The research on the internal combustion engines has been motivated in recent
decades by the air pollution and its effect on environment and the energy scenario
of the world. The development of advanced combustion techniques that would combine
high thermal efficiencies and ultra low pollutant emission is yet to become
technically and commercially viable. One of the combustion methods that look
promising to meet the above requirements is Homogeneous Charge Compression Ignition
(HCCI) (Najt and Foster, 1983).
HCCI has high thermal efficiencies on par with diesel engines due to low wall
heat losses (Djavareshkian et al., 2008) and
emits virtually no NOx. However, high heat release rates and lack of control
combustion are the major issues that limit the load range of engine operable
with HCCI combustion. The higher end of load is limited by rapid combustion
that eventually results in knocking.
There are researches going on all over the world to find a practical means of combustion control. Several methods of combustion control such as high exhaust gas recirculation, operation with two different fuels, use of additives, multi stage injection, variable valve timing, controlling intake temperature and pressure, variable compression ratio and injection of reaction inhibitors. This paper discusses the effect of EGR on the combustion of gasoline and CNG which have different fuel properties such as octane number, autoignition temperature etcetera.
The use of two fuels having different autoignition temperatures is an effective
strategy of achieving and controlling the combustion. While the fuel with lower
autoignition temperature lowers the intake temperature required, the other fuel
will burn later causing a slower overall combustion (Oakley
et al., 2001; Topgu et al., 2006;
Megaritis et al., 2007; Mack
et al., 2005; Yao et al., 2006; Kong,
2007; Yao et al., 2006).
The task of intake air heating can be done using Exhaust Gas Recirculation
(EGR). Retaining some of the exhaust gases in the cylinder can do this by negative
valve overlap. This method is called internal EGR or residual gas trapping and
can be achieved by Variable Valve Timing (VVT) with electronic control of lift
and timing of the valves operation (Yap et al., 2005).
The other method is to re-circulate exhaust gases externally called external
EGR (Kim and Lee, 2006). The re-circulated gases have
many effects on the charge characteristics and combustion phenomenon (Zhao
et al., 2001).
When burnt gases are mixed with cooler inlet mixture of fuel and air the hot gases heat up the charge. This promotes autoignition and this is found to be responsible for ignition timing.
The charge dilution and the presence of chemical substances have some effects
on the combustion duration than ignition timing. As dilution occurs some of
O2 is replaced with burnt gases. This suppresses any chemical reactions
resulting in extended combustion and reduced Nox due to reduced oxygen availability
(Zhao et al., 2001; Oakley
et al., 2001a).
The heat capacity of the burnt gases is higher than that of the fresh charge and this has a very significant effect on increasing combustion duration and retarding ignition. Higher heat capacity is due to the higher specific heat values of carbon dioxide and water vapor.
Replacement of some O2 by CO2 and H2O reduces the ratio of specific heats (γ value) of the cylinder charge. This results in less temperature at the end of compression stroke resulting in ignition delay. Combustion duration increases due to the thermal cushioning caused by higher energy absorption and less pressure and heat release rates.
The mixing of EGR and charge in the cylinder is less complete and that creates
charge and thermal stratification inside the cylinder (Ghasemi
and Djavareshk, 2010). This feature of EGR can be used for control of combustion
by thermal stratification (Morimoto et al., 2001).
MATERIALS AND METHODS
Operation of the engine in HCCI mode: As gasoline has a high octane
number (RON 92) and the high auto ignition temperature, the intake air had to
be heated to achieve HCCI combustion for the given engine of geometric compression
ratio 14. Therefore, an intake air heater was added to the intake manifold of
the engine and the charge was preheated (Lu et al.,
2007; Kim and Lee, 2006). After several attempts
of altering various engine operating parameters, HCCI combustion could be achieved
at an inlet air temperature of 320°C.
The ignition of CNG that has high octane number is by the heat released by
the combustion of gasoline. The CNG, that is otherwise difficult to autoignite,
gets ignited by the heat liberated by the combustion of the gasoline (Yap
et al., 2005).
The exhaust gas is re-circulated by using a valve which can be controlled by
the engine ECU. The amount of EGR was varied from 0% to the limit of misfire
that results at 53%.
The effects of EGR and CNG injection on the combustion has been studied at a constant engine speed of 1500 rpm and at a load of brake torque 8 Nm are presented in this study.
Experimental setup: The experiment setup is based on the CNG Direct injection engine of specifications listed in Table 1. Figure 1 shows the schematic of the experimental setup with the added components to the existing CNG DI engine.
