It is a simple equation – the more air and fuel you pump through an automotive engine, the more power it can produce. Since the history of American automobile production has revolved around large displacement power plants that make gobs of torque and horsepower, there has not been a great outcry for powerful small displacement engines. That is until lately.
There are many influences pushing the car buying public and automakers alike into smaller engines, such as the recent gas crisis, which pushed prices through the $4 per gallon mark, and new emission and fuel economy standards. Yet performance is one factor the public looks for when buying a vehicle and one the automakers use to sell vehicles.
So how do you get an engine, which has (literally) half the amount of cylinders as a typical production V8, to provide equivalent performance? While it’s been obvious for decades in Europe, it has recently become a sobering reality here in the states: you pump it up with a turbocharger. With one-third of GM’s North American engine volume being four-cylinders by 2011, and 21 percent of the four-cylinder volume will be turbocharged (a seven-fold increase over today’s volume of turbo engines), turbochargers are fast becoming the American automaker’s answer to producing small powerful engines while still delivering decent fuel economy and cleaner emissions.
History
The history of turbocharging is almost as old as that of the internal combustion engine (ICE). While automotive icons Gottlieb Daimler and Rudolf Diesel investigated increasing the power of their engines by pre-compressing the combustion air in the late 1800s, it wasn’t until 1925 when Swiss engineer Alfred Büchi was the first to be successful with exhaust gas turbocharging. And this began a gradual introduction of turbocharging into the automotive industry.
The development of turbocharged production engines in the U.S. hasn’t been a smooth process, but more a series of short bursts of breakneck activity followed by years of inactivity. In fact, some of the first American turbocharged cars, like the Chevrolet Corvair Monza and Oldsmobile F-85 Jetfire introduced by GM for the 1962 model year, led to dismal sales. But since the oil crisis of the mid-1970s, select turbo cars have pushed the technology forward significantly.
With the turbocharger’s entry into motorsports in the 1970s, especially into open wheel racing like Indy Car and Formula 1, engineers disproved the old adage that cubic inches are everything. No longer was it necessary to stuff massive engines into vehicles for performance, because smaller displacement turbocharged engines could provide equal or better performance.
PAGE 2Today, turbocharging is seen not only from a performance perspective, but is viewed as a means of reducing fuel consumption and emissions. Automakers now are looking at ways to combine turbocharging with technologies such as Direct Gasoline Injection and Hybrid technology to further refine their engines.
Operation
A naturally aspirated automobile engine uses the downward stroke of a piston to create an area of low pressure in order to draw air into the cylinder through the intake valves. Because the pressure in the atmosphere is no more than 14.7 psi, there is a limit to the amount of airflow entering the combustion chamber.
A turbocharged engine uses a radial fan pump driven by the engine’s exhaust that consists of a turbine and a compressor on a shared shaft. The turbine converts exhaust gases exiting the engine into rotational force, which is used to drive a compressor that draws in ambient air and pumps it at high pressure into the intake manifold to improve the engine's volumetric efficiency. This results in a greater mass of air entering the cylinders on each intake stroke.
There are four main components to a turbocharger: the housing, the impeller/turbine wheels, the center hub and the bypass.
The size and shape of the housings fitted around the impeller and turbine dictate the performance characteristics of the overall turbocharger. This allows the engine system’s designer to tailor the compromises between performance, response and efficiency to application or preference.
The impeller and turbine wheel sizes also dictate the amount of air or exhaust that can be flowed through the system. Generally, the larger the turbine and compressor wheels are, the larger the flow capacity. The shape, curvature and number of blades on the wheels allow infinite variability in design to tailor a turbocharger to a given engine.
The center hub connects the compressor impeller and turbine and uses a bearing lubricated by a constant supply of pressurized engine oil. While some systems are cooled by the engine oil, the preferred method is to use engine coolant to keep the lubricating oil cooler, avoiding possible oil coking from the extreme heat found in the turbine.
A bypass or wastegate is used to prevent over pressurizing the system. Once a specific boost pressure is achieved, part of the exhaust gas flow is fed around the turbine via a bypass. A spring-loaded diaphragm in response to the boost pressure usually operates the wastegate, which opens or closes the bypass.
Evolution
The turbocharger’s basic function has not fundamentally changed since the times of Büchi. However, according to turbocharger manufacturer BorgWarner, “The basic development goals for future automotive engines makes more refined turbocharging systems necessary.”
The design of a charging system leads to a conflict between the rated output of the engine and the combination of momentary response and maximum power. While a relatively large turbocharger is needed to attain maximum power, a smaller turbocharger is needed to maintain high boost pressure even at low engine speeds means.
PAGE 3To resolve this conflict, BorgWarner has developed a Regulated 2-Stage Turbocharging system (R2ST) that currently is being used on BMW 3-Series models. R2ST consists of two turbochargers connected in series: a smaller turbocharger (known as the high-pressure turbocharger) and a larger turbocharger (known as the low-pressure turbocharger). At low engine speeds, the smaller high-pressure turbocharger delivers spontaneous engine response without turbo lag. The fast build-up of boost pressure improves pull-away torque and accelerating from a standing start.
With increasing engine speed, the larger low-pressure turbocharger gradually takes over more of the work, providing more power when accelerating at higher speeds. The system continuously delivers high boost pressure over the entire engine speed range, ensuring optimum engine response at all times while reducing fuel consumption.
Servicing
Turbochargers are designed to last the lifetime of the engine and normally do not require any special maintenance. However, strict adherence to the engine manufacturer’s service instructions must be observed. Ninety percent of all turbocharger failures are due to either foreign bodies entering into the turbine or the compressor, dirt in the oil, inadequate oil supply or high exhaust gas temperatures.
The most important maintenance factor is clean oil. Because turbochargers can be easily damaged by dirty or ineffective oil, most manufacturers recommend more frequent oil changes for turbocharged engines. The use of synthetic oils, which tend to flow more readily when cold and do not break down as quickly as conventional oils, also is a common practice.
Because the turbocharger generate heat when running, many automakers recommend letting the engine idle before shutting off the engine if the turbocharger was used shortly before stopping. (Most manufacturers specify a 10-second period of idling before switching off to ensure the turbocharger is running at its idle speed to prevent damage to the bearings when the oil supply is cut off.) This lets the turbo rotating assembly cool from the lower exhaust gas temperatures and ensures that oil is supplied to the turbocharger while the turbine housing and exhaust manifold are still very hot. Otherwise coking of the lubricating oil trapped in the unit might occur when the heat soaks into the bearings, causing rapid bearing wear and failure when the car is restarted. Even small particles of burnt oil will accumulate and lead to choking the oil supply and failure.
The easiest way to diagnose a weak turbocharger is to observe the turbo boost. If the turbocharger does not show normal boost at full throttle (typically 9 to 14 psi), the system should be further diagnosed. One common, but overlooked, condition is excessive exhaust backpressure (often due to a clogged catalytic converter), which can prevent the turbo from developing its normal boost pressure.
Turbocharging doesn’t require automakers or consumers to give up the performance that they have come to expect from their vehicles. By harnessing exhaust gas that otherwise would be wasted, a turbocharger allows for more power from smaller engines with lower levels of emissions. It’s truly a case of having the best of both worlds – clean power that maintains or improves a vehicle’s performance and allows for significant gains in fuel economy and significant reductions in emissions.
