Increasing regulation for reducing emissions has forced the automotive industry to accept different technologies over the years in order to stay ahead of the market. In an industry that is so accustomed to internal combustion engines, new solutions such as electric motors and turbocharger systems have allowed experts in other industries to cultivate an influence in the automotive market. Specifically in the realm of turbomachinery, increased development has gone into designing turbochargers in order to minimize the effect and size of internal combustion engines. Design challenges are inherent in the fact that an engine is a positive displacement device whereas the turbocharger falls under aerodynamic turbomachinery. The two separate machine types have distinctly different flow characteristics, and the proper sizing of a turbocharger for its parent engine requires proper modeling of the engineering system as a whole.
In general, initial turbocharger sizing becomes a matter of obtaining the necessary boundary conditions required for a preliminary design. A thermodynamic cycle analysis of an ICE-Turbocharger system will allow the designer to obtain an initial idea of the bounds
Back when the California high-speed rail project was announced, Elon Musk (CEO of SpaceX and Tesla Inc. and perhaps the most admired tech leader of present day) was not only disappointed with this project, but also introduced an alternative to this system called the Hyperloop in 2012. Since the abstract of this project was introduced, many engineers around the world have started to evaluate the feasibility of this “5th Mode of Transportation”.
The general idea for the Hyperloop consists of a passenger pod operating within a low-pressure environment suspended by air bearings. At the realistic speeds estimated by NASA of 620 mph, the pod will be operating in the transonic region. While Japan’s mag-lev bullet train has succeeded at achieving speeds of up to 374 mph, the scale and complexity of a ground transportation system rising above 600 mph bring to surface an unusual number of engineering challenges. As well, brand new designs such as the one proposed by Musk have a certain amount of risk involved due to this technology inherently having no previous run history on a large scale.
One of the main setbacks in scaling different turbochargers for diesel, petrol, and gas engines is the inherit variability that different turbochargers would exhibit at low or high RPMs. In order to understand this further, a common term used to describe a flow characteristic of these machines is the A/R ratio. Technically, this ratio is defined as the inlet cross-sectional area divided by the radius from the turbo center to the centroid of that area (Figure 1). This ratio is essentially a metric for the amount of air that is allowed into the turbine section of the turbocharger.
For smaller turbochargers, lower A/R ratios allow the fast exhaust velocities to drive the turbine at lower speeds. This results in a more responsive engine and overall higher boosts at lower RPMs. However, once a vehicle starts to navigate at a higher RPM, smaller turbochargers experience a significant reduction in performance due to the high backpressure present in the system. This occurs because of the low A/R ratio limits the flow capacity and does not allow a sufficient amount of air to feed into the turbine. The same effect is present for larger turbochargers, only in reverse. They will perform most efficiently at higher RPMs, but in turn exhibit a significant reduction in performance at lower RPMs.
In order to overcome this phenomenon, many engineers have developed more complex turbocharger systems over the years, which attempt to leverage the benefits of each type of turbo. One of the first solutions to this dilemma was the twin turbo: simply comprised of two separate turbochargers operating in the system in parallel or in series. The problem with this system is that it disproportionately increases the cost, complexity, and space necessary for implementation.
An opportunity to discuss turbocharger usage and design with Softinway engineer Ursula Shannon in a question and answer format:
What are some of the major current turbocharger design challenges?
When it comes to turbocharger design, there are two challenges that engineers generally face. “Turbo lag” and turbo boost power at varying engine RPMs. “Turbo lag” is the time that it takes for the engine to produce enough exhaust to start the turbocharger “working”. This can vary greatly depending on engine size, turbocharger geometry, exhaust output etc. Ideally, engineers want to reduce this “Turbo lag” by as much as possible in any given situation, as during that time, the exhaust is “wasted” in a sense. Finding the most efficient configuration with all of the parameters in mind can be a very challenging scenario from a design perspective.
The turbo boost design challenge is one of efficiency at variable exhaust outputs. A smaller charger for example will start to boost at lower engine speeds while a larger one will start to boost at engine speeds. The trade off however is that a smaller turbo will start to create what is known as back pressure at higher speeds, and this results in a loss of potential power. A larger turbocharger, will be able to create more overall boost at higher speeds, however the “Turbo Lag” is more pronounced as more engine exhaust is required. Minimizing these trade offs is another key challenge in turbocharger design.
