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Engine: An engine is a device which transforms one form of energy in to another form. Normally most of the engines convert thermal to mechanical work and are called heat engines.

Heat Engines: It is a device which transforms chemical energy of a fuel into is Thermal energy and utilizes this to perform useful work. Thus, this Thermal energy converted to mechanical energy and are called heat engines.

Heat engines are classified in to two types:
1. IC engines
2. EC engines.

Whether the engines are IC or EC the movement or function which takes place is reciprocatory, and rotary. External combustion engines are those in which combustion takes place outside the engine where as IC engines takes place outside the engine where as IC engines combustion takes place with in the engine.
Eg: steam turbine heat generated due to combustion of fuel is employed to generate high pressure steam which is used as working fluid in a reciprocating engine or turbine.
In case of diesel engine or gasoline the products of combustion generated bu combustion of fuel and air with in the cylinder form the working fluid .
C.I: Compression ignition.
S.I: Spark ignition.

In conventional engines common fuels used are diesel and petrol. These fuels are becoming scare and scare and becoming costly day by day. 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 are used for starting the engine and the engine is shifted to unconventional fuels which are cheap.

Speed of IC engine limited to 2000 to 5000 R.P.M in case of C.I and S.I due to reciprocating parts and problems of balancing and vibrations. The pollution is one of the main problem faced by the designers of I.C engines. This can be partly reduced by using different non-conventional engines such as sterling engine. The purpose of this non- conventional I.C engines is just to introduce it and to know about their advantages and disadvantages.


1. Dual fuel and multi fuel engines
2. Gases and dual fuel engines
3. Anken bio gas engine
4. Multi fuel engines
5. Satisfied charge engine
6. Adiabatic engines
7. Variable compression ratio engines
8. Free piston engines
9. Sterling engine
10. Wrankel-rotary engine.

Now, I am going to discuss among the one above mentioned engines i.e. the nothing but
Free piston engines.

Free piston engines:

A free piston engine combines the feature of high thermal efficiency of reciprocation diesel engine and low weight/power ratio of gas turbine.
To get more power from the engine the engine was supercharged as better fuels and better materials were made available. Further improvement in the supercharged engine was
Turbo-charged engine which used the exhaust gas heat to drive the super charger. This idea of using exhaust gas energy has developed and it is used in free piston engine. In this system, free piston engine is highly supercharged and using its exhaust gases, power is developed in a gas turbine. The energy developed by the engine is used to compress the air required for supercharging the engine. No external power is available from the engine and it produces only high pressure, high temperature gas which is further used for developing the power in gas-turbine. Therefore, this engine arrangement is known as gasifier and the whole arrangement is known as free piston engine.

In this arrangement, the connecting rod, cam shaft and crank shaft are avoided and the valves are operated by pressure differences only. The pistons are connected by a synchronizing mechanism.


Fig(1) shows the basic components of free piston engine having inward compression in the engine. It consists of a gasifier and a gas turbine. The gasifier is like an opposed piston uniflow scavenged two stroke cycles diesel engine with compressor of air is directly achieved by compressing the air through piston rod of the engine which is directly connected to the compressor piston as shown in fig.

The motion of the piston is dictated by the gas pressure, and compressor space and main diesel combustion chamber space and not by mechanical linkages. This freedom of motion from mechanical linkage provides variable stroke according to the load so it is called as

Free piston engine.

The stoke of the piston is in the gasifier is variable and depends upon the amount of fuel injected. The gasifier is provided with a safety device in case of over stroking.


Referring to the Fig(1), the inside is the opposed piston uniflow engine. The I.P is the inlet air port and EP is the exhaust port. Each small piston of the engine is connected by a piston rod to the compressor piston. The engine pistons P1 reciprocates in cylinder c1 and compressor pistons P2 reciprocates in cylinder c2.

The working steps are described below:

1. Consider, both the pistons P1 are at the centre of the cylinder and volume between the two pistons is filled with high pressure air (similar to two stroke diesel engine charged with high pressure air when the piston is at TDC). At this position of pistons P1, the pressure in the cylinders c2 is below atmospheric pressure and air is sucked inside the compressor cylinder through valves V1.
2. As the fuel is injected in the compressed air in the compressed air in the engine cylinder, it starts burning and pushes the engine pistons outward allowing the gases to expand. During outward motion of the cylinder pistons, the compressor piston also moves outward pressurizing the air in the compressor cylinders. At a particular position of the engine pistons, the ports which are provided as shown in figure open. The pressure of the cylinder is sufficient to open the valves V2 and high pressuer air through the I.P.
(inlet ports) enters into the engine cylinder. At the same time, the exhaust ports (EP) open and the exhaust gses enter in to the receiver as shown in Fig. a part of air entering into the engine cylinder also enters into the receiver with the exhaust gases.

