1000

1500 2000

Engine speed (rev/min)

Figure 2.56 Rematching the turbine to improve low speed torque and smoke, with no change in fuel delivery consumption and high b.m.e.p. of match 3 at 1000 rev/min, with the low cylinder pressure and low fuel consumption of match 1 at 2500 rev/min. This is an ideal that has attracted turbocharger and engine manufacturers for many years, since it reduces the speed dependence of turbine specific available energy shown in Figure 2.55. The problem is one of engineering a cheap, reliable and effective system, and has not been solved to date. Prototypes exist, but no system is currently available in mass production (1983).

The problem of overspeeding the turbocharger and coping with high cylinder pressures becomes prominent when engines which operate over a very wide speed range are turbocharged and matched for good torque back-up. The small turbocharged passenger car diesel engine falls into this category. A method of avoiding this problem is to bypass some of the exhaust gas around the turbine (through a waste gate) at high speed and load. Thus, when a small turbine is fitted to achieve good low speed boost, the massive increase in specific available energy at the turbine at high speed is alleviated by increasing the effective flow area out of the exhaust manifold. This has two effects. Firstly, only part of the exhaust gas flow goes through the turbine. Secondly, the increase in flow area reduces the exhaust pressure that would otherwise build up. Both measures reduce turbine work and hence boost pressure. In addition, the second factor reduces pumping work during the exhaust stroke and would, for example, moderate the loss in b.m.e.p. and deterioration in fuel consumption shown in Figure 2.56 match 3, at high speeds.

The waste gate valve may be built into the turbine casing, and will consist of a spring loaded valve acting in response to the inlet manifold pressure acting on a controlling diaphragm. Different combinations of spring load, diaphragm area and valve area can be used to achieve a wide variety of boost pressure variations with engine speed. Disadvantages are increased cost, potential unreliability and the restriction to a single entry turbine housing.

2.6.7.3 Fuel delivery and engine speed range

Development of fuel injection pumps and associated equipment has introduced additional freedom to vary fuel delivery over the speed range of an engine. The turbocharger matching process must be closely linked with fuel system matching even after optimum injection rates, pressures, nozzle sizes and swirl have been achieved.

Tailoring of the fuel delivery characteristic is a method of achieving good torque back-up within the framework of engine and turbocharger limitations (Figure 2.54). For example, maximum fuelling can be restricted at high speeds in order to limit the maximum turbocharger speed with a small area turbine. Thus impressive torque back-up would be achieved, but at the expense of a low maximum power output.

At the other end of the speed range, excessive smoke can be reduced by restricting fuelling until sufficient boost is available to generate a reasonable air-fuel ratio. Thus a spring loaded diaphragm senses boost pressure and allows the maximum fuel stop to open as engine boost increases. Since fuelling is restricted only when the boost pressure is zero or low, torque is only reduced at very low speeds, that is, below that at which maximum torque is achieved. The device is commonly called an 'aneroid' (fuel controller), and is described in Chapter 10.

The difficulty of achieving a satisfactory match over a wide speed range has been explained. In certain circumstances it may be advantageous to reduce the rated speed of an engine whilst increasing b.m.e.p. to achieve the same maximum power output.

By reducing the turbine area and increasing fuelling, high b.m.e.p is obtained. If maximum rated speed is reduced from 2500 rev/min to 2000 rev/min (Figure 2.56, but an extreme case), excessive turbocharger speed is avoided. Naturally the final drive gear ratio of the truck must be raised to compensate, hence the engine is working at a higher load than would normally be the case at the same vehicle speed and load. Since specific fuel consumption reduces with load, fuel savings are possible.

2.6.7.4 Compressor matching

Since the truck engine operates over a wide speed and load range, the air flow requirements cover large areas of the compressor map. A typical superimposition of engine air flow on the compressor map is given in Figure 2.57, showing lines of constant engine speed (1000, 1500,1900,2400 and 2800 rev/ min) and load (lower 3.85,6.17 and 8.48 bar), and the maximum torque curve.

Selection of the correct compressor is largely a matter of ensuring a sufficient surge margin (A/B, Figure 2.57) and that the operating points at maximum torque and power (points X and Y, Figure 2.57) occur at reasonable compressor speed and efficiencies. Thus the compressor shown in Figure 2.57 is a satisfactory choice, since the operational area is clear from the surge line and lies in an area of high efficiency. However, a slightly larger compressor might result in more of the operating regime experiencing higher compressor efficiency, with a less generous surge margin. A small improvement in low speed could also be obtained if maximum compressor efficiency occurred at a lower pressure ratio (e.g. 1.6 compared with 1.8 in Figure 2.57).

Reducing turbine area will raise the boost pressure, reducing the surge margin (compare Figures 2.57 and 2.58) and therefore a smaller compressor trim may be required in some cases. However, the surge margin shown in Figure 2.58 is adequate, the compressor being well matched.

Figure 2.59 shows a poor compressor match, using too small a compressor trim on an intercooled engine. At maximum speed and load (point Y) the compressor efficiency is low. Boost pressure actually falls as the engine speed increases from 2400 to 2800 rev/min as a result. It may occasionally be convenient to deliberately match into an inefficient region at maximum engine speed and load, in order to hold the boost pressure, and therefore maximum cylinder pressure, down. However, an excessive reduction in efficiency occurs in the extreme case shown, and piston pumping work during gas exchange will suffer.

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