Hello.
I have question please.

If I can put equal amount of air into an engine is it better to be hot or cold? Please remember I say equal amount, I understand that cold air is denser but thats not my question please, quantity air is equal. Do know what difference for emissions with this same question?

Thank you for time

I think flow and resonance will help define how much air eventually gets to the engine...do hot and cold air behave in different ways? I think the resonance quality of each is potentially different.

ok I make bad question, sorry I try again.

Engine has intaked same amount of air is now inside engine. One air is hot one air is cold but same quantity. What will difference be when ignition, power and what emissions difference please?

Cold air is generally prefered for several reasons.

You must also ask yourself how you're going to compensate for the lower density of hot air? Aside from that, I think you also need to define the problem more clearly. Are we talking about a gasoline engine or a diesel engine? Turbocharged or naturally aspiranted? What is the engine used for, what kind of load conditions and what is the goal of the engine?

A given mass of air at a higher temperature in the same volume - your intake and cylinders - will have to create a higher pressure, by definition. Therefore, your engine is under higher stress.

hot air is better for fuel vaporisation therefore at part throttle it could give better emissions and efficiency.

cold air is better for getting as much air in to the cylinder as possible and is therefore better for WOT.

Colder air will require less work to compress, so will have a better cycle efficiency. Cold is good, hot is bad.

Like the aircraft turbofan paradox: It produces less power at sea level, 40c, when it needs it the most. And is more powerfull at 30Kft, -40c, when it's cruising.

The OP has made a couple of attempts but still hasn't specified what "same quantity" of air means - same mass, same volume, both (and therefore different pressures)?

Unfortunately not - Whilst it's good for them to be sucking nice cold air, the ambient air pressure at cruise levels is usually only about 30% as much as at sea level and so they produce far less thrust. True enough a little is picked up from ram effect & maybe a tiny bit of pressure recovery, but I'd be surprised if it's much more than about 35% of max sea level thrust.
Also, most modern jet engines are flat-rated to 30° and so don't often suffer too much of a performance penalty on a warm-ish day.

Not much to add to the OP, other than after having a read of Smokey Yunick's autobiography he went into this a bit when talking about his adiabatic engine ..... had a lot of trouble stopping it from pinging due to the hot air it ingested.

I am guessing but I suspect the op means same mass although it would also have the same (cylinder) volume.
I used to work for a company that made air preheaters for the air going into coal fired powerstations, heating the air (from exhaust heat) improved the thermal efficiency enough to make it worthwhile fitting them. My understanding was that the air will be heated up to combustion temperature by energy from the fuel, higher inlet air temperature needs less heat from the fuel so more heat is left to do useful work. Different thermodynamic cycle but the concept is still valid.

You'd want hot air when starting the engine. Cold when it's warmed up and running.

Very happy much people interest my question, many thanks.

But I am confused say cold is good hot is good! I have building engine for test with supercharger 2 stroke and high pressure gasoline injection in head not come in with air. Best if not cool air because large intake heat exchanger no easy to work. I hope hot air is better for emissions but this I worry. I know cold air more power but no important as emission and size weight because of large heat exchanger in middle of engine and supercharger. Cold air better emission then I need do it.

Thanks you very friends.

The reason the answer "depends" is that you have not specified the reason for the question, which would provide insight into the design goals in mind for your hypothetical engine.

Cold air is good for maximum power.

Hot air is good for thermodynamic efficiency.

Living in a non-English speaking country surrounded daily by broken English makes the question a bit easier for me to understand.

The OP wants to know if he needs to fit an intercooler or not to improve emissions by virtue of the cooler air charge

or

Run the engine supercharger direct feed with the higher temp intake charge if that offers an emission advantage.

Just to illustrate what I said above: http://en.wikipedia.org/wiki/File:Adiabatic.svg , from this article: http://en.wikipedia.org/wiki/Adiabatic_process

The graphic considers an adiabatic process i.e., there is no heat exchange during compression or expansion - work needed to take a given gas from p1v1 to p2v2 will increase with increasing t1. Compression is right to left, expansion is left to righ.

So cooler air will aways be more efficient mecanically.

I don't know if hotter air will have some effect on the burn process and amount of specific emissions though.

