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The jet engine was one of the most important inventions of the 20th century. It brought a complete change in aviation and flight.
Jet engines can be classified into 5 types, namely : the turbofan, turboprop and the turbojet.
This paper compares the efficiency of turboprop engines and turbojet engines.
Before going into the details, we should discuss the basic difference between the two.
A turboprop (short for Turbo Propeller) engine uses a gas turbine core to run a propeller. The main source of thrust from the engine is the large mass of air that propeller pushes. This accounts for about 80% of the total thrust. The rest of the thrust comes from the jet of exhaust gases exiting from the back. Since, the jet is used mainly used to run the turbine, it can only provide little amount of additional thrust.
A turbojet, on the other hand, derives all of its thrust from expansion of gases in the engine and releasing them from a nozzle. The engine does not have any turbines. It may have a separate compressor to force compressed air into the engine for combustion.
Because of the presence of propellers, the turboprop engines, inherently produces lesser thrust. Conversely, the turbojet, due to reliance purely on expansion of gases produces far more thrust than any turboprop.
Coming to the topic of fuel consumption, turboprops are more efficient that turbojets. Lesser amounts of fuel is needed to turn a propeller than to generate thrust purely based on expansion of gases.
Also, turboprops generate lesser thrust. Aircrafts using turboprops are not as fast as their counterparts who use turbojets. Addison Schonland (2011), in his article while comparing the two engines says that fuel economy is the killer blow which turboprops can deliver to the turbojets. Over short distances (of around 300 to 350 nautical miles) the turboprop has a significant advantage over the turbojet.
Andrew Compart (2013) thinks that though turboprops are old, they are very valid for operations today. Their economy and efficiency is what keeps them in demand. For short distance flights, they are unbeatable.
But, here lies the dilemma. Over large distances, turbojets may turn out to be more economical. They may consume more fuel, but due to their phenomenal speed, they can cover large distances in small amounts of time. Turboprops on the other hand, may lose their upper hand due to longer and slower flight times. They may end up costlier than turbojets.
Corporate players compare pricing at every step. Most of the time, airliners use turbofan engines. Though these are costly to run that turboprops, they also produce more power and hence, more people and goods can be transported in a single trip. This turns the scales against turboprops.
Another area where turboprop aircrafts have advantage is the fact that they can take off and land in shorter distances when compared to turbofans. This aids in reducing the flight time.
An article on turboprops (n.d.) says that, turboprops are efficient because they accelerate only a small mass of air to a slightly higher velocity while turbojets accelerate the same mass to a much higher velocity. Turboprop aircrafts can also take off faster due to the variable pitch of their propeller blades. Due to this ability, they can also reverse the direction of air flow which helps in reducing landing distances.
The same article while explaining the disadvantage of turboprops states that the design of the propeller is responsible for the slow speed of turboprops. Up to speeds of mach 0.6, propellers are aerodynamic. But above this speed, the drag on the propeller blades increases tremendously. It slows the aircraft. Hence, turbo props are efficient to be flown only at heights of 25ooo feet and at speeds of 415 to 450 miles / hour. Since, turbofans and turbojets do not have propellers, they can sustain flight at high speeds on 0.8 mach for the turbofan to more than 2 mach for the turbojet.
The above graph shows the consumption of fuel of various engines based on their altitudes. It is clearly visible that the turboprops hold the advantage.
In today’s world, turboprops are still being used to ferry passengers and cargo over short distances. Turbojets are mainly used on military aircrafts where speed is of paramount importance.
The bypass ratio of jets is the ratio of amount of air which bypasses the combustion chamber to the amount of air that enters the combustion chamber.
The next graph compares consumption based on bypass ratio of engines. Again the turboprops prove to be more efficient than turbojets.
The following experiment by NASA (n.d.) compares the Thrust Specific Fuel Consumption of turbojets and turbofans. It can be modified to compare TSFC of turbojets and turboprops.
TSFC is an efficiency factor use to determine the efficiency of jet engines.
Mathematically, it is the ratio of the engine fuel mass flow rate (mf) to the amount of thrust produced by burning the fuel.
ASSUMPTIONS :
1. Consider 2 engines, a turbojet (A) and a turboprop (B).
2. Let the rate of fuel consumption (mf) by both engines be same.
FORMULAE :
1. Fuel Mass Flow Rate : (mf) (kg / hr) or (lbm / hr).
2. Net Thrust : F. (N) or (lbs).
3. TSFC = mf / F. [kg / (N * hr)] or (lbm / hr*lbs).
CALCULATIONS :
The TSFC can be calculated by plugging the values of thrust and fuel flow rate.
CONCLUSION :
A low TSFC means higher efficiency and a high TSFC means poor or low efficiency
Comparing the values of turbojets and turboprops, we can say in general, that turboprops are more efficient. TSFC of turbojets lies in the range of 1.5 to 2.3 whereas the TSFC of turboprops lies in the range of 0.3 to 0.6.
In conclusion, we can say that both the engines are important in their respective fields and comparing both directly is not fair. Though the turboprop is slow and produces very less power, it is very useful in short trips and since the world is facing fuel crises, turboprops are here to stay.
Similarly, though turbojets are fast, they are very noisy and consume lots of fuel. Running turbojets economically needs speeds of over 1.2 mach which are usually avoided over land and cities. The military and research applications of turbojets though are endless as all future aircrafts may have efficient and quieter turbojets.
REEFERENCES / WORKS CITED :
1. Schonland, Addison. (2011) 19th August. Jet vs. Turboprop – a debate that dates from the early 1950s. [Online] Available at <http://airinsight.com/2011/08/19/jet-vs-turboprop-a-debate-that-dates-from-the-early-1950s/#.VX0vEvmqqko> [14th June 2015]
2. Compart, Andrew. (2013). 1st April. Turboprop Efficiency Has User’s Holding On. [Online]. Available at <http://aviationweek.com/awin/turboprop-efficiency-has-users-holding> [14th June 2014].
3. Turboprops. [Online]. Available at <http://what-when-how.com/flight/turboprops/> [14th June 2015].
4. Thrust Specific Fuel Consumption. [Online]. Available at <https://www.grc.nasa.gov/www/k-12/airplane/sfc.html> [16th June 2015].
5. Difference Between Turbojet And Turboprop. (2012) [Online]. Available at <http://www.differencebetween.com/difference-between-turbojet-and-vs-turboprop/> [!6th June 2015].
6. Stoll, Alex. What Benefits do Turbofans offer over Turboprops. (2011) [Online] Available at <http://www.quora.com/Jet-Engines/What-benefits-do-turbofans-offer-over-turboprops>. [16th June 2015].
7. Are Jets Safer than Turboprops..? (5th Feb 2014). [Online]. Available at <http://www.schubachaviation.com/blog/3227/are-jets-safer-than-turboprops/> [16th June 2015].]
8. Turbojets. (5th May 2015). [Online]. Available at <https://www.grc.nasa.gov/www/k-12/airplane/aturbj.html> [16th June 2015].
9. Brain, Marshall. How Gas Turbine Engines Work. [Online]. Available at <http://science.howstuffworks.com/transport/flight/modern/turbine2.htm> [16th June 2015].
10. Thrust Specific Fuel Consumption. (2015) [Online]. Available at <https://en.wikipedia.org/wiki/Thrust_specific_fuel_consumption> [16th June 2015].