The CNG DI engine was suitably modified to operate with two fuels at a time and on HCCI mode. To supply the gasoline to the engine, a manifold injection system was added to the existing CNG DI engine. As intake air is to be heated to achieve HCCI combustion, an electrical air heater of 2.4 kW had been added to the intake system.
Gasoline injection system: The gasoline injection system consists of a fuel pump, fuel rail, fuel pressure regulator and an injector. The fuel was injected into the intake manifold at a pressure of 3 bars. The injector was powered by an injector driver circuit and controlled by the pulse train generated by a LABVIEW program.
CNG direct injection: CNG was injected by the existing direct injection
system at a fuel rail pressure of 12 bars. CNG DI engine has the piston with
a bowl on its top which creates stratified CNG concentration for late injection
timings (From the angle of inlet valve closure, 132 to 60° BTDC).
|| Experimental setup of the dual fuel HCCI engine test facility
|| Engine specifications and operating parameters
However, early injection timings (300° BTDC) had resulted in relatively
RESULTS AND DISCUSSION
There are significant effects of EGR on both gasoline and dual fuel HCCI combustion as shown in Fig. 2 and 3. The rate of pressure rise and peak pressure are affected the presence of CNG in the mixture. When CNG is added to the charge and with no EGR, it results in higher peak pressures and pressure rise rates as shown in Fig. 3. However, EGR reduces the peak pressures and retards the autoignition timing for both gasoline and dual fuel combustion. The EGR results in higher peak pressures within 30% and further EGR reduces peak pressures. The effect of EGR is more significant after 30 to 53%.
From the heat release analysis as shown in Fig. 2 and 3, it is observed that dual fuel HCCI has higher heat release rates than the single fuel gasoline combustion. Up to 30% of EGR, the heat release rate increases for the gasoline HCCI and beyond that EGR reduces the heat release rate. The presence of CNG in the mixture results in earlier autoignition of the whole mixture at no EGR conditions. However, the EGR reduces heat release rates and retards the autoignition for the both single and dual fuel combustion. However, in all the cases, at a given EGR percentage, the dual fuel combustion has higher heat release rates than the single fuel gasoline HCCI.
The exhaust temperature is higher for dual fuel HCCI, as the heat release rates are higher. Figure 4 shows the effect of EGR on the exhaust temperature of gasoline and dual fuel HCCI combustion. The exhaust temperature is reduced at 20% which may be due to the increase in specific heat capacity of the mixture. However, the effect of increased specific heat capacity diminishes and there is corresponding increment in exhaust temperature as the EGR rate increases.
||Pressure and heat release rates for gasoline HCCI combustion
with various levels of EGR
||Pressure and heat release rates for gasoline and CNG dual
fuel HCCI combustion with various levels of EGR
From Fig. 5, it can be seen that EGR helps improve the of
variation of IMEP. There is marginal difference effect of EGR on the exhaust
temperature of gasoline and The exhaust temperature is higher for dual fuel
HCCI, as the heat release rates are higher. Figure 4 shows
the between the single fuel and dual fuel HCCI and for both the cases the COV
is very high at 10% EGR. Figure 6 shows that the indicated
thermal efficiency is higher regardless of EGR for single fuel gasoline HCCI
combustion and this may be due to poor combustion efficiencies with dual fuel
operation as shown in Fig. 7. When EGR is increased the up
to 10% it improves the combustion efficiency and then decreases again. It may
be due to the heat capacity effect. More than 10% EGR may result increased in
energy absorption and reduced oxygen availability.
Figure 8 and 9 show the mass fraction burned
and the effect of EGR on the combustion process.
||Exhaust gas temperatures with single and dual fuel operation
|| Co-efficient of variation of IMEP
The combustion process is rapid for the dual fuel combustion and EGR retards
the combustion. Up to certain point around 0.85, there is rapid combustion for
dual fuel HCCI and after that the curve becomes flat. This indicates that the
combustion of CNG depends on the heat released by the heat liberated by the
combustion of gasoline.
||Effect of EGR and ignition timing on indicated thermal efficiency
||Role of EGR on combustion efficiency
||Effect of EGR on the mass fraction burned with single fuel
||Effect of EGR on the mass fraction burned with gasoline and
Gasoline and CNG dual fuel HCCI results in higher peak pressures and higher heat release rates compared to single fuel gasoline HCCI combustion. Dual fuel operation was found to be of no significant benefits at the given proportion of mass flow rates of gasoline and CNG. EGR reduces the peak pressures and a heat release rate for both and this effect is more significant above 30% of EGR. About 10 of EGR addition increase the thermal efficiency and combustion efficiency. Overall, EGR is found to be an effective tool in reducing the characteristic high heat release rates of HCCI for both single fuel and dual fuel combustion with similar trends.