Turbocharged Engines

Almost all auto manufacturers are moving towards direct-injected turbocharged engines. This is because turbocharged engines are more efficient compared to naturally aspirated engines, in terms of fuel efficiency and power. Turbochargers allow smaller displacement engines to have more power compared to their naturally aspirated counterparts. I have created this article for those who are considering a vehicle with a turbocharged engine, as well as those who already own one and willing to learn how they operate and perhaps troubleshoot any issues. As you read this article, you will get an idea on how complicated turbocharged engines really are, but learning how a turbocharged engine works will at least give you the necessary knowledge on how to approach any issues should they arise. So my goal with this article is to help those who have any sort of issues with their turbocharged engine, or those who just want to learn about them!

How it Works:

A turbocharger is simply an air compressor that forces air into the engine to increase its volumetric efficiency, and therefore, power. The turbocharger itself has two main components, the impeller and the turbine. The impeller is found on the cold side (or compressor side) of the turbo, and it's responsible for sucking in air to force into the engine. The turbine is found on the hot side (or exhaust side) of the turbo, and it's connected to the impeller by a shaft and center bearings. The exhaust gases from the exhaust manifold are used to spin this turbine, and therefore, powers the turbo by spinning the impeller.

Now that we know how a turbocharger works, let's look at the turbocharged system as a whole:

We just saw that the turbocharger compresses the air it sucks in, but when the air is compressed it generates heat, which increases the temperature of the air and lowers its density. You might be wondering...what's the point of a turbo if it forces in hot air into the engine? Hotter air doesn't make more power, does it? Well, the compressed air needs to get cooled down before it's forced into the engine. This is where the intercooler comes into play. The intercooler is a heat exchanger (usually air-to-air, but can also be coolant-to-air) that is responsible for cooling down the compressed air before it is forced into the engine, and this cooled air is known as charged-air, and the extra air pressure created by this charged-air is known as boost. This is why turbocharged engines use additional MAP sensors (manifold absolute pressure) along with a MAF sensor to meter the air entering the engine. This is the air that provides the engine with more power. Cooler air is more dense, and denser air has more oxygen molecules in a fixed volume. The more oxygen an engine can get, the more fuel it can inject, and therefore, the more power it can make. Remember that this air is being forced into the engine, and this ultimately increases the volumetric efficiency of the engine.

Volumetric efficiency is exactly what it sounds like: the engine's efficiency of sucking in the proper amount of air to fill up the cylinders' volume. Naturally aspirated engines typically have a volumetric efficiency of around 80% (80% of the cylinder's volume) to a maximum of 100% (full volume of the cylinder), and this is because naturally aspirated engines use vacuum to intake air into each cylinder. The piston traveling downwards during the intake stroke acts like a "syringe" that sucks in air, but most of the time this effect doesn't completely fill the cylinder's volume with air (i.e. 80% of the volume is filled with air), especially at higher engine speeds when the valves and pistons are moving faster. Now in the case of turbocharged engines, the air is being forced into the cylinder rather than being sucked in by the piston, and this basically allows 100% of its volume to be filled with air and perhaps even more air than the given volume as the turbo builds boost (more than 100%). This is why turbocharged engines are known to have a volumetric efficiency of 100% or more. It is the increased volumetric efficiency that makes the engine more powerful.

Now let's talk about the two other components that are used in a turbocharged system: the bypass/blow-off valve and the wastegate. The bypass/blow-off valve is used to release excess boost out of the pipes found between the turbo outlet and throttle body. When you let off the accelerator pedal, the throttle plate closes, leaving the pressurized air inside the pipes with no where else to escape other than back out the turbo impeller. Boosted air escaping out of the turbo impeller is known as compressor surge, and it is detrimental to the turbo's health because it puts stress on the impeller and bearings. So the bypass/blow-off valve that is found either on the turbo housing or charged-air pipe releases the excess boost when the throttle plate closes to prevent compressor surge. This valve operates off a vacuum line connected to the intake manifold, so when the throttle plate closes, the manifold pressure drops relative to the pressure of the charged-air pipe, which causes a pressure imbalance that allows the piston to open up to release the excess boost.

The wastegate is responsible for regulating the amount of exhaust gases used to spin the turbine on the exhaust side, which regulates the amount of boost a turbocharger can generate. It is a physical "gate" that opens and closes an alternate exhaust path that bypasses the turbine side of the turbo. Again, it uses a vacuum line connected to the intake manifold to monitor boost, so when the intake manifold reaches maximum boost (determined/set by the pneumatic wastegate's spring force), the wastegate opens so that the turbo doesn't over-boost the engine (safety feature). 

*Note: Modern turbocharged engines now use electronically-controlled bypass valves and wastegates instead of pneumatically-operated ones. So the ECU uses various MAP sensors instead of vacuum lines to monitor boost, which helps the ECU determine when to open/close the bypass valve and wastegate.

