a 4.3.4 Effect of various parameters on combustion in IC engines: Compression ratio: A higher compression ratio increases the pressure and temperature of the working mixture which reduce the initial preparation phase of combustion and hence less ignition advance is needed. High pressures and temperatures of the compressed mixture also speed up the second phase of combustion. Increased compression ratio reduces the clearance volume and therefore increases the density of the cylinder gases during burning. This increases the peak pressure and temperature and the total combustion duration is reduced. Thus engines having higher compression ratios have higher flame speeds. Natural gas as a vehicle fuel has low energy density and exists in a gaseous state in the intake manifold. The lack of latent heat of evaporation of natural gas decreases volumetric efficiency by about 3% compared to sequential injection gasoline engines. Unless a direct injection fuelling approach is adopted, the volumetric efficiency of a port fuel injection engine is reduced by between 10% and 15%, depending on engine optimization. Hence, a port fuel injection CNG engine will suffer power output losses over 10% compared to the same size gasoline engine due to the displacement of the air by the gas fuel. In fact, these performance losses can be recovered in part by increasing the compression ratio, since natural gas engines can be safely operated at compression ratios approaching 15:1 provided gas quality is maintained. Higher antiknock quality of LPG and CNG also provides an opportunity for use of a higher compression ratio and improvement of engine performance and thermal efficiency. Speed Variation At lower speed, the thermal efficiency is less for LPG as compared to gasoline. The flame speed increases with an increase in engine speed due to increased turbulence. The compression of charge occurs rapidly for the engine running on high speeds and reduces the delay period and knock tendency. This is because, at high speeds, the flame front reaches end of the cylinder and burns even last part of the charge more quickly before temperature of end mixtures is below
b self-ignition temperature. Also at lower speed air-fuel ratio is higher and fuel consumption is less as compared to higher speed for CNG as well as LPG. Load Variation With increase in engine load the cycle pressures increase. Hence the flame speed increases in SI engines with decrease in load and power of an engine is reduced by throttling. Due to throttling the initial and final compression pressures decrease and the dilution of the wording mixture due to residual gases increases. This makes the smooth development of self propagating nucleus of flame difficult and unsteady and prolongs the ignition lag. The difficulty can be overcome to a certain extent by enriching the mixture at low loads but still it is difficult to avoid after-burning during a substantial part of expansion stroke. Poor combustion at low loads and the necessity of mixture enrichment are among the main disadvantages of SI engine which cause wastage of fuel and discharges of a large amount of products of incomplete combustion like carbon monoxide and other poisonous substances. Ignition advance angle In the explosion cycle maximum useful work is obtained with combustion at constant volume, with maximum pressure occurring near the dead centre. Since a certain time element elapses after ignition and before the maximum pressure is attained, the charge must be ignited before the dead centre. This time period for which charge must be ignited before the piston reaches TDC, is known as ignition advance time or when measured in terms of angle of crankshaft, it is termed as angle of spark advance. Cylinder pressure of gasoline can reach nearly up to 46.27 bar and for CNG it is approximately 38.56 bar. Ignition timing for CNG should also be adjusted for about 7 to 18º more advanced than gasoline. The reason is that methane has a low flame speed. It needs more time to reach the peak pressure just after TDC. However, the negative effect of advance timing will increase NO x emission. As a result, CNG fuelled engine produce less work compared to gasoline Air-fuel ratio variation The composition of the working mixture influences the rate of combustion and the amount of heat evolved. With hydrocarbon fuels the maximum flame
