subaru impeza wrx sti

technical description about Engine
Introduction
The new Impreza WRX STi is a sophisticated technology package which draws directly on engineering skills gained in Subaru’s All- Wheel Drive World Rally Championship programme, producing a car with outstanding sports performance, handling and ride. The Impreza WRX STi is much more than a performance upgrade to the standard Impreza WRX, it is a complete engineering re design in all mechanical areas to not only enhance performance but also durability and reliability. The engine not only provides an output of 195kW @ 6000 rpm with 343 Nm of torque @ 4000 rpm but is also significantly strengthened to cope with the increased thermal loads. It also features an Active Valve Control System (variable valve timing) for improved torque delivery and engine breathing. The transmission is a six speed constant mesh, multiple synchromesh unit with significantly larger gears for
added strength and durability. Similarly the suspension has also been upgraded to
match the improved engine performance along with larger ‘Brembo’ brakes for increased stopping power. Key Mechanic a l c h a n g e s i n t h e I m p r e za WRX S T i a r e : -
• Engine power & torque increase with strengthened components:-
* Semi closed deck cylinder block.
* Forged pistons.
* Stronger con-rods & big ends.
* Revised camshaft timing and lift.
* Hollow stem intake valve with sodium filled exhaust valves.
* Variable valve timing.
* Larger more efficient IHI turbo charger.
* Larger inter cooler.
• Stronger close ratio six speed All-Wheel Drive transmission with centre viscous LSD differential and high performance clutch.
• Front & rear axle “Suretrac” limited slip differentials.
• Strengthened & inverted front & rear suspension struts.
• Brembo Brakes with super sports ABS and Electronic Brake force Distribution (EBD).
• Dual stage PIN operated six-way immobilisation supplementary security alarm system.
• Minimum Fuel Requirement for this vehicle is 98 RON Fuel.***
Engine Construction

The phase II engine used in the new Impreza WRX STi is a strengthened variant of the traditional Subaru horizontally opposed boxer engine featuring a semi closed deck cylinder block and variable valve timing. The cylinder heads feature more aggressive camshafts with increased valve lift and duration with lower weight valve mechanism for reduced valve
inertia. This provides for more accurate tracking of the cam profile particularly at high engine speeds. The low emission cylinder heads continue to feature ‘tumble swirl’ intake ports for improved combustion efficiency. In the unique configuration of the boxer engine, the pistons move in the horizontal plane from left to right with low levels of noise, vibration and lower power loss. This is due in part to the cancellation of the inertia forces created by the downward force of the pistons that act in opposite directions. With an in-line engine all four pistons are moving in the same direction and therefore a larger and heavier crankshaft is required to counteract this inherent imbalance. Structurally the horizontal design also yields a more rigid cylinder block because the crankshaft is sandwiched between the
left and right hand crankcases and supported by five main bearings. This provides for long life with little wear and tear. The Phase II engine also features the relocation of the crankshaft thrust bearing to the rear of the crankshaft. This provides for a reduction in the transfer of natural engine frequencies to the transmission and driveline thereby improving N.V.H. levels in the passenger compartment. The natural balance of the horizontally opposed engine along with the lightweight crankshaft provides for excellent rotational balance, rotating smoothly all the way up to high engine speeds without the use of balancer shafts that are necessary with in-line engines. This feature along with the aluminium construction achieves a lightweight compact engine that allows for a great deal of freedom in positioning the engine in the vehicle. Its low height also makes a low centre of gravity possible with a more balanced left/right and front/rear weight distribution for improved vehicle handling.
Engine

Engine EJ20B 43R STi
Max power = 195 kW @ 6000 rpm
Max torque = 343 Nm @ 4000 rpm
Bore x stroke = 92 x 75 mm
Compression ratio = 8:1
Power to weight ratio = 7.54 kg/kW
The increased power output of the STi engine is achieved through a combination of a larger turbo charger with increased air charge pressure and volume plus the adoption of variable
valve timing. This provides for a wider and flatter torque band with less turbo lag as the turbocharger can be tuned for better low speed response. Maximum power output of 195kW is the result of the engine management ECU being tuned for operation on 98 RON fuel compared to the 206kW output of the Japanese version which is tuned on 100 RON fuel. The result is a better all round power delivery because the ECU is specifically tuned for the available fuel. Similarly maximum torque output is 343Nm at 4000rpm with torque delivery in excess of 300 Nm commencing with 308Nm @ 2800 rpm and still maintaining 310Nm @ 6000 rpm.