Misconceptions:

The biggest misconception about turbochargers is that they wear out the engine faster due to the the extra power and pressure associated with them. Well, this isn't exactly true because engines are built specifically for a turbocharged application. Auto engineers are not taking naturally aspirated engines and just slapping turbos on them. The engine itself has to be built with stronger internals in order to handle the turbocharger's boost. When a person turbocharges a naturally aspirated engine, does it blow up most of the time? Yes, because that engine wasn't built to handle boost from a turbocharger. When you buy a turbocharged engine, does it blow up most of the time? No, because it was specifically designed and built to handle boost. Now I am NOT saying that turbocharged engines are trouble free. In fact, turbocharged engines have a bigger potential for issues compared to a naturally aspirated engine, and this is because of the complexity (which we just observed). For example, if you get a code for running rich on your naturally aspirated engine, then either it's getting too much fuel or not enough air or sometimes both. In terms of diagnosing air restrictions, you just have to deal with the air box, intake pipe, or the MAF sensor depending on the issue. But if you get a code for running rich on your turbocharged engine, then you have a whole turbocharger to deal with because it's part of the air intake system. It could be the bypass valve stuck open, it could be the wastegate stuck open, it could be the physical turbine/impeller that is stuck. In other words, there are a lot more components to check when you have an issue relating to the turbo. In my opinion, if you know exactly how a turbocharged engine works, you'll have a significantly better time diagnosing and fixing the issues. When I first bought my BMW 428i, I learned the ins and outs of how the turbocharged system works specifically on my car (and in general), so if an issue ever comes up, I will be better prepared to deal with it. And it will also give you a better idea on how to take care of it to prevent issues. Turbocharged engines don't like slow engine speeds, they like to be driven hard to get the turbocharged system working to its full potential. Carbon build-up is the main killer for turbocharged engines, and so you want to get the turbo nice and hot to burn off any carbon before it builds up and causes problems with the wastegate, turbine, or impeller. The PCV system on these modern turbocharged engines vent the crankcase pressure into the intake before the turbo, leading to potential carbon build-up on the impeller itself. Direct injection paired with turbochargers only exacerbates the carbon build-up issue. Direct injection is well-known for causing carbon-build up on the intake valves because gasoline doesn't "wash" over them like it would in port injection. This is why carbon cleaning is a MUST on turbocharged engines every 60-80k miles or so. 

Troubleshooting Tips:

When it comes to turbocharger issues, you NEED to data-log to see how the engine is operating while driving. Turbocharged engines operate completely different at idle and under acceleration. Data-logging is necessary in order to see how the turbo is functioning in terms of the boost it is making. Do some research to find out how much peak boost your turbo is supposed to provide and at what engine speed. Then compare and see if your turbo is providing peak boost, under-boosting, or over-boosting.

Some example trouble codes relating to turbocharger issues:

• P0299 -- Turbocharger Under-boost condition
• P0234 -- Turbocharger Over-boost condition
• P0045 -- Turbocharger boost control solenoid circuit/open
• P0047 -- Turbocharger boost control solenoid circuit low
• P0172 -- Engine rich running condition
• P0171 -- Engine lean running condition

Things to check:

1. Induction piping and Intercooler: You want to make sure all the clamps/C-clips from the turbo all the way to the throttle body are sealing the pipes and intercooler tightly. Turbocharged engines operate with a pressurized system, so loose piping/clamps WILL cause boost leaks. There are many ways to test for boost leaks, and you can search up some potential ways on YouTube and Google.
2. Vacuum lines/Electrical connections: Vacuum lines are VERY important on turbos that are pneumatically operated (depends on your application...refer to the note stated earlier). If any vacuum lines are cracked or disconnected, it will cause issues with how the turbo operates. Now if your engine uses an electronically-controlled bypass valve and wastegate, then check the condition of the wires and harnesses to make sure they are not corroded.
3. Wastegate: The wastegate determines how much exhaust gas is diverted to the turbo to spool it, and therefore determines how much boost the turbo makes. You want to make sure the wastegate is operating properly. You can have someone rev the car while you look at the wastegate. You should see the wastegate arm "push" or "pull" as the person revs the engine. If you don't, then the wastegate may be your problem. But of course, you can search up other ways to test your wastegate for your specific engine on YouTube and Google. You should check the wastegate for an under-boost or over-boost condition.
4. Turbo Bypass Valve/blow-off valve: You want to make sure the bypass valve is working as it should. This valve controls when to relief excess boost pressure when you let off the throttle. If it is stuck open, then the system won't build any boost in the first place. Check the vacuum lines (or electrical connections if electronically-controlled) associated with it. You should check this valve for an under-boost condition.
5. MAP sensor(s): You want to make sure your MAP sensors are giving correct data to the ECU on how much boost the turbo is producing. Maybe give it a cleaning as it may be super dirty. 

As a final note, I highly recommend using an API SN+ rated oil in ANY turbocharged engine to prevent LSPI (Low Speed Pre-ignition). SN+ rated oils use a different formulation (the phrase "calcium down, magnesium up") to reduce the risk of LSPI. It was determined through various experiments that the calcium content in oil increased its ability to ignite under high pressure and heat, which therefore, causes LSPI. You can learn more about SN+ oils here and LSPI here. 

I hope this article was useful to those who are considering or already own a turbocharged engine.

- Razmig Bartassian (@razmig)