Turbochargers Explained: How They Add Power and More
Turbochargers are becoming more prevalent on new cars in the automotive industry, partly due to increasing government fuel efficiency requirements. Turbochargers allow smaller engines to bring the power of a larger naturally aspirated engine. It does this without sacrificing fuel economy under conservative driving conditions.
However, don’t expect improved fuel efficiency if your turbocharged engine gets driven spiritedly or tows heavy loads. When a turbo creates boost (increasing PSI), the engine requires significantly more fuel than operating at partial throttle and lower rpm (revolutions per minute, a measure of engine speed).
Getting 30+ mpg on the highway while still having a bit of fun once in a while makes turbocharged 4-cylinder engines popular in specific automotive segments. The turbo size will determine the engine’s boost threshold, which is the rpm required to start spooling the turbo. Large turbochargers will offer a higher boost threshold and can make more power. In contrast, small turbochargers have a lower boost threshold but don’t produce as much horsepower and torque. Increasing the size of the turbo will allow for an increase in power output at the cost of putting more stress on the engine and possibly shortening its lifespan.
How Does a Turbocharger Work?
Turbochargers work by using exhaust gas to spin a turbine that is attached to a second turbine that sucks air into the engine. Think of a turbocharger as an air compressor that runs on exhaust instead of electricity. When making boost, turbos can increase the PSI inside of the engine to pressure greater than the atmospheric pressure. A turbo needs enough exhaust gas to overcome its boost threshold, which both throttle position and engine rpm impact.
The hot exhaust gas heats the turbo raising the intake air temperature. Hot air has decreased density and less oxygen than cold air, which causes reduced engine performance. Before air enters the engine, it travels through an intercooler to lower the intake air temperature. Intercoolers primarily utilize air-to-air cooling since it’s reliable and inexpensive. In some high-performance and limited-space applications, air-to-water intercoolers are superior due to increased turbo responsiveness and compact footprint.
Types of Turbochargers
There are six main turbocharger designs, and all have their benefits and drawbacks. A twin-turbo engine can provide a wider power band than a single turbo engine at the expense of additional complexity and money. Turbochargers are expensive, and the more complex designs can result in a repair bill costing thousands of dollars if they fail.
Single turbo – A single turbo setup is most commonly found on inline engines because all of the exhaust ports are on one side of the engine. A large single turbo can make as much boost if not more than a twin-turbo setup. The tradeoff for maximum power output is a high boost threshold, creating a narrow power band.
Twin turbo – Twin turbos are usually on V engines with two banks of exhaust ports. Most of the time, the turbos will live on each side of the engine bay except for engines that utilize a hot V layout, and place the turbos in the engine valley. Two turbos allow smaller turbines to be used, which can widen the power band and improve low-end torque due to the lower boost threshold.
Twin-scroll turbo – By using two separate exhaust paths to the turbo, the impact of negative pressure due to valve overlap causes less performance degradation. Pairing cylinders that don’t fire consecutively helps eliminate interference in exhaust gas velocity. It leads to performance gains compared to a single-scroll turbo. Engines not initially designed with twin-scroll turbos will also require a new exhaust manifold to be compatible.
Variable twin-scroll turbo – A variable twin-scroll turbo builds upon the twin-scroll turbo’s performance gains by adding a second turbine. Turbines can operate independently to maximize exhaust velocity or simultaneously generate maximum power. Both turbines operate at higher engine rpm when the throttle position reaches a certain point. Variable twin-scroll turbochargers combine the benefits of small and large turbos while eliminating their drawbacks.
Variable geometry turbo – The addition of adjustable vanes around the turbine allows variable geometry turbos to provide a wide power band. The vanes are mostly closed during low engine rpm, allowing the turbo to spool quickly. The vanes open up at high engine rpm to reduce restrictions that would otherwise cause a decline in performance at the engine’s redline. Variable geometry turbos deliver outstanding performance at the cost of added complexity, creating more points of failure.
Electric turbo – Want a big turbo boost without the high boost threshold? Electric-assisted turbos can help get the turbine spinning. It does this when the engine operates at low rpm and doesn’t produce enough exhaust gas to spin the turbo effectively. E-turbos add complexity and weight since an electric motor with an additional battery is necessary.
Some turbos will solely run on electricity, but they are still in the early stages of development and can’t match the power output of exhaust-powered turbos. The battery required to power an electric turbo is significant, adding weight and complexity to a car. Manufacturers use small electric turbos to help lower the boost threshold on the larger exhaust-powered turbo.
Are Turbochargers Reliable?
With proper maintenance and good driving habits, a turbocharged engine shouldn’t have any significant reliability concerns compared to a naturally aspirated engine. Frequent oil changes become exponentially more important for a turbocharged engine because of the extra heat a turbo adds to the engine bay. If oil is past its recommended change interval, it can cause sludge buildup that may block oil passages that feed the turbo.
