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Old 31st December 2014, 14:27   #91
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by rev_rohit View Post

You are absolutely right in use of terminology.

But I am still unable to get the concept right. I don't think fuel intake varies/gets adjusted according to the density of air.
Correct me if I'm wrong as I'm not a very technically sound person. If it was so then there has to be a device to measure air density and a computing device to adjust fuel injection accordingly.
And secondly if fuel and air are in exact predefined ratio in all air densities, how do you explain incomplete fuel combustion and soot at higher altitudes.
MAF sensor and the O2 sensor at the exhaust together are used to make sure the A/F ratios are maintained.

Now more soot? Id attribute it to this.

High altitude>Less Oxygen>Less overall power or Less effective engine>Hence richer mixture in an attempt to balance out the requirement of power.

Yes richer mixture (in a certain window) will result in less efficient engine but will give more output. This can be the cause for the soot.

Simple example: All diesels smoke under hard acceleration. Because of sudden demand for power which results in a rich mixture.

Does it make any sense??

EDIT:
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Originally Posted by Jeroen View Post
Just about all modern car, or anything with an ECU has an air mass flow meter.

No air mass flow meter or piece of electronics can make up for poor driving technique.

Jeroen
I saw your reply after I'd written mine.

I think the habit of maintaining low rpms is a result of the overbearing need to increase fuel economy. To the uninitiated, high gear-low rpm=maximum fuel economy!

Last edited by rangakishen : 31st December 2014 at 14:35.
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Old 31st December 2014, 14:56   #92
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by rev_rohit View Post

But I am still unable to get the concept right. I don't think fuel intake varies/gets adjusted according to the density of air.
Correct me if I'm wrong as I'm not a very technically sound person. If it was so then there has to be a device to measure air density and a computing device to adjust fuel injection accordingly.
The quantity of fuel varies according to the quantity of air, (by the help oxygen sensor) in this case density of the air (mass/volume) is less so the fuel is introduced accordingly.

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Originally Posted by rev_rohit View Post
And secondly if fuel and air are in exact predefined ratio in all air densities, how do you explain incomplete fuel combustion and soot at higher altitudes.
One might think in higher altitudes there won't be excess air to facilitate soot formation, but we should remember both fuel & air supplied to the engine is less.
The charge and combustion by-products travel in a predetermined way in the combustion chamber(based on combustion chamber design), With the decrease in density, that predetermined compressible flow undergo changes. The mixing of Air & fuel won't take place effectively like before. Also we must note soot formation is less in the case of turbo charged engines.
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Old 1st January 2015, 20:41   #93
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Default Re: Engine size inversely proportional to Fuel efficiency?

The thread has gone OT - to emissions etc. There are other threads for that.
Also I think posters should indicate whether their opinions are relevant for petrols, diesels, or both. Very confusing otherwise.

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Old 2nd January 2015, 09:46   #94
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Default Re: Engine size inversely proportional to Fuel efficiency?

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The quantity of fuel varies according to the quantity of air, (by the help oxygen sensor) in this case density of the air (mass/volume) is less so the fuel is introduced accordingly.

One might think in higher altitudes there won't be excess air to facilitate soot formation, but we should remember both fuel & air supplied to the engine is less.
The charge and combustion by-products travel in a predetermined way in the combustion chamber(based on combustion chamber design), With the decrease in density, that predetermined compressible flow undergo changes. The mixing of Air & fuel won't take place effectively like before. Also we must note soot formation is less in the case of turbo charged engines.
I totally disagree, reducing fuel intake to compensate for fall in air pressure cannot be justified. Otherwise no aeroplane would ever be able to achieve high altitude.
Engines loose power at high altitude due to inefficient mixture of air and fuel. Turbocharger compensates for fall in air density /mass by increasing air intake into the engine rather than reducing fuel intake.
Sensors are there to make best air :fuel ratio.
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Old 2nd January 2015, 15:36   #95
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Default Re: Engine size inversely proportional to Fuel efficiency?

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I totally disagree, reducing fuel intake to compensate for fall in air pressure cannot be justified.
Why? Do you think engines at higher altitude is running on richer air-fuel ratios? Then what's your take on the little or no difference in efficiency of engine at higher altitudes ?

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Originally Posted by rev_rohit View Post
Otherwise no aeroplane would ever be able to achieve high altitude.
Jet engines seen on aeroplanes are designed to working in higher altitudes & they compress air in different stages. When old piston engines were replaced by Jet engines, there undergoes a significant increase in the cruising altitudes. This was not possible in the case of piston driven or propeller-shaft air-crafts.

