THERMAL ENGINEERING-1 (UNIT-1)

UNIT-1

Actual Cycles and their Analysis: Introduction, Comparison of Air Standard and Actual Cycles, Time Loss Factor, Heat Loss Factor, Exhaust Blow down-Loss due to Gas exchange process, Volumetric Efficiency. Loss due to Rubbing Friction, Actual and Fuel-Air Cycles of CI Engines.

Introduction: 

  The actual cycles for IC engines differ from the Fuel-air cycles and air standard cycles in many respects. The actual cycle efficiency is much lower than the air-standard cycle efficiency due to various losses .

The major losses are followed as

1. Variation of specific heat with temperature
2. Dissociation of the combustion products
3. Progressive combustion
4. Incomplete combustion of fuel
5. Heat transfer into the walls of the combustion chamber
6. Blow down at the end of the exhaust process
7. Gas exchange process

Comparison of Air Standard and Actual Cycles:

  The Actual cycle in internal combustion engine is different when compared to Air standard cycle. The differences are mainly due to 

1. The working medium being a mixture of air and fuel with the products left in previous cycle in actual cycle , but we are not taking the consideration in standard cycles.
2. The change in chemical composition of the working medium
3. The variation of specific heats with temperature.
4. The change in the composition, temperature and actual amount of fresh charge because of the residual gases.
5. The progressive combustion rather than instantaneous combustion.
6. The heat transfer from the working medium and to the working medium.
7. Loss of work on the expansion stroke due to early opening of exhaust valve.
8. Gas leakages, fluid friction etc., in actual engines.  

Out of all of these the major influences are

• Time loss factor: Loss due to time required for mixing of fuel,air and also for combustion
• Heat Loss factor: loss of heat from gases to cylinder walls
• Exhaust blow down factor: loss of work on the expansion stroke due to early opening of the exhaust valve

Exhaust blow down loss Factor:

• The cylinder pressure at the end of exhaust stroke is about 7 bar depending on the compression ratio employed
• If the exhaust valve is opened at the BDC, the piston has to do work against high cylinder pressure during the early part of the exhaust stroke.
• If exhaust valve is opened too early , a part of expansion stroke is lost.
• The best optimum value to open the exhaust valve is 40° to 70° before BDC which it reduces cylinder pressure to half the previous value i.e 3.5 bar

Loss due to gas exchange process:

• The difference of work done in expelling the exhaust gases and the work done by the fresh charge during the suction stroke is called as Pumping work.
• In other words loss due to the gas exchange process is due to pumping gas from lower inlet pressure Pi to higher exhaust pressure Pe. 
• The pumping loss increase by increasing the throttling process because of the throttling process reduces the inlet pressure in suction.
• Pumping loss also increases with  speed. The gas exchange process largely effects volumetric efficiency of the engine.

Loss due to Rubbing friction:

• The losses are due to friction between the piston and the cylinder walls , friction in various bearings and operating equipment like cooling water pump,ignition system , fan etc,.
• The piston rings friction increases rapidly with engine speed and slightly with mean effective pressure.
• The bearing friction and auxiliary friction also increases with engine speed.
•  The efficiency of engine is maximum at full load and decreases with part load because of energy loss in forms of heat loss, friction losses.

Time Loss Factor:

• It is due to the time required for mixing of air-fuel mixture and complete combustion.
• Heat addition is not instantaneous and spread over a period (30° to 40° of crank shaft revolution). Therefore ,Pmax is not at TDC , but just after TDC.
• Time loss depends up on flame velocity which it again depends on type of fuel used , A/F ratio and shape of combustion chamber.

Spark advance: This to have Pmax at TDC. If the spark is initiated at TDC peak pressure would be low due to expansion of gases.Further if the spark is initiated too early additional work is required to compress the burning gases,which is a direct loss. Therefore the optimum time to start the combustion is 15° to 30° before TDC.

Heat Loss Factor:

• Heat loss is due to the transfer of heat through water jackets and cooling fins.
• Some heat is being transferred during compression and expansion process.
• In some cases heat energy is loss by gas blowby through piston ring gaps.
• Because of heat loss temperature Tmax decreases and specific heat get reduced. So efficiency automatically comes down. 

Volumetric Efficiency:

     Volumetric efficiency (in simple terms) is how much air gets filled in the engine. To explain this, consider an engine with a displacement of say 600cc. Now during the intake stroke, air is filled in because the intake valve is opened. Theoretically air of 600cc must fill the combustion chamber. This would make the engine 100% efficient in terms of volume i.e. volumetric efficiency of 100%. But there are some limitations like intake manifold, valve timings, etc; which provide a passage to air into engine. But due to design constraints, air of say 500cc enters the engine. This makes the volumetric efficiency of 83.3%.

By formula:

Volumetric efficiency=(Air entering engine / Engine displacement)X100

In short it displays, the percentage volume of air entering the combustion chamber.

Note: 

1. Volumetric efficiency can be increased also above 100% by using a turbocharging and supercharging methods.
2. Volumetric efficiency can be increased by increasing the inlet valve hole cross sectional area
3.  Volumetric efficiency can be increased by increasing the out let valve closing time.

Actual and Fuel-Air Cycles of CI Engines:

• In the diesel cycle the losses are lesser than Otto cycle. The main loss is due to incomplete combustion and it is the major difference between fuel-air cycle and actual cycle of a diesel engine.
• In a fuel-air cycle the combustion is supposed to be completed at the end of constant pressure burning where as in actual practice after burning continues up to half of the expansion stroke.
• The ratio between the actual cycle efficiency and the fuel-air cycle efficiency is about 0.85:1 in diesel engines
• In fuel-air cycles the allowances are made for the presence of fuel and combustion products , there is reduction in cycle efficiency. But in the actual cycles allowance are also made for the losses due to heat transfer and finite combustion time. This reduces the cycle efficiency further.

New Technique: For complete analysis of actual cycles computer models are being developed. These models are helpful in understanding the various processes that are taking place in SI and CI engines.