The turbocharger

What is a turbocharger?

A turbocharger is a device fitted to internal combustion engines to increase power. In a normal car engine the amount of power the engine produces depends on how much fuel is being burnt in the cylinders. In a non-turbocharged engine a mixture of air and fuel is drawn into the engine as the piston moves down in the cylinder. The ideal mixture is 14.7:1 air to fuel (by weight) for gasoline. This is called the stoichiometric ratio. If you always try to maintain this ratio then if you add more air to an engine you must add more fuel. And if you are burning more fuel you will generate more power. The turbocharger is simply a device to force more air into the engine.

To increase the amount of air in the engine the turbocharger uses a compressor. The compressor consists of a finned wheel that spins at high speed in a specially shaped housing called a volute. Air is drawn into the center of the compressor wheel and accelerated as it is flung to the outside of the wheel. The volute channels and slows the air which causes its pressure to increase. Increasing the pressure means you can now have more air in a given space, such as the space inside a cylinder. The amount by which the air is compressed is called 'boost'.

The compressor wheel must run at very high speeds (up to and over 100000 rpm) to give useful levels of boost. The compressor wheel is connected to one end of a shaft which runs through the central core of the turbocharger. The shaft usually runs in plain bearings which need constant lubrication. Oil under pressure must be pumped through the central core constantly. When it is turning the shaft is essentially 'floating' on a cushion of oil. The oil also helps remove heat generated by friction. Without proper lubrication a turbocharger will very quickly fail. The core of the turbocharger may also contain passages through which cooling water is circulated.

At the opposite end of the shaft from the compressor is the turbine wheel. The turbine wheel is also contained in a volute housing but in this case hot exhaust gases from the engine are fed in from the edge of the housing and flow out from the centre of the wheel. The flow of hot gas causes the wheel to accelerate to the very high speeds the compressor needs to provide a lot of boost. Once the gases have passed the turbine wheel they flow through the normal exhaust system of the engine. Because too much boost can actually be damaging to an engine a way of limiting the turbine wheel speed is often needed. One way of doing this is with a wastegate. The wastegate allows the hot exhaust gases to bypass the turbine wheel. Instead of driving the turbine the gases simply flow through an alternate passage in the turbocharger directly into the exhaust.

Turbocharger Diagram

Simplified turbocharger diagram.

Cold air is drawn in from the left into the compressor (blue). The compressed air (light blue) then goes exits the turbo and it is fed into the engine. Hot exhaust gas from the engine (yellow) is fed back into the turbo. The hot gas flows past the turbine (red) rotating it as it passes. From there the exhaust exits the engine. The turbine is connected via a shaft (black) to the compressor.

Turbocharged diesel engine models:

A normally aspirated gasoline engine model cannot be modied to describe the turbocharged

diesel engine. However, the models reviewed in the previous section provide us with useful

techniques to approach our particular modeling task. Moreover, drawing a comparison

between the SI and CI engines, one can nd similarities as well as dierences, because as

mentioned earlier, there are common principles valid for all internal combustion engines. Several

models have been studied, covering the range from quasi-steady to lling-and-emptying

methods, and including intermediate levels of complexity.

The model developed by Winterbone (1977) is a wholly dynamic one, which represents the

engine and the turbocharger (TC) gas ows by a set of 30 interconnected rst order nonlinear

dierential equations. It employs the lling-and-emptying technique and, being based mostly

on physical principles, it gives an accurate description of the engine processes. Hendricks

uses the mean value method to create a series of models for dierent types of diesel engines

in (Hendricks 1986, 1989, Jensen et al. 1991). The mean value models are similar to the

quasi-steady ones with respect to simplicity. However, an important aspect of the mean value

method is the use of physically based models. In Hendricks' opinion, the fact that a given

system is complex does not necessarily imply that there are no underlying physical principles

which can give a simple overall picture of engine operation. The resulting models predict

