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94-414: Electronic automatic transmissions have been changing in design from purely hydromechanical controls to electronic controls since the late 1980s. Since then, development has been iterative and today designs exist from several stages of electronic automatic transmission control development. Transmission solenoids are a key component to these control units. The evolution of modern automatic transmission and

188-408: A hydrostatic transmission . Directional control valves route the fluid to the desired actuator. They usually consist of a spool inside a cast iron or steel housing. The spool slides to different positions in the housing, and intersecting grooves and channels route the fluid based on the spool's position. The spool has a central (neutral) position maintained with springs; in this position

282-418: A 565 hp (421 kW) diesel engine can produce up to 600 hp (450 kW) of braking power at 2,100 RPM. Normally, during the compression stroke, energy is used as the upward-traveling piston compresses air in the cylinder; the compressed air then acts as a compressed spring and pushes the piston back down. However, with the jake brake in operation, the compressed air is suddenly released just before

376-482: A Jacobs brake or "jake brake"), is the type of brake most commonly confused with real engine braking; it is used mainly in large diesel trucks and works by opening the exhaust valves at the top of the compression stroke, so the large amount of energy stored in that compressed air is not returned to the crankshaft but is released into the atmosphere. It is very effective method of braking, creating large amounts of braking force which significantly extends friction brake life –

470-470: A certain amount of engine braking (viscous losses to the engine oil and air pumped through the engine and friction losses to the cylinder walls and bearings) when no accelerator pedal is applied. The term "engine braking" refers to the braking effect that occurs in gasoline engines when the accelerator pedal is released. This causes fuel injection to cease and the throttle valve to close almost completely, greatly restricting forced airflow from, for example,

564-445: A common example. In this type of machine, hydraulic fluid is pumped to various hydraulic motors and hydraulic cylinders throughout the machine and becomes pressurized according to the resistance present. The fluid is controlled directly or automatically by control valves and distributed through hoses, tubes, or pipes. Hydraulic systems, like pneumatic systems , are based on Pascal's law which states that any pressure applied to

658-417: A few milliseconds to reduce load on the transmission during heavy throttle. This allows automatic transmissions to shift smoothly even on engines with large amounts of torque which would otherwise result in a harder shift and possible damage to the gearbox. The TCU provides information about the health of the transmission, such as clutch wear indicators and shift pressures, and can raise trouble codes and set

752-413: A floating piston. On the one side of the piston there is a charge of pressurized gas, and on the other side is the fluid. Bladders are used in other designs. Reservoirs store a system's fluid. Examples of accumulator uses are backup power for steering or brakes, or to act as a shock absorber for the hydraulic circuit. Also known as tractor fluid , hydraulic fluid is the life of the hydraulic circuit. It

846-429: A fluid inside a closed system will transmit that pressure equally everywhere and in all directions. A hydraulic system uses an incompressible liquid as its fluid, rather than a compressible gas. The popularity of hydraulic machinery is due to the large amount of power that can be transferred through small tubes and flexible hoses, the high power density and a wide array of actuators that can make use of this power, and

940-484: A freewheel device on the transmission to make engine braking optional. Most two-stroke motorcycle engines since the 1970s have had lubrication by an oil pump , independent of the throttle and fuel system, such as Suzuki's Posi-Force system. In electric and hybrid vehicles, electric motors provide the drivetrain resistance, recharging the onboard battery using energy recovered from the vehicle's kinetic motion which would otherwise have been wasted. In hybrid vehicles ,

1034-413: A hydraulic rotary pump with the displacement 10 cc/rev is connected to a hydraulic rotary motor with 100 cc/rev, the shaft torque required to drive the pump is one-tenth of the torque then available at the motor shaft, but the shaft speed (rev/min) for the motor is also only one-tenth of the pump shaft speed. This combination is actually the same type of force multiplication as the cylinder example, just that

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1128-426: A network of tubes of coolant in a thermodynamic system) or to control fluid pressure (as in hydraulic amplifiers). For example, hydraulic machinery uses hydraulic circuits (in which hydraulic fluid is pushed, under pressure, through hydraulic pumps , pipes, tubes, hoses, hydraulic motors , hydraulic cylinders , and so on) to move heavy loads. The approach of describing a fluid system in terms of discrete components

