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Old 3rd October 2013, 18:37   #76
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by Aroy View Post
As far as I can recollect, the idea of turbo is to increase the volume of air so that more fuel can be accommodated (constant air/fuel ratio) in a charge.
The volume of air remains exactly the same whether turboed or not since that is governed by the size of the cylinder when piston is at B.D.C. It is the density and mass of air that is increased which in effect means more molecules of Oxygen to combine with the Hydrocarbons.
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Old 3rd October 2013, 18:40   #77
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AFAIK the intercooler decreases the air temperature and helps normalize it because when a turbo pressurizes the air, its temperature rises. And it is some law of thermodynamics and the carnot heat engine thing that states the lower the temperature of air the better is the extraction of thermal energy. So this is done to decrease the temp of air, and this in turn increases the density also. Hence allowing for more pressurizing oppertunity, I believe.
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Old 3rd October 2013, 18:45   #78
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by katchkamalesh View Post
Aroy, I am of the belief that denser air and high injection pressures leads to a fine atomization

Suppose a turbo is designed to kick in at 1500 RPM and the engine idles at 700 RPM. Now, you are driving at a slow pace (you do not need to cross 1300 RPM), in this case, you will still be pressing the accelerator (to go from idle to 1300 RPM) and the turbo will never going to kick in. Is this still called a turbo lag?
ON the contrary denser air makes it very difficult to atomize the injected fuel since the droplets tend to clump together.

Turbo lag is exactly what you mentioned i.e the time between idle and turbo spooling up to overcome the hindrance of the impeller. Till spool up the turbo is actually an hindrance to exhaust flow and power. Turbo is always an hindrance but after spool up it helps overcome the loss in power caused by the hindrance in the first place.
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Old 3rd October 2013, 20:43   #79
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by katchkamalesh View Post
1. Aroy, I am of the belief that denser air and high injection pressures leads to a fine atomization and higher the degree of atomization, higher the completeness of combustion. What is your view on this?

........

2. Again using a inter cooler as the name suggest is to cool the intake air (whose temperature has been increased due to compression) and the desired end result is increase in the density of air (which I had pointed out).

3. You are right with a zillion links and websites, As a market researcher it is my duty to filter good sites from the bad ones. The reason I had pasted the links, is that you can look at the content and decide on the worthiness of the website for yourself.

By the way, times have changed and there are a bunch of people like me (and also in IT industry) who rely on WWW for information. Whether you take the information or not is at your own discretion. To me, the WWW has given me access to knowledge.

4. I also read some article sometime back, it had mention of a type of turbo using exhaust gas recirculation to harness the energy from the waste gases. Here the end result is increase in fuel efficiency and not increase in power.

5. Turbos running on electricity is new to me. Wont these put pressure on the batteries/electricals which have been strained all ready?

.....
I have just numbered the relevant paragraph to ease answering them.

1. Actually as noted in a couple of posts before denser air has more mass. The air-fuel mixture is mass based, hence denser the air the more fuel you can utilize. Do note that at higher altitudes the air is less dense hence less mass which results in an extremely rich mixture if you do not adjust the fuel. It is done in ECU controlled injection systems, but lacking in the older carburetor system. This is one of the reasons that engines with turbos fare better at high altitude.

2. Yes, and to prevent pre-ignition in extreme cases.

3. The data on web has to be taken with a pinch of salt, and verified with cross references. In today's age Web based search can still yield useful and authoritative information provide the source is verified. You have both frivolous sources (which are just personal beliefs with no rational) as well as authoratative sources - professional journals, proceedings of conferences, etc. The onus of vetting information is on you. Just as you will verify critical (to you) information from doctors, lawyers and CA's with multiple sources, so must it be with WEB.

