03-10-2012, 02:57 PM
Specific Heat Capacity
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Specific heat is the amount of kcals needed to raise the temperature of 1 kg of oil by 1oC. The
unit of specific heat is kcal/kgoC. It varies from 0.22 to 0.28 depending on the oil specific
gravity. The specific heat determines how much steam or electrical energy it takes to heat oil
to a desired temperature. Light oils have a low specific heat, whereas heavier oils have a
higher specific heat.
Calorific Value
The calorific value is the measurement of heat or energy produced, and is measured either as
gross calorific value or net calorific value. The difference being the latent heat of
condensation of the water vapour produced during the combustion process. Gross calorific
value (GCV) assumes all vapour produced during the combustion process is fully condensed.
Net calorific value (NCV) assumes the water leaves with the combustion products without
fully being condensed. Fuels should be compared based on the net calorific value.
The calorific value of coal varies considerably depending on the ash, moisture content and
the type of coal while calorific value of fuel oils are much more consistent.
Specific Heat Capacity
Specific heat capacity is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius. Every substance has its own specific heat capacity, with the specific heat capacity of water being 1 cal/(g°C).
The specific heat capacity of water is much higher than that of other common substances. For the sake of comparison, the specific heat capacity of oil is about 0.5 cal/(g°C) and the specific heat capacity of aluminum is about 0.2 cal/(g°C). This means that it takes a lot more heat to raise the temperature of water compared to the amount of heat it would take to raise the temperature of oil or aluminum.
The high specific heat of water helps the earth's temperature remain moderate since water traps heat during the day and releases it slowly at night. As a result, the temperature on earth's surface does not vary very widely, ranging from extremes of 134°F to -129°F. For comparison, the moon has no liquid water and its temperatures can range from 240°F to -290°F. (The lack of atmosphere on the moon, along with other factors, also contributes to the wide range of temperature.)
There are three types of scavenging on the basis of the flow of air:
1. Direct or cross scavenging
2. Loop scavenging, using Schnuerle porting
3. Uniflow scavenging
in petrol engine air and fuel (petrol) pre mixed before enter in to the combushion (burning)chamber. it has low compression ratio needs external ignition. if petrol engine has more compression ratio, it burns before compressing the air fuel mixture (flash and fire points of petrol are less) means advanced to the required time. so knocking sound will come and there is no optimum utilisation of petrol .
where as in diesel engine, high compression ratios are used (flash and fire points of petrol are more) . after compression of air, fuel (diesel) fuel injected with high pressure