Turbocharger Modelling using GT Power

Aim:-> Study of turbocharger and its modeling using GT Suit.

Theory:-

A turbocharger (technically a turbosupercharger), colloquially known as turbo, is a turbine-driven, forced induction device that increases an internal combustion engine's power output by forcing extra compressed air into the combustion chamber This improvement over a naturally aspirated engine's power output is because the compressor can force more air—and proportionately more fuel—into the combustion chamber than atmospheric pressure (and for that matter, ram air intakes) alone.

Turbochargers were originally known as turbosuperchargers because all forced induction devices are classified as superchargers. Technically, turbochargers are superchargers, however today, the term "supercharger" is typically applied only to mechanically driven forced induction devices. The key difference between a turbocharger and a conventional supercharger is that a supercharger is mechanically driven by the engine, often through a belt connected to the crankshaft, whereas a turbocharger is powered by a turbine driven by the engine's exhaust gas. Compared with a mechanically driven supercharger, turbochargers tend to be less responsive. By comparison, a turbocharger does not place a direct mechanical load on the engine, although turbochargers place exhaust back pressure on engines, increasing pumping losses. This is more efficient because while the increased back pressure taxes the piston exhaust stroke, much of the energy driving the turbine is provided by the still-expanding exhaust gas that would otherwise be wasted as heat through the tailpipe.

In contrast to supercharging, the primary disadvantage of turbocharging is what is referred to as "lag" or "spool time". This is the time between the demand for an increase in power (the throttle being opened) and the turbocharger(s) providing increased intake pressure, and hence increased power. Throttle lag occurs because turbochargers rely on the buildup of exhaust gas pressure to drive the turbine. In variable output systems such as automobile engines, exhaust gas pressure at idle, low engine speeds, or low throttle is usually insufficient to drive the turbine. Only when the engine reaches sufficient speed does the turbine section start to spool up, or spin fast enough to produce intake pressure above atmospheric pressure.

A turbo is made up of two halves joined together by a shaft. On one side, hot exhaust gasses spin the turbine that is connected to another turbine which sucks air in and compresses it into the engine. The turbocharger's compressor draws in ambient air and compresses it before it enters into the intake manifold at increased pressure. This results in a greater mass of air entering the cylinders on each intake stroke. The compression is what gives the engine the extra power and efficiency because as more air can go in the combustion chamber, more fuel can be added for more power.

Step by step turbocharger working-

1. Cool air enters the engine's air intake and heads toward the compressor.
2. The compressor fan helps to suck air in.
3. The compressor squeezes and heats up the incoming air and blows it out again.
4. Hot, compressed air from the compressor passes through the heat exchanger, which cools it down.
5. Cooled, compressed air enters the cylinder's air intake. The extra oxygen helps to burn fuel in the cylinder at a faster rate.
6. Since the cylinder burns more fuel, it produces energy more quickly and can send more power to the wheels via the piston, shafts, and gears.
7. Waste gas from the cylinder exits through the exhaust outlet.
8. The hot exhaust gases blowing past the turbine fan make it rotate at high speed.
9. The spinning turbine is mounted on the same shaft as the compressor (shown here as a pale orange line). So, as the turbine spins, the compressor spins too.
10. The exhaust gas leaves the car, wasting less energy than it would otherwise.

Types of turbochargers are as follows:

• Single turbo
• Twin-turbo
• Twin-scroll turbo
• Variable geometry turbo
• Electric turbo

1. Single turbo:

single turbocharger consists a single turbine, compressor and its related mechanism. The setup of a single turbo is much simpler than other types of turbochargers. By differing the size of the elements like the compressor wheel size and turbine within the turbo, completely different torque characteristics can be achieved. Large turbos will bring on high top-end power, but smaller turbos will provide better low-end grunt as they spool faster. They are a cost-effective way of increasing engine power and efficiency, and as such have become increasingly popular, allowing smaller engines to increase efficiency by producing the same power as larger naturally-aspirated engines, but with a lower weight.

Advantages:

1. Cost effective way of increasing an engine’s power and efficiency. Simple, generally the easiest of the turbocharging options to install.
2. Allows for using smaller engines to produce the same power as larger naturally-aspirated engines, which can often remove weight.

Disadvantages:

1. Single turbocharger tends to have low effective RPM range. This makes sizing an issue, one need to choose between good low-end torque or better high-end power.
2. Turbo response may not be as quick as alternative turbo setups.

2. Twin turbo:

Twin-turbo or bi-turbo designs have two separate turbochargers which implies as per its name, turbos which may operate either in a sequence or in parallel. In a parallel configuration, both turbochargers are fed one-half of the engine's exhaust. In a sequential setup one turbocharger runs at low speeds and the second turns on at a predetermined engine speed or load. Sequential turbochargers further reduce turbo lag, but require an intricate set of pipes to properly feed both turbochargers. Two-stage variable twin-turbos employ a small turbocharger at low speeds and a large one at higher speeds. They are connected in a series so that boost pressure from one turbocharger is multiplied by another, hence the name "2-stage." The distribution of exhaust gas is continuously variable, so the transition from using the small turbocharger to the large one can be done incrementally. Smaller turbochargers have less turbo lag than larger ones, so often two small turbochargers are used instead of one large one. This configuration is popular in engines over 2.5-litres and in V-shape or boxer engines.

Advantages:

1. For sequential turbos or using one turbo at low RPM and both at high RPM, this allows for a much wider, flatter torque curve. Better low-end torque, but the power won’t taper at high RPM like with a small single turbo.