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Originally Posted by rev_rohit View Post
Engines loose power at high altitude due to inefficient mixture of air and fuel. Turbocharger compensates for fall in air density /mass by increasing air intake into the engine rather than reducing fuel intake.
Sensors are there to make best air :fuel ratio.
You are saying turbo-charger in cars can so something Jet-engines can't ?? I find it amusing, since both of them are basically gas turbines.
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Old 2nd January 2015, 15:44   #96
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Originally Posted by rev_rohit View Post
I totally disagree, reducing fuel intake to compensate for fall in air pressure cannot be justified. Otherwise no aeroplane would ever be able to achieve high altitude.

.

Not quite sure what you mean with that statement, but here is how it works for cars, petrol and diesel and for planes, with reciprocating propellor and or jet engines with or without TC. They all produce less power as they climb. TC on reciprocating engines make up for some as it crams more air into the cylinder, allowing more fuel. Due to the less dense air, there is also less drag on a plane. This is actually a huge factor in aircraft performance. Whilst indicated air speed for a certain throttle setting remains more or less the same, speed over the ground increases as you gain altitude and the engine actually produces less power but the drag reduces as well. Simply put cars and planes produce the most power whilst on terra firma. For planes this is great as you need all the power you can muster to take off. Anorak fact, planes produce the most power at Amsterdam airport as it is the only international airport in the world below sea level, imagine what that does for the power output!

As you climb either the pilot or a little computer continuously needs to lean the air mixture on a reciprocating engine.
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Old 2nd January 2015, 16:47   #97
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by Jeroen View Post
I've been in India now for 3 years and I can honestly say, Ive never been in a car where the engine got revved over 1500 RPM.
well, even i was under that impression(low rpm= good FE) but then a Chinese made ELM327 dispelled all such notions.

The ECU just pumps in more diesel in to compensate for the driver who keeps pressing the A pedal instead of downshifting, like the pilot of AF447 who kept pulling the stick.
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Old 2nd January 2015, 18:19   #98
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Originally Posted by Aromal View Post
Jet engines seen on aeroplanes are designed to working in higher altitudes & they compress air in different stages. When old piston engines were replaced by Jet engines, there undergoes a significant increase in the cruising altitudes. This was not possible in the case of piston driven or propeller-shaft air-crafts.

s.

Actually there are plenty of propellor driven planes that can reach altitudes upwards of 40.000 feet, so comparable with commercial airliners. Even the little propellor planes such as the Cirrus SR22, I fly can easily reach 25.000 feet. You need to put an oxygen mask on as the cabin is not pressurized. And this is only the certified ceiling. The physical ceiling would be higher.

The big advantage of jet engines is their thrust specific fuel consumption, so that rate of fuel flow is proportional to drag, rather than power. See my earlier post on drag.

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Old 2nd January 2015, 20:23   #99
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by Aromal View Post
Why? Do you think engines at higher altitude is running on richer air-fuel ratios? Then what's your take on the little or no difference in efficiency of engine at higher altitudes ?
Yes! Engines (not turbocharged ones) run on richer mixtures at high altitudes. Turbocharged engines increase the air intake according to the demand put on them (may it be in the form of hard throttling or in the form of low atmospheric pressure), so that the air-fuel mixture is always in optimum ratio for best combustion.
Efficiency and effectiveness are two different things as pointed out earlier in this discussion. Richer mixture at high altitudes makes engine less effective rather than less efficient.



Quote:
Jet engines seen on aeroplanes are designed to working in higher altitudes & they compress air in different stages. When old piston engines were replaced by Jet engines, there undergoes a significant increase in the cruising altitudes. This was not possible in the case of piston driven or propeller-shaft air-crafts.
I do agree that high altitude is not possible with proprller-shaft aircrafts, but it is for the same reason that air at very high altitudes is so thin that it just refuses to burn the fuel or you can say air-fuel mixture becomes so rich that there is not enough oxygen to cause a blast.



Quote:
You are saying turbo-charger in cars can so something Jet-engines can't ?? I find it amusing, since both of them are basically gas turbines.
I'm not saying but it's a fact that turbos are doing what jet-engines can't do, turbos are driving small cars which jet-engines can't even imagine!!!
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Old 3rd January 2015, 03:38   #100
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Originally Posted by rev_rohit View Post
Yes! Engines (not turbocharged ones) run on richer mixtures at high altitudes. Turbocharged engines increase the air intake according to the demand put on them (may it be in the form of hard throttling or in the form of low atmospheric pressure), so that the air-fuel mixture is always in optimum ratio for best combustion.

