Turbocharger

  • Turbochargers allow the engine to burn more fuel and air by packing more into the existing cylinders.
  • The¬†turbine¬†in the turbocharger spins at¬†speeds¬†of up to 150,000 rpm.
    • In order to handle this speed, the turbine shaft has to be supported very carefully. Most bearings would explode at speeds like this, so most turbochargers use a¬†fluid bearing. This type of bearing supports the shaft on a thin layer of oil that is constantly pumped around the shaft. This serves two purposes: It cools the shaft and some of the other turbocharger parts, and it allows the shaft to spin without much friction.
  • The typical boost provided by a turbocharger is 6 to 8 psi.
    • Since normal atmospheric pressure is 14.7 psi at sea level, you can see that you are getting about 50 percent more air into the engine. Therefore, you would expect to get 50 percent more power. It‚Äôs not perfectly efficient, so you might get a 30- to 40-percent improvement instead. One cause of the inefficiency comes from the fact that the power to spin the turbine is not free. Having a turbine in the exhaust flow increases the restriction in the exhaust. This means that on the exhaust stroke, the engine has to push against a higher back-pressure. This subtracts a little bit of power from the cylinders that are firing at the same time.

 

Boost Pressure Sensor

  • The¬†Boost Pressure Sensor¬†is used to monitor the turbocharger boost and shut down system¬†if it has too much boost.¬†Typically this sensor is screwed in to the intake system and may have 1, 2, or 3 wires attached¬†to it. If it fails the system may not add fuel for under boost and the vehicle may feel like it has less power.
  • MAP vs Boost pressure sensor
    • Map¬†sensor reads¬†absolute air pressure, i.e.. atmospheric is 14.7 psi.
    • Boost¬†gauge reads¬†pressure relative to atmospheric. e.g. 10 psi boost is 10 psi above atmospheric, which equals 14.7 + 10 = 24.7 psi of absolute pressure.

Blowoff Valve

  • A¬†blowoff valve¬†(BOV),¬†bypass valve¬†or¬†dump valve¬†is a pressure release system present in most¬†turbocharged¬†engines.¬†Its purpose is to¬†prevent¬†compressor surge, and¬†reduce wear¬†on the turbocharger and engine. Blowoff¬†valves¬†relieve the damaging effects of compressor ‚Äúsurge loading‚ÄĚ by allowing the compressed air to vent to the atmosphere, making a distinct hissing sound, or recirculate into the intake upstream of the compressor inlet.
  • When the throttle plate is open, the air pressure on both sides of the piston in the blow-off valve is equal and the spring keeps the piston down.
  • When the throttle is closed, a vacuum forms in the manifold. This in combination with the pressurized air from the turbocharger moves the piston in the valve up, releasing the pressure into the inlet of the turbo (Recirc.) or the atmosphere (BOV).
  • Without the blow-off valve, the exhaust turbocharger would work against the ram pressure of the closed throttle valve and become slower. On opening the throttle valve, the exhaust turbocharger would react with a delay.
  • Blowoff Valve and MAF Problems
    • In the case where a¬†MAF sensor¬†is used and is located¬†upstream¬†from the blowoff valve, ECU¬†will inject excess fuel because the atmospherically vented air is not subtracted from the intake charge measurements. The engine then briefly operates with a fuel-rich mixture after each valve actuation.¬†The rich mixing can lead to¬†hesitation or even stalling¬†of the engine when the throttle is closed, a situation that worsens with higher boost pressures. Occasional events of this type may be only a nuisance, but frequent events can eventually foul the¬†spark plugs¬†and destroy the¬†catalytic converter, as the inefficiently combusted fuel produces¬†soot¬†(excess carbon) and unburned fuel in the exhaust flow can produce soot in the converter and drive the converter beyond its normal¬†operating temperature¬†range.
    • Methods for utilizing both MAF and blowoff valve:
      • Have the MAF located down stream between the intercooler and the throttle plate. This is known as Blow-through rather than the traditional Draw-through set up. Care must be taken as to the position of the MAF to prevent damage to the sensitive element.
      • Using MAP sensor for fuel metering system.¬†The MAP sensor monitors the absolute pressure in the manifold at all times and will correctly detect the change that occurs when the valve vents, allowing the ECU to reduce fuel metering accordingly.