Finally, the process of turbocharger design process itself is complex, and requires highly specialized software such as our own here in Softinway (AxSTREAM).
AxSTREAM Turbocharger Design Software ( Flowpath Design and Optimization )
AxSTREAM Turbocharger Design Software (Compressor 1D Design and Analysis)
What are some design changes do you see coming to turbochargers in the future?
As I mentioned some of the challenges engineers face in turbocharger design, currently many technologies and methods are being developed to alleviate some of the issues faced.
Two stage turbochargers are good example of trying to offer a solution to the boost powers at varying engine outputs, using a smaller turbocharger that operates at low RPMs and a larger turbocharger that operates at higher RPMs.
Electronic energy storage setups are currently being developed and used in European race cars which uses the output side of the turbocharger as a sort of generator which stores energy in a battery from turbocharger operations and acts as a boost during a turbocharger’s lag period.
Almost every car produced nowadays is propelled by a Reciprocating Internal Combustion Engine (RICE). Fueled by gasoline or diesel, these engines have pistons inside the cylinders which move up and down, compressing and expanding the mixture. They are connected to a crankshaft that converts the movements into a rotational motion to turn the wheels that move the car.
Big engine makers are constantly researching and developing to make engines lighter, more powerful, more fuel efficient, and more environmentally friendly. But isn’t there a better way to power the automobile Industry?
After WWII, the gas turbine (GT) engine (turbojet) was a trend for aircraft propulsion. A few companies did some research and explored the idea of using a GT to power a car. The GTs mentioned here are evidently not turbojets, but turboshafts where almost any power is used from exhaust. Instead there is a power turbine activated by the combustion gases that would be connected to a gearbox and consequently to the wheels.
The first company to ever build a GT car was Rover in 1950 with the JET 1. A few years later GM also built a number of futuristic prototypes called the Firebirds.
While some companies came up with GT cars, it was Chrysler that invested the most in this concept, spending a lot of time and money doing R&D for almost 20 years (from 1950 to 1970).
For the first time ever in 1963, more than just a prototype came out and fifty-five cars were built and given to people to try as a daily mode of transport. Although reviews were generally good, the project did not go any further.
The car used the A-831 GT, a dual spool, and free shaft engine with an output of 130 horse power, weighing 410 lbs. It comprised a single stage centrifugal compressor rotating at a maximum of 44,600 rpm (CR=4:1), the air, after leaving the compressor, would go through 2 regenerators working as heat exchangers using hot gases from the exhaust to increase temperature before the combustion to reduce fuel consumption. From the combustion chamber, the gases travelled by a single stage axial turbine that activated the compressor and the accessories and posteriorly through a variable geometry power turbine nozzle, to control the amount of gas that would go through, before the free single stage axial power turbine that was connected to a Torqueflit, 3 speed automatic transmission.
Chrysler ended up destroying all but nine of the cars. Today they are in museums or in Jay Leno’s garage.
Why didn’t a car with a well-reviewed engine and a futuristic concept stick? Why are GTs present in so many industries but not in Automotive? They’re faster, simpler, have a better power-to-weight ratio and require less maintenance.
While they have advantages, however, they also have some disadvantages. Some of the Chrysler car users mentioned a lack of engine brake, lack of support when maintenance was needed and noise. This could easily be solved, and Chrysler did fix some of this issues. What ultimately killed the project was the low throttle response in comparison to RICE and fuel consumption. GTs are very fuel efficient for high speeds with constant throttle, but cars operate at relativity low speeds with a big vary of throttle. This has a big impact in the GT fuel efficiency. Although the company tried to resolve this issue, the 1970’s oil crisis made the scenario even worse.
