Forced Induction Series - Part 2: The Turbocharger
Forced induction has been on the rise in new cars for the last 30 years, and turbocharging has taken the lead as the mainstay for boosting performance and efficiency over the last 10 years. Even though mechanically turned superchargers were the first mainstream form of forced induction, they are connected to the engine's crank, thus limiting the system's efficiency. At the turn of the 20th century Dr. Alfred Buchi invented the exhaust gas powered turbocharger. Invented to improve power of diesel engines, turbochargers were created to use exhaust gas to spin a turbine that is connected to a compressor that increases the volume and density of air supplied to the engine. The higher density air will increase the compression ratio inside of a cylinder and generate more horsepower and torque. The intake side of the process works similar to a supercharger with the main difference being that air is compressed at a variable rate. Where a supercharger is dependent on engine speed, turbos are dependent on volume and speed of exhaust flow. Because power is generated from the wasted heat of the exhaust, there is a large efficiency benefit to including a turbocharger on and engine.
Turbochargers have to be connected to the exhaust system of an engine. Many vehicles have the turbo connected to the exhaust manifold, just inches from the engine. As exhaust pressure builds against the turbine and spins it faster, the compressor is able to build higher pressures of air, this is called boost. The higher the pressure on the exhaust side the more boost that can be generated. In order to keep exhaust pressure regulated, a waste gate can be installed on the exhaust side to allow exhaust gasses to bypass the turbo. A blow off valve is used on the intake side to reduce back pressure by recirculating or venting excess boost. Compression of air will generate heat, and the temperature of the air will increase after it passes the compressor. To lower intake temperatures and improve fuel burn, the intake air will pass through an intercooler system, where either high flowing outside air or liquid from the coolant system will be used to exchange the heat and cool the boosted air. Extra fuel can also be applied to lower intake temperature, but this will make the air/fuel ratio rich and this can increase exhaust emissions.
Just like supercharging, turbocharging has its own set of flaws and drawbacks. The most prominent is called turbo lag. This is the amount of time it takes for boost pressure to build in the intake. Because pressure is dependent on engine exhaust flow rate, this can cause power surges and a sluggish takeoff from a standstill. Many new advances have reduced turbo lag, including twin scroll designs that separate which cylinders are pushing exhaust against the turbine to more efficiently speed up the compressor at lower RPMs. There is also more piping required with turbocharging. Extra exhaust and intake piping is needed, which requires more engine compartment space, as well as room for an intercooler and oil cooler. Also every turbo has a boost threshold, which is the minimum exhaust pressure required to start generating positive boost. Newer turbos are finding ways to lower this threshold by using a clutch system between the turbine and compressor in order to modulate speed, or an electric motor to spin the compressor at lower RPMs.
The original purpose of the turbocharger was to improve efficiency of a diesel engine, but compressing air with wasted exhaust gasses from an engine could easily increase the compression ratio for a more powerful burn. But materials and fuels of the era were not up to the task. The first successful use of turbocharging was with aircraft. Because outside air pressure decreases as altitude increases, turbos could help increase intake pressure to a constant level and maintain engine power in all environments. World War II saw an increase in aircraft using turbocharged engines, including the invaluable B-17 bomber. This lead the way for new developments and technology advancements in turbocharging.
It would take until 1962 when Oldsmobile introduced the Cutlass Turbo Jetfire V8 and a few weeks later Chevrolet added the Corvair Monza Spyder with a turbo flat 6. Even though many Turbo Jetfire owners eventually replaced the complex and unreliable turbo system with a conventional four-barrel carburetor intake system, turbos would soon take off due to the popularity of the Corvair and a German sports car company called Porsche replicating the success of the turbo flat 6 engine in 1975 on the 930 turbo model. Today’s turbo technology is more complex and reliable, allowing automakers to downsize engines and improve both power and fuel economy. Many leading sports cars are using turbocharged four cylinder engines, including the ever competing pony cars, Ford Mustang and Chevrolet Camaro. Porsche has even moved the venerable 911 to only turbo engines to reduce engine sizes, increase power, and improve emissions.
With such strict mandates on vehicle fuel economy and reduced emissions, the turbocharger is an ideal solution for squeezing maximum power and efficiency out of today’s and tomorrow’s internal combustion engines. If we’re lucky, the turbocharger might become as common in new car engine compartments as spark plugs and oil filters.