Inside Lexus Turbo

Transcription

Inside Lexus Turbo

Handbook
Lexus Turbocharger System
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Naturally Aspirated Engines
Since 1989, Lexus has used naturally aspirated engines in all of the vehicles they have produced.
Naturally aspirated engines, whether they are carbureted or fuel injected, use atmospheric pressure
in our environment to push air into the cylinder for proper combustion. At sea level, atmospheric
pressure is rated at approximately 14.7 PSI absolute (101.3 kPa). This means that there is a constant
pressure of 14.7 PSI pushing on all objects at sea level. Atmospheric pressure also changes as
elevation or altitude is increased. As we start to increase in elevation or altitude, the atmospheric
pressure decreases. For example, the city of Denver (which is 5,280 feet above sea level) has an
atmospheric reading of 12.2 PSI. So how does the atmospheric pressure have an effect on the
engine?
When the piston in the engine moves down in the cylinder bore it creates an area of low pressure. At
this point, the high atmospheric pressure pushes air into the cylinder that has a low pressure. The air
that is pushed into the cylinder mixes with fuel to create a mixture for combustion to occur. The
atmosphere can only push as much air into the cylinder as the atmosphere pressure will allow. Since
the atmospheric pressure is lower at higher elevations, there is a reduced amount of air entering the
engine for the combustion process. During normal operation of the engine at a high elevation
(Denver), the atmosphere can only push 12.2 PSI of air into the cylinder which is less than the
atmospheric pressure could at sea level. Having less air in the cylinder, means that a lesser amount of
fuel is needed for our stoichiometric combustion mixture. This lack of air and fuel dilutes the proper
combustion mixture and causes the engine to lose power at higher elevations.
Overall, the naturally aspirated engine is a tried and true method of induction. This system has less
components, is reliable and cost friendly. There are a few downfalls though as power output is
restricted to atmospheric pressure and power output changes as elevations change.
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Naturally Aspirated Engine Benefits –
-Less Components
-Reliable
-Lower Cost
Naturally Aspirated Engine Downfalls -Less power output
-Changes in Power based on elevation
-Rely on Atmospheric Pressure for proper combustion mixture
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Forced Induction
Forced induction is the process of compressing the intake air and pressurizing the intake manifold of
the engine with a higher than atmospheric pressure. This allows more air to be pushed into the
cylinders during the combustion process. By allowing more air to be pushed into the engine, and in
turn fuel, a larger, denser air-fuel mixture enters the cylinder for combustion. With this large, dense
air-fuel mixture in the combustion chamber the engine can create additional power and torque.
There are two ways to use forced induction:
Supercharging – Supercharged engines use a mechanical device (typically either belt or gear) driven
by the engine to energize the supercharger. The supercharger than compresses the intake air
supplied to the engine.
Benefits – more power especially at lower RPMs, no lag, heat is generally not an issue
Downfalls – can be noisy, cost more, consumes power to create power
Turbocharging – Turbocharged engines use exhaust gas to energize a turbocharger which
compresses the air supplied to the engine.
Benefits – more power especially at higher RPMs, better efficiency, lower emissions
Downfalls – intake air is hotter, turbo lag, higher cost and complexity
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Turbo
Beginning with the 2015 Lexus NX200t, Lexus has adopted forced induction into the vehicle line-up
by the means of turbocharging. The benefits of smaller turbocharged engines with less weight, better
power output and increased fuel economy will help keep Lexus in front of the competition.
Another concern for car manufacturers is the looming CAFÉ standards for 2025. By 2025, car
manufacturers are required to meet a combined fuel economy of 54.5 MPG across their vehicle
lineup and reduce the amount of CO2 for each vehicle. Turbocharging allows manufacturers to use
smaller displacement engines, that get better fuel economy and lower CO2 emissions but still have
the power output of a larger engine when it is needed.
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Basic Turbo Operation
Turbochargers use spent exhaust gases from the combustion process to energize or propel the
turbine wheel inside the turbocharger. The turbine wheel is placed in the engine exhaust stream and
as the exhaust pulses from each cylinder flow out of the cylinder head, they contact the turbine wheel.