1222-424: A problem due to limited exchange of oil flow. High power closed loop systems generally must have a 'flush-valve' assembled in the circuit in order to exchange much more flow than the basic leakage flow from the pump and the motor, for increased cooling and filtering. The flush valve is normally integrated in the motor housing to get a cooling effect for the oil that is rotating in the motor housing itself. The losses in

1316-484: A ratio between the turbine speed sensor (TSS) and wheel speed sensor (WSS) which is used to determine when to change gears. If either the TSS or WSS fails or malfunctions/becomes faulty, the ratio will be wrong which in return can cause problems like false speedometer readings and transmission slipping. To test these parts, check the resistance to make sure it's within manufacturer specs. Modern automatic transmissions also have

1410-401: A restriction in the exhaust, much like the intake throttle causes in a gasoline engine. In simple terms, it works by increasing the back-pressure of the exhaust. Nearly all of these brakes are butterfly valves similar to a throttle valve, mounted downstream of the turbocharger if there is one. Modern diesels are subject to many strict controls on emissions and often have many obstructions in

1504-508: A shiftlock solenoid to stop a driving range being selected if the brake pedal is not depressed. Modern electronic automatic transmissions have electrical solenoids which are activated to change gears. Simple electronic-control designs (such as Ford's AOD-E, AXOD-E and E4OD) use the solenoids to modify the shift points in an existing valve body, while more advanced designs (such as the Chrysler Ultradrive and its follow-ons) use

1598-519: A similar way to road cars), and are responsible for operating electronic throttle control , clutch and gearshift actuation (via an electric , hydraulic , or pneumatic actuator ), gearshift time and speed , sensors , switches , solenoids , and other hydraulic , pneumatic , and electronic sub-systems that control and constitute the transmission control unit in a racecar. Hydraulic machinery Hydraulic machines use liquid fluid power to perform work. Heavy construction vehicles are

1692-419: A turbocharger. The restriction causes a strong manifold vacuum which the cylinders have to work against, sapping much of the potential energy out of the system over time and producing the majority of the engine-braking effect. This vacuum manifold effect can often be amplified by a down-shift, which induces a faster spinning drivetrain to engage with the engine. Engine braking is a viable method of controlling

1786-409: A unique trademark depending on the manufacturer of the valves, for example "LSC" (Linde Hydraulics), "LUDV" ( Bosch Rexroth Hydraulics) and "Flowsharing" (Parker Hydraulics) etc. No official standardized name for this type of system has been established but flowsharing is a common name for it. Hydraulic pumps supply fluid to the components in the system. Pressure in the system develops in reaction to

1880-463: A wheel speed sensor input to determine the true speed of the vehicle to determine whether the vehicle is going downhill or uphill and also adapt gear changes according to road speeds, and also whether to decouple the torque converter at a standstill to improve fuel consumption and reduce load on running gear. The TPS sensor along with the vehicle speed sensor are the two main inputs for most TCUs. Older transmissions use this to determine engine load, with

1974-444: Is a generally accepted practice and can help save wear on friction brakes. It is even used in some motor sports to reduce the risk of the friction brakes overheating. Additionally, fuel injection engines generally do not use fuel while engine braking. This is known as deceleration fuel cut-off (DFCO). Although no longer in production in most countries, there are still plenty of carbureted engines in service, with which engine braking

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2068-403: Is around 2 MPa (290 psi). If the pump flow is high the extra loss can be considerable. The power loss also increases if the load pressures vary a lot. The cylinder areas, motor displacements and mechanical torque arms must be designed to match load pressure in order to bring down the power losses. Pump pressure always equals the maximum load pressure when several functions are run simultaneously and

2162-519: Is inspired by the success of electrical circuit theory . Just as electric circuit theory works when elements are discrete and linear, hydraulic circuit theory works best when the elements (passive components such as pipes or transmission lines or active components such as power packs or pumps ) are discrete and linear. This usually means that hydraulic circuit analysis works best for long, thin tubes with discrete pumps, as found in chemical process flow systems or microscale devices. The circuit comprises

