincent Allan Engine brake with controlled valve closing
AU694703B2 (en) * 1996-10-11 1998-07-23 Mitsubishi Fuso Truck And Bus Corporation Engine-brake assisting system
US5809964A (en) 1997-02-03 1998-09-22 Diesel Engine Retarders, Inc. Method and apparatus to accomplish exhaust air recirculation during engine braking and/or exhaust gas recirculation during positive power operation of an internal combustion engine
DE19728350A1 (de) * 1997-07-03 1998-12-03 Daimler Benz Ag Verfahren und Vorrichtung zur Erhöhung der Motorbremsleistung von Brennkraftmaschinen
US5996550A (en) * 1997-07-14 1999-12-07 Diesel Engine Retarders, Inc. Applied lost motion for optimization of fixed timed engine brake system
DE19733322A1 (de) 1997-08-01 1999-02-04 Mwp Mahle J Wizemann Pleuco Gm Ventilsteuerung einer als Antrieb oder Bremse schaltbaren Brennkraftmaschine
KR20010031821A (ko) 1997-11-04 2001-04-16 디이젤 엔진 리타더스, 인코포레이티드 공전 밸브 작동기 시스템
US6647954B2 (en) * 1997-11-17 2003-11-18 Diesel Engine Retarders, Inc. Method and system of improving engine braking by variable valve actuation
KR100564296B1 (ko) * 1997-11-21 2006-03-29 디이젤 엔진 리타더스, 인코포레이티드 최소한 포지티브 동력 작동 모드 및 엔진 제동 작동 모드를 갖는 내연기관 및 내연기관 작동방법
US6085705A (en) 1997-12-11 2000-07-11 Diesel Engine Retarders, Inc. Variable lost motion valve actuator and method
US8820276B2 (en) 1997-12-11 2014-09-02 Jacobs Vehicle Systems, Inc. Variable lost motion valve actuator and method
US6293237B1 (en) 1997-12-11 2001-09-25 Diesel Engine Retarders, Inc. Variable lost motion valve actuator and method
US6000374A (en) * 1997-12-23 1999-12-14 Diesel Engine Retarders, Inc. Multi-cycle, engine braking with positive power valve actuation control system and process for using the same
US6718940B2 (en) 1998-04-03 2004-04-13 Diesel Engine Retarders, Inc. Hydraulic lash adjuster with compression release brake
FR2780446B1 (fr) * 1998-06-29 2000-12-29 Inst Francais Du Petrole Procede de controle de l'admission d'un moteur turbo-compresse et moteur associe
DE19833147C2 (de) * 1998-07-23 2000-05-31 Daimler Chrysler Ag Verfahren zur Einstellung der Motorbremsleistung einer aufgeladenen Brennkraftmaschine
US6234143B1 (en) 1999-07-19 2001-05-22 Mack Trucks, Inc. Engine exhaust brake having a single valve actuation
ATE456737T1 (de) 1999-09-10 2010-02-15 Diesel Engine Retarders Inc Kipphebelsystem mit totgang und integrierter motorbremse
US6394050B1 (en) 1999-09-15 2002-05-28 Diesel Engine Retarders, Inc. Actuator piston assembly for a rocker arm system
US6394067B1 (en) 1999-09-17 2002-05-28 Diesel Engine Retardersk, Inc. Apparatus and method to supply oil, and activate rocker brake for multi-cylinder retarding
WO2001020150A1 (en) 1999-09-17 2001-03-22 Diesel Engine Retarders, Inc. Captive volume accumulator for a lost motion system
US6216667B1 (en) 1999-11-12 2001-04-17 Frank J. Pekar Method and device for a supercharged engine brake
EP1242735B1 (de) 1999-12-20 2010-10-13 Jacobs Vehicle Systems, Inc. Verfahren und vorrichtung zum hydraulischen an- und loskoppeln einer motorbremse mittels totgang
US6386160B1 (en) 1999-12-22 2002-05-14 Jenara Enterprises, Ltd. Valve control apparatus with reset
US6321717B1 (en) 2000-02-15 2001-11-27 Caterpillar Inc. Double-lift exhaust pulse boosted engine compression braking method
US6868824B2 (en) * 2000-03-31 2005-03-22 Diesel Engine Retarders, Inc. System and method of gas recirculation in an internal combustion engine
US6584954B2 (en) 2000-03-31 2003-07-01 Robb Janak Use of external exhaust gas recirculation (“EGR”) to improve compression release braking and method for EGR valve and system cleaning
DE10020884A1 (de) 2000-04-28 2001-10-31 Mahle Ventiltrieb Gmbh Steuereinrichtung für ein Ein- oder Auslaßventil eines Verbrennungsmotors
WO2002018761A1 (en) 2000-08-29 2002-03-07 Jenara Enterprises Ltd. Apparatus and method to oprate an engine exhaust brake together with an exhaust gas recirculation system
US6446598B1 (en) 2000-12-11 2002-09-10 Caterpillar Inc. Compression brake actuation system and method
US6609495B1 (en) 2000-12-19 2003-08-26 Caterpillar Inc Electronic control of engine braking cycle
US6691674B2 (en) 2001-06-13 2004-02-17 Diesel Engine Retarders, Inc. Latched reset mechanism for engine brake
US6622694B2 (en) * 2001-07-30 2003-09-23 Caterpillar Inc Reduced noise engine compression release braking
US6418720B1 (en) 2001-08-31 2002-07-16 Caterpillar Inc. Method and a device for engine braking a four stroke internal combustion engine
US6805093B2 (en) 2002-04-30 2004-10-19 Mack Trucks, Inc. Method and apparatus for combining exhaust gas recirculation and engine exhaust braking using single valve actuation
SE522146C2 (sv) * 2002-05-07 2004-01-20 Volvo Lastvagnar Ab Metod för regenerering av ett partikelfilter vid motorbromsning samt fordon i vilket en sådan metod utnyttjas
DE10250771B4 (de) * 2002-10-30 2014-09-11 Iav Gmbh Ingenieurgesellschaft Auto Und Verkehr Motorbremseinrichtung und Verfahren zu deren Steuerung
SE525932C2 (sv) 2003-11-27 2005-05-31 Volvo Lastvagnar Ab Förfarande vid bromsning med en förbränningsmotor med variabelt turboaggregat
CN1839250B (zh) * 2004-07-17 2011-01-05 玛勒阀门有限公司 用于阀,尤其是内燃机的气体交换阀的控制器
US7954465B2 (en) * 2004-08-17 2011-06-07 Jacobs Vehicle Systems, Inc. Combined exhaust restriction and variable valve actuation
DE102007060822A1 (de) 2007-12-18 2009-06-25 Daimler Ag Verfahren zum Steuern einer Motorbremseinrichtung einer Brennkraftmaschine und Brennkraftmaschine für ein Kraftfahrzeug
AT506338A1 (de) * 2008-02-04 2009-08-15 Man Nutzfahrzeuge Oesterreich Verfahren zur regenerierung eines im abgasstrang eines fahrzeug-dieselmotors angeordneten partikelfilters
US7789065B2 (en) * 2008-07-09 2010-09-07 Zhou Yang Engine braking apparatus with mechanical linkage and lash adjustment
US20100037854A1 (en) 2008-08-18 2010-02-18 Zhou Yang Apparatus and method for engine braking
CN102003240B (zh) * 2009-08-31 2013-01-16 上海尤顺汽车部件有限公司 一种发动机制动装置的改进结构
WO2010078280A2 (en) * 2009-01-05 2010-07-08 Shanghai Universoon Autoparts Co., Ltd Engine braking devices and methods
EP2444602B1 (de) * 2009-01-05 2015-06-24 Shanghai Universoon Autoparts Co., Ltd Motorbremsvorrichtungen und -verfahren
US7984705B2 (en) 2009-01-05 2011-07-26 Zhou Yang Engine braking apparatus with two-level pressure control valves
US7712449B1 (en) * 2009-05-06 2010-05-11 Jacobs Vehicle Systems, Inc. Lost motion variable valve actuation system for engine braking and early exhaust opening
CN102261283B (zh) 2010-05-27 2013-10-09 上海尤顺汽车部件有限公司 一种固链式发动机制动装置
CN107829791B (zh) * 2010-07-27 2021-01-05 雅各布斯车辆系统公司 组合发动机制动和正功率发动机空动阀致动系统
WO2013005070A1 (en) 2011-07-06 2013-01-10 Renault Trucks Valve actuation mechanism and automotive vehicle comprising such a valve actuation
WO2013014490A1 (en) 2011-07-22 2013-01-31 Renault Trucks Valve actuation mechanism and automotive vehicle comprising such a valve actuation mechanism
CN103688028B (zh) 2011-07-22 2016-10-19 沃尔沃卡车集团 阀致动机构和包括这种阀致动机构的机动车辆
US9163534B2 (en) 2011-07-22 2015-10-20 Volvo Trucks AB Valve actuation mechanism and automotive vehicle comprising such a valve actuation mechanism
CN104685170B (zh) 2012-09-25 2017-06-30 沃尔沃卡车集团 阀促动机构和配备有此阀促动机构的机动车辆
SE539214C2 (sv) * 2013-12-05 2017-05-16 Scania Cv Ab Förbränningsmotor, fordon som innefattar en sådan förbränningsmotor och förfarande för att styra en sådan förbränningsmotor
DE102013022037A1 (de) * 2013-12-20 2015-06-25 Daimler Ag Verfahren zum Betreiben einer Hubkolben-Verbrennungskraftmaschine
DE102015016526A1 (de) * 2015-12-19 2017-06-22 Daimler Ag Verfahren zum Betreiben einer Hubkolben-Verbrennungskraftmaschine
SE540139C2 (en) * 2016-07-11 2018-04-10 Scania Cv Ab Method of changing gear ratio in a gearbox of a vehicle
US10859007B2 (en) 2016-10-06 2020-12-08 Volvo Truck Corporation Internal combustion engine and a method for controlling a braking torque of the engine
DE102016015457A1 (de) * 2016-12-22 2018-06-28 Daimler Ag Verfahren zum Betreiben einer Hubkolben-Verbrennungskraftmaschine
US11220963B2 (en) 2017-03-16 2022-01-11 Volvo Truck Corporation System for improved engine braking
DE102017004819A1 (de) * 2017-05-18 2018-11-22 Man Truck & Bus Ag Betriebsverfahren für ein Fahrerassistenzsystem und Kraftfahrzeug
KR20200063453A (ko) * 2018-11-28 2020-06-05 현대자동차주식회사 자동차의 배기브레이크 자동 제어 시스템 및 방법
WO2021098959A1 (en) 2019-11-20 2021-05-27 Volvo Truck Corporation Method for controlling engine braking of an internal combustion engine

