Non-Conventional IC Engines

Introduction: (Part-I)

The main purpose of this blog is to introduce the non-conventional IC engines. In conventional engines, the common fuels used are diesel and petrol. These fuels are becoming scare and costly also. In addition to this, there are limited stores of these fuels and engineers are in constant search to find the fuels which can be used to replace the conventional fuels. As in multi fuel engines, conventional fuels are used for starting and then the engine is shifted to unconventional fuels which are relatively cheap.

In C.I. and S.I engines due to many reciprocating parts and problems of balancing and vibrations, it is also seen that the speed of IC engines are limited to 2000 to 5000 RPM. A few rotary types of engines are developed which can solve the problem of balancing. This also reduces the size of engines per kW as engine can be designed to run at a higher speed of 10,000 RPM. The problem of knocking associated with conventional engines can also be avoided in dual fuel and multi fuel engines even by using inferior fuels.

The pollution is one of the main problem faced by the designers with conventional IC engines. This can be partly reduced by using different non-conventional engines as stirling engines.

In addition to the methods mentioned above for developing the power using non-conventional engines, a variable-compression ratio engine is developed and used for research purpose for conducting the trials using different types of non-conventional fuels and their effects on the performance of the engine.

Dual fuel and MultiFuel Engines:

The dual fuel engine was developed from the diesel engine to take advantage of relatively cheap gases like- producer gas, biogas and natural gas. The engine uses high compression ratio and runs with high A:F ratio like diesel engine. The gaseous fuel is ignited by injecting ‘’pilot oil’’ into the heated mixture as it has high-self ignition temperature compared with diesel ignition temperature.

The storage of liquid fuels and large availability of the gaseous fuels led to an increased attention on the dual-fuel engine. The development of transporting facilities of liquefied gases has made natural gas available to most part of the world at considerably cheaper rate than the conventional liquid fuels. In addition to this, biogas for local uses, sewage gas, and producer gas are finding the partial substitute for the conventional diesel engines. The large amount of industrial gases can also be used for running IC engines and enhance the overall economy of the plant.

The natural gas contains 95% CH₄ and has self ignition temperatures of 730⁰C compared to 470⁰C of petrol. Therefore, all gaseous fuels are mainly used with diesel engines.

The dual fuel engine combines the use of diesel fuel and gaseous fuels such as natural gas, sewage gas and biogas. The engine can be shifted from dual fuel operation to diesel operation almost instantaneously in case of emergency.

Advantages of Dual Fuel Engines:

Following are the major advantages of dual fuel engines:

1.      It is preferred when cheap natural gas is easily available,

2.      The exhaust of dual fuel engines is clean as it does not contain any residue. The pollution from the engines is very much reduced,

3.      The wear and tear of engine parts is reduced as well as consumption of lubricating oil because of clean combustion in the engine,

4.      The engine can run on either of the fuel and the diesel requirement is hardly 5% if it runs on gas,

5.      The utility of the engine is increased considerably as instantaneously change over from gas to diesel and vice-versa is possible,

6.      These engines are suitable from the point of using total energy concept. Because, the exhaust heat of the engine can be used to digest the sludge in a sewage disposal plant and the sewage gas produced can be used to run the plant,

7.      These engines are best suited for low pressure liquefied gas (LPG) which evaporated very easily,

8.      A typical use of dual fuel engine is to produce synthetic gas by burning CH₄ and simultaneously developing power,

9.      Lot of conventional fuel can be saved if dual fuel engines are used particularly for irrigation purposes as more than 3 million diesel powered pump sets of 2 to 35 kW capacity are currently.

The diesel engine can be operated on gaseous fuels with minor modifications and has comparable efficiency and therefore, it is very attractive power generating system because of its great flexibility of operation compared with conventional diesel engines.

