The theoritical maximum amount of work that can be obtained from a system at any state P₁ and T₁ when operating with a reservoir at the constant pressure P_o and temperature T_o is called the Exergy or Avalibility.

Consider a system consisting of a liquid in a cylinder behind a piston. The fluid expanding reversibly from initial condition of P₁ and T₁ to final atmospheric condition of P_o and T_o. Imagine also that the system works in conjuction with a reversible heat engine which receives heat reversibly from the fluid in the cylinder such that the working substance of heat engine follows the cycle O1OA as shown in fig. Where s₁ = s_A and T_o = T_A.

The only possible way in which this could occur would be if an infinite number of reversible heat engines were arranged in parallel, each operating in a carnot cycle, each one receiving heat at a different constant temperature, and each one rejecting heat at To. The work done by this engine is given by :

-W = Q

Therefore

-W = Q – T_o (s₁-s_o)

The heat supplied to the engine is equal to the heat rejected by the fluid in the cylinder. Therefore for the fluid in the cylinder undergoing the process 1 to 0 we have:

-Q + W_fluid = u_o – u₁

W_fluid = (u_o – u₁) + Q

Therefore the total work output is given by:

-W – W_fluid = -(u_o-u₁) – Q+Q-T_o(s₁-s_o)

-W – W_fluid = (u₁-u_o) – To(s₁-s_o)

The work done on the fluid by the piston is less than the total work done by the fluid, -W_fluid, since there is work done on the atmosphere which is at constant pressure P_o. I.e.

Work done by the fluid on atmosphere is given by:

-W_atm = P_o (v_o-v₁)

When a fluid undergoes a complete cycle then the net work done on or by the atmosphere is zero. Hence;

Max. Work available = Work done by the fluid on piston

Work done by the engine

= -W_fluid + W_atm – W

= (u₁-u_o) – T_o(s₁-s_o) – P_o(v_o-v₁)

= (u₁ + P_ov₁ – T_os₁) – (u_o + P_ov_o – T_os_o)

= a₁ – a_o

The property a=u+P_ov-T_o is called the Specific Non flow exergy.

Let the fluid flow steadily with a velocity C₁ from reservoir in which the pressure and temperature remain constant at P₁ and T₁ through an apparatus to atmospheric pressure of P_o. Let the reservoir be at height Z₁ from the datum, which can be taken at exit from the apparatus i.e. Z_o = 0. For a maximum work output to be obtained from apparatus the exit velocity C₀, must be zero. It can be shown as for non flow system above that a reversible heat engine working between the limits would reject T_o(s₁-s_o) units of heat, where To is the atmospheric temperature.

Therefore we have;

Specific exergy = (h₁ + C₁²/2 + Z₁g) – h_o – T_o(s₁-s_o)

In many thermodynamic system the kinetic and potential energy terms are negligible,

i.e. Specific exergy = (h₁ – T_os₁) – (h_o – T_os_o) = b₁ – b_o

Instead of comparing a process to some imaginary ideal process, as is done in the case of isentropic efficiency for instance, it is a better measure of the usefuleness of the process to compare the useful output of the process with a loss of exergy of the system is given by the increase of exergy of surroundings i.e.

Effectiveness = Increase of exergy of the surrounding

Loss of exergy of system

For a comparison or heating process the effectiveness becomes;

Loss of exergy of surrounding

In the four stroke cycle engine, four stroke of the piston are require to complete one cycle intake, compression, power, exhaust.

Start your observation with the piston in the raise position. As the piston moves own, a mixture of gasoline and air flows into the cylinder. The first downward stroke is called "Intake stroke".

On the next second stroke the piston moves upward into the close upper part of the cylinder, to squeeze the gasoline air mixture into a tiny space. This is called the compression stroke.

The rapid bind up of burning gases in the cylinder forces the piston down, gives us a power stroke.

When the piston reaches the bottom of its stroke, the cylinder is filled with burned gases, an the piston moves upward to push them out. This is called the exhaust stroke.

The four stroke cycle has been completed an the cycle is repeated.

Only one stroke out of four is power stroke. Turning effort provided by a heavy rotating fly wheel keeps the crankshaft turning through the other three strokes.

On the intake stroke, the intake valve is open and exhaust valve is closed. The fuel air mixture flows into the cylinder.

On the compression stroke, both valves are closed. Valve spring pressure and pressure of gases within cylinder keep valves tightly closed.

On the power stroke, both valves are closed. The fuel air mixture is ignited, and piston moves downward.

