By Jim Kerr
Engine technology is an evolving art. Designs or systems that worked only partially a couple of decades ago now work superbly, largely due to the increased computer power in our vehicles. A great example is Chrysler's Multiple Displacement System (MDS), or GM's Active Fuel Management (AFM). Both of these systems de-activate valves and fuel delivery on half the engine's cylinders during light load conditions so the engine can achieve better fuel economy. GM tried this concept without success back in the early 1980's on Cadillac V8's, but the fast computers of today combined with computer-controlled throttle make the current systems work seamlessly.
Other technologies, such as turbocharging and supercharging, have been used for decades. Even back in the 1920s, there were cars with superchargers. Both turbochargers and superchargers pump air into the engine cylinders to increase efficiency and power but use different methods to do it. A turbocharger is an exhaust gas-driven air pump. As hot exhaust gases leave the engine, they are directed to a turbine wheel that spins very fast - in the 30,000 to 120,000 r.p.m. range! This turbine is connected by a shaft to another wheel with vanes, called the compressor. The compressor takes outside air and pumps it into the engine under pressure.
An advantage of turbocharging is that it uses the waste energy of the exhaust gases to produce more power. Another advantage is that the exhaust turbine "chops" the exhaust pulses up so that a more even exhaust sound comes from the tailpipe. A turbocharged engine doesn't need as big a muffler to keep the vehicle quiet.
There are disadvantages of a turbocharged engine too. It costs more to build than a normally aspirated (non-turbocharged) engine because of the additional parts required and the need to make internal engine components stronger to handle the increased power output. Another disadvantage is that turbochargers keep a lot of heat near the engine. Cooling airflow through the engine compartment must be an integral part of vehicle design.
One of the biggest disadvantages of turbocharging is turbo "lag". This is the time delay between stepping on the throttle and the moment the engine starts to accelerate. In older vehicles, this lag could almost feel like the engine was stalling. By using smaller turbochargers, variable vane turbochargers or combinations of two turbochargers in series, current vehicle designs have all but eliminated lag. BMW's twin turbo six-cylinder engine is a great example of how current technology enables an engine to produce power instantly at any time or speed.
Finally, anytime air is compressed, it also produces heat in the air. This is true of turbocharged or supercharged engines. Hot air entering the engine can cause pre-ignition, that damages the engine. By adding intercoolers, the manufacturer can cool the compressed air before it enters the engine, but this again adds complexity and cost.
Superchargers also pump air, but they are mechanically driven, typically by a belt from the engine crankshaft. The belt turns two rotors inside the supercharger housing that pump air into the intake manifold. There is less piping and plumbing necessary to install a supercharger than a turbocharger, but a supercharger does place a tremendous load on the crankshaft and drive belt. Computers now control bypass valves on these engines, so the supercharger only compresses the air when the driver demands power. The rest of the time, the bypass is open, allowing the supercharger to freewheel without any load.
The new Corvette ZR-1 uses a supercharger integrated with a liquid-to-air intercooler to help the engine produce 620 horsepower from a 6.2-litre V8 engine. Even though superchargers have been around a long time, engineers have now found how to increase supercharger efficiency by changing the angle of the lobes on the supercharger rotors. Sometimes even a small change can make huge differences.
Small displacement, high horsepower engines cost more to build than the monster V8 engines we used in the past, but these small engines can provide both fuel economy and power and I predict we will see a lot more of them in the future. Because of improvements in materials and assembly technologies, these new engines also require less maintenance than ever. By changing the oil, air filters and belts, modern engines can outlast the body of a new vehicle.
Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).