2256-621: Is no muffler on the intake manifold of the engine, loud enough to disturb the surrounding area. Anecdotally, it sounds similar to a jackhammer , however, the loudness is between 10 and 20 times the sound pressure level of a jackhammer (10 to 13 dB greater). Numerous cities, municipalities, states, and provinces have banned the use of unmuffled compression brakes, which are typically only legal on roads away from populations. In Australia, traffic enforcement cameras are currently being tested that automatically photograph heavy vehicles that use compression braking. An exhaust brake works by causing

2350-418: Is one of the most expensive and sensitive parts of a hydraulic circuit. The hydraulic fluid reservoir holds excess hydraulic fluid to accommodate volume changes from: cylinder extension and contraction, temperature driven expansion and contraction, and leaks. The reservoir is also designed to aid in separation of air from the fluid and also work as a heat accumulator to cover losses in the system when peak power

2444-418: Is stainless steel. Components of a hydraulic system [sources (e.g. pumps), controls (e.g. valves) and actuators (e.g. cylinders)] need connections that will contain and direct the hydraulic fluid without leaking or losing the pressure that makes them work. In some cases, the components can be made to bolt together with fluid paths built-in. In more cases, though, rigid tubing or flexible hoses are used to direct

2538-414: Is surrounded by multiple layers of woven wire and rubber. The exterior is designed for abrasion resistance. The bend radius of hydraulic hose is carefully designed into the machine, since hose failures can be deadly, and violating the hose's minimum bend radius will cause failure. Hydraulic hoses generally have steel fittings swaged on the ends. The weakest part of the high pressure hose is the connection of

2632-473: Is to provide in each component a female-threaded port, on each hose or tube a female-threaded captive nut, and use a separate adapter fitting with matching male threads to connect the two. This is functional, economical to manufacture, and easy to service. Fittings serve several purposes; A typical piece of machinery or heavy equipment may have thousands of sealed connection points and several different types: Engine braking Engine braking occurs when

2726-402: Is used. Reservoirs can also help separate dirt and other particulate from the oil, as the particulate will generally settle to the bottom of the tank. Some designs include dynamic flow channels on the fluid's return path that allow for a smaller reservoir. Accumulators are a common part of hydraulic machinery. Their function is to store energy by using pressurized gas. One type is a tube with

2820-410: Is usually a major problem when using hydrostatic transmissions at high vehicle speeds for longer periods, for instance when transporting the machine from one work place to the other. High oil temperatures for long periods will drastically reduce the lifetime of the transmission. To keep down the oil temperature, the system pressure during transport must be lowered, meaning that the minimum displacement for

2914-410: Is usually petroleum oil with various additives. Some hydraulic machines require fire resistant fluids, depending on their applications. In some factories where food is prepared, either an edible oil or water is used as a working fluid for health and safety reasons. In addition to transferring energy, hydraulic fluid needs to lubricate components, suspend contaminants and metal filings for transport to

Transmission control unit - Misplaced Pages Continue

3008-494: The gearshift , that is activated whenever the internal transmission control unit senses driver touching the gearshift to switch gears, which then primes a sensor or solenoid to impel a clutch servo , and in turn, disengages the clutch actuator so the driver can change gears. The internal clutch actuator in a semi-automatic transmission can be powered by either hydraulic , pneumatic , or electric means. Later examples of clutchless manual transmissions used in road cars include

3102-438: The malfunction indicator lamp on the instrument cluster if a serious problem is found. An output to the cruise control module is also often present to deactivate the cruise control if a neutral gear is selected, just like on a manual transmission . The transmission control unit (TCU) in older automobiles with a clutchless manual transmission (without a clutch pedal) typically consists of an electrical switch connected to

3196-438: The "B" mode acts like a lower gear, using the higher RPM of the internal combustion engine to waste energy, preventing the battery from being overcharged. Almost all electric and hybrid vehicles are able to convert kinetic motion into electricity, i.e. regenerative brakes , but since the internal combustion engine is not used to slow the vehicle when using regenerative braking, it is not the same as engine braking. Engine braking

3290-463: The CP system. The LS system generates a constant power loss related to the regulating pressure drop for the pump regulator : Power loss = Δ p LS ⋅ Q tot {\displaystyle {\text{Power loss}}=\Delta p_{\text{LS}}\cdot Q_{\text{tot}}} The average Δ p L S {\displaystyle \Delta p_{LS}}