Each exhaust pulse pushes on the turbine wheel blades causing it to rotate inside the turbocharger
housing. Inside the turbocharger, the turbine wheel is connected via a shaft to a compressor wheel
that is isolated on the intake air side. When the compressor wheel is spun, due to the turbine wheel
spinning, intake air is compressed into the intake housing and pressurized to a higher than
atmospheric pressure. This allows the engine to push more air than is available from the atmosphere
into the engine during the combustion process.
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Types of Turbocharger Systems
Single or Dual Turbochargers –
Single turbochargers can be added to vehicles to increase performance. There are many aftermarket
turbocharger applications that are available and it is a common accessory to add to a vehicle to
increase performance. One downfall to a vehicle equipped with a single turbocharger is that the
vehicle will experience a phenomenon known as turbo lag. Turbo lag is the amount of time that is
takes for the turbocharger to spool up before it starts creating boost pressure in the intake manifold.
Turbo lag varies and is dependent on the turbocharger that is being used. For example a large
turbocharger will take a longer time to spool up than a smaller turbocharger.
There are a few ways to overcome the effects of turbo lag on a vehicle. One way is to use two
turbochargers. This type of setup can be accomplished by using a smaller turbocharger that will spool
up quickly and provide quick response at low rpm’s and a larger turbocharger that will continue to
provide boost and the air volume needed for response at high RPM’s. By having both turbochargers
the vehicle gets great response and performance across all RPM ranges with a minimal amount of
turbo lag if any is felt at all. This type of dual turbocharger set-up is also known as sequential
turbocharging. Sequential turbocharger systems provide a way to decrease turbo lag without
compromising ultimate boost output and engine power. Lexus has not used a dual turbocharger setup in its history, but Toyota used the dual turbocharger setup in the infamous Toyota Supra.
Variable Vane Turbochargers Variable vane turbochargers allow the same benefits of a sequential turbocharger system, but with
only the use of a single turbocharger. Inside a variable vane turbocharger there are plates or vanes
that direct the exhaust flow in the turbocharger. These vanes are placed before the turbine wheel and
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are adjustable. The adjustability of these vanes allow them to direct more or less air to the turbine wheel and
also how the air is directed to the turbine wheel. In low RPM situations, the turbine wheel needs to spool up
quickly to reduce turbo lag, so the vanes are adjusted to close off more creating a higher back pressure in the
exhaust system which increases the velocity of the exhaust gases to the turbine and directs the exhaust gases
into the turbine blades. At higher RPM’s, the vanes open up more to allow a larger volume of exhaust gases
to move through the turbocharger and directs the exhaust gases to flow onto the turbine blades. By
constantly changing the angle of the vanes inside the turbocharger, the vehicle has great response and
power throughout the full RPM range of the engine.
Twin Scroll Turbocharger –
The twin scroll turbocharger was also developed to provide power throughout the RPM range of the engine
with reduced turbo lag. As with the variable vane turbochargers, the twin scroll turbocharger is a self
contained single turbocharger. What makes the twin scroll turbocharger unique is that is separates the
cylinder exhaust streams to the turbine wheel in the turbocharger. One exhaust stream is designed to
increase velocity of the exhaust gases to the turbine wheel for low RPM performance and the other exhaust
stream is designed for high exhaust volume which helps provide power at higher RPM’s. To provide a
comfortable ride with almost no turbo lag, Lexus has decided to use a twin scroll turbocharger for the
NX200t.
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Twin Scroll Turbocharger Details
Like a VGT, a twin scroll turbocharger can be used to maximize the amount of boost across a larger
RPM range, this is accomplished in two ways. First, in the case of a 4 cylinder engine, the exhaust
gases from cylinders 1 and 4 are separated from the exhaust gases from cylinders 2 and 3. This is done
to prevent the exhaust gas pulses that are output by the cylinders from interfering with each other.
When the piston is on the exhaust stroke there is a spike in exhaust pressure from that cylinder,
immediately following the spike in pressure there is a moment of vacuum caused by valve overlap. This
moment of vacuum occurs at the same time that the next cylinder is on its exhaust stroke causing an
interference. By separating the exhaust from the cylinders, this interference can be avoided. Now that
the exhaust is separated into two channels it can be routed through two different size/shape scrolls
(channels) before the exhaust gas energizes the turbine.
• One of the channels is larger to accommodate larger amounts of exhaust gases at higher RPM
ranges. This channel routes larger amounts of exhaust gases at a high enough velocity to spin the
turbine effectively and create effective boost at higher RPMs.
