Exhaust flow and temperature?

[xpensive]

kilcoo;
I fail to see how you can neglect the addition of fuel, when you already have calculated the air-intake to be 0.180 m^3/s, which is 180g/s at 20C, and fuel addition can be estimated to at least 50g/s at 18000 rpm.

That is more than 20% of the massflow, how could that possibly be irrellevant?

But nevermind, if assumed an exit-temp of 1000C, does that mean a volumetric exit-flow of 50 times the volumetric intake-flow at 20C?

[kilcoo316]

kilcoo;
I fail to see how you can neglect the addition of fuel, when you already have calculated the air-intake to be 0.180 m^3/s, which is 180g/s at 20C, and fuel addition can be estimated to at least 50g/s at 18000 rpm.

0.220 kg/s of air = 0.180 m^3/s of air @ density of 1.2256 kg/m^3

0.05 kg/s of fuel = 0.0000625 m^3/s of fuel @ density of 800 kg/m^3

See now why I'm leaving it out?

That is more than 20% of the massflow, how could that possibly be irrellevant?

Because its density is over 500 times greater.

But nevermind, if assumed an exit-temp of 1000C, does that mean a volumetric exit-flow of 50 times the volumetric intake-flow at 20C?

*Assuming* a subsonic exit - that gives an air density of

P/RT = rho

101325/(287*1273)= 0.277 kg/m^3

So using the massflow rate from earlier, 0.220 kg/s - which gives a volume flow rate of ~ 0.97 m^3/second...

That is ~ 0.48m^3/second per exhaust

Now... how big is an exhaust exit area? Say its 5cm diameter circle, thats then 0.0020m^2

0.48/0.0020 = 244 m/s

Or 880 kph.

[xpensive]

Your postings confuses me somewhat kilcoo.

Mixing of Apples and Pears are always annoying, what counts is of course the intake mass-flow and with your airflow-numbers, fuel represents 22% and should not be ignored. All mass is transformed to gas-form in the combustion process anyway.

It also seems that you are not totally comfortable with how a four-stroke engine operates. Your first number, 0.72 m^3/s should not be divided by four as in your second attempt, but with two, which gives a volumetric inlet flow of 0.36 m^3/s.

Other than that, air-density at 0C with five digits looks almost like copy'n paste to me, 1 kg/m^3 at 20C is a good enough approximation at this stage.

All in all 360 g/s of air, mixed with 50g/s of fuel results in a 12% mix, which sounds reasonable for a combustion engine, don't you agree?

[kilcoo316]

Your postings confuses me somewhat kilcoo.

Mixing of Apples and Pears are always annoying, what counts is of course the intake mass-flow and with your airflow-numbers, fuel represents 22% and should not be ignored. All mass is transformed to gas-form in the combustion process anyway.

What counts is the volume flow - volume flow/area defines speed.

As I have said a number of times now - the density of fuel is so high it is negligble. When it combusts with air, it adds negligble volume to the flow.

It is not considered in calculations for sizing the throat or exit area of a turbofan engine core - I don't think it should be worried about here.

It also seems that you are not totally comfortable with how a four-stroke engine operates. Your first number, 0.72 m^3/s should not be divided by four as in your second attempt, but with two, which gives a volumetric inlet flow of 0.36 m^3/s.

Correct - 4 strokes is 2 revolutions, therefore the earlier division by 4 is non-sensical.

Other than that, air-density at 0C with five digits looks almost like copy'n paste to me, 1 kg/m^3 at 20C is a good enough approximation at this stage.

You want to include the effects of fuel, but are happy to throw away 20% of the air mass?

1.2256 is a bit much ok, 1.225 would do. Those numbers are not cut and paste, they are second nature, like 101325, 287 and 288. A bit nerdy, but hey, not as bad as knowing IP addresses!

All in all 360 g/s of air, mixed with 50g/s of fuel results in a 12% mix, which sounds reasonable for a combustion engine, don't you agree?

Not really, sounds very very very fuel rich to me. However, this is an F1 engine, not your average road-car.


Then again, I'm used to turbofan combustor mixes, not reciprocating. A 12% mix will not get a turbo(jet)fan very far!

[xpensive]

What is your engineering-degree again, kilcoo?

[kilcoo316]

What is your engineering-degree again, kilcoo?

Aeronautics - not mechanical.


Why?

'Cos I don't give a --- about engines and counted the revs to exposed cylinder volumes wrong you think I'm not actually an engineer?

[xpensive]

No, because you use five digits for perhaps the most uncertain of all parameters involved.

[kilcoo316]

No, because you use five digits for perhaps the most uncertain of all parameters involved.

Air density into the engine?

At roughly sea level?

At below Mach 0.3?



Maybe you could argue it will be hotter than 15 deg C - but apart from that - the density of air is pretty set in stone to be honest. See them ISA tables .

[Scotracer]

Scotracer, is there a point I'm missing as I can't immediately think why the exhaust velocity can't exceed mach 1. Granted the shockwave might not help, but there are already a number of shockwaves developed.
Anyhow the volumetric rate will be proportional to engine speed.

