Beginner's Guide to Propulsion
Beginner's Guide to Propulsion
Mar 29, 2019 - Much of this weight to be lifted is an unavoidable part of making flight possible. These extra weights represent the load on the aircraft that a. The standard weight empty will indicate the weight of the aircraft with no fuel, passengers or payload on board. Find the maximum gross weight of the aircraft. Lower and Higher Heating Values of Fuels. The higher heating value (also known gross calorific value or gross energy) of a fuel is defined as the amount of heat released by a specified quantity (initially at 25°C) once it is combusted and the products have returned to a temperature of 25°C, which takes into account the latent heat of vaporization of water in the combustion products.
Range and Fuel Consumption Activity
If so instructed by your teacher, print out a worksheet page for these problems.
Before you begin:
Airplane specifications generally include pieces of information about the range, cruise speed, and fuel capacity of a given airplane. These three items make it possible to calculate fuel consumption, range, and cruise speed.
Equations:
Equations:
- To find the time it takes to travel a given distance at a given speed, use the following equation:
- To find the airplane's fuel consumption:
Note: Total fuel is considered the total amount of fuel excluding any fuel reserves. Reserves can generally be considered 10% of the total amount of fuel capacity for the airplane. For airplanes that are able to fly by 'IFR' (Instrument Flight Rules), or with instruments, the reserves will be higher. If this is the case, the specifications will show the IFR quantity reserves. Total time is the number that you calculated in Equation 1.
Directions:
- Using these two equations and the data from the Airplane Information sheet on four types of aircraft, answer the questions on the worksheet. (Ask your teacher if you can print the worksheet.)
- Use the Conversion Web site for assistance with speed, distance, and/or weight conversions.
Answer the following questions:
- You are an acrobatic pilot getting ready to go to an air show. The air show is being held at an airport 500 nautical miles (NM) away. Given the cruising speed of the Extra 300 you are flying, how long will it take to fly to the air show?
- In the situation above, will you be able to fly to the air show without refueling along the way? Why or why not?
- A Cessna Skylane has a range of 820 NM and a cruising speed of 140 KTS. How many hours will it take you to fly 820 NM?
- Since the Skylane has a fuel capacity of 88 US GAL (including 10% in reserve -- Don't forget to subtract the 10% before figuring your consumption.), how many gallons of fuel per hour does it use?
- You are flying a Cessna Skylane at a speed of 120 KTS. You are flying on a trip that will take you 520 NM. How long will it take you to get to your destination?
- In the situation above, given that your Cessna holds 88 US GAL of fuel (with 10% held back as reserve fuel), how much fuel will you use? (Hint: you will need your final answer from Question 4 to find this number.)
- You are the pilot of a Learjet 31A leaving Chicago's Meigs airfield under dense fog conditions. You must fly using your instruments (IFR). How much fuel do you have for this flight? (Don't forget about your reserves.)
- You've been flying the Learjet for 1.5 hours, at a speed of Mach .76 (Visit a conversion page, like Online Conversion, to convert Mach numbers into Knots.), under visual flight conditions (VFR). How much fuel do you have left (excluding reserves)?
- How much farther could you fly on the fuel you have left from Question 8?
- You are a WW-I Sopwith Camel fighter pilot chasing the infamous 'Red Baron.' You are traveling at a speed of 115 KTS. You know that your enemy is 40 NM away from you. How many minutes will it take to reach him?
A general expression for the efficiency of a heat engine can be written as:
We know that all the energy that is put into the engine has to come out either as work or waste heat. So work is equal to Heat at High temperature minus Heat rejected at Low temperature. Therefore, this expression becomes:
Where, QHot = Heat input at high temperature and QCold= Heat rejected at low temperature. The symbol is often (Greek letter eta) used for efficiency this expression can be rewritten as:
The above equation is multiplied by 100 to express the efficiency as percent.
French Engineer Sadi Carnot showed that the ratio of QHighT to QLowT must be the same as the ratio of temperatures of high temperature heat and the rejected low temperature heat. So this equation, also called Carnot Efficiency, can be simplified as:
Note: Unlike the earlier equations, the positions of Tcold and Thot are reversed.
The Carnot Efficiency is the theoretical maximum efficiency one can get when the heat engine is operating between two temperatures:
- The temperature at which the high temperature reservoir operates ( THot ).
- The temperature at which the low temperature reservoir operates ( TCold ).
In the case of an automobile, the two temperatures are:
- The temperature of the combustion gases inside the engine ( THot ).
- The temperature at which the gases are exhausted from the engine ( TCold ).
