Literature Review
2.3 INVESTIGATIONS WITH VARIABLE COMPRESSION RATIO ENGINE Variable compression ratio is the method by which the compression ratio could be altered to
2.3.2 Findings with gaseous Fuel
2.3.2.8 Effect of EGR and Intake air temperature
In this method the exhaust gases from engine were taken and supplied back to the inlet port of engine through manual valve. To reduce the volumetric efficiency of EGR the gas was cooled before supplying to inlet port. Initially engine runs at fixed speed with lean air/ fuel ratio i.e. excess air with equivalence ratio around 0.35, when it runs smooth, the exhaust gas recirculated slowly by manual valve to the inlet port reducing the excess air till the mixture becomes stoichiometric. This is depicted by Xie et al., 2013; Park et al., 2011; Alger et al., 2012; Hu et al., 2009; Lee et al., 2009.
a. Combustion
Xie et al., (2013) study the effect of ignition timing on the spark ignition methanol engine with wide open throttle at 1400 rpm and full load with compression ratio 18.5. The authors found that when the engine runs at 18, 15 and 120 bTDC the engine runs smoothly without knock. The authors suggested that the ignition timing could be adjusted to get better performance of engine at variable load especially when variable EGR was supplied. There findings were for lower EGR and higher load; ignition timing should be delayed for avoiding knock whereas higher EGR with lower load, ignition timing should be advanced to maintain good combustion quality and low cycle-by-cycle variations.
b. Efficiency
It can also be seen that both brake thermodynamic efficiency and torque decreased as the EGR percent increased (Hu et al., 2009). The authors found that with lean mixture, torque was 94 N-m decreased to 88 N-m with EGR. Similarly brake thermodynamic efficiency for lean burn is 36 % decreased to 32 % for EGR. This was thought to be due to the reduction in volumetric efficiency brought on by the increase in MAT (Manifold Air Temperature) and the additional work required by the engine to pump the water through the combustion chamber.
Hu et al., (2009) reported that the addition of hydrogen with natural gas would increase the power output at large EGR also. But the effective thermal efficiency was found to be decreasing at small EGR rate while increased with large EGR rate. One reason being the combustion improvement of natural gas hydrogen blend. The other reason was the free radicals produced during combustion processing and the presence of radicals will activate the pre-ignition reaction and leads to the improvement of combustion efficiency.
Xie et al., (2013) studied ignition timing with EGR for spark ignition methanol engine and found that with increasing EGR rate and reducing load, ignition timing needs to be advanced in order to make the fuel economy of engine to optimum. The optimal ignition timing for the
minimum BSFC was constantly changing with the changing of EGR rate. When the EGR rate was between 0 and 0.1, the minimum BSFC was obtained at ignition angle of 18 0 bTDC;
when the EGR rate was between 0.1 and 0.2, it is 21 0 bTDC; when the EGR rate was between 0.2 and 0.32, it was 240 bTDC. Ignition timing should increase to 33 - 36 0 bTDC to reach minimum fuel consumption.
Fig. 2.14 NOx variation with EGR Rate for different composition of biogas (Lee et al.,2010) Lee et al., (2010) reported the effect of low pressure exhaust gas recirculation on generating efficiency using different gaseous fuels like natural gas, model biogas +5 % hydrogen and model biogas. The experiment carried out at fixed spark timing and optimum spark timing.
Author did experimentation of optimum spark timing for the listed fuels without EGR.
Utilizing this optimum spark timing which was 14 0 bTDC for natural gas, 16 0 bTDC for model biogas and 13 0 bTDC for model biogas with 5% hydrogen. The generating efficiency in model biogas engine test had a tendency to decrease from 30.15 % to 29.02 % as EGR rate increased from 0 % to 15 % since incomplete fuel combustion occurred because of CO2 in mixture. When 5 % hydrogen was added to model biogas, generating efficiency was improved by 1.18 % without application of EGR.
Different methods were considered by different authors to mitigate the exhaust gas emission.
One such technique is reported by Tartakovsky et al.,2015 where the gaseous hydrogen rich methanol reforming products were used in direct injection SI engine. This technique has a great potential of emission mitigation as compared with gasoline. The NOx in exhaust was found to be reduced by factor of 7 as a result of lean combustion and lower cylinder temperature.
Another technique of high level EGR for high efficiency engines was discussed by Alger and Mangold, 2009. They experimented the use of high level EGR for reduction in emission and
improvement of efficiency. In this proposal, out of 4 cylinders, exhaust of one of the cylinders was used for recirculation. Hence, that exhaust manifold was directly connected to the intake manifold leading to constant 25% EGR rate. This concept showed large gain in fuel efficiency over the baseline performance and also had significantly reduces the emission from engine.
c. Emission
Author Haffel, (2003) studied the effect of EGR on exhaust gases leaving the hydrogen fueled engine specifically NOx emission. Author uses the engine at 3000 rpm constant speed with varying fuel supply from 1.63 to 2.72 kg/ h. The maximum equivalence ratio limited in lean burn strategy by NOx emissions (maximum 0.4). As per author, with the increase of torque the NOx emission increases. So for zero NOx emission the EGR gives more torque than lean burn fuel air mixture.
Alger et al., (2012) studied the effect of addition of exhaust gas recirculation on the performance and emission of spark ignition gasoline engine. They reported that EGR improves the fuel consumption of gasoline engine by reducing pumping losses and knock by eliminating enrichment regions. The EGR substantially reduces the emissions of nitrogen oxide (NOx) and CO. Authors tested 2.4v litre multi-point injection engine and a 1.6 liter gasoline direct injection with high levels of both cooled and uncooled EGR. The results showed that an improvement of 5 and 30 percent in fuel consumption is possible. Emissions like NOx reduce by 80 % and CO by 30 %.
Xie et al., (2013) studied ignition timing with EGR for spark ignition methanol engine and found that for optimum EGR rate and ignition timing, NO emissions decreased with reduction of engine load. HC and CO emissions deteriorate significantly at lower load due to excessive EGR.
Lee et al., (2010) reported the effect of low pressure exhaust gas recirculation on NOx emission using different gaseous fuels like natural gas, model biogas + 5 % hydrogen and model biogas as shown in Fig.2.14. The NOx emission of model biogas generally lower compared to those of natural gas test. In addition, NOx concentration of exhaust gas decreased from 221 ppm to 38 ppm as recycled exhaust gas increased to 11.8 %. Addition of hydrogen to model biogas increases the NOx emission in comparison to those generated by model biogas. The NOx production of hydrogen with model biogas was decreased to the level of model biogas when EGR rate increased to 15 %.
Hu et al., (2009) reported the effect of natural gas hydrogen blend with EGR on the exhaust gas emissions. They found that for a specified hydrogen fraction, the NO fraction decreases with increase in EGR and HC emission increased with increasing of EGR rate. The CO and CO2 emissions show little variation with EGR but they decrease with increase of hydrogen fraction.