Turbocharger Basics
Turbocharged combustion engines make use of the high-temperature exhaust gasses generated from an internal combustion engine. Both spark ignition and a diesel engine can benefit from using a turbocharger. Typical exhaust temperatures can vary from as low as 600 up to 1,600 Fahrenheit. This depends on the type of engine you’re running and how hard you’re pushing it. Because the exhaust gas has such a high temperature relative to the temperature of the air surrounding your race car, it can be put to work.
The major benefit of using a turbocharger versus a supercharger for example is that a turbocharger makes use of the waste heat from the engine. A turbocharger by contrast draws power directly from the crankshaft. The backpressure of a turbocharger, however, can result in reduced power output of the engine. This is sometimes called a parasitic loss. The parasitic loss from a turbo is typically less than that from the supercharger. The reason for this is that the supercharger is connected directly to the crankshaft via a belt, and does not take advantage of the waste heat coming out of the exhaust. This is like starting a new campfire to cook your marshmallows instead of just using the waste heat from the fire you’re already using to cook your delicious steaks. Well…not exactly but you get the point.
In any case, the benefits and drawbacks of using a turbocharger versus a supercharger are worth saving for another post. The reason I mention it here is simply to spotlight the potential upside when using exhaust gas instead of adding a load directly to the crankshaft for your forced induction approach.
In a turbocharged engine, the exhaust gas from each cylinder is routed from the combustion chamber, through the exhaust manifold, then to the turbine inlet of the turbocharger. A classic turbocharger is split into two sides, the exhaust side and the intake side. As illustrated in Figure 1 below, the exhaust side of the turbo uses a turbine to extract as much mechanical work from the hot gas as possible. It then transfers that work, via a shared shaft, to the intake side where a compressor is used to compress ambient air and create a high-density high-pressure stream of air.
This compression process increases the temperature of the intake air. This effect is similar to how your bicycle pump gets hot when you’re pumping your bicycle tire to high pressure, or why most pumps require cooling fins. In turbocharged engines, the compressed air is routed to an intercooler, essentially a small radiator, to cool the air prior to reaching the engine. This further increases the density of the air. Denser air allows the engine to use more fuel and therefore produces more power.
For gasoline fuel, the target chemical balance between air and fuel is an air-to-fuel ratio of 14.7 to 1. So for every 14.7 grams of air that the turbocharger can compress into the combustion chamber, the injectors can add 1 more gram of fuel into the combustion chamber. Despite the fact that the denser air results in more fuel consumption, the power (work per unit time) extracted from the exhaust gas provides significant fuel economy due to overall reduced fuel consumption. And it’s not just sports cars that take advantage of this increased power. A lot of heavy-duty high tow capacity diesel engines use turbochargers as well to increase power and provide increased fuel efficiency.
Figure 2 illustrates the flow of gasses and major components that make up a conventional turbocharger setup. Figure 2 shows the setup for a single turbo. For smaller engines like in-line 4 or 6-cylinder engine configurations, it is typical to use a single turbo since the exhaust manifold is typically located to one side of the engine. There are of course high-end dual turbo in-line engines. On V-shaped engines, many car manufacturers use a twin-turbo configuration since there are typically two exhaust manifolds.
How Does an Electric Turbocharger Work?
While the concept of turbochargers and superchargers has been around for decades, (more than a century if you consider aircraft engines) electric turbochargers have only recently begun showing up in production cars of big original equipment manufacturers (OEMs). In this section, I’ll describe potential use cases when implementing an electric motor into a turbo and how OEMs are deciding to introduce electric turbos into their current production models.
Electrified turbochargers offer an opportunity to reduce or completely eliminate turbo lag. Turbo lag occurs at low engine speeds, or revolutions per minute (rpm), when the boost levels are too low for the compressor to provide any significant amount of boost (increased intake pressure) on the intake side. In part, this is a result of the work that can be extracted by the turbine. At lower engine speeds the exhaust gas is less hot and the turbine wheel speeds are also much lower. This results in significantly less work available to transfer to the compressor. Low boost results in minimal additional horsepower output from the engine at a time when parasitic losses from the turbo are highest.