4. Exhaust gasses are used mainly to extract thermal rather than mechanical energy in large engines. In fact in cold climates the electric generators are used both to generate electricity as well as to give heat (from exhaust gasses). Similarly large turbines are at times connected in series, each successive one having lower operating pressure to extract maximum energy from the fuel. You could go on increasing stages, but as the size of turbine grows as the pressure is lowered, you stop at some point to reduce the capital costs.

5. Electric turbos are used mainly at lower RPM. Exhaust driven turbos slowly take over as the RPM increases. This is to ensure that the engine is always has "turbo" operating. The results are better power as well as FE at lower RPM.
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Old 3rd October 2013, 21:48   #80
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Default Re: Engine size inversely proportional to Fuel efficiency?

Rushing in foolishly where angels fear to tread.
@Katchkamalesh: Except for the density thing, must say I'm more or less in agreement with Jeroen.
Always willing to learn, so please don't hold back on the details. Will try to keep up. (If over our heads, will ask you to clarify, and am sure you'll oblige)

@Jeroen: Any thoughts as to why a turbo is De rigueur for modern diesels?

Regards
Sutripta

Last edited by Sutripta : 3rd October 2013 at 21:49.
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Old 3rd October 2013, 22:05   #81
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Default Re: Engine size inversely proportional to Fuel efficiency?

Right guys, now we're rocking!!! Lots of members kicking in with opinions and facts. That's what a forum like these should be about!!

Great job! Keep it coming!

Quote:
Originally Posted by katchkamalesh View Post
Aroy, I am of the belief that denser air and high injection pressures leads to a fine atomization and higher the degree of atomization, higher the completeness of combustion. What is your view on this?
Love to hear more opinions and facts on this as it is key to the discussion we are having. Mind you, I don't believe the air is denser as the compression ratio tends to be lower.

Still, begs the question, does denser air allow for better, more efficient combustion?? Any takers?

Quote:
Originally Posted by katchkamalesh View Post
By the way, times have changed and there are a bunch of people like me (and also in IT industry) who rely on WWW for information. Whether you take the information or not is at your own discretion. To me, the WWW has given me access to knowledge.
There is nothing wrong with relying on the web for information, but it is really down to how you validate whether its relevant and true, ie what are your criteria to conclude it's true?

To be honest, I don't have a very scientific method. I look at the text, the source, I look at how it;s written, whether I feel it is generic information or backed up by imperial or scientific facts. Then I decide whether I believe it or not. Not very scientific I grant you that. But on topics I consider myself knowledgable, I am very critical, and on this particular topic there is an awfull lot of nonsense out there. True, in the eye of the beholder. But that is where a forum likes this comes into its own. Here we have real individuals that can make a point, drill down into facts or the lack there off.

As stated before I'm an old git, so I don't necessarily get the WWW, still struggling with Facebook for that matter. I am of the opinion though that the Internet offers a wealth of information, but that the more classic way of gathering knowledge, ie studying, books, classes, research, discussions is more meaningfull than googling for a particular topics/answer. The reason is very simple. It takes me a few minutes to type this whole reply. But to write a book to teach for an hour a college take far more time in preparation. And is likely to be much more thorough. The real problem with the net is that everybody can pose as a specialist and quite a few do, but building real knowledge, experience and competence takes much more than surfing the web. I know I'm old git, but I do believe very strongly in classical education, where you discuss and probe real experts to broaden your own horizon.

I have been called many names, quite a few of them not very flattering. But nobody that knows me would call me an academic. Still, I have come to appreciate the very essence of a more academic approach to anything in life. And it's really at the level of a toddler. When somebody explains or argues a point and you can still ask: Why? You know you haven't really understood the topic at hand at all. That doesn't mean you need to be an expert on every topic, but be asking "why" you'd be surprised how many so called experts shut up. Again, opinion, I think they just haven't got a clue and at best googled around the WWW a bit. No harm in that, but it doesn't bring any real value. Insight, the attitude to dig in and to want to understand a topic at hand is what I find is relevant. Again, just an old git's opinion. My kids think I'm going mental. I don't think they know how to read a book, but they are way ahead of me on their smart phones though!