Engine
Cylinder Block

The cylinder block used in the STi is what is known as a semi-closed deck design. This design provides for the cooling efficiency of a completely open deck design with the strength of a closed deck. By providing additional reinforcement at the top of the cylinder liner the bore is less susceptible to bore distortion under the increased pressures generated by the increased power output of the STi MY02 engine over a wider engine speed operating range.
Additional reinforcement ribs have also been provided in the crankcase housing to further strengthen the block.
P i stons

High strength lightweight forged aluminium alloy pistons with high heat resistance and low thermal expansion. The piston crown thickness has been increased again to cope with the increased thermal loadings created by higher combustion temperatures and pressures. A solid slipper-type skirt with molybdenum coating is used to minimise friction.

Engine
Connecting Rods


The connecting rods are made from forged high carbon steel with increased shaft crosssectional area, along with big end cap dowel pins and set screws to improve mating accuracy, to meet the demands of a high performance engine.
Valve Operation

Valve operation is via direct acting twin camshafts per cylinder head using hollow valve stems to reduce valve mass and as a consequence valve inertia. Similarly shim less cam followers are used to further reduce the mass of the moving valve mechanism. As a result improved conformity of the cam profile is obtained particularly at high engine speeds. Graded size cam followers are used to obtain adjustment of valve clearance. The exhaust valve stem is sodium filled to assist heat transfer away from the valve head into the cylinder head where it is
dispersed by the coolant. Use of sodium filled valves is an essential requirement to prevent valve stem and or follower damage due to the higher thermal loadings as a result of the higher power output.
A c t i v e Valve Control S y s t e m


The purpose of Active Valve Control System is to increase engine power and torque output while at the same time gaining improvements in fuel consumption, exhaust emissions and idling stability. By controlling the intake valve timing to suit the engine load and speed
conditions this system optimises the engine volumetric efficiency and combustion process.
This is achieved by rotating the camshaft sprocket relative to the intake camshaft within a maximum range of 35 crankshaft degrees. This movement is controlled by the engine management computer (ECM) based on input signals from the air flow sensor, engine coolant temperature sensor, throttle position sensor and camshaft position sensors. The ECM then generates a duty ratio electrical output signal to an Oil Control Valve (OCV) positioned at each intake camshaft sprocket to control engine oil pressure which is supplied to advance and retard chambers within the AVCS actuator. Valve timing is continuously and infinitely variable within the 35 crank degree range and controlled according to engine speed and load conditions. There are three computer maps that are used depending on the conditions to provide for optimum valve timing for stable idling, improved fuel consumption in the medium speed range and maximum power at high engine speed and load.
Idle stability control

Engine

Medium Speed range
When the engine is running in the medium speed range and the engine load is small advancing the intake valve timing reduces intake air blow back thereby improving fuel consumption. Increasing the intake and exhaust valve overlap also promotes exhaust gas recirculation (EGR) reducing NOx exhaust gas emissions. When the engine load increases, advancing the intake closing time takes advantage of the intake air inertia to create a supercharge effect on the incoming intake air.
High Speed and Load range

At high engine speed and load conditions intake and exhaust valve overlap utilises the scavenging effect produced by the exhaust gas pulsation to draw intake air into the cylinder. Since the intake valve is closed at the end of the intake stroke, air intake efficiency is
improved and engine power output is boosted.

Engine

Turbocharger
The turbocharger unit consists of two sections, an exhaust side and an induction side. On the exhaust side is a turbine wheel with vanes that are shaped to harness the exhaust gas energy
causing the turbine wheel and centre shaft to rotate. On the induction side there is an impeller wheel attached to the common shaft which also has vanes but shaped in the opposite direction so as to compress the induction air. With increasing engine speed and
load the level of kinetic energy in the exhaust gas also increases and as a consequence the turbine rotates faster. This causes the impeller to also rotate faster causing greater
compression of the induction air. Rotational speeds of the turbine are in the region of 20,000 rpm. at idle to approx. 165,000 rpm. at full engine load. The limiting of boost pressure is achieved by the use of a ‘wastegate’, which bypasses the exhaust gas around the turbine wheel when the desired level of boost is reached. The wastegate is a simple flap valve, which is opened by diaphragm to which boost pressure is applied.