Applications of IC Engines

Introduction:

Now-a-days, IC Engines are used in all fields of industry and transportation. They are universally used for land, water and air transport also. Many CI engines are used for power generation in small capacity (2 to 20 MW). Brief applications of different types of engines are given below:

a.      Petrol Engines:

Automobiles, Marine, Aircraft.

b.      Diesel Engines:

Automobile, Locomotive, Marine and Power Generation.

c.      Gas Engines:

Industrial purposes and Power Generation.

d.      Gas Turbines:

Air craft, Marine, Industrial and Power Generation.

1.      Use of Two Stroke Petrol Engines:

These types of engines are used for scooters, mopeds and lawn movers in the capacity of 1.5 to 5 kW at 2000 to 5000 RPM. They are preferred for such applications because of their simplicity in working and low maintenance cost. They are also used for small electric generating sets; pumping sets and outboard engines.

The details of engines used for 2-wheelers are listed below:

a.      Moped (50 CC) with 1.5 kW at 5000 RPM,

b.      Scooter (100 CC) with 3.5 kW at 5000 RPM,

c.      Scooter (150 CC) with 5 kW at 5300 RPM,

d.      Motorcycle (250 CC) with 9 kW at 4500 RPM

2.      Use of 4-stroke Petrol Engines:

These engines are extensively used in cars, jeeps, buses and trucks. They are also used in pumping sets and mobile electric generating sets. But these are replaced by now-a-days by diesel engines because of high cost of petrol.

The details of engines used for 4-wheelers are listed below:

a.      Fiat car (1089 CC) 4-cylinder, 32 kW at 5000 RPM,

b.      American car (6-cylinder) 185 kW at 5000 RPM,

c.      Jeeps (4000 CC) 6-cylinders 90 kW.

The present common trend is to shift towards diesel cars because of high price of petrol and high running cost.

3.      Use of 4-Stroke Diesel Engines:

This is most commonly used prime mover because of low running cost. The speed of this engine can vary from 100 to 4500 RPM and power capacity varies from 1 kW to 100 kW per cylinder.

Small capacity diesel engines are used for pumping sets, construction machinery, drilling rigs and air compressors. In a middle range capacity, they are used for Jeeps, tractors and buses and trucks. Higher capacity diesel engines are used in railways, ships and small electric generating sets.

4.      Use of 2-Stroke Diesel Engines:

These engines in high capacity range are generally preferred for ship propulsion as the weight of the engine is one of the major considerations in the design of ship. Theoretically, the weight of 2-stroke engine is 50% of four stroke engine when both are running at same speed and developing same power.

Generally all engines above 60 cm bore are 2-Stroke, uniflow with exhaust valves or loop scavenged diesel engines. A Nordberg, 2-Stroke, 12 cylinder diesel engine develops 20MW at 120 RPM. This low speed of the engine allows to couple directly to the propeller of the ship without any gear mechanism.

IC Engine Cooling System

Introduction:

Generally, for safe working of the engine, it is necessary to carry 30% of the total heat generated by the combustion of fuel through cooling of the engine. The temperature of the gases in a reciprocating IC engines varies from 40⁰C to 2500⁰C during the cycle. In addition to this, the heat is also carried by lubricating oil which also accounts for 1 to 1.5%.

If the engine is not cooled, then the cylinder and piston temperatures may exceed 1500⁰C. At such higher temperatures, the metals will lose their properties and expansion of piston will be considerable and seize the liner. The lubrication of the engine will be badly affected if the engine cylinder exceeds 80⁰C because the lubricating oil will start evaporating and piston and cylinder will be badly damaged. In addition to this, high stresses will be induced and damage many parts of the engine. Therefore, it is essential to maintain the temperature of engine parts below some limit by proper cooling.

The cooling is provided to avoid the bad effects of overheating as listed below:

a.      The high temperature reduces the strength of the piston and piston rings and uneven expansion of cylinder and piston may cause the seizer of the piston,

b.      The high temperature may cause the decomposition of the lubricating oil and lubrication between the cylinder wall and piston and may break down resulting in a scuffing of the piston,

c.      If the temperature around the valve exceeds 250⁰C, the overheating of the valves may cause the scuff of the valve guides due to lubrication break down,

d.      The tendency of the detonation increases with an increase in temperature of the cylinder body,

e.      The pre-ignition of the charge is possible in spark ignition engines if the ignition parts initially are at higher temperature.