3384-627: The Saab Sensonic transmission, used in the 900 NG , and the Ferrari Valeo auto-manual transmission, used in the Mondial T . Both systems used a computer-controlled ECU or microprocessor , connected to a sensor embedded in the gearshift, that would detect when the driver was going to change gear (i.e., by touching the gearshift), and would actuate the clutch automatically, allowing the driver to change gear. Saab's Sensonic system

3478-509: The TCU and the ECU are combined into a single unit as a powertrain control module (PCM). The typical modern TCU uses signals from engine sensors, automatic transmission sensors and from other electronic controllers to determine when and how to shift. More modern designs share inputs or obtain information from an input to the ECU, whereas older designs often have their own dedicated inputs and sensors on

3572-401: The application or release of hydraulic control elements. If the vehicle is fitted with cruise control the TCU may also have a connection to cruise control system. This can modify shift behaviour to take into account the throttle is not being operated by the driver to eliminate unexpected gearchanges when the cruise control is engaged. This is also used to inform the cruise control system about

3666-426: The brakes from overheating or excessive wear. If it is applied before the brakes have been used, it can leave the brakes available to make emergency stops. The desired speed is maintained by using engine braking to counteract gravitational acceleration. Potential transmission wear caused by engine braking can be mitigated by certain techniques. Slipping the clutch to complete a downshift wears the clutch plate as it slows

3760-459: The diesel engine rpm while reducing the vehicle speed in order to increase the available hydraulic power output for the working hydraulics at low speeds and increase the tractive effort. The function is similar to stalling a converter gearbox at high engine rpm. The inch function affects the preset characteristics for the 'hydrostatic' gear ratio versus diesel engine rpm. The closed center circuits exist in two basic configurations, normally related to

3854-407: The efficiency and developments in the software have also improved the characteristics, for example selectable gear shifting programs during operation and more gear steps, giving them characteristics close to the hydrostatic transmission. Hydrostatic transmissions for earth moving machines, such as for track loaders, are often equipped with a separate ' inch pedal ' that is used to temporarily increase

Transmission control unit - Misplaced Pages Continue

3948-426: The engine components. Modern TCUs are so complex in their design and make calculations based on so many parameters that there are an indefinite amount of possible shift behaviours This sensor sends a varying frequency signal to the TCU to determine the current speed of the vehicle. The TCU uses this information to determine when a gear change should take place based in the various operating parameters. The TCU also uses

4042-480: The engine is often made to provide extra braking power to take some strain off the vehicle's regular brake system and to help avoid overheating the brakes. In its simplest form this consists of a butterfly valve that restricts the exhaust flow. This is referred to as an exhaust brake and mostly found on older trucks. It has a limited effect, and more advanced systems as described below are near universal on newer heavy vehicles. A compression release brake (also known as

4136-431: The engine runs on electric power to dissipate excess energy when the battery has been fully recharged. As soon as the accelerator is released enough to slow the engine, engine braking comes into effect as long as the wheels remain connected via the transmission to the engine. A slipping or disengaged clutch , or a torque converter , would disengage the wheels or absorb braking energy. The braking force varies depending on

4230-460: The engine, and the gear the transmission is in. The lower the gear, the higher the braking effect due to higher rpm and the torque transferred through the transmission (higher torque is delivered from the engine in lower gears). Engine braking avoids wear on brakes, and can help a driver maintain control of the vehicle. Active use of engine braking by shifting into a lower gear can help control speed while driving down very steep and long slopes, saving

4324-477: The exhaust, which cause them to feel like they have some engine braking like a gasoline engine. The main ones are: Engine braking in a premix two-stroke engine can be extremely harmful to the engine, because cylinder and piston lubricant is delivered to each cylinder mixed with fuel. Consequently, during engine braking, the engine starves not only of fuel but also lubricant, causing accelerated wear. Many old two-stroke cars ( Saab Automobile , Wartburg 353 , etc.) had

4418-407: The filter, and to function well to several hundred degrees Fahrenheit or Celsius. Filters are an important part of hydraulic systems which removes the unwanted particles from fluid. Metal particles are continually produced by mechanical components and need to be removed along with other contaminants. Filters may be positioned in many locations. The filter may be located between the reservoir and