3. Once the engine cylinder C1 is fully charged as pistons P1 reaches BDC, then the
pistons P1 start moving inward compressing the air which is taken in . As the engine pistons P1 move inward, the compressor piston P2 also move inward causing vacuum inside the compression of air in the engine cylinder is completed. Then the fuel injector injects the fuel in the compressed air in the engine cylinder, it burns and starts expanding. Thus the cycle is repeated. The exhaust gases from the engine and part of scavenging air enters into the receiver and the mixture at high pressure(3 to 5 bar) and high temperature (500c) is passed through the turbine to develop the power.

The speed of the engine is controlled by controlling the amount of fuel supplied as per the load on the turbine. As per the amount of fuel supplied, the stroke length of the engine piston changes. A relatively small change in the piston stroke can vary the amount of air supply to the engine up to 40% to 45%.


Fig (2) shows the schematic diagram for the flow of gases in the free piston engine and the processes are represented on T-s diagram.
Referring to the T-s diagram,
1-2 is adiabatic compression of m kg of air in the compressor.
2-3 is adiabatic compression of m1 kg of air in the engine.
3-4 is combustion in the engine at at constant pressure.
4-5 is adiabatic expansion in the engine cylinder.
5-6 and 2-6 represent the mixing of m1 kg gases from the engine and m2 kg of air from compressor.
6-7 represents the work output from the turbine with mass flow of m kg.
Generally, m2=3m1

The specific fuel consumption of free piston engine at part loads is higher than the conventional diesel engine. This can be improved by recirculation part of the air in air-box case, back to the intake of the compressor. This method raises the temperature of the incoming air under part load conditions and makes the combustion possible even at compression ratio 10.

As the gas-turbine develops the powe, the enginehas excellent high torque even at low speed weight to power ratio which varies from 0.7 to 0.4.



1. The thermal is considerably higher than simple gas turbine unit.
2. Higher volumetric efficiency can be obtained than conventional diesel engine.
3. The system is highly simplified due to absence of connecting rod, crankshaft, valves and valve mechanism required for conventional diesel engine.
4. The engine is perfectly balanced and gives practically vibration less performance.
5. Weight /B.P. is considerably less (0.5) compared with conventional diesel
engine (2.5).
6. The turbine in this system need not supply compressor work, the maximum temperature for better efficiency is considerably lower. The temperature lies between 450c to 500c against 700c to 800c in conventional gas- turbine. Therefore, cost of the turbine is also low as cheap materials can be used.
7. The free piston engine adjusts the ignition requirements by changing the compression ratio (Rc varies from 20 to 50). This property allows the system to use a wider variety of fuels than the conventional diesel fuel in diesel engine.
8. As the system is better balanced, the vibration and noise are lower than conventional diesel engines. Therefore, the lubrication requirements are also reduced.
9. The starting of the engine is easy compared with conventional diesel engine as air required for starting is hardly 50% of the conventional diesel engine of the same capacity.
10. Acceleration characteristics are much superior sompared to diesel engine as it has little thermal as well as mechanical inertia. It s output can be varied form full load to idling and back to full load hardly with in 15 to 20 sec only.


1. The specific fuel consumption is higher than conventional diesel engine. Particularly at part load conditions.
2. The starting of free piston engine is difficult as there is no fly wheel.
3. The free piston engine requires a certain amount of fuel per stroke according to the load on turbine to maintain stable operation. The fuel supply can be varied only in a limited range. Any fluctuation in the fuel supply will make the operation unstable.
4. As the supercharge is considerably high, the combustion rate per m of cylinder volume is 4 to 5 times higher than conventional diesel engine. This intensifies lubrication and cooling problems.
5. A gearing is necessary to couple high speed turbine with low speed marine propeller. This further increases the capital cost and weight of the plant. The reduction gear of 30 is generally required as speed of 9000 R.P.M is to be reduced to 300 R.P.M
please read and for getting more information about NON CONVENTIONAL I.C ENGINES and Free Piston Engine

to get information about the topic"NON CONVENTIONAL I.C ENGINES full report" refer the page link bellow

This is really helpful information, especially for those people who study about engines. I remember having a homework about this before and not having enough information about it. That was really hard. So I really think that this is great and helpful.


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