The Combustion Section

The combustion chamber of a jet engine is where the well-known, powerful “bang!” is produced. Fuel mixes with the air that comes from the compressor and ignites, which is what produces this loud sound.. Large quantities of fuel are burned within the combustion chamber, which is supplied by fuel spray nozzles. Heat is released to cause an expansion of the air within, as well as accelerating it for a stream of heated gas. Maximum heat release and minimum pressure loss is required.

The combustion chamber experiences a very high rise in temperature; just how much of an increase in temperature is determined by the amount of fuel within the chamber. In our J85 engines, the exit temperature of the combustion chamber can reach temperatures up to 1,000 °C! A maximum temperature allowance is determined by the material used for the chamber, turbine blades, and nozzles. The combustion chamber must be able to maintain a stable and efficient combustion over a varying range of engine operating conditions since the temperature of the mixed gas determines the engine’s thrust. Our jet racing applications make combustion temperatures even hotter. To manage that, we go to Florida Tech’s Center for Advanced Coatings where we can take advantage of ceramic coatings that provide resistance to extreme heat.

J85 general electric engines contain an annular combustion chamber, which means that its combustion takes place in a full 360 degrees around the engine. This makes for a more uniform combustion. Annular combustion chambers typically have a more consistent exit temperature as well. As mentioned before, a minimum pressure drop is desired within the combustion chamber, and annular combustors have the lowest pressure drop of all the types of combustion chambers, making them very efficient. These aspects make it one of the most commonly used types of combustion chamber.

The exit of the combustion chamber is the hottest region of the engine, therefore a material that is able to withstand these high temperatures needs to be used. Inconel is typically the material of choice because of its thermal properties. It is high strength and corrosion resistant as well. Also, it is quite easily fabricated, making it convenient for manipulation into complex shapes.

Next stop on our journey through a jet engine will be the turbine section. If you haven’t already, watch my video about the combustion section, and stay tuned for the next vlog and blog about the turbine section!

Anatomy of a Jet Engine: The Compressor Section

Jet engines take in a great amount of surrounding air through their inlet. Behind the inlet is the compressor section. The compressor section, as you may have already guessed, compresses this incoming air. The compressor’s job is to increase the pressure of the incoming air before it reaches the combustion section.

There are two main types of compressors: axial and centrifugal. Many modern turbojet and turbofan engines have axial compressors because of their performance efficiency. Centrifugal compressors can increase the total pressure of the air by a factor of 4, and axial-flow compressors can only increase the pressure by a factor of 1.2. So, how do axial compressors become more efficient? They’re made to be multi staged, therefore, the pressure increase is multiplied row by row. Axial compressors have an advantage over centrifugal compressors because of their ability to have multiple stages. These advantages are ideal for an application where the thrust of the engine itself is the driving force of the aircraft, or in our case the driving force of the vehicle.

A J85 engine is made up of 8 stages, and uses an axial-flow compressor. If you multiply the 8 stages by the factor of 1.2, that makes for an overall pressure increase factor of 4.3. In an axial-flow compressor, the flow enters in an axial direction, which means it is parallel with the axis of rotation. This compressor first compresses the incoming fluid by accelerating it and then diffusing it, which creates that increase in pressure.

A fluid is anything that flows, so do not be confused with the term “fluid” in referral to air. Although liquids are most commonly known as fluids, anything that is loosely held together by gas particles is in fact a fluid. Since air is a gas, it does flow and it will take the shape of its container.

Figure: General Electric J85 Cutaway

“Stages” within the compressor refers to the rotors and stators. Rotors are the blades that rotate and accelerate fluid, and stators are stationary blades that do the diffusing. In axial flow compressors, the air flows from stage to stage. The pressure increases in the direction of flow, and the stages allow for incremental increase in pressure to eliminate the risk of the engine stalling. Incremental pressure increases also allow for higher engine efficiency. Throughout the compressor, the flow area decreases. The blades get smaller and smaller, and this compensates for the increase in fluid density, creating a constant axial velocity.

Through the front of the engine, at the inlet, air enters the compressor at about 14.69 psi, which is the standard atmospheric pressure. Standard atmospheric (air) pressure at sea level is 14.7 psi, give or take. Air pressure is simply the force exerted against a surface by the weight of the air. Once the air reaches the back of the compressor, its pressure can reach around 70 psi. This is a very high pressure for air that is coming through a wide open inlet and exiting through another wide open area. Along with a pressure increase of the air within the compressor, there is a velocity increase too. Air entering the inlet is flowing at speeds higher than 200 mph, and once it reaches the back of the compressor it can reach speeds close to 700 mph!

The back of the compressor leads to the combustion section, which will be covered in the next segment of the jet engine breakdown. Check back next time to see how the combustion section of the engine works!

Planes to Race Cars

Have you ever wondered how we take jet engines out of planes and put them into our jet dragsters? To take these engines from an airplane and make them fit into a racing application, we “put the engines on a diet.” A handful of changes are made to the engine to lighten up the weight and make it more efficient for drag racing.

J-85 engines weigh just a little under 400 pounds when they are taken straight off of the plane. We place our own custom afterburner on the jet engine. Starting from the exhaust flange back, we remove the original exhaust in order to apply our custom racing afterburner. The new afterburner adds about 60% more power to the engine itself.

Some other parts that we remove from the engines when they come off of the plane are the engine anti-ice system, bleed air plumbing components, factory engine mounts, the exhaust gas temperature probes and harness, and any rear components that must be removed to place our custom afterburner on.

With the plane to race car transformation also comes maintenance differences. The biggest difference between the two applications is how the engine is cooled after being shut off. Airplanes equipped with jet engines can fly for almost 1,000 hours before having to stop for major maintenance routines. This involves an inspection of the hot section of the engine. This time frame is not the same for our racing application. We inspect the hot section of our engines for every 90 seconds of full power. A race run takes about 6 seconds, so it takes about 15 runs down the track until the dragster reaches 90 seconds of full power. For a hot section inspection, the combustion section goes through a disassembly in order to look for cracks inside from shock cooling.

So let’s talk about engine reliability, because you obviously cannot make these changes to a jet engine without making sure that it is still going to be reliable. To answer this question in short, yes, the reliability of the engine is affected. Take a plane with a jet engine, for example. When a plane lands its engine is still running, but it is running at a lower power setting while taxiing into the gate, all while cooling the engine. Once our jet dragsters reach the end of the drag strip, the engine is fully shut off. Cold air goes down the inlet which shock cools certain parts of the engine. These parts require additional maintenance attention.

Taking engines from planes and putting them into jet cars is not necessarily hard, but it is important to keep safety, efficiency, and reliability in mind. Here at Larsen Motorsports, we do a very good job at keeping our engines in top shape so we can continue racing as often and as successfully as we do!