• The other channel is smaller in order to effectively use smaller amounts of exhaust gases at lower
RPM ranges to create boost. This channel routes smaller amounts of exhaust gases through a
smaller space, which increases the velocity of the exhaust gases used to spin the turbine to create
effective boost at lower RPMs.
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Turbocharger System Components
The turbocharger assembly is the main component that provides the additional air need to increase
performance in a turbocharged engine. For the turbocharger to function correctly and accurately
there are other components that are needed to help control the turbocharger and the boost pressure
that it provides to the engine. Listed below are a few of the main components that help the
turbocharger control boost:
1.
Wastegate Valve
2. Wastegate Actuator
3. Vacuum Regulating Valve
4. No. 2 Check Valve
5. Vacuum Pump
6. No. 1 Turbo Pressure Sensor
7. No. 2 Turbo Pressure Sensor
8. Intercooler
9. Air Bypass Valve
10. Ejector Valve
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Wastegate Valve and Actuator
The wastegate valve controls turbo pressure and is opened and closed via the wastegate valve
actuator. The default position of the actuator with no vacuum applied is the OPEN position, this is
achieved with a spring in the actuator assembly; conversely, when vacuum is applied to the actuator
the valve is closed. This type of vacuum actuator control is opposite from other turbocharger control
systems that were adopted by other vehicle manufacturers. By having the wastegate valve normally
open, the chances of mechanical failure that will cause an over boost condition are minimized and it
creates an area of opportunity to make the engine more fuel efficient. By timing the wastegate valve
and the boost pressure, Lexus can reduce fuel consumption of the engine and increase the MPG. The
ECM cooperatively controls the throttle valve, VVT-iW, VVT-i and wastegate valve to achieve a driving
condition with optimal fuel efficiency and power during normal driving.
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Wastegate Valve Control
During Engine Start - The ECM controls the vacuum regulating valve assembly to block the vacuum
from the vacuum pump assembly to the actuator and open the wastegate valve. As a result, high
temperature exhaust gases are supplied to the catalyst and warm-up time has been reduced.
During Light Loads - The ECM controls the vacuum regulating valve assembly to block the vacuum
from the vacuum pump assembly to the actuator, opening the wastegate valve and decreasing the
exhaust manifold pressure. As a result, fuel economy has been enhanced due to reduced pumping loss
and improved combustion (reduction of remaining gas).
During Acceleration and High Loads - The ECM controls the vacuum regulating valve assembly to
control the vacuum from the vacuum pump assembly to the actuator and close the wastegate valve.
As a result, engine torque increase and response during acceleration have been achieved.
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Vacuum Regulating Valve
The Vacuum Regulating Valve (VRV) is a ECM duty cycled controlled device that limits the amount of
vacuum going to the wastegate actuator. The VRV receives vacuum from the vacuum pump and
check valve and provides this vacuum source to the wastegate actuator through vacuum lines in the
system.
During diagnosis of the turbocharger system, the VRV can be active tested. This active test is labeled
as Wastegate Valve Control Duty Ratio.
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Vacuum Pump Assembly
A vacuum pump assembly is used on turbocharged engines since intake vacuum will not be sufficient
at all times due to the use of a turbocharger. The vacuum pump supplies vacuum to the Vacuum
Regulating Valve (VRV), which in turn controls the operation of the wastegate valve. The vacuum
pump also supplies vacuum to the brake booster. The pump assembly is installed on the rear of the
engine and is driven by the exhaust camshaft (No. 2 camshaft). Inside the vacuum pump is a rotor with
two end caps. The rotor spins inside the housing and creates a vacuum for the turbocharger and brake
booster systems.
The Vacuum Pump also has a maintenance interval of 124,000. At this time, the pump is supposed to
be disassembled and checked for wear. If any wear is noticed on the rotor, end caps, housing or cover
the components should be replaced.
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Turbo Pressure Sensors
There are two turbo pressure sensors in the intake system.
• No.1 Turbo Pressure Sensor – Is located after the throttle body assembly. It is comprised of the
turbo pressure sensor and a thermistor type intake air temperature sensor. The ECM uses the
temperature sensor in the No.1 turbo pressure sensor along with the temperature sensor in the
MAF sensor to monitor the turbo charger intercooler efficiency. The pressure sensor in the No.1
turbo pressure sensor is only used for on F-Sport models as an input for the turbo boost gauge on
the multi information display.