It’s possible that soon electric hybrid vehicles will mean the GT finally becomes a viable power source for cars. Whether braking or accelerating, the micro gas turbine runs at a relatively constant rpm and generates electricity to be stored in batteries. Those batteries are connected to electric motors (4 in the Jaguar C-X75 case, one on each wheel) that run the car. Two known prototypes are the Jaguar C-X75 using two 70kW micro turbines produced by Bladon Jets, and the Capstone CMT 380 using a single 30 kW micro gas turbine
A turbocharger (TC) has to provide a required pressure ratio for efficient combustion and operation of an internal combustion engine (ICE). The turbocharger consists of a turbine and a compressor sides on the same shaft. The turbine utilizes the energy of exhaust gases while the compressor forces the air into the engine. The compressor with a wide operating range is a strict requirement in the automotive industry because the unit has to operate across all of the ICE regimes.
Even though any compressor has a design point, the ability to operate at low and high mass flows is critical for TC compressors. To satisfy the operating range requirement, a designer tries increasing mass flow at choke and decreasing mass flow at surge. This is quite a challenge. For smaller mass flow rates, the impeller outlet and diffuser should be optimized. The choice of a vaneless diffuser is always justified by increased flow range at the cost of efficiency.
To increase the right-most mass flow limit, a designer optimizes the compressor inlet. The common practice is to design blades with large inlet metal angles. Increase in inlet angles open larger area for the flow to pass. This, in turn, leads to large incidence angles at design point. Therefore, many TC compressors are designed with large positive incidence in the design point. The incidence angle increases for every speedline going toward the surge line. Incidence distribution on a TC compressor map is shown in the figure below. It is equal to +12 deg (with respect to tangent) in the design point.
Design point: An operating condition where a compressor reaches maximum efficiency
Compressor Map: Pressure versus mass flow characteristic at different rotational speeds and isoefficiency contours
Speedline: Dependence of pressure on mass flow rate for a given shaft speed
Surge: Left-most point on a compressor map for a given shaft speed
Choke: Right-most point on a compressor map for a given shaft speed
Incidence: The difference between inlet flow and metal angles. If an incidence is small, the flow has less resistance to enter the impeller.
The history of turbochargers in Formula 1 is pretty fascinating. Turbochargers were initially introduced in 1905, applied to large diesel engines in the 1920’s and found their way into commercial automobiles in 1938. However, it took a few more decades for the turbochargers to be used in Formula 1 car racing.
When Renault decided to enter the sport in 1977, they started their engines based on the novel turbocharger concept. As one would expect, their first design suffered from constant reliability problems through all the races it competed in. As Renault focused their development entirely on the engine, the car’s aerodynamics worsened; it suffered a huge turbolag under acceleration, and when the boost finally triggered the tires were not able to handle it . “So the engine broke and made everyone one laugh”, Jean-Pierre Jabouille, the driver, admitted in an interview. At the time, everyone was looking at the turbo engines as something that no one would ever hear about again.
Turbochargers, nowadays, are becoming increasingly common in the internal combustion engines of automobiles in order to improve fuel economy and meet government emission regulations. A turbocharger must provide a designed increase in pressure under load condition (design point) while generating enough power at the low end (loss mass flow region). Internal combustion engine working characteristics, however, prevent a centrifugal compressor from generating enough boost at the low end when radial turbine rotational speed is low. Continue reading “Free Webinar: Maximizing Turbocharger Boost with Advanced Design Features”→
Yes, the Formula 1 races have begun. The world is three races in with the fourth Grand Prix scheduled for April 20 in China. As the world watches in awe at the versatility and speed (let’s face it, the races are all about the cars, right?), engineers marvel at the aerodynamics, energy recovery systems, turbochargers and internal combustion engines (because we love engineering).
With the ongoing movement toward global environmental protection, regulations related to the exhaust emissions and fuel consumption of automobiles are being strengthened. To cope with these requirements, turbochargers are an effective tool to improve fuel consumption and reduce carbon dioxide emissions, by reducing the engine weight and friction loss.
Since a turbocharger supplies compressed air to an engine, it can reduce the engine displacement relative to an atmospheric engine for the same power. Variable geometry turbochargers, which can control the boost pressure according to the engine operating conditions, are becoming increasingly popular, creating a demand for a centrifugal compressor with a wide and stable operational range. Continue reading “At a Glance – Turbochargers”→