I'm very lucky as tomorrow I'm leaving for a course at the Sante Fe Institue, NM, USA to do just that. http://www.santafe.edu. Spending a whole week with top experts in different fields, disciplines and expertise discussing, probing their minds and learning about how to deal with complexity and innovation. So I might drop out of this discussion for the next week or so.

Quote:
Originally Posted by katchkamalesh View Post
Turbos running on electricity is new to me. Wont these put pressure on the batteries/electricals which have been strained all ready?

With BMW recommending people to drive their cars for at least 20 KM per week to maintain the life of the battery, I have reasons to believe that electric turbos will strain the system.
Was new to me as well, at least for automative applications. It will put additional strain on the dynamo/battery as it is an additional load. Its the engine, (indicated horse power as I stated before) that will make need to make up for it. So you loose a bit of power to the wheels, because you need to drive an electric motor to drive the turbo. Very similar to a supercharged engine where the same is applicable for driving the supercharger. The difference with the electric driven turbo is that you can hook it up to the ECU and control it very efficiently. I suppose you could hook up a supercharger to an electric motor too?

Quote:
Originally Posted by drpullockaran View Post
Yes another mind bender. The intercooler brings the air density UP or more or in other words increases air density and brings the temperature down.

Not to worry I too go through these episodes more often than not; but thankfully not when I am treating patients.
Oh man, this is getting embarrassing!! Put it down to me being an old git, who can't remember his thermodynamic lesson anymore!!

Please be careful with those patients!

Jeroen

Last edited by Jeroen : 3rd October 2013 at 22:14.
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Old 3rd October 2013, 22:36   #82
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Guys, isolate the engine from other systems, otherwise there are too many variables which effect the Fuel consumption .

Heck , two same cars , one run in and the other brand new both can have variations up-to 10% (sometimes also more ) when tested under same conditions. I remember we testing one in a chassis dyno.

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Old 3rd October 2013, 22:38   #83
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by Jeroen View Post

Still, begs the question, does denser air allow for better, more efficient combustion?? Any takers?
Combustion or its efficiency in an IC engine is a function of oxygen availability, flash point of the fuel and its calorific value. It does not have anything to do with density of the medium in the cylinder. But yes, rate of discharge of power/energy after combustion is a function of pressure and higher the pressure faster is its propagation.

In my opinion, denser the medium more difficult to propagate the combustion, thus greater the inefficiency as well as it necessitates greater mechanical intervention to pump the fuel into the cylinders in form of higher pressure fuel pumps, MPFI and et el.
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Old 5th October 2013, 16:33   #84
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by Jeroen View Post
Still, begs the question, does denser air allow for better, more efficient combustion?? Any takers?
Not directly! Density of air has absolutely nothing to do with combustion efficiency.
As mentioned above density of air is directly proportional to the amount of oxygen available for combustion. This will allow for MORE fuel to be burnt, hence more power but not better combustion efficiency.

But it can also be argued that soot (from diesels) and other gases are a result of incomplete combustion. And that addition of more oxygen give more opportunity for better combustion.

There are quite a few papers published on the oxygen enrichment of intake air in an IC engine and its effects. In almost all the papers the result has been increased efficiency and decreased emission with the ONLY exception being NOx gases owing to their Chemical mechanisms of formation.

sciencedirect.com has a couple of papers on the aforementioned topic.

Last edited by rangakishen : 5th October 2013 at 16:35.
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Old 20th December 2014, 23:49   #85
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Not directly! Density of air has absolutely nothing to do with combustion efficiency.
In my opinion density of air definitely makes a difference in combustion, that's why we get less power and more soot out of our diesel cars when on a high altitude terrain.
In other words, because ratio of oxygen in air is constant (~21%), less dense air means less amount of oxygen available for combustion per litre of air intake into the engine. Therefore if density of air is less, efficiency of engine would fall due to incomplete combustion.
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Old 30th December 2014, 21:41   #86
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Default Re: Engine size inversely proportional to Fuel efficiency?

