Turbochargers are one of the more common turbomachines out there today! As everyone is making efforts to lower carbon dioxide emissions in automobiles, and the automotive OEMs engage in a “horsepower war”, the turbocharger will likely continue to grow in popularity for both civil and commercial uses.
But how did these machines get so popular? That’s what we’ll be exploring in this blog miniseries! Today’s blog will introduce the concept of the turbocharger, and the beginnings of its development around the turn of the 20th century.
Turbocharging engines and the idea of forced induction on internal combustion engines are as old as the engines themselves. Their intertwined history can be traced back to the 1880’s, when Gottlieb Daimler was tinkering with the idea of forced induction on a “grandfather clock” engine. Daimler was supposedly the first to apply the principles of supercharging an engine in 1900, when he mounted a roots-style supercharger to a 4-stroke engine.
The birth of the turbocharger, however, would come 5 years later, when Swiss engineer Alfred Büchi received a patent for an axial compressor driven by an axial turbine on a common shaft with the piston of the engine. Although this design wasn’t feasible at the time due to a lack of viable materials, the idea was there.
Turbochargers vs Superchargers
What idea was that, exactly? And how did it differ from supercharging?
I think it’s important to quickly go over the basic differences between turbocharging and supercharging. Both offer “forced induction” for piston engines. A naturally aspirated engine simply will draw in atmospheric air as the intake valve opens, and the piston travels down to bottom dead center. A forced induction engine, pushes more air into the cylinder than what the dropping of the piston would pull in, allowing more air to be combusted, and thus generating more power and efficiency. While turbochargers and superchargers are both forced induction , how superchargers and turbochargers go about compressing that air is different. Superchargers are driven by the engine themselves, typically via a belt or gear. This uses some of the engine’s available horsepower, but doing so provides more horsepower back to the engine. The compressors can be either positive displacement configurations (such as a Roots or Twin-Screw), or a centrifugal supercharger.
Turbochargers, as mentioned before, use the air from the exhaust of the engine to drive a turbine, and the work of the turbine is transmitted on a common shaft to a compressor. The most common configuration is a radial turbine driving a centrifugal compressor similar to the one above in the supercharger diagram. However, there are other configurations ,seen in larger examples, such as an axial turbine driving a centrifugal compressor.
While it doesn’t need mechanical power from the engine to be driven, the turbocharger does rely on the exhaust gasses to run. As a result, the compressor may not be operating at its boost-threshold at lower engine speeds, and quick changes in engine speed are delayed as the turbine “spools up”. This is known as “turbo-lag”. While turbo-lag hampered turbocharger implementations in everyday cars, modern engineering has created workarounds for this, including twin scroll turbines, and variable geometry turbochargers; but that’s a discussion for a different blog.
Getting back on topic, the supercharger and turbocharger both found their first real world testing and application in the aviation world, especially as World War I began in Europe. Just as the First World War marked the culmination of an arms race in small arms and naval dreadnoughts, it also saw an aviation race; something that would be seen again in the Second World War 30 years later.
War Time Innovations in Turbochargers
As airplanes took to the skies, engineering teams on all sides of the global conflict were looking for ways to improve the performance of their airplanes. One particular challenge engineers were facing was the power loss these planes faced at higher altitudes, due to lower air density. Alfred Büchi decided to put his turbocharger to work. Unfortunately, this prototype would not work consistently, as Scoltock explains in Automotive Engineering, saying, “Although the turbocharger worked, it was less than reliable and could not maintain the boost pressure required.” (1) Büchi was not the only engineer looking to turbocharge military aircraft, however.
Enter August Rateau, a French engineer working to see how warplane performance at higher altitudes could be addressed. He decided to follow Büchi’s idea and turbocharge the Renault engines of several French aircraft, including one of the popular SPAD warplanes
Rateau’s experiment was partly successful. The turbochargers implemented to these airplanes significantly improved their performance, however reliability was an issue. Sherman writes, “One turbocharged Renault engine improved the rate of climb at 14,000 feet by 15 percent and boosted top speed from 104 to 120 mph. The British evaluated Rateau’s equipment, noting a 23 percent improvement in the rate of climb, but suspended research after a catastrophic turbine failure at 13,500 feet.” (2) So the idea was there, but reliability was still a serious problem.