Disadvantages:

1. Cost and complexity are increased, as nearly double turbo components are added in system.

3. Twin scroll turbine:

Twin-scroll also called as divided turbochargers. Twin-scroll turbochargers are better in nearly every way than single-scroll turbos. By using two scrolls, the exhaust pulses are divided. This turbocharger has two exhaust gas inlets and two nozzles, a smaller sharper angled one for quick response and a larger less angled one for peak performance. With high-performance camshaft timing, exhaust valves in different cylinders can be open at the same time, overlapping at the end of the power stroke in one cylinder and the end of exhaust stroke in another. In twin-scroll designs, the exhaust manifold physically separates the channels for cylinders that can interfere with each other, so that the pulsating exhaust gasses flow through separate spirals (scrolls).

Advantages:

1. A wider RPM range of effective boost can be achieved by varying scroll designs.
2. Large valve overlap design can be implement without hampering exhaust scavenging.

Disadvantages:

1. Feasible for large size engines due to cost and complexity of system.

4. Variable geometry turbo:

Variable-geometry or variable-nozzle turbochargers use moveable vanes to adjust the air-flow to the turbine, imitating a turbocharger of the optimal size throughout the power curve. The vanes are placed just in front of the turbine wheel like a set of slightly overlapping walls. Their angle is adjusted by an actuator to block or increase air flow to the turbine. This variability maintains a comparable exhaust velocity and back pressure throughout the engine's rev range. The result is that the turbocharger improves fuel efficiency without a noticeable level of turbocharger lag. A VGT Turbocharger can also work as an exhaust brake by closing off the exhaust entirely.

Advantages:

1. Provide number of variations in flow rate which can be adjusted very easily.

Disadvantages:

1. Cannot use for high temperature applications as nozzle may get damage due to large amount of heat.

5. E-turbo:

An E-turbo is a turbocharger that is propelled by both exhaust gas, like a traditional turbo, and electric power to spin the turbines and create positive air pressure (boost). The electric power is fed to two motors which are capable of running at speeds of 200,000 rpm and also should work at extreme temperatures of almost more than 1,000 °C. For the average commuter E-Turbo will use the electric power to allow the engine to run more efficiently. This electric power will be used to spool the turbo when there isn't enough exhaust gas present, this sensation is commonly known as "turbo lag". Since E-Turbos can get rid of the lag associated with traditional turbos, the overall size of the engine can be reduced and produce the same results. The turbo is no longer completely reliant on energy that is coming from the exhaust gasses. Traditionally turbo chargers would often reach peak performance closer to the top of an engines RPM range than the bottom. The E-Turbo will allow drivers to have the same amount or torque available through the whole RPM range.

Advantages:

1. By directly connecting an electric motor to the compressor wheel, turbo lag and insufficient exhaust gases can be virtually eliminated by spinning the compressor with electric power when needed.
2. E turbo allow for a more continuous torque output at varying speeds and loads, as well as 4x better transient response than a normal turbocharger,
3. E-Turbos also have the advantage of using the cars wasted exhaust gas energy and converting it back into electrical energy to be used later on.

Disadvantages:

1. Cost and complexity, as you now must account for the electric motor and ensure it remains cool to prevent reliability issues.
2. Packaging and weight become an issue, especially with the addition of a battery on board, which will be necessary to supply sufficient power to the turbo when needed.

Turbo-charger modelling:

~Model:

~Case setup:

~Geometry parameter:

~Compressor and turbine speed map:

	Compressor Map	Turbine map
Case 1
Case 2
Case 3

~Turbine parameter comparison:

~Compressor parameter comparison:

~Engine performance:

~Mass flow rate contour:

VGT EGR model:

~Circuit model:

~Case setup:

~Engine geometry:

~Compressor speed map:

~Turbine parameter comparison:

~Compressor parameter comparison:

~Engine performance:

~Mass flow rate contour:

Application of VGT:

VGTs tend to be much more common on diesel engines, as lower exhaust temperatures mean they are less prone to failure. Early gasoline-engine VGTs required significant pre-charge cooling to extend the turbocharger life to reasonable levels, but advances in technology have improved their resistance to high-temperature gasoline exhaust, and they have started to appear increasingly in gasoline powered cars. Typically, VGTs are only found in OEM applications due to the level of coordination required to keep the vanes in the most optimal position (complex geometry design) for whatever state the engine is in. However, there are aftermarket VGT control units available, and some high-end aftermarket engine management systems can control VGTs as well.

In trucks, VGTs are also used to control the ratio of exhaust recirculated back to the engine inlet (they can be controlled to selectively increase the exhaust manifold pressure until it exceeds the inlet manifold pressure, which promotes EGR (exhaust gas recirculation)). Although excessive engine backpressure is detrimental to overall fuel efficiency, ensuring a sufficient EGR rate even during transient events (such as gear changes) can be sufficient to reduce nitrogen oxide emissions down to that required by emissions legislation. Another use for sliding-vane turbochargers is as a downstream exhaust brake, so that an extra exhaust throttle valve is not needed. The mechanism can also be deliberately modified to reduce the turbine efficiency in a pre-defined position. This mode can be selected to sustain a raised exhaust temperature to promote "light-off" and "regeneration" of a diesel particulate filter (this involves heating the carbon particles stuck in the filter until they oxidize away in a semi-self-sustaining reaction - rather like the self-cleaning process some ovens offer). Actuation of a VGT for EGR flow control, or to implement braking or regeneration modes in general, requires hydraulic actuators or electric servos.

Conclusion:

Hence modelling of various types of turbochargers is completed and effect of changing RPM, injected mass and rail pressure on turbine and compressor speed map.