by Jim Kerr
This is the third in a series of eight articles on new technology presented this Fall to the Automobile Journalists of Canada (AJAC) journalist panel as part of their annual Canadian Car of the Year testing. The Presenters highlighted what’s new for the automotive consumer, each vying for the title of “Best New Technology”.
This week the Subaru WRX 2.0 litre turbocharged engine is the subject of our attention. The WRX sedan and wagon are a couple of Subaru’s hot performance cars and incorporate the character of their World Championship rally cars. Under the hood you will find a 227 horsepower 2.0 litre engine with technology to both improve performance and at the same time reduce emissions. This is a Low Emissions Vehicle.
To the performance part first. This 2.0-litre opposed 4-cylinder uses an intercooled turbocharger to force-feed air to the engine. Compared to a normally aspirated or even a supercharged engine, turbocharging is more efficient. The power lost out the exhaust with the heat is reduced from 30 – 35% of a normally aspirated or a supercharged engine to 28 -30%.
Mechanical losses are in the range of 5-10% for a turbocharged engine, while a supercharger uses 10-15% to compress the air. What this means in the end it that net horsepower after turbocharging is in the 28 -30% rate, while other methods are less efficient.
Add an intercooler to the car and performance takes another jump. The intercooler can reduce intake air temperature from 120-130 degrees C to about 70-80 degrees C. Cooler, more dense air provides more oxygen to the cylinders and the potential for more power. The spark knock limit can be raised, and there are lower exhaust gas temperatures and reduced NOx produced because of lower cylinder temperatures.
There are several ways Subaru has reduced emissions on this engine. Some of them help economy as well. The most unique is their “Tumble Generator Valve”. These valves look like extra throttle plates for each cylinder, but they are located directly above the fuel injector. At temperatures below 60 degrees C, the computer closes the valves, forcing all the intake air to flow through a small idle air passage directly past the end of the injector. This high velocity, swirling air mixes with the fuel to provide excellent fuel delivery to the cylinders. The system works so well, that the engine can run at a stoichiometric 14.7:1 air fuel ratio as soon as the engine has started. Normally engines must run on the rich side during a cold start, but not this Subaru. After the engine has warmed up, the valve opens allowing full airflow to the engine.
Another innovation is a metal honeycomb pre-catalyst located directly below the exhaust manifolding. Because of its location it heats up very quickly to reduce emissions, yet the metal honeycomb has less resistance to flow than a conventional ceramic honeycomb converter. The metal honeycomb converter can also withstand exhaust temperatures 400 -500 degrees C higher than ceramic ones. But to prevent damage, Subaru has placed a temperature sensor in the pre-converter to sense catalyst damaging engine misfires. If the converter starts to overheat, the computer will reduce fuel delivery.
Last but not least is the WRX’s variable flow rate fuel pump. Three different flow rates are controlled by the computer to match driving conditions. At idle, the pump is operated at a 33% duty cycle. During cruise, the pump increases to 67%, and at wide-open throttle the pump runs at 100%. Reducing the movement of gasoline from the tank to the engine with a variable flow rate pump reduces the amount of heat added to the fuel and slows the evaporation rate. The load on the vehicle’s canister purge system is reduced so vapours are not released to the atmosphere.
Subaru’s technology is mostly hidden on the vehicle and perhaps not as exciting as some others, but it does provide customers with exiting engine performance and at the same time keeps emissions to a minimum.
This technology could easily be adapted to other vehicles, so it could have a large impact on future vehicles. Will it win AJAC’s “Best New Technology” award? We will have to wait and see.