Superb acceleration and ample torque is the charm of an engine with a turbocharger. I am sure the owner of a car with a turbocharged engine is already aware of the merits of a turbocharger. They also know that the turbocharger utilizes exhaust gas to spin the turbine rotor and thereby increase air pressure to the engine.
Why did the Subaru engineers fit a turbocharger to the Horizontally-Opposed Engine? Why are the Horizontally-Opposed Engine and a turbocharger well matched? I think, this is an area, which might not be well known. I posed these questions to Takumi Ikeda, who is in charge of turbochargers at Subaru Engineering Division.
"Fitting the turbocharger was our answer to Subaru's quest for overall car development embodied in the concept "a car that is enjoyable to drive." To achieve this, we need a better engine. We have confidence in the naturally aspirated Horizontally-Opposed Engine, however, with a turbocharger we can gain an even more powerful engine with greater available capacity. The turbocharged 2.0-liter, 4-cylinder, Horizontally-Opposed Engine of the Impreza WRX has a performance approximately equivalent to a 3.0-liter, naturally aspirated engine. However, the turbocharged engine is lighter and has better fuel efficiency than a 3.0-liter naturally aspirated engine."
The turbocharger uses exhaust gas to spin the turbine. Another method to spin the turbine is a supercharger. A supercharger rotates the turbine by means of the engine revolution. Subaru fits superchargers to some of the Pleo and Sambar models, its mini vehicle models for the Japanese market. The supercharger proves a good match for engines with a small displacement.
Both turbochargers and superchargers control the volume of air to the engine. When combined with the fuel injection system, the air-fuel mixture can be freely controlled.
The main parts that make up the turbocharger are the turbine, the compressor and the bearing. The rotating parts are made with especially high precision. The turbine rotates using the energy of exhaust gas. Air is sent to the engine by spinning the compressor that is connected to the turbine on the same shaft.
The turbine of the turbocharger fitted to the Impreza turns 130,000 times per minute for an engine running 3,000 revolutions per minute. This is an incredibly high speed of revolution and the parts require precision machinery.
Along with its high-speed revolutions, the turbocharger is also exposed to high temperature exhaust gas. Therefore, oil and water from the engine are used to lubricate and cool the turbine shaft bearing. In addition, when the intake air is compressed (by the compressor), during the course of doing work the temperature increases further. An intercooler is used to lower the temperature of the intake air and enhance the air intake efficiency.
This is a rough outline of the operation of a turbocharger. Now to the question of how well the turbocharger suits the Horizontally-Opposed Engine.
"Since the Horizontally-Opposed Engine is compact, there is a large amount of room to fit the turbocharger. We are able to relatively freely choose the size of the turbocharger we require. We do not have to sacrifice the size of the turbocharger, when we need more supercharged air. Furthermore, the Horizontally-Opposed Engine has very little vibration compared to an in-line 4-cylinder engine. This provides greater stability for the turbocharger, and allows high-speed revolution. These factors reduce the chance of it breaking down."
When I questioned Mr. Ikeda further I learned of a range of advantages to turbocharging the Horizontally-Opposed Engine offers a range of advantages.
"Intake efficiency is a factor for the performance of a turbocharger, and this efficiency is influenced by the layout of the air intake pipes both upstream and downstream of the turbocharger. In a Horizontally-Opposed Engine we have a high degree of freedom with respect to the layout of the turbocharger and therefore can make the air intake port straight allowing for the volume of the air-pipes between the turbocharger and the engine to be small. These factors increase the efficiency and enhance the response of the air intake. The lack of resistance and speedy delivery of compressed air are important benefits for the turbocharger arising from the good layout in the Horizontally-Opposed Engine."
Making use of these advantages, the Subaru turbocharger is created according to a highly precise design. A slightly modified turbocharger is fitted the Impreza that competes in the World Rally Championship. In addition, so as to prevent unpleasant noise from the spinning turbocharger, noise and the vibration of each spinning part has been reduced. The result is a characteristic hum that has sometimes been called angels singing.
Mr. Ikeda continued by noting that the developments of the turbocharger is highly relevant to the technology of Subaru in the near future.
"Subaru's turbocharger technology has not been developed just to deliver greater horsepower and torque. Taking advantage of the benefits derived from a high level of freedom in selecting size and optimum layout of the turbocharger for the Horizontally-Opposed Engine, we would like to continue developing technology for fuel efficiency and low emissions."
The turbocharger has been likened to the shape of a snail and projects a slightly cumbersome image. However, the precision technology involved is on a micro-level. In addition, the turbocharger provides not only increased power and torque, but also enhanced fuel efficiency and potential to lower emissions.