244 Principles of pulse converter and other turbocharging systems

Pulse converter turbocharging systems have been developed to improve the performance of those engines that suffer from low turbine efficiency with pulse turbocharging due to long windage periods and partial admission (multiple turbine entries) losses. They attempt to preserve the advantages of the pulse system, with its inherent high available energy and unsteady flow at the exhaust port, with steadier and more efficient flow at the turbine.

A pulse converter system, in its simplest form, is shown in Figure 2.34, applied to a four-cylinder engine. A conventional pulse manifold is used, but a carefully designed junction connects the two branches of the manifold to a single entry turbine. Figure 2.35 shows pressure diagrams recorded from an automotive two-stroke engine with the pulse converter of Figure 2.34 and a conventional pulse system. By connecting all four cylinders to a single turbine inlet, windage periods between exhaust pulses are totally avoided. Turbine entry conditions are not steady, as per the constant pressure system, but the very low efficiency operating points of the pulse system are avoided. The junction is designed to minimize pressure pulse transmission from one branch of the exhaust manifold to the other, thereby avoiding a blow-down pulse from one cylinder destroying the scavenge process of another. This is achieved by accelerating the gas as it enters the junction, reducing its pressure at the junction, and minimizing its effect on the other branch. In Figure 2.35, an exhaust pressure pulse from cylinder 3 arrives at cylinder 1 at the end of its scavenge period, but the junction has reduced its amplitude.

Varying the pipe cross-sectional areas at the inlets to the junction can control the influence of pressure pulses in one branch on the pressure in the other, but substantial area reductions must be avoided or turbulent mixing at the junction will reduce available energy at the turbine.

Pulse converters of this type are fitted to many medium speed diesel engines, particularly those with 4,8,16 and other awkward (for pulse turbocharging) numbers of cylinders. Invariably the normal pulse-type manifold is used with the pulse converter joining the branches to a turbine. In the case of an eight-cylinder engine, two pulse converters are used, each connected to one of the two entries of the turbine. A sixteen-cylinder engine must use four-pulse converters with two twin entry turbochargers or one four entry unit. See Table 2.1 for a summary of the advantages and disadvantages relative to pulse and constant pressure turbocharging.

Alternatively a multi-entry pulse converter (Multi-stoss, Brown Boveri patent) can be used, with three or four manifold branches joining in the pulse converter and to a single entry turbine. Provided that the firing intervals of the cylinders are equally spaced, then in principle the more cylinders connected to the turbine, the steadier the junction and turbine inlet pressure will be. Figure 2.36 compares exhaust pressure diagrams with a four-entry multi-pulse system and a conventional pulse system, on a medium speed V8 engine. The pressure fluctuation at turbine inlet is very small, hence the turbine is operating under conditions just as favourable as a constant pressure system. Although the turbine inlet pressure is not following the 'ideal' path as discussed with reference to Figure 2.18, the system does fully utilize the available energy associated with area 5-7-13 in Figure 2.18. This energy is transmitted to the pulse converter in the pressure pulse shown in Figure 2.36, where it is largely converted to kinetic energy in the converting inlet section of the pulse converter. Most of this kinetic energy is transmitted directly to the turbine, although some is made unavailable to the turbine due to inefficient mixing at the junction.

By creating an almost steady pressure in the junction, the influence of a blow-down pulse from one cylinder on the

Single entry turbocharger

Single entry turbocharger

Pulse Converter System For Turbocharger

Figure 2.34 A simple pulse converter and Its application to a four-cylinder engine

Area ratio (nozzles) = AJAV (0.65-0.85) Area ratio (throat) = /4th/2/4p (0.5-1.0)

Figure 2.34 A simple pulse converter and Its application to a four-cylinder engine

Turbine Entry

Turbine Entry

Pulse Converter Diesel Engine
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Responses

  • mira
    What is pulse convter turbochargers in automobile?
    2 years ago

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