The 2nd law of Thermo dictates that if you have pipe flow entering subsonic it cannot exceed Mach 1 as that would require a drop in entropy. And gas flow in a pipe actually increases velocity along its length, due to friction. It will reach Mach 1 then choke. The exhausts will be below the critical length.

At a Mach number of around 0.3 you can assume the density of air doesn't change due to the flow - kilcoo is right.

[xpensive]

I never thought I would see myself writing this kilcoo, so savour this one:

Bone-dry density of air, at sea-level, goes from 1.292 at 0C to 1.165 at 30C.

My approximation to 1 in our previous calculations was one or two digits short, I give you that much, now accept that the 12% mass addition from fuel plays a role?

Aeronautical engineers...

[kilcoo316]

I never thought I would see myself writing this kilcoo, so savour this one:

Bone-dry density of air, at sea-level, goes from 1.292 at 0C to 1.165 at 30C.

Duly savoured...

But when is the last time you seen an F1 race in freezing conditions?

From 15deg C (ISA sea-level) to 30 deg C is a change of under 5%.

My approximation to 1 in our previous calculations was one or two digits short, I give you that much, now accept that the 12% mass addition from fuel plays a role?

The mass addition will play a small role IMO.

As I said - we are really after volume flow at the exit here. The fuel is that dense compared to air that it doesn't occupy significant volume. In combustion, the molecules formed are attached to the prior nitrogen (NOx) or oxygen (CO2/H2O) content of the atmospheric air. Therefore they do not add significant volume to the exhausted gas.

I also said earlier that aircraft nozzles are sized based on the air through the engine, not air+fuel, as the extra volume is insignificant. I don't see how a reciprocating engine will be that much different.

[PlatinumZealot]

xpensive,

The exhaust gas local velocity and temperature varies widely depending upon where you measure it in the exhaust tract. The max velocity and temperature will occur in the flow area between the exhaust valve and seat just as the exhaust valve opens. The velocity and temperature quickly drop as the gas flow expands into header primary tube volume, but being a compressible fluid, the pressure will also rise.

The acoustic pressure wave front produced by the valve opening event then progresses down the header pipe at the sonic velocity conditions created by the local conditions. When the wave front hits the increased cross section of the collector, a reverse wave front heads back down the header pipe in the opposite direction.

As for temperatures, the blue flame color you see would indicate near-stoichiometric combustion conditions at the exhaust outlet (likely due to over-scavenging of the intake charge and excess fuel in the exhaust flow). Stoichiometric combustion means adiabatic flame temps around 2800K at constant volume.

Interesting..

I give it 1200C max. ~ 1400K..Average of all the air, Because the gas already expanded in the pistons..so the particles that burn afterward; the luminous particles that you see orange and blue... will be colder.. my guess.

[outer_bongolia]

Scotracer, is there a point I'm missing as I can't immediately think why the exhaust velocity can't exceed mach 1. Granted the shockwave might not help, but there are already a number of shockwaves developed.
Anyhow the volumetric rate will be proportional to engine speed.

The 2nd law of Thermo dictates that if you have pipe flow entering subsonic it cannot exceed Mach 1 as that would require a drop in entropy. And gas flow in a pipe actually increases velocity along its length, due to friction. It will reach Mach 1 then choke. The exhausts will be below the critical length.

At a Mach number of around 0.3 you can assume the density of air doesn't change due to the flow - kilcoo is right.

Actually, with a de Laval nozzle (http://en.wikipedia.org/wiki/De_Laval_nozzle) in the pipe, you should be able to locally exceed the speed of sound. But that will come at a major punishment (as a pressure increase) to the feeding flow.

Especially when talking about systems that require the maximum efficiency, not the maximum exit speed, one has to avoid forming such traps in the flow. I would assume any engineer that designs an F1 engine with a supersonic exhaust flow will not be keeping his job for too long.

[riff_raff]

expensive,

A) you asked for volumetric flow, which is by definition displacement x cyclic frequency, for a positive displacement device like a recip piston engine. As an example, a 2.4L 4-cycle engine at 19,000 RPM would have a volumetric flow rate of 22,800 L/min. Volumetric flow is not the same thing as mass flow.

B) you asked for exhaust gas flow velocity. I can only say two things for certain: 1. max velocity will occur at the flow orifice choke point created between the exhaust valve and its seat. 2. exhaust flow velocity in the rest of the exhaust tract will never exceed the sonic velocity at ambient pressures and temperatures.

C) you asked for exhaust gas temps. The only data I have to base my assumptions on is the color of the flames in the picture. Based on those colors, the exhaust is close to adiabatic conditions at constant volume combustion. So the temp I gave you is probably correct.

Hope that helps.

[riff_raff]

expensive,

From your original post:

"With all the aerodynamacists frequenzing this forum, I was wondering if anyone could quantify the volumetric flow, exit-speed and temperature of the exhausts from an F1 engine on full song?

Sorry if you disagree with my answer to your question, but your question was rather ambiguous. Instantaneous exhaust gas velocities, temperatures and (thus) mass flow rates vary widely at any given point in the exhaust manifold (including the exit point), of any piston engine, throughout the course of any given engine cycle. The only constant being that compressible flows in a duct will always tend to stagnate at sonic velocities.

If you are still annoyed by my "knowledge", please feel free to ignore my response.

Regards,
Terry