4512-434: The flow from one component to the next. Each component has entry and exit points for the fluid involved (called ports) sized according to how much fluid is expected to pass through it. There are a number of standardized methods in use to attach the hose or tube to the component. Some are intended for ease of use and service, others are better for higher system pressures or control of leakage. The most common method, in general,

4606-427: The fluid based on temperature in order to improve shift comfort, and also to determine regulation of the torque converter lock-up clutch. One of the most common inputs into a TCU is the kick down switch which is used to determine if the accelerator pedal has been depressed past full throttle. Traditionally this was required on older transmissions with a simple logic in order to ensure maximum acceleration. When activated

4700-473: The fluid temperature inside the transmission. This is often used for diagnostic purposes to check ATF (Automatic Transmission Fluid) at the correct temperature. The main use of this has been as a failsafe feature to downshift the transmission if the ATF becomes extremely hot. On more modern transmissions this input allows the TCU to modify the line pressure and solenoid pressures according to the changing viscosity of

4794-422: The following components: For the hydraulic fluid to do work, it must flow to the actuator and/or motors, then return to a reservoir. The fluid is then filtered and re-pumped. The path taken by hydraulic fluid is called a hydraulic circuit of which there are several types. Open-loop: Pump-inlet and motor-return (via the directional valve) are connected to the hydraulic tank. The term loop applies to feedback;

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4888-410: The force exerted on C1 is 10 lbf , the force exerted by C2 is 1000 lbf because C2 is a hundred times larger in area ( S  = π r ²) as C1. The downside to this is that you have to move C1 a hundred inches to move C2 one inch. The most common use for this is the classical hydraulic jack where a pumping cylinder with a small diameter is connected to the lifting cylinder with a large diameter. If

4982-481: The hose to the fitting. Another disadvantage of hoses is the shorter life of rubber which requires periodic replacement, usually at five to seven year intervals. Tubes and pipes for hydraulic n applications are internally oiled before the system is commissioned. Usually steel piping is painted outside. Where flare and other couplings are used, the paint is removed under the nut, and is a location where corrosion can begin. For this reason, in marine applications most piping

5076-456: The huge multiplication of forces that can be achieved by applying pressures over relatively large areas. One drawback, compared to machines using gears and shafts, is that any transmission of power results in some losses due to resistance of fluid flow through the piping. Joseph Bramah patented the hydraulic press in 1795. While working at Bramah's shop, Henry Maudslay suggested a cup leather packing. Because it produced superior results,

5170-660: The hydraulic press eventually displaced the steam hammer for metal forging. To supply large-scale power that was impractical for individual steam engines, central station hydraulic systems were developed. Hydraulic power was used to operate cranes and other machinery in British ports and elsewhere in Europe. The largest hydraulic system was in London. Hydraulic power was used extensively in Bessemer steel production. Hydraulic power

5264-408: The input shaft or torque converter . The TCU uses the input shaft speed to determine slippage across the torque converter and potentially to determine the rate of slippage across the bands and clutches . This information is vital to regulate the application of the torque converter lock-up clutch smoothly and effectively. This may also be known as Transmission Oil Temperature. This sensor determines

5358-500: The integration of electronic controls have allowed great progress in recent years. The modern automatic transmission is now able to achieve better fuel economy, reduced engine emissions , greater shift system reliability, improved shift feel, improved shift speed and improved vehicle handling . The immense range of programmability offered by a TCU allows the modern automatic transmission to be used with appropriate transmission characteristics for each application. On some applications,

5452-566: The introduction of drive-by-wire technology, this is often a shared input between the ECU and TCU. The input is used to determine the optimum time and characteristics for a gear change according to load on the engine. The rate of change is used to determine whether a downshift is appropriate for overtaking, for example, the value of the TPS is also continually monitored during the journey and shift programmes are changed accordingly (economy, sport mode, etc.). The TCU can also reference this information with

5546-414: The larger diameters the pipe can usually be inspected internally after welding. Black pipe is non-galvanized and suitable for welding . Hydraulic hose is graded by pressure, temperature, and fluid compatibility. Hoses are used when pipes or tubes can not be used, usually to provide flexibility for machine operation or maintenance. The hose is built up with rubber and steel layers. A rubber interior