c velocities occur when mixture strength is 110%. When the mixture is made leaner or is enriched and still more, the velocity of flame diminishes. Lean mixtures release less thermal energy resulting in lower flame temperature and flame speed. Very rich mixtures have incomplete combustion which results in production of less thermal energy and hence flame speed is again low. Stoichiometric air-fuel ratio for CNG is 17.11, for LPG is 15 and for gasoline is 13.6. Emission parameters like CO, HC & NOx With natural gas operation, large reductions in engine-out emissions compared to either Gasoline or diesel fuel operation can be achieved. It could be mentioned that most light-duty SI natural gas engines are stoichiometric similar to their Gasoline-fuelled counterparts. With natural gas, mixture enrichment during cold starting is not required unlike the Gasoline operation. Hence, use of natural gas results in lower unburned fuel emissions during cold staring and warm-up phase. CNG buses without after-treatment have high emissions of formaldehyde, which is considered a possible human carcinogen. The formaldehyde emissions can be reduced with an oxidation catalyst but not to the low level of a diesel bus equipped with catalytic regeneration particulate trap (CRT). In addition to emissions benefits, NGV has other differences from the vehicles operating on the conventional Petroleum fuels as below: (i) Compared to Petroleum fuels, emissions of carbon dioxide, a green house gas are lower in the dedicated natural gas engines as a higher engine compression ratio can be used. (ii) Low emissions of non-methane hydrocarbons from natural gas vehicles result in low photochemical reactivity and ozone forming potential of the exhaust gases. Emissions of air toxics such as benzene and 1-3, butadiene are very low. In case of LPG fuel, emissions are substantially lower compared to Gasoline vehicles. LPG has disadvantage compared to natural gas in respect of nonmethane hydro carbon (NMHC) emissions as these consists of higher amounts of reactive olefinic hydrocarbons. LPG has significantly lower smog formation
d potential compared to Gasoline and Diesel fuels. LPG operation results in negligible PM emissions compared to Diesel. LPG is relatively a low sulphur fuel. Any other parameters: Natural gas and propane are generally considered to reduce engine maintenance and wear in SI engines. The most commonly cited benefits are extended oil change intervals, increased spark plug life, and extended engine life. Natural gas and propane both exhibit reduced soot formation over Gasoline. Reduced soot concentration in the engine oil is believed to reduce abrasiveness and chemical degradation of the oil. Gasoline fuelled engines particularly carbureted engines require very rich operation during cold starting and warm up. Some of the excess fuel collects on the cylinder walls, "washing" lubricating oil off walls and contributing to accelerated wear during engine warm up. Gaseous fuels do not interfere with cylinder lubrication. Engines powered by gaseous fuels are generally considered easier to start than Gasoline engines in cold weather. Because gaseous fuel are already vaporized before inducted into engine. However, under very cold temperatures, cold-start difficulty occurs for propane and natural gas. This is probably due to ignition failure caused by very difficult ionization conditions, sluggishness of mechanical components. Hot starting can cause difficulties for gaseous fuelled vehicles, especially in warm weathers. After an engine is shut down, the engine coolant continues to draw heat from the engine, raising its temperature. If the vehicle is restarted within a critical period after shutdown, (long enough for the coolant temperature to rise, but before the entire system cools), the elevated coolant temperature will heat the gas more than normal, lowering its volumetric heating value and density. This would cause mixture enleanment. Gasoline shows very little change over the normal temperature or pressure range. Propane, however, is gas at ambient conditions. Its physical properties depend mainly on the temperature and pressure at which they are being stored.
e There must be space left in a propane fuel tank. As the temperature rises, the volume of liquid increases significantly. Due to this, propane system has some kind of safety fill stop device to prevent tank fills to not more than 80 to 85%. This provides room for liquid expansion if the temperature increases after the tank is filled. Due to low viscosity of propane and its storage under pressure, it may leak through small cracks, pumps, seals and gaskets more readily than Gasoline. CNG lacks the latent heat of evaporation to cool intake charge. Accordingly, the temperatures of piston, cylinder wall, valve and valve seat increase. Because of the absence of the liquid fuel spray acting as a cooling agent or lubrication property for CNG engines like that of gasoline, intake valves run hotter, which results in increased intake valve and seat wear rates. When run on gaseous fuels, standard gasoline exhaust valves are also subject to increased recession. Exhaust valve and seat materials should be developed for natural gas engines.