This stuff is a wee bit over my head, but there was a great piece about the SR71 and the adjustable engine nose cones. The outcome was the faster the thing went the less fuel it used...so ironically, if the pilot wanted to save fuel he needed to go faster.

I have often wondered why an adjustable piece like this has not been incorporated into auto engine intake systems?

Saudoso:
Not sure if you're responding to my post ... but ... gordy's post made me think of recuperators: http://en.wikipedia.org/wiki/Recuperator

I know they are used in gas turbines but am not sure they are used in ICE piston engines.

The intake spike is used to "shock down" low density, high speed air in supersonic flow. They would not be useful in anything going slower than the speed of sound in the working fluid.

Further, I recently learned more about the intake system on the SR-71 (through a book I recently purchased). The intake system is FAR more complex than just the variable geometry intake spike.

KJ was a genius! An incredible airplane.

Ok, so I went a bit far with the more power at cruise altitude thing. I was just parroting the tales my professors told 2 decades ago.

http://www.gepower.com/prod_serv/products/...ds/ger3567h.pdf

See page 8, the effect of temperature: Standard 16c. 20% net power gain at -18c and 20% loss at 49c. I didn't grasp the altitude one. Where is PII when he is needed?

Dosco, yes it was kind of directed to you. I see what you mean with the recuperators, but that is good after compression. You want to compress cooler air, aways. Keep in mind that what happens in a gas turbine is equivalent to the compressions stroke on the ICE. So you would need to recover heat within the combustion chamber.

EDIT Have a look here at page 8/9 also: http://www.gepower.com/prod_serv/products/...ds/ger3695e.pdf

And intecooler in these beasts, interesting.

In my opinion the emissions will probably improve overall with hotter air. HC and CO will almost certainly reduce. NOx will probably increase slightly. The effect of adding an intercooler will depend to some extent on the type of supercharger. If it is positive displacement, the intecooler will probably reduce boost pressure along with temperature and NOx will reduce. If it is a centrifugal compressor (but not turbo), the boost will probably not drop much, the mass flow will increase, the power will increase and NOx will probably not reduce.

As mentioned above, a car doesn't quite go fast enough.
Not even this one ....

I think you are wrong with SR 71, perhaps the engine efificncy improved (thrust per flowrate of fuel) but I'd really expect the v^2 law to win.

You can check for yourself at http://www.sr-71.org/blackbird/manual/appendix/a5-4.php etc.

The nose cone /ramp is only to decelerate supersonic air to subsonic. you'd be a very silly boy if your intake on your car went suspersonic (exhaust sytems do a bit, locally, sometimes)

I think he's trotting out the overused "the intake system produced over 40% of the thrust at cruise" thing.

That concept is only valid for internal combustion engines if you can heat the air after it has been compressed as the air + fuel also is the working fluid (which is not the case with the coal fired powerstation). If you heat the air prior to compression it will have a negative effect on the efficiency and one reason for this is that the increased temperature during combustion will increase the heat losses. At high loads additional fuel might also be required to keep the temperatures under control and to prevent knockning, this also reduce the efficiency.

Pre heating of air have been used with piston engines though, often to aid vaporisation after cold starting. Hot intake air will also increase the exhaust temperature, which also means it reduces the catalyst light off time after a cold start, but that is today handled with very late ignition timing instead.

With hotter air I would also expect NOx emissions to increase. HC and CO might improve if fuel vaporisation is an issue, but otherwise I would expect them to stay about the same. CO is mainly dependant on fuel mixture and HC by other factors, such as combustion chamber design.



The SR-71 operated their engines sort of like a turbojet/ramjet hybrid. At high velocity the intake provided a large part of the compression of the intake air, and the adjustable nose cones was a part of that system. The engine also had bypass ports, not too unlike modern turbofans, this prevented choking in the latter compression stages. As the intake provided much of the compression at high speeds, this also raised the intake temperature which could exceed 400 degC.

I would also suspect that the SR-71 consumed more fuel at high speed even if the engines operated more efficiently; when the velocity of the plane is closer to the exhaust velocity of the engines.