5640-466: The larger sizes and pressures), welding cones/nipples (with o-ring seal), several types of flare connection and by cut-rings. In larger sizes, hydraulic pipes are used. Direct joining of tubes by welding is not acceptable since the interior cannot be inspected. Hydraulic pipe is used in case standard hydraulic tubes are not available. Generally these are used for low pressure. They can be connected by threaded connections, but usually by welds. Because of

5734-400: The linear force in this case is a rotary force, defined as torque. Both these examples are usually referred to as a hydraulic transmission or hydrostatic transmission involving a certain hydraulic "gear ratio". A hydraulic circuit is a system comprising an interconnected set of discrete components that transport liquid . The purpose of this system may be to control where fluid flows (as in

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5828-399: The load and operating conditions. The hydrostatic transmission is generally limited to around 200 kW maximum power, as the total cost gets too high at higher power compared to a hydrodynamic transmission. Large wheel loaders for instance and heavy machines are therefore usually equipped with converter transmissions. Recent technical achievements for the converter transmissions have improved

5922-614: The load. Hence, a pump rated for 5,000 psi is capable of maintaining flow against a load of 5,000 psi. Pumps have a power density about ten times greater than an electric motor (by volume). They are powered by an electric motor or an engine, connected through gears, belts, or a flexible elastomeric coupling to reduce vibration. Common types of hydraulic pumps to hydraulic machinery applications are: Piston pumps are more expensive than gear or vane pumps, but provide longer life operating at higher pressure, with difficult fluids and longer continuous duty cycles. Piston pumps make up one half of

6016-399: The low pressure side. Closed-loop circuits are generally used for hydrostatic transmissions in mobile applications. Advantages: No directional valve and better response, the circuit can work with higher pressure. The pump swivel angle covers both positive and negative flow direction. Disadvantages: The pump cannot be utilized for any other hydraulic function in an easy way and cooling can be

6110-413: The manifold vacuum caused by the air-flow restriction. On an automatic transmission, engine braking often spontaneously increases the engine RPM, causing a sudden revving to occur even without applying the accelerator pedal. Diesel engines in personal cars provide little engine braking as they are not equipped with a throttle body and thus cannot draw a vacuum in the intake manifold. In heavy vehicles

6204-483: The more correct term is open versus closed "circuit". Open center circuits use pumps which supply a continuous flow. The flow is returned to the tank through the control valve's open center; that is, when the control valve is centered, it provides an open return path to the tank and the fluid is not pumped to a high pressure. Otherwise, if the control valve is actuated it routes fluid to and from an actuator and tank. The fluid's pressure will rise to meet any resistance, since

6298-399: The motor housing from rotating effects and losses in the ball bearings can be considerable as motor speeds will reach 4000-5000 rev/min or even more at maximum vehicle speed. The leakage flow as well as the extra flush flow must be supplied by the charge pump. A large charge pump is thus very important if the transmission is designed for high pressures and high motor speeds. High oil temperature

6392-410: The motor must be limited to a reasonable value. Circuit pressure during transport around 200-250 bar is recommended. Closed loop systems in mobile equipment are generally used for the transmission as an alternative to mechanical and hydrodynamic (converter) transmissions. The advantage is a stepless gear ratio (continuously variable speed/torque) and a more flexible control of the gear ratio depending on

6486-425: The need for mechanical gears or levers, either by altering the effective areas in two connected cylinders or the effective displacement (cc/rev) between a pump and motor. In normal cases, hydraulic ratios are combined with a mechanical force or torque ratio for optimum machine designs such as boom movements and track drives for an excavator. Cylinder C1 is one inch in radius, and cylinder C2 is ten inches in radius. If

6580-414: The piston begins its downward travel (this sudden release of compressed air creates audible sound waves similar to the expanding gases escaping from the muzzle of a firearm ). Having lost the energy stored within the compressed air, there is no "spring back" so the engine must expend yet more energy pulling the piston back down again. This type of brake produces extreme amounts of noise pollution if there