Wastegate

  • Most automotive turbochargers have a¬†wastegate, which allows the use of a smaller turbocharger to reduce lag while¬†preventing it from spinning too quickly¬†at high engine speeds. The wastegate is a valve that allows the exhaust to bypass the turbine blades. The wastegate senses the boost pressure. If the pressure gets too high, it could be an indicator that the turbine is spinning too quickly, so the wastegate bypasses some of the exhaust around the turbine blades, allowing the blades to slow down.
  • ūüďé Turbo. Boost Pressure Control Solenoid N75 | Audi VW
      • In the event of failure (or disconnection of N75), then responsibility for boost pressure regulation is delegated to a mechanical safety feature of the wastegate which operates at lower boost pressure than N75 permits.
      • Without Pressure Control Solenoid,¬†turbo¬†would not be able to produce more than ~0.3-0.4 bar of boost,¬† because it would shut itself down immediately.
      • When the ECU wants more boost (because you are stepping on it for example), it VENTS some air from this loop (turbo‚ÄĒ>wastegate) so the wastegate will not open at all, or will open later.
      • The N75 has got 1 boost input, and 2 outputs. At the input, boost is entering into the N75 and exits on one of the outputs going to the wastegate. When N75 is closed (default state), all the boost goes to the wastegate. When ECU opens the N75 valve (because it has got an electrical plug too coming from ECU), so when ECU opens it, some of this boost (which would operate the wastegate) is leaving/leaking back to the inlet (and not opening wastegate). Since the N75 valve has NO state in between, is is opened or closed. The ECU switches it ON/OFF all the time (several times a second) to generate the desired amount of boost. Doesn‚Äôt it reminds you to something???? ‚Ķ Yeah, it‚Äôs surging. But it is absolutely normal, this is how ECU controls the wastegate. When a car is chipped, the boost is a LOT higher than it is designed to operate on, so this kind of boost control will be noticeable.
      • By adjusting the N75 valve, you can adjust the amount of air which should leave when the N75 is opened. If you adjust the N75 to the right, you will ‚Äúclose‚ÄĚ the N75 valve, so just a few amounts of air is vented from the wastegate which will result in a lot smoother drive (eliminating surging, because the boost will be more constant), BUT at the same time it will introduce more boost to the wastegate, causing LESS overall boost. Adjusting the N75 valve to the left will ‚Äúopen‚ÄĚ it, enabling more air to escape from the wastegate when N75 is opened, which will result in more boost (coz wastegate wont open), BUT at the same time it MAY cause surging as the N75 opening/closing will be more obvious.

 

Intercooler

  • An¬†intercooler¬†or¬†charge air cooler¬†is an additional component that looks something like a¬†radiator, except air passes through the inside as well as the outside of the intercooler. The intake air passes through sealed passageways inside the cooler, while cooler air from outside is blown across fins by the¬†engine cooling fan.
  • Air heats up when its compressed.
  • The intercooler further¬†increases the power of the engine¬†by cooling the pressurized air coming out of the compressor before it goes into the engine. This means that if the turbocharger is operating at a boost of 7 psi, the intercooled system will put in 7 psi of cooler air, which is denser and¬†contains more air molecules¬†than warmer air.

 

Downsides

  • Too much boost can cause¬†knocking.¬†Knocking¬†happens because as you compress air, the temperature of the air increases. The temperature may increase enough to ignite the fuel before the¬†spark plug¬†fires. Cars with turbochargers often need to run on¬†higher¬†octane¬†fuel to avoid knock. If the boost pressure is really high, the compression ratio of the engine may have to be reduced to avoid knocking.
  • If a turbocharger with too much boost is added to a fuel-injected car, the system may not provide enough fuel ‚ÄĒ either the software programmed into the controller will not allow it, or the pump and injectors are not capable of supplying it.
  • Turbochargers provide boost to engines at high speeds.¬†They do not provide an immediate power boost when you step on the gas. It takes a second for the turbine to get up to speed before boost is produced. This results in a feeling of lag when you step on the gas, and then the car lunges ahead when the turbo gets moving.¬†One way to decrease turbo lag is to reduce the¬†inertia¬†of the rotating parts, mainly by reducing their weight and size.
  • Warning: Do not switch off the engine immediately after revving as the turbocharger speed will remain high while the oil pressure supplied to the turbocharger will fall rapidly. This will lead to the undesirable condition of the turbocharger rotating at speeds in excess of 100,000 rpm with no oil supply/lubrication. After free revving a turbocharged engine, let the engine idle for 30 seconds before you switch it off.

 

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