Please watch the following 4:40 presentation about how automobile engines work:
Click Here for Transcript of How an automobile engine works
From this diagram we are going to learn how, basically, an automobile engine works. This is a cylinder. This is basically one cylinder that we have and these are the walls of the cylinder and this is the piston that we have. And this piston is what moves up and down and we have two inlets. One inlet is for fuel and oxygen, fuel and air to come in and mixture. And this is a valve that opens and closes. And there is another outlet here, that takes out the exhaust gases and there is another valve that opens and closes at appropriate times. And when the fuel and air mixture is inside, this spark plug provides a spark to ignite the mixture. Now let's see in sequence what happens in this cylinder. This piston basically moves up and down. So the first step is to bring the fuel and oxidizer or air inside the mixture. So this valve opens up. This valve of course is closed. And this fuel and air mixture comes in as this piston is pushed down. This acts like a syringe, a medical syringe. When we push this down, the fuel and air mixture is basically sucked in and it fills up this entire wall unit as this piston comes down. That is the first step or stroke. This is a four stroke engine that we are talking about. First stroke is intake. And the second stroke now, the second step is we close both of these valves and push this mixture; this mixture is again, by pushing the piston up, this mixture that is in here is compressed. It is compressed in very high pressures. Roughly about 8 times to 9 times and this piston moves back up here and again we have to spend energy to move this piston up or compress this mixture here. When this mixture is compressed to the predetermined pressures, at that time, the spark plug ignites. This mixture and this flame front travels all the way and burns all this fuel and this fuel reaches very high temperatures and it pushes the piston all the way down. All the way down. So this is actually the power stroke. The first stroke is intake, the second stroke is compression - compression. And the third stroke is the power stroke. That is the stroke that gives us the power from this engine. In the first stroke and the second stroke, we have to supply the energy that is required to bring in the oxidizer and fuel and also to compress. After this mixture expands, or pushes the piston down, this valve, this exhaust valve will open and of course this valve is still closed and the piston again moves back up for which we have to supply the power and all these gasses go out through this exhaust. And once the exhaust gas surrounds this cylinder, this value closes again and this valve opens again and the entire cycle again repeats. So the fourth stroke is the exhaust stroke. So this is a four, basically, a four stroke engine that we are talking about. We need to know the four strokes and the important components of this engine cylinder. And there generally will be multiple cylinders. We are talking about one here and there could be four, there could be 6, there could be 8 or even 10 or even 12 sometimes. So as the engine becomes bigger and bigger you have more cylinders and that gives us more power.
Then, why should we operate the automobiles at low efficiencies?
It is not that we cannot achieve high temperatures, but we do not have the engine materials that can withstand the high temperature. As a matter of fact, we do not let the engine gases go the maximum that they can go even now and instead try to keep the engine cool by circulating the coolant.
So we are taking the heat out of the gases (thus lowering the Thot) and making the engine operate at cooler temperatures so that the engine is protected - but lowering the efficiency of an automobile.
It's like Taxes. The more money you earn (heat), the more money is taxed (cold), leaving you with less money to take home (efficiency). However, if you could earn more money (heat) and find a way to have less taxes taken out (better engine material), you would have more money to take home (efficiency).
Below are two temperature scales. The first scale, labeled 'HOT,' shows the range of temperatures for the combustion of gases in a car engine. The second scale, labeled 'COLD,' shows the range of temperatures at which gases are exhausted from the car engine.
Instructions: Select numbers from the range on the 'HOT' scale and enter them (one at a time) into the text box labeled 'Hot' below. At the same time, select numbers from the range on the 'COLD' scale and enter them (one at a time) into the text box labeled 'COLD' below. Try various combinations of hot and cold numbers and observe the graph to see the temperatures' effect on efficiency.
Example
For a coal-fired utility boiler, the temperature of high pressure steam (Thot)would be about 540°C and Tcold, the cooling tower water temperature, would be about 20°C. Calculate the Carnot efficiency of the power plant:
Solution:
Carnot efficiency depends on high temperature and low temperatures between which the heat engine operates. We are given both temperatures. However, the temperatures need to be converted to Kelvin:
Practice
For a coal fired utility boiler, the temperature of high pressure steam would be about 540 degrees C and Tcold, the cooling tower water temperature, would be about 20 degrees C. Calculate the Carnot efficiency of the power plant.
Step 1
Convert the high and low temperatures from Celsius to Kelvin:
Convert the high and low temperatures from Celsius to Kelvin:
Step 2
Determine the efficiency using the Carnot efficiency formula: From the Carnot Efficiency formula, it can be inferred that a maximum of 64% of the fuel energy can go to generation. To make the Carnot efficiency as high as possible, either Thot should be increased or Tcold (temperature of heat rejection) should be decreased.
Practice Problem
Use the following link to generate a random practice problem.