Family Cites Families (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE227925C (de) *
DE202998C (de) *
CH118905A (de) * 1926-02-11 1927-02-16 Franz Brozincevic & Cie Zum Bremsen benutzbarer Viertaktmotor für Fahrzeuge.
CH310325A (de) * 1952-11-28 1955-10-15 Saurer Ag Adolph Motorbremse an 4-Takt-Brennkraftmaschinen.
DE3003566A1 (de) * 1980-02-01 1981-08-06 Klöckner-Humboldt-Deutz AG, 5000 Köln Bremsvorrichtung fuer eine ventilgesteuerte brennkraftmaschine
DE3428627A1 (de) * 1984-08-03 1986-02-13 Daimler-Benz Ag, 7000 Stuttgart Viertakt-brennkraftmaschine
DE3506894A1 (de) * 1985-02-27 1986-08-28 Klöckner-Humboldt-Deutz AG, 5000 Köln Motorbremsvorrichtung fuer brennkraftmaschinen
AT404288B (de) * 1986-10-30 1998-10-27 Avl Verbrennungskraft Messtech Motorbremse bei einer brennkraftmaschine für kraftfahrzeuge
DE3900739A1 (de) * 1989-01-12 1990-07-19 Man Nutzfahrzeuge Ag Verfahren zur steigerung der motorbremsleistung bei viertakt-hubkolben-brennkraftmaschinen