To avoid all the adverse effects mentioned above, it is necessary to cool the engine. The cooling system used for IC engine generally carries 30 to 35% of the total heat generated in the cylinder due to the combustion of air fuel mixture. It is also necessary that the temperature of engine should be maintained above a particular temperature. This is essential for easy running and better evaporation of the fuel.

Factors affecting the temperature of the cylinder and piston and Engine Heat Transfer:

The factors which affect the temperature distribution in the different parts of the engine are discussed below:

1.      Air-Fuel Ratio:

The burned gases temperature and indirectly the temperatures in the piston and the cylinder depend upon the A:F ratio. The maximum gas temperature occurs at A:F ratio of 13.5:1 in SI engine. Therefore the heat transfer from the gas to the engine parts will be maximum. At equivalence ratio of one, the increase in piston temperature is small but increase in gas temperature and hence, the valve temperature is significant. This is due to slow burning of the fuel.

2.      Compression ratio:

The gas temperature increases as the compression ratio increases. But higher pressure ratio allows the gas to expand more and therefore gas temperature becomes minimum at the end of expansion and therefore, the heat rejected during blow down will also be less.

3.      Engine speed:

At higher speed, the gas velocity increases and heat transfer coefficient from the gas to the engine parts increases and also the temperatures. If the load on the engine is same, but the speed is increases, the heat input per cycle increases and therefore, the gas temperature also increases.

4.      Engine output

The fuel supply per cycle increases with increase in load on diesel engine and mixture mass in petrol engine. Therefore, heat released and indirectly heat transferred increases with load on the engine. This increases the temperature of piston and liner temperatures. The common trend observed in diesel engine, the piston temperature is always lower for supercharged engine than naturally aspirated engine at all speeds.

5.      Ignition timing:

The engine efficiency becomes maximum when the heat release occurs at TDC. As combustion takes finite time, the ignition has to start a few degrees before TDC. Higher the angle of advance, the pressure and the temperature in the cylinder increase. This increase the piston as well as cylinder temperature.

6.      Effect of coolant:

Mostly water and air are used as coolant for petrol as well as diesel engines as per the capacity of the engine. Small capacity engines are air cooled where higher capacity engines are water cooled. The heat carrying capacity of water is much higher than air, the temperatures of the ports are lower in water cooled engine compared with air cooled engines.

7.      Turbulence:

The heat transfer for different combustion systems may differ by a factor of 10. This is mostly because of high turbulence created in these combustion chambers. In pre-combustion chambers of diesel engine, high turbulence occurs simultaneously with high pressure and temperature. This increases the instantaneously heat transfer.

8.      Bore-Stroke ratio:

With increased speed of the engine, the duration of all events, during each cycle, is decreased but the increased piston speeds create higher turbulence and as a result, heat rejected to the jacket remains more or less same or a marginal increase.

Magneto Ignition System and Problems in Aero Engine Ignition System

Spark ignition system

Introduction:

Some of the major drawbacks of battery ignition system are listed below:

a.      There are chances of discharging of battery,

b.      There are chances of misfiring at higher speed of the engine,

c.      It requires complicated wiring,

d.      There are chances of failure of system,

e.      There are many mechanical complications in the operation of the system.

These difficulties experienced with battery ignition system can be avoided by using magneto ignition system.

Magneto Ignition System:

It is exactly same like battery ignition except the source of generation is magnet instead of the battery. There are three types of magneto ignition system, namely:

a.      Rotating magnet system,

b.      Rotating armature type system,

c.      Polar inductor magneto.

