24 Turbocharging systemsprinciples

Successful design of a turbocharged diesel engine is highly dependent on the choice of system for delivering exhaust gas energy from the exhaust valves or ports, to the turbine, and its utilization in the turbine.

Virtually all the energy of the gas leaving the cylinders arrives at the turbine. Some is lost on the way, due to heat transfer to the surroundings, but this is unlikely to exceed 5% unless water cooled exhaust manifolds are used, and will usually be much less. However, the design of the exhaust manifolds between the exhaust valve and turbine influence the proportion of exhaust gas energy that is available to do useful work in the turbine. An important parameter is the pressure in the exhaust system. Equation (2.18) shows that turbine power increases with pressure ratio (Po3/P4), hence the exhaust manifold pressure should be high. However, this implies that the pistion has to push the combustion products out of the cylinder against a high 'backpressure', reducing the potential power output of the engine. Various turbocharging systems have been proposed to rationalize these apparently conflicting requirements. The most commonly used will be described herein. More complex systems that have been developed for special purpose applications are described in Chapter 3.

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