Turbo lag can be quite noticeable in some engines and is the reason a lot of race car drivers prefer the faster and more predictable boost response of a supercharger. The boost from a supercharger is directly related to the engine speed since it is belt driven by the crankshaft. As the crankshaft speed increases, so does the boost pressure. As a side note, it’s interesting to note and compare the rpm speeds of engines versus turbos. Engines typically redline around 5,000-7,000 rpm. Some turbos on the other hand can max out at 300,000 rpm.
The turbo lag effect can be quite dramatic. A classic example of this is the introduction of Porsche’s first turbocharged 911 in 1975. The tricky part with turbocharging the 3.0 liter 200 hp engine (the turbo brought it up to 260 hp) is that due to the increased back pressure from the turbo, the engine was designed with a much lower compression ratio. The lower compression ratio allows engine designers to maximize the boost produced by the turbo. Engines can only hold so much boost. Without reducing the compression ratio, you risk blowing the engine or creating engine knock effects when using lower octane fuel. For the turbocharged 911, this meant that until you got to 4,000 rpm, you had little power to work with. Then all of a sudden, the turbo kicks in with a sudden surge of power and acceleration. Many drivers saw themselves losing control during this sudden surge. Imagine going around a corner and you get an unexpected kick in acceleration. It’s a good thing I didn’t have a 911 turbo when I was young. I wouldn’t have made it past 22. The 911 turbo earned its name as The Widowmaker due to the deaths of many drivers of the 911 turbo. Dax Shepard has a cool segment on this in Top Gear America (S02E02).
Now, how does an electric motor help with all this?
One approach to incorporating an electric motor into an engine system is to activate the compressor using an electric motor during the turbo lag speeds of the engine. For example, the electric motor could come on from engine idle and up to engine speeds where the turbo is generating less than 60-90% of peak power. Once the conventional turbo’s turbine is generating enough work, the electric motor could shut off and begin recharging for the next time the turbo boost is near zero. Figure 4 below illustrates what the setup for an electrified turbocharger might look like. In this scenario, the electric turbo is acting as a compressor while the conventional turbo gets up to speed. Using an electric turbo can not only eliminate turbo lag but also eliminates the need to reduce the size of the turbo for fear of increased turbo lag. Turbo size is no longer a factor. This allows turbo designers to maximize turbo boost.
This electric exhaust gas turbocharger technology has most recently been developed in F1 engines. Mercedes recently announced the release of the world’s first series-production engine to feature an electric exhaust-gas turbocharger which is technology directly adapted from the Mercedes-AMG Petronas F1 team. The F1 engine even uses the waste heat to generate electrical power and transfers that power to the drivetrain. With innovations in electric vehicle battery technology, there is no doubt car manufacturers will continue to iterate on the electric turbo design and continue to increase power with smaller engines.
How much Horsepower Does an Electric Turbo Add?
As much as an additional 60 horsepower! Electric exhaust gas turbochargers like the one Mercedes has developed have only recently begun hitting the sports car market. An interesting case study conducted by Motortrend looked at the BorgWarner e-turbo demonstration with a 2017 Porsche 718S Boxster. The BorgWarner e-turbo has similar technology to that used by the F1 teams. In fact, it also powers an additional motor and transfers power to the drivetrain. It also has a larger compressor wheel than the factory version without any turbo lag. Table 1 below illustrates some of the improvements. With these kinds of engine performance improvements, it will be no surprise when Porsche announces their own version of the electric-turbo-powered engine.
Stock | E Turbo | |
Horsepower | 350 hp | 400 hp |
Torque | 309 ft lb | 450 ft lb |
Highway Miles per Gallon | 26 mph | 39 mph |
Can I Install an Electric Turbo on My Car?
Not quite. The list of aftermarket suppliers of electric turbos (see Table 2) is small but sure to grow over the coming years. While the benefits are obvious, so is the added complexity in implementing it correctly. Luckily we at Autosports Tech have a project car to experiment on! A how-to series of articles are coming soon. We will start by rebuilding the engine of our 1997 E36 M3 in order to get it ready for our forced induction experimentation. We too will have to lower the piston compression ratios before we can add a turbo. Sign up for our newsletter and follow along as we experiment with getting our M3 project car powered up with an electric turbocharger.
Aftermarket Electric Turbo Manufacturers |
torqamp.com |
duryeatechnologies.com |