Adiabatic Engines, Concept and Performance Analysis

Introduction:

Many development programs have been arranged in many countries during the last 20 years to improve the efficiency of the IC engines, particularly diesel engines. Adiabatic engine is one of the program to develop an engine with higher efficiency.

The feature of an adiabatic engine is that the combustion takes place in an insulated cylinder and the engine works at a considerably higher temperature than conventional engines. This reduces the loss of heat going to cooling water. In addition to this, the part of the heat carried by exhaust gases can also be converted into work passing through a turbine. This further reduces the weight of the system. The use of low conductivity material for cylinder wall increases wall temperature which increases the danger of knocking in SI engines but decreases knocking tendency in CI engine. Therefore, this concept is considered only for diesel engine.

Concept of Adiabatic Engine:

In thermodynamics, a adiabatic process is defined as a no heat loss process. An adiabatic engine is a system where Q=0, when the system is fully functional. Generally, it is difficult to have adiabatic engine in practise. However, an adiabatic engine has no conventional cooling and strives to minimise heat loss. The adiabatic engine combustion chamber is made of material which allows the operation of the engine with minimum heat loss. Fundamentally, the adiabatic engine is more efficient than conventional diesel engine because it converts the fuel heat into additional useful output.

The hot or insulated high temperature components include piston, cylinder head, valves, cylinder liner and exhaust manifold. Additional power and improved thermal efficiency from an adiabatic engine are possible because, thermal energy, normally lost to the cooling water and exhaust gases, is converted into useful power through the use of high temperature materials and a small gas turbine.

Lubrication of Adiabatic Engines:

The high temperature lubricant is the major problem in the development of adiabatic engine. The ceramic liner temperature reaches to 5000C against the conventional engine liner temperature of 2000C. The lubricant used must resist such high temperature and maintain the viscosity property required for low friction.

The following methods can be used:

i.                    Gas lubricated piston-ring-liner combination.

ii.                 Unlubricated ceramic roller bearings for the wrist pin, crank pin and main bearings.

iii.               Solid lubricant coating for gears, valve guides, rocker arm and push rod assembly.

The following synthetic oils are suggested for lubrication of hot engine surfaces:

i.                    Ester + synthetic chain,

ii.                 Polyolester + synthetic HC

Performance of Adiabatic Engines:

The performance of the engine mostly depends upon the effective insulation used. An increase in engine wall temperature reduces the heat loss through the walls while increasing the exhaust gas energy loss.

For engines, where the exhaust gas is also used to develop power, there is an improvement in thermal efficiency of the engine. Calculations have shown that 40% reduction in heat flow through the cylinder wall increases the thermal efficiency by 2%.

The lower volumetric efficiency resulting from high degree of insulation has negative effect on power output which can be overcome by turbo-charging. The frictional losses were increased because of lower viscosity of the lubricating oil at elevated temperatures.

The adiabatic engine is more suitable for bi-fuel operation with alcohols which have high latent heat of vaporisation. The hot walls of combustion chamber help for better and quick vaporisation. This makes it possible to utilize a higher percentage of alcohol in an adiabatic engine compared to a normal dual-fuel engine.