The First Reliable Turbocharger
Around the same time as Europeans were making progress around WWI, an American engineer by the named Sanford Alexander Moss, was also working with the idea of forced induction to improve aircraft performance at high altitudes. Up to this point, Moss had a history of experimenting with utilizing turbines that extracted work from expanding combustion gasses in different applications. After authoring a thesis paper on gas turbines, he went to work for General Electric at just around the time that Sir Charles Parsons’s steam turbine was beginning to take the world by storm. (2) He spent the first several years at GE on gas turbine research and development, but unfortunately, this research didn’t pan out. His designs weren’t efficient enough, and in a similar problem to what Sir Frank Whittle encountered years later, there was a lack of available materials that could withstand the high temperatures. (2) After Rateau’s turbocharger was used in an experiment on a Liberty V12 aircraft engine in Ohio, Moss was asked by a colleague to help in an experiment to improve the Liberty’s performance at altitude, and thus came the Pike’s Peak climb.
Moss was able to design a 10-inch diameter turbocharger (both the turbine and impeller were 10 inches wide!) which spun at 20,000 RPM, had two bearings lubricated by engine oil at each end of the rotor. (2) The design also incorporated blow off valves to avoid compressor surge when the engine changes speed, and placed the turbo up near the front of the engine near the propeller, where it would get the most cooling flow from the propeller’s slipstream. (2) In short, Moss created a turbocharger very similar in principle to what is used in the modern day and age.
Since aircraft typically do not spend much time at sea level, and the entire purpose of forced induction was to improve engine performance at high altitudes, it made sense that the turbocharger would need to be tested at altitude without posing the risk of a catastrophic failure to a pilot.
Moss and his team chose Pike’s Peak in Colorado as the venue, since it is one of the tallest mountains with a road all the way to the top. Moss and his team built a test rig on the bed of a Packard truck, with a propeller mounted on the engine to properly simulate the losses from running a propeller.(2) This would provide a safe way of testing the engine and turbocharger’s performance without putting a pilot’s life on the line with a test flight. In 4 weeks, they completed 25 test runs up the 28-mile road to the top of Pike’s Peak. Over the course of these tests, mechanical failures were fairly low, however notably there were, “clogged carburetor jets, leaks in exhaust manifold joints, a leak in the compressor housing attributed to casting flaws, some broken turbocharger thrust washers, and some failed stay bolts that were supposed to keep the exhaust manifolds from warping in the heat”. (2) The performance results, however, were worth the work. As the engine ran at the absolute limits, it was able to crank out 377 horsepower, a larger figure than anything achieved in Ohio when the engine was tested at sea level while also running the risk of complete destruction. (2) In a 4-hour endurance test of the turbocharger, the engine was able to run at 313 horsepower consistently, and performed significantly better than with the turbocharger shut down, during which it could only put out 230 horsepower.
Thanks to his efforts, the groundwork was laid for the future of aviation at high altitudes, and in the wake of these tests, high-performance aircraft which relied on the turbocharger’s capabilities would become common.
In the next edition of this series, we’ll be looking at the interwar period with regards to turbochargers, as well as how World War II would see not just the creation of the jet engine, but the widespread implementation of the turbocharger in everything from high-end cars and heavy duty trucks, to numerous fighters and bombers.
Do you have a turbocharger project that you’re working on? Or perhaps you’re looking to design a turbocharger from scratch? SoftInWay is here to help! Reach out to us at firstname.lastname@example.org to learn more about AxSTREAM, as well as SoftInWay’s engineering services for turbocharger projects.
- Scoltock, J. (2010, July 15). Alfred Büchi the inventor of the turbocharger. Retrieved from Automotive Engineer : https://web.archive.org/web/20150405003800/http://ae-plus.com/milestones/alfred-bchi-the-inventor-of-the-turbocharger/page:1
- Sherman, D. (2001, May). Hill Climb. Retrieved from Air & Space: https://www.airspacemag.com/history-of-flight/hill-climb-2023375/?all