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
2.3.1 Liquid fuels in VCR engines
The disadvantages listed above could be reduced and concept if applied in the spark ignition engine, then it is definitely helpful for the fossil fuels to reduce their consumption. Alongside Variable compression ratio Engine (VCR) is one promising technology with increasing focus towards practical implementation in the existing engines or new engines to enhance the efficiency of the engine. The compression ratio of an engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity (Heywood, 1988). With higher compression ratio same combustion temperature can be reached with lesser fuel, while giving a longer expansion cycle, lowering the exhaust temperature and creating more mechanical power output. But petrol engines have a limit on the maximum pressure during the compression stroke, after which the fuel/air mixture detonates rather than burns which is called knocking. So higher compression ratios may cause knocking (Sayin , 2012).To achieve higher power outputs at the same speed, more fuel air mixture must be burnt but this would result in detonation unless the compression ratio was decreased. However, increase in power is possible. The effect of lowering the CR is that under light loading, the engine can lack power and torque. So higher loads require lower CR to be more efficient and vice versa. Thus all IC engines have a fixed compression ratio with optimal value to prevent knocking. This prevents the engine to perform with highest efficiency all the time as the load is varying. Variable compression ratio technology allows adjusting the compression ratio of an internal combustion engine to suit the requirement of the load and increase fuel efficiency. When the compression ratio is increased comparatively higher gas temperature is produced during the latter part of the expansion stroke. Thus producing higher oxidation of hydrocarbons in the cylinder increasing the emission of exhaust hydrocarbons. But the effect of compression ratio on NO emissions is small.
Caris and Nelson,(1959) conducted experiments on a series of eight-cylinder 5.3 dm3 displacement engines at a throttle of 2000 rev/min using gasoline fuel. The experimental results showed that the mechanical efficiency and the volumetric efficiency essentially remained constant over the full compression range. Both indicated fuel conversion efficiency and MEP showed a maximum at compression ratio. But for higher compression ratios the efficiency and MEP decreased slightly. This trend was explained as being due to increase in surface/volume ratio and slower combustion and is also due to increasing importance of crevice volumes. The heat losses to the combustion chamber decreased as the compression ratio are increased.
Kerley and Thurston, (1962) studied the effect of compression ratio on fuel conversion efficiency. They studied the variation of ratio of fuel conversion efficiency at given
compression ratio divided by the efficiency at CR8 and found some results that are good till CR < 14. Over the compression ratio range that are accessible by SI engines with available fuels (r < 12) he found that the fuel conversion efficiency increases to about 3% per unit of compression ratio increase.
Abdel and Osman, (1997) studied the effect of varying the compression ratio on the engine performance with different ethanol-gasoline fuel blends using a variable compression ratio engine. The results obtained show that ethanol addition up to 10% improved the indicated power at CR 10. When the ethanol addition increased beyond 20% as the CR increases, the indicated power increases. For each fuel blend, there is an optimum compression ratio that gives maximum indicated power. In this study, the optimum compression ratios were found to be 8, 10, 12 for 10%, 20% and 30% ethanol and above respectively.
Maher et.al., (2010) made an analytical model of spark ignition engine operating on hydrogen fuel and then studied the effects of varying compression ratio, equivalence ratio and engine speed on the performance of engine against experimental data of engine. In summary
1. Highest power occurred at a compression ratio of 11:1 and with further increase, the power reduced due to unstable combustion.
2. The optimum spark timing decreased as compression ratio is increased.
3. The indicated thermal efficiency increased with compression ratio, reaching maximum between 10:1 and 11:1.
4. The specific fuel consumption decreased with increase in compression ratio until 11:1 and after 11:1 specific fuel consumption increased.
5. The NOx emissions increased with compression ratio for all equivalence ratios less than 0.8 due to high combustion temperature with abundance of oxygen and decreased with compression ratio for all equivalence ratios over 0.8 due to decreasing amount of oxygen.
Yuh and Tohru, (2010) conducted research to observe the effect of higher compression ratios in 2 stroke engines. The results showed that actual fuel consumption was improved by 1-3 % for each unit rise in compression ratio range 6.6 to 13.6. It was concluded that the rate of improvement was smaller when compared to the theoretical values and the difference was mainly due to increased mechanical and cooling losses. Power output was also increased, but the maximum compression ratio was limited due to knock and increase in thermal load.
The author studied the theoretical behavior of combustion in Otto cycle for variable compression ratio as plotted in Fig. 2.5. According to the author, the efficiency of the engine keeps on increasing with compression ratio. However there is certain CR which gives maximum work and found reduced for other CRs.
Srinivas et al.,(2010) tests on variable compression ratio mechanism for a 2 stroke petrol engine concluded that with increase on compression ratio specific fuel consumption reduced,
brake thermal efficiency and mechanical efficiency increased. It was also possible with their design for the driver to operate the engine at compression ratio of his choice based on the terrain he is driving.
Aina et al.,(2012) conducted research on a 4 stroke spark ignition variable compression ratio engine having a maximum compression ratio of 9. Although there were differences in theoretical and experimental engine performance characteristics due to mechanical and cooling losses in the engine. The conclusion was that an increase in the compression ratio increases the brake power, brake thermal efficiency, brake mean effective pressure and reduction in specific fuel consumption which means that higher compression ratios makes it possible to improve the performance characteristics of spark ignition engines.
Researcher (Ozcan and Yamin, 2007; Adams et al., 1987) reported the modification in the SI engine in terms of variable compression ratio to find the best performance and least emissions conditions. Li et al., (2010) study the effect of injection timing and ignition timing of methanol in direct injection stratified charge spark ignition engine and found the optimum timings to get complete combustion, maximum cylinder pressure and temperature. Author explained the methodology of experimentation with all measuring devices clearly in this paper.
Fig.2.5 Net work done/cycle and efficiency variation with compression ratio(Klein,1991) Turkoz et al., (2013) investigated experimentally the best ignition timing in an 4-stroke, 4- cylinder SI engine. The author experimented E85 (15% Gasoline + 85% ethanol) ethanol fuel blends by altering the timing angle with respect to gasoline use. The output performance parameters of interest were power, efficiency and energy distribution of engine. The ignition timing was successively delayed in 20 increments up to 60 (denoted as -2, -4, -6, respectively) and then successively advanced by 20 up to 60 (denoted as +2, +4, +6, respectively) with
respect to that ignition advance values used with gasoline (called the ‘‘original advance values’’) at full load operation within 2000 to 4000 rpm at increment of 500 rpm The best performance like brake torque and brake power were obtained with + 40 crank advances at 3500 rpm which results in better overall efficiency.