• No. 2 Turbo Pressure Sensor – Is located before the throttle body assembly and detects turbo
pressure. It is connected via a hose to the intercooler assembly on the back side of the engine.
The turbo pressure sensors have a semiconductor that utilizes the characteristics of a silicon chip
which changes its electrical resistance when pressure is applied to it. The sensor converts the intake
air pressure into an electrical signal and sends it to the ECM in an amplified form.
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Air Bypass Valve
The air bypass valve is designed to reduce the surge phenomenon and turbo lag. When the
accelerator pedal is released, the throttle valve closes and compressed air from the turbocharger hits
the closed throttle valve, increasing intake pressure.
The surge phenomenon occurs when the intake air flow reverses, resulting in an abnormal noise
during deceleration.
Turbo lag is caused when this extra pressure slows the compressor turbine.
On deceleration the ECM commands the air bypass valve to open, diverting excess pressure
upstream to the turbocharger, reducing both of these conditions.
The air bypass valve includes:
• Air bypass valve -- controlled by ECM
• Resonator – to reduce sound
Lexus decided to use a bypass valve instead of a blow-off valve for two reasons:
1. Blow-off valves vent to the atmosphere and provide that distinct turbocharger sound. Being a
luxury model, Lexus did not want to introduce any unwanted noises into the vehicle. The bypass
valve provides a direct path back to atmosphere within the intake side of the turbocharger
housing reducing noise.
2. The bypass valve can be precisely controlled allowing the system to spool the turbo up quicker
and reduce turbo lag.
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Air Bypass Valve
During Turbo Operation – The Bypass Valve is closed allowing all intake air to be pressurized in the
intake before the throttle body. This allows the compressor wheel to create a pressure higher than
atmospheric for the engine.
During Deceleration – When the throttle plate is closed while the vehicle is by turbocharged, the
intake air surges to a higher pressure. This happens because the turbocharger is still spinning and
creating boost, but the throttle plate is closed, so the air is trapped in the intake. With the sudden
increase of intake air, the pressure surges and tries to flow backwards through compressor wheel.
This incorrect path of air flow puts stress on the compressor wheel, slows it down considerably and
creates a whistle type noise. This is known as the surge phenomenon and can damage the compressor
wheel if not controlled.
To allow the air to escape when the throttle plate is closed, the air bypass valve is opened. When the
air bypass valve is opened it provides a path for the high intake air pressure to escape back to
atmospheric pressure, reducing the pressure in the intake and minimizing the surge phenomenon.
The high pressure air is routed back to the atmospheric side of the turbocharger allowing the air to
enter the turbocharger again when it is needed.
During Acceleration – If the vehicle is requesting a higher than atmospheric pressure, the air bypass is
closed and the turbocharger will spool back up in a relatively short amount of time reducing turbo lag.
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Intercooler
A turbocharger compresses and forces more air into the cylinders of the engine to produce a larger,
denser combustion charge. During the intake air compression process the intake air is heated up.
Cooling the incoming intake air is needed to help maintain a dense air/fuel charge for proper
combustion and to also minimize possible pre-detonation or knock within the engine.
Lexus has equipped the NX200t with a small, but efficient intercooler that is mounted to the backside
of the engine assembly. This intercooler is an air to water intercooler. An air to water intercooler uses
an outside coolant source to cool and maintain the proper temperatures for the intake air. Lexus
found that using an air to water intercooler over an air to air intercooler decreased the vehicles 060MPH times by 0.3 seconds.
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Turbocharger/Intercooler Cooling System
Heat is one of the worst enemies for a turbocharger system and precautions need to be taken to
prevent the turbocharger from constantly being overheated. Lexus has developed
a dedicated turbocharger and intercooler cooling system that is separate from the engine cooling
system. This system has its own radiator, electric water pump and reservoir. The ECM varies the
pump motor speed based on various inputs in order to achieve an optimal coolant flow rate. The
turbocharger/intercooler cooling system makes sure that the system stays at the right temperatures.
If the turbocharger is constantly overheated, oil coking and turbocharger bearing failure may occur.
The capacity of this system is 3.1 quarts of SLLC.
