N. Watson

Turbocharging the Internal Combustion Engine

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The Axial Flow Turbine

In the early days of turbochargers, axial flow turbines were used exclusively, assisted by experience gained from aircraft gas turbine technology after 1945. Indeed, it was largely turbine blade material developments, pioneered for gas turbines, that made diesel engine turbochargers practicable.

Turbocharging the Petrol Engine

Turbochargers have commonly been used on diesel engines for many years. In contrast, few turbocharged petrol engines have been built until recently and it is unlikely that a large fraction of the world’s petrol engines will be so equipped. It is the difference in the combustion system between diesel and petrol engines that prevents the full potential of turbocharging from being obtained on the latter.

The Radial Flow Compressor

Throughout the history of turbocharging, the type of compressors and turbines used have reflected the general state of the art, at any particular time, in the field of turbomachinery. The very first turbochargers (chapter 2) were built with radial flow single or two-stage compressors, incorporating backswept vaned shrouded impellers (figure 3.1a). Since the Second World War, due to great advances made in the development of the axial flow compressor, a few turbochargers have been built using multi-stage axial flow compressors, for experimental purposes. However, from an economic point of view their usage as turbocharger components is not attractive and has never reached the production stage.

The Radial Flow Turbine

There are two basic types of turbine suitable and used at present in turbochargers, the radial flow and the axial flow. The radial flow turbine is mainly used for small automotive or truck turbochargers; the axial type is commonly used for the large turbochargers applied to medium-speed stationary and railway traction engines and large marine engines.

Diesel Engine Exhaust Emissions and Noise

The upsurge of interest in environmental quality during the 1970s has resulted in much existing and proposed legislation for diesel exhaust emissions and noise. The bulk of this legislation is aimed at vehicles, but stationary power sources are not immune. The movement towards current legislation had two separate sources. First came general public hostility to the black smoke emitted from a substantial number of diesel engined trucks in the United Kingdom and Europe, followed by complaints directed at all noisy transportation means (aircraft, diesel trucks, cars and motorcycles). Meanwhile, in the United States the public nuisance of the Los Angeles ‘smog’ prompted the start of a major series of environmental quality laws. Although the number of diesel engined vehicles is increasing in the United States, the total proportion of vehicles so equipped (relative to petrol engines) remains low compared with that in Europe. Thus it was the petrol engine rather than the diesel engine that was originally singled out for criticism.

Pulse Converters and Summary of Turbocharging Systems

The pulse turbocharging system has been found to be superior to the constant pressure system on the majority of today’s diesel engines. Generally, it is used on all but highly rated engines designed for constant speed and load or marine applications. In chapter 7 it was made clear that the pulse turbocharging system is usually most effective when groups of three cylinders are connected to a turbine or turbine entry. When one or two cylinders are connected to a turbine entry, the average turbine efficiency and expansion ratio tend to fall due to the wide spacing of exhaust pulses. The ‘pulse converter’ has been developed to overcome some of these disadvantages on certain engines (although some versions are suitable for use on any engine) as a compromise between the pulse and constant pressure turbocharging system.

Charge Cooling, the Inlet and Exhaust Systems

The principal reason for turbocharging is to increase the power output of an engine without increasing its size. This is achieved by raising the inlet manifold pressure, hence increasing the mass of fresh air drawn into the cylinders during the intake stroke and allowing more fuel to be burnt. However, it is impossible to compress air without raising its temperature unless the compressor is cooled. Since the objective is to raise the density of the air, this temperature rise partly offsets the benefit of increasing the pressure, since (9.1) The objective must therefore be to obtain a pressure rise with a minimum temperature rise. This implies isentropic compression (figure 9.1) in which case the temperature rise will be given by the equation (9.2) Unfortunately due to inefficiencies in practical compressors, the actual temperature rise will be greater than that given by equation 9.2. In terms of the isentropic efficiency of the compressor (ηc) it will be given by (9.3) The more efficient the compressor, the closer the temperature rise approaches the isentropic temperature rise (figure 9.1).

Transient Response of Turbocharged Engines

The major increase in diesel engine ratings that has occurred over the last 30 years (figure 1.5) has largely come about due to turbocharging and subsequent charge cooling. Supercharging pressures have risen steadily, and it has become evident that some difficulties associated with turbocharging have become more significant at these higher ratings. The principal limitations — mechanical stresses of engine and turbocharger, thermal loading, turbocharger flow range and efficiency — have been discussed in chapter 11. An additional problem that has become more serious at the high pressure ratios now in use, is poor performance under transient conditions.

High-output Turbocharging

The demand for higher and higher power outputs from a specified size of diesel engine continues. Higher outputs may be achieved by increasing speed and/or BMEP. In the last 25 years there has been a steady increase in rated mean effective pressure (figure 1.5) but only a relatively small speed increase. There are many reasons why it is difficult to increase the speed of an engine substantially. Limitations arise due to the inertia of reciprocating parts, deterioration of volumetric efficiency, air/fuel mixing problems, etc. Some limitations are absolute, in that engine damage will result if a certain speed is exceeded; others result in a progressively deteriorating engine fuel consumption (efficiency). Furthermore different limitations affect different types (and sizes) of engine. However, the potential for increasing engine speed is rather limited, although gradual progress will be made (figure 11.1).