Advantages:

a.      It is more reliable compared to coil ignition system, because there is no maintenance problem in magneto ignition system,

b.      It is used for medium to high speeds. Also it is very popularly used in racing cars,

c.      Space required is less as compared to coil ignition system,

d.      By providing suitable shunts on magnet, the danger of burning of spark plug is minimised,

e.      Very light in weight and compact in size,

f.       Automatic time adjusting of ignition can be affected.

Disadvantages:

a.      Initial cost is very high as compared to coil ignition system,

b.      To start with, 75 RPM is necessary,

c.      For high power engines, some other devices are necessary to start an ignition.

Problems in Aero Engine Ignition System:

Problems are met in providing ignition for aircraft engines which do not arise in connection with automobile engines and certain features are incorporated in aeroengine ignition systems to meet the special requirements.

Long periods of continuous operation impose severe mechanical strain on the rotating parts of the magnet and distributor units and these are designed to combine maximum strength with minimum weight in order to insure almost reliability under working conditions. Contact breakers and sparking plugs are sources of radio interference because they transmit electromagnetic waves and a method known as screening is used to prevent the waves interfering with radio reception. Their effect of screening and the use of weaker fuel mixtures in aero engines make it necessary for the ignition system to produce a higher H.T voltage than with other types of engines.

It is usual duplicate the ignition system by using two magnets, each cylinder being provided with two sparking plugs. Each system is quite independent of the other, but in most cases they are synchronized so as to give simultaneous sparks in the cylinder.

This arrangement gives a slight increase in power and if one system fails, the engine will operate quite well on the other.

The effects of high working temperatures tend to cause leakage of the H.T currents as stated previously. The resistance of insulating materials decreases with the rise in temperatures. Special consideration is given to the kinds of insulation materials employed in aero ignition system.

Variation of spark timing in accordance with engine speed, load and mixture conditions is a necessity and an arrangement whereby this is carried out automatically is essential.

Magneto ignition system is the general rule in aircraft and since the H.T voltage of a magnet is largely dependent on the speed at which the magnetic spindle is rotated, there is difficulty in starting an aero engine without special apparatus to ensure an efficient spark.

Spark Ignition System, Requirements, Pros and Cons

Spark Ignition System:
Introduction:

 Ignition is of fuel is only concerned with starting the combustion and not with the behaviour of the combustion wave or flame. It is necessary to ignite the beginning of the combustion process in the mixture of air fuel taken in during the suction stroke of the engine. The ignition must add sufficient energy for starting and sustaining the burning of air fuel mixture.

Ignition System

The basic requirement of the ignition system is to supply the minimum required energy within a small volume in a very short period of time to ensure that minimum energy is lost other than needed to establish the flame. Therefore the rate of energy supply is very important to generate and sustain the flame.

The intensity of the spark should be sufficiently high to raise the temperature of the surrounding mixture above its ignition temperature to initiate and sustain the combustion process. If the spark is weak, the energy from the point of spark will be quickly dissipated to the surrounding mass of mixture and ignition will not be initiated as the released energy is not sufficient to raise the temperature of the surrounding mixture above its ignition temperature.

Requirements of Ignition System:

The important requirements of the spark ignition system are listed below:

1.      The voltage across the spark plugs electrodes should be sufficiently large to produce an arc required to initiate the combustion. The voltage necessary to overcome the resistance of the spark gap and to release the enough energy to initiate the self propagating flame front in the combustible mixture is about 10,000 to 20,000 volts.

2.      The intensity of the spark should lie in the specified limits because too high intensity may burn the electrodes and too low intensity may not ignite the fresh mixture properly.

3.      The volume of the mixture at the end of compression should not be too large, otherwise the spark produced may not be sufficient to ignite the whole charge. There is definite relation between the size of the produced spark and the clearance volume.

4.      There should be no missing cycle due to failure of spark.

5.      In a multi-cylinder engine, there must be arrangement to carry this voltage to the right cylinder at the right time.

Battery ignition system:

Spark Plugs

The function of battery ignition system is to produce high voltage spark and to deliver it to the spark plugs at regular intervals and at the correct time with respect to the crank position. The required components of the battery ignition system are listed below:

a.      Battery of 6-12 volts,

b.      Induction coil,

c.      Contact breaker,

d.      Condenser,

e.      Distributor,

f.       Spark plugs.

