Ram jet

We have studied almost every engine used in automobile. Now we are going ahead towards some jet engines among which today we will discuss Ram jet.

So the first question comes in our mind is why to use different engines for aircrafts while we have several i.c. engines available in automobile? The answer is very simple as the altitude increases the efficiency of automobile i.c. engine decreases, also with increase in altitude the density of air decreases which is not preferred for normal automobile engine as it results in incomplete combustion of the fuel in cylinder.

Ram jet (also known as athodyd) is considered as a very simple air breathing engine hence we are starting our study from ram jet.

Ram jet was patented by French inventor Rene Lorin in 1913. As it doesn’t contain any moving parts like turbine, compressor so it is a key reason behind calling it as simplest jet engine.

Construction:

Ram jet consists of a subsonic (Mach number(M) < 1) diffuser, supersonic (Mach number(M) > 1) diffuser, a combustion chamber and a discharge nozzle. Before going further one should have clear idea on mach number. This arrangement is shown in figure of ram jet. 

Working 

Air enters in a diffuser at very high speed and gets compressed with the help of supersonic diffuser placed in front of ramjet at the same time velocity of air will reduced to subsonic speed. As air proceeds further it enters in to subsonic diffuser where it is further compressed and air pressure is increased. This pressure is also known as ram pressure. Now this air enters in combustion chamber where combustion takes place the oxygen required for combustion is taken from the compressed air. Now this air temperature will increases and sent to the exhaust nozzle where speed of gases will increased greater than it was at inlet. This results in change in momentum which is thrust required in vehicle direction in order to get motion in direction of flight. Ram jet works on brayton cycle. At high speed specific fuel consumption will reduce as at high speed compression is higher however with high altitude the efficiency of ram jet will reduces due to unavailability of oxygen in proper quantity.

Advantages

-Simple in construction

-Cheaper and requires less maintenance

 -Greater thrust per unit weight of engine compared to other jet engine

-Higher maximum cycle temperature is possible

-At high speed and high altitude specific fuel consumption is less.

Disadvantages

-This engine cannot start on his own. It has to be accelerated at some high flight velocity.(hence they are mounted on other vehicle)

-At low speed fuel consumption is higher.

Application

Ram jets are widely used for guided missiles due its high thrust, it is also used in some aircraft.

Indian cruise missile BrahMos is based on ramjet. Bomarc, Akash missile, Bristol Bloodhound are the other name of missiles which are based on ram jets.

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