IMO, cooler air = denser air (intercooler), denser air = more oxygen for better / more complete burning of fuel, therefore increasing efficiency.

From personal, actual experience:

I lived in the US for a long time and owned a Camry 3.0L NA V6 5-speed automatic (torque converter). Was getting 9.8 kmpl in city use. I live in Bangalore now and drive a Linea T-Jet with a 1368 cc turbocharged engine with a 5-speed manual. I get 7.5 kmpl in city use.

Traffic conditions are similar. Even on the highway, I got better mileage with the Camry. It was difficult to accept the low mileage of the Linea, but I've come to terms with it.

Go figure.
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Old 30th December 2014, 23:06   #87
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The term people should refer here is Compression ratio .
The problem is that this air has to enter the engine in the mili seconds it has .
It is not colder air that has to enter.
Colder air will have less volume so more amount of air can enter in that short amount of time.
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Old 31st December 2014, 03:56   #88
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We need to get our terminology right. Engines don't become less efficient at higher altitudes. If anything, at least theoretically they become more efficient. This is due to theoretical thermal efficiency.

This measure of efficiency is determined by the difference between the temperature of the burning fuel, and the temperature of the heat sink to which the burnt gases may be exhausted. The higher the altitude, the lower the temperature and hence at least a theoretical improvement in efficiency.

What is happening is as the air is less dense there are fewer oxygen molecule around, but the amount of fuel injected gets adjusted for that. So the engine produces less output, ie it is less effective, not less efficient.
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Old 31st December 2014, 13:29   #89
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Default Re: Engine size inversely proportional to Fuel efficiency?

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Originally Posted by Jeroen View Post
We need to get our terminology right. Engines don't become less efficient at higher altitudes. If anything, at least theoretically they become more efficient. This is due to theoretical thermal efficiency.

This measure of efficiency is determined by the difference between the temperature of the burning fuel, and the temperature of the heat sink to which the burnt gases may be exhausted. The higher the altitude, the lower the temperature and hence at least a theoretical improvement in efficiency.

What is happening is as the air is less dense there are fewer oxygen molecule around, but the amount of fuel injected gets adjusted for that. So the engine produces less output, ie it is less effective, not less efficient.
Jeroen

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.
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Old 31st December 2014, 14:23   #90
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Default 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.

Just about all modern car, or anything with an ECU has an air mass flow meter. Thats exactly what it does, measures the mass of air so the ECU can calculate the required amount of fuel to be injected. It constantly allows for changes in atmospheric pressure to be taken into account.

See http://en.m.wikipedia.org/wiki/Mass_flow_sensor

There is a misconception about atmospheric pressure. Everybody sort of knows it will decrease as your altitude goes up. Here's some of the basics

http://en.m.wikipedia.org/wiki/Atmospheric_pressure

Never the less, atmospheric pressure changes all the time, no matter where you are or how high or how low you are. Just check the weather forecast. The difference between a high a low pressure area can easily be measured in tens of millibars. For an engine its all the same, it doesn't care about altitude. It cares about atmospheric pressure.

I see soot even in Delhi, I don't need to go to the Himalayas for that. Usually a combination of not particularly well maintained engines, poor fuel, overloading and poor driving technique. You try to haul any car up a hill in the wrong gear at too low an rpm and you will get soot. For some reason Indian drivers appear to be very reluctant to rev their engines. Even when the situation calls for it, such as a steep incline. 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.

When we visited Leh, Ladakh, our driver took us all the way up to nearly 18.000 feet to the worlds third highest motor-able pass, Changla, at 1200 RPM all the way! plenty of soot coming out of his Innova. No air mass flow meter or piece of electronics can make up for poor driving technique.

Jeroen

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Last edited by Jeroen : 31st December 2014 at 14:39.
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