4

Represent Brayton cycle on PV and TS diagram. Name the processes completing the cycle

4

Draw p-v and T-S diagram for Diesel cycle. Name the processes involved in it.

Diesel Cycle on P-V and T-S diagram :

Processes : 1-2 : Isentropic compression 2-3 : Heat addition at constant pressure 3-3 Isentropic expansion 4-1 Heat rejection at constant volume

6

Draw a neat labelled sketch of fuel injection pump. Give its function.

Fuel injection pump : Fuel injection pump is used widely for the supply of fuel under high pressure in diesel engines

4

Differentiate supercharging and turbocharging in I.C. engine.

6

Draw superimposed p-v diagram of Otto cycle, Diesel cycle and Dual cycle to compare their efficiencies for same compression ratio (R c ) and heat rejection (Q r ).

Superimposed P-V Diagram of Otto, Diesel & Duel Cycle: A comparison of the cycles (Otto, Diesel and Dual) on the p-v and T-s diagrams for the same compression ratio and heat supplied is shown in the Fig. Since all the cycles reject their heat at the same specific volume, process line from state 4 to 1, the quantity of heat rejected from each cycle is represented by the appropriate area under the line 4 to 1 on the T-s diagram. As is evident from the cycle which has the least heat rejected will have the highest efficiency. Thus, Otto cycle is the most efficient and Diesel cycle is the least efficient of the three cycles

4

Represent ‘Brayton Cycle’ on P-V and T-S diagram.

4

What is ‘Scavenging’ ? List any two types of ‘scavenging’.

Scavenging: At the end of expansion stroke the combustion chambers of a two stroke engine is left full of products of combustion. This is because unlike four stroke engines, there is no exhaust stroke available to clear the cylinder of burnt gases in two stroke engine the process of clearing the cylinder after the expansion stroke is called scavenging process. The scavenging systems are as follows : 1. Uniform scavenging system 2. Cross scavenging system 3. Loop or reverse scavenging system

4

Explain in brief the constructional features of MPFI engine

Absence of Venturi – No Restriction in Air Flow/Higher Vol. Eff./Torque/Power • Hot Spots for Preheating cold air eliminated / Denser air enters • Manifold Branch Pipes not concerned with Mixture Preparation • Better Acceleration Response • Fuel Atomization Generally Improved. • Use of Greater Valve Overlap • Use of Sensors to Monitor Operating Parameters/Gives Accurate Matching of Air/fuel Requirements: Improves Power, Reduces fuel consumption and Emissions • Precise in Metering Fuel in Ports • Precise Fuel Distribution Between Cylinders • Fuel Transportation in Manifold not required so no Wall Wetting • Fuel Surge During Fast Cornering or Heavy Braking Eliminated • Adaptable and Suitable For Supercharging • Increased power and torque.

6

A petrol engine working on constant volume cycle has compression ratio of 8 and consume 1 kg of air per minute, if minimum and maximum temp. during cycle is 300 °K and 2000 °K respectively. Find power developed by engine. Assume γ = 1.4 and Cv = 0.71 kJ/kg °K.

4

State four assumptions made for air standard cycle.

Assumption made in air standard cycle Following assumption made in actual cycle to analysis as air standard cycle. 1. The working fluid is perfect gas. 2. There is no change in mass of the working medium. 3. All the process that constitutes the cycle is reversible. 4. Heat is assumed to be supplied from a constant high temperature source and not from chemical reaction during the cycle. 5. There are no heat losses. 6. The working medium has constant specific heats throughout the cycle.

4

Name four sensors used in I.C. engine and explain working of any one.

Sensor used in IC engines ( Explanation of any one )

A sensor is an input device that provides variable information on an engine function. Examples of sensors include the airflow sensor (AFS), crank angle sensor (CAS), throttle potentiometer sensor (TPS) etc, and these provide data on load, rpm, temperature, throttle opening etc. This data is signaled to the ECM, which then analyses the results and computes an output signal. The output signal is used to actuate an n output device.

