Cowl induction for jet aircraft engines , is it possible?

Why can't jet aircraft engines have a reverse cowl induction as a few cars have used on their air intake systems? Putting a cover on the front of the engine and letting the air come in from the rear of it seems to work fine with autos, so why not use the same design on jet engines?:shrug:

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Notice the air comes in from the rear and the hood can be extended to fit over the entire jet engines frontal area.

 

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I amsure people have thought of it.

 

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thermodynamics fail.

 

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How do therodynamics fail? It works perfectly on a cars intake, so why not expand the idea for a jet engine?:shrug:

 

please explain in detail, with more than just a picture...but how exactly this thing works with a car.

I know nothing about cars.

 

Probably possible, but not very practical. Think of the air plane as still and the air streaming by it. There is always at least one point on the front where one stream line of the air goes to the left and a very nearby air stream goes to the right to flow around that part of the body of the airplane. In case of a jet, there is a set of stream lines that does neither, but is "swallowed" by each jet engine. This air, as it enters the engine, is slowed and compressed by the kinetic energy. If a flat plate were covering the engine entrance the pressure would be the "stagnation pressure" which depends upon the plane speed and density of the air mainly. The "stagnation pressure" is not much above the local atmospheric pressure until the speed of sound is approached, but the cowling you suggest would drop the intake pressure well below the local atmospheric pressure. Think of the individual air molecules and the forces (pressure gradients) need to accelerate them. I.e.

If the air stream flowed over some cowling and then it must rapidly turn around and flow back into the cowling not only would all of the natural described above compression be lost but the pressure at the reversed intake of the cowing would be much less than atmospheric at that altitude. (To supply the accelerating pressure gradient that turned the air flow 180 degrees.)

Few realize that most of the power of a jet engine generates is used by the jet engine to compress the air it is "swallowing" - I forget the typical percent but recall is nearly 90% of all the power the jet produces as it flows thru the turbine part of the shaft is "harvested" and transferred forward by that shaft to turn the blades in the compressor section of the engine. Thus with a cowling, even more compression would be required and a smaller fraction of the total power would be available to throw the burnt fuel mass out of the jet at high velocity.

 

Picture a cone of a space shuttles front end. Picture that over the front of a jet engine with the rear facing cowl induction all the way around the back of the cone. Now place rear facing louvers everywhere on the cone without sacrificing the structural integrity of the cone allowing more air to enter that just from the rear facing cowl induction. That way there's plenty of air coming in from all around the cone not just from the rear. The cone is aerodynamically made so it cuts through the air easily but deflects anything that hits it. It should be made from titanium or other very light but strong metal or perhaps carbon fiber. Can you picture that?

 

What advantages would such a system confer to the design of the jet? The current models seem to work just fine (save for bird strikes)

 

I think so, but regardless of the details, when you force a fast moving (wrt the airplane) mass of air (the air stream) to change flow direction by 180 degress, there is a huge pressure drop that the compressor section of the jet engine must restore. Read post 6 for more detailed discussion.

 

How about a umbrella shaped object, solid of course put onto the center of the jets turbine fan? Imagine a smaller than the engine about 3 feet away from it extended out that would deflect anything from going in but the air could still go around it and into the engine.

 

Turbine (particularly jet) engines need to be able to suck air in. That reverse cowl hood, at airline speeds, offers absolutely no way for the engine to take air in. In fact, the way the cowl's opening is in the rear there, would almost deprive the engine of it's air intake. The manner in which the air flows into the engine is extremely important. If the airflow gets messed up too much, you can stall out the engine. (think of Top Gun when Mav and Goose got into that flat spin. Their engine stalled because the airflow got out of whack).
Also, at airline speeds, the faster the jet goes, the more air gets 'forced' into the engine giving it more thrust (think of a ramjet engine).

 

I'm a senior in aerospace engineering, maybe I can offer my two bits...

mikenostic's got it pretty much spot on: redirecting airflow to a new direction takes energy, and trying to redirect the air 180 degrees, then 180 degrees again (to inevitably exit with the flow going out the rear of the plane) would devestate an engine's performance.

The reason they do this for cars is the same reason many cars today have some sort of goofy duct thing: style. I think the superchargers that poked out of the hoods of classic muscle cars inspired today's fad of having a hugenormous scoup on the hood. If you want to know if the scoop is doing anything particularly useful, look into the thing. If you see another radiator or a dedicated air intake positioned right behind the duct, some engineering occured in regards to the duct, and it might be providing extra cooling and/or air. Some aircraft will have extra ducts on the sides of an engine where an oil cooler or something is positioned (however, even aircraft get some ducts that are intended for style only, marketing, it sucks).

However if all you see is the naked top of an engine, it's just a stylish gimick. If you see a fake grate behind the duct and it's not a real duct at all, it's a crime and shame (I'm looking at you, recent-model Ford Mustangs...). Some will say it's helping to cool the engine, but it won't do that too well without a radiator. Some will say it provides more air to the engine, which is true, but whatever horsepower you gain, there's going to be some loss to the aerodynamic drag the duct places on the car.

Often, I think it's best to have a better-designed front grill then another duct on the car. Look up a pic of a NASCAR, to my knowledge, they don't use ducted hoods, just a single front grill. Formula One cars have giant ducts but only because the engine is in the rear, and needs air brought to it, so to speak.

Keep in mind, my limited experience is with aircraft mostly, if a gearhead here knows better, call me out please.

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...come in various forms. They all need compressed air, which is why an air intake at the front makes sense. Most use a turbine to assist the compression, however at speeds over about mach 2.2 you can built a jet with no fan ('ram jet').

Cars can afford stylish designs because they do not need as much oxygen as a jet engine of the same size and they don't need the same level of compression.