I'm not sure if the relationship of fuel-flow to TAS was linear around the designed cruise speed of M 3.2, but I too remember reading 'Sled Driver' and the author saying that if they were a little thin on fuel they'd bump the power up to run at M 3.3 for a while. I've got the book here, I'll try to find the paragraph in question if you like.

That would be novel - an aircraft where the cruise speed was lower than the best economy speed. I can understand however that factors other than fuel economy would limit the continuous speed of the Blackbird (any one of a number of critical temperatures for a start).

That's right, just about everything they did above M3 was dominated by temperature concerns, airframe and engine both.

As Greg mentioned, they were temperature limited. The obvious limit was the mach number and so skin temperature. There was also engine inlet temperature I think, and one of the big limits was fuel temperature into the engine - as you can imagine it had a number of fuel tanks placed around the airframe and it'd automatically shift the fuel around front-to-rear to keep the centre of gravity where it was needed, to keep the elevons in the neutral position for minimum drag, etc. But they also used fuel for airframe cooling and it'd automatically suck fuel from the hottest fuel tank available, and that was pretty clever as it'd help dump airframe heat out the exhaust pipes.

Sorry me I am not understand your airplane meaning for me.

My University try builds like Sarich 2 stroke. We have 2 cylinder and supercharger we make ourselves like General Motors diesel type we copy but small. Pressure go direct in intake port when piston bottom open port. We do not go under piston before. When compression we put gasoline injector in cylinder head then burn. Now we decide and research use heat exchanger or no for emission level. Engine small I think no push airplane.

Thank you friend

Page 65 of 'Sled Driver' by Brian Shul ->
"The Blackbird loved being up high. She came into her own up there and never ceased to impress me with what she could do. Because of the design of the inlet system, the faster the jet flew the more efficient it became. Better range was attained by increasing the speed. This was the opposite of other jets I had flown, where fuel flow increased at higher speeds. Once the SR-71 was at cruise speed, I continually adjusted the throttles back to keep the speed down. The jet cruised in afterburner, but rarely was maximum power needed."

Not quite as descriptive as I'd hoped but it's pretty much as I remember it. I don't know if the increasing efficiency thing would continue up to the point where the speed would stabilise at full power though and for sure something would over-temp quite quickly like that anyway.

Seems you could fly several miles at full throttle without overheating anything:

"I was piloting the SR-71 spy plane, the world's fastest jet, accompanied by a Marine Major (Walt), the aircraft's reconnaissance systems officer (RSO). We had crossed into Libya and were approaching our final turn over the bleak desert landscape when Walt informed me that he was receiving missile launch signals. I quickly increased our speed, calculating the time it would take for the weapons-most likely SA-2 and SA-4 surface-to-air missiles capable of Mach 5 - to reach our altitude. I estimated that we could beat the rocket-powered missiles to the turn and stayed our course, betting our lives on the plane's performance. After several agonizingly long seconds, we made the turn and blasted toward the Mediterranean . 'You might want to pull it back,' Walt suggested. It was then that I noticed I still had the throttles full forward. The plane was flying a mile every 1.6 seconds, well above our Mach 3.2 limit. It was the fastest we would ever fly. I pulled the throttles to idle just south of Sicily , but we still overran the refueling tanker awaiting us over Gibraltar."

So do you have poppet exhaust valves in the cylinder head? Do you inject air with the fuel like Orbital (Sarich)?

As I mentioned earlier, this is somewhat of a misleading statement due to its simplicity. If you take a look at pages 96-101 of the book, Design and Development of the Blackbird, From Archangel to Senior Crown by Peter W. Merlin, you will see what I mean. If you are familiar with the SR-71, the only externally visible feature of the complex intake system (other than the spike) are the vents on the outside of the engine nacelles. Inside the nacelle are a variety of ducts, louvres, doors and other devices to manage the airflow.

I had no idea about the complexity of the system until I read the book, assuming it was a simple geometry that I learned about in school. In retrospect, I would now assume all supersonic aircraft have some degree of additional complexity that is not apparent by simple visual inspection ... (understatement, I know).

I haven't read the book but I would guess the complication is needed to cope with the wide range of intake conditions (from zero speed thru subsonic to Mach 3.2).