The exhaust valves open and piston moves upward to remove burned gases. The intake valves is closed.

Four stroke automotive an diesel type engines generally have four or more cylinders. The cylinders are connected to the crankshaft so each delivers one power stroke during a complete cycle. For each cycle the crankshaft must make two turns. During the first revolution of the crankshaft the inlet valve is open on intake stroke. During the second revolution, the exhaust valve is open on the exhaust stroke. Both valve remain closed during the compression and power strokes.

Engine valves are closed by springs, and are opened by camlobes (ear shaped projection) on a rotating camshaft. The cams push down on the valves to open them as camshaft turns, on four cycles engines, the small gear in the crankshaft goes around twice energy time the larger gear on the camshaft turns once. Lobes on the camshafts operate both the intake and exhaust valves.

In a two stroke cycle, as the name indicates, there are two strokes to each cycle. One stroke is up, one down. Each down stroke is a power stroke.

In the two-cycle diesel, both valves are exhaust valves. Ports (holes) in the cylinder wall which are opened an closed by the movement of piston, permit air to be blown into the cylinder. When the piston is at the bottom of the stroke, the ports are open, and air is forced into the cylinder under high pressure, by a blower (air pump). At the same time exhaust gases are being blown out through the open valves at the top of the cylinder.

As the piston rises, the intake ports are covered, the exhaust valves close, and air in the cylinder is compressed.

Fuel injected when the piston is near the top of the stroke is ignite by heat developed from compression of the air, an the expending gases force the piston own to develop power.

We have made our assignment up on a two stroke motor bike engine. Basically a two stroke motor bike engine consist of five main parts:

1. Piston
2. Plug
3. Carbrator
4. Field Magnet
5. Gears box

The energy change takes place in the piston when it vibrates, fuel fall upon the plug an huge spark of plug made the piston to vibrate in such a way that the fuel energy changes into mechanical function.

This is the most useful and essential part of the motor bike engine, in contact with our control accelerator. It is the part of the engine in which the mixture of combustion is prepared. When we rise the accelerator, the amount of fuel flowing in the carbrator also increases and in the mean time the carbrator get the oxygen from the atmosphere and prepare the mixture for combustion. Carbrator also control the ratio of the fuel and air.

It is also a main part of motor bike. Basically this field magnet is used to produce a high voltage current. When we made a kick, the kick rotates the gears and the gears rotate the field magnet. Due to the rotation of field magnet the e.m.f. induced in the coil placed inside the hole of a magnetic cylinder. Thus the current is transformed into the piston where it is used for the combustion.

This is a very essential part of the motor bike engine. Basically it is the part of the engine in which the thermodynamical change takes place and the fuel changes into mechanical energy. When the carbrator provide the mixture forr combustion the mixture fall upon the plug where due to the huge spark the mixture rapidly burns due to which an explosion is produced. Due to this explosion the piston vibrates and performs the mechanical work. When the current come from the magnet the plug also concentrate the current and take it to a high voltage level which is the requirement for the fast combustion.

This is the part of the engine in which the mechanical work of the piston is being used. In this part the gears are used to rise the acceleration.

There are four types of gears are present, first two are low speed gears and rest are high speed gears. The gears are only power transmitters. When the piston vibrates the gears of the engine changes the vibratory function into the rotatory function, then it is transmitted to the cluch plates. These cluch plates are connected with gears. When we press the cluch the piston is separated with the gears box and we can change the gears with our choice.

1. Energy Change:

When the fuel drop on the piston, then due to vibration of the piston the fuel changes into mechanical work.

 

2. Motion of the Bike:

The working of the piston consist of two main parts:

1. procedure before the energy change
2. procedure after the energy change.

This process consists of two steps, first the preparation fuel an air mixture and second the outlet by the nozel up on the plug. First the fuel comes from the storage tank to the carbrator. The function of the carbrator is to provide the dust free air form the filter which is used for the preparation of the fuel mixture.

This fuel is then taken to the plug by means of a small nozel, which drop the fuel on the plug. When we kick the bike it rotates the magnet inside the enigne. Due to this rotation the coil inside the mangetic cylinder induces e.m.f. This induced current is then transformed to the plug where the plug make this current to very high voltage. When the fuel from the nozel is dropped on the plug due to the high voltage an explosion is produced. This vibration is then converted into rotation by means of the weight of the bike. Now this force is transformed to the gear box where it is used for the motion of bike.