Turbo and Supercharger Basics
The more fuel that is burned in the cylinder, the more power an engine will make. However, fuel needs air (oxygen) to burn. Simply allowing more fuel into a naturally aspirated engine (a “non-turbo-d” engine) would simply be wasteful. Most of the extra fuel would not be able to burn and just end up in the exhaust system. The only thing pushing air into the cylinder is atmospheric pressure. The absolute most air that a cylinder of a naturally aspirated engine could possibly take in, is its cylinder displacement. (total displacement÷number of cylinders) However, most naturally aspirated engines volumetric efficiency is only about 80%, meaning the engine only takes in 80% of its full displacement per 720° cycle. A typical turbocharged engines volumetric efficiency is about 120%. This is why they make such a big difference in power. Forced induction is simply an air pump used to get as much air into the cylinders as possible, (when we want to) so we can add the extra fuel and burn it in the cylinder creating more power than what the engine would normally be capable of. Forced induction does this without adding much weight to the vehicle, compared to swapping the engine for one with a higher displacement. The extra air also increases compression pressures (when the engine is running). This also helps engine power by squeezing the air/fuel mixture much harder before combustion but too much compression can destroy an engine. That is why modern forced induction sytems are so complex. Boost pressures need to be regulated to prevent engine damage. There are two main types of forced induction, turbochargers and superchargers. Both of them have different advantages and disadvantages but both serve the same purpose: To get more air into the engine. Engines that were designed for turbochargers are generally made a bit more robust to take the added abuse. They usually have lower compression ratios to avoid high compression pressures while the turbo is active.
Turbocharger System Basics
Turbochargers are sometimes refered to as “free-power.” That is because they don’t use power to make power. They use exhaust gasses to spin a turbine, which spins a compressor wheel that pumps air to the engine. On a turbocharged engine, exhaust gasses come out of the engine still expanding from combustion. These exhaust gasses are directed to the turbine, or hot side, of the turbo charger. The exhaust spins the turbine before flowing through the exhaust system. The turbine is connected by a shaft to the compressor wheel. So, when the turbine gets spun by the exhaust gasses, the compressor wheel also gets spun, sometimes 10x+ the speed of the engine. The compressor wheel pulls air in the inlet and uses centrifugal force to push it out of the turbocharger and into the intake air stream. Since the can turbocharger displace more air than the engine can take in, pressure builds between the turbo and the engine. This is called “boost.” The more air the turbo forces into the engine, the more exhaust gasses come out of the engine and end up in the turbos turbine. This is why turbos are sometimes said to “run themselves.” All turbochargers will have an oil feed line to supply oil to the turbo to lubricate the shaft, and a return line to allow oil to return to the pan. Most turbos will also have coolant lines going to/from it for cooling. Turbos without coolant lines are cooled by oil alone.
Pieces of a Turbocharger System
A wastegate is a bypass for the exhaust gasses to go around the turbos turbine. When boost pressures approach the maximum allowable pressure, the wastegate opens to allow exhaust gasses to bypass the turbine and regulate boost pressure. The wastegates actuator can be connected directly to the intake manifold by a vacuum hose to sense boost pressure. Modern systems will have a control valve between the manifold and the actuator. These systems use a MAP sensor to sense boost pressure so the PCM can tell the control valve when to move the actuator. When the boost pressure becomes too much, the actuator will compress and open the wastegate. The wastegate can be a part of the turbo or a completely separate piece.
As pressure in the intake system builds, so does heat. The more you compress air, the more heat is generated. Hot air is less dense than cold air, so the heat actually hurts engine performance, by allowing less air to be delivered to the engine while maintaining maximum boost pressures. The intercooler cools the intake air charge to counteract this. The intercooler acts just like a radiator for the intake air charge. They use cool air passing through them while the vehicle is in motion to cool the intake air charge. They can be front mount, side mount, or top mount. Top mount intercoolers sit above the engine with a hood scoop which directs air to it. A side mount intercooler sits beside the radiator and condenser and gets its own airflow, but they tend to be smaller than the other two. Front mount intercoolers are the most common (pictured). They can sit in front or behind the condenser but always in front of the radiator.
