Turbocharging – Back to the Future
By Ian Grasso
We are in the midst of another turbocharging renaissance. It seems that every 10 years or so we see an uptick in turbo development, usually coinciding with some sort of government regulation on fuel economy. BMW recently returned to turbocharging after a 26-year absence with the outstanding and award winning N54 3.0L bi-turbo, which produces over 300HP and 295 Lb. ft of torque. Ford, GM, and Chrysler are all back in the fold – promising new turbo powerplants for their entire lineups from compacts to trucks, while many European manufacturers are building on already solid turbine technology and incorporating it into new cars. But unlike BMW, the latter manufacturers are not promising better performance along with their new engines—because of CAFE they are looking to replace their heavy and inefficient naturally aspirated engines with smaller and lighter engines with equal, or lesser, performance.
Because soon we all might be driving 4-cylinder turbo minivans or V6 turbo F-150s, it seems appropriate to look at the capabilities and limitations of turbocharging.
The turbo was invented to provide a higher manifold pressure to piston-powered aircraft during and after World War II, a process known as Turbo-normalizing. Turbo-normalizing is different from automobile applications in that its goal was simply to maintain the air pressure inside of an aircraft engine closer to 1 atm/101 kPa (the air pressure at sea level) while allowing aircraft to take advantage of the benefits of high altitude travel. Early turbo-normalizing was incredibly complex, as operators had to manually operate the wastegate and normalizing control for each engine. In automobile and commercial diesel engines, turbos are used for boosting – delivering a higher air pressure than that produced by the simple suction created by cylinder motion.
The classic boosting problem is that a sufficient amount of exhaust gas is needed to overcome the rotational inertia of the compressor – this is the dreaded “turbo lag.” Lag is realized by the driver quite simply—a lack of engine response at low engine speeds. Some cars are famous for their laggy performance, namely late 70-80’s domestics, the BMW 2002 Turbo, and early Saabs and Volvos. There were other reasons for performance issues – including unsophisticated valve timing and port fuel injection, but the size of the turbo was a considerable factor. Saab was and is known for strapping huge turbochargers onto small engines, unfortunately, the huge rotational inertia of those big turbines necessitated high idle and a heavy right foot to move the car from a stop. However, Saabs were also famous for having breakneck acceleration at passing speeds – in some cases faster than Porsche 911’s of the day.
To combat lag most manufacturers are now using small turbochargers. The smaller compressor allows it to spin up to boost speed quickly and limits turbo lag, but this workaround has its own pitfalls. Small turbos limit maximum engine speed – they do not let enough exhaust gas leave the engine during high power operation and produce the familiar turbo “wheeze” while they rapidly lose power at high RPM. Those who owned an early gasoline turbo or turbodiesel are very familiar with this lack of power at the high end. Of course, engineers design the engine management software to limit the driver’s use of high engine speeds in cars with small turbochargers. A good example is that BMW N54 engine – it produces its maximum power at 5800 RPM, while the mechanically similar, non-turbocharged N52 creates maximum power at 6650 RPM (272HP). Another example is the VW/Audi 1.8T engine, which was available in a wide range of configurations- from 150 to 225 HP. As HP and torque numbers increase, the maximum effective speed of the engine decreases (except in the case of the 225HP 1.8T used in the Audi TT – which used a different, larger turbocharger).
The holy grail of boost, which solves many of the problems of lag and wheeze, is the variable geometry turbocharger. The variable geometry turbocharger uses guide vanes (also called variable stator vanes) prior to the turbine that change the airflow of the exhaust gas based on engine speed. The volumetric efficiency of the engine is maximized through this method. Variable stator vanes are a technology from aviation jet engines that prevent turbine stall at high airspeed or angle of attack. This technology has been used since 1996 in commercial truck engines to great success, and is now making its way to automobiles.
The only manufacturer currently using this technology in cars is Porsche, in its 911 Carrera Turbo (997). The 997 produces maximum torque over a wide power band – 1,950 to 5,000 RPM– and has the ability to overboost the intake manifold for more power. Compared to the naturally aspirated Carrera, the Turbo actually produces maximum power at a higher RPM – something almost unheard of before.
Sophisticated fuel programs and reinforced components are absolutely vital to high performance turbo engines. Because these engines operate at high compression ratios, often close to 12:1, they generate more heat their naturally aspirated counterparts. Turbo engine blocks often contain more iron than those without forced induction and make heavy use of charge cooling. This method of cooling the combustion chamber by enriching fuel mixture can decrease the inherit efficiency of high compression ratio engines, but is mostly an issue for high performance cars and heavy duty trucks.
BorgWarner and Honeywell (Garrett) are the leading developers and suppliers of turbochargers, and they stand to profit from an increased use of the technology in automobile engines. But while turbocharging is a way to get more performance out of a smaller, lighter, engine– it is not the answer for fleets of 35 MPG plus automobiles. It is important to note that all other things being equal, a turbocharged version of a 4 cylinder engine does not get better gas mileage than the same engine naturally aspirated, but it will provide better torque and horsepower over a limited powerband. Like the fuel crisis of the late 70’s spawning a turbo Mustang, they will be used as a replacement for higher displacement engines – sometimes to the detriment of performance.
Additionally, turbocharging an engine does not simply mean strapping a turbo to the exhaust system. An intercooler, boost controller, and a myriad of other gadgets and reinforcements for high compression must also be installed. Overall, a turbo engine is more expensive than its naturally aspirated counterpart, even if it has less cylinders. The day may soon come where marking the “Turbo” box on the option sheet means a lower performance, albeit more expensive, option—if you even have the choice.
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