Crank angle sensor: A permanent magnet inductive signal generator is mounted in close proximity to the flywheel, where it radiates a magnetic field. As the flywheel spins and the pins are rotated in the magnetic field, an alternating (AC) waveform is delivered to the ECM to indicate speed of rotation. If a pin is intentionally omitted at two points on the flywheel, or by contrast a double pin is used, the signal will vary at these points, and a reference to TDC will be returned to the ECM. The location of the positional signal is not at TDC, but may be some other point fixed by the VM. When used, the CAS provides the primary signal to initiate both ignition and fuelling.

Air Flow Sensor (AFS): The AFS is normally located between the air filter and the throttle body. As air flows through the sensor, it deflects a vane (flap) which wipes a potentiometer resistance track and so varies the resistance of the track and generates a variable voltage signal.

Manifold absolute pressure (MAP) sensor: The MAP sensor measures the manifold vacuum or pressure, and uses a transducer to convert the signal to an electrical signal which is returned to the ECM. The unit may be designed as an independent sensor that is located in the engine compartment or integral with the ECM.

Coolant temperature sensor (CTS): The CTS is a two-wire thermistor that measures the coolant temperature. The CTS is immersed in the engine coolant, and contains a variable resistor that usually operates on the NTC principle.

Throttle Position Sensor (TPS): TPS is provided to inform the ECM of idle position, deceleration, rate of acceleration and wide-open throttle (WOT) conditions. The TPS is a potentiometer which varies the resistance and voltage of the signal returned to the ECM. 01 03 MAHARASHTRA STATE BOARD OF TECH From the voltage returned, the ECM is able to calculate idle position, full-load and also how quickly the throttle is opened.

Oxygen sensor (OS): An oxygen sensor is a ceramic device 'placed in the exhaust manifold on the engine side of the catalytic converter. The oxygen sensor returns a signal to the ECM, which can almost instantaneously (within 50 ms) adjust the injection duration.

4

Explain the process of combustion in diesel engine.

Combustion in CI Engines :The combustion in CI engines is taking place in following stages as shown in figure 1. Ignition delay period: During this period, some fuel has been admitted but not yet ignited. The delay period is a sort of preparatory phase. It is counted from the start of injection to the point where P-ɵ curve separates from air compression curve. 2. Rapid or uncontrolled combustion : In this stage , the pressure rises rapid because during the delay period the fuel droplets have time to spray and have fresh air around them. This period is counted from end of delay period to the max pressure on indicator diagram. 3. Controlled combustion : uncontrolled combustion is followd by controlled combustion stage. The period of this stage assumed to be at the end of max cycle temperature. 4. After burning : It is expected to end combustion process after third stage. Because of poor distribution of fuel particles combustion still continues during remaining part of expansion stroke. This is after burning .

4

A diesel engine has a compression ratio of 14 and cut-off takes place at 6% of stroke. Find the air standard efficiency.

4

Write the equations for air standard efficiency of otto cycle and diesel cycle and state various terms involved in it.

4

Explain turb charging with a neat sketch.

6

Explain with neat sketch turning moment diagram for a four-stroke engine.

4

An engine of diameter 250 mm and 375 mm stroke works on otto cycle. The clearance volume is 0.00263 m3, find the air standard efficiency of cycle also sketch the cycle on P-V plane.

6

A petrol engine working on otto cycle has compression ratio 8 and consumes 1 kg of air per minute. If maximum temperature during the cycle is 2001 k and minimum temperature is 299 k. Find power developed by engine.

4

Explain different stages of combustion in C.I. engine with sketch.