Efficiency and effectiveness are two different things as pointed out earlier in this discussion. Richer mixture at high altitudes makes engine less effective rather than less efficient.







I do agree that high altitude is not possible with proprller-shaft aircrafts, but it is for the same reason that air at very high altitudes is so thin that it just refuses to burn the fuel or you can say air-fuel mixture becomes so rich that there is not enough oxygen to cause a blast.







I'm not saying but it's a fact that turbos are doing what jet-engines can't do, turbos are driving small cars which jet-engines can't even imagine!!!

No, they don't run rich, you would burn the valves and do a lot of damage very quickly and it would give very poor fuel efficiency. TC on not, reciprocating plane engine get leaned, either manually or automatically. As a pilot you are constantly monitoring your engine parameters to avoid running rich for too long.

High altitude IS possible with propellor aircrafts, plenty of aircraft that can do,wel over 40.000 feet. Second world war B29 were already flying and bombing from 32.000 feet!

Jeroen
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Old 3rd January 2015, 08:49   #101
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by Jeroen View Post
No, they don't run rich, you would burn the valves and do a lot of damage very quickly and it would give very poor fuel efficiency. TC on not, reciprocating plane engine get leaned, either manually or automatically. As a pilot you are constantly monitoring your engine parameters to avoid running rich for too long.

High altitude IS possible with propellor aircrafts, plenty of aircraft that can do,wel over 40.000 feet. Second world war B29 were already flying and bombing from 32.000 feet!

Jeroen
Let's consider an imaginary situation where you take a Royal Enfield Bullet in your plane which has non pressurised cabin like Cirrus SR 22 and start that bullet over 40,000 feet. Will it run on rich mixture or not?
I think it would not start only!!!
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Old 3rd January 2015, 15:13   #102
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by rev_rohit View Post
Yes! Engines (not turbocharged ones) run on richer mixtures at high altitudes.
In that case, Could you explain soot formation at high altitudes?

Quote:
Originally Posted by rev_rohit View Post
Turbocharged engines increase the air intake according to the demand put on them (may it be in the form of hard throttling or in the form of low atmospheric pressure), so that the air-fuel mixture is always in optimum ratio for best combustion.
Efficiency and effectiveness are two different things as pointed out earlier in this discussion. Richer mixture at high altitudes makes engine less effective rather than less efficient.
Actually effectiveness is a bad term, In many textbooks Effectiveness & Efficiency is considered same. Terms Effectiveness are often confused. We all talk about Theoretical efficiency, Thermodynamic, Actual efficiency and never about "thermodynamic effectiveness". Usually effectiveness is often used with Heat ex-changers, Heat pumps etc.
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Old 3rd January 2015, 15:24   #103
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Quote:
Originally Posted by rev_rohit View Post
Let's consider an imaginary situation where you take a Royal Enfield Bullet in your plane which has non pressurised cabin like Cirrus SR 22 and start that bullet over 40,000 feet. Will it run on rich mixture or not?

I think it would not start only!!!

There is a difference between the engine on the Cirrus and the engine on a Bullet. The engine on the Cirrus has, next to the throttle lever, an additional lever to lean the mixture. Never seen that on my Bullet, whereas all the Cirrus, and every other plane I have piloted have a mixture/lean control

http://www.gaceflyingclub.com/member...ingArticle.pdf

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Old 3rd January 2015, 18:17   #104
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by Jeroen View Post
There is a difference between the engine on the Cirrus and the engine on a Bullet. The engine on the Cirrus has, next to the throttle lever, an additional lever to lean the mixture. Never seen that on my Bullet, whereas all the Cirrus, and every other plane I have piloted have a mixture/lean control

http://www.gaceflyingclub.com/member...ingArticle.pdf

Jeroen
Does that mixture/lean control lever increase air intake or reduce fuel intake to make the mixture leaner?

It has to be increase in air intake rather than reducing the fuel intake.

Then the mixture/lean control is doing the same thing as that of a Turbocharger. Just controlled manually with a separate lever or with a microprocessor or microprocessor overridden by pilot's command.

Whatever it is, it has to be a Turbocharger may be with a fancy name which takes compressed air into the engine to make it burn the fuel completely to compensate for loss of power at high altitudes.
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Old 3rd January 2015, 19:07   #105
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by Jeroen View Post
High altitude IS possible with propellor aircrafts,
BIGGEST examples:
TU95 (Bear) Turboprop
Convair B36 Piston (initially at least) prop.

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