Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 66

There are some important differences in the combustion characteristics of alcohols and hydrocarbon derivatives. Alcohols have higher flame speeds and extended flammability limits. Also, alcohols produce a great number of product moles per mole of fuel burnt; therefore, higher pressure is achieved.

The alcohols mix in all proportions with water due to the polar nature of OH group. Low volatility is indicated by high boiling point and high flash point. Alcohols burn with no luminous flame and produce almost no soot, especially methanol. The tendency to soot formation increases with molecular weight.

Combustion of alcohol in presence of air can be initiated by an intensive source of localized energy, such as a flame or a spark, and also, the mixture can be ignited by application of energy by means of heat and pressure, such as happens in the compression stroke of a piston engine. The energy of the mixture reaches a level sufficient for ignition to take place after a brief period of delay called ignition delay, or induction time, between the sudden heating of the mixture and the onset of ignition (formation of a flame front which propagates at high speed throughout the whole mixture). The high latent heat of vaporization of alcohols cools the air entering the combustion chamber of the engine, thereby increasing the air density and mass flow. This leads to increased volumetric efficiency and reduced compression temperatures. Together with the low level of combustion temperature, these effects also improve the thermal efficiency by 10%.

Alcohols have higher flame speeds and extended flammability limits than hydrocarbons. Also, alcohols produce a great number of product moles per mole of fuel burnt; therefore, higher pressure is achieved. The higher flame speed, giving earlier energy release in the power stroke, results in a power increase of 11% at normal conditions and up to 20% at the higher levels of a compression ratio (14:1). The power continues to rise steadily as the mixture is enriched to an equivalence ratio of approximately 1 to 4. Because of the low proportion of carbon in alcohols, soot formation does not occur and therefore alcohols burn with low luminosity and therefore low radiation. In conjunction with lower flame temperature, approximately 10% less heat is lost to the engine coolant. The lower flame temperature of alcohols results in much lower NOx (nitrogen oxides) emissions. The wider flammability limits of alcohols permit smooth engine operation even at very lean mixtures. But aldehyde emissions are noticeably higher. For ethanol, emissions are acetaldehydes, and for methanol, emissions are of formaldehydes. Increasing compression ratio from 9 to 14, aldehyde emissions can be reduced by 50%, to a level compared to that for gasoline. An addition of 10% water reduces aldehyde emissions by 40% and NOx by 50%. Addition of 10% water in the alcohol can be tolerated without loss of thermal efficiency.

The oxygen content of alcohols depresses the heating value of the fuel in comparison with hydrocarbon fuels. The heat of combustion per unit volume of alcohol is approximately half that of isooctane. However, the stoichiometric fuel-air mass ratios are such big that the quantity of energy content based on unit mass of stoichiometric mixture becomes comparable with that of hydrocarbon derivatives.

Methanol is not miscible with hydrocarbon derivatives, and separation ensues readily in the presence of small quantities of water, particularly with reduction in temperature. Anhydrous ethanol, on the other hand, is completely miscible in all proportions with gasoline, although separation may be affected by water addition or by cooling. If water is already present, the water tolerance is higher for ethanol than for methanol, and can be improved by the addition of higher alcohols, such as butanol.

The high heat of vaporization and constant boiling point make cold starting difficult with neat alcohols. The problem is not as severe as in case of alcohols blended with gasoline. Ethanol has a constant boiling point of 80°C (176°F). Gasoline which has a high vapor pressure (therefore highly volatile) can be used for cold start.

See also: Alcohols, Butanol, Ethanol, Methanol, Propanol.