1) Ignition delay period : During this fuel has already admitted but has not yer ignited. This is counted from start of injection to the point where P-O curve separates from pure air compression curve. 2) Rapid or uncontrolled combustion : In this stage pressure rise because of during the delay period the fuel droplet have time to spread over a wide area and fresh air around them. 3) Controlled combustion : The temperature and pressure rise in the second state is quite high, hence droplet of fuel injected in stage burn faster with reduced ignition delay as soon as they find necessary oxygen and further pressure rise is controlled by injection rate. 4) After burning : This sage may not be present in all cases. Because of poor distribution of fuel particles, combustion continues during part of the remainder of the expansion stroke.

4

Draw and explain Battery ignition system.

Battery Ignition system : I t consists of six or twelve volt battery, ignition switch,induction coil, circuit breaker condenser and distributor. All the circuit parts are shown in figure. One terminal of battery is ground to engine frame and other is connected through the ignition switch to one primary terminal of induction coil. The other primary connection is connected to one end of contact point of circuit breaker and through closed points to ground. The ignition switchis made on and engine is crancked. When the contacts touch, the current flows the battery to the switch. Due to this primary winding of induction coil to circuit breaker points and circuit is completed. A condenser prevents sparking of this point. The rotating cam breaks open the contacts immediately and breaking of this primary circuit brings about change of magnetic field causing very high volt about 8000 to 12000 volts. Due to this high voltage the spark jumps across the gap in the spark plug and thereby it ignites mixture of air and fuel.

4

Draw P-V and T-S diagram for dual cycle. Name the processes involved in it.

4

Why does the Carnot heat engine not exist in practice? Give any four points

Carnot heat engine is an ideal heat engine and is not possible in practice due to following reasons. i) Alternate adiabatic and isothermal process is not possible. ii) Heat addition and heat rejection at constant temperature is not possible. iii) All processes are reversible which is not possible in practice

4

Draw super imposed PV and TS diagrams of otto cycle, diesel cycle and dual cycle to compare their efficiencies under the following conditions: (i) for same compression ratio and heat rejection (ii) for same maximum pressure and temperature and heat rejection.

Same compression ratio and same heat rejected heat rejection

ii) For same maximum pressure and temperature and heat rejection

4

Define cut off ratio. Express it in terms of compression ratio and expansion ratio.

4

State the functions of following components used in battery ignition system: 1) capacitor 2) ballast register 3) contact breaker 4) distributor

Function of Components used in battery ignition system 

4

Which is more effective way to increse the C.O.P. of refrigerator, to increase T2 keeping T1 constant or to decrease T1 keeping T2 constant? (T1 > T2). Give justification to your answer.

4

In gas turbine plants, Brayton cycle is more suitable than otto cycle, even though both cycles have equal thermal efficiency for same compression ratio. Justify

In gas turbine plant – it works on brayton cycle where the heat added & heat rejected at constant pressure. It consists of compressor, combustion chamber & a turbine. The efficiency of Brayton cycle rotor cycle is same for but efficiency is of gas it temperature & pressure is increasing. High temperature & pressure require for ignition & fuel consumption for bray ton cycle. It is not possible in Oto cycle because the heat added & rejected at constant volume so bray ton cycle is most suitable than Otto cycle for gas turbine plant.

4

Draw P-V and T-S diagram for Diesel cycle. Name the processes involved in it.

4

What is supercharging ? State advantages of supercharging.

Superchargers are pressure boosting devices (compressors) which increase the pressure of the air before inletting it get into cylinder of the internal combustion engine, and the process of increasing the pressure OR forcing more air to get into engine is called as supercharging. This gives each intake cycle of the engine more oxygen, letting it burn more fuel and do more work, thus increasing power.

Advantages 1. Higher power output. 2. Reduced smoke from exhaust gases. The extra air pushed into cylinder, helps the air to complete combust leading to lesser smoke generation. 3. Quicker acceleration of vehicle. Supercharger starts working as soon as the engine starts running. This way the engine gets a boost even at the beginning leading to quicker acceleration. 4. Cheaper than turbocharger.

6

Explain with neat sketch working principle of four stroke petrol engine.