Blue, MEng

 

Didn't I just say that a few posts above?

You forgot to take into account the forced induction recip engines (supercharger, turbosupercharger). A turbo recip engine works on the same princible as a turbine engine; as you mention above, like on a turbine engine, it assists with the compression.
Let me clarify, a recip engine does not need the same TYPE of compression.
One exception is the kind of engine you find in an F1 race car. That big ass intake cowl right over and behind the driver's head serves a functional purpose. Those engines spin at over 18000 rpm and have at least a 12:1 compression ratio. Engines like those suck in air comparable to a turbine engine. At speeds, the air is being forced into the intake plenum, giving the car a noticeable power increase. If you disrupted that airflow, or if by chance that cowl was facing to the rear, that engine would not perform to the specs that it should.
Howver, most cars do not travel at the same speed as a turbine powered aircraft, so the air flow at those kinds of speeds physically behaves a bit different. Take a look at this bike, it's powered by a turbine engine. Jay Leno own's one.

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You'll notice that the only place for the intake is right behind the front wheel like on a regular bike. From an aircraft standpoint, that would never fly (no pun intended), as the wheel/fork would sorely disrupt the airflow at flight speeds. Since the motorcycle wont' reach those kinds of speeds, airflow at lower speeds isn't as big of a factor at high speeds.
If there are any physicists on here, correct me if I'm wrong.

Also OP, that reverse cowl in the pic you posted is not designed specifically to let air into the engine, it's to allow the heat buildup in the engine bay to escape; the air comes in (as it does on most cars w/o a hood scoop) through the grille.
Now, there are some vehicles that route their air intake from the intake plenum/carburetor to the grill area; that allows air to come in faster and colder and can nominally increase engine performance and fuel mileage.

 

I thought the reverse cowling was intended to assist laminar flow. Air coming directly into a forward intake on a car would tend to be turbulent, while on an aircraft it tends to be clear.

 

Not sure that I did, actually. Happy to be corrected though.

 

The opening post is a two part comment/question. This post only addresses one aspect.

It has been entertaining to read what so many sincere thinkers have speculated on the subject of car scoops facing rearward.

In the late 1960s Pontiac engineers executed wind tunnel tests. These tests showed that there was an air pressure build-up in the windshield region of a moving car. It was speculated that this pressure rise could be exploited to provide a small but realizable increase in air the pressure, therefore, the air flow, into the carburetor of a moving car. The 1970 1/2 ( there is controversy about whether the model year should properly be called the 1970 1/2 or the 1971 ) Pontiac Firebird Trans Am was introduced with a rearward facing hood scoop ( the "shaker hood" ) which had a sealed duct connection to the carb. This was reported to give a 1 or 2 percent horsepower advantage at 100 MPH. Since the standard Trans Am motor was rated at roughly 500 horsepower, this means that about 5 or 10 horsepower was due to the shaker hood scoop. In professional car racing, even 1 or 2 horsepower gain is golden. And, of course, at the stock car's top speed of over 150 MPH, the horsepower gain was more in the realm of 20 horsepower or so.

I was fortunate enough to have owned and driven a 1973 Trans Am and I testify that the factory stock car would GLH ( go like hell ) from 110 MPH on up.

Now I will dip my toe into the second half of the opening post and remark that there have been many jet engines which have a surprising peculiarity in their design. Armchair physicists who have never seriously studied the subject, but are happy to speculate in ignorance, may remark that it is taboo to make the airflow reverse 180 degrees. The surprise is there have been many jet engines in widespread manufacture and use which have reversed the airflow 180 degrees from the compressor turbines to the combustion chambers. Of course, the combustion products were then again turned round to eventually flow through the exhaust turbine blades and then out the tailpipe.

Don't take my word for it. Actually do some research.

 

Uno Hoo

Thank you for confirming my thoughts. :thumbsup:

 

All will agree with Uno Hoo's observation that when air flow streams into some object, like a vertical windshield on a car, there is a pressure increase in the air as it tends to stagnate against that object. Yes, it can be advantages to collect some of that higher pressure air for combustion, but if the windshield were not needed for other reasons, no one would put up some obstacle in the air stream and hope to make the car more efficient by INCREASING the drag coefficient.

In fact, the modern window shields have all become quite sloping to lower the drag coefficient, compared to the Henry Ford's model T's vertical wind shield. A vertical design was cheaper as used the least glass and the Model T did not need to be concerned with drag coefficients.

Summary: No not otherwise not needed obstacle or impediment to smooth, laminar air flow is desirable. Making one that turns the air flow 180 degrees is the worst possible way to INCREASE the car's DRAG COEFICIENT.

 

First of all, I'd appreciate not being called an "armchair physicist". I do actually build UAVs for my school, and have been involved in a project that utilized a turbojet system to get a plane up to 300 mph. I'm also specializing my degree toward aerodynamics, and know a thing or two about engine design. I can appreciate that I don't know the nuances of the aerodynamics of a Trans Am, and am willing to be corrected on that basis, but I won't take name-calling.

You are correct, though, there are a number of turbine engine systems that actually duct the air completely 180 degrees and then some, particularly earlier designs that utilized centrifugal compressors and turbines. These systems are also sometimes in use today since centrifugal-flow systems are often cheaper then axial-flow systems. This cheaper form of turbojet is what we used for our plane, and by cheap I mean an eight-inch-long turbojet costs a mere $4,000 or so. However, serious turbine engines and powerplants that are designed to be efficient performers utilize axial flow to maintain as high an engine efficiency as possible, and redirect the airflow as little as possible.

In summary, the 180 degree flow redirection is only done to cut costs, not to add performance gain, which is why my gut told me it didn't make much sense in this application. I did, however, forget about that turbulent flow region that occurs where the windshield meets the hood.