Many approaches have been taken to determine the burning velocity in internal combustion engines. Experimentally, the burning velocity has been determined in optically accessible gasoline engines by tracking the propagation of the flame front from the spark plug to the end of the combustion chamber. These experiments are costly as they require special imaging techniques and major modifications to the engine structure. Another approach to determine the burning velocity is using 3D CFD simulation models. These models require basic information about the mechanisms of combustion which is not available for distillate fuels in addition to many assumptions that have to be made to determine the burning velocity. Such models take long periods of computational time to execute and have to be calibrated and validated through experimentation.
This section presents a new technique to determine the burning velocity in a production engine without making any modification to its structure or use any high speed imaging equipment. This technique is based on the combustion-produced ionization measured at two defined locations in the combustion chamber. In the engine used in this investigation, the spark plug is used as an ion current sensor which indicates the time the flame kernel is formed around the spark plug gap. In addition, the fuel injector is used as an ion current sensor, after which it is electrically insulated from the cylinder head.
The time between the start of the ionization at the two locations is used to calculate the burning velocity.
The use of this technique to determine the cycle to cycle variation in the burning velocity and other combustion parameters is demonstrated with the engine operation at a steady speed and load.
Demands for improved fuel economy and high power density, in addition to reduced emissions, require an improved understanding of combustion processes in direct injection gasoline engines, which are widely used in passenger cars and light duty vehicles. This requires a detailed analysis of complex fluid dynamic, thermodynamic and chemical processes which affect the combustion process. Due to
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changes in these processes during the engine cycle, there is a need for computer simulations to determine the effect of different design and operating variables on combustion. Computer simulations enhance the development of strategies for the engine control unit (ECU) to optimize engine operation and achieve the production targets. Of primary importance is the validation of the simulation predictions with experimental data to ensure the validity and accuracy of the models. One of the key parameters that would indicate the quality of a simulation model is the burning rate of the fuel-air mixture.
6.2 Ionization in the combustion chamber
Figure 6.1 shows zoomed in traces for cylinder gas pressure and ion current from the spark plug and from the MSFI. The signal from the spark plug indicates the timing of the ignition coil charge and discharge. Point (a) represents the start of ionization at the spark plug. Point (b) is the first peak of the spark plug ion current, point (c) is the start of the ion current due to flame arrival at the injector, point (d) is the second peak of spark plug ion current, point (e) is the peak of the ionization from the fuel injector, and point (f) is the end of detection of ionization at the injector. The location of the piston at each of the above points is given in Figure 6.2.
Figure 6.2, shows the location of the piston in six positions from the flame kernel formation around the spark plug to the end of the ion current detection by the MSFI.
Figure 6.1 Zoomed in traces at 1500 rpm and 175 N.m (11.0 BMEP)
Figure 6.2 Piston location at key points of figure 6.1
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Figure 6.3 Cycle to cycle variation of 100 consecutive cycles
Figure 6.3 shows the variation of the burning velocity with the variation of the IMEP for 100 consecutive cycles. The figure shows the peak pressure, engine speed, IMEP, burning velocity, MBF 5%, MBF 10%, MBF 50% and MBF 90% for each cycle for 100 consecutive cycles. These data showed that the burning velocity for each cycle showed a variation that agreed with the variation of the IMEP of each cycle. That could lead to a conclusion that the burning velocity for each cycle represent the cyclic variability of IMEP.
6.3 Effect of Speed and Load on Burning Velocity
Figure 6.4 shows the effect of engine load on the average burning velocity at different engine speeds. The average burning velocity increases slightly with the increase in load up to about around 50%
of a full load, followed by a sharper increase up to 75% of a full load, and it continues to increase at a lower rate until there is a full load. This sharp increase is because the turbocharger started to act to increase the boosting pressure and in consequence increasing the tumble and turbulence. This shows that the turbulence increases the burning velocity and enhancing the combustion process.
Figure 6.4. The effect of engine load on burning velocity at different engine speeds
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The results shown in Figure 6.4 are plotted in Figure 6.5 to show the effect of speed on burning velocity at different engine BMEP. The burning velocity increases continually with speed but at different rates which depend on the engine load.
Figure 6.5. Effect of engine speed on burning velocity at different engine loads 6.4 Effect of injection timing and spark timing on Burning Velocity
Figure 6.6 shows the effect of ignition timing on the burning velocity at a constant operating condition of 1750 rpm and 50 N.m. The burning velocity decrease with spark advance or retard as well the IMEP decreased too. This results showed that the point that has the highest IMEP which represent the MBT, also, is the point that has the highest burning velocity. This conclusion can be used in engine control to keep the engine running on the MBT continually.
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Burning Velocity (m/s)
Engine Speed (rpm)