IC Engine - 2 - Mechanical Engineering MCQ

When cut off ratio (α) is equal to zero then the efficiency of Diesel cycle approaches to Otto cycle efficiency.

The normal range of compression ratio for diesel engine is 16 to 20 whereas for spark ignition engines it is 6 to 10. Due to higher compression ratios used in diesel engines the efficiency of a diesel engine is more than that of the gasoline engine.

A dynamometer is a device used for measuring the torque and brake power required to operate a driven machine. It has a device to measure the frictional resistance.

Following are the two types of dynamometers, used for measuring the brake power of an engine.

1. Absorption dynamometers: The entire energy or power produced by the dynamometer is absorbed by the friction resistances of the brake and is transformed into heat, during the process of measurement.

Example: Prony brake dynamometer, Rope brake dynamometer, Hydraulic dynamometer

2. Transmission dynamometers: The energy is not wasted in friction but is used for doing work. The energy or power produced by the engine is transmitted through the dynamometer to some other machines where the power developed is suitably measured.

Example: Epicyclic-train dynamometer, Belt transmission dynamometer, Torsion dynamometer.

CH₄ + 20₂ → CO₂ + 2 H₂O

∴ 1 m³ of methane requires 2 m³ of oxygen

∴ 0.6 m³ of methane requires 1.2 m³ of oxygen

2 CO + O₂ → 2 CO₂

1 m³ of CO requires 0.5 m³ of oxygen, so 0.3 m³ of CO requires 0.15 m³ of oxygen

∴ Total oxygen requires = 1.2 + 0.15 = 1.35 m³

Oxygen already present = 0.1 m³

∴ Stoichiometric oxygen needed = 1.25 m³

Dieseling is the phenomenon in which an SI engine sometimes continues to run for a very small period even after the ignition is switched off. Dieseling may take place due to following reasons:

• Engine overheating
• Too high spark plug heat range
• High engine idling speed
• Oil entry into the cylinder
• Increase in compression ratio due to carbon deposits
• Incorrect adjustment of idle fuel-air mixture
• Sticking of throttle
• Requirement of tune up of engine

Dieseling is the phenomenon in which an SI engine sometimes continues to run for a very small period even after the ignition is switched off. Dieseling may take place due to following reasons:

• Engine overheating
• Too high spark plug heat range
• High engine idling speed
• Oil entry into the cylinder
• Increase in compression ratio due to carbon deposits
• Incorrect adjustment of idle fuel-air mixture
• Sticking of throttle
• Requirement of tune up of engine

Otto cycle ⇒ 1 - 2 - 3 - 4

Diesel cycle 1 - 2’ - 3’ - 4’

It is clear from T - s diagram that the heat rejected by the Diesel cycle (area below 4’ - 1) is less than the heat rejection by the Otto cycle (area below 4 - 1); hence the diesel cycle is more efficient than Otto cycle for the condition of same maximum pressure heat input. The dual cycle efficiency lies between the Otto and Diesel cycle.

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 ratio have higher flame speed.

Volumetric efficiency is defined as the ratio of actual volume flow rate of air into the system to the rate at which the volume is displaced by the system.

It indicates the breathing capacity of the system. It is to be noted that the utilization of the air is that determines the power output of the engine. Intake system must be designed in such a way that the engine must be able to take in as much air as possible.

Catalytic converters are called three way converters because it reduces the concentration of CO, HC and NO_x in the exhaust (three concentrations).

Octane number of natural gas is around 110 which makes it a very good SI engine fuel. Because of this high octane number the flame speed is higher and engine can operate with high compression ratio.

Brakepower=

Friction power = Indicated Power – Brake Power = 1.8 - 1.508 = 0.292

If a very high compression ratio is used, than the petrol will reach its self to the ignition temperature before the spark and the knocking will also occur. So compression ratio of petrol engine is less.

Mean effective pressure is the average pressure inside the cylinder of an internal combustion engine based on the calculated or measured power.

The ratio of actual fuel air ratio to stoichiometric fuel air ratio is called equivalence ratio.

The highest flame velocities are obtained with somewhat richer mixture of equivalence ratio 1.1 to 1.2. When the mixture is made leaner or richer the flame speed decreases. Less thermal energy is released in case of lean mixture resulting in lower flame temperature. Very rich mixture leads to the incomplete combustion which results again in the release of less thermal energy.

For CI engines, the normal paraffin’s are the best fuels and aromatics are the least desirable.

For SI engines the aromatics are the best fuel and the paraffins are least desirable.

The reason for this is normal paraffins exhibit the poorest antiknock quality when used in SI engines. But the antiknock quality improves with the increasing number of carbon atoms and the compactness of the molecular structure. The aromatics offer the best resistance to knocking in SI Engines.

In diesel engine intake is only air. The air is compressed and then the fuel is injected to the high pressure and temperature compressed air.

Volumetricefficiency=

∴ Actual intake volume = 200 × 0.85 = 170 cm³

For a combustion process that takes place adiabatically with no shaft work, the temperature of the products is referred to as the adiabatic flame temperature. This is the maximum temperature that can be achieved for given reactants. The maximum adiabatic flame temperature for a given fuel and oxidizer combination occurs with a stoichiometric mixture (correct proportions such that all fuel and all oxidizer are consumed). The amount of excess air can be tailored as part of the design to control the adiabatic flame temperature.

For any particular engine, operating at a given speed and power output, there will be a specific mean effective pressure, mep. This mep is used as a basis to compare the performance.