INTERNAL COMBUSTION ENGINE - Development of a Methodology for Performance Optimisation and Manu

The internal combustion engine (ICE), reciprocating or rotary (i.e. Diesel, Otto, Brayton cycle), is a heat engine used to transfer chemical energy into useful mechanical energy. Fuel is burned in the presence of an oxidiser (usually air) in the confined combustion chamber of an ICE to produce high-temperature and

2.2.1 Standard Diesel Engine

Figure 2.13a is a schematic of the internal combustion engine for a diesel engine. The internal combustion engine of a marine diesel engine undergoes a four stroke cycle - intake, compression, power and exhaust as shown in Figure 2.13b. During the intake stroke, the intake valve opens up allowing air to flow in while the piston moves downwards, then closes. The piston moves back up during the compression stroke, compressing the trapped air in the process. As the piston reaches the top dead centre (TDC), fuel is injected and ignited. Compared to gasoline engines where a spark plug is used to ignite a compressed mixture of fuel and air, the fuel ignites by itself in a diesel engine because of higher compression and temperatures up to 800o_{C. The hot gas expansion (at constant pressure for a diesel engine) forces the}

piston to the bottom dead centre (BDC). In the exhaust stroke, the exhaust valve opens and the upward movement of the piston pushes the exhaust gases out of the combustion chamber and the cycle repeats continuously. The pressure volume diagram of a typical diesel engine is shown in Figure 2.14.

Figure 2.14 – P-V Diagram for the ideal Diesel cycle

2.2.2 Diesel Engine with Miller Timing

In marine diesel engines, a technique called “Miller timing” is widely used. Using this technique, the engine cylinder inlet valve closes just before the piston reaches the bottom dead centre. This reduces the work of compression and the combustion temperature, which results in lower emissions and higher engine efficiency. Since the rate of NOx formation is increased at higher temperatures, Miller timing reduces the rate of NOx formation, resulting in a high thermal efficiency [18]. However, the application of Miller timing decreases the mean effective pressure in the engine cylinder due to expansion in the intake stroke. This also reduces the power density of the engine. Figure 2.15 is a cycle diagram of a diesel engine with Miller timing. The formation of NOx at high temperature is explained by the Zeldovich mechanism [107] as shown in Equations 2.5 and 2.6

Figure 2.15 – P-V Diagram for a Diesel cycle with Miller timing

2.3 THE NEED FOR TWO-STAGE

TURBOCHARGER

In a two-stage turbocharger, two compressors are connected in series to deliver higher pressure ratio as shown in Figure 2.16. The compressors in series are powered by two turbines in series. The exhaust gas leaving the engine first drives the HP turbine which drives the HP compressor. The exhaust gas then flows through the LP turbine stage which powers the LP compressor stage. Ambient air is admitted into the LP compressor where the density is raised. A further gain in density can be achieved through inter- stage cooling. The higher density air is further compressed in a smaller HP compressor stage to even higher pressure. The air is then cooled again by a charge air cooler or after-cooler to increase density. The HP compressor in a two-stage turbo charger is smaller because of mass conservation between the LP and HP stage. The HP stage then requires a smaller volume flow rate because the density of air is already increased from the LP stage. On the turbine side, complete expansion of the exhaust gases does not occur in the HP stage, making it smaller than it would be for a single stage turbocharger. Further expansion occurring in the LP stage, means a larger LP turbine.

Figure 2.16 - A schematic of a two-stage turbo charger

2.3.1 High Pressure Ratio Compression

The drawback of reduced cylinder air mass when Miller timing is used in a diesel engine can be compensated for by increasing the engine intake manifold pressure so that when induction occurs in the intake stroke, the increased manifold density is sufficient to produce the needed air mass for combustion. In the marine industry, a higher boost pressure (usually >= 8:1) is needed to take advantage of the benefits of Miller timing and this requires the use of two stage turbo charging. This is also due to material limits of Aluminium at high temperatures if a single stage turbocharger is used. Research developed over the past 40 years have shown that no further gain in pressure is possible beyond 6:1 for Aluminium when in a single stage compression as shown in Figure 2.17. This is due to high temperatures greater than 200o_{C at}

pressure ratios > 5, resulting in high thermal expansion and softening of aluminium. In Figure 2.17, the

LPC HPC ICE LPT HPT 1 2 3 4 5 6 7 8 IC AC

LPC: Low Pressure Compressor HPC: High Pressure Compressor LPT: Low Pressure Turbine HPT: High Pressure Turbine IC: Intercooler

AC: After cooler or charge air cooler ICE: Internal Combustion Engine

to machine, it is not economically feasible for use in the marine industry. Moreover, attempting to achieve a very high compression ratio in a single stage compressor requires a considerably larger compressor thereby increasing the system inertia, which is particularly disadvantageous during transient ship operations or rapid acceleration when the compressor is required to respond quickly to engine demand for more air.

Figure 2.17 - Pressure Ratio with single stage Aluminium compressor [Modified from [19]]

2.3.2 Combining Two-Stage Turbo charging with Miller

Timing

Using a two stage turbocharger with inter cooler overcomes the draw backs of achieving high compression ratios in a single stage system using aluminium and provides the additional benefits of using Miller timing. In a two-stage system, the HP compressor is smaller than the LP and has a lower inertia so it responds quicker during transient operations/rapid acceleration. The use of two stage turbo charging also provides the additional benefits of better overall turbo charging system efficiency, afforded by inter cooling between the compressor stages [18]. However, the effect of lower charge air temperature has the

system. Two-stage turbo chargers also favour smaller mass flows at higher pressures due to the smaller size of the HP compressor resulting in wider compressor map in the surge region. Two-stage turbo chargers also allow for easy integration of emission reduction devices such as EGR and SCR [1]. Figure 2.18 shows the higher engine intake pressure when a diesel engine with Miller timing is used with a two- stage turbocharger.

Figure 2.18 - P-V diagram of a Miller cycle diesel engine with two-stage turbo charging intake

In document Development of a Methodology for Performance Optimisation and Manufacturing Sensitivity Studies for Radial Flow Turbocharger Compressors for 21st Century Legislation (Page 39-45)