6674-402: The position of the selector lever so that the cruise control can be deactivated if the lever is shifted out of a driving range. A wide variety of information is delivered to the TCU via Controller Area Network communications or similar protocols (such as Chrysler's CCD bus, an early EIA-485 -based vehicle local area network ). In older vehicle designs, as well as in aftermarket TCUs sold into

6768-452: The power input to the pump equals the (max. load pressure + Δ p LS ) x sum of flow. Technically the down-stream mounted compensator in a valve block can physically be mounted "up-stream", but work as a down-stream compensator. System type (3) gives the advantage that activated functions are synchronized independent of pump flow capacity. The flow relation between two or more activated functions remains independent of load pressures, even if

6862-453: The pump has a constant output. If the pressure rises too high, fluid returns to the tank through a pressure relief valve. Multiple control valves may be stacked in series. This type of circuit can use inexpensive, constant displacement pumps. Closed-loop: Motor-return is connected directly to the pump-inlet. To keep up pressure on the low pressure side, the circuits have a charge pump (a small gear pump) that supplies cooled and filtered oil to

6956-407: The pump intake. Blockage of the filter will cause cavitation and possibly failure of the pump. Sometimes the filter is located between the pump and the control valves. This arrangement is more expensive, since the filter housing is pressurized, but eliminates cavitation problems and protects the control valve from pump failures. The third common filter location is just before the return line enters

7050-430: The pump reaches the maximum swivel angle. This feature is important for machines that often run with the pump at maximum swivel angle and with several activated functions that must be synchronized in speed, such as with excavators. With the type (4) system, the functions with up-stream compensators have priority, for example the steering function for a wheel loader. The system type with down-stream compensators usually have

7144-413: The racing and hobbyist markets, the TCU receives only the signals needed to control the transmission (engine speed, vehicle speed, throttle position or manifold vacuum, shift lever position). The typical modern TCU sends out signals to shift solenoids, pressure control solenoids, torque converter lockup solenoids and to other electronic controllers. Many automatic transmissions lock the selector lever via

7238-434: The regulator for the variable pump that supplies the oil: Load-sensing systems (LS) generate less power losses as the pump can reduce both flow and pressure to match the load requirements, but require more tuning than the CP system with respect to system stability. The LS system also requires additional logical valves and compensator valves in the directional valves, thus it is technically more complex and more expensive than

7332-697: The reservoir. This location is relatively insensitive to blockage and does not require a pressurized housing, but contaminants that enter the reservoir from external sources are not filtered until passing through the system at least once. Filters are used from 7 micron to 15 micron depends upon the viscosity grade of hydraulic oil. Hydraulic tubes are seamless steel precision pipes, specially manufactured for hydraulics. The tubes have standard sizes for different pressure ranges, with standard diameters up to 100 mm. The tubes are supplied by manufacturers in lengths of 6 m, cleaned, oiled and plugged. The tubes are interconnected by different types of flanges (especially for

7426-485: The retarding forces within an internal combustion engine are used to slow down a motor vehicle , as opposed to using additional external braking mechanisms such as friction brakes or magnetic brakes . The term is often confused with several other types of braking, most notably compression-release braking or "jake braking" which uses a different mechanism. Traffic regulations in many countries require trucks to always drive with an engaged gear, which in turn provides

7520-419: The shift solenoids themselves to provide precise pressure control during shifts by ramping the solenoid on and off. The shift pressure affects the shift quality (too high a pressure will result in rough shifting; too low a pressure will cause the clutches to overheat) and shift speed. Most electronic automatic transmissions utilize a TCC solenoid to regulate the torque converter electronically. Once fully locked,

7614-453: The solenoids to control the clutches indirectly, by way of a greatly simplified valve body. Modern electronic automatic transmissions are still fundamentally hydraulic. This requires precise pressure control. Older automatic transmission designs only use a single line pressure control solenoid which modifies pressure across the entire transmission. Newer automatic transmission designs often use many pressure control solenoids, and sometimes allow

7708-404: The speed at which a vehicle travels downhill. By shifting to a lower gear in a manual transmission , or applying "low" mode on an automatic transmission , engine braking reduces the need to repeatedly apply the foot brake , lowering the risk of the brakes overheating. While some of the braking force is produced due to friction in the drive train , this is negligible compared to the effect from