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Turbocharger/Intercooler Cooling System
In addition to the turbocharger\intercooler cooling system, the Owner’s Manual for the Lexus
NX200t outlines some precautions to take after driving the vehicle. In order to make sure that the
turbocharger is not overheated after certain high speed driving conditions, it is required that the
vehicle be idled for a period of time before shutting off the vehicle. The graph above shows the
amount of idling time required for each driving scenario.
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Blowby Gas Ventilation System
Because a larger volume of blowby gas is present in a turbocharged engine compared to an normally
aspirated engine, and because naturally aspirated engines rely in intake manifold vacuum to draw the
blowby gases from the crankcase, the 8AR-FTS uses two methods to control blowby gases. A
conventional PCV valve is used and is effective when vacuum is available (when turbocharger is not
being utilized) in the intake manifold. An ejector is also used to forcibly ventilate the crankcase when
the turbocharger is operating. The ejector uses the venturi effect to draw gases from the crankcase:
pressurized intake air supplied by the turbocharger is routed through the ejector nozzle which
creates venturi vacuum in the ejector to draw the blowby gas out of the crankcase.
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Turbocharger Reference Information
In good repair practice, it is critical to know and understand how a system actually works before trying
to repair any of the components. A component that you think may be faulty because of its operation,
may actually be working as designed. For example, the Lexus wastegate actuator is normally open. If
you did not understand the system, you may think that this is at fault since most other manufacturers
keep the wastegate valve normally closed. The repair manual and new car features books can help
you understand the system.
When diagnosing an actual concern with the vehicle, use the repair manual to help guide you through
the repair process. The repair manual lists Diagnostic Trouble Codes that are related to the
turbocharger system and will have you perform specific tests in a particular order to determine the
root cause of the concern. The repair manual also lists out repair and test procedures for customer
concerns that are not related to a specific failure with a DTC, such as turbocharger noise and white
smoke from the turbocharger system.
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Turbocharger Diagnosis
After thoroughly understanding the turbocharger system and using the reference information to
come up with a specific repair process, the technician will have to perform a few tests. Most of the
tests for the turbocharger system can be performed with some basic hand tools, a vacuum gauge and
a vacuum tester. Since the control part of the turbocharger system uses vacuum, most of the tests that
are performed are checking for a constant source of vacuum or making sure that components can
hold a source of vacuum.
The Techstream scan tool is also a vital tool that is needed in the repair and diagnosis of the
turbocharger system. The Techstream allows a technician to retrieve DTC’s, perform active tests and
monitor live data on the vehicle to see exactly what is happening.
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Techstream Diagnosis
Using the Techstream the technician can monitor the specific data lines that allow them to see what is
happening in the turbocharger system. The data lines listed below are data that is directly related to
the turbocharger system and should be used during the diagnosis of the turbocharger system.
Intake Air Temperature B1S1 (Turbo) – Intake temperature coming from the No. 1 Turbo Pressure
Sensor located in the intake manifold. This sensor reports the intake air temperature after going
through the intercooler. Can be used to diagnose intercooler efficiency.
Wastegate Valve Control Duty Ratio – Target duty cycle that the VRV is receiving from the ECM to
control the wastegate valve actuator.
Intercooler Water Pump Speed – The RPM of the intercooler water pump. The intercooler water
pump can range from 2,000 to 7,000 RPM’s when operating.
Intercooler Water Pump - The drive request duty ratio of the electric water pump assembly.
Target Boost Pressure – The intake air pressure that the ECM is trying to receive from the
turbocharger system. This data list item is labeled in absolute PSI reading. Therefore the technicians
will see a target boost of close to 15 PSI when the vehicle is at idle. This is a normal condition, because
atmospheric pressure is close to 15 PSI absolute. During boost conditions under WOT, the target
boost will reach approximately 30 PSI, which means the ECM is requesting the maximum boost
pressure of 17.4 PSI from the turbocharger system.
Boost Pressure Sensor - The absolute pressure inside the intercooler. This pressure reading comes
from the No. 2 Turbo Pressure Sensor. The data list item is also read in absolute PSI.
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Air Bypass Valve Control - Operation status of the air by-pass valve assembly. Determines if the air bypass
valve is requested open or closed. The air bypass valve is normally off which means that it is closed in the
turbocharger system.
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Inside Lexus Turbo

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