Additionally (if I read an earlier post correctly) it seems the engine had the ability to bypass not only the turbine (as is commonly done to feed compressed air to the afterburner section) but also the compressor, when ram-air compression alone produced sufficient pressure to feed the after burner - thus creating a ram jet in parallel with the turbo-jet. This would only function at higher speeds, so would require some complex ducting and controls.

Here's the data from the flight manual http://www.sr-71.org/blackbird/manual/appendix

M Max Range from 50000 lb to 10000 lb page ref
3.2 2260 a6-4
3.15 2280 a6-15
3.1 2240 a6-27
3.0 2160 a6-39
2.8 2000 a6-51
2.4


Bugger, I was misreading the graph before. So yes, up to 3.15 the range increases with speed. Wow.

This must be the worst hijacked thread I have seen in the tech forum, you Guys should get a hotel room.

Re the SR71 fuel mileage...I caught this on the TV. I cannot debate this for sure...but their words went something like, "...if the pilot needed to conserve fuel he had to go faster..." I have no idea how this works but ram air was part of the reason this worked...they explained this procees in a TV format

Re a supersonic intake...what kind of vacuum would an engine have to generate to create a velocity over 650mph in the intake? An amusing question at best. I imagine that a rotory would have better theoretical chance...

Correct - It's relatively (!!) easy to design a supersonic inlet that works at a specific speed. FAR more difficult if the engine has to go from a standing start and that's why a lot of the early-ish fast jets had various ramps & diverters to help control the airflow into the engine. I'm not sure how it's done these days but they typically fly a little slower so it's less of a problem and I also suspect they have good pre-planned control over the shock waves that travel down the inlet to slow the incoming air to subsonic.
Anyway, here's a simplified diagram of the J58 propulsion system of the SR-71.

That's one of the pages I was referring to in the book I mentioned. Look closely at the centerbody bleed, shock traps, etc. Much more complicated than just a simple shock cone.

I don't think of it as the engine generating a vacuum as much as I think of the ejector creating thrust that pushes the plane which then experiences ram air.

The intake system must be matched to the compressor demand.

I was actually referring to an automotive engine...do F1 engines for example move air at the speed of sound?

A vacuum of approx 0.5 atm will produce Mach 1 at the smallest restriction. Increasing the vacuum beyond this level will not increase the flow ie Mach 1 cannot be exceeded by increasing vacuum. This is referred to as choked flow and is useful for regulating engine output in "restrictor" classes.

In the flight manual there is a graph of ambient temperature vs. mach number as limited by the 427 degC maximum compressor inlet temperature. Mach 3.3 / 1925 knots air speed is given as the maximum usable speed.

http://www.sr-71.org/blackbird/manual/5/5-10.php

According to the flight manual mach 3.17 is the recommended cruise speed, and when authorized speeds up to mach 3.3 can be used under condition that the maximum inlet temperature isn't exceeded.

http://www.sr-71.org/blackbird/manual/5/5-8.php

So it is a uniflow two stroke?

Aftercooling will reduce the power consumed by the compressor, so that alone is a reason for cooling the incoming charge.

Do you mean CJ? I presume you refer to Clarence L "Kelly" Johnson?
http://en.wikipedia.org/wiki/Clarence_Johnson

Will it?

My understanding is that hot air intakes do not make practical improvements in efficiency because they limit the compression that a piston engine can use, and in order to use higher compression ratios requires more fuel to be used to cool the charge.

This is not a problem in gas turbine engines.

Yes - it allows a lower boost pressure (so less compressor work) for the same mass flow (power).


It is a problem in gas turbines because they are limited by turbine inlet temperature and higher air inlet temperature means higher temps throughout the cycle.

On the other hand, if the turbine outlet temperature is significantly higher than the compressor outlet temperature, some of this waste heat can be used to heat the air for free, prior to adding expensive heat (fuel). This process is known as regeneration. (not applicable in jet engines where the hot exhaust is doing useful work anyway)

It also reduce piston compressing work



Reused heat is passed after compression. And check a few posts back GE's intercooler system, controlling temperature during the compression phase.

I was but I didn't know his name was Clarence...I wonder if there is a hidden joke in the 'Airplane' movies?