7802-401: The spool left or right. A seal allows part of the spool to protrude outside the housing, where it is accessible to the actuator. The main valve block is usually a stack of off the shelf directional control valves chosen by flow capacity and performance. Some valves are designed to be proportional (flow rate proportional to valve position), while others may be simply on-off. The control valve

7896-539: The supply fluid is blocked, or returned to tank. Sliding the spool to one side routes the hydraulic fluid to an actuator and provides a return path from the actuator to tank. When the spool is moved to the opposite direction the supply and return paths are switched. When the spool is allowed to return to neutral (center) position the actuator fluid paths are blocked, locking it in position. Directional control valves are usually designed to be stackable, with one valve for each hydraulic cylinder, and one fluid input supplying all

7990-399: The torque converter no longer applies torque multiplication and will spin at the same speed as the engine. This provides a major increase in fuel economy. Modern designs provide partial lockup in lower gears to improve fuel economy further, but this can increase wear on the clutch components. Many TCUs provide an output to the ECU to retard the ignition timing, or reduce the fuel quantity, for

8084-425: The traditional need for this switch. This input is used to determine whether to activate the shift lock solenoid to prevent the driver from selecting a driving range with no foot on the brake. In more modern TCUs this input is also used to determine whether to downshift the transmission to increase engine braking effect if the transmission detects that the vehicle is going downhill. Many TCUs now have an input from

8178-403: The transmission downshifts into the lowest permissible gear based on current road speed to use the full power reserves of the engine. This is still present in most transmissions though is no longer necessary to use in most circumstances because the TCU uses the throttle position sensor, the rate of change, and driver characteristics to determine whether a downshift may be necessary, thus eliminating

8272-491: The valves in the stack. Tolerances are very tight in order to handle the high pressure and avoid leaking, spools typically have a clearance with the housing of less than a thousandth of an inch (25 μm). The valve block will be mounted to the machine's frame with a three point pattern to avoid distorting the valve block and jamming the valve's sensitive components. The spool position may be actuated by mechanical levers, hydraulic pilot pressure, or solenoids which push

8366-402: The vehicle speed sensor to determine vehicle acceleration and compare this with a nominal value; if the actual value is much higher or lower (such as driving uphill or towing a trailer) the transmission will change its gearshift patterns to suit the situation. Known as an input speed sensor (ISS). This sensor sends a varying frequency signal to the TCU to determine the current rotational speed of

8460-477: The vehicle will not regain traction until the wheels are allowed to turn more quickly. If the driver reduces engine braking by shifting back up, or disengaging the clutch on a manual transmission, traction can be regained. In hybrid electric vehicles , like the Toyota Prius , engine braking is simulated by the computer software to match the feel of a traditional automatic transmission. For long downhill runs,

8554-544: The vehicle's traction control system. If the TCS detects unfavourable road conditions, a signal is sent to the TCU. The TCU can modify shift programmes by upshifting early, eliminating the torque converter lock-up clutch application, and also eliminating the first gear totally and pulling off in 2nd. These simple on/off electric switches detect the presence or absence of fluid pressure in a particular hydraulic line. They are used for diagnostic purposes and in some cases for controlling

8648-404: The vehicle, doing the job of the brake pads or shoes. A well-executed rev-match in the same setting minimizes stresses on transmission components, so engine braking does the work of slowing the vehicle. Improper engine braking technique can cause the wheels to skid (also called shift-locking), especially on slippery surfaces, as a result of too much deceleration. As in a skid caused by overbraking,

8742-483: Was also used for elevators, to operate canal locks and rotating sections of bridges. Some of these systems remained in use well into the twentieth century. Harry Franklin Vickers was called the "Father of Industrial Hydraulics" by ASME . A fundamental feature of hydraulic systems is the ability to apply force or torque multiplication in an easy way, independent of the distance between the input and output, without

8836-426: Was electro-hydraulic, using an electric motor or solenoid connected to a hydraulic clutch actuator , whereas Ferrari's Valeo system was electro-mechanical , using an electric motor or solenoid, connected to the mechanical clutch system. Similar TCU or GCU systems are used in racecars with paddle-shift transmissions . These electronic systems typically work in conjunction with the engine control unit (in

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