Suppose the turbo isn’t properly lubricated and cooled by the engine oil. In that case, it can cause damage and lead to a catastrophic failure that may destroy the entire engine. As a result, it can effectively leave the car totaled. Making sure a turbocharged engine stays happy and healthy involves a few requirements.
Turbocharger Reliability Tips:
- Don’t use low-octane gas
- Don’t boost with cold oil
- Don’t floor the accelerator at low rpm
- Don’t spool the turbo before shutting off the engine
Don’t use low-octane gas – Premium gas with 91 or 93 octane offers more resistance to engine knocking than regular gas with 87 octane. Turbocharged engines generate more heat and pressure than naturally aspirated engines and are more prone to detonation. Premature ignition of the gas, or detonation, can cause massive issues. It can effectively destroy an engine in severe and prolonged instances. Some turbo engines, however, can run on lower-octane fuel. Always be sure to heed the manufacturer’s recommendations when it comes to fueling your car.
Detonation happens while the cylinder is on the compression stroke of the combustion cycle and hasn’t reached the top dead center yet. The issue with detonation is that fuel combustion is fighting the compression stroke and placing opposing forces on the engine’s rotating assembly instead of powering it through the combustion stroke.
Don’t boost with cold oil – Cold oil is thicker than hot oil and causes extra stress on the engine. Don’t rely on the engine temperature gauge, as this measures coolant temperature instead of oil. If the car doesn’t use an oil temp gauge, it’s better to play it safe and wait a certain amount of time after the engine coolant reaches operating temperature.
Don’t floor the accelerator at low rpm – This mainly only applies to manual transmissions. That’s because most automatic vehicles will cause the transmission to downshift to a lower gear. Accelerating a car in its tallest gear will require the turbo to stay spooled longer under wide-open throttle compared to downshifting to a shorter gear lower in the range. The longer a turbo stays spooled at maximum boost, the more heat it will generate. When this happens, it can shorten the life of engine components ranging from the wiring harness to oil.
Don’t spool the turbo before shutting off the engine – Driving any engine hard or revving it before shutting it off isn’t a good idea. This is true regardless if the engine is turbocharged or not. A hot turbo is particularly effective at oil coking and must be cooled down before the engine stops running. Some cars utilize turbo timers, which allow the engine to stay running for a few minutes after removing the key from the ignition. Another method for cooling the turbo is an electric pump that continues to circulate oil or coolant without the need to keep the engine running.
Turbocharger Pros and Cons
Turbochargers are effective at adding power and efficiency to engines, but they also add complexity. It’s essential to weigh the pros and cons when deciding if a turbocharged engine will be the right choice. Turbos make an excellent choice for drivers who value performance. They’re also great for drivers not willing to make a big sacrifice in fuel efficiency under everyday driving conditions.
Improved power output – Turbochargers add additional power to an engine and allow a smaller engine to match the power output of a larger displacement engine. Increasing the turbo size can add more power and raise the boost threshold, effectively narrowing the power band.
Better fuel economy – Turbochargers can improve fuel economy by allowing a smaller displacement engine to produce adequate power. Don’t expect to see many extra miles per gallon when adding a turbo to a naturally aspirated engine. A naturally aspirated 2.0-liter 4-cylinder engine would probably get better fuel economy than a turbocharged 2.0-liter 4-cylinder engine. But that essentially compares apples to oranges.
Decreased throttle response – Turbochargers suffer from decreased throttle response, known as boost threshold and turbo lag. Boost threshold is the minimum rpm an engine needs to spool the turbocharger. Turbo lag is the time it takes to pressurize the air ducting that leads to the throttle body when the engine rpm is above the boost threshold.
Increased engine complexity – A turbocharged engine uses extra parts compared to a naturally aspirated engine. The turbo, intercooler, blowoff valve, and boost hoses are just a few of the parts necessary to turbocharge an engine. These additional parts can make a cramped engine bay a little claustrophobic and raise the level of complexity involved in some repairs.
Higher repair cost – Turbochargers aren’t cheap, and it’s not uncommon for them to cost upwards of $1,000. If a turbo fails, it can send pieces of metal into the engine and require a complete rebuild or replacement. A destroyed engine will cost thousands of dollars to repair and might sometimes exceed the car’s value.
Modified exhaust note – Turbochargers disrupt the exhaust gasses flow and change the exhaust sound. Comparing the exhaust note of a Porsche 911 GT3 and a Porsche 911 Turbo is one of the most notable exhaust comparisons. Yes, the induction noises bring a nice tradeoff for the muted exhaust note. But, it’s hard to beat the screaming sound of a naturally aspirated engine high in the rpm range.