Engine
Turbocharger Characteristics
A turbocharger uses exhaust gas energy to rotate the turbine wheel and as a consequence rotate the impeller which compresses the intake air. Exhaust gas pressure however is low at low engine speeds and as a result the turbine does not respond immediately when the throttle is opened. This phenomenon is referred to as ‘Turbo Lag’. In an attempt to overcome this phenomenon, design characteristics of the turbocharger are matched to the prospective use of the vehicle. Two important design criteria are size and the A/R ratio.

Size.

Smaller turbocharger’s require less rotating energy (exhaust pressure) to rotate due to the smaller mass of the turbine and impeller, and therefore provide improved throttle response at lower engine speeds. The use of a small turbocharger however will result in a lower power output at high engine speeds due to the smaller volume of compressed air that is generated by the smaller impeller. A large turbocharger is capable of supplying a larger volume of compressed air, and is therefore more suitable in providing maximum power output. The increase in turbocharger size however will result in a large amount of turbo lag, as more exhaust energy that is dependent on engine speed is required to rotate the bigger turbine and impeller.
A/R Ratio.

The A/R ratio of the turbocharger determines the characteristics of boost pressure production. ‘A’ represents the smallest area of the inlet of the turbine housing, and ‘R’ represents the distance from the centre of the turbine shaft to the centre
of the turbine-housing inlet. By reducing the area ‘A’, exhaust velocity is increased. With higher exhaust velocity acting upon the turbine, response time is reduced because the turbine spins faster at lower engine speeds. If the area is too small, flow is restricted at higher engine speeds thereby limiting power output. An increase in dimension ‘R’ will improve turbine startup response but will diminish maximum turbine speed. Small A/R ratios are suitable for low speed applications where fast startup response is required at the expense of high speed power. Large A/R ratios are suitable for high speed applications where low speed response is not a priority. In the Impreza WRX STi due to the use of variable valve timing it has been possible to tune the turbocharger to perform better at low speed and hence reduce turbo lag while still retaining high top end power output. This is due to the improved breathing performance of the active valve control system engine as described earlier.
Engine
Turbocharger Characteristics

The compressor wheel to housing seal has also been improved to increase turbocharger efficiency. This new seal reduces the clearance by 66% compared to the previous Impreza WRX STi and therefore pumping losses are reduced meaning a better boost pressure response time.
Turbocharger Specifications


Engine
Intercooler
The temperature of the intake air is increased as it is compressed by the turbocharger. This rise in temperature causes a corresponding expansion of the air, leading to a reduction in air density. The intercooler is designed to transfer the heat of the compressed intake air to the external air flowing through the intercooler as the vehicle is in motion. There are two positive by-products of decreased air temperature and increased air density, one; a reduction in combustion chamber temperature allowing for more advanced ignition timing, and two; improved volumetric efficiency due to the increase in air mass for a given air volume. With adenser air charge in the combustion chamber, more fuel can be injected leading to greater power output. The STi intercooler due to the increased boost pressure and power output has a significantly bigger cooling capacity than the standard WRX to cope with additional heat load. It is also equipped with a manually operated water spray which can be operated by the driver in 2 second bursts to provide additional cooling under high engine load or high ambient temperatures.
Intercooler Specifications



Engine
Immob i l i s e r a n d S e c u r i t y S y s t e m
The factory immobiliser system interfaces directly with the engine management computer(ECU). This system is a transponder type that utilises a rolling code for additional security. Once the key is inserted in the ignition lock and the ignition turned on an antenna amplifier positioned around the ignition lock reads the transponder code and transmits it to the engine management and immobiliser computer (ECU). The ECU then compares the transmitted code for the correct sequence and, if correct, allows the engine to start. Remote central locking transmitter is now incorporated as one unit into the key along with the immobiliser transponder. If a duplicate key is required, the transponder code needs to be registered with the ECU. This teaching operation can only be performed with special equipment and the software is only available to authorised personnel. In addition to the factory immobiliser system the STi is fitted as standard with a Subaru Australia designed ‘Dual Stage Security System’. This system provides two separate security systems. The remote locking transmitter operates one system and the other by a PIN operated keypad. This provides for additional six points of immobilisation, anti hijack mode, automatic re-arm, Intrusion Alert, False Alarm Prevention, Internal Screamer, Infrasonic sensor, valet mode and also features anti cross pollination software for additional theft protection.

terato.com drift.com.my

Related Posts by Categories