correctly the steady state operating points as well as the most important aspects of dynamic

engine response. The model built by Jennings and Blumberg (1986) stands at a level between

the quasi-steady and the lling-and-emptying. It presents a method to calculate the engine

brake torque on an average rather than on an instantaneous basis, proving that there is no

loss of accuracy. A linearized model by Krutov is discussed in (Kullkarni et al. 1992). Flower

and Gupta (1974) give a state-space form of a simple discrete-time representation of the TC

diesel engine. Recently, Kao and Moskwa (1993) have tried to summerize the modeling

eorts at dierent levels of complexity presenting two models: mean torque production and

cylinder-by-cylinder. The rst one uses basically the quasi-steady approach and the latter

employs the lling-and-emptying technique.

Despite their dierences, all models describe the turbocharged diesel engine with charge

cooling, which is schematically presented on gure 1.4. The various modeling approaches for

the subsystems depicted in the diagram will be presented in parallel.

Compressor and turbine

Standard steady-state performance maps, which relate the mass ow rate and eciency of

the compressor to its pressure ratio, inlet temperature and the TC rotor speed are used to

model this subsystem. The disadvantage of this representation is its complete reliance on

empirical data provided by the compressor manufacturer. Unfortunately, it seems to be the

only possible approach, since it was used by all studied models. The turbine submodel is

built in a similar manner.

Intercooler

The process of compression raises temperature as well as pressure. Since the objective is to

increase inlet air density, intercoolers are often used to cool the air between the compressor

delivery and the cylinders, so that the pressure increase is achieved with the maximumrise in

density. The more involved intercooler models (Jennings et al. 1986, Winterbone et al. 1977)

employ energy balance on the gas and coolant ows through the intercooler to determine

their respective outlet temperatures. We adopted the simpler approach suggested in (Kaoand Moskwa 1993) to determine the outlet temperature using the cooling eciency.

Intake manifold:

Different approaches are used to determine the intake manifold (IM) pressure. The llingand-emptying method used in (Jennings et al. 1986, Winterbone et al. 1977) provides detailed

consideration of all factors associated with the IM. Assuming that the heat transfer is negligible,

one can achieve certain simplication as in (Hendricks 1986, 1989, Jensen et al. 1991,

Kao and Moskwa 1993). In addition to that the IM can be viewed as a no volume component

(i.e., no temperature change occurs), which leads to an even simpler model. The air massow into the cylinder depends on the engine speed and on the volumetric eciency. Jensen

et al. (1991) have stated the fact, that in the presence of a turbocharger, the volumetric e- ciency is a function only of the engine speed as opposed to the case of a normally aspirated engine, where it also depends on the intake manifold pressure.

Combustion and torque production

The major dierence in computational complexity of the models is due to their dierent representation of the torque production process. The combustion can be described based on physical principles as in (Winterbone et al. 1977). The advantage of this approach is its capability to simulate the instantaneous uctuations of the engine speed. However, they have no eect on the vehicle dynamics, therefore applying this method will only increase the computational burden, without contributing to the accuracy of our model.

The other commonly used technique for computation of the indicated torque is based on using steady-state data. The engine speed and the air/fuel ratio are the factors that aect the combustion process. Empirical characteristics, specic to the engine, are used to determine the thermal eciency and respectively the produced torque.

Exhaust manifold:

The importance of this subsystem comes from the presence of the turbocharger.

The pressure and temperature in the exhaust manifold are input conditions for the turbine. As in the intake manifold model, several levels of detail are possible. The temperature can be determined either from complete thermodynamic analysis of the combustion process, or using empirical data for the temperature rise through the engine.

The pressure calculation depends on the turbocharging method. There are two different ways in which the energy of the exhaust gases can be utilized to drive the turbine.

With constant pressure turbocharging, the exhaust ports from all cylinders are connected to a single exhaust manifold whose volume is suciently large to ensure that