When the operator is driving the engine hard, and the turbo is spooled up making boost, and suddnely lets off (to shift gears or slow down), the turbo continues to spin from momentum. However, if the throttle plate is closed air has nowhere to go. A blow-off valve will simply dump this air pressure into the atmosphere. This type of valve is what makes that nice “cheee” sound when you shift gears, but what you are hearing is wasted boost. A diverter valve is connected to the airstream before the turbo. When these are active air is recirculated through the turbo until the pressure is used up or the throttle plate opens again. They keep the turbo spooled between gears and do not waste any boost. A splitter valve is a “hybrid” of the two. Most of the air is recirculated but the valve dumps a little bit of pressure to the atmosphere to get the nice sound.
Turbo actuators can work in three ways. The first way is with pressure and is the most common. This system has a vacuum line from the actuator to the intake manifold. Normally the actuator holds the wategate closed with a spring. When the pressure gets too high, the actuator is pushed open, opening the wastegate to regulate boost pressure. The second type is the vacuum actuated type. In this system the turbo wastegate or veins stay in the “turbo-off” position until vacuum is applied to the actuator. This system is most common on diesels because diesels have a vacuum pump to create vacuum even under boost conditions. The third kind of actuator is a fully electronic actuator. This system is directly controlled by the PCM.
Variable Geometry Turbos or Variable Nozzle Turbine turbos are turbos without a wastegate. Insted of a wastegate, veins around the turbine move to change the angle that exhaust gasses come into contact with the turbine blades. This system makes high boost pressures at low engine RPM with little exhaust flow possible. This is why they are common on many diesel engines.
Turbo lag is the time between the operator demanding wide open throttle and the turbo actually starting to make pressure in the intake manifold. The turbo takes a little bit of time to spool up and start to displace more air than the engine normally takes in. One of the biggest factors in turbo lag is the volume of the intake system between the turbo and the engine. The more plumbing between the turbo and the engine the longer it will take to start to compress the air.
A supercharger is a device that pumps air into the engine by using some of the engines rotational power. They can be belt driven, chain driven or driven by meshed gears. Superchargers may have a by-pass valve. A by-pass valve allows air from the intake manifold to be recirculated through the supercharger if manifold pressures get to high. It works similar to a diverter valve in a turbocharged system. The by-pass may also be open when the engine is idling to prevent a lean air/fuel mixture. There are 4 basic types of superchargers: roots type, twin screw, G-lader and centrifugal. A roots type is the oldest and uses lobes on two shafts to trap air between the lobes and the case to pull air in towards the engine. This design makes good power at low RPM but is not that good at high RPM. Twin screw superchargers operate on the same principles as the roots type. The only difference is that the shape of the rotors resemble screws that wind into one another. These superchargers are more efficient than the roots type. The G-lader was used by VW for several years. G-laders are basicly a scroll-type supercharger. They use 2 sets of spirals, one fixed, the other rotates, to kind of slap air into the engine. The last kind of supercharger is the centrifugal type. This type has a compressor wheel like a turbocharger but has a belt driven pulley on the other side rather than a turbine. These make good power at high RPM.
Some superchargers will have their own lubrication system and some will need an oil feed and return line. Roots and twin screw type superchargers usually sit right on top of the intake manifold. These are the ones you see sticking out of the hood on some hot rod muscle cars. In a setup such as this, there is no intercooler. The engine is being fed air at the same temperature as the supercharger outlet. The purpose of having them stick out of the hood like that is to let them take in as cool of air as possible. Centrifugal and G-lader superchargers can be placed anywhere that a belt can drive them so they typically have room for intercooler plumbing.
Turbocharger Vs. Supercharger
- -High RPM power
- -Doesn’t cost power to make power (“runs itself”)
- -Room for an intercooler
- -Turbo lag
- -Complex plumbing
- -Lots of heat
- -No lag
- -Less heat
- -May have its own lubrication system
- -Costs power to make power
- -May not be able to run an intercooler
- -Max boost is limited to engine RPM
So, which is best? Depends on the application, and what is demanded from the car. There is no right answer here. If they can make a turbo with zero lag or a supercharger that doesn’t consume power we may have a clear winner. Until then, pick your favourite and run with it.
So how do I turbo my whip?
If you are looking at turboing an engine that did not come with one from the factory, do your homework first to make sure the engine can handle the boost you are giving it. Consider lower compression pistons or a head spacer. Installing a turbocharger on an engine that wasn’t designed for it can also be very costly. There is all kinds of custom pipe work you must be prepared for. I’m not going to tell you not to do it, but I am going to tell you that it won’t be the easist project. If you have an engine that came with a turbo from the factory, you can most likely upgrade to a slightly bigger turbo without much worry.