Also Published As

Publication number Publication date
DK0458857T3 (da) 1994-03-07
BR9007110A (pt) 1991-11-12
SE8900517L (sv) 1990-08-16
CA2047219C (en) 1999-11-30
DE69003094T2 (de) 1994-04-07
EP0458857A1 (de) 1991-12-04
KR0158458B1 (ko) 1998-12-15
SE8900517D0 (sv) 1989-02-15
SE466320B (sv) 1992-01-27
JP2931090B2 (ja) 1999-08-09
WO1990009514A1 (en) 1990-08-23
JPH04503987A (ja) 1992-07-16
KR920701618A (ko) 1992-08-12
AU637352B2 (en) 1993-05-27
EP0458857B1 (de) 1993-09-01
AU5105490A (en) 1990-09-05
DE69003094D1 (de) 1993-10-07
ES2044564T3 (es) 1994-01-01
US5146890A (en) 1992-09-15
CA2047219A1 (en) 1990-08-16

Similar Documents

Publication Publication Date Title
ATE93929T1 (de) Verfahren und vorrichtung zum bremsen eines viertaktverbrennungsmotors.
SE9201855L (sv) Förfarande och anordning för motorbromsning med en flercylindrig förbränningsmotor
EP1136678A3 (de) Steuersystem für eine Brennkraftmaschine mit variabler Ventilsteuerung
ES2000277A6 (es) Un metodo para controlar el ciclo de trabajo de un motor de combustion interna de pistones, de cuatro tiempos
PL313582A1 (en) Method of using a four-stroke internal combustion engine for braking
EP0779427A3 (de) Verfahren zum Ab- und Zuschalten einzelner Zylinder einer Mehrzylinderbrennkraftmaschine
ATE258648T1 (de) Verfahren zum betrieb einer dieselbrennkraftmaschine
SE8700116L (sv) Forfarande och anordning for styrning av arbetsforloppet i en forbrenningskolvmotor
DE3860050D1 (de) Verfahren und vorrichtung zur kontrolle des arbeitszyklus einer brennkraftmaschine.
TW332239B (en) A two-stroke internal combustion engine and a method of controlling the same
ATE75815T1 (de) Viertaktbrennkraftmaschine mit aufladung und verfahren zu ihrem betrieb.
SE9801544L (sv) Anordning för avgasåterföring vid förbränningsmotor samt en motor innefattande en sådan anordning
SE9802510D0 (sv) Förbränningsmotor
SE8903220L (sv) Anordning foer att minska gasvaexlingsfoerluster hos en foerbraenningsmotor
ATE452282T1 (de) Vorrichtung für einen verbrennungsmotor
EP1522690A3 (de) Verbesserte Brennkraftmaschine und Arbeitstakte
ES8405886A1 (es) Motor diesel de pistones proporcionante de un funcionamiento como mando de suministro de aire
SE9503236L (sv) Arrangemang och metod för avgasåterföring i en fyrtakts förbränningsmotor

Legal Events

Date Code Title Description
UEP Publication of translation of european patent specification
EELA Cancelled due to lapse of time