Advantages:

a.      Its initial cost is low compared to magneto. This is the main reason for the adoption of the coil ignition on cars and commercial automobiles.

b.      It provides better spark at low speeds of the engine during the starting and idling. This is because maximum current is available throughout the engine speed range including starting.

c.      The maintenance cost is negligible except for the battery.

d.      The spark intensity remains unaffected by advance and retard position of the timing control mechanism.

e.      The simplicity of the distributor drive is another factor in favour of the coil ignition system.

Disadvantages:

a.      The engine cannot be started once the batter is dead.

b.      The overall weight of the battery ignition system is greater than magneto which is major consideration in adopting the system in aero-engines.

c.      The wiring involved in the coil ignition is more complicated than used in magneto ignition and therefore there is more likelihood of defects occurring in the system.

d.      The sparking voltage drops with increasing speed of the engine.

Two Stroke Engine and Scavenging

Introduction:

The purpose of this blog is to introduce the readers the further details of scavenging methods for better performance of engine and discuss relative factors which are responsible for the same.

Analysis of two stroke engine is more difficult than four stroke engine as the overlapping period for the exhaust and inlet ports is considerably large. The pattern flow is more difficult and uncertain. Therefore, it is more difficult because of randomness in formation of pattern during the exhaust and charging the engine cylinder.

Two stroke engine is easy in construction as there are very few moving parts compared to the four stroke engine but more difficult for analysis because of overlapping the inlet and exhaust. As there are only two strokes for performing four basic operations, the overlapping process cannot be avoided. During the downward stroke, expansion is carried out during part of the stroke and during remaining part of the stroke, the exhaust and charging are carried out simultaneously. This process is also continued during the upward motion of the piston for the part of stroke, and for the remaining part of the stroke, the compression is carried out.

Use of two stroke SI engines is universally adopted with two wheelers because of its simplicity in construction and maintenance free operation. Two stroke diesel engines are used in bigger capacity 2000 kW and above for generating the power or they are used for ships.

Scavenging Process and Scavenging methods:

The method of removing the exhaust gases with the help of fresh charge in petrol engine or by the inlet air in diesel engine is known as Scavenging. This process is carried out during the overlapping of inlet and exhaust ports. The basic requirement of an ideal scavenging system is to remove exhaust gases without any loss of fresh charge or air. Such ideal system is practise is impossible but utmost care should be taken to reduce the loss to minimum.

The best air path is achieved through scavenging in which air is admitted at one end of the cylinder and exhaust gases are discharged from the other end. Ideally, this method results in perfect scavenging in which the incoming air displaces the exhaust gas without mixing. In cross-scavenging, care must be taken to avoid short circuiting. Without the hump on the piston top, the incoming air or mixture would have a tendency to simply go in and out of the cylinder without displacing the exhaust gases. This is known as Short circuiting. From experiment it is suggested that the best scavenging that can be achieved via the cross method occurs when there is perfect mixing in which the fresh air introduced successively dilutes the residual exhaust gases. If the sufficient air is used, at the end of scavenging, an acceptable scavenging efficiency can be achieved.

In an ideal scavenging process, the fresh mixture should push the residual gases without mixing and exchanging heat and this process should continue until all burned gases are replaced with fresh charge. At this point, the flow must stop. The pressure and temperature of the charge in the cylinder should be same as that inlet. The cylinder is expected to be filled with fresh charge when the piston is at BDC. Also fresh charge should not escape through the exhaust post.

Scavenging methods:

1.      Cross-scavenging

2.      Combined port and valve scavenging system

3.      Auxiliary exhaust valve arrangement

1.      Loop scavenging system

a.      Full loop or MAN

b.      Tangential loop scavenging system

c.      Combination of loop and tangential scavenging

2.      Uniflow scavenging system

3.      Kadency scavenging